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Schmalzried.pdf
Copyright 1999 by The Journal of Bone and Joint Surgery, Incorporated
Current Concepts Review
Wear in Total Hip and Knee Replacements*
BY THOMAS P. SCHMALZRIED, M.D.†, LOS ANGELES, CALIFORNIA, AND JOHN J. CALLAGHAN, M.D.‡, IOWA CITY, IOWA
Before the advent of total joint replacement, patients
who had end-stage arthritis of the lower extremities had
unremitting pain and a greatly decreased functional capacity. In addition, they often were confined to a wheelchair and were dependent on the care of others. Today,
the outcomes of primary total hip and knee replacement
are predictable and usually excellent35,45,57,64,212,221. Prosthetic joint replacement has dramatically improved the
lives of millions of people worldwide.
As the fixation of total joint implants has become
more reliable and durable and as the technology of total
joint replacement has been applied to younger and
more active patients, the current limitations of total
joint arthroplasty are related to the wear of the components248. Wear is the removal of material, with the generation of wear particles, that occurs as a result of the
relative motion between two opposing surfaces under
load. In complex mechanical-biological systems such as
total hip and knee replacements, there can be many
types of wear. Although the mechanical consequences
of wear, such as progressive thinning of polyethylene
components, can limit the functional life of a joint replacement, the clinical problems from wear more frequently are due to the release of an excessive number
of wear particles into a biological environment. When
particles within a certain size-range are phagocytized in
sufficient amounts, the macrophages enter into an activated state of metabolism, releasing substances that can
result in periprosthetic bone resorption. Progressive loss
of periprosthetic bone can necessitate a reoperation,
which is the definitive measure of clinical failure of a
joint arthroplasty.
Wear Modes
It is important to distinguish among the fundamental
mechanisms of wear (adhesion, abrasion, and fatigue);
*One or more of the authors has received or will receive benefits for personal or professional use from a commercial party related
directly or indirectly to the subject of this article. In addition, benefits have been or will be directed to a research fund, foundation,
educational institution, or other nonprofit organization with which
one or more of the authors is associated. Funds were received in
total or partial support of the research or clinical study presented in
this article. The funding source was National Institutes of Health
Grant AR43314.
†Joint Replacement Institute, 2400 South Flower Street, Los
Angeles, California 90007.
‡Department of Orthopaedic Surgery, University of Iowa College of Medicine, Iowa City, Iowa 52242-1008.
VOL. 81-A, NO. 1, JANUARY 1999
the changes in the appearance (the morphological characteristics) of the bearing surfaces, which are referred to
as wear damage; and the conditions under which the
prosthesis was functioning when the wear occurred,
which have been termed the wear modes156. Adhesion
involves bonding of the surfaces when they are pressed
together under load. Sufficient relative motion results
in material being pulled away from one or more surfaces, usually from the weaker material. Abrasion is a
mechanical process wherein asperities on the harder
surface cut and plow through the softer surface, resulting in removal of material. When local stresses exceed
the fatigue strength of a material, that material then
fails after a certain number of loading cycles, releasing
material from the surface. One or more of the classic
mechanisms of wear may be operating on the prosthesis in a particular wear mode, and a prosthesis may function in several wear modes over its in vivo service life.
The predominant type of wear of one prosthetic joint
can differ from that of another. Furthermore, in a specific joint, there may be different types of wear occurring
at different times over the service life of the implant. The
damage to an implant is a result of all of the mechanisms
of wear that have acted on it over its service life, with
the most recent events having the greatest influence.
Mode-1 wear results from the motion that is intended
to occur between the two primary bearing surfaces, such
as the motion of the prosthetic femoral against the polyethylene acetabular bearing surface. Mode-2 wear refers to the condition of a primary bearing surface that
moves against a secondary surface that it is not intended to move against. For example, mode-2 wear occurs when a femoral component penetrates through
a modular polyethylene bearing and rubs against the
metallic tibial base-plate or acetabular shell8,22,67,131,148,231.
Mode-3 wear refers to the condition of the primary
surfaces as they move against each other but with the
interposition of third-body particles. In mode-3 wear, the
contaminant particles directly abrade one or both of
the primary bearing surfaces. This type of wear is known
as three-body abrasion or three-body wear. The primary
bearing surfaces may be transiently or permanently
roughened by this interaction.
Mode-4 wear refers to two secondary (nonprimary)
surfaces rubbing together. Examples of mode-4 wear
include impingement of the prosthetic femoral neck
on the rim of the acetabular component249; motion at
115
116
T. P. SCHMALZRIED AND J. J. CALLAGHAN
the stem-cement or bone-cement interface or relative
motion of a porous coating, or other metallic surface,
against bone2,6,30,90,115,164; relative motion of the external
surface of a modular polyethylene component against
the metal support (so-called backside wear); fretting
between a metallic substrate and a fixation screw67,110,142,188;
and fretting and corrosion of modular taper connections232 as well as that of extra-articular sources104,133,218.
The particles that are produced by these types of wear
may be composed of bone, polymethylmethacrylate,
metal alloys, metallic corrosion products, or hydroxyapatite2,24,214,232. Wear particles produced by mode-4 wear
can induce an inflammatory reaction and can be transported to the bearing surfaces and induce three-body
wear (mode-3 wear)104,181,232.
Mode-1 wear is necessary for the joint prosthesis
to function, whereas modes 2, 3, and 4 are generally
unintended. The operating conditions in vivo are variable, and several types of wear can occur simultaneously. The clinical importance and interaction of
wear modes may be clarified by a hypothetical example. A total hip or knee prosthesis that is well fixed
and well functioning has a low rate of mode-1 wear
of the polyethylene articular surface or surfaces. The
gradual release of polyethylene particles into the periprosthetic tissues can result in a slow rate of interfacial
bone resorption201, which can lead to an increase in the
relative motion between the implant and the adjacent
bone. Such relative motion causes mode-4 wear, which,
depending on the type of joint prosthesis, can generate
particles of bone, cement, metal, or hydroxyapatite.
These hard particles can affect the baseline mode-1
wear by passing through the articulation and resulting in a transient three-body wear mechanism (mode-3
wear). The femoral component can be scratched by this
interaction. Additionally or independently, hard particles may become embedded in the polyethylene and
act as an ongoing abrasive source. The increased roughness of the bearing surface of the femoral component
can then increase the rate of polyethylene wear in
mode 1 because of the increased two-body abrasive
wear42,64,66,153,171,238. This wear of the polyethylene can increase the rate of production of polyethylene particles,
which increases the rate of periprosthetic bone resorption, leading to additional relative motion and eventually to the possible clinical failure of the implant. The
time-course for such a sequence of events is variable.
From a practical perspective, a problem with fixation
leads to a problem with wear and vice versa.
Friction and Frictional Torque
Friction is the resistance to movement between two
surfaces in contact. The degree of resistance is proportional to the load. The ratio between frictional force and
load (friction/load) is the coefficient of friction (µ)19.
Frictional torque is the force created as a result of the
friction of bearing. Charnley et al. initially selected a
stainless steel-on-polytetrafluoroethylene bearing couple because of a low coefficient of friction39,43. The small,
22.25-millimeter-diameter head (subsequently referred
to simply as a twenty-two-millimeter head) was selected
to minimize the moment arm of the frictional forces
and, thus, to minimize the frictional torque (coefficient
of friction [µ] × r2). Thick walls of polyethylene increased
the outer diameter of the Charnley socket, which distributed the frictional torque over a large fixation area.
Unfortunately, Charnley hip components with the polytetrafluoroethylene bearing uniformly failed because of
rapid wear with the release of polytetrafluoroethylene
wear particles, formation of granulomas, and loosening
of the component86,238. Thus, despite a sound theoretical
premise supported by laboratory investigations, this
early clinical experience demonstrated the difficulty of
predicting wear resistance in vivo as well as the adverse
effects of wear particles.
Contrary to theoretical considerations, frictional
torque has not been demonstrated to be important in
the initiation of aseptic loosening of either femoral or
acetabular components161,207. Accumulating evidence indicates that wear particles have a far greater effect on
the durability of the fixation of the implant than does
frictional torque. From this perspective, the subsequent
success of the Charnley low-friction arthroplasty with
a polyethylene acetabular component is primarily a
function of the low volumetric wear of the twenty-twomillimeter metal bearing, not of low frictional torque.
Bearings with a larger diameter can be successful if
the rate of wear is low161. This is an important consideration as methods to reduce polyethylene wear, such
as extensive cross-linking of polyethylene or utilization
of hard-on-hard bearings (such as ceramic-on-ceramic
and metal-on-metal), are being investigated. The coefficient of friction for the metal-on-metal bearing of the
McKee-Farrar hip prosthesis is roughly two to three
times greater than that of the Charnley prosthesis. The
larger diameter of the McKee-Farrar bearing couple,
which is approximately forty millimeters, amplifies this
difference, and as a result the frictional torque is as
much as ten times greater than that of the Charnley
bearing couple207. This value is still only about onetenth the static torque to failure of an acetabular component immediately after it has been freshly implanted
with cement5,63,160,235.
Polyethylene Wear
Most total hip replacements and effectively all total
knee replacements have one primary bearing surface
made of ultra-high molecular weight polyethylene (referred to simply as polyethylene). Although there are
many potential sources of wear particles, in most total
hip and knee replacements the greatest contribution is
from mode-1 wear — that is, polyethylene particles generated by the intended motion of the joint at the primary bearing surfaces. For practical purposes, wear of
THE JOURNAL OF BONE AND JOINT SURGERY
WEAR IN TOTAL HIP AND KNEE REPLACEMENTS
the much harder bearing surface of the femoral component is negligible in mode 1. Polyethylene wear is distinct from creep, which is plastic deformation due to
loading. Creep contributes to the deformation of a polyethylene bearing but does not produce wear particles.
The rate of creep decreases rapidly over time, becoming
negligible by the first twelve to eighteen months after
implantation113,253. Wear accounts for most of the change
in the surface of a polyethylene bearing over the longer
term113,253.
There are a myriad of variables that affect the wear
of a polyethylene bearing in vivo, including the wear
resistance of the materials as well as the loads, lubrication, sliding distance, motion pattern, specifics of
the design and manufacturing of the polyethylene
component, implantation techniques, type of wear, and
amount and type of use of the joint. The wear resistance of polyethylene is a function of the base resin,
the manufacturing, and the method of sterilization of
the polyethylene component48,143,223. Oxidation reduces
the static strength and elongation properties of polyethylene and decreases the resistance of polyethylene
bearings to fatigue, leading to higher in vivo rates of
wear. A time-dependent increase in the amount of oxidation can result from gamma irradiation in air, which
until recently was the most common method of sterilization in the orthopaedic industry. Gamma irradiation breaks molecular bonds in the long polyethylene
chains, giving rise to free radicals. When present, oxygen can combine with these free radicals. Peak levels
of oxidation typically occur about one to two millimeters below the surface of a polyethylene component.
As the degree of this subsurface oxidation increases,
so does the occurrence of fatigue cracking and delamination, as has been observed in retrieved tibial components46-48. Components that have been sterilized less
than one year before implantation exhibit lower in vivo
oxidation and better in vivo performance than components with a longer so-called shelf life before implantation52. In laboratory wear tests, polyethylene that had
been sterilized with gamma irradiation in air and had
been aged exhibited a higher rate of wear than material
that had not been irradiated71,159.
When free radicals are formed in polyethylene, such
as by gamma irradiation, cross-linking of polyethylene
molecules is a competing reaction to oxidation. Crosslinking also changes the material properties of polyethylene, but it can improve the resistance to wear. The
relative amount of oxidation and cross-linking varies
with the depth from the surface of the component and
results in a corresponding variation in the resistance of
the material to wear as a function of the depth from the
surface. In general, as oxidation increases, cross-linking
decreases and vice versa159. Methods for controlled
cross-linking include the use of chemicals (peroxide),
variable-dose gamma irradiation, and electron-beam irradiation. Clinical studies have indicated a substantial
VOL. 81-A, NO. 1, JANUARY 1999
117
reduction in wear associated with cross-linked polyethylene185,253. Recent studies with use of a wear simulator have indicated that, with optimum cross-linking, the
type of wear that occurs in acetabular cups can be reduced by more than 95 percent120,159,236. However, crosslinking may not have the same degree of benefit with
regard to the types of wear that occur in total knee
replacement236.
Polyethylene contact stresses are a function of the
thickness of the component as well as the load and the
contact area12. A minimum thickness of six millimeters
is recommended for a conforming articulation such as
a total hip replacement. In general, from a materials
standpoint, a thickness of more than six millimeters is
preferred for bearings with less conformity13. Polyethylene wear is also a function of the motion pattern.
In wear tests that use a linear motion path, such as
a reciprocating pin-on-disk, the rate of polyethylene
wear for a given set of test conditions is ten to 100
times lower than that in wear tests that use crossing
motion paths151. Wear tests that use crossing motion
paths have been shown to produce more closely the
type and amount of wear occurring in vivo in total hip
replacements156,191.
The clinical manifestations of polyethylene wear in
total hip and total knee replacements include the
removal of material, which results in progressive penetration of the femoral head into the polyethylene acetabular component and a reduction in the thickness of
the polyethylene tibial bearing. A reoperation may be
necessary because of thinning or wear-through of a
polyethylene bearing or because of frank failure of the
component8,22,67,231. Progressive penetration of the femoral head into the polyethylene acetabular component
can lead to impingement between the femoral neck
and the socket, which may contribute to loosening of
the acetabular component43,249. Asymmetrical wear of
a tibial polyethylene bearing can alter the mechanical axis of the knee and thereby increase the rate of
wear in that compartment because of the increased
load33,190,237.
Roughness of the Countersurface
When a joint prosthesis has one polyethylene bearing surface, the other bearing surface is commonly referred to as the countersurface. The base material and
the specifics of manufacturing, such as the method of
polishing, determine the initial (as-manufactured) surface characteristics of a femoral head or those of a
femoral component of a total knee prosthesis. The
microtopography of the surface determines its roughness. A contact or noncontact (laser) stylus can be used
to scan the surface and make an analog recording of the
peaks and valleys over a specified length of the surface.
As a result of mode-3 wear, the original surface can be
damaged in vivo, resulting in a rougher surface.
Increased roughness of the femoral countersurface
118
T. P. SCHMALZRIED AND J. J. CALLAGHAN
may dramatically accelerate two-body abrasive wear
of polyethylene61. Experimental studies have indicated
that a threefold increase in the roughness of the femoral countersurface can cause at least a tenfold increase
in the rate of polyethylene wear50,70. Polyethylene wear
is sensitive to the specific type of surface damage. A
scratch of two micrometers in depth with an average
lip height of one micrometer may not substantially increase the measured average roughness of the surface,
but in laboratory wear tests such scratches increased
polyethylene wear thirty to seventyfold, depending on
the motion pattern72. The combined effects of increased
roughness of the countersurface and increased oxidation on wear may be synergistic rather than simply
additive16.
Damage to the countersurface is common and was
found on 89 percent of fifty-four cobalt-chromiumalloy femoral heads after a service life that ranged
from eight months to nineteen years119. The affected
areas were usually discrete, measuring from one to
twenty square millimeters. Similarly, in an analysis of
retrieved Charnley femoral components, 76 percent of
seventy-one stainless-steel heads had an average surface roughness (Ra) that was greater than the specification of the as-manufactured surface112. Increased
roughness of the countersurface of the femoral head
has been associated with substantially higher rates of
polyethylene wear in vivo, with the magnitude of the
increase dependent on the nature of the damage to the
countersurface89.
The mechanisms for generating an increased roughness of the countersurface in vivo apply to all materials, but the susceptibility to scratching is a function of
the hardness of the material. The decreased hardness
of titanium alloy, compared with stainless-steel and
cobalt-based metal alloys, results in decreased resistance
to abrasion. Although the initial surface roughness of
a titanium-alloy femoral head may be equivalent to
that of other bearing materials, there is a greater potential for its surface roughness to increase in vivo. In
an environment with few or no hard third bodies, the
wear performance of titanium alloy against polyethylene can be comparable with that of other metals152,153,158;
however, the presence of hard third bodies, such as
particles of cement or metal, has a greater adverse
effect on the performance of titanium alloy against
polyethylene148,153,181.
Ceramics are harder than stainless-steel and cobaltbased metal alloys and are therefore more resistant
to damage by third-body particles than are metal countersurfaces49. For this reason, the increased hardness of
ceramic materials is advantageous. Ceramic materials did not demonstrate a substantial advantage in a
laboratory joint simulator in which there were few or
no third bodies154,155. However, in clinical comparisons,
in which the operating environment of the articulation is more variable, ceramic heads have demon-
strated rates of wear that are lower than those of metallic heads62.
Polyethylene Wear Particles
Not until the 1990s was it recognized that most polyethylene wear particles are less than one micrometer in
size and are produced in very large numbers, even by
well functioning joints. Because the resolution of light
microscopy is limited by the wavelength of visible light
(0.4 to 0.7 micrometer), objects that are of submicrometer size cannot be clearly seen with a light microscope. With polarized light microscopy, submicrometer
polyethylene particles appear as a fine, diffuse, background birefringence in the cytoplasm of macrophages
and giant cells86,198. For this reason, polyethylene wear
particles may have been present but not appreciated
in some early studies of tissue from osteolytic lesions198.
Techniques have been developed to analyze wear particles generated in vivo by retrieving them from periprosthetic tissues33,100,156,166,168,198,214. The concentration of
wear particles from prosthetic joints can extend into the
billions per gram of tissue100,166,168,230. As suggested by light
microscopic observations, most of these are polyethylene wear particles166,214 that have linear dimensions that
are less than one micrometer 34,100,166,168,199,213,230.
Wear particles are a function of the type of wear that
produces them87,205. For example, a smooth, polished
femoral component moving against polyethylene in the
absence of third bodies, as in a laboratory joint simulator or in an optimally functioning total joint replacement, produces a visually smooth, so-called burnished
or polished-appearing polyethylene surface. This intended type of wear is the source of numerous submicrometer polyethylene wear particles156,198,201,205. There
are case-to-case differences in the mode and the frequency distribution of these small particles in total hip
replacements, and some data have indicated that the
distribution is influenced by the type and amount of
surface damage of the femoral head230.
Differences in the articulating surfaces and motion
patterns of total knee replacements as compared with
those of total hip replacements have important effects
on the wear of the polyethylene. Decreased conformity
can result in substantially increased contact stresses that
can exceed the yield strength of polyethylene4,13,46. Furthermore, in a total knee replacement, the motion pattern can include rolling, sliding, and rotation on the
same surface. The combination of these factors results
in differences in the mechanisms of wear for total knee
replacement compared with those for total hip replacement. In total hip replacement, the predominant wear
mechanisms appear to involve microadhesion and microabrasion with the generation of many polyethylene particles less than one micrometer in length156. In
contrast, subsurface delamination, pitting, and fatigue
cracking, with the release of much larger particles of
polyethylene, have been recognized as important mechTHE JOURNAL OF BONE AND JOINT SURGERY
WEAR IN TOTAL HIP AND KNEE REPLACEMENTS
anisms of wear in total knee replacement46,140,247. These
mechanisms result in the visually striking surface damage of some retrieved tibial polyethylene bearings140,245.
More variety is seen in the size, shape, and texture
of polyethylene particles associated with posteriorcruciate-retaining total knee replacements compared
with those associated with total hip replacements100,105,205.
Submicrometer granules are less prevalent in specimens
obtained from the tissues around total knee replacements than in specimens taken from the tissues around
total hip replacements. Larger flake-shaped particles,
measuring several micrometers in length and width, are
relatively common in association with total knee replacements but are not common in association with total
hip replacements. The overall average area of particles
generated by total knee replacements has been reported
to be about twice that of particles generated by total hip
replacements209. There are data indicating that specific
features of the type of total knee prosthesis can influence the size of the polyethylene particles that are
produced100. Furthermore, modular total hip and knee
components have several surfaces, such as the so-called
backside of the polyethylene, which can contribute polyethylene wear particles to the periprosthetic tissues. In
one study, tissues adjacent to failed total knee replacements had more particles per gram than those adjacent
to failed total hip replacements. However, the knee replacements had a longer service life and there was no
difference in the apparent rate of particle deposition100.
Polyethylene Wear in Vivo
The clinical assessment of the wear of a polyethylene bearing has traditionally been based on radiographic studies. As measured on standard radiographs,
the degree of penetration of the femoral component
into the polyethylene component is the linear wear of
the bearing. Because of practical issues related to the
alignment and reproduction of x-ray-beam projection, it
is more difficult to assess wear routinely on radiographs
of total knee replacements than it is on radiographs of
total hip replacements. For this reason, there is less information in the literature on the radiographic assessment of wear of total knee replacements45.
For total hip replacements, the method of radiographic measurement that is most commonly referenced
is a variation of the duoradiographic technique originally described by Charnley and Halley42 and reported
by Livermore et al.144. On standard anteroposterior radiographs of the pelvis, a compass is used to identify
the shortest distance from the center of the femoral head
to a reference point on the acetabular cup on the followup radiographs. A measurement is then made between
the same reference points on the initial postoperative
radiograph. After correction for magnification, the difference between the measurements on the initial postoperative and follow-up radiographs is the linear wear,
which is conventionally expressed in millimeters. The
VOL. 81-A, NO. 1, JANUARY 1999
119
linear wear rate is then calculated by dividing the linear
wear by the duration of time after implantation. The
linear wear rate is conventionally expressed as millimeters per year. This method only measures wear that occurs in the plane of the radiograph, and it cannot detect
any component of the wear vector that occurs out of the
plane of the radiograph. The maximum variation in the
measurements made by a single observer with use of the
Livermore technique was initially reported to be 0.1
millimeter144. However, other investigators have found
the variability to be between three and four millimeters32.
Electronic digital calipers can measure to the nearest
0.05 millimeter, and the mean intraobserver error with
this device is reported to about 0.08 millimeter243.
A more accurate term for this measurement is linear
penetration. The term linear wear has historically been
used synonymously with the term linear penetration
in discussions of polyethylene bearings. However, the
measurement of linear penetration includes several
factors in addition to wear, which is the removal of
material, with the generation of wear particles, from
the articulation of the femoral and acetabular components. Linear penetration includes creep (plastic deformation) and wear113. The contribution of creep to linear
penetration is greatest in the early postoperative period and decreases with time, becoming negligible by
twelve to eighteen months. Additionally, there is an initial running-in of the bearing, which results in better
conformity, lower contact stresses, and a lower rate of
wear12,13,253. In longer-term studies, decreasing patient activity over time can result in decreased wear; also, there
may be a survival selection for the better-functioning
implants (those with lower wear rates)210. For these reasons, short-term rates of linear penetration are higher
than long-term rates226,253. Modularity is another factor
that can contribute to linear penetration. Because of
issues related to creep, manufacturing tolerances, and
the in vivo assembly of modular acetabular components,
the acetabular liner may change position relative to the
acetabular shell, resulting in a change in the position of
the femoral head and an increase in the short-term rate
of linear penetration226. So-called backside wear of the
modular polyethylene liner could also contribute to
higher rates of linear penetration87,110.
Volumetric wear is a measure of the amount of material removed from the bearing surface. The simple
formula v = πr2w, in which v is the change in the volume
of the polyethylene bearing, r is the radius of the femoral head, and w is the measured linear wear, has commonly been used to calculate volumetric wear144. This
formula assumes a single, cylindrical wear track, but this
assumption is not supported by some retrieval studies254.
In the 1990s, techniques for computer-assisted measurement of wear have been developed54,169,216,227. Standard radiographs can be digitized to create a computer
model of the femoral head and the acetabular component. Use of both anteroposterior and lateral ra-
120
T. P. SCHMALZRIED AND J. J. CALLAGHAN
diographs allows construction of a three-dimensional
model. Comparison of serial radiographs gives both the
magnitude and the direction of the femoral head displacement. Such computer-assisted techniques can reduce measurement variability that is due to difficulties
related to the identification of a single reference point,
the angle of the radiographic beam, and positioning of
the patient. Edge-detection techniques that infer the
margins of the components by evaluating gray-scale intensity on digitized images of radiographs have been
developed. These techniques minimize the potential for
intraobserver and interobserver variability and should
enable more accurate comparisons of results between
institutions169,216.
Assuming a negligible contribution by creep in the
long term, the wear of retrieved polyethylene acetabular
components has commonly been measured with variations of the so-called shadowgraph technique132,249. With
this technique, a cast of the acetabular bearing surface
is made and the profile (the so-called shadow) of the
cast is used to measure the wear track. This method
allows determination of the angle of wear relative to the
mouth of the component so that a wear vector (magnitude and direction) can be determined. Single-plane radiographic measurements have slightly underestimated
the linear wear measured on the corresponding retrieved implants. This may be due, at least in part, to the
fact that the wear vector is not consistently in the plane
of the radiographs. Additionally, the wear track is not a
complete cylinder. Volumetric wear calculated from linear wear with use of a formula for a hemicylinder with
a correction factor for the direction of wear is about 47
percent less than that based on the simple cylindrical
formula132. Multiple wear vectors have been identified in
30 percent of one type of polyethylene acetabular component. The vectors do not appear to be a result of
loosening of the cup, but impingement between the edge
of the cup and the neck of the femoral component may
be a factor254.
Fluid-displacement methods have also been used
to measure the wear of retrieved polyethylene acetabular components121,225. A femoral head of appropriate
size is placed into the original articulating contour, and
the volume of fluid that is required to fill the remaining contour (the worn area) is measured. The interobserver variability is reportedly within five cubic
millimeters, and the accuracy is reportedly within fifteen cubic millimeters121.
The amount of wear of hard-on-hard bearing surfaces, such as metal-on-metal or ceramic-on-ceramic, is
typically so small that it cannot be measured on routine
clinical radiographs. Furthermore, the amount of wear
can also be too small to be measured on retrieved specimens with use of the shadowgraph technique207. Consequently, computerized coordinate measuring devices
have been employed to quantify the amount of wear.
These devices can be used to assess the sphericity of the
bearing surfaces by comparing the measured dimensions
in multiple planes with the best-fit sphere. The volume of
wear can be calculated by integrating the depth of multiple individual wear points on the worn surface157.
Studies of Wear in Vivo
The rates of polyethylene wear in total hip replacements have been reported in a number of previous studies10,11,32,36,39-41,55,56,84,98,104,112,121,132,144,145,180,183,184,201,216,225-227,243,249,250,252,253.
(The findings of those studies were summarized in table
form by one of us [T. P. S.] and colleagues in a previous
issue of The Journal of Bone and Joint Surgery211.) Many
variables influence polyethylene wear in vivo and, consequently, the rates are highly variable. These studies
have demonstrated substantial variability not only with
regard to the average rates of linear wear but also with
regard to the ranges (when reported). Regardless of the
duration of follow-up, a number of hip replacements
have no radiographically measurable wear and some
demonstrate wear that is several times the average for
the study. These large patient-to-patient variations in
rates of wear have not been explained by differences in
the wear resistance of the polyethylene239. This is not
surprising considering the number of variables that contribute to wear in vivo.
Patient-related variables include age, gender, weight,
general health, and activity as it relates to the use of
the hip prosthesis210. Variables related to the hip reconstruction include the implanted materials (including,
but not limited to, the polyethylene bearing material);
the design and manufacturing of the prosthesis; and the
characteristics of the implantation procedure, such as
the operative techniques, biomechanical considerations,
and the initial as well as the long-term fixation of the
implants. These variables are important with respect to
wear as they can affect the loads and the motions of
the bearing and the degree of three-body-wear mechanisms. There is also variability in wear measurements
because of differences in the methods of assessment and
limitations of the measurement techniques, as already
discussed. For these reasons, the strength of comparisons made between different studies is limited. Furthermore, in cohort studies, the rates of wear do not
follow a Gaussian distribution, and appropriate methods should be used in statistical analysis to account for
these distributions186.
In one study, in which serial radiographs of each hip
replacement were analyzed and compared over a minimum of five years, the rate of penetration of the femoral head into the polyethylene liner tended to decrease
with time, reaching a steady state after the sixth postoperative year226. Despite substantial differences in the
rates of penetration over the first three years in subsets
of hip replacements with high and low rates of penetration, the rates became similar over time. These results indicate that, for individual patients and prosthetic
hip systems, multiple assessments of wear over time are
THE JOURNAL OF BONE AND JOINT SURGERY
WEAR IN TOTAL HIP AND KNEE REPLACEMENTS
more valuable than a single measurement226 and that
caution should be exercised when comparing rates of
penetration in hip replacements after different durations of follow-up113.
Theoretical models and retrieval analyses have
shown that the rate of volumetric wear of polyethylene
components increases with an increase in the diameter
of the femoral head. This is because of an increase in the
contact surface and because, during the same gait cycle,
the sliding distance (the motion of the surface of the
femoral head relative to the surface of the socket) increases with an increase in the size of the bearing171. With
use of the simple cylindrical formula, v = πr2w, it is shown
that for any given amount of linear wear the volumetric
wear increases exponentially with increases in the radius of the bearing. In one retrieval study, for each millimeter increase in the diameter of the head, there was
an increase of 6.3 cubic millimeters per year in volumetric wear132. In another report, the rate of volumetric wear
increased from 7.5 to 10 percent for each millimeter
increase in the diameter of the head121.
In clinical studies, the linear wear rates of polyethylene against thirty-two-millimeter-diameter femoral heads have been equivalent to or greater than those
of polyethylene against heads with a smaller diameter115,144, and they have been associated with bone resorption and loosening of the acetabular component175,201,204.
Strictly with respect to wear, heads that have a thirtytwo-millimeter diameter do not compare favorably with
heads that have a smaller diameter. Because of their
large diameters, surface-replacement components have
rates of volumetric polyethylene wear that are four to
ten times higher than those of conventional total hip
replacements with twenty-eight-millimeter heads132. It
had been hoped that the reduction in polyethylene
stresses due to the larger contact area would result in reduced linear wear, but this has not been the case. Within
the range of contact areas for metal-on-polyethylene
prosthetic hip bearings, it appears that the stresses in
these conforming bearings are all so low that any differences do not appreciably affect wear in vivo. The increased contact area and sliding distance of larger heads
result in increased volumetric wear. The relationship
between the size of the head and the rate of wear may
be confounded by the use of a relatively thin polyethylene component with some thirty-two-millimeter bearings and in surface replacements.
The association between volumetric wear and periprosthetic bone resorption is related to the number of
polyethylene wear particles that have been released into
the so-called effective joint space, which includes all
periprosthetic spaces and tissues in communication with
joint fluid198. A twenty-eight-millimeter-diameter bearing with a conservative linear wear rate of 0.05 millimeter per year (a volumetric wear rate of about thirty
cubic millimeters), with individual wear particles equal
in volume to a 0.5-micrometer-diameter sphere, generVOL. 81-A, NO. 1, JANUARY 1999
121
ates a total of 500 billion particles. Assuming that a
patient takes one million steps per year, this translates to
500,000 particles per step. Such estimates are very sensitive to particle size. The number of particles produced by
a given volume of wear varies with the cube of the diameter of the particle. A single ten-micrometer-diameter
spherical particle has the same volume as 8000 0.5micrometer-diameter particles. There would be only
about sixty-three million ten-micrometer-diameter particles for the same amount of volumetric wear (thirty
cubic millimeters). At one million steps per year, this
translates to only sixty-three wear particles per step156.
Studies of wear particles retrieved from periprosthetic tissues and analyses of worn polyethylene surfaces have demonstrated findings that are consistent
with an average particle size in the 0.5-micrometerdiameter range34,100,156,166,168,205,214,230. If the size of the wear
particles is constant, then increases in volumetric wear
lead to increased numbers of polyethylene wear
particles. From a combined mechanical and biological
perspective, optimization of in vivo wear requires not
only a reduction of wear volume but also a reduction
in the generation of the most biologically active wear
particles. A lower rate of wear may not necessarily be
preferred clinically if a higher number of biologically
active wear particles is generated.
Compared with the assessment of wear on the hemispherical bearing of a total hip replacement, the assessment of wear in a total knee replacement is more
complex because of differences in the geometry of the
numerous femoral and tibial components and because
of differences in kinematics. Studies have indicated that
the rates of polyethylene wear in total knee replacements are even more variable than the rates of wear in
total hip replacements and that they are a function of
design, including the conformity of the articulation7,190,245;
operative technique, such as the mechanical alignment
of the knee and the fixation of the components33,190,237;
the presence of abrasive third bodies103,140; the thickness
of the polyethylene component; and the polyethylene
manufacturing process13,67,143,245,246.
The geometry of an articulation can be generally
described as a convex surface on a concave surface. In
total knee replacement, the degree of conformity between the two surfaces can be described as the ratio of
the radius of curvature of the tibial component (R2) to
the radius of curvature of the femoral component (R1),
or R2/R1. Such an analysis can be done for both the
sagittal and the coronal plane geometry. As the value of
this ratio approaches one, the conformity of the articulation increases. Thus, the most conforming articulation
(such as in a total hip replacement or a flat-on-flat surface) would have matched radii and a ratio of one. As
R2 becomes larger than R1, the conformity decreases and
contact stress increases. For a metal-on-polyethylene articulation, the contact stress is roughly doubled when
this ratio increases from one to five220. Because the ra-
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T. P. SCHMALZRIED AND J. J. CALLAGHAN
dius of curvature of a femoral component may not be
constant over the entire sagittal profile, the contact
stresses for the same load may vary throughout the
range of motion of the knee. Constraint is distinct from
conformity. Constraint is the restriction of motion. A
flat-on-flat articulation is completely conforming and
has no constraint to motion in any plane, whereas a
dished articulation of matched radii is completely conforming but motion is constrained to one plane.
Condylar designs with a conforming tibiofemoral
articulation have large contact areas, lower contact
stresses, and more favorable wear characteristics45,111, but
they may not allow physiological translational and rotational movements. Relatively flat tibial articulations
can accommodate such motions, but they have smaller
contact areas, higher contact stresses, and higher rates
of wear46,80,81,95. Experience indicates that, when an attempt is made to maximize function through design
innovations, priority must be given to the material
limitations of the prosthetic implants18. The apparent
dilemma of balancing the goals of conformity and multidirectional motion is addressed by designs with mobile
tibial polyethylene bearings. These designs have a high
degree of tibiofemoral conformity and allow for rotation (a so-called rotating platform) or rotation combined
with anterior-posterior translation (so-called meniscal
bearings)7,25-27,80. Dislocation of the mobile bearings was
reported in 2 percent (three) of 123 knees and was
associated with instability in flexion and with revision
total knee replacement25.
Rather than quantifying a change in dimensions,
as is done in assessments of wear in total hip replacements, most investigators who have performed retrieval studies of total knee components have analyzed
the relative amounts of several types of surface damage, including pitting, scratching, abrasion, burnishing,
delamination, and embedded third-body abrasive particles103,140. The degree of surface damage is related to
the weight of the patient and the duration of time
after implantation103,245. In total knee replacements,
peak stresses occur at about one to two millimeters
below the surface. The combination of high subsurface
stresses and decreased mechanical properties due to
oxidation at roughly this same depth predisposes to
delamination, which is a frequent type of surface damage in tibial bearings with lower conformity 47,140,245 and
is associated with rapid rates of wear66,67,131,172,231.
Pitting was the most common type of surface damage seen in association with a conforming total condylar
design; only two of forty-eight components showed evidence of delamination103. The highest scores for damage
were in the central, weight-bearing portions of the medial and lateral tibial concavities. There was greater
damage on the anterior aspect than on the posterior
aspect, which is probably due to the kinematics of this
design. No delamination was seen in the standard tibial
polyethylene components of twenty retrieved posterior
cruciate-substituting total knee replacements. Burnishing was the most common type of surface damage reported for patellar components103. It was roughly equal
on the medial and lateral aspects of the patellar component, where contact occurred with the femoral component, and the severity of the damage was related to the
weight of the patient. In metal-backed patellar components, the polyethylene may be quite thin and demonstrate rapid wear, fracture, or dissociation from the
metal backing147.
In a retrieval study190 of a posterior cruciate ligamentsacrificing total knee replacement with a high degree of
tibiofemoral conformity in the sagittal plane and with
polyethylene that had a minimum thickness of six millimeters, the rate of linear penetration of the femoral
component averaged 0.025 millimeter per year, which is
considerably lower than that reported for the Charnley
total hip prosthesis249,250,252,253. The calculated rate of volumetric wear was sixteen cubic millimeters per year, which
is also lower than that reported for the Charnley total hip
prosthesis245,249,252,253. The relative amount of penetration
on the medial side was increased in varus knees and
decreased in valgus knees. Symmetrical wear was most
likely to occur when the knee was in about 5 degrees of
tibiofemoral valgus. In a study of one design of a so-called
meniscal-bearing knee replacement, which has a high
degree of tibiofemoral conformity in the sagittal plane,
bearings retrieved at reoperation because of loosening
or dislocation had an average rate of penetration of only
0.026 millimeter per year, which is quite low7. The contact
area available for load transmission in each tibiofemoral compartment was 5.7 square centimeters, which is
larger than that of the Charnley hip prosthesis. Such low
rates of polyethylene wear are consistent with a linear
motion path.
Higher rates of wear in total knee replacements
have been associated with some designs characterized
by lower conformity, especially when the tibial polyethylene is less than six millimeters thick 46,245 and when socalled heat-pressing of the tibial articular surface has
been used131. Younger, more active, and larger (predominantly male) patients are at risk for accelerated wear
in association with these prosthetic variables67,134,172,231,237.
This type of rapid wear tends to involve only the medial
or lateral compartment, suggesting a role of asymmetrical loading and high localized stresses from a suboptimum mechanical environment. The rate of progression
of the femoral component into the polyethylene can be
dramatic, as much as one millimeter per year, resulting
in wear-through of a four-millimeter-thick component
in four years66,131. The type of wear that occurs is predominantly due to subsurface fatigue resulting in delamination (and the production of larger particles of
polyethylene)205. Wear-through or gross mechanical failure of the polyethylene component can result in mode-2
wear with abrasive damage to the femoral component
and substantial generation of metal particles.
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WEAR IN TOTAL HIP AND KNEE REPLACEMENTS
The clinical triad of effusion, pain, and progressive
change in the coronal alignment of the knee (most commonly into varus alignment) is characteristic of accelerated polyethylene wear131. The joint fluid is laden with
polyethylene particles of various sizes, and aspiration
can confirm the diagnosis of polyethylene-induced synovitis23,88. Arthroscopy can be helpful in assessing the
degree of polyethylene wear and the damage of the
femoral component as well as in planning a revision
operation. Arthroscopic débridement may provide temporary relief, but most patients who have this triad need
a revision172.
In clinical studies, wear rates generally have been
retrospectively compared on the basis of a specific variable, such as the type of femoral component or the
type of fixation (with or without cement). The tremendous number of potentially confounding variables in
such clinical studies is a fundamental limitation to this
type of comparison. Furthermore, great care should be
taken when extrapolating the results to other reconstructions with the same generic variable. An example
is the issue of metal-backed acetabular components.
One specific type of metal-backed acetabular component inserted with cement has been reported to have
an increased rate of polyethylene wear compared with
an all-polyethylene component inserted with cement36.
However, a different type of metal-backed acetabular
component has been reported to be associated with a
lower rate of polyethylene wear in other studies 32,104.
The issue cannot be as simple as the presence or absence of metal backing. Specific details of the design,
materials, and manufacturing of the components as well
as any differences in the populations of patients, durations of follow-up, measurement techniques, and statistical analyses all must be carefully considered when
evaluating and comparing rates of polyethylene wear
in vivo.
Clinical rates of wear traditionally have been expressed with use of a denominator of time because of
convenience, not accuracy. More appropriately, investigators performing in vitro studies involving laboratory
wear simulators have always used the number of loading cycles as a denominator. Similar to the use of a set
of automobile tires, the wear of a prosthetic hip or knee
is a function of use or the number of cycles and not a
function of time in situ. The assumption made in clinical
studies is that all patients with a joint replacement have
about the same level of activity — that is, the actual use
of or the number of cycles on the bearing is about the
same — or that any differences average out over a large
sample size. The limitations of this assumption must be
recognized.
One of us (T. P. S.) and colleagues studied, with use
of an electronic digital pedometer, the walking activity
of 111 patients who had a total joint replacement210. We
calculated an average of about 0.9 million cycles for each
joint in the lower extremity per year. This number is close
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123
to the average of 1.0 million cycles per year proposed by
Seedhom et al.213 on the basis of their study of the gait
cycles of nine elderly individuals, on vacation, who did
not have a joint prosthesis. The most important finding
of our study, however, was not the average but the fact
that there was a forty-fivefold difference in the range of
gait cycles between the least active and the most active
individual. The most active individual averaged 3.2 million cycles per year, about 3.6 times higher than the
average. These data indicate that individual differences
in activity are a substantial source of variability in rates
of wear. A forty-fivefold difference in rates of wear as
well as rates that are more than 3.5 times the average can
be accounted for by differences in an individual’s activity.
Age was associated with daily walking activity but with
a high degree of variability (the standard deviation was
3040 steps per day). Individuals who were less than sixty
years of age walked about 30 percent more on the average than those who were sixty years of age or older (p =
0.023). The men walked 28 percent more on the average
than the women (p = 0.037), and the men who were less
than sixty years old walked 40 percent more on the
average than the other individuals (p = 0.011). Thus, the
variation in an individual’s activity contributes to the
variability in rates of wear that is consistently seen in in
vivo studies. These results should be considered when
analyzing studies of in vivo wear rates that have a denominator of time in situ.
Periprosthetic Bone Loss
Periprosthetic bone loss can occur as a result of
a reduction in the load transmitted to bone, so-called
stress-shielding. Periprosthetic bone loss also occurs as
a result of an inflammatory reaction to small particles,
such as those produced by the various wear modes. To
varying degrees, both processes occur simultaneously in
complex mechanical-biological systems such as joint replacements, and the adverse effects can be additive. Bone
with decreased density secondary to stress-shielding may
be more susceptible to osteolysis64. Relative motion between an implant and bone can cause bone loss through
both mechanical and biological mechanisms79.
Cellular Mechanisms
The tissue adjacent to total hip and knee prostheses consists of synovial tissue, variably organized and
variably vascularized fibrous tissue, lymphocytes (occasionally), and foreign-body inflammatory cells (macrophages and giant cells) that are present roughly in
proportion to the number of small particles79,105,173,198. Prosthetic particles elicit a cascade of responses at the cellular and tissue levels. The cell whose function is central to
the biological reaction to prosthetic wear particles appears to be the macrophage1,196,197. The mononuclear stem
cell, which originates in the bone marrow, is the progenitor for both mononuclear macrophages and osteoclasts.
Macrophages phagocytose small wear particles and may
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T. P. SCHMALZRIED AND J. J. CALLAGHAN
fuse to form foreign-body multinucleated giant cells,
usually in association with larger particles. Osteoblasts
and fibroblasts also may be important in the response
to wear particles resulting in altered formation of bone
and connective tissue126. Although lymphocytes are occasionally present, their role in the inflammatory reaction is unclear.
Most periprosthetic bone resorption is effected by
osteoclasts, but there is evidence that macrophages and
foreign-body giant cells are capable of direct, low-grade
bone resorption9,102,177. In vitro studies have indicated
that activated macrophages release cytokines, including
interleukins and prostaglandins, which play a role in
the recruitment and differentiation of cells and stimulate bone resorption178, but the specificity of the cytokine response and the regulatory mechanisms have not
been defined. Under certain conditions, macrophages
appear to directly release interleukin-1 beta and tumor
necrosis factor. Several cytokines, including interleukin1 beta, stimulate osteoclast maturation. Although cytokines released by macrophages may directly stimulate
bone resorption by osteoclasts, other effects may be
mediated by intermediary cells such as fibroblasts or
osteoblasts106,126,248. Matrix metalloproteinases (collagenase, gelatinase, and stromelysin), which are capable of
effecting bone resorption, are also produced by interfacial membrane tissue around failed total hip and
knee replacements44,59,79,105. On the basis of the knowledge of such cellular and biochemical mechanisms of
bone resorption, there has been increasing interest in
and investigation of pharmacological agents that may
modify these cellular responses20,94,107,215. Other work indicates that, in addition to bone resorption, there is
also a decrease in bone formation in association with
periprosthetic osteolysis82,255.
It appears that all of the materials used in total joint
replacement are capable of inducing an inflammatory
foreign-body reaction if the particles are within a certain
size-range and there are enough of them. The boneresorbing ability of macrophages in vitro is a function of
the size, shape, and composition of the particles, and it
is dose-dependent214. It has been previously recognized
that there is an upper size limit for particle reactivity106,
but there may also be a lower size limit. For a given
concentration of particles, the stimulatory effect of polyethylene particles in vitro decreases when the particles
are larger than about seven micrometers or smaller than
about 0.2 micrometer83. This information suggests that
polyethylene wear particles generated by current designs of total hip and knee replacements are especially
apt to cause problems because they are produced in
very high numbers and are predominantly of a size
within the range of peak biological activity. This also
suggests that the biological effects of the very small
wear particles (linear dimensions measured in nanometers) produced by metal-on-metal articulations may be
different than those of polyethylene wear particles 58. In
aggregate, these very small particles have a large surface
area and the systemic levels of metal ions are elevated
in patients who have metal-on-metal articulations. The
clinical importance of this is not known114.
Osteolysis
The classic descriptions of osteolysis adjacent to total joint replacements have been based on radiographs
and have referred to a nonlinear, scalloped, or erosive
form of femoral endosteal bone resorption in association with total hip replacements inserted with cement 38,90.
The term osteolysis has subsequently been used generically in reference to bone resorption that occurs in
association with a foreign-body response to particles
from a prosthetic joint3. Although there may be similarities at the cellular level, the pattern, location, extent,
and rate of progression of inflammatory periprosthetic
bone resorption are variable because of specific differences in the anatomy and physiology of the joint, the
operative procedure, and the prosthetic implants.
The evolution of the understanding of the anatomical, mechanical, and biological components of osteolysis
associated with total joint replacement has been one of
the more fascinating sagas in the history of orthopaedic surgery and a topic of controversy and confusion.
It should be acknowledged that the conclusions of retrieval analyses are a function of the material available
for study. Furthermore, the questions to be answered
determine the methods with which the specimens are
processed, which influence the observations and conclusions that can be made. In addition, even the most complete retrieval analysis can describe only one moment
in time of a complex mechanical and biological process
for an individual specimen. An appreciation of these
limitations may at least partially explain the apparent
differences in observations that have been reported in
various studies.
Osteolysis in Association with
Total Hip Replacement
In 1968, Charnley et al. reported a pattern of nonlinear endosteal erosion in association with the Charnley
total hip prosthesis inserted with cement and they suspected that this type of localized bone loss was due to
infection38. Charnley also observed cystic erosion of bone
in the femoral diaphysis in association with fracture of
the stem and believed that a deficient cement mantle
was probably the main cause of these fatigue fractures41.
The tissue was reported to contain fragments of polymethylmethacrylate, but polyethylene particles were not
identified. In 1976, Harris et al. reported a similar pattern of nonlinear localized bone resorption (osteolysis)
in the proximal part of the femur around loose total hip
replacements90. Lesional tissues were characterized by
osteoclastic bone resorption, a high concentration of
macrophages, and occasional foreign-body giant cells,
with spherical spaces of variable size that were consistent
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WEAR IN TOTAL HIP AND KNEE REPLACEMENTS
with particles of polymethylmethacrylate. This appears
to be the origin of the concept of so-called cement disease97,106,130. Jasty et al. subsequently reported osteolysis
in association with polymethylmethacrylate particles adjacent to well fixed total hip replacements that had been
inserted with cement115. Polyethylene wear particles were
not identified. It was not until endosteal osteolysis was
identified around femoral components inserted without
cement163 that polyethylene, metals, and other materials
in particulate form were seriously considered as potential contributors to the inflammatory reaction and associated bone resorption.
In 1990, Anthony et al.6 demonstrated a communication between the articulation and the endosteal surface of the femur, through a space between the stem and
the cement (as described by Fornasier and Cameron 73),
and then through a defect in the cement mantle. Particles
of metal, cement, and polyethylene were identified in
macrophages in these osteolytic lesions. Arthrography
of the hip, performed after the patients had exercised
the hip joint, demonstrated transfer of contrast material from the femoral-acetabular articular space to the
area of the osteolysis. Studies116,118,162 of well functioning
total hip replacements retrieved at autopsy have indicated that some degree of separation of the femoral stem
from the cement mantle, so-called debonding, occurs
frequently and as early as two weeks after implantation.
Fractures of the cement mantle were seen in association
with debonding and had originated at the metal-cement
interface, most often at the corners of the mantle or in
association with pores in the cement. Cement fractures
most frequently occurred in areas where the cement was
thin or in association with frank defects in the mantle.
A common finding in patients who have femoral
endosteal osteolysis around a cemented component appears to be a defect in the cement mantle. The risk of
osteolysis can be decreased when technical errors are
avoided92,176. Separation of the stem from the cement
mantle does not lead to localized endosteal osteolysis if
the cement mantle remains intact, and such separation
appears to be compatible with satisfactory long-term
function of certain types of femoral components77,212
but not others174. The relationships between osteolysis
and the geometry of the stem, the surface finish of the
stem, and the operative techniques have continued to
be investigated91,149,174,243,244.
Compared with femoral components that have been
inserted with cement, femoral components that have
been inserted without cement and that have a limited
proximal porous coating can be associated with earlier,
more frequent localized endosteal bone resorption in
the femoral diaphysis92,163,229. The size of the lesions can
increase with time as can the prevalence, which has
exceeded 30 percent in some reports92,163,229. This appears
to be due to the fact that a limited proximal porous
coating, especially if it is not circumferential, allows
joint fluid and wear particles relatively easy access to
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125
the diaphyseal endosteum21,198,233. Diaphyseal endosteal
osteolysis has not been reported in association with extensive, circumferential, porous coating even in the presence of substantial stress-shielding28,64.
Inflammatory bone resorption (induced by small
particles) also occurs in a more linear pattern, which may
progress along the cement-bone interface and cause or
contribute to loosening of the implant, especially in the
acetabulum101,203,256. There may be mechanical factors that
contribute to interfacial particle transport. Once stability
has been lost, additional motion can only be detrimental203. Even with the relatively low rate of volumetric wear
of the Charnley total hip prosthesis, accumulated generation of wear particles eventually decreases the stability
of the acetabular implant. A highly significant association (p < 0.01) has been demonstrated between the depth
or rate of polyethylene wear and loosening of the acetabular component75,137,250-252. A high rate of polyethylene wear
precludes a long service life for the implant112.
A similar process of linear osteolysis can occur in
association with acetabular components inserted without cement188,198-200,202,204,206. The integrity of the peripheral
implant-bone interface governs the ingress of joint fluid
and wear particles. Progressive radiolucent lines are
more likely to develop adjacent to components with
initial peripheral interface gaps than they are likely to
develop adjacent to components without such initial
peripheral gaps199. Tissues from the peripheral acetabular implant-bone interface contain macrophages laden
with small polyethylene wear particles198. Insertion of
acetabular components with a tight peripheral press-fit
has led to a reduced prevalence of initial peripheral
interface gaps, a reduced prevalence of progressive
peripheral interface radiolucency, and a higher prevalence of hip replacements without any interface radiolucency206. Regardless of other design features, a similar
process of inflammatory bone resorption can occur in
the proximal part of the femur after hip replacement
with or without cement85,128,129,198,203. This process can cause
or contribute to the loosening of femoral components
inserted with cement79. Inflammatory bone resorption
may play a role in loosening, after previous rigid fixation
by bone ingrowth, of a component inserted without cement and with a limited proximal porous coating117. This
same process has not resulted in loosening of a component with an extensive porous coating64.
Acetabular components inserted without cement
have a lower prevalence of interface radiolucency than
components inserted with cement199,206,242. However, the
patterns of bone resorption around acetabular components inserted without cement are different than
those seen around components inserted with cement.
The bone loss associated with acetabular components
inserted with cement typically occurs predominantly
along the interface, following the contours of the cement mantle. The bone loss associated with acetabular
components inserted without cement typically pro-
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T. P. SCHMALZRIED AND J. J. CALLAGHAN
gresses away from the interface into the cancellous
bone of the pelvis, resulting in localized bone resorption, the classic nonlinear or expansile form of osteolysis165,196,204,256. Although less common, this form of pelvic
osteolysis can also occur in association with acetabular
components inserted with cement43,189. Pelvic osteolysis
is associated with a younger age256, vertical positioning
of the acetabular component, and high volumetric wear
of the polyethylene8,204. Substantial bone resorption
around well fixed components may be asymptomatic
until a pelvic fracture occurs. For this reason, annual
evaluations with radiographs made in multiple planes
are recommended167,204,256.
Other concerns associated with acetabular components inserted without cement are wear of the convex
surface of the modular polyethylene liner against the
metal shell (so-called backside wear) and fretting of
the fixation screws placed through the shell8,110. On the
basis of reports of several series of acetabular components inserted without cement, no consistent relationship has been demonstrated between the presence of
screw-holes and the development of osteolysis behind
the component165,256. Furthermore, particle access by way
of screw-holes cannot be the cause of isolated osteolysis
in the ischium and pubis. It appears that an interplay of
specific design and implantation variables is involved in
the development of pelvic osteolysis14,199,202,204,206.
Osteolysis in Association with
Total Knee Replacement
Classic, expansile osteolysis associated with total
knee replacements in which all components were inserted with cement has been relatively unrecognized
in the literature53,69,193,194 despite the fact that retrieval
studies have demonstrated substantial wear of the tibial
polyethylene103,246. Although uncommon, when osteolysis
has occurred in association with components inserted
with cement, it has generally been associated with a flat
tibial articulation. Long-term follow-up studies have not
indicated that inflammatory bone resorption, either
classic osteolysis or interfacial bone resorption, is a frequent cause of failure of cemented total knee replacements with a more conforming articulation 45.
Osteolysis necessitating reoperation has developed
in association with several designs of total knee replacements without cement15,31,135,136,142,182,187. Osteolysis has been
reported in the proximal part of the tibia in as many as
16 percent (twenty-seven) of 174 patients142,187, in the
distal part of the femur in 11 percent (thirty) of 271
patients31,135, and in the patella in 80 percent (twentyfour) of thirty patients who had a prosthesis implanted
without cement136. Radiographs may underrepresent the
true extent of the bone loss194. The development of osteolysis is not a direct function of the absence of cement but is related to other design, operative technique,
and patient-related variables that are associated with
so-called first-generation total knee replacements with-
out cement. These variables include the mechanism of
attachment of the modular polyethylene insert to the
metal base-plate and the interface between the polyethylene and the metal base-plate as well as the design and
location of holes in the base-plate and the presence of
fixation screws142. With limited distribution of porous
coating, wear particles can enter the implant-bone interface and metaphyseal bone by way of the unbonded
interface between the smooth metal and bone. Fully
porous-coated components are associated with a lower
prevalence of osteolysis than those with a less extensive
or discontinuous porous coating240. In addition, access to
tibial metaphyseal bone may be gained by means of
holes through the tibial base-plate or along the course
of tibial fixation screws67,187,240. Increased rates of polyethylene wear have been associated with fixation without
cement due to the use of thin polyethylene components
with low tibiofemoral conformity and metal-backed patellar components as well as with a younger age, male
gender, and greater body weight31,69,136,147,194,195,222. In aggregate, this information indicates that the fundamental
variables for the long-term success of a total hip or knee
arthroplasty include the attainment and maintenance of
the integrity of the implant-bone interface, which provides not only mechanical stability but also a barrier to
joint fluid and particulate debris, and a low rate of generation of biologically active particles from all sources.
These findings illustrate the interplay between fixation
and wear, and they show that many factors, including the
materials and design of the implants, the operative techniques, and the amount and type of the patient’s activity
contribute to the long-term success (or failure) of a
total joint replacement.
The Effective Joint Space
In 1977, Willert reported his classic observations
after reoperations for the removal of failed joint replacements241. He found that capsular tissue has some
capacity to transport particles through the lymphatic
system by means of perivascular lymph spaces, leading
to regional and systemic distribution. If the capacity
for elimination by this mechanism is exceeded, then
particles accumulate in the periarticular tissues. The
pseudocapsule appears to be the primary location for
phagocytosis of particles. This process results in the
development of foreign-body granulomas with areas
of necrosis and fibrosis that are somewhat proportional to the amount of particles. In principle, the
whole environment of the implant could become involved. Extension of this foreign-body response into
the cement-bone interface could cause loosening of the
implant. The findings from subsequent experimental
models108 and analyses of specimens from well functioning implants retrieved at autopsy are in accord with
this principle188,198,201,203,233. Aside from the observations
that particles larger than about ten micrometers in linear dimension tend to remain localized and that rapid
THE JOURNAL OF BONE AND JOINT SURGERY
127
WEAR IN TOTAL HIP AND KNEE REPLACEMENTS
area, leading to an expansile lesion. If the rate of particle
production is low, or if joint fluid is more evenly distributed, the rate of bone resorption is slower and is accompanied by a fibroblastic response, which results in a
more linear pattern of bone loss that often follows the
contours of the implant.
In addition to wear particles, soluble factors capable of directly or indirectly effecting bone resorption
have been identified in joint fluid. These factors include
prostaglandins, interleukins, and matrix metalloproteinases59,79,138,228. Elevated levels of these factors may play
a central role in rapidly destructive arthropathy of the
hip138. While proteolytic enzyme activity in interfacial
tissues contributes to local bone resorption, pseudocapsular tissues also release bone-resorbing factors into
joint fluid228. Cytokines and matrix metalloproteinases
can be elevated in the fluid around total joint replacements59 and could effect bone resorption at distant sites
from the articulating surfaces of the joint.
Variations in the pressure of the joint fluid play a
role in transporting the joint fluid and wear particles
around the effective joint space. Hendrix et al.96 reported large variations in the pressure of the intracapsuFIG. 1-A
Diagram showing the effective joint space, which includes all
periprosthetic regions that are accessible to joint fluid. The operative
implantation procedure alters the natural anatomy of a joint. In
prosthetic total hip and knee replacement, some bone as well as the
implant-bone interfaces are exposed within the new joint space.
Contraction of muscles, such as the psoas and abductor muscles, and
changes in joint position (flexion, extension, abduction, adduction,
and rotation) can alter the volume of the effective joint space,
resulting in changes in intracapsular joint-fluid pressure.
wear with the production of an extreme amount of
particles is associated with systemic distribution, the
variables that influence local accumulation compared
with systemic distribution of wear particles have not
been defined29,43,65,99,217.
Polyethylene wear particles are dispersed in the
fluid around a prosthetic joint. Conceptually, the effective joint space includes all periprosthetic regions that
are accessible to joint fluid and, thus, accessible to wear
particles199. The limits of the effective joint space are
determined by the intimacy of the contact between the
prosthesis and the bone and the variability of this contact within a given reconstruction. This variability determines the access routes for joint fluid and particles, to
and along various interfaces, into bone, and through soft
tissues as well. Joint fluid flows according to pressure
gradients, following the path of least resistance. Patterns
of joint-fluid flow influence the shape and extent of
osteolysis. The local concentration of particles is a factor
in the local inflammatory reaction and, hence, the degree of bone resorption in that location. As bone is
resorbed, a larger space is produced, encouraging preferential flow of joint fluid and wear particles into that
location, which fuels additional bone resorption in that
VOL. 81-A, NO. 1, JANUARY 1999
FIG. 1-B
Diagram showing wear and the release of wear particles into the
effective joint space. Capsular tissue has some capacity to transport
particles through the lymphatic system (large straight arrow). In the
effective joint space, joint fluid and wear particles follow the path of
least resistance, which is dependent on the prosthetic and anatomical
details of each specific reconstruction. The pseudocapsule becomes
thickened (small straight arrows) because of phagocytosis of wear
particles and the development of foreign-body granulomas. The effective joint space can extend along interfacial planes, expand into
bone, or expand into soft tissues or a variety of combinations is
possible (curved arrows).
128
T. P. SCHMALZRIED AND J. J. CALLAGHAN
lar fluid around total hip replacements during activities
of daily living, with peak pressures of more than 750
millimeters of mercury (100 kilopascals). During reoperations for osteolysis, intracapsular pressures of more
than 500 millimeters of mercury (sixty-seven kilopascals) have been measured204. Fluid pressure of 198 millimeters of mercury (twenty-six kilopascals) has been
measured in a femoral diaphyseal lesion6. In addition to
moving joint fluid around the effective joint space, the
fluid pressure may contribute to expansion of the effective joint space. The expansile and balloon-like nature
of classic osteolytic lesions may be a consequence of
fluid pressure. The recorded pressures are high enough
to interfere with the normal perfusion and oxygenation
of bone6, and a physical effect of pressure may account
for cases of osteolysis in which no prosthetic particles
are identified in the lytic lesions165,234.
In normal synovial joints, such as the hip or knee,
bone is not exposed to joint fluid. The boundaries of the
joint are defined by the capsule and, within the capsule,
bone is covered by cartilage or synovial tissue150,219. In
disease processes such as osteoarthritis, and in total joint
replacement, the normal anatomical and physiological
compartmentalization of a synovial joint can be disrupted. Osteoarthritic cysts, or so-called geodes, are a
form of periarticular bone resorption that occurs in the
absence of prosthetic implants. Elevated intracapsular
fluid pressure can lead to intrusion of joint fluid into
cancellous bone through gaps in degenerated articular
cartilage. Numerous investigations have shown evidence of a role for intrusion of joint fluid and fluid
pressure in the enlargement of these cysts74,139,192. Intracapsular fluid pressure is a function of many variables,
including capsular compliance, fluid compartmentalization, joint position, and muscle action. The compliance
of the osteoarthritic joint capsule is reduced. Fluid pressures are increased in osteoarthritis and vary with use
of the joint. In addition, these pressures can be transmitted to periarticular cysts68,123-125,141,146,179. An important
difference between the etiology of geodes and that of
osteolysis associated with total joint arthroplasty is
activated macrophages; geodes develop without this
foreign-body response to prosthetic particles208 but demonstrate a cytokine profile similar to that of the osteolytic lesions around total joint implants 127.
The effective joint space can expand into soft tissue
as well as bone17,51,93,109,170,224. Increased intracapsular fluid
pressures can be painful and may result in the formation
of synovial cysts or rupture of the capsule78,122. The presence of an effusion or synovial cysts may be apparent
on examination of the knee because of the relatively
limited amount of overlying soft tissues. Such evidence
of fluid accumulation is less often appreciated in the
hip. Synovial cysts may have a protective function by
accommodating fluid volume and limiting the increase
in intra-articular pressure, thus protecting bone from
pressure damage. An inverse relationship has been dem-
onstrated between the development of periarticular
bone cysts and the development of synovial cysts76. The
natural hip joint can communicate with the iliopsoas
bursa37. Pressure-driven synovial fluid may be pumped
into the bursa, causing distension that may be symptomatic17. Similar events can occur after total hip and knee
arthroplasty109,170,172. In the effective joint space, joint fluid
seeks the path of least resistance; the path is variable
depending on the anatomical and physiological specifics of the reconstruction and includes soft tissue as well
as bone198.
Thus, after total hip or knee replacement, the fluid
in the effective joint space has at least three components
that can contribute to periprosthetic bone resorption:
wear particles, soluble factors, and the physical effects
of fluid pressure. Wear particles are released into joint
fluid and are variably distributed throughout the effective joint space (Figs. 1-A and 1-B). These particles
cause inflammation and contribute to the development
of an effusion. Chronic inflammation causes fibrosis and
decreased capsular compliance, which can result in elevations of intracapsular joint-fluid pressures. The flow
of joint fluid is an active process driven by fluctuations
in joint-fluid pressures that occur with activities of daily
living and can contribute to progressive expansion of
the effective joint space. This fluid also contains various
soluble factors that are capable of effecting bone resorption at distant sites. Fluid pressure can cause bone erosion from a purely physical effect or necrosis due to
circulatory compromise. In contrast to osteoarthritic
joints, in which the progression of the osteoarthritis limits the use of the joint, a well fixed and otherwise well
functioning prosthetic joint may be asymptomatic; the
patient remains active and maintains the driving forces
for osteolysis, which may account for the progression
and extreme size of some osteolytic lesions associated
with total joint replacement.
Overview
In summary, improving the durability of total hip and
knee replacements requires a reduction in the total production and release of small particles into the biological
environment. There is a need not only for more wearresistant bearing materials to decrease mode-1 wear but
also for concomitant improvements in the design and
manufacturing of the implant and the operative techniques to minimize the occurrence of mode-2, 3, and 4
wear. Another limitation of current prosthetic joint arthroplasty is the variable disruption of the anatomy and
physiology of the joint that occurs as a result of the prosthetic implantation and that may allow variability in the
access of joint fluid (and wear particles) to bone. Although a reduction in the production of particles is desirable, limiting access of joint fluid to bone is necessary
to decrease periarticular inflammatory bone resorption.
NOTE: The authors thank Christian D. McClung, M.Phil., for his assistance in obtaining
and organizing the references and in the preparation of this manuscript.
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129
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