history of total joint replacement has been about basically
Firstly, finding the right materials that can function
safely and effectively as biomaterials in long-term implants.
Early attempts have used such disparate materials as ivory,
cellophane, fascia lata, chromacized pig bladder, skin, other
plastics, glass and metals in various forms.
The interface between the implant and bone, termed the fixation,
has been the second issue. Press-fit, cement and bone ingrowth/ongrowth
have been the principal approaches that have been tried.
Finally, the bearing surface has been an issue since
the single side replacement was abandoned early on in favor
of 'total' replacement.
The term 'total joint replacement' (TJR) is a bit of a misnomer,
since the totality of a joint includes ligaments, tendons,
muscle, synovium and bone. It would be more accurate to label
what currently passes as TJR something that is more descriptive
of what is done, such as 'complete bearing surface renewal', but
everybody understands that this is what the term 'total joint
replacement' actually means.
the initial attempts to renew the painful or damaged joint
surface, interpositional membranes with various tissues or
materials listed above, were employed with the
intent of guiding reformation of the natural articular surfaces.
Surprisingly, many of these did enjoy limited success, but
too frequently, were failures. This
was followed by single sided resurfacing or replacement, of
which the Austen-Moore endoprosthesis, and the Aufranc Cup
in the hip, and the MGH distal femoral resurfacing, the McKeever
and Macintosh prostheses in the knee were examples. Although
resurfacing or replacing one side of the joint with a hard
bearing surface enjoyed greater success than pliable interpositional
membranes, this approach still did not produce the high level
durable success that was being sought.
modern era of TJR was entered in the late 1950's with the
McKee-Farrar metal on metal (MOM) prosthesis and most notably
with the introduction of the low friction arthroplasty employing
a metal stem and high-density polyethylene cup, fixed to host
bone with polymethylmethacrylate (PMMA) by Sir John Charnley.
Over a very short time, this became the 'gold standard' for
joint replacement, starting with total hip replacement and
quickly spilling over to total knee replacement. Over
the next 20 years, the vast majority of prostheses consisted
of metal or metal and plastic prosthetic components, fixed
to bone by PMMA. Short-term results rose almost immediately
from the 70% range to the mid 90%, as measured by lack of
need for a second (revision) operation in 2-5 years. A great
wave of enthusiasm for joint replacement swept the developed
world in the 70's, as THE solution for previously insolvable
problems seemed to be at hand.
wasn't long before 2 longer term problems raised their collective
heads. As joint replacements were being offered to younger
and more active patients, component loosening, and osteolysis
become clinically evident, with 50% failure with a minimum
5 year follow-up of THR's done in patients younger than 40
reported in 1980. Most of these failures were associated with
loosening, and also showed in some cases, significant bone
was assumed that the loosening, that is, degradation of fixation,
was the problem and that the bone loss from osteolysis was secondary.
Attention turned to finding ways to eliminate loosening. This
proceeded along 2 paths: understanding how to use cement more
effectively and cementless fixation. Both
paths have produced dramatic success and aseptic loosening,
of either cemented or cementless components designed for bone
ingrowth or direct bone attachment, has largely been eliminated.
Permanent fixation of prosthesis to bone is still technique
dependent, but those techniques are well understood and largely
the way it was discovered that loosening was not an obligatory
prelude to osteolysis. This had been missed for probably a
decade because initially most of the osteolysis was associated
with loosening, and also because the polyethylene particles
were so small that the majority of them could not be visualized
by conventional microscopy, whereas the fragments of PMMA
were readily apparent.
There were a few other problems along the way that were also
resolved. Metal stem breakage disappeared as metallurgical
advances produced 'unbreakable' prostheses; metal sensitivity
abated with the discontinued use of high nickel content stainless
steel and the introduction of titanium based alloys for non-cemented
devices, while techniques and instruments advanced to the
point where attentive surgeons could achieve proper alignment
and fixation of the components. Five and 10-year results were
widely reported in the 90+% range for both hip and knee replacements.
However after 10 years, polyethylene wear and osteolysis began
to appear in increasing numbers and it started to look like
15 - 20 years was the most that could be expected from a TJR.
Efforts have been made along the way to improve the wear capacity
of ultrahigh molecular weight polyethylene (UHMWPE), which
had broadly replaced HDPE (high density polyethylene), including
heat-pressing, introduction of carbon fibers, and altered
processing. None of these proved successful; in fact they
all showed equivalent or worse (in the case of knee replacements)
wear in the clinical setting in spite of the fact that laboratory
mechanical testing predicted improvement in wear.
Ultrahigh Molecular Weight Polyethylene (UHMWPE)
is now sterilized by either gamma irradiation or gas sterilization.
Gamma irradiation produces cross-linking of the polyethylene
chains and increases resistance to wear. However it also produces
free radicals, which, when combined with oxygen, produce chain
scission reducing wear resistance. When components were irradiated
and stored in air the integrity of the component began to
deteriorate in the box on the shelf, and this continued in
the body after implantation.
the mid 90's 2 separate solutions to the problem were undertaken,
1. Sterilize with gas (in preference to gamma irradiation)
and 2. Irradiate and store in an inert gas, such as nitrogen.
The first avoided creation of free radicals, but failed to
obtain cross-linking, and the second achieved cross-linking
and prevented the free radicals from combining with oxygen
to cause chain scission. In the mid 90's the concept of increasing
cross-linking by increasing the irradiation was introduced
and components implanted. Those
components have now been in clinical use for minimum 5-year
studies to appear, documenting significantly reduced wear.
But they have not been in use long enough to know that this
translates into reduced osteolysis, because even with conventional
UHMWPE, osteolysis is not usually seen at 5-year follow-up.
However, numerous studies correlate increased wear with
increased incidence of osteolysis, especially in the hip.
Therefore the goal to minimize wear appears a reasonable means
to reduce/eliminate osteolysis.
recently advanced processing has been introduced which permits
strengthening UHMWPE by cross-linking with little or no remaining
free radicals. However, clinical use of these advanced materials
is still too recent to reach any conclusions.
Other Solutions (MOM & COC)
have been 2 other roads taken to reduce wear: MOM (Metal-on-Metal)
bearing surfaces, and COC (Ceramic-Ceramic) bearing surfaces.
Both these concepts have been around for a long time, but
enjoyed limited popularity.
of the very early MOM hip prostheses, the McKee-Farrar prosthesis,
was a metal-metal couple using a cobalt based alloy as both
the structural and bearing surface material. However manufacturing
techniques of the time did not allow uniformity of prosthetic
component production and many of these devices failed prematurely.
The bearing surfaces wore excessively producing a black metal
sludge. Also presumably high friction from prosthetic components
that did not mate properly ('stiction friction'), led to an
increased incidence of loosening as, presumably, the increased
friction forces were translated to the bone-cement interface
and degraded it. MOM
bearing surfaces have enjoyed resurgence lately as manufacturing
tolerances and bearing finishes have been improved to provide
a low friction surface with extremely low wear rates. Five
year follow-up reports have also been published for this bearing
couple with very low loosening and osteolysis rates.
bearing surfaces have also been in clinical use for many years.
One formerly popular design, the Mittelmaier prosthesis, had
a screw in all ceramic acetabular components that had an elevated
aseptic loosening rate. Ceramic
as components have also been plagued periodically by fracture,
which presents some unique problems that will be reviewed
below. However, several large series, particularly from France,
have reported outstanding multi-decade results with a low
incidence of both fracture, loosening and wear. Recently
acetabular components have been introduced where the ceramic
is embedded in a porous surfaced metal shell. This protects
the ceramic liner from damage during insertion and provides
an ingrowth surface. Again results at 5+ years show high success,
low aseptic loosening and low osteolysis rates.
appears that in 2006 there are 4 candidates to choose from
for an ultra-durable hip replacement prosthesis: MOM, COC
and metal or ceramic on highly cross-linked UHMWPE. On the
basis of both laboratory testing and available clinical evidence,
it would appear that ANY of these could be the ultimate long-lasting
How are surgeons and patients supposed to choose?
do not believe that there is any evidence today that one is
more likely to be more successful in the long run than the
other three. It is possible that all 4 will be equally successful,
as measured by clinical outcome, and that is what I think
is a reasonable supposition. It
is possible that with 10 and 15 years follow-up that one of
the 4 will be more successful than the other 3, but such documentation
will have to await the definitive follow-up, which cannot
be expected for another decade.
question is what to do now? Perhaps
it doesn't make any difference, but there are differences.
MOM and COC solutions are significantly more expensive than
metal on highly crosslinked UHMWPE, while substituting a ceramic
head for a metal head is intermediate in cost. Translated
to the whole population of TJR candidates, these cost increases
could be quite significant.
has a history of fracture. Manufacturers claim that this has
been largely eliminated through the use of higher strength,
smaller grain size materials, and certainly it is rare but
not zero. However, ceramic fracture is a clinical disaster.
Manufacturers strongly recommend replacing the fractured femoral
head with a new ceramic component, only if a new trunion can
be provided, and discourage use of a more conventional UHMWPE
acetabular component after such failures. This means revising
an otherwise perfectly good femoral stem (and an occasional
acetabular cup) with all the potential complications that
prostheses have clearly demonstrated both high short-term
success rates and low wear/osteolysis rates. However, all
MOM bearings produce significantly higher metal ion levels
in both blood and tissues, and these elevated levels, that
can be 5 - 10 times normal, persist for as long as they have
been followed, in some cases over decades. It does not appear
to simply be an issue of 'bedding in', but rather ongoing
wear of the bearing surface. Promoters
of the MOM solution cite the extremely low wear volume of
the metal debris from the MOM bearing, noting that it is only
a fraction of the volume of that produced even by the highly
crosslinked UHMWPE, and this is certainly true. However the
typical particle size of the metal wear debris is also only
a tiny fraction of the particle size of UHMWPE. The
resulting high number of extremely small particles, even when
present in such low total volume, exposes a very large surface
area of metal to body fluids, which undoubtedly contributes
to the increased chromium, and cobalt levels measured in local
and distant tissues and body fluids. Both Chromium and Cobalt
are presumptive human carcinogens. Epidemological studies
of patients with older MOM hip replacements have produced
mixed results: some show slight elevations of blood related
malignancies and others show no adverse effects. The situation
is further complicated since the very low dose rates produced
by these devices present the possibility of late effects,
due to the very long, multi-decade latency known for Chromium
induced malignancies in industrial exposures.
is no compelling indication for MOM prostheses in the average
patient presenting for THR (average age 70) and the higher
cost cannot be justified. However, the rationale for use in
the young, because of low wear, is also problematic because
of the potential for late stage malignancy. There
is also a new potential problem that has recently surfaced,
one of metal allergy. This is an unknown long-term liability.
Because the exposure to the metal ions is continuous, it is
apparently something that can develop at any time, related
to transient sensitization episodes produced by some illness
or medications. Although conventional prostheses are often
made of cobalt-based alloy, the incidence of allergy is extremely
low and does not appear to increase with time. However the
higher level of metal ion exposure with the MOM prosthesis
apparently makes this a very real potential problem.
the introduction of ceramic femoral heads (to replace conventional
metal heads) while apparently reducing clinical wear rates,
does not appear to reduce the need for revision surgery, and
thus, it seems hard to justify their higher cost and the rare
risk of fracture.
on an overview of the available options it appears to me that
metal-highly crosslinked UHMWPE bearing surface offers the
best risk/benefit ratio of the 4 potentially long-wearing
bearing surfaces in hip replacement. It is the least expensive,
the easiest to revise, should that become necessary, has a
long history of successful use as a bearing surface system
and is well tolerated as long as wear is kept below a threshold
that certainly appears to be achieved by the new highly crosslinked
UHMWPE offerings. Conventional
UHMWPE, with durability now improved by better sterilization
and storage techniques, bearing against cobalt-based alloy,
remains the best choice for knee replacement.
David Hungerford, MD