This article discusses current bearing-surface alternatives
for long-term total hip articulations involving metal-polyethylene,
ceramic-polyethylene, metal-metal, and ceramic-ceramic couples.
Previous attempts to improve the performance of ultra-high molecular
weight polyethylene have included carbon-fiber reinforcement (Poly-2)
and, more recently, polymer reprocessing to enhance mechanical properties
(Hylamer). The former was withdrawn from the market because of excessive inflammatory
response, whereas the latter has been linked to debris-induced osteolytic
responses in early reports.
Laboratory simulation has demonstrated that the resistance of
ultra-high molecular weight polyethylene to wear is improved with
increased cross-linking of the carbon-hydrogen polymer chains. A number
of thermal and chemical processing solutions have been described.
One such approach involves component storage at elevated temperatures
in an oxygen-depleted environment. This is done following irradiation
and encourages kinetic recombination of the carbon-hydrogen free
radicals created by the radiation process.
Other techniques deliver increased radiation doses to the component
material followed by remelting to quench free radicals. While this
results in dramatic wear reduction in laboratory simulations (Fig. 3), it also changes
the amorphous and crystalline regions of the polymer, affecting
mechanical properties and potentially reducing fatigue strength. Clearly,
clinical experience will demonstrate the in vivo viability
of these "new polys."
"Articulations ahead of their time" aptly describes the
metal-metal McKee-Farrar, Ring, Müller, and Sivash prostheses.
Short-term clinical failure in the face of growing success with
Charnley prostheses led to their disuse by the early 1970s.
Suboptimum design, inconsistent component manufacturing techniques,
poor fixation, and high equatorial frictional torques have been
mentioned as reasons for aseptic loosening of these designs, which
often occurs with little apparent wear of the bearing surfaces.
Despite these early experiences, many metal-metal implants have
survived twenty years or longer and still have exhibited highly
polished surfaces (Figs. 6 and 7).
Over the last decade, a resurgence of interest in metal-metal
articulations has evolved. Currently, upwards of 100,000 Sulzer
Metasul designs have been implanted in Europe, and on August 3,
1999, this device received Food and Drug Administration 510K approval
for commercial distribution. Other designs have recently been cleared
by the Food and Drug Administration for clinical use in the United States,
and reclassification efforts should make them generally available
in the near term.
The Metasul design includes an ultra-high molecular weight polyethylene
sandwich that theoretically dampens load transmission to periacetabular
bone as a means of preventing component subsidence given the high
rigidity of the metal-metal components (Fig. 8). This approach has also been adopted
in some ceramic-ceramic applications to accommodate high rigidity
as well as the low energy-absorbing capacity of the ceramic.
With other contemporary metal-metal designs, modularity is maintained
by direct assembly of the cobalt-chromium liner into its cementless
shell, with stability usually achieved by means of a Morse-taper
locking mechanism (Fig. 9).
Hip-simulator studies have demonstrated the importance of specific
diametrical clearances to facilitate polar bearing and access for
serum lubrication. Close control of component dimensions, sphericity,
and surface finish are also critical, but they add to manufacturing
costs. Currently, both cast and wrought cobalt-chromium-molybdenum alloys
of differing carbon content are used in this self-bearing application.
These alloys possess high hardness and a capacity to "self-heal" by
polishing out third-body scratches in contact areas. Simulator studies
have demonstrated a twentyfold to 100-fold reduction in the amount
of particle generation in comparison with that demonstrated in similar
evaluations of metal-ultra-high molecular weight polyethylene articulations,
suggesting their potential for longevity in in vivo use.
Despite their apparent advantage in younger patient populations,
there is a longer-term concern about metal particle and ion generation.
Toxicity, hypersensitivity, and carcinogenesis have all been mentioned
as potential adverse events, but a relationship has not been established. Figure 10 depicts increased
serum chromium concentrations at the time of long-term follow-up
of a population of patients with McKee-Farrar implants.
n The debris from standard polyethylene-metal bearings has been
responsible for aseptic loosening and osteolysis in many patients.
With the indications for hip replacement expanding to include younger
and more active patients and with the increasing recreational activities
and life expectancy of our senior population, the search for bearing
alternatives has intensified.
n Enhanced polyethylenes represent a class of emerging ultra-high
molecular weight polyethylene alternatives whose common denominator
is that they were created with an appreciation of the importance
of increased cross-linking of the polymer chains and the elimination
of free radicals to reduce component wear. A number have already gone
through the Food and Drug Administration regulatory process, and
their in vivo performance should be closely followed.
n Alumina and zirconia ceramic femoral head components substantially
reduce polyethylene wear volume but are highly taper-tolerance sensitive. Their
selection on the basis of patient age and activity level may well
justify their added cost. They are currently available for clinical
use in the United States.
n Both contemporary metal-metal and ceramic-ceramic hip-replacement
systems are widely used in Europe, with clinical experience dating
back ten years and longer. The poor clinical performance of first-generation
metal-metal designs appears to have been overcome through improved
metallurgy, design, and manufacture. There is lingering concern
about a causal relationship between malignancy and other systemic
problems and elevated levels of trace metals, but no relationship
has yet been established.
n Contemporary metal-metal hip articulations have been successfully
used in Europe for more than a decade. On August 3, 1999, the Sulzer
Metasul cup design received a 510K clearance for distribution in the
United States; it has been followed by several other designs. Reclassification
efforts should make these cups generally available in the near term. Ultimately,
their increased cost will be weighed against patient benefit and
will define which particular patients are candidates for their use.
n Ceramic-ceramic hip systems have good biocompatibility with
much improved material composition, designs, and manufacture. Matching
taper tolerances of the head-neck and cup-liner junctions reduces
the prospect of fracture. Precise technical placement of the hip
components during surgery is essential to avoid ceramic-ceramic
impingement and the potential for debris generation.
n Currently, ceramic-ceramic hip systems are investigational
and are not available for general clinical use in the United States.
However, in July 2000, a first design gained a Food and Drug Administration Advisory
Panel recommendation for approval. Ultimate release of these systems
by the Food and Drug Administration will depend on the demonstration
of their safety and effectiveness through ongoing clinical trials
and laboratory evaluation. Once this bearing-surface alternative
becomes available, its increased cost will have to be carefully
weighed against long-term patient benefit in the current reimbursement
climate.