Advance Bearings in Total
Hip Replacement: Hard-On-Hard
J. Dennis Bobyn, PhD. Associate Professor Division of Orthopaedics Faculty of Medicine,
McGill University Director of Orthopaedic
Research, Montreal General Hospital Montreal, Quebec Problems with wear debris generation in the
artificial hip and associated peri-implant osteolysis have led to a major
research thrust to improve the wear resistance of bearing materials through
advanced engineering and manufacturing procedures. The strategy is divided along two major approaches: improving
the wear properties of polyethylene or using
hard-hard bearing technology. Hard-hard bearing technology presently involves
either metal-metal or ceramic-ceramic designs. Two decades after the original McKee-Farrar prosthesis was
largely abandoned due to sub-optimum engineering and excessive loosening
rates, metal-metal articulations made of cobalt-based alloys have witnessed a
renaissance. The basis of this interest
is that cobalt-based alloys are extremely hard and wear-resistant in
self-bearing applications. With tight control over material specifications,
implant dimensions, and surface finish, wear volumes of < 1mm3
after several million cycles can reproducibly be obtained in hip simulator
tests. Clinical retrievals have
confirmed low linear wear rates on the order of < 5 mm/year. At present the cast, low carbon wrought,
and high carbon wrought cobalt-chromium alloys are all used clinically for
self-bearing applications, the latter material being the most common. Ceramic-ceramic bearing technology is a
well-established strategy for reducing wear in the hip, with extensive
clinical experience in Europe and continuous ceramic improvements occurring
over the past two decades. Ceramic-ceramic
bearings are typically made of alumina because its self-bearing properties
are more understood and thought to be superior to zirconia-zirconia. Much of the published information on
ceramic bearings involves older formulations of the material that possessed
coarser grain structures and poorer mechanical properties. Alumina-alumina bearings are sensitive to
stress concentrations; in the past burst fractures have been reported due to
stress risers at Morse taper connections and runaway wear has been reported
in cases of excessive cup verticality.
The current generation ceramics are finer-grained, stronger, and more
wear resistant. Recent information
from simulator testing has indicated very low linear wear rates in the range
of 1-3 mm/million
cycles. Zirconia possesses about
twice the burst strength of alumina and hence is more fracture
resistant. For this reason, zirconia
has been proposed for use as a head material bearing against alumina, a
combination that has also proven to be highly wear resistant in hip simulator
tests. With all of the advanced bearing technologies,
there is a need to elucidate issues related to the wear particles themselves,
both size and number, and their local and systemic effects on tissues. Also important, from a design standpoint,
is the ubiquitous issue of integrating the acetabular bearing surface into
the overall cup design. Modular
designs and those involving molding of a thin metal or ceramic liner into
polyethylene require careful scrutiny for mechanical durability and resistance
to motion and fretting. Finally,
hard-hard bearings tend to be less forgiving than those involving
polyethylene in terms of cup positioning and impingement. Notwithstanding these concerns, extremely
low wear in artificial hip replacement is currently possible from diverse
design perspectives.
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