METAL-ON-METAL
TOTAL HIP ARTHROPLASTY
David J. Zukor, M.D.,
F.R.C.S.(C) Associate Professor of
Surgery McGill University Chief, Dept. of
Orthopaedic Surgery The Sir Mortimer B. Davis
Jewish General Hospital Montreal, Quebec While
Total Hip Arthroplasty is undoubtedly one of the most successful surgical
procedures available, it is clear that its longevity is limited by material
considerations, i.e. wear of the implants.
It has also become apparent that loosening of implants is highly
correlated with wear debris accumulation.
Currently, for a variety of reasons, the demand for Total Hip
Arthroplasty has increased, especially among younger patients who desire
“perfect results”, in terms of quality of life, high functional demand and longevity
of the implant. It is apparent that,
to date, metal-to-polyethylene arthroplasties can provide excellent function
but are limited by wear. As
a result, orthopaedic surgeons and industry are striving to prolong the life
of these implants. Various approaches
including the search for alternative bearing surfaces are being employed. It has been suggested that
some improvements to arthroplasties should be “evolutionary and not
revolutionary”. An interesting
question arises: Is metal-to-metal THR a revolutionary idea? In fact, some of the
original designs of THR (Wiles 1938, McKee 1950’s, Huggler and Müller and
Ring 1960’s) employed metallic articulations. These designs were abandoned largely because of some poor early
results (related to design) paralleled by the great success of Charnley low
friction arthroplasty. However, three
decades of employing polyethylene as the “gold standard” has led to
significant knowledge regarding polyethylene wear, debris accumulation and
subsequent osteolysis. Considerable evidence
exists based on long-term outcome studies of original metal-metal THR, as
implants can function for prolonged periods of time with good clinical
results, minimal wear and little if any osteolysis. In the 1980’s, a new
generation of metallic articulating surfaces was created employing
sophisticated engineering technology which vastly improved the performance of
the articulating surfaces. Short to
mid-term results have been highly encouraging. Retrieval studies have confirmed the low rate of wear and the
paucity of adverse histological reactions in peri-prosthetic tissues. Despite this, concerns
still exist regarding the possible dangers of metallic particles and ions in
large numbers for long periods of time.
Ironically, while the wear rates of metallic articulations are
approximately 50-100 times less than that of metal-polyethylene
articulations, the number of metallic particles is approximately 100 times
greater than the number of polyethylene particles present. To a certain extent, this large volume of
particles is “neutralized” by corrosion and dissolution of particles,
systemic spread, excretion and agglomeration of particles decreasing their
available surface area. However, it
is also known that the latency period for metals can approach 15 to 35 years
and therefore long-term evaluation is required. Of especially great
concern is the possibility of development of cancers either adjacent to Total
Hip Arthroplasties with metallic bearing surfaces or at distance sites. Conflicting data to date have revealed
little if any increased incidence of lymphoma and hematopoietic cancers but
further ongoing studies are definitely required. The early clinical results of 41 metal-on-metal Total Hip Arthroplasties at our institution are very promising. We currently believe that the potential benefits outweigh the potential risks as understood today. We continue to use these implants in selected patients who are closely monitored. |
