Updated 10/2/2014

Metal-on-metal (MoM), ceramic-on-ceramic (CoC), and ceramicized
metal-on-polyethylene (CMoP) bearings all were developed as a reaction to the
well recognized mediocre long-term results with metal-on-polyethylene hips in
younger, more active patients.

Polyethylene wear increases with increased
activity and load, and often elicits an aggressive response from the body where
loosening and bone loss occurs (osteolysis), making subsequent revision more
difficult.

Alternate bearings (MoM, CMoP, and CoC) are more resistant to
wear, and their wear debris does not usually appear to stimulate near as much
osteolytic bone loss response as does polyethylene wear. Metal-on-metal bearings
have the added benefit that they can withstand high impact without shattering,
as do ceramic heads, and the larger metal head sizes render them statistically
about 10 times more stable to dislocation than traditional small femoral head on
poly and ceramic-on-poly hips from a posterior approach.

Resurfacing
metal-on-metal hip arthroplasty, in addition, offers much less bone sacrifice
than a stemmed prosthesis, since most of the femoral head and neck are retained.
Resurfacing is also associated with the attainment of a greater activity level
as compared to a traditional hip replacement after the initial healing period.

Metal-on-metal big femoral head total hip arthroplasty (THA), which is a
variety of a stemmed prosthesis, has been utilized in patients with avascular
necrosis or too much deformity of the femoral head. It has also been used in
older patients with decreased bone density and decreased activity demands, where
the increased stability of the femoral head is desired. It offers the greatest
theoretical stability of all the common total hip arthroplasty options because
the larger head relative to the small neck design gives the least impingement.

Recently, however, concerns have been increasingly raised about the body’s
reaction to the wear products (metal ions) of metal-on-metal bearings and the
early failure of some metal-on-metal products. This discussion paper addresses
recent issues regarding bearing choices and presents some education and
perspective in making decisions as to bearing preferences.

Each bearing
option – metal-on-polyethylene (MoP), ceramic-on-ceramic (CoC), ceramicized
metal-on-polyethylene (CMoP), and metal-on-metal (MoM) – has pros and cons and
risks and gains that need to be considered in selecting a type of replacement
that is appropriate for each individual patient.

Metal-on-polyethylene
total hip arthroplasty (aka “traditional total hip arthroplasty”) was developed
by Sir John Charnley in England and has been successfully used since the ’60s in
older patients. Polyethylene wear, osteolysis, loosening, and recurrent
dislocation have been the most common problems with traditional total hip
arthroplasty. Activity restrictions are necessary and common to prolong the life
of a metal-on-polyethylene prosthesis. Non-cemented cups and stems and
cross-linking of the polyethylene are now commonly utilized to try to increase
prosthesis longevity by decreasing wear. Cross-linking definitely makes the
polyethylene more resistant to wear, but also more brittle, and as femoral head
size has increased in an effort to reduce dislocation, fracture of the
polyethylene is being increasingly reported. Long-term results on cross-linking
are not yet available. Few orthopaedists generally recommend
metal-on-polyethylene hips in young, active patients, but if utilized,
significant lifelong restrictions of high-impact activities are generally
indicated.

Because of the recognized polyethylene wear problems, several
alternate bearings, as mentioned above, have been developed. Ceramic-on-ceramic
bearings have the least tested wear of the bearings available, but have several
drawbacks. Fractures of the ceramic head continue to be reported, and
high-impact activities are not recommended with these replacements. They are
more susceptible to squeaking, with some series reporting up to 17% frequency.
If problems such as squeaking or instability arise, they cannot be readily
revised to a metal-on-polyethylene replacement later because the minute ceramic
wear particles left in the soft tissues at revision rapidly degrade the softer
polyethylene in a metal-on-polyethylene revision THA.

Ceramicized metal
(Oxinium-Smith & Nephew) on polyethylene bearings resolve the fracture and
impact problems for the femoral head and seem to give excellent wear data in
simulators. The surface of zirconium metal is oxidized to a thin ceramic layer
and conveys excellent wear characteristics. This is presently my prosthesis of
choice in patients allergic to metal. Mid- and long-term data is not available
yet in the hip, but results in total knee arthroplasty are promising. Whether it
holds up to high-impact over time remains to be seen.

Metal-on-metal
bearings, which have been a big part of my practice over the last seven to eight
years, have been in use in England since Charnley popularized the
metal-on-polyethylene total hip arthroplasty. When fabricated and performed
correctly, many of the early prostheses have lasted 30 to 40 years, with little
or no osteolysis seen on follow-up. Because of the large size ball and the small
neck, they are extremely stable. They are, however, more sensitive to the
position of implantation, particularly pertaining to the cup. If the cup is too
vertical or horizontal or facing too forward (anteversion) or backward
(retroversion), then edge loading and impingement can occur and increased metal
wear is generated.

This can cause a local reaction to the metal wear in
the form of a fluid-filled sac or soft tissue mass, often referred to as a
“pseudotumor”, which can injure surrounding structures. In addition,
significantly increased metal ions in the blood from increased wear may produce
systemic injury to the liver or heart.

Recent evidence suggests that the
larger femoral head size may produce increased wear at the head-neck junction of
the stem. The United Kingdom equivalent of the FDA recently published a
recommendation to continue yearly monitoring of metal-on-metal big femoral head
total hip arthroplasty (not for hip resurfacing) for metal ion levels.

There are also occasional patients that develop actual allergic responses to
metal-on-metal bearings that may require revision. These true allergic reactions
to metal are rare, with most adverse response to metal-on-metal bearings usually
being secondary to increased local metal wear often associated with less than
optimal position of the cups.

I have been using the Wright Medical
Conserve Plus metal-on-metal big femoral head arthroplasty since 1999 and have
had generally very good results. We have seen, however, six modular neck metal
fractures in very heavy, or very active patients and now use a one-piece
non-modular stem. We have also seen two metal allergies, with subsequent cup
loosening and two pseudotumors (in patients who had the components in good
position) that have required re-operation. Although our results with
metal-on-metal big femoral head arthroplasty have been very good, the recent
findings of increased neck trunnion wear with larger femoral head sizes is
concerning and has not been previously recognized or reported. Corroborative
studies are presently needed, but because of this concern, I am now beginning to
look at alternatives to the metal-on-metal big femoral head arthroplasty. The
Smith & Nephew ceramicized Oxinium head on cross-linked polyethylene alternative
is what I use in older, less active patients who need stability.

In
contrast, in younger, more active patients the metal-on-metal hip resurfacing
performed with the Smith & Nephew Birmingham prosthesis has been extremely
successful in our practice. Sixteen year data in England and Australia and
Scandinavia shows excellent mid-term results in a patient population that had
previously performed poorly with metal-on-polyethylene components. Our results
with the BHR have been excellent, with over 765 implanted since 2006.

In
our series to date, we have a 98.6% success rate. Because of our experience and
Dr. McMinn’s experience in females with hip dysplasia, I now lean to metal on
poly hip replacement from an anterior approach for this small subgroup.

Although the Birmingham hip resurfacing has been performing very well when
implanted in the correct alignment, some other resurfacing products have
performed dismally. Both the DePuy ASR and Zimmer Durom prostheses were released
to the market in the U.S. without FDA approval. Because of design and metallurgy
issues, they have performed poorly and had a very high early revision rate. The
FDA recently recalled these two prostheses from distribution, but because they
were implanted without FDA approval, much legal activity has been directed
toward the companies and the physicians that implanted them. Numerous TV ads by
lawyers seeking clients with complications related to the DePuy or Zimmer
metal-on-metal bearings are inundating the airwaves.

These concerns have
subsequently exploded in the lay press. When statistics for longevity for the
DePuy ASR and Zimmer Durom and others are combined with the BHR results, the
overall success rate doesn’t look as good as conventional metal-on-polyethylene.
Careful analysis, however, shows that the BHR has continued to perform extremely
well when implanted in the correct position.

One needs to be careful to
compare apples to apples and not “throw out the baby with the bath water”
regarding metal-on-metal resurfacing. When it comes to young, very active
individuals, which is a new demographic that we are seeing more commonly over
the last 15 years, what other viable options are available?
Metal-on-polyethylene doesn’t hold up well to high-impact and excess activity,
and instability is still a concern, even with an anterior approach to the hip.
Ceramic is fragile to high-impact and very hard to revise if complications
occur. Ceramicized metal-on-polyethylene may be a good option for patients who
are metal allergic or have significant avascular necrosis, but this operation
still sacrifices good bone of the femoral head, neck, and shaft, and makes later
revision much more difficult.

The anterior approach to total hip
arthroplasty also has pros and cons. It may make posterior dislocation less
common, but can be associated with anterior instability. Technically, it is very
difficult to perform this procedure in heavy or well-muscled patients,
especially for resurfacing. Commonly, the femoral stem used with the anterior
approach has been modified to allow easier insertion, and this may potentially
compromise later stem stability. At present, I generally recommend the anterior
approach for thin, metal allergic female patients. This is a metal-on-poly
component that limits future high-impact activities.

Overall, for the
physiologically young and very active individuals with good bone density, I
still firmly believe that the Smith & Nephew Birmingham hip resurfacing is a
superior choice and continue to use it in this demographic population with
excellent results.

We are happy to address any concerns you may have
regarding implant options, choices, and techniques, and will continue to strive
for the optimum outcome for each of our individual patients.

John S.
Rogerson, M.D. September 2014