Alternative Bearing Surfaces in Total Hip
Arthroplasty: Metal on Metal and Ceramic on Ceramic
William J. Hozack, MD Fabio Orozco, MD
Total hip arthroplasty (THA) is very successful when it comes to
pain control and the restoration of function and the ability to
walk. The most common bearing surface used in THA is
polyethylene-on-metal. However, nowadays, with young and more
active patients undergoing the procedure, other bearing surfaces
such as metal-on-metal and ceramic-on-ceramic have been proposed
as an alternative to metal-on-polyethylene as a technical
solution to the need for reducing wear debris production and
subsequent osteolysis leading to surgical revision. Some of the
recent clinical results and wear debris consequences with these
bearing surfaces were presented at the American Academy of
Orthopaedic Surgeons 69th Annual Meeting and are summarized
here.
Clinical Results
Metal-on-Metal
Short- and intermediate-term
follow-up of metal-on-metal bearing surfaces revealed a low wear
rate and good clinical results in general. Lombardi and
colleagues[1] had previously compared short-term results of
polyethylene and metal liners in a multicenter, randomized,
controlled study. Seventy-two polyethylene liner THAs and 78
metal liner THAs were followed for a minimum of 2 years. Harris
hip scores were similar in both groups: 95.5 for the
polyethylene liner vs 95.2 for the metal liner. There was no
evidence of early failure in either group and no acetabular
components have been revised or are pending revision. The
authors concluded that metal-on-metal bearing surfaces may
represent a viable alternative for THA in younger, more
demanding patients.
As an adjunct to this study, Dorr and colleagues[2] reviewed the
clinical performance at 4 to 7 years of 70 THAs with
metal-on-metal articulation. At an average follow-up of 5.2
years after the operation (range, 4 to 6.8 years), the Harris
hip score for the 53 patients who did not have a revision was
89.6 points (range, 62 to 100). One patient had revision of a
loose cup, but there were no other loose acetabular components
in the series. Two patients had revision of the acetabular
component because of dislocation. No femoral components were
loose or revised. The mechanical failure rate was 2%(1 patient).
Wear was not measured, but there was not evidence of osteolysis.
The authors concluded that at intermediate-term follow-up, the
clinical results of metal-on-metal THAs are similar to those
seen with metal-on-polyethylene articulation. In addition, given
the lack of osteolysis, metal-on-metal articulation may play a
role in reducing the wear that occurs with "classic" total hip
replacement.
Ceramic-on-Ceramic
Three clinical studies evaluated the performance of
ceramic-on-ceramic bearing surfaces in THA. Hamadouche and
colleagues[3] retrospectively studied 108 consecutive
alumina-on-alumina bearing surfaces in THA at 20-years
follow-up. Both components were cemented in 85 hips, a hybrid
was used in 4 hips, and cementless sockets were used in 29 hips.
The mean age of the patients in the series was 62.2 years.
At 20 years follow-up, 46 patients (comprising 51 hips) were
still alive and had not undergone revision, 25 patients (25
hips) had undergone revision, 29 patients (30 hips) had died
from unrelated causes, and 12 patients (12 hips) were lost to
follow-up. Using revision as the end point, the survival rate
was 88.5% for cementless sockets, vs 59.7% for cemented sockets
(P = .0058). Survival of the stem was 88.0% for cementless stems
vs 83.5% for cemented stems (log-rank, P = .65). On x-ray, no
wear could be measured and osteolysis was not significant. The
authors concluded that uncemented sockets performed
significantly better than cemented sockets and the absence of
osteolysis in this long-term study is most likely related to a
low wear rate. Accordingly, they propose that ceramic-on-ceramic
bearing surface in THA is a safe option with good long-term
results...
...Local and Systemic Effects of Wear Debris
Metal-on-Metal
The concern of metal ion release in THA with metal-on-metal
bearing surfaces was addressed by 2 groups of investigators. In
the first study, Brodner and colleagues[6] presented a
randomized trial of metal levels in 100 uncemented THAs, 50 with
metal-on-metal and 50 with ceramic-on-polyethylene articulation.
All patients received titanium acetabular and femoral
components. Measurements of metal blood levels were done
preoperatively and at 3, 6, 12, and 24 weeks postoperatively;
measurements were then repeated every 6 months.
Serum cobalt was analyzed using atomic absorption spectrometry.
In the metal-on-metal group, the median serum cobalt
concentration was 1 mcg/L at 1 year, 0.55 mcg/L at 2.5 years,
and 0.7 mcg/L at 5 years. The median serum cobalt in the
ceramic-on-polyethylene group was always below the detection
limit. The difference in median serum cobalt between the 2
groups was highly significant (P < .0001). This study clearly
demonstrated a systemic cobalt release from metal-on-metal
bearing surfaces over a 5-year postoperative period.
A second study prospectively evaluated metal serum and urine
levels in 14 metal-on-metal THAs. Skipor and colleagues[7]
measured the levels of serum chromium (SrCr), serum cobalt (SrCo),
and urine chromium (UrCr). According to the results of this
study, SrCr and SrCo levels increase as postoperative time
increases. UrCr showed increases at 6 months, but decreased at
12 months. Concentrations at 12 months were 19 times greater for
SrCr, 14 times greater for UrCr, and 6 times greater for SrCo
compared with values seen in patients with metal-on-polyethylene
bearing surfaces. The authors concluded that there is a
substantial elevation in serum and urine metal content following
metal-on-metal THA, but the toxicologic significance is unclear.
Indeed, although studies have shown an increased incidence of
certain cancers in patients with metal-on-metal THA, other
cancers were significantly reduced in the treated population.[8]
These data indicate that the clinical significance of the
elevated metal levels in metal-on-metal THA has not been
established, and its relevance in terms of toxicity and
carcinogenesis remains unclear.
Ceramic-on-Ceramic
Ceramic-on-ceramic bearings have demonstrated very low in vivo
wear. However, as with other particles, ceramic debris appears
to induce granulation tissue along the implant bone interface,
potentially resulting in osteolysis and aseptic loosening. In
the past, breakage of ceramic articulations was a significant
problem, but the use of the newer ceramics has rendered this a
very uncommon complication. There are no studies in the
literature reporting systemic adverse effects with the use of
ceramic-on-ceramic THA.
Conclusions
In addition to the "classic" metal-on-polyethylene bearing
surfaces used in THA, metal-on-metal and ceramic-on-ceramic
bearings have become increasingly popular over the last few
years. Metal-on-metal THA appears to have good clinical results
and a very low rate of wear, but high blood levels of cobalt and
chromium particles have been detected in patients using these
devices. Nevertheless, the toxicologic importance and
carcinogenic effect of trace metal elevations has not been
established. Long-term prospective studies are needed to
determine the risk of this implants as a potential cause of
cobalt and chromium toxicity. Ceramic-on-ceramic bearings appear
to be a safe option for young and active patients, demonstrating
a very low wear rate and good clinical results. However,
additional long-term studies are needed in order to completely
elucidate the role of this type of articulation in THA.
