by Thomas P. Gross, MD (d),

Fei Liu, PhD (d) 

In 2009, the Oxford Group reported a 1.8%
rate of revision for pseudotumours in a series of 1419 hip
resurfacings performed by multiple surgeons within a mean 4
year (0-9 years) time period and a 6% Kaplan-Meier failure
rate for pseudotumour by 8 years. We were surprised by this
high rate of failure due to a primary inflammatory reaction
to metal bearings (pseudotumour) and we therefore wanted to
evaluate our database for the incidence of failure we
experienced due to pseudotumours.

Methods

Between July 1999 and August 2011, a
single surgeon performed 2559 metal- on-metal hip
resurfacing arthroplasties in 2109 patients. The mean length
of follow-up was 4±3 years. We were able to achieve a
follow-up rate of 92%. This study was compared to the
multi-surgeon Oxford report on 1419 cases with 4 years
follow-up. The Corin Cormet 2000 (393 cases) and Biomet
Recap implants (2166 cases) were used in our series. All
adverse wear failures had metal ion levels tested and had
acetabular inclination angles measured on both supine and
standing pelvis xrays. Similar to the Oxford Group, we
analyzed wear related failures with “symptoms severe enough
to cause revision”. A detailed comparison between the Oxford
report and our database has been compiled (Table I).

Results

In this study, seven (0.27%) cases (six
in female and one in male) out of 2559 cases were diagnosed
as failures due to adverse wear with soft tissue and fluid
accumulation, and metallosis seen at the time of surgery
(Table II). The wear related failure rate was 0.27%
(7/2559). Failures due to all other causes occurred in 70
(2.7%) cases. Revisions for adverse wear represented 9.1% of
all failures. At 8 years post-operatively, our Kaplan-Meier
cumulative revision rate for adverse wear was 1% for all
patients (Fig I), 0.2% for men, 2.6% for women, and 9% for
patients with a diagnosis of dysplasia. All adverse wear
failures had component sizes ≤ 48 mm. All had metal ion
levels of at least
15 ug/ml and acetabular inclination angles (AIA) >50° on
standing pelvis radiographs. 5/7 had AIA > 60°. All had
severe metallosis found at the time of revision. There were
no failures from pseudotumours without evidence of adverse
wear in this series (0/2559). In table III, we report the
chief similarities and differences between the present study
and the Oxford study.

Conclusions

In summary our data suggests the following conclusions:

1.) Adverse wear related failures (metallosis) are rare
with the Biomet and
Corin HRA systems (0.27%, Kaplan-Meier 1% at 10 years).
2.) Pseudotumours not related to adverse wear are rare with
metal bearings.
3.) Adverse wear failures were seen with the Biomet and
Corin systems only if
the AIA on standing pelvis xray was > 50 degrees. (5/7
failures had AIA>60).
4.) A Safe zone for placing the Biomet and Corin devices is:
AIA< 50 degrees on standing pelvis xrays. This likely also
applies to other well-designed systems.
5.) Adverse wear failures are more common in women,
dysplastics, and when
femoral components ≤ 48 mm are required. These factors are,
of course,
interrelated.

We therefore conclude that hip
resurfacing with Biomet and Corin implants (and likely most
others with a similar bearing design) is safe and effective
and is exceedingly unlikely to result in adverse wear
failure as long as the acetabular component is placed such
that the AIA is < 50° on standing AP pelvis xray. In women
with dysplasia that require small femoral components there
is less room for error in acetabular component positioning.
Adequately designed components exist; it is now up to
surgeons to learn to place them accurately and reproducibly
to avoid adverse wear failures.