Presented at the 2nd Annual Total Hip Resurfacing
Arthroplasty Course in LA Oct. 2008

By: John Fisher
Co-Authors: Ian Leslie, Sophie Williams, Eileen Ingham, Graham
Isaac
Institute of Medical and Biological Engineering
University of Leeds

Background: There are considerable variations in metal ion
levels and metallic wear rates in patients with metal on metal resurfacing. In this in
vitro study the effect of design variables of head diameter and bearing clearance and
patient variables of cup and head position on metal ion levels and wear rates are were
investigated. Methods: Hip joint simulator studies were carried out on size
39mm and size 55mm metal on metal resurfacing with the same design. Size 55mm
diameter bearings with 110 micrometer diametrical clearance were compared to size 54mm
diameter with larger >250 micrometer diametrical clearance. The wear rates of
size 39mm bearings with a standard cup position of 45 degrees, were compared to a
steep cup position of 60 degrees and to a steep cup position combined with micro
separation associated with head offset deficiency.

Results: Initial bedding in wear rates and ion levels were
higher with size 39mm bearings compared to size 55mm, but in long term after 15
million cycles there was no difference in the steady state wear rates. Bearings with the
larger diametrical clearance had higher initial wear and steady state wear rates at
five million cycles. Cup position and head position resulted in much greater
increases in wear. For the 39 mm bearings, a 60 degree cup position resulted in a 9 fold
increase in wear. A steep cup and microseparation resulted in a 17 fold increase in wear
after two million cycles and a 39 fold increase in wear compared to the long term steady
state wear rate.

Conclusions: The study confirmed clinical ion level studies of
increased wear with larger clearance bearings. The results also confirm clinical ion
level measurements that steep cup angles can substantially increases wear. The
study also indicates that offset deficiency and microseparation may be responsible for
extremely high wear rates and ion levels found in some retrievals and some patients.
Further work is needed to investigate effect of different head sizes with steep
cups and microseparation and the effect of version.