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Metal ion release following hip
replacement not strongly related to patient activity
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The amount of patient activity following metal-on-metal
hip resurfacing arthroplasty seems to show little
correlation to metal ion release, according to study
presented here.
Amir Kamali, PhD, presented his team's findings at the 56th
Annual Meeting of the Orthopedic Research Society.
Kamali reported that his group looked at 25 consecutive male
patients who underwent a unilateral, 50-mm diameter
metal-on-metal hip resurfacing. The devices implanted
consisted of a hydroxyapatite-coated acetabular cup and a
cemented femoral head, both made of high-carbon cobalt
chrome alloy in the "as cast" microstructure state. To
reduce confounding variables, study patients were required
to have unilateral end-stage arthritis of the hip needing a
femoral head size of 50 mm.
Activity measurements
Patient step activity was then recorded at 1, 2 and 4-year
follow-up stages, wherein the patients were required to wear
a step monitor just above the ankle of the appropriate leg
for a period of 7 days during their waking hours. The device
recorded maximum sustained activity over a designated time
of 1, 20 or 30 minutes.
Finally, whole blood samples were collected from the
patients at all time points and analyzed for metal ion
levels.
Kamali said all patients in the study had well-functioning
hips at the 4-year follow-up stage, with overall patient
step activity remaining unchanged up to the 4-year mark.
Weak trends
At the 1-year follow-up stage, the whole blood cobalt and
chromium concentrations showed no significant correlation to
the average number of steps taken per day. According to
Kamali, for a function of body weight and peak index vs. the
whole blood metal-ion concentration, there was a weak trend
of increasing metal ions with an increase in patient
activity and weight.
"I should mention here that at 1, 2, and 4 years, the
average metal ion levels were similar between these
implants, meaning they had gone well into their steady
state," Kamali said.
He added the results may be due to the effect of
individuals' stop-start motion, walking speed and the effect
of varying kinetics and kinematics on the lubrication
conditions – all of which contribute to different levels of
metal ion release.
Reference:
Daniel JT, Kamali A, Ziace H, et al. The effect of patient
activity on metal ions levels, in patients with
metal-on-metal hip resurfacing. Paper 323. Presented at the
56th Annual Meeting of the Orthopaedic Research Society.
March 6-9, 2010. New Orleans.
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