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Larger cups and optimal positioning produced lowest ion
levels and wear In a review of 585 blood serum evaluations following hip
resurfacing, only femoral size and cup inclination were
found to have an effect on ion levels, according a study by
orthopedic investigators.
The findings were presented at the 2010 Annual Meeting of
the American Academy of Orthopaedic Surgeons.
David J. Langton, MRCS, and his colleagues also found
that the size of the coverage angle of the acetabular
component contributed significantly to its tolerance of
suboptimal positioning.
"Larger joints, it must be emphasized, tolerated
suboptimal cup position," he said. "This must be taken into
account in all analyses."
Inverse relationships
Using routinely obtained blood serum metal ion levels
from patients under the care of the two senior authors of
the paper being presented - both highly experienced hip
resurfacing surgeons - metal ion results were analyzed
regarding their relationship to femoral and acetabular
component size and orientation, UCLA activity score, age,
time post surgery and postoperative femoral head/neck
ratios.
Langton reported an inverse relationship between metal
ion levels and femoral size. A smaller acetabular coverage
arc was associated with higher metal ion levels.
Another significant inverse correlation was noted by
Langton between metal ion levels and contact patch to rim
(CPR) distance. CPR is a measurement that relates the
position of the articular contact patch with the patient in
standing position to the cup rim. According to the abstract,
CPR less than 5 mm is associated with a 50% chance of ion
levels greater than 30 mg/L.
Words of warning
Langton warned the audience, "To increase metal ion
levels as quickly as possible, use as small a bearing
diameter as possible, use a cup with the smallest coverage
arc, and combine very high anteversion with high
inclination."
He concluded, "Cups placed with angles between 40° and
50° inclination and 10° to 20° anteversion have the lowest
ion levels and the lowest rates of volumetric wear."
Langton D, Jameson S, Joyce T, et al. A review of 585
serum metal ion results post hip resurfacing: cup design
and position is critical. Paper 006. Presented at the
2010 Annual Meeting of the American Academy of
Orthopaedic Surgeons. March 9-13. New Orleans.
Langton has received research or institutional
support from DePuy, a Johnson & Johnson Company, DJ
Orthopaedics; he has also received miscellaneous
non-income support (e.g., equipment or services) from
DePuy, a Johnson & Johnson Company.
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