|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.”
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.”