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Wednesday, February 28, 2001

Poor positioning can limit range of motion

Component impingement can limit range of motion after total hip arthroplasty due to poor positioning. Researchers in scientific exhibit 57 used a computer kinematic model to determine the effects of component position and varying head-neck ratios on prosthetic impingement and hip range of motion, and a finite model to compute polyethylene stresses at different cup positions.

They found that cup abduction angles between 45 degrees and 55 degrees when combined with appropriate acetabular and femoral anteversion resulted in maximum overall range of motion and stability with respect to prosthetic impingement.

A three-dimensional total hip prosthesis with a hemispherical acetabular cup, femoral neck diameter of 12.25 mm and head sizes ranging from 22-32 mm was generated. Maximum hip range of motion was measured before impingement of the neck on the cup liner for acetabular abduction angles ranging from 35 degrees to 55 degrees, acetabular anteversion angles 0 degrees to 30 degrees and femoral anteversion angles 0 degrees to 30 degrees. Contact areas and stresses were calculated in the acetabular liner utilizing hip forces measured during gait from published telemetry data.

They found that acetabular abduction angles less than 45 degrees reduce flexion and abduction, while higher angles tend to reduce adduction and rotation. Femoral and acetabular anteversion increased flexion, but reduced extension. Acetabular abduction angles between 45 degrees and 55 degrees permitted better overall range of motion and stability when combined with appropriate acetabular and femoral anteversion.

According to the polyethylene stress analysis, a consistent decrease in contact area with increasing acetabular abduction angle (from 35 degrees to 55 degrees) with a corresponding increase in mean and peak contact stress was predicted using the finite element model. At each acetabular abduction angle, increasing cup anteversion from 0 to 30 degrees resulted in an increase in contact area and a decrease in stresses. The stresses at the posterosuperior edge of the liner also increased with increasing abduction.

The researchers are Darryl D. D'Lima, MD; Peter Chen, PhD; and Clifford W. Colwell Jr., MD, all of Scripps Clinic, LaJolla, Calif.

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2001 Academy News February 28 Index B

Last modified 20/February/2001 by IS