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Friday, March 2, 2001

Larger femoral heads provide greater motion

Larger femoral heads provide greater motion

With the advent of cross-linked polyethylenes, larger femoral heads have been advocated to increase the stability of the hip without excessive wear of the bearing surface. In the experimental study in scientific paper 74, researchers said Thursday they examined the impact of head size on the range of motion of total hip (THR) to impingement and dislocation. They found that large femoral heads can provide significant improvement in the range of motion of the hip to impingement and dislocation. However, these benefits are dependent on the design of the neck of the prosthesis.

A THR was performed on six cadavers using two designs of femoral prosthesis with the same heel (shaft angle 132 degrees): One implant (A) had a 12 mm diameter neck with 48 mm head offset while the second component (B) had a 14 mm diameter neck with 41mm head offset.

Each specimen was instrumented with cables simulating the action of seven muscles controlling the hip joint during rising from a low chair, and then mounted in an MTS machine. The hip was placed in 15, 20, 25, 30, 35, and 40 degrees of adduction and flexed from 80 degrees until frank dislocation. The position of the hip at impingement and dislocation were monitored electronically. The test procedure was performed with modular heads of 38 mm and 28 mm diameter.

In changing the head size from 28 to 38 mm, hip flexion at impingement increased by an average of 6.0 degrees with implant A (94.6 degrees vs. 100.6 degrees ; p < 0.0001), and 6.4 degrees with implant B (97.7 degrees vs. 104.1 degrees ; p < 0.0001). At dislocation, the 38 mm head increased hip flexion of component A by an average of 6.8 degrees (105.2 degrees to 112.0 degrees ; p < 0.0001). However, flexion of the prosthesis with the 14 mm neck only increased by 2.3° (108.4 degrees to 110.7 degrees; p = 0.048).

With both neck designs, 38 mm heads increased the incidence of bony impingement prior to dislocation, say the researchers Philip C. Noble, PhD; Vibor Paravic, BS; and Sabir Isnaily, BS, all of Houston, Texas.

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Last modified 15/February/2001 by IS