Soft tissue tension is four times lower in the unstable primary total hip arthroplasty

The aim was to compare patients who suffered recurrent dislocation following total hip arthroplasty (THA) with those who did not to clarify the degree of soft tissue tension in dislocation patients.

The subjects were 18 hips with recurrent dislocation (unstable THA group) and 37 hips without dislocation (stable THA group). To evaluate soft tissue tension, radiographs were taken while applying distal traction at traction forces of 40, 30, and 20% of the body weight (BW) and femoral head displacement was measured. Acetabular offset, femoral offset, limb offset, and leg length discrepancy were measured in patients with a normal contralateral hip joint.

The mean femoral head displacement in the unstable THA group was 5.6 mm at 40% of the BW, 4.6 mm at 30% of the BW, and 3.5 mm at 20% of the BW. In the stable THA group, the mean femoral head displacement was 1.4 mm at 40% of the BW, 1.1 mm at 30% of the BW, and 0.9 mm at 20% of the BW. Significant differences were seen between the groups at all traction forces. Furthermore, on comparing the unstable and stable THA groups, femoral offset was found to significantly be smaller in the affected side than in the healthy side in the unstable THA group.

We found that soft tissue tension is approximately fourfold lower in patients exhibiting recurrent dislocations following THA than in patients exhibiting no dislocations and that femoral offset was related to decreased soft tissue tension.