Abstract
Introduction
The incidence of dislocation after total hip arthroplasty (THA) was reported to be 0.5 to 10% in primary THA and 10 to 25 % in revision THA. The main causes of instability after THA were reported to be implant malalignment and inappropriate soft tissue tension. However, there was no study about quantitative data of soft tissue tension of unstable THA. The purpose of this study is to clarify the features of soft tissue tension of unstable THA in comparison to stable THA.
Methods
The subjects were 15 patients with 15 THAs who had developed recurrent dislocation after primary THA. Thirty four patients with 37 THAs who developed no dislocation for one year after surgery were recruited as a stable THA group. In both group, all THAs were performed through posterolateral approach. In order to assess the soft tissue tension of THA, we recorded antero-posterior radiographs of the hips while applying distal traction to the leg with traction forces of 20?, 30%, 40% of body weight (BW). The distance of separation of the head and the cup after traction was measured under correction of magnification. Nine of 15 THAs in the unstable THA group and 32 of 37 THAs in the stable THA group were unilateral involvement. In the hips with unilateral involvement, the femoral offset difference between the healthy hip and the reconstructed hip were evaluated. Statistical analysis was performed with χ2 testand Mann-Whitney U test, and statistical significance was set at P<0.05.
Results and Discussion
The average separation distance of the head and the cup was 5.2 ± 3.4mm (SD) at 40%BW, 4.3±3.2mm at 30%BW, and 3.2±2.8mm at 20%BW in the unstable THA group. The average separation distance of the head and the cup was 1.4±1.5mm at 40%BW, 1.1±1.4mm at 30%BW, and 0.9±1.2mm at 20%BW in the stable THA group. There were statistically significant differences in the separation distance between the groups in all ranges of traction force. The femoral offset difference between the operated side and the healthy side was −1.2±5.6mm in the unstable THA group and 3.1±4.8mm in the stable THA group. There were no significant difference in the femoral offset difference, however the femoral offset tends to be small in the unstable THA group compared to the stable THA group (P=0.05). The leg length discrepancy was −3.1±11.6mm in the unstable THA group and 2.7±7.1mm in the stable THA group. There were no significant difference in the leg length discrepancy (P=0.12).
Conclusion
The separation distance of the head and the cup during leg distal traction in the unstable THA group is about four times larger than that in the stable THA group. The femoral offset tended to be smaller in the unstable THA group compared to the stable THA group.
