The orientation of the acetabular cup is a critical factor for prevention of various postoperative complications in total hip arthroplasty (THA). Although most patients are treated in either supine or lateral position during surgery, it is still unclear which position is superior to achieve more accurate cup positioning. Our study was conducted in order to answer the following questions: (1) does the supine position provide a higher accuracy of cup positioning than the lateral position in THA using modified Watson-Jones approach? (2) is there any difference in the distribution of cup position between the two positions?

Hypothesis Our hypothesis was that the supine position would provide a higher accuracy of cup positioning than the lateral position in THA using modified Watson-Jones approach.

A single-center prospective randomized study (registration number: UMIN000021627) was conducted between May 2016 and December 2017. We recruited a total of 60 participants undergoing unilateral primary cementless THA using modified Watson-Jones anterolateral approach based on the result of the sample size calculation. They were randomly assigned to either supine position (n=29) or lateral position (n=31). The cup alignment was targeted using a goniometer during surgery. The radiographic cup inclination was targeted to 40° and the radiographic cup anteversion was targeted considering the femoral stem anteversion during surgery. Postoperative cup alignment was measured by plain radiography and computed tomography. We defined the difference between postoperative and target cup angle as target error and our primary outcome was the absolute value of the target angle. As secondary outcome, the distribution of the target error was evaluated. The target errors of each inclination and anteversion were divided into 3 groups; neutral (−3°≤the target error≤3°), positive error (3°<the target error), and negative error (the target error<−3°).

The assessment of primary outcome for all recruited patients showed that supine group was significantly more accurate than lateral group in terms of radiographic inclination (2.4° vs. 4.5°, respectively, mean difference 2.1°; 95% confidence interval, 0.7 to 3.5; p<0.01). There was no significant difference in terms of radiographic cup anteversion (5.6° vs. 5.2°, mean difference 0.4°; 95% confidence interval, −1.8 to 2.6; p=0.69). The rate of positive error of anteversion in supine and lateral group was larger than that of negative value of anteversion (51.7% vs. 10.3% and 48.4% vs. 12.9%, respectively). Any acute complication (dislocation, fracture, and infection) was not found in both groups during postoperative 3 months.

In this randomized-controlled trial, higher accuracy of acetabular cup inclination was provided by supine position than by lateral position in THA. On the other hand, there was no significant difference between both groups in terms of cup anteversion. In both groups, most cups were placed with larger anteversion than we targeted. Modified Watson-Jones approach in both positions should be performed considering these results.

UMIN000021627.

Level of evidence II, randomised controlled study (population-limited).