Cup alignment in total hip arthroplasty using the muscle-sparing modified Watson-Jones approach-comparison between lateral and supine positions.

The present study aimed to compare the cup alignment outliers in total hip arthroplasty (THA) using the same surgical approach with the patient in the supine position versus the lateral position. THA using the muscle-sparing modified Watson-Jones approach was performed in 142 consecutive hips. THA was performed with the patient in the lateral position in 84 hips (lateral group) and in the supine position in 58 hips (supine group). The cup alignment was aimed at 40° inclination and 20° anteversion by referring to the mechanical alignment guide. Cup alignment and outliers (10° > aimed alignment) were assessed using post-operative 3D-CT. The absolute error from the aimed inclination was 6.0 ± 4.7° in the supine group and 4.2 ± 3.6° in the lateral group (p = 0.01). The absolute error from the aimed anteversion was 4.1 ± 3.2° in the supine group and 5.1 ± 3.7° in the lateral group (p = 0.12). The supine group showed a higher rate of outliers than the lateral group for the cup inclination (22% vs 5%; p < 0.01). Inclination and BMI were positively correlated in the spine position group (p < 0.01, R = 0.48), but were not correlated in the lateral position group. THA performed with the patient in the supine position has a higher risk of outliers of cup alignment compared with the lateral position, even when the same surgical approach is used. BMI affected the cup inclination in the supine position.