Surgical procedure and postoperative evaluation

All surgical procedures were performed in the lateral decubitus position, and a CT-based navigation system (Stryker CT-Hip System V1.1, Stryker-Leibinger GmbH & Co. KG, Freiberg, Germany) was used as the acetabular component. The posterior articular capsule and short external rotator muscles were repaired in all cases [15]. All surgeries were performed by either of the two senior authors.

A CT scan was performed at 1 week postoperatively as a routine protocol to evaluate implant placement and to confirm the presence of fractures. The postoperative CT data were imported into the template for 3D analysis. The postoperative placement angles of the acetabular and femoral components were measured using the template. Matching with the preoperative template data, a template of the same size as the actual implant was overlapped on the postoperative CT for measurement (Fig. 1a, b). Based on these placement angles, the postoperative CA values were calculated using the formulas of Widmer et al. (cup radiographic anteversion + 0.7 × stem anteversion = 37°) and Yoshimine et al. [10, 11]. The mean CA values of the dislocated and non-dislocated groups were calculated and compared. Cup radiographic inclination within 35°–55° and CA values within ± 10% of the recommended values (Widmer: 37° ± 4°, Yoshimine: 90.8° ± 9°) were defined as good CA, and those outside the range were defined as poor CA. The dislocation rates in the good and poor groups were compared. The absolute values of the difference between CA values calculated for each case and the recommended CA values using the formulas of Widmer et al. and Yoshimine et al. (37° and 90.8°, respectively) were calculated. Moreover, the cutoff values of CA for dislocation were examined. All offset measurements were made by projecting the distance on a horizontal plane using the template based on the preoperative and postoperative CT findings.

Measurement of the postoperative acetabular component and femoral component placement angles. a Widmer’s combined anteversion; b Yoshimine’s combined anteversion

The horizontal distance from the pubic symphysis to the center of the femoral head was measured as the acetabular offset, and the horizontal distance from the center of the femoral head to a line passing through the center of the femoral shaft was measured as the femoral offset [16]. The sum of these offsets was defined as the total offset.

The mean values of the total, acetabular, and femoral offsets on the operated side were compared between the dislocated and non-dislocated groups. The difference between the preoperative and postoperative values of each of these three offsets was divided into the postoperative increase and non-increase groups, and their dislocation rates were compared. The number of dislocations in each combination of the total offset increased/non-increased group and the CA good/poor group was investigated. The dislocation rates were also compared in cases with inner heads < 32 mm and ≥ 32 mm.