CA is widely used as an index of implant orientation. To the best of our knowledge, no study to date has examined the permissible range of CA values to satisfy the ROM criteria for activities of daily living without prosthetic impingement with CA after THA. This study demonstrated that the range of postoperative Yoshimine’s CA and Widmer’s CA satisfied the ROM conditions for activities of daily living after THA.
Few studies have evaluated ROM on simulation after THA clinically, as in this study. One study has shown impingement-free ROM after THA with the stem-first technique using imageless navigation . Their ROM criteria to be satisfied were defined as: flexion > 110°, internal rotation at a flexion 90° > 30°, extension > 30°, abduction > 45°, abduction > 50°, and adduction > 30°. Out of the 57 cases using imageless navigation, 48 (84%) met all the ROM criteria. The ROM criteria were different from the strict criteria in our study, except for external rotation. However, 83.5% (111/133 hips) of the cases in our study met our ROM criteria. This report has not evaluated implant orientation before or after surgery and the target CA and did not indicate the range of postoperative CA to meet the ROM criteria. We evaluated the impingement-free ROM by postoperative CT after THA with a cup-first technique using CT-based navigation targeted by Yoshimine’s CA, according to a previous study . We reported that 45 (88%) of the 51 hips met all the Yoshimine’s ROM criteria. Moreover, postoperative Yoshimine’s CA within the target range of 90.8±10 would significantly meet all ROM criteria. However, we have not examined whether postoperative Yoshimine’s CA within 10° from the target value would be suitable as a range to meet the Yoshimine’s ROM criteria.
Yoshimine’s CA and Widmer’s CA theories recommended a target CA value of 90.8 and 37.3. Given that navigation systems have potential to implant components in an optimal orientation, there have been many reports on the accuracy of implant orientation in THA using these navigation systems. Inaba et al.  reported absolute differences between preoperative and postoperative cup inclination as 3.2°±2.3°, cup anteversion 4.0°±3.5 °, stem anteversion 3.9°±5.0°, and Widmer’s CA 5.3°±5.2 ° using a CT-based navigation system. Dorr et al.  showed that the average CA was 37.6°±7.0°, and 45 cases (96%) out of 47 were within the target CA of 25°–50° using an imageless navigation system. Although tools such as navigation systems were used for accurate implant orientation, errors may occur between preoperative and postoperative implant orientation. It has been difficult to place components to set target CA accurately during THA. Therefore, we consider that it is important to examine the range of postoperative CA obtaining the target ROM criteria to be indexes evaluated for implant orientation after surgery and to set an implant position intraoperatively. This study revealed that the absolute value of difference between the target and postoperative values of Yoshimine’s CA was a significant factor in whether or not the ROM criteria were met using univariate and multivariate analyses. This study showed that postoperative Yoshimine’s CA and Widmer’s CA should be within 90.8°±6.0° and 37.3°±6.9°, which is likely to meet the Yoshimine’s ROM criteria for activities of daily living. Our results indicated a useful target range of Yoshimine’s CA and Widmer’s CA in considering the error related to implant orientation in THA.
The absolute error of Yoshimine’s CA and Widmer’s CA was 5.7±4.6 and 4.4±3.6, respectively, in our study, which is comparable to Widmer’s CA 5.3°±5.2 ° of previous studies using CT-based navigation . The use of CT-based navigation could predict whether the postoperative CA will place the implant within the target range of CA.
There were two cases in which the prosthetic ROM did not meet the Yoshimine’s ROM criteria, even though the Yoshimine’s CA was in the range of 90.8°±6.0°. The Yoshimine’s CA in two cases was 88.5° and 92.6°. These cases did not reach the ROM boundary only for one direction each of internal rotation at flexion 90° and external rotation. These cases did not reach the ROM benchmark due to a difference of 1°. Three-dimensional templating software has been shown to have excellent interobserver and intraobserver reliability for component alignment in THA . Inoue et al.  reported that error measurement might be performed using this software when manually matching the reference points between the preoperative plan and postoperative evaluation on CT. Two studies have reported that the sagittal alignment of the stem had an influence on impingement-free ROM [20, 21]. Therefore, it was considered that a measurement error with the three-dimensional templating software occurred, which influenced the sagittal alignment of the stem.
This study has several limitations. First, we excluded the cases wherein the stem anteversion was >40° or <10°. For cases wherein the stem anteversion was >40°, cup radiographic anteversion was set to <10°, according to the CA theory, and if errors occurred in these cases, cup radiographic anteversion may have led to retroversion. Widmer et al.  did not recommend cup radiographic anteversion of <10° to be incompatible with the intended ROM. When stem anteversion was <10°, cup radiographic anteversion was >30° and the acetabulum could not sufficiently cover the posterior area of the cup. A cementless stem was implanted manually without the use of navigation systems in this study. Anteversion of the cementless stem was hard to control because of the anatomy of the proximal femur. A broad range of postoperative stem anteversion has been reported in the literature [22–24]. The postoperative error may cause the stem anteversion to become retroverted. Dorr et al.  reported similar results showing that 2 of 47 hips exhibited postoperative CA of <25° due to the retroverted native femoral anteversion that was not in the safe zone. Therefore, stem anteversion should be avoided.
Second, two types of implant designs and oscillation angles were included. The formula of Yoshimine’s CA was calculated based on the assumption that the implant design had an oscillation angle of 135° . The study showed that the safe zone for an oscillation angle of 120° was extremely small, and implant designs with a greater oscillation angle were recommended. Two implant designs had an oscillation angle of ≥135° in our study. Many varieties of implant designs have been used widely in THA. Our study showed that the use of two types of implants with an oscillation angle of ≥135° could fulfill the Yoshimine’s ROM criteria within the range of ±6.0° according to Yoshimine’s CA. However, this study may not be accurate when using implants with an oscillation angle of ≤135°. Third, this simulation study investigated only ROM without prosthetic impingement. Clinically, ROM without bone and soft-tissue impingements was also a risk factor of dislocation. However, we did not evaluate them. The factors that the surgeon could control to avoid dislocation were to place the optimum implant positions and to increase prosthetic ROM.