We investigated the measurement accuracy of an instrument for measuring the angles of cemented stems during THA and examined the accuracy of stem positioning performed using the angle-measuring instrument. The mean absolute measurement accuracy was 4.9°, and the mean absolute implantation accuracy was 4.3°. As for implantation accuracy, 68% of cases were within 5°, and 93% were within 10°.
Generally, the intraoperative estimation of stem anteversion is performed based on surgeons’ visual assessments. Several studies have reported that there can be considerable differences between surgeons’ visual assessments of stem anteversion and postoperative measurements of stem anteversion [18, 22, 25]. Wines and McNichol found that compared with postoperative measurement stem anteversion measurements surgeons underestimated stem anteversion by a mean of 1.1° (standard deviation: 10.4). Estimated measures may range from as low as 21.9° below (–2 standard deviations) to 19.7° above (2 standard deviations) the CT calculated measurement. They believed that these differences were clinically unacceptable [22]. Dorr et al. reported that there was a weak correlation between surgeons’ visual assessments of stem anteversion and postoperative stem anteversion measurements [18]. In addition, Woerner et al. found that compared with postoperative measurement stem anteversion measurements surgeons underestimated stem anteversion by a mean of 7.3°±9.8° [25].
Several studies have investigated the accuracy of measurements obtained using a goniometer or navigation systems during THA [12–16] (Table 4). They reported that the mean absolute measurement accuracy ranged from 4.5°–7.3°. Thus, the mean absolute measurement accuracy value obtained in the present study (4.9°) seems to be acceptable. Several studies have reported that knee OA and the femorotibial angle significantly influenced measurement accuracy during THA [13, 16]. In the current study, knee OA was shown to significantly influence measurement accuracy during THA, which agrees with the latter studies.
Only one navigational study, which involved a CT-based navigation system, has examined stem implantation accuracy during THA [17]. Kitada et al. reported that the mean target angle was 34.2°±12.4°, and the mean postoperative stem anteversion angle was 31.7°±11.7°. Thus, the mean implantation accuracy value was –2.5°±6.1°. The absolute implantation accuracy was not reported. Our mean implantation accuracy value was 1.4°±5.6°, which is similar to the abovementioned value. In a previous study, it was reported that stem anteversion implantation accuracy of within 5° was achieved in 60% of cases. In our study, stem anteversion implantation accuracy of within 5° was achieved in 68% of cases, suggesting that the angle-measuring instrument can be used to achieve accurate stem implantation. However, the stem anteversion implantation accuracy was >5° in almost 30% of cases. This suggests that the angle-measuring instrument should be used with caution, and that the instrument requires further improvement.
Improvements in our understanding of femoral morphology and anteversion and their influence on implant impingement and dislocation have caused surgeons to re-evaluate component positioning during THA [26]. Several studies have examined stem anteversion during THA [18, 27]. Dorr et al. proposed that stem anteversion should exhibit an approximate range of 10°–20°, whereas D’Lima et al. stated that it should display an approximate range of 10°–30°. The mean target angle in the present study was 37.4°. Although this was larger than those described in other studies, it was similar to some previously target angle [17]. As the present study included many cases of developmental dysplasia of the hip, in which the degree of femoral anteversion was significantly large, we employed larger target angles [28]. Recently, the concept of the IR angle was reported to be a useful method for predicting hip stability after THA, and an IR angle of 51° was suggested to be an appropriate cut-off point for such predictions [21]. A recent study reported that stem anteversion had the strongest effect on the IR angle [29]. We fine-tuned stem anteversion prior to the final implantation, with the goal of achieving an IR angle of >51°. To achieve an appropriate IR angle, it is necessary to obtain precise intraoperative information about stem anteversion, and it is very important that surgeons are able to place cemented stems accurately during THA. We developed a simple angle-measuring instrument for performing intraoperative assessments of cemented stem anteversion. To evaluate the feasibility of using the angle-measuring instrument to aid femoral stem placement, the authors investigated the measurement accuracy of the angle-measuring instrument when it was used to assess cemented stem anteversion and the accuracy of stem positioning performed using the angle-measuring instrument. Our angle-measuring instrument is very simple, does not need a large computer console, is easy to use, and is cost-effective.
Our study had several limitations. First, it did not involve a case-control group. Second, only the posterolateral approach was examined. Danoff et al. reported that when a posterior approach is employed the cup anteversion safe zone is larger than was previously believed [30]. Thus, the optimal degree of stem anteversion might differ according to the approach employed. Third, the target angle was decided by fine-tuning the femoral broach to acquire an appropriate IR angle; however, the concept of combined anteversion has been proposed [31, 32]. Finally, we did not evaluate postoperative clinical outcomes; however, this was beyond the purpose of this study.
In conclusion, we developed a simple angle-measuring instrument to intraoperatively measure cemented stem anteversion and facilitate the accurate placement of cemented stems during THA. The angle-measuring instrument is expected to allow cemented stems to be placed accurately during THA.