Previous studies had limitations in their validations of anteversion measurement methods. First, many of the previous studies used computed tomography (CT) axial scans to validate the anteversion measured in plain anteroposterior radiograph (19-23). However, this may cause potential bias as the anteversion measured in CT scans reflects anatomical anteversion while the reference anteversion utilized in previous anteversion measurement methods using simple radiograph varied. Also, depending on how the simple AP radiograph is taken, the measured anteversion value may be significantly different. In simple pelvis AP radiographs, the radiation beam is projected toward the center of the triangle formed by the ASIS and the symphysis pubis. Thus, the radiation beam received by the hip joint in simple pelvis AP radiograph is deviated by about 6°. In contrast, the hip joint receives a perpendicular radiation beam in hip-centered AP radiographs.
The results of our study indicated that the anteversion measurement methods described by Pradhan, Lewinnek, and Liaw showed anteversion that differed by less than 4° from the real anteversion, while the methods proposed by Widmer, Hassan, and Ackland showed discrepancies of up to 29°. Moreover, measurement using hip-centered AP did not show superior accuracy compared to conventional AP; therefore, additional hip-centered AP radiographs are not recommended to improve the accuracy of anteversion measurement.
The correct positioning of the acetabular cup plays a significant role in minimizing wear and maintaining stability. The recommended inclination and version of the acetabular cup are controversial but the literature often refers to inclinations between 30 and 50° and anteversion between 5 and 30° as “safe zones” (24-27).
The inclination angle of the acetabular cup is defined as the angle formed by the tear drop line and the elliptical long axis of the entrance of the acetabular cup; thus, the inclination angle can be directly measured by simple radiography (15). However, anteversion of the acetabular component may be less accurate and difficult to obtain with two-dimensional simple radiographs (11, 13). Therefore, a number of studies have attempted to accurately measure the position of the acetabular components in plain AP radiographs using complex mathematics and trigonometric functions to describe the ellipses of the acetabular cup boundaries (1, 14-18). However, most of these studies did not specify where the center of the beam was directed or the reference plane that was utilized (Table 4). Several studies have tried to validate the accuracy and reliability of these methods but have reported inconsistent results (11-13). Marx et al compared five proposed formulas (Pradhan, McLaren, Hassan, Ackland, and Widmer) to measure anteversion in AP radiographs utilizing a CT-based navigation system as a reference (11). The study concluded that all five formulas had substantial differences in anteversion angles. Nho et al compared six formulas (Lewinnek, Widmer, Hassan, Ackland, Liaw, Woo, and Morrey) by CT axial scan, reporting that the Lewinnek, Hassan, Liaw, Woo, and Morrey methods provided satisfactory results (11). Nomura et al compared five formulas (Lewinnek, Widmer, Liaw, Pradhan, Woo, and Morrey), concluding that the values from Widmer’s method were most similar to those measured using CT (12). It should be noted that study by Nomura et al is the only study to utilize the functional coronal plane as a reference while the other two studies used CT axial scans to measure anteversion (11). Compared to the previous studies, our results show that, while all measurements had high reliability, the accuracy was high only in the Pradhan, Lewinnek, and Liaw methods. We observed significant differences from the reference anteversion values for the Widmer, Hassan, and Ackland methods. We were unable to identify the reasons why the result of our study differed from those of the previous studies; however, we believe that we added precision to the reference anteversion values by adding a goniometer to the model.
We also found no significant difference in comparisons of anteversion measurements between conventional pelvis and hip-centered AP views. Depending on the measurement method, one radiograph measurement tended to be closer in value to the reference; however, the difference was minimal. We believe that this difference is based on which reference radiograph was used when the formula was first developed.
Another finding of our study was that the anteversion measurements tended to be closer to the reference anteversion in the inclination between 30° to 50°. As most of the cup during total hip arthroplasty procedure is targeted in this range, the Liaw, Pradhan, and Lewinnek methods can be used with relatively high accuracy if the inclination is not excessively malpositioned.
Compared to the previous literature, our study showed a very high ICC. We believe this is because we did not utilize an in vivo model. As our model did not include soft tissue or a metal femoral head, we were able to more accurately identify the boundaries of the reference variables. Also, unlike taking X-rays in real humans, it can be hypothesized that the radiograph was taken with more accuracy. However, we also acknowledge that this may also be a potential limitation of the current study since the accuracy may be lower in real total hip arthroplasty scenarios due to the interference of the soft tissue of the metallic head, which may result in haziness in the radiograph. Another limitation of the current study was that we used only one type of cup. Thus, our results may only be applied to a cup which is perfectly hemispherical.