To maximize the accuracy of intraoperative fluoroscopy, the influence of pelvic tilt and rotation needs to be eliminated during surgery. We generally confirmed the following two points: the pubic symphysis must be vertical and overlying the coccyx, and the obturator foramen has a similar appearance on fluoroscopy as on preoperative standing plain pelvic radiography. These results are consistent with the protocol reported in a previous study [12]. However, some unavoidable factors of intraoperative fluoroscopy may still create errors [20]. First, intraoperative fluoroscopy was performed with a posteroanterior beam direction. Second, the film focus distance is small. Third, the central beam is usually centered on the femoral head. Previous studies using the intraclass correlation coefficient (ICC) have shown that the measurements obtained on intraoperative fluoroscopy are correlated with those obtained on postoperative radiography [12]. However, correlation analysis cannot be used to assess the agreement between the two methods of clinical measurement [21, 22]. In this study, Bland–Altman plots were used to analyze the agreement between intraoperative fluoroscopy and postoperative radiography in assessing the outcome of PAO. The results indicated that the bias between the imaging modalities could be neglected.
This study has some limitations. First, because we excluded some patients who had undergone a previous hip surgery, who underwent simultaneous PFO, or who had a nonspherical femoral head or subluxated contralateral hip, our findings cannot be generalized to patients with these conditions. In addition, all measurements were performed by a single observer (J.P.). Since multiple prior studies have reported on interrater and intrarater reliability in measuring the LCEA, AI, AWI, PWI, EI, and MO, we did not repeat such assessments. Third, we determined that the bias of the AI, PWI, and MO as measured on fluoroscopy was acceptable based on the reference normal values reported in previous studies. Further studies are needed to determine whether the bias of these parameters affects hip function after PAO. Fourth, we did not investigate the radiograph in the supine position after the operation. However, the stability of the hip joint in the weight-bearing position is more important regarding the occurrence of pathological hip changes and symptoms. Fifth, we did not use intraoperative 3D images to evaluate femoral head coverage. Intraoperative 3D evaluation may be a more accurate method. However, this is a relatively expensive method that has not been widely used in China. Since 3D images can more accurately evaluate the coverage of the acetabulum to the femoral head, we will add postoperative 3D image data in future studies to further evaluate the accuracy and reliability of intraoperative fluoroscopy.
The LCEA of Wiberg, AI, and EI was used to assess the lateral coverage of the femoral head. Correction of the LCEA between 25° and 40°, AI between 0° and 10°, and EI ≤ 20% were defined as the target ranges after PAO based on previously published normative values [8, 23]. Charles’ study indicated that EI was less strongly correlated, with an ICC of 0.66 (0.46–0.79) [12]. Unlike the results reported by Charles, we found high agreement for the EI between intraoperative fluoroscopic images and postoperative standing AP pelvic radiographs. The LCEA and AI have demonstrated a strong correlation between intraoperative fluoroscopy and postoperative plain radiography in previous studies [12, 13]. However, a strong correlation does not imply good agreement between the two methods; correlation analysis quantifies only the degree to which two variables are related [21]. Stefanie’s [24] study indicated an acceptable agreement between the two imaging modalities using kappa statistics; however, intraoperatively, they inclined the C-arm by approximately 5° to imitate a pelvic-centered image, differing from conventional methods. In this study, the LCEA also showed high agreement between intraoperative fluoroscopic images and postoperative standing AP pelvic radiographs. In contrast to previous studies, our study found that the AI acquired by postoperative radiography was larger than that measured on intraoperative fluoroscopy (p < 0.05). We suspect that this difference may be due to the difficulty in determining the medial margin of the acetabular sourcil on fluoroscopic images. Charles [12] also considered fluoroscopic images to have poorer resolution than plain radiographs, potentially making it more difficult to find the necessary landmarks for measurement. Through Bland–Altman analysis, we considered this difference to be acceptable (mean bias: − 0.97°).
Proper acetabular reorientation includes not only lateral but also anterior and posterior coverage. Excessive anterior coverage is a detriment to posterior coverage and may cause impingement and adversely affect the long-term survival of the joint after PAO [6]. An anterior center-edge angle of Lequesne (ACEA), created on the false-profile view, of < 20° can be indicative of structural instability [23]. Most surgeons prefer to obtain an oblique view of the iliac crest during surgery to achieve a false-profile view. Previous studies have shown that the intraoperative ACEA is strongly correlated with that obtained on postoperative radiography, with ICCs of 0.71 (95% CI: 0.54–0.82) [12] and 0.80 (95% CI: 0.71–0.86) [15]. We chose not to measure the ACEA intraoperatively to assess the improvement in anterior coverage; although we can imitate the version of the standing pelvis by tilting the C-arm beam, we cannot simulate the version of the standing pelvis when obtaining an oblique image. Klaus [19] recommended the AWI and PWI to quantify anterior and posterior coverage. According to their report, the mean AWI and PWI were 0.41 and 0.91, respectively, for normal hips. Because these parameters for judging anterior and posterior coverage are measured on images simulating the standing pelvic version, we prefer this method to using the ACEA. In this study, the AWI obtained on intraoperative fluoroscopy strongly agreed with that obtained on postoperative radiography. Although the mean PWI obtained on intraoperative fluoroscopy was smaller than that obtained on postoperative radiography, this difference was acceptable since the mean bias was only 0.11.
A lateralized hip center is considered to be a sign of structural instability. The hip center is considered lateralized if the medial aspect of the femoral head is greater than 10 mm from the ilioischial line [25]. Medialization of the fragment could decrease the joint contact forces by decreasing the bodyweight lever arm. Troelsen [26] found that an MO distance greater than 20 mm correlated with a poor 6.8-year survivorship of PAO. Charles [20] recommended placing the medial aspect of the femoral head only 5 to 15 mm lateral to the ilioischial line. In their study, the MO showed the weakest correlation (ICC: 0.46) between measurements obtained by intraoperative fluoroscopy and postoperative AP pelvic radiography. Our data indicate that the femoral head was more medial on intraoperative fluoroscopy than on postoperative AP pelvic radiography. This difference is partly due to the different central beam positions. On the other hand, the effect of the imaging magnification ratio on the MO was more significant than that on the angle (LCEA, AI) measurements. To eliminate this effect, the MO was calibrated using the ratio of the femoral head diameter as measured on preoperative CT to that measured on intraoperative fluoroscopy and pelvic radiography. In this study, the mean bias was only − 1.11 mm, and the 95% LOA was − 5.55 mm-3.29 mm. Compared to the acceptable range of 15–20 mm [20, 26], this error is completely negligible.
