Restoring the anatomical hip center of rotation and equalizing lower limb length are paramount to pathologic hip joint reconstruction. To achieve this goal, several authors defined the anatomical hip center in different ways. Ranawat and colleagues18 developed a method for locating the anatomical position of the acetabulum in deformed hips by drawing an isosceles right triangle on the anteroposterior pelvis radiograph with a length of the two sides that equals one-fifth of the height of the pelvis. The midpoint of the hypotenuse was defined as the anatomic hip center. Lewalle19 placed the theoretic center of the hip 25 mm above the highest point of the obturator foramen. Boudriot et al20 defined anatomic rotation center of the hip by the vertical and horizontal distances in relation to the point of intersection between Koehler's line and a horizontal line between the superior rims of the two obturator foramen. Numerous authors9,21–28 recently located the center of the acetabular component using the vertical and horizontal distances of the center of rotation in relation to the acetabular teardrop. The reference points described above except for the teardrop are limited to preoperative templating and are not available intraoperatively. In the majority of deformed hips, the location of the radiographic teardrop is visible intraoperatively and can be used as a simple marker to position the acetabular component. The teardrop, the structure located in the anteroinferior portion of the acetabular fossa at the acetabular notch, consists of cortical and medullary bone contributed to primarily by the ischium and to a much smaller degree by the superior pubic ramus.29 The inferior border of the teardrop is just the superior border of the obturator foramen, the so-called acetabular notch, while the superior border of the teardrop corresponds to the superior border of the acetabular fossa. The medial branch of the teardrop is the lateral border of the obturator canal, while the lateral branch corresponds to the floor of the acetabular fossa.
Our study showed that an acetabular component with the inferior edge of cup at the same level as the inferior edge of the teardrop did not exactly replicate the anatomic hip center height. In 106 unilateral THA included in our study, however, 99 absolute differences (93.4%) between two hip center heights were in the range of 0–5 mm, while only 7 (6.6%) were >5 mm. Although the differences were statistically significant, the difference may be acceptable in clinical practice for performing THA. Otherwise, it might be the result of normal bilateral anatomic variation in the hip rotation center. As a result, it is essential that we perform an investigation of the bilateral hip center heights in standard anteroposterior pelvic radiographs without hip joint pathology based on a large sample. In the current study, abduction inclination angle and cup anteversion correlated positively with the superior cup center displacement distance in relation to the normal contralateral hip center, implying that a cup position with greater inclination and anteversion may increase the proximal cup center displacement. However, interestingly, the outer cup diameter did not influence center cup height.
We acknowledge the following limitations of this study: first, this was a retrospective selected analysis and not a prospective randomized study, increasing the possibility of selection bias. The latter is considered the most accurate research method for assessing acetabular component position. A second limitation is that our sample size was small. A prospective study with a larger sample may produce a more accurate result and allow extrapolation of the results. Third, we selected the contralateral normal hip as the control, which might underestimate the study's accuracy as a result of normal anatomic variation between an individual's two hips.
In conclusion, although our study demonstrated that an acetabular component position with its inferior edge at the same level as the inferior edge of the teardrop did not statistically replicate the height of the anatomic hip center, its clinical significance is minimal because most of the differences between cup center height and anatomic hip center height were much smaller. As such, this acetabular component placement remains the correct option because of its simplicity, reliability, and stability.
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