CORR Insights®: Medialization of the Hip's Center with Periacetabular Osteotomy: Validation of Assessment with Plain Radiographs

Where Are We Now?

The medialization of the hip joint’s center of rotation improves hip biomechanics by increasing the moment arm and force generation of the hip abductor muscles and decreasing joint reaction forces [2, 3, 5]. This is true in both native and prosthetic hips and is particularly important to consider during the surgical correction of dysplastic hips that are often lateralized or subluxated as a result of the deformity. Periacetabular osteotomy (PAO) surgery has been shown to effectively medialize the hip to varying degrees [1, 8], but assessing medialization of the femoral head after PAO can be difficult because the ilioischial line often is obscured when the acetabular fragment is repositioned. Additionally, we lack a consistent technique for measuring hip medialization with advanced imaging beyond plain radiographs.

In the current study, Fowler et al. [4] propose an alternative measurement method for radiographic assessment of medialization of the femoral head following PAO: measuring the distance between the ilioischial line and the most medial point of the inferior third of the femoral head. This is at a cranial-caudal level where the ilioischial line is less altered by PAO surgery, and therefore is more easily identified postoperatively.

Fowler et al. [4] also assess femoral head medialization on postoperative CT scans, demonstrating that radiographs tend to underestimate the amount of femoral head medialization after PAO compared to CT scan. Importantly, this study validates intraoperative fluoroscopic assessment of femoral head medialization, indicating that fluoroscopy can reliably guide intraoperative acetabular fragment medialization during PAO surgery.

Where Do We Need To Go?

Although we understand that medialization of the hip joint improves the biomechanical loading characteristics of the joint, the ideal amount of medialization for an individual hip remains unclear. How much medialization is enough? The current study suggests that we can expect more severely dysplastic hips to undergo more medialization than less severely dysplastic hips, but can we predict how much medialization to aim for during surgery? Prior studies have investigated the normal range of hip coverage and target zone of acetabular correction during PAO surgery in terms of the lateral center-edge angle (LCEA), anterior center-edge angle, and Tönnis angle [6, 7], but we still lack information regarding exactly how many millimeters the femoral head ought to be positioned relative to the ilioischial line or the body’s midline. Without knowing how much medialization to aim for during PAO surgery, we are still operating based on educated assumptions.

How Do We Get There?

Now that we have a reliable way to measure hip joint medialization on an AP pelvis radiograph, the first step in better understanding the role of femoral head medialization in the correction of hip dysplasia is for surgeons to consistently assess joint medialization before, during, and after PAO surgery. Much intraoperative attention is paid to improving lateral hip coverage and sourcil obliquity during surgery, but medialization of the acetabular fragment is less emphasized. Surgeons should routinely make a conscious effort to medialize the hip joint during acetabular fragment positioning and to assess femoral head medialization in intraoperative and postoperative imaging.

Ultimately, the effect of joint medialization on the longevity of the hip needs to be studied by examining the long-term PAO outcomes in terms of patient-reported pain and function and radiographic evidence of joint degeneration over many decades. We may be able to determine an ideal range of joint medialization for hips of varying degrees of dysplasia, stratified by LCEA or Tönnis angle ranges, and to determine the ideal position of the femoral head relative to the ilioischial line. It may very well be that there is a range of femoral head medializations that are appropriate, depending on an individual’s hip morphology (including the shape of the acetabulum, proximal femur, and pelvis). However, sorting out the ideal range of hip joint medialization based on hip morphological parameters and long-term PAO outcomes would require large numbers of dysplastic hips undergoing PAO surgery, and involve a multicenter collaborative effort over many years. It may therefore be more practical to estimate the effects of varying degrees of joint medialization through computer modeling and finite element analysis studies. The information from such a study would help to translate biomechanical principles to improved clinical care.