CORR Insights®: Periacetabular Osteotomy Improves Pain and Function in Patients With Lateral Center-edge Angle Between 18° and 25°, but Are These Hips Really Borderline Dysplastic?

Where Are We Now?

Professor Gunnar Wiberg was the first to suggest that a structural hip abnormality could predispose a patient to develop idiopathic hip osteoarthritis [12]. Since his original description of the lateral center edge angle (LCEA) in 1939, this single radiographic parameter has become the gold standard to assess acetabular coverage of the femoral head. During the past 80 years, our understanding of the three-dimensional (3-D) shape of the acetabulum and proximal femur has evolved, but physicians continue to depend primarily on the LCEA to assess acetabular development. Other commonly used parameters include the Tönnis acetabular roof angle, the anterior center-edge angle (ACEA) measured on the false profile radiograph of the hip, and more recently, the anterior and posterior wall indices and the femoro-epiphyseal acetabular roof (FEAR) index [10].

These measurements are particularly important when a patient presents with hip pain associated with activity and has radiographic evidence of mild acetabular dysplasia as defined by a LCEA between 18° and 25°. If nonsurgical treatment does not produce meaningful improvement, surgical treatment options include hip arthroscopy to address intra-articular pathology including labral tears, cartilage delamination, and even some structural abnormalities of the femur, like cam lesions. Open surgery can also manage proximal femoral deformities, as well as acetabular dysplasia, which in most such patients is best addressed using periacetabular osteotomy (PAO). Sometimes, arthroscopy in combination with PAO is used, as well. The challenge comes in determining the best procedure to address each patient’s symptoms.

In the current study, McClincy and colleagues [3] perform a retrospective analysis of clinical and radiographic data to identify and define additional radiographic parameters of dysplasia in patients with LCEA between 18° and 25°, describe the perioperative complications and 2-year revision and survival rates in these patients, and report the 2-year patient-reported outcome measures (PROMs) after surgical management. The authors divided patients with hip pain into those with hip symptoms predominantly associated with femoroacetabular impingement (FAI) and those with symptoms suggestive of instability such as pain with upright activities, abductor fatigue, and a positive anterior apprehension test. The authors concluded that a thorough preoperative radiographic analysis of the hip including not only the LCEA, but also the ACEA, the Tönnis angle, the anterior and posterior wall index, and the FEAR index is required to appreciate the 3-D features of hip dysplasia; and that surgical management (PAO with or without an arthroscopy) can result in good short-term PROMs with few complications.

Where Do We Need To Go?

McClincy and colleagues highlight several gaps in our approaches to assessment and management of patients with hip pathology. Most importantly, we need to move beyond relying on one-dimensional assessment of 3-D structures.

Alternative assessment options include advanced axial imaging with CT and MRI. However, MRI can be expensive and difficult to obtain in resource-limited practices, and CT exposes the patient to ionizing radiation. Additionally, analyzing the acetabular-femoral head relationship in a supine position may be very different from looking at it in a more-functional, upright position. Recent advances in upright biplanar radiography [6, 9] have improved our understanding of 3-D disorders of the spine, however, this technology has not yet been used in the hip.

Additionally, we need to improve our diagnostic capabilities to better determine the underlying etiology of hip symptoms. In the current study, McClincy and colleagues [3] propose clinical and physical examination techniques to differentiate between patients with impingement and instability. However, there is an overlap of these entities in many patients with these conditions. Diagnostic injections and functional ultrasonography need to improve and become less technician-dependent in order for us to better understand the pathology of the hip joint. We also need to better understand the role of ligamentous laxity in the etiology of hip dysfunction in these patients with “borderline dysplasia.” Several studies have recommended [1, 2] the use of the Beighton score [4], but generalized joint hypermobility represents a wide spectrum of disease, and standard surgical techniques to normalize radiographic parameters, such as the periacetabular osteotomy, may not completely resolve the “microinstability” present in patients with severe hypermobility.

How Do We Get There?

In the near-term, CT will likely be best suited to define the 3-D shape and pathology of the hip joint. We could better define normal hip morphology in 3-D by using techniques that minimize radiation exposure and standardize pelvic position. Peterson and colleagues [7] demonstrated this by examining 3-D acetabular morphology to define acetabular version, tilt, and coverage angles in a cohort of 157 normal patients between the ages of 8 and 17. This study demonstrated the importance of comparing pathologic states such as hip dysplasia or overcoverage to age- and sex-matched controls.

The next step should be to perform this similar analysis in a cohort of patients with acetabular dysplasia to more-specifically define the patterns of dysplasia present in each individual patient. Recently, published studies have worked towards that goal [5, 11], but they lack a control cohort, or their results are difficult to reproduce. Three-dimensional analysis should eventually be automated, simplified, and distributed to multiple centers so that it can become routine practice in the treatment of hip pathologies.

Additionally, 3-D assessment of the femur is equally important when treating hip pathology, not only to understand the shape of the femoral head, but to define femoral torsion and how intra- or extra-articular impingement can occur with hip motion. Ultimately, this type of analysis would allow surgeons to perform patient-specific corrections to normalize 3-D parameters for an individual patient. Additionally, intraoperative navigation techniques should be refined to precisely quantify the movements of the acetabular fragment during the PAO procedure to consistently obtain anatomically corrected outcomes in 3-D.

Looking beyond CT, the currently available software programs are labor intensive and time consuming, and 3-D MRI analysis is only in its infancy. Advances in image acquisition to decrease slice thickness and to minimize gaps between slices are required to obtain high resolution data. Additionally, automation of image reconstruction is required to decrease the amount of manual input needed to define specific tissues such as the femoral head cartilage and the acetabular articular cartilage. On the other hand, upright biplanar radiography is more promising. EOS imaging can consistently obtain lower extremity 3-D reconstructions, but further advancements are necessary to better define proximal femoral and acetabular morphology.

In terms of improving our clinical diagnostic acumen, randomized controlled trials can be expensive and impractical, but large, multicenter prospective registries could address most of the clinical challenges described above. For example, a study that compares PAO outcomes in patients with similar preoperative 3-D radiographic features and varying severity of ligamentous laxity as documented by the Beighton score, would be informative. Additionally, identification of clinical and radiographic factors that correlate with microinstability documented on ultrasound evaluation would be useful to centers without access to skilled ultrasonographers. The Academic Network of Conservational Hip Outcomes Research (ANCHOR) study group [8] is in the process of collecting prospective, multicenter data on hip preservation procedures including outcome measures of hip function, pain, quality of life, and economic value. A thorough and standardized assessment of each patient needs to be performed and clearly documented. Dynamic assessment with fluoroscopy or ultrasonography should play a role in assessing complex patients. Intraoperative documentation of the specific pathology should be performed. Following the operation, surgeons should determine the accuracy of the anatomic deformity corrections by performing a 3-D assessment. Finally, long-term followup with relevant and sensitive PROMs should be collected to determine if our surgical interventions improve quality of life and effect the natural history of the disease.