Where Are We Now?
Our understanding of hip dysplasia has greatly evolved since 1939, when Wiberg’s monograph described the lateral center-edge angle (LCEA) . While his writings concentrated on lateral acetabular coverage, numerous papers have since examined the comprehensive evaluation of acetabular coverage [5, 13]. A comprehensive radiographic evaluation of acetabular coverage now includes LCEA, Tönnis angle, anterior and posterior wall index on the AP pelvis radiograph, and anterior center-edge angle (ACEA) on the false-profile radiograph . Three-dimensional (3-D) imaging is also more commonly ordered in the young adult with hip pain, where measurements like acetabular version at 1, 2, and 3 o’clock, coronal and sagittal center-edge angles, and femoral version can be obtained to further understand the 3-D anatomy. Some researchers have even quantified the cartilage surface area of the acetabulum, which is important in order to understand the true weight-bearing surface that makes up the lunate cartilage, and ultimately, whether the socket is deficient or not . This allows us to compare the degree of dysplasia in patients with anterior versus posterior acetabular deficiency or a large acetabular fossa.
Improved radiographic evaluation and advanced imaging has led us to better understand anterior, posterior, and lateral undercoverage of the acetabulum. In the current study, Tachibana and colleagues  add sector angles to quantify geometric coverage. The sector angles used in this study and the correlation to radiographic measures give us a powerful new tool to evaluate 3-D acetabular coverage on CT, and validates our radiographic measures of anterior and posterior coverage, the anterior and posterior wall indicies.
Regarding the differing morphologies of hip dysplasia [5, 13], one study found that women more commonly presented with anterolateral undercoverage while men presented more commonly with posterior undercoverage . Tachibana and colleagues build off of this by measuring sector angles on CT scans to examine femoral head coverage of the acetabulum in multiple planes. In addition, they found differences in both CT and radiographic measures from the supine to standing position and found an increased posterior pelvic tilt in the standing position, which increases the functional acetabular anteversion compared to the supine position. This is illustrated by their decreased anterior and anterior-superior sector angles on CT imaging and decreased anterior wall index on radiographs. While there are small differences in LCEA and Tönnis angle, these are likely not noteworthy changes. This is similar to prior reports of minimal differences in LCEA and Tönnis angle in different degrees of pelvic tilt .
Where Do We Need To Go?
Should we measure acetabular coverage based on standing or supine films? Given the changes in both radiographic and CT measures, this question is the most important clinical consensus that we still need. When an expert panel at the 2018 Bern Hip Symposium was asked this question, an unofficial count of hands showed that about half of the experts answered for the former and half for the latter.
A dual-plane fluoroscopy study suggests that the acetabular coverage during gait is similar to that of the standing AP pelvis radiograph . In addition, since hip dysplasia causes weight-bearing overload and shear injury to the acetabular rim, I believe the pelvis should be measured on a standing weight-bearing radiograph. Further studies confirming that the acetabular coverage during functional activities corresponds to the standing weight-bearing AP pelvis would help to determine the right diagnostic images to use; I believe such studies likely would push the consensus towards use of the standing radiograph.
After deciding which AP pelvis radiograph to use, we need to use them to define more clearly the natural history of the dysplastic hip or the hip with femoroacetabular impingement (FAI), so that we can ascertain whether the natural history is modified by surgery. Although we have generally concentrated on the natural history of hip dysplasia as defined by the LCEA, we still need to determine the importance of the anterior and posterior wall indexes, the ACEA on the false profile radiograph, and the specific sector angles in the acetabulum on CT. Should we examine the combination of borderline LCEA (18°-25°) with a deficient anterior or posterior wall? How do these newer measures affect hip preserving surgery outcomes for hip arthroscopy or periacetabular osteotomy? A recent report showed that anterior wall deficiency is a risk factor after hip arthroscopy in women with an LCEA < 25° . Similarly, a separate study showed that patients with borderline dysplasia and femoral anteversion greater than 20° did worse with hip arthroscopy compared to a control group . More studies are needed to examine the patient reported outcomes and treatment failures of specific subpopulations of hip morphology.
How Do We Get There?
Studies on acetabular orientation during functional activities may help sway opinion on whether the standing or supine AP pelvis film should be used to quantify acetabular coverage. Advanced techniques like dual-plane fluoroscopy and marker-based motion capture during functional activities will likely give us the best answer to the functional orientation of the pelvis. The difficult part will be convincing surgeons to agree on a consensus. This is partly because surgeons have many years of data and clinical insight evaluating coverage in either the supine or standing position and it may be difficult to translate that if a change in radiographic protocol is needed.
There have been multiple mid- and long-term prospective studies on the natural history of hips with cam type FAI, acetabular dysplasia (defined by the LCEA), and to a lesser extent pincer type FAI [1, 2, 9]. If available, the images from these prior natural history studies could be re-evaluated with our further understanding of acetabular coverage to investigate the natural history of certain hip morphologies. We could use our more-recent understanding of 3-D acetabular coverage [5, 11], for example borderline dysplastic women with a deficient anterior wall or men with posterior wall deficiency, to further understand the effect of these morphologies on the natural history of the native hip. These new radiographic measures and our 3-D understanding of acetabular coverage will also need to be used to investigate the effect of these baseline imaging measures on the patient-reported outcomes and failures of hip preserving surgeries.
There are large cohorts of patients who underwent hip arthroscopy and periacetabular osteotomy that could be examined retrospectively to further investigate the effect of 3-D acetabular coverage on the outcomes of these surgeries. An example of this is a recent paper reporting worse outcomes in women with borderline dysplasia and anterior wall deficiency . Similarly, Vahedi and colleagues  reported that patients with acetabular retroversion do worse after arthroscopy than those without. Is this due to anterior overcoverage or posterior wall deficiency? In addition, future prospective studies should take the comprehensive evaluation of acetabular coverage presented above into consideration when designing their radiographic or CT analysis for their cohorts.
Reporting on young adults undergoing hip preservation surgery will need to be more complete and more focused on homogenous patient populations of hip morphology. For example, we should report patient-reported outcomes and failures of hip preserving surgeries in women separately from men. The morphology of a young man’s hip is markedly different than a woman’s, with men having larger cam lesions and women having more-subtle cam lesions, less 3-D acetabular coverage, and more ligamentous laxity. In my view, gender-specific reporting of outcomes would uncover unique gender-specific findings compared to examining the hips of men and women together as a large heterogeneous group.
A more-comprehensive approach to looking at hip morphology and its effects on the natural history of the native hip and the results of hip preserving surgery will help us in surgical decision making and allow us to better take care of the young hip patients that we serve.
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