A Three-View Radiographic Approach to Femoroacetabular Impingement
Author’s Disclosure Statement: The author reports no actual or potential conflict of interest in relation to this article.
Dr. Halawi is an Assistant Professor, Department of Orthopaedic Surgery, University of Connecticut Musculoskeletal Institute, Farmington, Connecticut.
Address correspondence to: Mohamad J. Halawi, MD, Department of Orthopaedic Surgery, University of Connecticut Health Center, 263 Farmington Avenue, Farmington, CT 06030-4037 (fax, 860-679-6649).
Am J Orthop. 2018;47(9). Copyright Frontline Medical Communications Inc. 2018. All rights reserved.
Femoroacetabular impingement (FAI) is an abnormality of the hip joint that is increasingly being recognized as a cause of athletic disability and early degenerative hip disease. Despite significant advances in the knowledge of FAI, it remains a frequently unrecognized cause of hip pain in adolescents and young adults among orthopedic providers. The purpose of this article is to present a simple 3-view radiographic approach to young adults with hip pain. The radiographs include a standing anteroposterior view of the pelvis, a cross-table lateral view, and a false profile view. Good quality radiographs showing the common sites of potential impingement combined with a basic understanding of certain radiographic parameters may allow faster diagnosis, eliminate unnecessary studies, and allow earlier referral and management.
- FAI is a frequently unrecognized cause of hip pain in adolescents and young adults.
- Understanding the potential sites of impingement and the specific radiographs to visualize these sites can help avoid unnecessary imaging and delayed diagnosis.
- A simple radiographic approach consisting of a standing AP view of the pelvis, a cross-table lateral view, and a false profile view is often a sufficient screening tool.
- While we tend to classify FAI into cam and pincer osseous bumps, alterations in hip dynamics can result in functional impingement even in the absence of the osseous bumps.
- Advanced imaging is reserved for patients who have failed conservative management or are considering surgical intervention.
The prevalence of femoroacetabular impingement (FAI) in the general population is estimated at 23.1%.1 While FAI is often bilateral,2 patients usually present with unilateral symptoms.3 Young, highly active individuals are most commonly affected.3 Despite significant improvement in our understanding of FAI in recent years, it remains a poorly recognized cause of hip pain among orthopedic providers. Clohisy and colleagues3 found that the average time to diagnosis was 3.1 years (range, 3-15 years) and the average number of providers seen before correct diagnosis was 4.2 (range, 1-16) with nearly half those providers being orthopedic specialists. This is likely attributed to limited training and lack of appropriate imaging. Multiple comprehensive radiographic approaches have been described, including plain films, computed tomography, and magnetic resonance imaging.2,4 The objective of this article is to present a simple 3-view plain film approach for young adults with hip pain. While history and physical examination remain key to FAI diagnosis, a basic knowledge of the common sites of impingement with appropriate radiographic views to visualize these sites may help eliminate unnecessary imaging and delayed diagnosis.
Standing Anteroposterior View of the Pelvis
An anteroposterior (AP) view of the pelvis, as opposed to an AP view of the hip, is an important first radiograph in the evaluation of young patients presenting with hip pain. Not only does it permit visualization of the contralateral hip for comparison, but it also allows more accurate measurements of several radiographic parameters (Table). An AP view of the hip often gives the false impression of global over coverage, such as coxa profunda2 and protrusio acetabuli (Figures 1A, 1B), and may overestimate the amount of acetabular anteversion.2
A good quality radiograph is important for accurate assessment. The X-ray beam should be perpendicular to the coronal plane of the pelvis. Neutral rotation of the pelvis is a prerequisite and can be confirmed by the presence of symmetric obturator foramina, iliac wings, and coccyx vertically in line with the pubic symphysis. Deviations from this configuration can significantly affect the ability to accurately assess the acetabular version. This is because the rotational profile of the acetabulum is sensitive to pelvic rotation.5,6
While the AP view of the pelvis can be obtained in either supine or standing positions, the standing position is recommended. A supine view tends to increase the likelihood of finding a crossover sign that often disappears in the standing position (Figures 2A, 2B). This is attributed to the posterior tilt of the pelvis in the sagittal plane with standing, which functionally increases acetabular anteversion, eliminating the crossover sign.5,6 In contrast, a crossover sign that persists in the standing position combined with other abnormal radiographic parameters, such as a negative Tonnis angle and/or increased lateral center edge angle, are concerning for pincer-type FAI (Figures 3A, 3B). An isolated crossover sign may be a normal variant in young asymptomatic patients7 and is not a reliable indicator of acetabular retroversion.5
In addition to assessing the acetabular coverage and version (Figures 1A, 1B, 3A, 3B, and 4A, 4B), the AP view of the pelvis can provide valuable information regarding the proximal femur. One should pay attention to the sphericity of the head (pistol grip cam lesions are most obvious on this view), congruency between the femoral head and the acetabulum, femoral offset, and neck-shaft angle. While we tend to traditionally classify FAI into cam and pincer osseous bumps, alterations in hip dynamics (i.e., coxa vara and coxa breva) can result in functional impingement even in the absence of the osseous bumps.
A cross-table lateral of the affected hip is another important radiographic adjunct in the evaluation of hip pain in young patients. This view provides AP axial visualization of the hip joint identifying potential pathologies such as anterior cam lesions that may not be apparent on frog-leg lateral radiographs (Figures 5A, 5B and 6A, 6B). The cross-table lateral view can also show posterior impingement and/or joint space narrowing from countercoup lesions associated with pincer-type FAI (Figures 3A, 3B). In addition, the rotational profile of the proximal femur is best assessed in this view (Figure 4B). The challenge with a cross-table lateral, however, is that it is operator-dependent. In circumstances where a good quality cross-table lateral cannot be obtained, we default to a frog-leg lateral to avoid excess radiation exposure.
False Profile View
A false profile view provides a good visualization of the anterosuperior aspect of the acetabulum. It can show anterior acetabular over or under coverage. It may also show sub-spine impingement (Figures 7A, 7B). Sub-spine impingement is characterized by a prominent anterior inferior iliac spine (AIIS) that extends to the level of the anterosuperior acetabular rim. The prominent AIIS can impinge on the femoral head-neck junction during hip flexion. A prominent AIIS has also been shown to give the false impression of a crossover sign.8
Even to the trained eye, radiographic findings of FAI can be quite subtle and easily missed. A systematic approach when interpreting plain radiographs is important. Radiographic assessment starts with good quality X-rays with the pelvis in neutral rotation. Because of the young age of most patients, radiation exposure should be minimized. An understanding of the potential sites of impingement and the specific radiographs to visualize these sites minimizes radiation exposure and other unnecessary imaging. In our experience, the 3-view radiographic approach presented combined with supportive history and physical examination findings are highly sensitive to identify cases of FAI. Advanced imaging is reserved for patients who have failed conservative management or considering surgical intervention.