A Three-View Radiographic Approach to Femoroacetabular Impingement
TAKE-HOME POINTS
- 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.
CROSS-TABLE LATERAL
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
CONCLUSION
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.
