Hip impingement, or femoroacetabular impingement (FAI), describes a condition where abnormal contact between the femoral head and the acetabular rim leads to pain and restricted movement. Interpreting an x-ray for this diagnosis requires a systematic analysis of bony morphology, as the deformities causing the impingement are often structural. A dedicated hip impingement x-ray series provides the necessary views to identify the specific type of cam, pincer, or combined lesion present.
Understanding the Biomechanics of FAI
The foundation of interpreting a hip impingement x-ray lies in understanding the biomechanics of FAI. In a healthy hip, the spherical femoral head fits perfectly within the acetabulum, allowing smooth articulation. In FAI, excess bone grows on one or both components, preventing the femoral head from rotating smoothly inside the socket. This abnormal contact damages the labral cartilage and underlying articular cartilage, leading to pain, stiffness, and eventually osteoarthritis if left untreated.
Key Projections for Evaluation
While an anteroposterior (AP) pelvis x-ray is the standard initial screening tool, specific projections optimize the visualization of impingement. The false profile view, also known as the modified Judet view, is arguably the most critical for assessing pincer lesions. It provides a direct profile image of the acetabulum, allowing for the accurate measurement of acetabular retroversion or overcoverage. A lateral view of the hip, often the Dunlop or frog-leg lateral view, is essential for evaluating cam lesions, which involve excess bone on the femoral neck.
Identifying Cam Lesions on X-ray
Cam lesions are characterized by a loss of the normal spherical contour of the femoral head-neck junction. On an x-ray, this appears as an aspherical femoral head or increased femoral neck offset. The alpha angle, measured on an AP view, is a quantitative indicator; an alpha angle greater than 60 degrees is often used as a threshold for significant cam impingement. The goal of measurement is to identify the asphericity that causes shear forces on the acetabular labrum during hip flexion.
Identifying Pincer Lesions on X-ray
Pincer lesions result from an overcoverage of the femoral head by the acetabulum, often due to acetabular retroversion. On an x-ray, this may present as a crossover sign, where the anterior edge of the femoral head crosses the line of the acetabular roof, or as a posterior wall sign, where the posterior wall is prominent relative to the femoral head. These osseous changes lead to an increased native joint contact pressure, which can cause labral tears and cartilage damage on the anterosuperior aspect of the acetabulum.
Measuring Femoral Anteversion and Neck Shaft Angle
Beyond the primary signs of cam and pincer, the x-ray evaluation must include an assessment of femoral version and the femoral neck shaft angle. The crossover sign and the posterior wall sign on an AP view provide indirect information about femoral anteversion. An increased femoral neck shaft angle, often termed coxa vara, can also contribute to abnormal impingement mechanics. Accurate measurement of these parameters is vital for surgical planning, particularly if a periacetabular osteotomy (PAO) is being considered.
Differential Diagnosis and Reporting
When reviewing a hip impingement x-ray, it is crucial to differentiate FAI from other conditions that may mimic its symptoms. Osteoarthritis, stress fractures, synovitis, and intra-articular loose bodies must be considered. A comprehensive radiographic report should not only describe the presence of cam or pincer morphology but also note the joint space width, presence of osteophytes, and sclerosis. This detailed analysis ensures that the clinical team has a complete picture of the bony pathology and its implications for treatment.
