M u s c ul o s kel et a l I m ag i n g • R ev i ew
Femoroacetabular Impingement: Radiographic Diagnosis—What the Radiologist Should Know
Moritz Tannast1 Klaus A. Siebenrock1 Suzanne E. Anderson2,3
Tannast M, Siebenrock KA, Anderson SE OBJECTIVE. The purpose of this article is to show the important radiographic criteria that indicate the twotypes of femoroacetabular impingement: pincer and cam impingement. In addition, potential pitfalls in pelvic imaging concerning femoroacetabular impingement are shown. CONCLUSION. Femoroacetabular impingement is a major cause for early “primary” osteoarthritis of the hip. It can easily be recognized on conventional radiographs of the pelvis and the proximal femur. emoroacetabular impingement(previously also called “acetabular rim syndrome”  or “cervicoacetabular impingement” ) is a major cause of early osteoarthritis of the hip, especially in young and active patients [3–6]. It is characterized by an early pathologic contact during hip joint motion between skeletal prominences of the acetabulum and the femur that limits the physiologic hip range of motion, typically flexion and internalrotation. Depending on clinical and radiographic findings, two types of impingement are distinguished (Fig. 1): Pincer impingement is the acetabular cause of femoroacetabular impingement and is characterized by focal or general overcoverage of the femoral head. Cam impingement is the femoral cause of femoroacetabular impingement and is due to an aspherical portion of the femoral head–neckjunction (Fig. 2). Most patients (86%) have a combination of both forms of impingement, which is called “mixed pincer and cam impingement,” with only a minority (14%) having the pure femoroacetabular impingement forms of either cam or pincer impingement . During sports activities and activities of daily living, repetitive microtrauma of these osseous convexities occur. As a consequence of thisrecurring irritation, the labrum degenerates  and irreversible chondral damage occurs that progresses and results in degenerative disease of the hip joint if the underlying cause of femoroacetabular impingement is not addressed [9, 10]. In the initial phase of this recently described entity, patients with femoroacetabu-
lar impingement do not have classic radiographic signs of osteoarthritissuch as joint space narrowing, osteophyte formation, subchondral sclerosis, or cyst formation. Thus, this article will familiarize radiologists with this pathophysiologic concept and describe the radiographic findings that are helpful for the correct diagnosis and evaluation before potential surgical treatment of femoroacetabular impingement. In addition, potential pitfalls simulatingfemoroacetabular impingement are discussed, and some “pearls” for diagnosis are offered. Clinical Findings Patients with femoroacetabular impingement are young, usually in their 20s–40s. The estimated prevalence is 10–15% . Patients present with groin pain with hip rotation, in the sitting position, or during or after sports activities. Some patients describe a trochanteric pain radiating in the lateralthigh. Typically, they are aware of their limited hip mobility long before symptoms appear. In the clinical examination, patients with femoroacetabular impingement have a restricted range of motion, particularly flexion and internal rotation [3, 8]. A positive impingement sign is present for anterior femoroacetabular impingement if the forced internal rotation/adduction in 90º of flexion isreproducibly painful, and for posterior impingement with painful forced external rotation in full extension [3, 12] (Fig. 3). The “Drehmann’s” sign is positive if there is an unavoidable passive external rotation of the hip while performing a hip flexion .
Keywords: bone, femoroacetabular impingement, hip, musculoskeletal imaging, orthopedic surgery, radiography DOI:10.2214/AJR.06.0921 Received...