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1540 AJR:188, June 2007
AJR 2007; 188:15401552
American Roentgen Ray Society
Tannast et al.Femoroacetabular Impingement
M u s c u l o s ke l e t a l I m ag i n g R ev i ew
Femoroacetabular Impingement: Radiographic DiagnosisWhat the Radiologist Should Know
Klaus A. Siebenrock1
Suzanne E. Anderson2,3
Tannast M, Siebenrock KA, Anderson SE
Keywords: bone, femoroacetabular impingement, hip, musculoskeletal imaging, orthopedic surgery, radiography
Received July 25, 2006; accepted after revision November 8, 2006.
1Department of Orthopaedic Surgery, Inselspital, University of Bern, Switzerland.
2Department of Diagnostic, Pediatric and Interventional Radiology, Inselspital, University of Bern, Switzerland.
3Present address: Royal Melbourne Hospital, University of Melbourne, Melbourne, Australia. Address correspondence to S. E. Anderson (email@example.com).
CMEThis article is available for CME credit. See www.arrs.org for more information.
OBJECTIVE. The purpose of this article is to show the important radiographic criteria thatindicate the two types of femoroacetabular impingement: pincer and cam impingement. In addi-tion, potential pitfalls in pelvic imaging concerning femoroacetabular impingement are shown.
CONCLUSION. Femoroacetabular impingement is a major cause for early primary os-teoarthritis of the hip. It can easily be recognized on conventional radiographs of the pelvis andthe proximal femur.
emoroacetabular impingement (pre-viously also called acetabular rimsyndrome  or cervicoacetabu-lar impingement ) is a major
cause of early osteoarthritis of the hip, espe-cially in young and active patients . It ischaracterized by an early pathologic contactduring hip joint motion between skeletal prom-inences of the acetabulum and the femur thatlimits the physiologic hip range of motion, typ-ically flexion and internal rotation. Dependingon clinical and radiographic findings, twotypes of impingement are distinguished(Fig. 1): Pincer impingement is the acetabularcause of femoroacetabular impingement and ischaracterized by focal or general overcoverageof the femoral head. Cam impingement is thefemoral cause of femoroacetabular impinge-ment and is due to an aspherical portion of thefemoral headneck junction (Fig. 2). Most pa-tients (86%) have a combination of both formsof impingement, which is called mixed pincerand cam impingement, with only a minority(14%) having the pure femoroacetabular im-pingement forms of either cam or pincerimpingement .
During sports activities and activities ofdaily living, repetitive microtrauma of theseosseous convexities occur. As a consequenceof this recurring irritation, the labrum degen-erates  and irreversible chondral damageoccurs that progresses and results in degener-ative disease of the hip joint if the underlyingcause of femoroacetabular impingement isnot addressed [9, 10].
In the initial phase of this recently de-scribed entity, patients with femoroacetabu-
lar impingement do not have classic radio-graphic signs of osteoarthritis such as jointspace narrowing, osteophyte formation,subchondral sclerosis, or cyst formation.Thus, this article will familiarize radiolo-gists with this pathophysiologic concept anddescribe the radiographic findings that arehelpful for the correct diagnosis and evalu-ation before potential surgical treatment offemoroacetabular impingement. In addition,potential pitfalls simulating femoroacetabu-lar impingement are discussed, and somepearls for diagnosis are offered.
Clinical FindingsPatients with femoroacetabular impinge-
ment are young, usually in their 20s40s.The estimated prevalence is 1015% .Patients present with groin pain with hip ro-tation, in the sitting position, or during or af-ter sports activities. Some patients describea trochanteric pain radiating in the lateralthigh. Typically, they are aware of their lim-ited hip mobility long before symptoms ap-pear. In the clinical examination, patientswith femoroacetabular impingement have arestricted range of motion, particularly flex-ion and internal rotation [3, 8]. A positiveimpingement sign is present for anteriorfemoroacetabular impingement if the forcedinternal rotation/adduction in 90 of flexionis reproducibly painful, and for posteriorimpingement with painful forced externalrotation in full extension [3, 12] (Fig. 3).The Drehmanns sign is positive if there isan unavoidable passive external rotation ofthe hip while performing a hip flexion .
AJR:188, June 2007 1541
Fig. 1Flowchart shows classification of types of femoroacetabular impingement.
Fig. 2Normal configuration of hip with sufficient joint clearance allows unrestricted range of motion (top). In pincer impingement, excessive acetabular overcoverage leads to early linear contact between femoral headneck junction and acetabular rim, resulting in labrum degeneration and significant cartilage damage. Posteroinferior portion of joint is damaged (contrecoup) due to subtle subluxations (center). In cam impingement, aspherical portion of femoral headneck junction is jammed into acetabulum (bottom).
Fig. 3Clinical tests to assess femoroacetabular impingement. Anterior impingement sign (left) is positive, with painful forced internal rotation in 90 of flexion. In extreme forms, there is unavoidable passive external rotation of hip during hip flexion (Drehmanns sign, center). Posterior impingement sign is positive when there is painful forced external rotation in maximal extension (right).
= Pincer impingement
= Cam impingement
Osseous bump Femoralretrotorsion,Coxa vara
Conventional Radiographic Imaging Technique
The role of imaging in femoroacetabularimpingement is to evaluate the hip for abnor-malities associated with impingement and toexclude arthritis, avascular necrosis, or otherjoint problems on radiographs. MRI or MR ar-thrography can then be used to confirm or ex-clude labral tears, cartilage damage, and otherpathologic signs of internal hip derangement ifimpingement is suspected. Alternatively, radi-ography is then usually followed by MRI forcartilage and labral disorders and a 3D under-standing of the bone anatomy.
Standard conventional radiographic imag-ing for femoroacetabular impingement in-cludes two radiographs (Fig. 4): an antero-posterior pelvic view and an axial cross-tableview of the proximal femur . An alternativeto the axial view, a Dunn/Rippstein view,preferably in 45 of flexion, can be obtainedto reveal pathomorphologies of the anteriorfemoral headneck junction . For the an-teroposterior pelvic radiograph, the patient isin the supine position with the legs 15 inter-nally rotated to compensate for femoral an-tetorsion and to provide better visualizationof the contour of the lateral femoralheadneck junction . The film-focus dis-tance is 1.2 m; the central beam is directed tothe midpoint between a line connecting bothanterosuperior iliac spines and the superiorborder of the symphysis (Fig. 4), which caneasily and reproducibly be palpated by the ra-diology technician [16, 17]. Accordingly, thecross-table view of the proximal femur istaken with the leg internally rotated, with afilm-focus distance of 1.2 m, and with thecentral beam directed to the inguinal fold. If these prerequisites of correct position-ing of the patient and accurate radiographictechnique are not fulfilled, the radiographsmust be interpreted with caution.
A faux profile of Lequesne and de Sze may be used for quantification of anterior over-coverage but is rarely indicated for femoroac-etabular impingement because it does not showthe relationship between the anterior and theposterior acetabular rims. Rather, it is used toassess the posteroinferior part of the hip joint todetect the so-called contrecoup lesions in pin-cer impingement described later.
To determine accurately the individual pel-vic tilt of a patient, a strong lateral view of thepelvis can be obtained (Fig. 5B). Correct in-terpretation of pelvic tilt is crucial for accu-rate description and radiographic assessmentof individual hip parameters. A neutral tilt is
Tannast et al.
1542 AJR:188, June 2007
Fig. 4Correct setting for anteroposterior and strong lateral (left) pelvic radiography. Cross-table axial radiograph of hip (right) is needed to visualize anatomy of anterior femoral headneck junction, which is not visible on anteroposterior pelvic radiograph.