Femoroacetabular Impingement

Femoroacetabular impingement, previously termed acetabular rim syndrome, is a major factor in early development of osteoarthritis of the hip.1-7 This syndrome is caused by abnormal contact between the osseous protrusions of the acetabulum and the femur during hip joint movement.1 In patients with femoroacetabular impingement, repetitive microtrauma from the osseous impingement leads to recurring irritation, degenerative changes of the labrum, and ultimately irreversible cartilaginous damage.1,2,4,5,8-11

Femoroacetabular impingement tends to involve young, active patients and can be debilitating.4,9,10 Femoroacetabular impingement is often bilateral, but may be symptomatic on only one side.1 Femoroacetabular impingement is diagnosed and classified into two types, pincer and cam, by the clinical and radiologic presentation.8 However, most cases of femoroacetabular impingement are a combination of the two subtypes.

Pincer impingement is caused by acetabular overcoverage with normal femoral head sphericity.1,2,4,8,9 The pincer effect is the linear contact between a normal neck and an overcovering acetabular rim, which can lead to labral damage.2,8,9 Pincer impingement is more common in middle-aged women with abnormalities of acetabular morphology.4,5,8,10,11

Cam impingement arises from a prominence of the femur that leads to an abnormal, aspherical shape of the femoral head-neck junction.1,4,8 This terminology arises from the cam, which is the projecting part of a rotating wheel or shaft that strikes a lever at one or more points on its circular path. The asphericity or abnormal head-neck offset in cam impingement is thought to result from subclinical slipped capital femoral epiphysis or a growth disturbance of the proximal femur.9 The cam effect leads to abrasion and chondropathy with only partial involvement of the labrum.2 Cam impingement is more common in young males with underlying abnormalities of the femoral head morphology.4,5,8,10,11

Etiology
The etiology of femoroacetabular impingement is not entirely clear and is felt to be either developmental or related to subclinical slipped capital femoral epiphysis.5,8 A number of conditions can predispose a patient to femoroacetabular impingement including Legg-Calve-Perthes disease, congenital hip dysplasia, slipped capital femoral epiphysis, avascular necrosis of the femoral head, ununited femoral neck fractures, coxa profunda, coxa vara, protrusion acetabuli, and acetabular retroversion.2,4,5,8,12 Slipped capital femoral epiphysis and femoroacetabular impingement have been proposed to be associated due to reduced clearance of the joint and detectable abutment of the metaphysis against the acetabular rim.4 Acetabular anatomic abnormalities (ie, retroverted acetabulum) or proximal femoral anatomic abnormalities (ie, coxa profunda) lead to the pincer-type femoroacetabular impingement.8-10 Those anatomic abnormalities that lead to abnormal sphericity of the femoral head (osseous prominence at the anterolateral femoral head and neck junction, slipped capital femoral epiphysis, and developmental dysplasia of the hip) can lead to cam impingement.8-11

Diagnosis
Clinical

The gold standard is clinical diagnosis with radiographic corroboration; however, patients can have radiographic findings and be asymptomatic due to early disease or overall decreased activity.1 Patients often report intermittent pain early in the course of the disease, followed by more consistent pain after demanding activities or prolonged sitting.4,6,10,13 Other exacerbating activities can include stair climbing, prolonged sitting, and athletic events.10,11

Patients with femoroacetabular impingement typically experience groin pain. Symptoms are typically unilateral and worse after prolonged periods of sitting or when significant stress is placed on the hip.8 Impingement mainly involves the anterolateral portion of the hip joint. Therefore, flexion and internal rotation lead to symptomatic impingement due to shear forces or compression of the acetabular labrum.2,4,8

The impingement test involves rotating a supine patient’s hip internally as it is flexed passively to approximately 90° and adducted.4 A positive “impingement sign” is pain in flexion-internal rotation. On examination, limited range of motion (ROM) frequently is encountered and is described as a loss of internal rotation out of proportion with other ROM deficits.1,4,8 Additionally, a grinding or popping sensation can be felt when the femur is externally rotated and the hip is maximally abducted.8 This sign is sensitive but not specific for femoroacetabular impingement, because the anterolateral labrum is frequently involved in other degenerative diseases of the hip.2,10

Radiographs

Standard standing anteroposterior and lateral radiographs of the pelvis are necessary for radiographic evaluation.2,4 Proper technique is required, as poorly obtained radiographs may lead to over- or underestimation of the degree of disease.1 Early osteoarthritic changes associated with femoroacetabular impingement are frequently atypical.1 For example, instead of the typical joint space narrowing, osteophyte formation, subchondral sclerosis, or cysts, radiographs of femoroacetabular impingement may demonstrate reactive ossification of the labrum or possibly acetabular rim fractures from repetitive stress.1 Radiographs of both types of femoroacetabular impingement demonstrate premature degenerative arthrosis, and up to one third of patients will have fibrocystic changes at the femoral head-neck junction (Figure 1).8,10 Herniation pits also may be found in the anterolateral portion of the femoral head/neck or morphologic changes affecting the acetabulum.2,4,8,11









Radiographic signs of pincer impingement include acetabular retroversion and evidence of impaction between the anterosuperior acetabulum and anterior femoral neck.8,9 Retroversion can be diagnosed by the presence of the “crossover” or “figure-of-eight” sign, in which focal retroversion of the superior hip joint exists. The superior aspect of the anterior acetabular rim extends lateral to the posterior rim on frontal radiographs, and computed tomography may be helpful in presurgical planning. Acetabular retroversion is associated with the development of hip osteoarthritis. The prevalence of radiographic acetabular retroversion is 20% among patients with idiopathic hip osteoarthritis and 5% among the general population.14

Lateral radiographs in cam impingement can demonstrate an osseous prominence at the anterolateral head-neck junction that extends beyond the spherical portion of the femoral head. In addition, the difference between the femoral head and neck planes aligned parallel to the femoral neck axis should normally be >7 mm. Cam impingement may have reduced offset of the femoral neck and head junction. Also, a head to neck ratio can be calculated by determining the difference between the maximal anterior radius of the femoral head and the anterior radius of the adjacent femoral neck.8,12 A pistol grip deformity of the proximal femur and changes of the acetabular rim may also be detected with radiographs.4,8,9,12






Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) and magnetic resonance arthrography can provide detailed views of the labrum and acetabular cartilage.4,8 Labral tears, paralabral cysts, and cartilaginous abnormalities are well evaluated with magnetic resonance arthrography.5,8,11 The degeneration of the labrum is characteristically anterosuperior (Figure 5).1,2,4,5,8-10 Acetabular cartilage lesions in cam impingement are characteristically anterosuperior, whereas lesions in pincer impingement are typically posteroinferior.6,12

Femoral head-neck morphology on MRI is assessed by measuring the anterior alpha (a) angle on the sagittal oblique image parallel to the femoral neck and passing through the narrowest portion of the femoral neck. The alpha angle is determined by first assessing the center of the femoral head. Two vectors are extended from the center of the femoral head along the femoral neck axis and the point at which the femoral neck intersects the spherical portion of the femoral head. The aspherical femoral head-neck junction due to a focal protuberance increases the alpha angle beyond 55° (Figure 6).5,6,8,10

Pincer impingement is caused by acetabular overcoverage, and MRI can demonstrate a deep acetabulum and signs of osseous impaction along the anterosuperior or superior femoral neck.6,8 The cartilage damage seen with pincer impingement tends to be smaller and more focal than the extensive cartilage abnormality seen with cam-type femoroacetabular impingement.8,10 Cartilage damage usually is adjacent to the acetabular labral tear and can be associated with subchondral cyst formation, bony sclerosis, and osteophyte formation.8





Figure 4: Pistol grip deformity of the proximal femur. Frontal radiograph demonstrates the abnormally sloped configuration of the lateral femoral neck, termed the "pistol grip" deformity. This finding may be the result of subclinical slipped capital femoral epiphysis or a growth disturbance of the proximal femur, and can be associated with femoroacetabular impingement. Figure 5: Anterosuperior labral tear associated with cam impingement. Sagittal magnetic resonance arthrogram demonstrates an anterior femoral head-neck junction protuberance (arrowhead) and associated anterior superior labral tear (arrow). Figure 6: Cam impingement. Oblique magnetic resonance arthrogram demonstrates an abnormally increased alpha angle (>55°) in this patient with cam impingement. The alpha angle is determined by first assessing the center of the femoral head (white circle). Two vectors are extended from the center of the femoral head along the femoral neck axis (line) and the point at which the femoral neck (arrow) intersects the spherical portion of the femoral head (line). The aspherical femoral head-neck junction due to a focal protuberance in this case increases the alpha angle.


Treatment
The first therapeutic step in treatment is nonsteroidal anti-inflammatory drugs and activity modification.2,9,10 Additionally, physical therapy is aimed at strengthening abdominal and gluteal musculature and stretching the paravertebral muscles to change posture or pelvic inclination.2 Specific movements that elicit symptoms should be avoided.

Surgical reconstruction is recommended as early as possible after first symptoms appear to prevent future damage.1 Surgical treatment is suitable only if there are no advanced degenerative changes or extensive articular cartilage damage.6 Once irreversible cartilage damage has occurred, pain will frequently persist after surgical intervention.8

Joint preserving surgery involves resection osteoplasty and less frequently osteotomy for reorientation.2-4,8 Surgical osteotomy involves removing the osseous protrusion by either surgical hip dislocation or arthroscopy (Figure 3).1,2,4,8-10 Surgical hip dislocation involves entering the hip joint anteriorly via a capsulotomy and reducing either the nonspherical portion of the femoral head in cam impingement or the acetabular rim portion in pincer impingement.8

Arthroscopic management involves a complete evaluation of both the central and peripheral compartments of the hip joint.9 An intertrochanteric flexion-valgus osteotomy involves increasing the distance between the femoral neck and acetabulum by performing an intertrochanteric osteotomy in patients with avascular necrosis or Legg-Calve-Perthes disease.8 Less commonly, reorientation of a retroverted acetabulum can be performed. This is called reverse periacetabular osteotomy.1,2,4,13 In patients with advanced osteoarthrosis, total hip arthroplasty is an option.2

References
Tannast M, Siebenrock KA, Anderson SE. Femoroacetabular impingement: radiographic diagnosis - what the radiologist should know. AJR Am J Roentgenol. 2007; 188:1540-1552.
Pierannunzii L, d’Imporzano M. Treatment of femoroacetabular impingement: a modified resection osteoplasty technique through and anterior approach. Orthopedics. 2007; 30:96-102.
Mardones RM, Gonzalez C, Chen Q, Zobitz M, Kaufman KR, Trousdale RT. Surgical treatment of femoroacetabular impingement: evaluation of the effect of the size of the resection. J Bone Joint Surg Am. 2006; 88:84-91.
Ganz R, Parvizi J, Beck M, Leunig M, Notzli H, Siebenrock KA. Femoroacetabular impingement: a cause for osteoarthritis of the hip. Clin Orthop Relat Res. 2003; 417:112-120.
Kassarjian A, Yoon LS, Belzile E, Connolly SA, Millis MB, Palmer WE. Triad of MR arthrographic findings in patients with cam type femoroacetabular impingement. Musculoskeletal Imaging. 2005; 236:588-592.
Pfirrmann CWA, Mengiardi B, Dora C, Kalberer F, Zanetti M, Hodler J. Cam and pincer femoroacetabular impingement: Characteristic MR arthrographic findings in 50 patients. Radiology. 2006; 240:778-785.
Klaue K, Durnin CW, Ganz R. The acetabular rim syndrome: a clinical presentation of dysplasia of the hip. J Bone Joint Surg Br. 1991; 73:423-429.
Beall DP, Sweet CF, Martin HD, et al. Imaging findings of femoroacetabular impingement syndrome. Skeletal Radiol. 2005; 34:691-701.
Guanche CA, Bare AA. Arthroscopic treatment of femoroacetabular impingement. Journal of Arthroscopic and Related Surgery. 2006; 22:95-106.
Wisniewski SJ, Grogg B. Femoroacetabular impingement: An overlooked cause of hip pain. Am J Phys Med Rehabil. 2006; 85:546-549
James SLJ, Ali K, Malara F, Young D, O’Donnell J, Connell DA. MRI findings of femoroacetabular impingement. AJR Am J Roentgenol. 2006; 187:1412-1419.
Beck M, Kalhor M, Leunig M, Ganz R. Hip morphology influences the pattern of damage to the acetabular cartilage: femoroacetabular impingement as a cause of early osteoarthritis of the hip. J Bone Joint Surg Br. 2005; 87:1012-1018.
Siebenrock KA, Schoeniger R, Ganz R. Anterior femoro-acetabular impingement due to acetabular retroversion. J Bone and Joint Surg Am. 2003; 85:278-286
Giori NJ, Trousdale RT. Acetabular retroversion is associated with osteoarthritis of the hip. Clin Orthop Relat Res. 2003; 417:263-269.
Authors
Drs Bathala, Bancroft, and Peterson are from the Department of Radiology and Dr Ortiguera is from the Department of Orthopedics, Mayo Clinic Jacksonville, Florida.

Drs Bathala, Bancroft, Peterson, and Ortiguera have no relevant financial relationships to disclose.

Correspondence should be addressed to: Elizabeth A. Bathala, MD, Mayo Clinic Jacksonville, 4500 San Pablo Blvd, Jacksonville, FL 32224.