Femoroacetabular impingement (FAI) is the term given to the mechanism by which two abnormal structural morphologies of the hip may lead to osteoarthritis (OA). FAI can result in early cartilage damage and labral tears, eventually leading to primary OA of the hip.

The two structural deformities that encompass FAI are the cam type and the pincer type morphologies. The two morphologic characteristics coexist in approximately 60 percent to 80 percent of cases, but isolated cam or pincer morphologies can be present and determine the sequence and type of damage that occurs in the hip joint.

Cam impingement
Cam impingement is mostly seen in younger athletic males. The abnormality is at the femoral head and extends into the head and neck junction (Fig. 1). An extension of the physis can lead to a loss of gradual transition from femoral head to neck, resulting in an aspherical femoral head and impeding hip range of motion, especially in flexion and internal rotation. The deformity is often antero- and posterolateral and has been described in the literature as a pistol grip deformity.

This deformity of the femoral head has been associated with slipped capital femoral epiphysis, which also occurs more frequently in males. In addition, Legg-Calvé-Perthes disease may also lead to an abnormal head structure that results in impingement, but in these patients, the head is usually misshaped and the damage to the hips also depends on the structural abnormality present on the socket side.

Most patients with cam impingement have no prior history of childhood hip disorders. The hip damage occurs in the impingement area, most commonly about the anterior superior region of the acetabulum. The increased radius of the femoral head causes constant abutment with the acetabulum, leading to a typical shearing injury of the cartilage, with preservation of the labrum in early stages. Eventually, the cartilage separates from the labrum and subchondral bone. With progression, the labrum will eventually tear at the transition zone.

Pincer impingement
Pincer-type impingement occurs secondary to acetabular-sided abnormalities (Fig. 2). It can occur when there is sectorial overcoverage of the acetabulum over the femoral head (acetabular retroversion) or global overcoverage of the femoral head by the acetabulum (ie, deep socket or protrusio). Acetabular retroversion is seen in both males and females, while the deep socket variety is commonly seen in middle-aged women.

Early on, pincer impingement is more circumferential and can degenerate and tear the acetabular labrum; long-standing impingement is associated with countercoup injury to the cartilage posteriorly and inferiorly in the femoral head and acetabulum. Bony apposition about the rim, due to repetitive microtrauma, can also occur with pincer-type impingement.

In contrast to the labral injuries associated with cam lesions, pincer impingement causes intrasubstance injury, which is more difficult to repair. However, there is less chondral damage with pincer impingement when compared to the deep chondral injury and delamination seen in cam impingement.

Diagnosing FAI
Early diagnosis and treatment of FAI is imperative to avoid additional hip damage and early development of hip OA. Insidious, intermittent groin pain is usually the first presenting symptom. Common misdiagnoses include adductor or hip flexor strains or pulls.

Pain occurs with high-demand activities such as sports or prolonged walking. Pain with prolonged sitting has also been described and attributed to the stress across the joint from hip flexion. Patients with associated labral tears may report catching, locking, and clicking at the hip joint, but these symptoms are rare. The impingement can be bilateral, but patients often have symptoms only on one side.

On physical exam, most patients have a positive impingement sign and limited range of motion, especially in flexion at 90 degrees and with internal rotation. This can be provoked with flexion, adduction, and internal rotation at the hip. Patients with pincer impingement and posterior cartilage damage may also experience pain with hip extension and external rotation.

The first diagnostic exam should be plain radiographs. For cam impingement, standard AP or lateral radiographs of the hip may not be sufficient, and a cross-table lateral and a modified or standard Dunn view of the hip are often necessary. With these radiographs, an alpha angle and the head-neck offset ratio can be used to diagnose cam impingement.

The alpha angle is the angle between the axis of the femoral neck and a line connecting the center of the head with a point 2 mm out of the initial asphericity. An alpha angle greater than 50 degrees is associated with cam deformity.

The femoral head-neck offset ratio is measured on the cross-table lateral view by dividing the anterior offset (the distance between a line adjacent to the anterior aspect of the neck and another line touching the most anterior part of the femoral head, both parallel to the femoral neck axis) by the femoral head diameter. An offset ratio less than or equal to 0.15 is sensitive and specific for cam deformity.

For pincer impingement, the presence of retroversion or a deep socket can be noted. The presence of the crossover sign, the posterior wall sign, or the prominence of the ischial spine (PRIS) sign is present with acetabular retroversion. The crossover sign is created when the anterior wall crosses the posterior wall distal to the roof. The posterior wall sign is seen when the center of the femoral head lies lateral to the posterior wall. The PRIS sign occurs when the ischial spine projects into the pelvic cavity.

Coxa profunda is diagnosed on an AP view when the medial wall of the acetabulum lies on or medial to the ilioischial line. This measurement is highly variable and should not be used as a surrogate for a deep socket. Protrusio, another cause of pincer impingement, is diagnosed on AP views when the femoral head crosses the ilioischial line.

Advanced imaging such as computed tomography (CT) and magnetic resonance imaging (MRI) is useful in further exploring the hip joint. The CT scan creates a three-dimensional image, which is helpful in surgical planning, while an MRI paired with a gadolinium arthrogram is useful in demonstrating acetabular pathology such as chondral damage and labral tears.

Treating FAI
Treatment of FAI can be nonsurgical or surgical. Nonsurgical management—activity modification, NSAIDs, and strengthening and range of motion exercises—can delay and sometimes decrease the necessity for surgical management.

Once conservative nonsurgical measurements have been exhausted, surgical management can address the structural factors that cause the impingement and the intra-articular pathology such as chondral and labral tears. Both open surgical interventions—including the mini-open approach, surgical hip dislocation, and periacetabular osteotomy (PAO)—and arthroscopic approaches are used to manage FAI. The technique chosen should take the patient’s age, range of motion, and activity level into account and be based on type and extent of pathology.

PAO is helpful when pincer impingement is due to acetabular retroversion with an associated posterior wall sign. Good to excellent results have been reported for such problems. If the impingement is due to the relative prominence of the anterior wall without the posterior wall sign, trimming the acetabulum arthroscopically or through a surgical hip dislocation is helpful.

Surgical dislocation of the hip with a trochanteric slide osteotomy was first described by Ganz and enables full circumferential visualization of the acetabular rim and femoral head-neck junction. The surgeon can then address the osseous deformity, perform labral débridements and osteochondroplasties, and treat other structural abnormalities that lead to extra-articular impingement. Although surgical dislocation has excellent results and patient satisfaction is high, recovery is slower than with arthroscopic treatment.

Improvements in arthroscopic techniques have made hip arthroscopy a viable treatment for FAI. These advances enable labral takedown and refixation, treatment of chondral injury, and osteochondroplasty of the femoral head-neck junction and acetabular rim. Studies have shown that surgical dislocation and arthroscopic treatments have comparable efficacy in surgical correction of FAI.

Ali Ashraf, MD; Rafael J. Sierra, MD; and Amy L. McIntosh, MD, are all affiliated with the Mayo Clinic in Rochester, Minn.

Bottom Line

• FAI encompasses two different abnormal hip morphologies that lead to hip OA.
• Cam impingement is a femoral head-neck junction abnormality commonly seen in young athletic males; pincer-type impingement is an acetabular deformity that causes overcoverage and is seen in middle-aged women.
• Surgical treatments include PAO, open surgical hip dislocation, and arthroscopic treatment and should be individualized to patient clinical and radiographic findings.
• Arthroscopic and surgical hip dislocations have comparable complication rates and results.

Putting sex in your orthopaedic practice
This quarterly column from the AAOS Women’s Health Issues Advisory Board and the Ruth Jackson Orthopaedic Society provides important information for your practice about issues related to sex (determined by our chromosomes) and gender (how we present ourselves as male or female, which can be influenced by environment, families and peers, and social institutions). It is our mission to promote the philosophy that male and female patients experience and react to musculoskeletal conditions differently; when it comes to patient care, surgeons should not have a one-size-fits-all mentality.

References

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