Introduction

The cause of hip osteoarthritis is a controversial topic. Historically, it was believed that arthritis was the result of intrinsic cartilage abnormalities and a biomechanical mismatch with axial loading that led to joint overload and subsequent cartilage wear. Recently, femoroacetabular impingement (FAI) has been shown to cause chondrolabral damage and leads to osteoarthritis. Although the pistol grip deformities have been described, the biomechanical concept of FAI as a unifying principle that explains how abnormal osseous parameters lead to arthritis is unique. FAI is a dynamic phenomenon that causes chondrolabral damage from repetitive hip motion, especially flexion and internal rotation. These “at-risk” hips have osseous structural abnormalities on either the femoral side, the acetabular side, or both. During the range of motion of the hip, particularly during flexion and internal rotation, these abnormalities produce the mechanical impingement of the femoral head against the acetabular cartilage or the femoral neck against the adjacent labrum. With time, repetitive impingement leads to acetabular cartilage damage, and this is followed by early osteoarthritis.

Two distinct mechanisms of FAI have been described; these are commonly referred to as cam disease and pincer disease (Figure 28-1, A through D). The descriptions of these conditions were based on the skeletal morphology and the pattern of chondrolabral damage observed during surgical hip dislocations. However, these morphologic patterns are not mutually exclusive; it is quite common for patients to have components of both cam and pincer types of impingements. With cam FAI, there is an abnormal bony prominence at the femoral head–neck junction that is often located anterosuperiorly. During hip flexion, the abnormal contoured femoral head engages the anterosuperior acetabulum and produces shear forces that lead to chondral abrasion, delamination, and, eventually, full-thickness cartilage loss. The natural history of this impingement process is initially acetabular cartilage injury, which is followed by labral injury and ultimately joint arthrosis. At first the labrum is uninvolved, but, with further impingement, labral injury results from the further loosening of the labrum at the transition zone between the peripheral cartilage and the labrum itself. With pincer FAI, there is increased acetabular coverage that leads to linear contact between the femoral head–neck junction and the acetabular rim. Acetabular overcoverage may be either generalized, as in coxa profunda and protrusio acetabuli, or localized, as in acetabular retroversion and anterior acetabular overhang. Unlike what occurs with cam FAI, the labrum is the first to get injured with intrasubstance degeneration, cyst formation, and bone apposition at the rim, which further deepens the socket and exacerbates the problem. The prominent acetabular rim abuts with the femoral neck and causes the femoral head to lever in the acetabulum. This chronic levering of the head generates shear forces and injury to the posterior cartilage. Over time, this contrecoup mechanism leads to posteroinferior chondral damage and joint space narrowing.

Figure 28–1 The two forms of femoroacetabular impingement (FAI) are shown. A, Cam FAI occurs when the lack of offset of the femoral neck (arrow) leads to mainly anterosuperior damage through repetitive inclusion of the femoral neck with subsequent shearing injury of the acetabular cartilage. B, During hip flexion, impingement causes chondral labral injury. C, Pincer FAI occurs as a result of acetabular overcoverage (arrow). D, Pincer impingement causes anterosuperior impaction and subsequent leverage with contrecoup injury to the cartilage of the posteroinferior acetabulum during hip flexion.

Redrawn from Leunig, M., Robertson, W., and Ganz, R. Femoroacetabular impingement: diagnosis and management including open surgical technique. Tech Sports Med 15:178-188, 2007.

Basic science

The major breakthrough in impingement surgery was the safe surgical dislocation of the hip. The key to this procedure is an in-depth understanding of the blood supply to the femoral head. A recent latex injection study demonstrated that the medial femoral circumflex artery (MFCA) is the main blood supply to the femoral head. The vessel was surgically dissected and found to cross the obturator externus posteriorly and to then pass anteriorly to the short external rotators before perforating the joint capsule at the level of the superior gemellus. The study also demonstrated that the vessel remained undamaged during a controlled surgical hip dislocation, providing that the short external rotators and obturator externus remained intact. As a result of this study, an operative technique was developed that allowed for the safe surgical dislocation of the femoral head. Unlike the commonly used posterior approach, which requires the division of the short external rotators, a surgical dislocation protects these structures and, in doing so, preserves the blood supply to the femoral head.

Another critical basic science concept is that, in most cases, chondral injury usually leads to labral tears (and not vice versa). On the basis of our observations during several hundred surgical dislocations, the pattern of disease in cam FAI is that osseous abnormalities create mechanical “outside-in” shear forces and chondral damage, and, subsequently, the uninvolved labrum gets injured. This concept is supported by the fact that the majority of so-called labral tears occur at the anterosuperior portion of the acetabulum at the transition zone between the labrum and the articular cartilage and not the capsular margin. In addition, in early cam FAI, chondral damage is often observed without labral tears. Intrasubstance labral tears that are less frequently associated with chondral injuries have been observed in patients with early pincer FAI. Labral tears in the anterosuperior portion of the acetabulum are the result of FAI and not isolated traumatic injuries; this idea is supported by the observations that the majority of labral tears are also associated with chondral damage and radiographic evidence of abnormal skeletal morphology.

Treatment, indications, and contraindications

Conservative treatment is usually attempted first, and this includes activity modification; rest; nonsteroidal antiinflammatory drugs; and a physical therapy regimen that focuses on core, lower back, and hip flexor strengthening. On occasion, one may use intra-articular injections for both diagnostic and therapeutic uses. We do not routinely perform intra-articular hip injections; however, for select cases in which the origin of a patient’s symptoms remains unclear, we will perform a diagnostic intra-articular injection. In many cases, conservative management strategies may only partially relieve symptoms, and often they only mask symptoms. Attempts by physical therapists to improve the passive range of motion are often not beneficial and in fact may be counterproductive, because the limitation of internal rotation in patients with FAI is the result of abnormal osseous morphology. Although some patients receive temporary benefits from these conservative measures, young athletic patients often have difficultly complying with activity modifications.

The most important indications for FAI surgery are physical examination and radiographic findings that are consistent with FAI (Box 28-1). Proper imaging studies are critical to confirm and quantify the deformity and to assess the degree of arthritis. Unnecessary treatment delays also should be avoided.

The next critical issue is determining the surgical technique that should be used. Although this decision depends on numerous patient factors, the type, magnitude, and location of the underlying osseous abnormality are important. Although arthroscopy is an emerging technique for the treatment of FAI, it is technically challenging, and it has its limitations. Arthroscopy can easily handle the soft-tissue or secondary effects of FAI (e.g., the chondrolabral pathology), although how well it can handle the underlying osseous abnormalities remains debatable. Alternatively, a surgical dislocation of the hip provides a wide, safe exposure with the complete visualization of both the acetabular and femoral pathologies. In addition, the structural morphologic changes (e.g., the lack of an anterior femoral neck offset, acetabular overcoverage) can be addressed with relative ease. It has also been shown that the failure to address the underlying bony abnormality is likely to lead to continued symptoms, progressive joint degeneration, and poor outcomes. Another advantage of a surgical dislocation is that it is provides versatile exposure of the hip joint. It enables the surgeon to perform numerous impingement procedures as well as other procedures for soft-tissue injuries, osseous Bankart lesions, avascular necrosis, hip resurfacing, loose bodies, osteochondromatosis, and other non-FAI diagnoses. Contraindications for this technique are few, but the most obvious are extensive arthritic changes (e.g., a score of more than 2 on the Tönnis scale) and extensive destruction and deformity of the femoral head. Other contraindications include significant acetabular protrusio and dysplasia.

History and physical examination

FAI typically presents with an insidious onset of groin pain in young adults. Although patients often mistakenly associate the condition with a traumatic event, it is a more chronic process with intermittent pain and occasionally with acute exacerbations from activities that require forceful hip flexion and internal rotation. As a result of the often subtle findings on routine radiographs, these patients may experience a delay in diagnosis and be subjected to extensive nonorthopedic workups. Pincer-type FAI is more common among women, and it is often quite painful as a result of the crushing of the sensitive labrum between the acetabular rim and the femoral neck. This symptom often acts as a warning sign that causes patients with the condition to seek earlier orthopedic evaluation before significant chondral damage occurs. Alternatively, cam-type FAI is more common among young males. These patients have a pain pattern that involves more deep-seated groin symptoms, which are usually less severe than those experienced by patients with pincer-type FAI, because the labrum is commonly spared. These patients often do not seek evaluation until they have developed significant chondral injury.

It is crucial that all FAI patients receive a thorough physical examination, because there are many extra-articular diagnoses that can present with hip pain. The examination begins with detailed motor and sensory examinations. Next, the range of motion is assessed. Limited internal rotation of the flexed and adducted hip is seen in both cam and pincer FAI, but a greater loss of internal rotation is seen with cam FAI. This is followed by FAI-specific tests. An impingement test (Figure 28-2) is performed with the patient in the supine position; the affected hip is adducted and internally rotated as it is passively flexed. In patients with FAI, the femoral head–neck junction and the acetabulum abut, thus producing shear forces on the labrum and reproducing a sharp pain in the groin. A posteroinferior FAI test is performed with the patient supine on the edge of the examination table with the legs dangling free from the end. The examiner then extends and externally rotates the affected hip. Deep-seated groin pain during this maneuver is indicative of posteroinferior FAI, and it is frequently combined with limited external rotation. Finally, other critical examination maneuvers are performed to find associated pathology of the psoas, the iliotibial band, the lower back, and other related structures.

Figure 28–2 Clinical examination of the impingement test.

Redrawn from Leunig, M., Robertson, W., and Ganz, R. Femoroacetabular impingement: diagnosis and management including open surgical technique. Tech Sports Med 15:178-188, 2007.

Imaging

A correctly performed anteroposterior pelvic radiograph is the most essential imaging study for the assessment of impingement. The radiograph should be standardized to ensure proper rotation (i.e., the sacrum bisects the pubis) and pelvic inclination (i.e., 2 cm to 5 cm of pubic–sacrococcygeal distance). Slight deviations in either of these parameters can lead to an inaccurate assessment of acetabular coverage, inclination, and anteversion.

After the adequacy of the radiograph has been verified, it should be reviewed in a systematic fashion. First, the radiograph should be assessed for the coverage of the femoral head (i.e., center edge angle and Tönnis angle) or for gross arthritic changes (i.e., Tönnis scale). Next, the acetabulum should be inspected for pincer-type FAI. Five radiographic structures must be identified: 1) the medial acetabular wall; 2) the ilioischial line; 3) the anterior wall of the acetabulum; 4) the posterior wall of the acetabulum; and 5) the femoral head. By understanding the relationship of these radiographic structures, all of the common causes of pincer FAI can be diagnosed. In a patient with coxa profunda, the medial acetabular wall approaches and overlaps (if it does not pass medial to) the ilioischial line, which causes a deep socket. In a patient with protrusio, the femoral head is medial to the ilioischial line. In a patient with true acetabular retroversion, the anterior and posterior acetabular walls overlap; they also have a positive crossover sign and a prominent ischial spine (Figure 28-3, A). In these cases, there may or may not be a sufficient posterior wall, but there is always a relative anterior overhang. Finally, one must assess for os acetabuli, which can represent either broken pincer lesions or unfused portions of the acetabulum (i.e., true os acetabuli).

Figure 28–3 A, Anteroposterior radiograph that shows a crossover sign indicating acetabular retroversion. B, Cross-table lateral view that shows a lack of femoral offset. L, left; R, right.

On the femoral side, cam FAI is readily diagnosed with the proper radiographs. Given its mostly anterosuperior location, the cam lesion is often underappreciated on a standard anteroposterior radiograph, and it may be obstructed by the greater trochanter on a frog-leg lateral view. The aspheric head–neck junction is best visualized with either a 45-degree Dunn view or a cross-table lateral view with the leg in 15 degrees of internal rotation. The Dunn view, which is also known as an extended neck lateral view, is taken with the patient’s hip in neutral rotation, flexed 45 degrees, and abducted 20 degrees. The internally rotated cross-table lateral view is often more practical for routine use, because positioning the patients for the Dunn view requires a leg holder or an assistant. Either image can be used to measure the head–neck offset and the alpha angle, both of which are abnormal parameters that can be used to assess cam FAI (see Figure 28-3, B). In addition, the femoral neck shaft angle should also be assessed for any significant varus deformities.

In addition to radiographs, we routinely obtain a magnetic resonance arthrogram to accurately assess chondral delamination and full-thickness cartilage defects. In many cases of FAI, hips that produce normal radiographs (as defined by Tönnis grade) in fact have extensive chondral injury. Magnetic resonance imaging also assesses for labral pathology or subtle signs of FAI, such as fibrocystic changes at the head–neck junction; these changes are also known as synovial herniation pits. The magnetic resonance imaging should involve the use of cartilage-specific sequences, and it should be performed in radial-directed sections for the accurate measurement of angulation and the translation of the impingement lesions in the radiologic reference planes (Figure 28-4). Standard magnetic resonance imaging of the pelvis is often less sensitive, because it does not occur in the correct sequence plane with a resolution being far too low. In addition, although computed tomography scans provide a superior assessment of femoral anteversion, acetabular version, and femoral offset, they require radiation exposure during 20 radiologic pelvic overviews; thus, computed tomography should be used quite judiciously. Other advantages of obtaining a magnetic resonance image include the evaluation of stress fractures and soft-tissue abnormalities as well as the measurement of the alpha angle and acetabular version.

Figure 28–4 Radial sequence magnetic resonance image of the right hip showing cam-type FAI with a decreased head/neck offset.

Surgical technique

The fundamental principle of the surgical treatment of impingement is an exposure that preserves the blood supply to the femoral head. The approach combined and made use of features from several previously described approaches, with the general principle being that of an anterior dislocation of the femoral head from a posterolateral approach. However, to protect the MFCA, the external rotators are left intact, and the joint capsule is exposed anteriorly with the use of a trochanteric osteotomy. The surgical technique is explained in detail later in this chapter.

General or spinal anesthesia is used. The patient is placed in the lateral decubitus position in well-padded bolsters, and care is taken to also protect the nonoperated limb. Correct orientation is important to allow for the accurate assessment of acetabular orientation during the procedure. The skin is cleansed with a standard preparation solution over the trochanteric region. The patient is prepared and draped in standard sterile fashion (Figure 28-5, A) with a free leg sterile bag drape placed on the opposite side of the operating table to receive the lower leg during hip dislocation (see Figure 28-5, B). A second-generation cephalosporin antibiotic is given for prophylaxis and continued for 24 hours. Image intensifying and laser Doppler flowmetry are not routinely used, but both can be helpful for either osteotomy fixation or to monitor the perfusion of the femoral head.

Figure 28–5 A, The lateral decubitus position for surgical hip dislocation. This technique provides for the exposure of the entire femoral head and the acetabulum, which allows for the identification and treatment of femoroacetabular impingement. B, With the hip in flexion and external rotation, the femoral head can be dislocated to allow for an almost circumferential inspection of the entire acetabulum.

Redrawn from Leunig, M., Robertson, W., and Ganz, R. Femoroacetabular impingement: diagnosis and management including open surgical technique. Tech Sports Med 15:178-188, 2007.