Femoroacetabular impingement

Femoroacetabular impingement is a suggested diagnosis for pain in the hip in the absence of other apparent causes, in which abnormal anatomy or superphysiologic motion produces eccentric loading of the joint or actual impingement on the femur by the acetabular rim. It is thought to be a cause of progressive athrosis of the hip.

Femoroacetabular impingement (FAI) is said to occur when there is abnormal contact between the proximal femur and the acetabular rim. This abnormal contact can be produced by activities that require repeated hip motion to an extreme range, or when there is an abnormal anatomic configuration to the bone such that contact is produced even with motion in the normal range.

There have been two types of FAI described, Cam and Pincer types.  Although they are described as separate entities, over 80% of patients have been shown to have components of both.  Cam type femoroacetabular impingement occurs when a femur with an abnormal head neck junction with insufficient head-neck offset is slammed into the acetabulum causing shear forces on the acetabular rim and causing damage to the acetabular cartilage and labrum.  This kind of impingement often occurs in patients with post-traumatic deformities, SCFE, coxa vara, and AVN.  Many patients are males who are physically active or heavy laborers.

The second type of impingement is designated as pincer type and is due to linear contact between the acetabular rim and the femoral head neck junction.  The labrum fails resulting from the continued impact.  It often ossifies, further worsening the situation by effectively deepening the acetabulum and decreasing the effective range of motion.  This type is often seen in patients with coxa profunda, protrusio, and acetabular retroversion. Many of these patients are middle aged females engaging in activities that require extreme ranges of motion such as yoga and ballet.

Femoroacetabular impingement is a diagnosis of increasing popularity (and awareness) though it has its critics

History

An open-ended questioning format should direct the history of present illness. The context, duration, severity, modifying factors, associations, radiation, and other characteristics of the patient's pain should be obtained. The location of pain should be noted as hip pain is often referred to the groin, thigh, and buttock, in accordance with the innervations of the hip joint from the obturator, femoral and sciatic nerves. Snapping, locking, and clicking are common symptoms in patients with hip disease. A true locking of the hip is a sign of labral disease.

Special considerations need to be made in each patient, as each has a unique history. In the case of a female patient, concomitant pelvic sources of pain must be considered.  For example, pain from an ovarian cyst is often referred to the groin. Symptoms of instability, such has frequent falls, should be discussed as patients with labral pathology may describe a feeling of instability, even if it has not actually given away.

Past Medical History

The patient's past medical history should be outlined.  Remote elements of the patient's history are also important, as hip disease may be concomitant with many degenerative conditions of the hip. Childhood conditions such as congenital hip dislocation, Legg-Calves-Perthes, Slipped Capital Femoral Epiphysis, inflammatory arthritis, and prior septic joints should be noted.

Risk factors, including athletic activities, medical history, family history and social history are of key importance to some hip disorders. Any history of trauma to the hip should be discussed. The use of glucocorticoids, history of hypercoagulable states, and barotrauma, are important, especially if osteonecrosis is suspected. If surgery has been performed previously, the examiner may wish to obtain old medical records or operative reports, if applicable.

Physical Exam

The physical exam should begin with a close inspection of the patient's posture, stance, and gait. Gait should be closely scrutinized looking for an abductor lurch, a shortened stance phase, circumduction, or steppage. Leg lengths should be measured using accurate methods. The patient's posture should be noted, including examination of the back for scoliosis. Skin should be inspected. Old incisions can give indication to previous procedures. The periarticular areas should be palpated. Here, trochanteric bursitis, iliotibial band syndrome, and trigger points may be elicited. The range of motion of the hip should be tested. A decreased range of motion of the hip, especially in cases of an external rotation contracture, can point to an intra-articular cause of the pain. Patients with hip pathology often also develop a flexion contracture. The Thomas test is a helpful maneuver. A thorough neurovascular exam should be conducted to rule out spine and other neural causes of the patient's pain.

In a patient with hip pain and negative X-rays were intra-articular sources of pain are suspected, impingement tests may assist in diagnosis. To look for anterior femoral-acetabular impingement the affected hip is flexed to ninety degrees and the leg is internally rotated and adducted. If there is abnormal contact between the anterior-superior acetabular rim and femoral neck, pain may be elicited. Posterior impingement is also possible, and may be tested by having the patient dangle their legs off the end of an examination table, with the affected leg externally rotated by the examiner, and the opposite limb held flexed by the patient. In a positive exam, the femur contacts the posterior acetabular rim eliciting pain.

Diagnostic Imaging

If an X-ray and MRI have already been obtained and are negative, many possible diagnoses can be excluded such as malignancy, osteonecrosis of the femoral head, osteoarthritis, and transient osteoporosis of the hip may be excluded. However, both the X-rays and MRI should be re-evaluated for subtle findings that may point to impingement or acetabular labral pathology.

Standard anterior-posterior pelvic, frog-leg lateral and cross-table lateral radiographs should be obtained. In certain situations a false-profile radiograph can asses anterior femoral head coverage.

Figure 1. False Profile radiograph.

The AP pelvis X-ray must be well centered and well developed as to show a clear outline of the acetabulum.  The coccyx should point toward the symphysis pubis, and there should be about 1-2 cm between them. The sourcil, the anterior and posterior walls, the tear drop and the lateral edge of the acetabulum should be noted. Measurements may be taken to evaluate for hip dysplasia including the Tönnis angle (abnormal > 10 degrees), the lateral center-edge angle of Wiberg (abnormal < 25 degrees), and the anterior center-edge angle of Lequesne (abnormal < 25 degrees) as measured on a false-profile radiograph. The neck shaft angle of the proximal femur is considered normal between 120 and 140 degrees.

Coxa profunda is present when the floor of the acetabular fossa is in line with the ilioischial line; protrusio is present when the medial most femoral head overlaps the ilioischial line. The crossover sign is a sensitive and specific indicator of native acetabular version.   On an AP pelvis radiograph, the outlines of the edges of the anterior and posterior walls of the acetabulum should meet superiorly and laterally.  In cases of acetabular retroversion, this crossover of the anterior and posterior acetabular wall outlines is more distal. Changes in the acetabular rim may also be noted.  A 'double line' is seen in labral ossification.  An os acetabuli may also be an indicator of pathology.

Alterations of the proximal femoral anatomy, such as head neck offset and bump formation can be observed in addition to acetabular and labral pathology. A pistol grip deformity of the femoral head is often seen in Cam Type impingement.  In this situation the superior-lateral head neck junction is convex instead of concave.  A high fovea can also indicate asphericity of the femoral head that is not able to be appreciated on the AP films. The alpha angle is a useful radiographic measurement for quantifying the head-neck junction deformity. This angle is most accurate when measured on MRI, however it can also be measured using plain films. The Dunn view (AP of hip in neutral rotation, 45 degrees hip flexion, and 20 degrees abduction) is the most sensitive x-ray view for detecting femoral head-neck asphericity . The alpha angle is formed by a line drawn from the center of the femoral head through the center of the femoral neck, and a line from the center of the femoral head to the femoral head/neck junction, found by the point by which the femoral neck diverges from a circle drawn around the femoral head.  At present, the upper end of normal is an alpha angle of 50 - 55 degrees.

Figure 2. Alpha Angle

Figure 3. Tonnis Angle (Normal)

Figure 4. Tonnis Angle (Abnormal)

Figure 5. Lateral Center Edge Angle

Figure 6. Crossover Sign.

Figure 7. Anterior Center Edge Angle. (On False Profile View)

Figure 8. Coxa Profunda

If closer examination of the radiographs indicates dysplasia, CT scanning will allow accurate observation of the bony anatomy and more precise quantification of the degree of dysplasia.  If impingement or labral pathology is still suspected, special MRI with specialized radial sequences perpendicular to the true plane of the acetabulum have been found to be useful.

Treatment

Non-operative

A course of non-operative treatment for most hip pathology may be tried first. Patients presenting with femoroacetabular impingement or labral disease may try modification of activity, avoiding excessive hip movement and regular non-steroidal anti-inflammatory medication.

Arthroscopy

Arthroscopic assessment of the hip can include examination of both the central and peripheral compartments. The central compartment includes the labrum and all structures located further medially. Tearing of the labrum anterolaterally and damage to the acetabular cartilage is characteristic. The lesions of the labrum and any areas of chondral damage are débrided. Labral repair may be possible for specific tears. For areas of exposed subchondral bone a microfracture technique may be performed. The peripheral compartment consists of all the structures that are lateral to the labrum but are inside the capsule. In the peripheral compartment, osteophytes located around the femoral head-neck junction can be resected to restore the concavity of the femoral neck. The external portion of the labrum can also be visualized and rim osteophytes can be resected.

Surgical Hip Dislocation- Osteoplasty

Surgical dislocation of the hip has been described.  The authors prefer a posterior incision (Kocher-Langenbeck) as this approach usually provides better access to posterior parts of the joint after the hip is dislocated. A trochanteric flip osteotomy is performed. The trochanter is osteotomized from a posterior to anterior direction, the cut exits superficial to the piriformis fossa superiorly and at the vastus ridge inferiorly. The gluteus minimus muscle is dissected carefully off the capsule starting at the piriformis interval. The capsulotomy is Z-shaped (for the right hip), with the superior limb located along the posterior acetabular rim, and the inferior limb located at the level of the anteromedial femoral neck. Careful protection retinacular vessels entering the capsule at the inferior area of the superolateral neck is mandatory to preserve the crucial blood supply to the femoral head. Before subluxation or complete dislocation of the hip, an evaluation of the ROM should be done. This step is crucial in the decision making process for treatment of FAI. After dislocation of the hip, the acetabular labrum and the adjacent articular cartilage are assessed, and the identified lesions are tested for partial or complete avulsions from the acetabular rim. The severity, extent, and location of these lesions should be defined, and their association with FAI should be confirmed by provocative maneuvers in flexion and internal rotation with the head relocated.  The combination of anterior over coverage and the status of the labrum and the acetabular articular cartilage will determine the type of treatment of the acetabular rim. In cases of anterior over coverage contributing to FAI, as is frequent with acetabular retroversion, a resection osteoplasty of the anterosuperior rim is done.  For patients presenting with FAI in the presence of acetabular retroversion, posterior wall deficiency, or at least lack of posterior over coverage, reverse periacetabular osteotomy is preferred. If the femoral head-neck junction is identified to be the cause of FAI, contouring of the femoral head and neck by excision osteoplasty is done.  There often is a clear demarcation between the area of normal femoral articular cartilage and the area of the head that is subjected to FAI.

Periacetabular Osteotomy

Reorientation of the articulating surfaces of the hip joint is an attractive procedure in the patient with hip dysplasia. Increased joint congruity after reorientation of the osteotomized fragment allows load transmission through a broader area subjected to less pressure. These changes can be expected to reduce pain and possibly protect the articular cartilage from degenerative changes. Joint reorientation, particularly of the femur, also may result in a more functional arc of motion.  Osteotomies should be offered to young patients who have symptomatic hip dysplasia without excessive proximal migration of the center of rotation, reasonably well preserved range of motion, and no more than mild degenerative changes of the articular surface. The primary abnormality in most patients with hip dysplasia is located on the acetabular side of the joint. Thus, pelvic osteotomy corrects the major anatomic abnormality and has the further advantage over femoral osteotomy of not creating a secondary femoral deformity. Femoral osteotomy may be added to pelvic osteotomy when coexistent femoral anatomic abnormalities are significant. The Bernese periacetabular osteotomy is indicated for patients with hip symptoms of mechanical overload, impingement, or hip instability as a result of insufficient acetabular coverage.  It is performed with a series of straight, relatively reproducible extra-articular cuts. This osteotomy allows large corrections of the osteotomized acetabular fragment in all directions, including lateral rotation, anterior rotation, and medialization of the hip joint. The posterior column of the hemipelvis remains intact; thus, minimal internal fixation is required, and early ambulation with no external immobilization with a cast or a brace is possible.

Reference

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