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From the question on page 536

Diagnosis

Femoroacetabular Impingement (FAI).

Findings

The plain radiograph shows a synovial herniation pit within the femoral neck (arrow, fig 1). The MR arthrogram found a tear of the anterior–superior labrum (arrow, fig 3). MRI also showed an osseous prominence or “bump” on the anterior femur at the femoral head–neck junction (fig 2, arrow), with associated subchondral cystic change, consistent with cam-type femoroacetabular impingement (FAI).

The alpha angle gives an estimate of the lack of sphericity to the femoral head–neck junction and is the angle between the axis of the femoral neck and the point where the femoral head–neck junction leaves the spherical contour of the femoral head. A good diagram of the alpha angle calculation is available elsewhere.1 Larger alpha angles are seen as the femoral head becomes less spherical at the level of the femoral head–neck junction. The patient had an abnormally increased alpha angle of 63 degrees.

The patient failed a nonoperative treatment course consisting of activity modification, nonsteroidal anti-inflammatory medication and physiotherapy. She was subsequently referred for surgery and underwent right hip arthroscopy, labral repair and an osteoplasty for cam-type FAI. An arthroscopic labral repair was performed using two suture anchors (Bio Mini-Revo; Linvatec, Naples, FL). A dynamic arthroscopic examination of the peripheral space found an impinging cam lesion, which was debrided arthroscopically with a burr.

DISCUSSION

FAI is a relatively recently described condition that is thought to predispose to early-onset osteoarthritis.24 Patients with FAI are thought to develop osteoarthritis because of damage to the articular cartilage and the acetabular labrum secondary to repeated abnormal contact between the anterior femoral head–neck junction and the acetabular labrum and anterosuperior articular cartilage.24 The impingement of the femur on the acetabulum can be caused by an abnormality of the femur, called cam-type impingement, or an abnormality of the acetabulum, referred to as pincer-type impingement.24 The classic triad of findings in patients with cam-type FAI are the osseous bumb (with or without accompanying marrow oedema), labral tear and chondral thinning.5 These findings are seen in >90% of patients with cam-type FAI.5

cam-type FAI results from an abnormally shaped femoral head–neck junction, secondary to an osseous bump arising from the anterior femoral head–neck junction. The cause of the osseous prominence remains uncertain, although one current theory is that it is the result of a subclinical slipped capital femoral epiphysis.4 The typical patient with cam-type type FAI is an active young adult man.

Pincer-type deformity includes conditions with abnormal increased coverage of the femoral head by the acetabulum.3 4 Pincer-type impingement can result from an abnormal acetabular angle as seen in acetabular retroversion, or from an increase in acetabular depth, as seen in acetabuli protrusio.24 The typical patient with pincer-type deformity is a middle-aged woman. Many patients with FAI have evidence of combined cam and pincer-type deformities.4

As in the case presented here, MR arthrography is the imaging test of choice in patients suspected of having FAI, as it allows visualisation of the anatomy of the femoral head–neck junction and diagnosis of an associated labral tear or chondral abnormality.3 4 Although plain radiographs can be normal in patients with cam-type FAI, a “pistol grip” deformity of the femoral neck may be seen on an anterior–posterior radiograph of the hip, and the osseous bump may be seen on a lateral or “frog’s-leg” radiograph.6 Although still controversial, there is some evidence that juxtaarticular fibrocystic changes (originally called synovial herniation pits or Pitt pit) within the femoral neck are associated with FAI and are in the same location as the osseous prominence.7 It has been proposed that the fibrocystic changes result from the impingement between the femoral neck and the acetabulum.7

There are certain findings on history and physical examination that should alert the clinician to the possibility of FAI and prompt further tests. The most common symptom reported by patients with FAI is groin pain.2 4 A considerable percentage of patients with FAI also indicate pain overlying the greater trochanter.2 6 Most cases of FAI describe gradual onset of pain; however in one series, a quarter of patients related a traumatic event as the inciting factor for the onset of pain.2 Pain is classically seen with activities requiring flexion and internal rotation of the hip, including prolonged sitting.2 4 In fact, pain provoked by long periods of sitting is common in patients with FAI.4 Another common problem is “clicking” or “popping” within the affected hip.4

Patients with FAI are very likely to have a positive anterior impingement test,4 and one study found a positive anterior impingement test in 99% of patients with FAI.1 For the anterior impingement test, the examiner flexes the patient’s hip to 90° and then adducts and internally rotates the hip, while the patient is supine. If pain is elicited, the test is considered positive. This manoeuvre results in contact between the femoral neck and the acetabulum, causing pain in those patients with FAI.

Patients with the combined cam and pincer-type FAI are often candidates for surgical intervention, such as a labral repair, anterior acetabular osteoplasty (trimming) or anterior femoral head–neck junction osteoplasty. It is possible that an early accurate clinical and radiographic diagnosis of patients with FAI will allow for earlier surgical intervention. This may allow for the prevention of further early-onset osteoarthritis in the hip with impingement.

REFERENCES

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Footnotes

  • Patient consent: Obtained.

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