Teaching Case

The patient presented with persistent bilateral hip pain, without radiation of pain to the knee or posterior thigh. Hip flexion and internal rotation was painful. There was no history of prior surgery or recent trauma. Because corticoid injections in the hip joint did not yield improvements, MRI was performed.

Signs of osteoarthritis, femoroacetabular impingement or osteonecrosis as possible causes for hip pain were absent on MR imaging. Figure 1 and 2 show marked oedema in the quadratus femoris muscle on both sides and a small accumulation of fluid against the quadratus femoris tendon on the right. There is bilateral narrowing of the ischiofemoral and quadratus femoris space.

The ischiofemoral space is measured as the shortest distance between the lateral cortex of the ischial tuberosity and the medial cortex of the lesser trochanter. The quadratus femoris space is measured from the superolateral surface of the hamstrings tendons to the posteromedial surface of the iliopsoas tendon or lesser trochanter [1].

On the right, intratendinous signal changes and thickening of the hamstring tendons are also visualized, compatible with insertional tendinopathy. Figure 3 demonstrates the narrowing of the ischiofemoral space in the coronal plane.

Ischiofemoral impingement is a relatively new entity. It was first suggested in 1977 by Johnson in patients with prior hip surgery, who experienced pain relief after lesser trochanter excision [2]. More recently, it has been reported in patients with no history of previous hip trauma or surgery [1, 3]. Its origin is found in compression of the quadratus femoris muscle between the lesser trochanter laterally, and the ischial tuberosity and the hamstrings medially [1].

It typically affects middle-aged women. The female predominance might be explained by the different configuration of the pelvis. Hip osteoarthritis, proximal femur fractures and intertrochanteric osteotomy may all narrow the ischiofemoral space. However, absence of these entities and bilateral hip involvement is reported in about one third of patients, also suggesting a congenital aetiology [1].

Patients may present with non-specific pain in the hip, groin or buttock. Incidentally a snapping sensation, crepitation or joint locking is mentioned. Irritation of the sciatic nerve with radiating pain to the lower extremity is less frequent [1, 4]. Since no specific diagnostic test exists, imaging should be obtained for proper diagnosis.

Radiographs are usually of little contribution to the diagnosis. Important MRI findings are the narrowing of the ischiofemoral space with abnormal signal intensity of the quadratus femoris muscle [1]. Impingement leads to oedema of the quadratus femoris muscle, and eventually to fatty degeneration and atrophy at an advanced stage. The transverse imaging plane is preferred to measure the ischiofemoral and the quadratus femoris space. Both are significantly reduced in patients with ischiofemoral impingement when compared with control subjects [1]. The normal ischiofemoral space measures about 19 mm in healthy females and 23 mm in males [5]. A cutoff of ≤ 15 mm for the narrowed ischiofemoral space and ≤ 10 mm for the quadratus femoris space yielded the best sensitivity and specificity in a recent meta-analysis [6]. The extent of the imaging findings is not always in correlation with the clinical findings. Patients may be asymptomatic even with extensive signs of impingement on MRI [7]. Furthermore, varying patient positioning during imaging might influence measurements adversely [1].

Treatment options include physical therapy, anti-inflammatory drugs, quadratus femoris steroid infiltrations or surgery in therapy-resistant cases.

In conclusion, ischiofemoral impingement should be considered in patients with non-specific hip pain, MR signal abnormalities of the quadratus femoris and a narrowed ischiofemoral space. However, thorough clinical correlation is needed, since imaging findings might be present in asymptomatic individuals as well.