Ischiofemoral Impingement

Ischiofemoral impingement (IFI) is an increasingly recognised but still under-diagnosed cause of posterior hip and deep buttock pain. The space between the ischial tuberosity and the lesser trochanter narrows, and the quadratus femoris muscle, and in some cases the sciatic nerve, is compressed within it. Johnson first described the condition in 1977, but it gained widespread attention only in the MRI era.

Who. Women outnumber men by approximately 3:1. Patients are typically aged 30 to 70, with a peak at 40 to 60, and approximately 25 percent of cases are bilateral. Impingement after total hip arthroplasty accounts for a significant proportion of secondary IFI.

Why it matters. The pain limits walking distance and sitting tolerance, and long-stride walking, stairs and running become difficult. Left undiagnosed it becomes chronic pain, much as FAI did before it was recognised, and a high index of suspicion is needed because the symptoms overlap with piriformis syndrome, hamstring origin tendinopathy and lumbar radiculopathy. It can also be iatrogenic, provoked by the position of the components after a hip replacement.

The muscle and its interval. Quadratus femoris runs from the lateral border of the ischial tuberosity to the quadrate tubercle of the proximal femur, acts as an external rotator and adductor of the hip, and normally glides freely in the ischiofemoral space as the hip moves. The sciatic nerve passes immediately posterior to it, which is why a narrowed space can compress the nerve as well as the muscle, and why some patients report radiating, sciatica-like leg pain rather than pure mechanical buttock pain.

The two spaces. The ischiofemoral space (IFS) is the narrowest distance between the lateral cortex of the ischial tuberosity, at the level of the quadratus femoris origin, and the medial cortex of the lesser trochanter. The quadratus femoris space (QFS) is the narrowest distance between the superolateral surface of the hamstring tendons and the posteromedial surface of the iliopsoas tendon or lesser trochanter. The muscle occupies this interval.

What narrowing does. Whether the cause is developmental bony morphology, post-surgical change or a pathological process, a narrowed space traps the muscle against the ischium and lesser trochanter during terminal extension and adduction of the hip. The interval is dynamic, narrowing in extension and adduction, so a measurement on a static scan is a snapshot of one position. Repetitive microtrauma produces muscle oedema and inflammation, and eventually fatty infiltration or tearing.

Why women. A wider pelvis increases the interischial distance and alters the ischiofemoral angle, and a greater average femoral anteversion positions the lesser trochanter closer to the ischium in extension. Women also have less quadratus femoris bulk, so there is less soft-tissue cushion in the interval, and hormonal and gait factors, a wider stance and coxalgic gait patterns, may exacerbate the narrowing.

View larger

Two classifications are in use: by cause, which determines what the treatment must address, and by MRI severity, which guides how aggressive it needs to be.

History. The pain is posterior, in the hip or deep in the buttock, and rarely radiates below the knee. Long-stride walking, running, prolonged sitting and extension with adduction aggravate it; rest, short strides and hip flexion relieve it. Ask specifically about prior THA, osteotomy or fracture fixation, and in a woman about an insidious onset, since female sex is strongly associated.

Examination. Inspection is usually normal, though the gait may be antalgic or Trendelenburg. Tenderness lies deep in the buttock, distal to the gluteal fold, and is difficult to localise. Terminal extension with adduction hurts; flexion is typically comfortable.

The provocation tests. No single test is pathognomonic, and the diagnosis needs clinical suspicion plus MRI confirmation.

• Long-stride walking test: standing, the patient takes a long stride with the affected leg trailing, which extends and adducts the hip and narrows the ischiofemoral space; reproduction of the characteristic deep posterior hip or buttock pain is positive
• Extension-adduction-external rotation (the IFI test): the supine alternative, passive extension of the hip with adduction and external rotation
• FAIR test (flexion, adduction, internal rotation): often positive because it compresses the quadratus femoris, but more classically associated with piriformis syndrome and deep gluteal syndrome

Deep gluteal syndrome. A common exam framing error treats IFI and deep gluteal syndrome as parallel diagnoses. Deep gluteal syndrome (DGS) is the umbrella term for non-discogenic, extrapelvic entrapment or irritation of the sciatic nerve in the subgluteal space, and IFI is one of its recognised causes, alongside piriformis syndrome, obturator internus/gemelli pathology, proximal hamstring origin pathology, fibrovascular adhesive bands and vascular lesions such as gluteal vein varicosities. Asked for the differential of posterior hip or buttock pain, frame it as DGS causes with IFI inside that family, then separate them by the discriminating manoeuvre (long-stride or extension-adduction for IFI, FAIR for piriformis) and by the MRI finding (quadratus femoris oedema in a narrowed IFS). This is also why a structured deep-gluteal-space MRI and, in selected refractory cases, sciatic nerve assessment are part of the workup.

After a hip replacement. Posterior hip pain after THA is a common exam scenario. Infection, loosening and instability head the differential, but a medialised acetabular cup, excessive femoral offset or leg-lengthening can narrow the ischiofemoral space, and IFI is increasingly recognised. When plain films and the infection workup are negative, consider MRI to measure the IFS; if implant position is the primary driver, management may require component revision.

Radiographs first. An AP pelvis, a lateral hip (cross-table or frog) and a Dunn view. Look for a prominent lesser trochanter, coxa valga, evidence of prior osteotomy, exostoses and, after THA, component position (a medialised cup, leg-lengthening). Radiographs cannot measure the IFS directly, which needs cross-sectional imaging, and normal films do not exclude IFI.

MRI is the gold standard. Axial T1 and T2 fat-saturated or STIR sequences through the hip at the level of the quadratus femoris are essential, because the IFS and QFS are measured on them. Quadratus femoris oedema, T2/STIR hyperintensity in the muscle belly, is the hallmark finding; look also for fatty infiltration, partial tears, hamstring tendon pathology and sciatic nerve signal change.

Read the numbers with the patient in mind. A measurement below threshold suggests IFI, but only with clinical correlation: asymptomatic narrowing of the IFS has been reported in up to 10 to 15 percent of routine hip MRI studies. The diagnosis is clinical plus radiological, oedema and a narrowed space matching posterior hip pain reproduced by extension-adduction. Treat the patient, not the MRI.

View larger

CT. For detailed bony anatomy when planning osteoplasty, with 3D reconstruction for surgical planning. It measures the bony IFS precisely, defines the lesser trochanter morphology and the ischial configuration, and maps the extent of resection needed for endoscopic or open decompression.

Image-guided injection. Corticosteroid and local anaesthetic into the quadratus femoris belly or the ischiofemoral space, under ultrasound or CT guidance, is both a test and a treatment. Pain relief of greater than 50 percent supports IFI as the pain generator, and the relief can last weeks to months, long enough to make rehabilitation possible.

What the evidence says, and does not. The 2020 systematic review reported no complications or adverse effects across its 17 included studies, a reassuring signal drawn from small uncontrolled series that are unlikely to capture rare harms. The figures in the table are those reported in case series.

Prognosis. The best outcomes follow a short duration of symptoms, primary IFI, a quadratus femoris preserved on MRI without fatty atrophy, a good response to injection and compliance with rehabilitation. The outlook is poorer with long-standing symptoms and fatty atrophy of the muscle, post-THA IFI with severe component malposition, failed prior surgery, and coexisting lumbar or intra-articular pathology.