Ischiofemoral impingement due to a lipoma of the ischiofemoral space

Abstract

Non-focal hip and groin pain can frequently be a diagnostic problem, particularly if it is related to uncommon causes such as ischiofemoral impingement. The vast majority of published cases of ischiofemoral impingement are caused by osseous changes of the ischiofemoral interval. We describe an unusual case of ischiofemoral impingement caused by an intermuscular lipoma. Surgical resection of the tumour and histology confirmed the lipomatous nature of the tumour, with subsequent resolution of symptoms. To the best of our knowledge, this is the first case of a lipoma causing ischiofemoral impingement described in the English literature and emphasises that impingement can occur on the basis of a soft tissue mass occupying the interval of otherwise normal osseous interval and boundaries.

Background

Ischiofemoral impingement (IFI) is an uncommon cause of buttock, hip and groin pain due to entrapment of the quadratus femoris muscle between the ischial tuberosity and the lesser trochanter. It is usually caused by an abnormally narrowed ischiofemoral space related to the configuration of the osseous boundaries. We report a case of 40-year-old woman with IFI secondary to an intermuscular lipoma, which was revealed on MRI and confirmed at surgery. Although it has been known as a cause of hip pain since it was first described by Johnson in 1977, IFI is frequently misdiagnosed or the correct diagnosis is delayed due to its rarity, and the absence of specific clinical findings and diagnostic tests. Familiarity with this syndrome and its possible causes, along with imaging findings, are important to facilitate appropriate diagnosis and treatment. This case highlights the fact that IFI can occur with a space-occupying lesion in the ischiofemoral space despite normal dimensions of the interval.

Case presentation

A 40-year-old woman presented to her general practitioner, with a 6-week history of left-sided lumbar pain radiating to the left buttock. She described the pain as a constant ache scoring 9/10 on the visual analogue scale with occasional sharp shooting pains triggered by prolonged sitting and walking. She had been unable to work or perform her daily activities. On examination, the patient could not sit with weight on the left gluteal area; on standing she showed a flattened lumbar lordosis and she walked with an antalgic gait. Forward flexion, lateral flexion and extension of the lumbar spine were all restricted. Flexion, abduction, lateral rotation and medial rotation of the hip were also all limited. A sciatic stretch test was positive but there was no neurological deficit of the left lower limb.

Naproxen, gabapentin and pregabalin, alongside physiotherapy and acupuncture, failed to improve symptoms.

Investigations

MRI of the lumbar spine was performed but revealed no neurological compression or cause for symptoms. An MR arthrogram of the hip was ordered as symptoms were subsequently thought related to a labral tear and femoroacetabular impingement (FAI). The study revealed a 2.7×2.6×0.5 cm lipomatous mass applied to the anterior surface of the quadratus femoris muscle (figure 1A, B). The deep surface of the lesion extended partly into the muscle and was associated with some surrounding ill-defined T2 hyperintensity, reflecting surrounding oedema/inflammation (figure 2A, B). The ischiofemoral space was not narrowed by its osseous boundaries. The symptoms were primarily exacerbated by external rotation and extension of the hip and, given the oedema/inflammation around a sizeable intermuscular lipoma, the lipoma and quadratus femoris were likely impinged in the ischiofemoral space during such movements. A small focal (<5 mm) partial tear of the anterosuperior labrum was also noted but there were no other features related to FAI present. Small focal anterosuperior labral tears are commonly found in asymptomatic individuals and therefore was unlikely the cause of symptoms.

Figure 1.

(A) Coronal T1-weighted MR arthrogram image at the level of quadratus femoris muscle showing the lobulated lipoma (arrow) in the ischiofemoral space. (B) Corresponding coronal T2 weighted with fat suppression showing signal suppression confirming a fat-containing lesion.

Figure 2.

(A) Sagittal proton density-weighted image of the same study demonstrating the lipoma abutting the anterior border of quadratus femoris. (B) Axial T2-weighted turbo spin-echo at the level of hamstring origin showing complete suppression of the fat signal from the lipoma and the oedema surrounding the lesion (arrow heads).

The patient suffered with severe discomfort for nearly a year before eventually being referred to a tertiary orthopaedic clinic. Clinical signs and symptoms were not typical, however, following MR investigation, the possibility of IFI due to an intermuscular lipomatous lesion was highlighted.

Treatment

Given the significant symptomatology and following discussion with the patient, it was decided to proceed with exploration of the left quadratus femoris and excision of the lipomatous tumour.

The patient was placed in the right lateral position and, through a posterior incision, the gluteus maximus was split along the fibres, exposing the lipomatous tumour, quadratus femoris and the sciatic nerve. The sciatic nerve was carefully protected and part of the quadratus femoris with the entire lipomatous tumour was excised (figure 3). The excised specimen measured 5 cm. Haemostasis was obtained, the sciatic nerve was ensured free and the wound was closed in layers.

Figure 3.

Excised lipomatous tumour with a portion of the quadratus femoris.

Outcome and follow-up

Histology confirmed the presence of a benign intermuscular lipoma of the quadratus femoris muscle. Following the operation, the patient’s symptoms improved markedly with her visual analogue pain score, at most, 0.5/10. She was able to sit without any discomfort and there was no sign of ongoing sciatic nerve irritation or IFI. The patient returned to full time work and no longer requires any analgaesia. One year on, the patient remains symptom free with no restriction of hip movement.

Discussion

The quadratus femoris muscle is a flat quadrilateral muscle running along the horizontal plane and acting as one of the external rotators of the hip. It arises from the inferolateral margin of the ischium along the anterior portion of the ischial tuberosity, just anterior to the origin of the hamstring tendons. Its insertion is along the posterior and medial aspect of the femur, along the quadrate tubercle of the posterior intertrochanteric ridge. The quadratus femoris is bordered by the obturator externus muscle anteriorly and by sciatic nerve posteriorly. Inferiorly, it is bordered by the adductor magnus and along its superior margin by the inferior gemellus.¹ Owing to this anatomy, the quadratus femoris muscle and/or the thin layer of adjacent intermuscular fat can be impinged between the ischium and the lesser trochanter.

IFI syndrome is characterised by chronic non-focal groin or buttock pain. It typically affects middle-aged to elderly women, but cases in children and young adults have been recently published.^2–4 Bilateral involvement is common.^{5 6} The pain can radiate distally,⁷ mimicking sciatica, in cases where irritation of the adjacent sciatic nerve is found. Snapping of the hip^{3 5} or crepitus and locking⁸ have also been described. On physical examination, the patient's symptoms can be reproduced not only by a combination of extension, adduction and external rotation of the hip,⁷ but also with flexion and internal rotation of the hip.^{2 6}

The aetiology of IFI can be related to the bone or soft tissue structures. The responsible bony abnormalities, which reduce the ischiofemoral space, can be further divided into congenital and acquired (table 1).

Table 1.

Ischiofemoral impingement aetiology

Bony origin
Congenital	Acquired	Soft tissue origin	Positional origin
Female osseous pelvis	History of previous surgery to the hip joint and proximal femur	Hamstring tendinosis Chronic apophyseal avulsion fracture	In high-level athletes, eg, gymnasts, and in ballerinas, involving repetitive extreme external rotation
Low ischiopubic ramus	Hip arthrosis	Chronic bursitis	Hip abduction dysfunction
Prominent/enlarged lesser trochanter	Intertrochanteric fractures	Lipoma or other space occupying lesion
Increased cross-sectional area of the femur at the level of the lesser trochanter	Exostosis or any expansile bone lesion involving the boundaries of the ischiofemoral space
Posteromedial position of the femur

In Johnson's initial report,⁹ two patients with total hip arthroplasty and one patient after proximal femur osteotomy experienced pain, which was attributed to abnormal contact between the ischium and lesser trochanter. Since then, the list with the acquired bony abnormalities has been expanded to include patients without prior surgery, with intertrochanteric fractures involving the lesser trochanter, valgus osteotomies and any cases with arthrosis leading to superomedial migration of the proximal femur.^{8 10 11} A more unusual bony abnormality that has been recently added to the list of causes is exostosis in cases with multiple hereditary exostoses.^{12 13} Any expansile bony lesion arising at the level of lesser trochanter or ischial tuberosity could reduce the ischiofemoral space and lead to quadratus femoris muscle impingement.

The osseous configuration of the female pelvis is one reason why the vast majority of published cases refer to female patients. The female pelvis is characterised by an overall greater width and smaller depth, with prominent and widely spaced ischial tuberosities in comparison with the male osseous pelvis.⁵ Other unusual causes of congenitally narrowed ischiofemoral space include a low ischiopubic ramus, prominent lesser trochanter, large cross-section of the femur at the level of lesser trochanter and increased posteromedial position of the femur.^{5 11}

Rarely, pinching of the quadratus femoris muscle can be of positional aetiology. This can be found in athletes mainly training in extreme external rotation, similar to ballet dancers,¹⁴ or in football players who perform extreme hip manoeuvres.¹¹ A rare functional cause has recently been proposed occurring secondary to abductor dysfunction and the abnormal gait it elicits.¹⁵

Although hamstring tendinosis is a common entity, it is rarely reported to cause narrowing of the ischiofemoral space, or to contribute or even cause IFI.^{5 15} Chronic apophyseal avulsion fracture of the ischium with excessive callus formation is another unusual cause of symptomatic IFI that has been recently published.⁴

In chronic cases of impingement, a bursa can develop between the quadratus femoris muscle and the lesser trochanter,^{11 16} and can be associated with snapping.¹⁶

To the best of our knowledge, the case presented here is the only one in the English literature where the cause of IFI was a lipomatous lesion in the ischiofemoral space.

Apart from cases with bony abnormalities, radiographs are typically normal. Rarely, cystic changes and sclerosis may be seen at the ischial tuberosity or the lesser trochanter.⁸ Ultrasound has not been proven particularly useful in the diagnosis of IFI. MRI is the modality of choice, since ischiofemoral space narrowing, quadratus femoris muscle changes and the course of the sciatic nerve can easily be assessed.

Torriani et al⁵ and Tosun et al⁶ have described some MR parameters, and define cut-off values of the ischiofemoral space. The space is defined as the distance between the lateral cortex of the ischial tuberosity and the medial cortex of the lesser trochanter. The quadratus femoris space is the smallest gap between the superolateral surface of the hamstring tendons and the posteromedial surface of the iliopsoas tendon or lesser trochanter. These measurements are dependent on the degree of hip rotation during image acquisition. With the hip in internal rotation, cut-off distances of 17 mm and 8 mm have been found for the ischiofemoral and the quadratus femoris space, respectively.⁵ In addition, the inclination angle between the femoral neck and femoral shaft, the hamstring tendon area and the total quadratus femoris muscle volume, have also been measured by Tosun et al,⁶ in an attempt to increase the accuracy of the diagnosis of the IFI.

On MRI, quadratus femoris muscle oedema is the prominent feature of impingement. Some caution is required when muscle signal abnormality is found, as muscle strains or tears can appear similar. Apart from cases with disruption and retraction of muscle fibres, the differentiation between grade 1 muscle strain and impingement could be based on the location of oedema; in cases of impingement, the oedema is found within the muscle belly, at the site of maximal impingement,¹⁷ whereas in cases of strain, the oedema is usually found at the myotendinous junction, predominantly on the femoral side.^{16 18} Isolated strains of the quadratus femoris are extremely rare, and true strains usually take place in association with strains of the adjacent tendons or muscles, typically with a history of sudden onset pain. Mild enhancement has been observed following intravenous gadolinium administration.^{5 8} Fatty replacement or even isolated quadratus femoris wasting has been found in chronic cases. Wasting of the muscle could also be secondary to muscle denervation, although isolated damage to the nerve of the quadratus femoris is unlikely.¹⁵ Associated findings include bone marrow oedema or cyst formation of the ischium or lesser trochanter, as well as oedema or tears of the hamstring tendons or the iliopsoas insertion.¹¹

The differential diagnosis of this entity principally includes quadratus femoris muscle tear, as aforementioned, chronic hamstring injury and adductor tendonitis due to the close proximity of these structures. Furthermore, not infrequently, patients have been referred with clinical suspicion of piriformis syndrome, snapping hip syndrome or sciatica.

The definitive treatment of IFI is not yet clear. Conservative treatment is usually the first approach, with rest, restriction of the provoking activity, anti-inflammatory drugs and rehabilitation exercises. In cases with no improvement, ultrasound¹⁹ and CT¹⁴-guided corticosteroid injections or prolotherapy²⁰ have been applied in a few patients, with promising results. Prolotherapy is recognised to offer some benefit in the management of osteoarthritis.²¹ However, there is no available evidence to support its use in the management of IFI caused by a lipoma. Resection of the lesser trochanter has been originally described by Johnson⁹ and, although there is the possibility of postoperative weakness of hip flexion, improved patient outcomes and no associated morbidity have been reported.^{3 9} Periarticular endoscopic decompression of the quadratus femoris may also be effective.⁷ Last, but not least, excision of any space occupying lesion, as in our case, or in cases of exostoses,¹³ has been proven to offer pain relief, with no adverse outcomes reported.

Learning points.

• Hip and buttock pain is non-specific and can be multifactorial in aetiology. Clinicians and radiologists should bear in mind ischiofemoral impingement as a possible cause of hip and buttock pain.

• MRI is the modality of choice to depict the soft tissue changes related to the impingement and identify the aetiology.

• Soft tissue lesions, such as a lipoma, as in this case presentation, in the ischiofemoral space, are potentially a cause for quadratus femoris muscle impingement in the ischiofemoral space.