A rare case of inter-compartmental giant lipoma of right distal forearm dorsal compartment invading anterior compartment via breaching inter-osseous membrane

Abstract

Lipomas are benign soft masses of adipose cells, which are often encapsulated by a thin layer of fibrous tissue and are most commonly present in head, neck, shoulders, and backs of patients. The tumors typically lie in the subcutaneous tissues of patients. Inter-muscular lipomas however, are rare and always occur deep within muscle compartments comprising 0.3% of all lipomas. Deep lipomas are most commonly found in the lower limbs followed by trunk, shoulders and arm. We present a rare case of deep dorsal inter-compartmental inter-muscular giant lipoma (>8cm) of right distal forearm leading to the formation of ganglion in the tendon sheath of Flexor Carpi Radialis (FCR) of anterior compartment as a result of breaching of inter-osseous membrane. MRI findings were indicative of a benign inter-compartmental lipoma of dorsal aspect of right distal forearm that erodes through distal inter-osseous membrane to enter the distal volar aspect of the wrist and therefore we did an excision biopsy and histo-pathological examination of the swelling reported an encapsulated tumour composed of mature adipose cells, which were uniformly arranged without high variations in size, consistent with benign lipoma. To the best of our knowledge, after an extensive search of literature we present this rare case of deep dorsal inter-muscular lipoma of forearm crossing over to the volar compartment resulting in ganglion formation in FCR tendon sheath.

1. Introduction

Lipomas are the most common benign musculoskeletal tumors comprising 50% of all soft tissue tumors¹ Inter-muscular lipomas however, are rare and always occur deep within muscle compartments comprising 0.3% of all lipomas². They are often asymptomatic, being identified only when they have reached a large size and/or compressing local neurovascular structures³. Deep lipomas are most commonly found in the lower limbs followed by trunk, shoulders and arm. We hereby present a rare case of deep dorsal inter-compartmental inter-muscular giant lipoma (>8cm) leading to the formation of ganglion in the tendon sheath of Flexor Carpi Radialis of anterior compartment via breaching the inter-osseous membrane.

2. Case presentation

A 45-year old female presented to us with a swelling involving dorsal aspect of her right wrist for the past one year, which was insidious in onset and size was progressively increasing in nature. The swelling was initially asymptomatic and later on started causing non radiating pain in distal dorsal forearm. The pain was present during both the periods of rest and activity. On examination, she had a soft & ill-defined palpable oval mass of size approximately 8cm over the distal forearm and wrist, which was tender to deep palpation, non-reducible, non-fluctuant & negative trans-illumination test. The surface of the swelling was smooth & was not associated with any distal neurovascular or functional impairment (Fig. 1).

Fig. 1.

Dorsal and volar swelling.

She underwent MRI, which revealed an encapsulated, lobulated (3 lobulations), non-septate lipoma of size 8cm in the dorsal aspect of distal forearm traversing into the volar compartment via inter-osseous membrane just above the wrist. The swelling was associated with minimal vascularity and was found to be kissing the tendon of Flexor Carpi Radialis anteriorly and thereby forming ganglion anteriorly (Fig. 2).

Fig. 2.

MRI revealing dorsal lipoma traversing into the volar compartment via inter-osseous membrane.

3. Surgical procedure

Under regional anaesthesia, dorsal incision was given over the radial aspect of the left distal forearm. Soft tissue dissection was done and the lipoma was found under the sub-tendinous plane (Fig. 3). The full extent of lipoma was dissected out and it was found to be breaching the inter-osseous membrane (Fig. 4)extending into the volar compartment (Fig. 5) and volar extension was kissing the FCR tendon sheath and thereby forming the ganglion which was excised in-toto (Fig. 6).

Fig. 3.

Lipoma in sub-tendinous plane of distal dorsal forearm.

Fig. 4.

On post resection of the dorsal extent of lipoma, it was found to be breaching the inter-osseous membrane.

Fig. 5.

Volar extention of Lipoma.

Fig. 6.

Lipoma after excision.

4. Histopathology

Histopathology reported an ill-encapsulated, non-septate tumour composed of mature adipose cells with minimal vascularity consistent with a benign lipoma. Histopathology report was consistent with MRI findings in terms of capsule, septa & vascularity.

5. Discussion

Inter-muscular lipomas are comparatively rare, comprising about 0.3% of fatty tumors and arise most often in the anterior abdominal wall². Inter-muscular lipomas are slow growing lesions that are referred to as giant lipomas when they exceed 5 cm in size¹. Due to the deep position of these lesions, they come to clinical attention only when they cause pain/discomfort because of stretching or pressure on adjacent structures³.

MRI is the imaging modality of choice for diagnosis of lipoma but definitive diagnosis can only be made by histology¹. MRI features such as size, depth, origin, septa, vascularity, fat content & enhancement are taken into account. Lipomas are divided into three subgroups as Benign Lipoma, Atypical Lipomatous Tumour and Lipo-sarcoma. Atypical Lipomatous Tumors (ALT) are considered low-grade lipo-sarcomas of the extremities, which rarely metastasizes have similar clinical and image characteristics compared to lipoma. They are generally found to have deep seated lipomas with thick/nodular septa on MRI. On Histopathology ALT shows adipocytes with marked size variations and many hyper-chromatic stromal cells are found around the thick septa⁴.

As far as Lipo-sarcomas are concerned MRI features such as large size, heterogeneity, irregular thickened septa, high degree of vascularity & low fat content go more in favor of lipo-sarcoma which warrant biopsy for further histological confirmation⁵. On Histopathology mono-vacuolated or multi-vacuolated lipoblasts are considered a hallmark of liposarcoma, although ALTs do not always contain lipoblasts⁶.

In our case, MRI findings were indicative of a benign inter-compartmental lipoma⁶ of dorsal aspect of distal forearm that erodes through distal inter-osseous membrane to enter the distal volar aspect of the wrist and therefore we did an excision biopsy and sent for histo-pathological examination, which reported an encapsulated tumour composed of mature adipose cells, which were uniformly arranged without high variations in size consistent with benign lipoma⁴.

The volar extention of lipoma was found to be kissing the tendon sheath ganglion of Flexor Carpi Radialis anteriorly. The most probable mechanism of FCR tendon sheath ganglion is due to the repeated grazing movements of the FCR tendon over the lipoma during wrist movements causing degeneration and inflammation of tendon sheath resulting in cystic out-pouching in weakened portion of tendon sheath.

6. Conclusion

Hence, we are reporting this case of benign inter-compartmental giant lipoma of distal forearm crossing the inter-osseous membrane into the volar compartment, because of its uniqueness on various aspects as deep dorsal inter-muscular lipoma of right distal forearm crossing over to volar compartment resulting in ganglion formation in FCR tendon most likely due to friction are unknown in the realm of medical literature, despite an extensive search to find a similar case.

Declaration of competing interest

In relation to this case report, I declare no conflict of interest.