Synovial Lipomatosis of Ankle with Sural Nerve Neuropathy: A Rare Case Report

Abstract

Synovial lipomatosis, also known as lipoma arborescens, is a rare benign condition characterized by villous fatty proliferation of the synovium with replacement of the subsynovial tissue by mature adipocytes. Although it predominantly affects the knee joint, involvement of the ankle is distinctly uncommon. We report the case of a 52-year-old woman who presented with progressive left ankle pain and swelling of 1 year duration, associated with mechanical symptoms and restriction of motion. Physical examination revealed a posterolateral ankle swelling with mild limitation of range of motion, while laboratory parameters and rheumatologic markers were within normal limits. Plain radiography demonstrated a soft-tissue swelling with a calcified/bony mass, and magnetic resonance imaging revealed diffuse synovial proliferation with associated bone marrow edema. The patient underwent open ankle arthrotomy with excision of a loose calcified body and synovectomy. Histopathological examination confirmed synovial lipomatosis, showing fibrocollagenous tissue lined by hyperplastic synovial membrane with infiltration of mature adipocytes in a myxoid stroma, without evidence of malignancy. Postoperatively, the patient achieved complete symptomatic relief with restoration of normal ankle function at 6 months of follow-up, and no recurrence was noted at 1 year. This case highlights synovial lipomatosis as an important differential diagnosis for chronic ankle swelling and supports surgical synovectomy as an effective treatment with excellent clinical outcome and low recurrence.

INTRODUCTION

Synovial lipomatosis (synovial lipoma, also known as lipoma arborescens) is a rare benign intra-articular disease with villous lipomatous growth of the synovium accompanied by diffuse infiltration of the subsynovial connective tissue with mature adipose cells.[1,2] The underlying mechanisms of synovial lipomatosis are not clear, as yet more often than not, it has been defined as a secondary reactive disease that may occur after long-term mechanical irritation, trauma, degenerative disease of the joint, or inflammatory arthritis.[1,2,3,4] Although the knee joint, in particular the suprapatellar pouch, is the most commonly affected area in the population (60%–70% of reported cases), hip, shoulder, elbow, wrist, and ankle joint involvement has been reported. Synovial lipomatosis of the ankle joint resulting in sural nerve neuropathy is very unusual, and to date, a small number of cases have been reported in medical literature.[5,6,7] Here, we describe a rare history of ankle-associated synovial lipomatosis, demonstrating the clinical picture, diagnostic imaging, histopathological symptoms, and surgical management of an uncommon disease.

CASE REPORT

A 52-year-old lady presents to the Orthopaedic Outpatient Department with left ankle pain and swelling for 1 year. There was no history of fall or trauma. Lateral numbness to the left heel and foot. There was diffuse swelling above the posterolateral aspect of the ankle on examination where progressively progressive development was noted. The skin over the top was normally smooth, with no evidence of erythema, warmth, or dilated veins. Pain started as a minor irritable pain and gradually worsened over time, affecting activities of daily living, especially during dorsiflexion and plantarflexions.

The swelling was mainly over the posterolateral axis, with 4 cm × 3 cm and no local increase in temperature. It was steady, firm, tender, nonpulsatile, with smooth borders and adherent on the underlying structure. The ankle joint had a limited range of motion in dorsiflexion of 15° and plantarflexion of 35°. It seemed to reduce over the lateral side of heel and foot supplied by the sural nerve compared to the contralateral foot.

Complete blood count, erythrocyte sedimentation rate, C-reactive protein, rheumatoid factor, HLA-B27, anti-CCP level, and serum uric acid were within normal limits. Radiographs of the left ankle indicate an ovoid bony mass with a radio-opaque margin superior to the calcaneum and posterior to the talus. Joint space was without any evidence of narrowing, soft tissue is intact, and there is no evidence of swelling [Figure 1]. The magnetic resonance imaging (MRI) showed a calcified mass that appeared superior to the calcaneum and posterior to the talus. It had a morphology such as Tibia, talus, and calcaneum to the periphery of the cortex. Suspicious cartilaginous connection appeared also between the bony mass and talus [Figure 2].

Figure 1.

Preoperative radiographs of the left ankle: (a) lateral view showing the posterolateral calcified/bony mass (red arrow), (b) anteroposterior view, and (c) mortise view

Figure 2.

Magnetic resonance images of the left ankle in multiple planes demonstrating the posterolateral lesion (red arrows): (a, b) axial images and (c, d) sagittal/coronal images showing synovial proliferation with the associated calcified mass

A preliminary diagnosis of intra-articular soft tissue pathology was reviewed (differential diagnosis of synovial lipoma, ganglion cyst, synovial hemangioma, pigmented villonodular synovitis (PVNS), synovial chondromatosis, synovial hemangioma, rheumatoid arthritis, gouty arthropathy). Precise differentiation is essential, as they differ in both treatment modalities and prognoses.

Recurrence and complete synovectomy would seem rare based on limited follow-up data in the literature. The clinical, radiological, and imaging analysis showed that surgical excision followed by biopsy should be performed in order to confirm the diagnosis on the bony mass. Informed written consent was given, and the patient underwent open excision at the site of the bony mass under spinal anaesthesia. A 5 cm wound was made using the posterolateral approach to the procedure inside. The sural nerve was found, and the nerve was compressed by the bony tissue. The nerve was gently withdrawn with a deep dissection. The peroneal tendons were retractively retracted, and the bony mass in the stalk was also identified and excised. The specimen was submitted for histopathological examination [Figure 3].

Figure 3.

Intraoperative and gross specimen photographs: (a) posterolateral surgical incision, (b) intraoperative exposure showing the excised loose calcified body (yellow arrow), and (c, d) gross specimen images of the removed calcified body

However, the specimen was grossly examined and a single grey-white-soft to hard tissue 3 cm × 2.5 cm × 1 cm (soft tissue measure = 2 cm × 1.5 cm × 0.6 cm). The specimen was prepared, and multiple sections were studied. Histopathological examination revealed fibrocollagenous tissue coated with villous and hyperplastic synovial membrane. The sections showed infiltration of mature adipocytes within the synovium, enclosed with myxoid stroma [Figure 4]. Significantly, there was no abnormality in cellular morphology, mitotic activity, or pathogenesis. Diagnosis was made as surgical excision of the left ankle loose body comprising histologically normal fragments of bone and fibrocartilagenous tissue, characteristic of synovial lipomatosis.

Figure 4.

Histopathology showing (a) hyperplastic synovial membrane lined by synoviocytes with subsynovial infiltration by mature adipocytes and (b) mature adipocytes with peripheral nuclei extending into the subsynovial myxoid stroma. No cellular atypia or mitotic activity is seen

The postoperative period was simple. The patient was placed on proper analgesics and physiotherapy on the second postoperative day. Two weeks later, weight bearing on the affected limb was done, and suture removal was performed. A 3-month follow-up showed full range of motion and a complete sensory recovery at 5-month follow-up with equal sensations felt on each side. At 1 year of follow-up, no recurrence is seen [Figure 5].

Figure 5.

Postoperative radiographs of the left ankle after excision of the bony mass: (a) lateral view, (b) anteroposterior view, and (c) mortise view

DISCUSSION

Synovial lipomatosis of the ankle is a very rare benign synovial disorder with only a low number of cases reported in the English literature. The term arborescens reflects the characteristic frond-like synovial proliferation composed of mature adipocytes.[1,2] Originally described by Hoffa in 1904, this lesion is considered a reactive, nonneoplastic process. There are two forms of synovial formation primary synovial lipomatosis, which develops spontaneously in younger people with no underlying pathology, and secondary synovial lipomatosis, typically seen in degenerative or inflammatory joint disease.[1,2,3,4] Although the knee joint accounts for approximately 60%–70% of cases due to its large synovial surface and abundant fat pad, involvement of other joints such as the hip, elbow, shoulder, wrist, and ankle has been recognized.[5,6,7] The comparatively rare occurrence of ankle involvement could either result from real anatomic discrepancies or from incomplete or erroneous reporting. In 2007, the first specific description of ankle joint disease was recorded, with the combined intra-articular and tenosynovial involvement emphasized. Clinically, patients have chronic, progressive swelling, pain, mechanical symptoms and joint limitation of motion. Since the presentation is nonspecific, diagnosis is often delayed, and symptoms are initially assigned to the following disorders: synovitis, osteoarthritis, and inflammatory arthropathies. Due to the presence of long-standing synovial lipomatosis, secondary osteoarthritis is potentially affected by mechanical impingement, chronic synovial irritation, and biomechanical disruption, confirming the importance of accurate diagnosis.

MRI remains the imaging modality of choice. Pathognomonic findings are villous or frond-like projections with signal intensity identical to fat on all pulse sequences and complete suppression on fat-suppressed imaging, and effusion at the joint surface.[6,8,9] Such features usually permit a confident preoperative diagnosis. Histopathology, however, continues to be the gold standard. Diagnostic criteria consist of mature adipocyte infiltration of the subsynovial tissue, villous synovial profile, and intermittent fibrosis with or without chronic inflammatory cells.[2] The myxoid stroma that appeared in this report is consistent with reported differences across stages of pathological evolution.

Conservative therapy typically offers only temporary relief and does not halt the progression of the disease. Nonsteroidal anti-inflammatory drugs or activity modification might lessen symptoms, whereas yttrium-90 has been tried as a first step in an experiment of radionuclide synovectomy but not widely accepted for clinical use.[10] Surgical excision (arthroscopically or open as needed, depending on the extent of the lesion) is the definitive treatment, and outcomes are favorable and rates of recurrence are low when synovectomy is complete.[5,7,10] The differential diagnosis is broad, including PVNS, synovial osteochondromatosis, synovial haemangioma, synovial lipoma, and inflammatory arthropathies.[6,8,9]

Most commonly, the following entities are distinguishable through MRI: – PVNS has a diffuse low T1/T2 signal owing to hemosiderin with blooming on gradient-echo sequences.– Multiple calcified or cartilaginous loose bodies (synovial osteochondromatosis).– Intermediate signal intensity is revealed with vascular channels and phleboliths in haemangiomas.– Synovial lipoma is manifested as a single encapsulated fatty tissue and not as diffuse subsynovial fat expansion. Our 52-year-old female patient presented with chronic ankle pain, swelling and tingling sensation for the past 12 months. The demographic profile reflects the typical spectrum of disease, which is characterized by predominantly middle-aged adults. Other cases reported related to these include Oesman et al. on 50-year-old women and in Wankhede et al. on a 28-year-old male who confirmed that, although the disease ranges at a wide age range, it mostly affects the 50-year-old population. The basis for this disparity is unclear, although male predominance is described in some series. The numbness of the lateral heel and foot in our patient was ascribed to sural nerve compression (an occurrence rarely mentioned in reports).

In this case, radiographs demonstrated a bony mass with radio-opaque margins, consistent with benign pathology, and without erosive change, narrowing of joint space, or loose bodies, as one would expect from more aggressive entities. MRI showed evidence of synovial mass formation with fatty infiltration, in accordance with synovial lipomatosis diagnostic criteria, which informed the treatment planning. Diagnosis was confirmed by histopathology with mature adipocyte infiltration, villous synovial proliferation, hyperplastic lining, and myxoid stroma. No atypia, mitotic activity, or malignant features were noted, thus ruling out liposarcoma. There has been considerable debate about its pathophysiological mechanism, however current evidence supports a reactive etiology secondary to chronic irritation or microtrauma and less of a true neoplastic and proliferative pathway. The lack of underlying trauma or degenerative disease in our patient supports a primary form of synovial lipomatosis. The surgical management was open ankle arthrotomy, excision of the loose body and synovectomy. For extensive lesions or joint area involving small joints, open excision is particularly advantageous as arthroscopy may be impractical in this setting. Synovectomy must also be done, and incomplete removal is at risk of recurrence. After surgery, we found 6-week significant improvement between surgery pain and swelling as well as full, pain-free range of movement, sensory recovery and complete absence of recurrence at 1-year follow-up. These results correlate to literature that suggests excellent outcome levels following successful completion of synovectomy with very low recurrence.

In this case, the presence of a loose calcified body is remarkable. Loose bodies, while more suggestive of synovial osteochondromatosis or degenerative disease, have been associated with synovial lipomatosis and probably signify chronic mechanical irritation or metaplastic change. Histology was in our case of benign bone and fibrocartilaginous tissue with no osteochondromatosis features, suggesting secondary and in contrast to an overt pathology in this patient. This case highlights the rationale for considering synovial lipomatosis in the differential diagnosis of chronic atraumatic ankle swelling. If symptoms persist or are atypical, early MRI examination should be recommended. It is crucial to have patient counselling regarding the benign behavior of the condition, a good surgical prognosis, and low recurrence risk when considering management.

CONCLUSION

Synovial lipomatosis of the ankle joint represents a rare benign condition that poses diagnostic challenges due to its infrequent occurrence and nonspecific clinical presentation of neurological deficit. A high index of suspicion is required for this entity in patients presenting with chronic atraumatic ankle swelling and pain. Timely surgical excision represents the treatment of choice and yields excellent clinical outcomes with low recurrence rates as evidenced by our patient’s symptom-free status at 1-year follow-up, underscores the benign nature of this condition.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient has given her consent for her images and other clinical information to be reported in the journal. The patient understand that name and initials will not be published and due efforts will be made to conceal identity, but anonymity cannot be guaranteed.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.