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Leicester Institute of Orthopaedics, University Hospitals of Leicester, Gwendolen Road, Leicester, Leicestershire, LE5 4PW, United KingdomLeicester Cancer Research Centre, University of Leicester, United Kingdom
The purpose of this pictorial review is to aid the clinician in distinguishing different benign lesions within the foot and ankle. We discuss the typical clinical and radiological findings as well as management options for intra and extra-compartmental lesions. Differentiation between sarcoma and benign lesions is imperative when presented with any mass in the foot or ankle.
Soft tissue and bone tumours can be differentiated through clinical acumen, imaging studies, and biopsy. For bone tumours radiographs are the primary modality of choice but for soft tissue tumours ultrasound and magnetic resonance imaging (MRI) are the modalities of choice.
The most important aspect when diagnosing soft tissue lesions is to not miss sarcoma. Unlike elsewhere in the appendicular skeleton and chest wall, acral sarcomas tend to be small (∼3 cm).
Wide excision of sarcomas is the primary treatment. When planning excision thought must be given to the surgical approach should further excision become necessary so as to contaminate as few compartments as possible.
The most common soft tissue tumours in the foot and ankle are ganglia and lipomata. This pictorial review illustrates typical clinical, radiological, and surgical appearances of common benign soft tissue swellings in foot and ankle. (Fig. 1)
Fig. 1(a) Clinical photograph, 1 (b) Surgical excision specimen, 1 (c) USS of ganglion, 1 (d) MRI of ganglion.
A ganglion is a cyst containing mucinous fluid. Although benign, ganglia can cause localised pressure effects resulting in pain or neurological signs. Symptoms due to mass effects are the most common primary presenting complaint.
Ganglia are more common in females and are most often located around the ankle and not within the sole or heel.
Ganglia can be associated with tarsal tunnel syndrome, a compressive neuropathy within the tarsal tunnel. Other causes for tarsal tunnel compression include lipoma, vascular engorgement, and synovial or bony abnormalities. Tinel's sign may be positive with percussion along the nerve. If wasting of abductor hallucis is noted then examination should look for a ganglion or soft tissue lesion within the tarsal tunnel.
Ultrasound of ganglia demonstrates a well-defined cystic lesion with mainly low internal echoic signals, posterior acoustic enhancement, and lobulations. MRI presents a well-defined mass with homogeneous signal, low in T1 and high in T2 weighting. Rim enhancement may be seen with contrast. The modality of choice is ultrasound scan (USS) which more clearly defines any space occupying lesions. Nerve conduction studies tend to show sensory changes opposed to motor.
Management of ganglia can be conservative if asymptomatic. Injection and aspiration can be considered as a temporising measure however the recurrence rate is close to 100%. If indicated then operative excision can be performed which has a 10% recurrence rate.
Ganglia are macroscopically firm, often oval in nature, with a smooth surface and dense on palpation, they contain differing volume of mucinous type substance. (Fig. 2)
Lipoma are benign fat cell tumour, commonly located in subcutaneous tissue (but can be intra or intermuscular). They most commonly occur in the fifth decade of life. Despite an equal sex distribution, males tend to have multiple lipomata where as women have solitary. Lipoma size can range from a few millimetres to greater than 5 cm, although they tend to be smaller in the foot. Patients usually present with a non-tender mass which is soft to palpation. Lipomata can often be present for many years without causing any noticeable effect.
Ultrasound demonstrates a hyperechoic mass, with the lesion usually being encapsulated. An MRI will have a mature fat signal with complete suppression on fat suppressed sequences. Incomplete fat suppressive images can be indicative of either an atypical lipomatous tumour or liposarcoma.
Lipomata can be managed non-operatively if asymptomatic. Marginal surgical excision is indicated if symptomatic, growing, or seen to have non-lipomatous components on imaging. (Fig. 3) Recurrence rates are 1–2%.
PNSTs are either schwannomas or neurofibromas. A schwannoma (neurilemmoma) is a benign tumour of the nerve sheath, arising from the schwann cells that produce myelin. Their underlying cause is unknown, schwannomata are slow growing, benign, and associated with neurofibromatosis type II. They are equally prevalent in males and females and occur most commonly between the third and sixth decades of life.
PNSTs can either present as a painful slow growing mass or remain asymptomatic for many years. Localised compression can produce paraesthesia along the distribution of the peripheral nerve and have positive Tinel's sign. The lesion itself is generally solid and fusiform.
Radiographically schwannomas have non-specific changes or may show diffuse soft tissue swelling with or without calcification dependent on degeneration and age of the lesion. Ultrasound may exacerbate pain and can be helpful in visualising the entering and exiting nerve. MRI is more helpful, demonstrating a well-defined mass along the neurovascular bundle, with or without enhancement based on the size and age of the lesion. The typical magnetic resonance (MR) appearance is iso-to-hyperintense (compared to muscle) on T1-weighted images, hyperintense on fluid-sensitive sequences, and often diffusely enhancing on contrast-enhanced images. Tissue heterogeneity is relatively common, particularly with cystic degeneration.
MRI will demonstrate a moderately bright signal on T1, that is oval in shape and less than 2.5 cm with an entering and exiting nerve often visible (Target sign). Nerve conduction studies can show prolonged sensory latency.
Features that favour schwannoma over malignant soft-tissue tumours include the split fat sign (rim of fat surrounding the tumour), the bright rim sign (high T2 signal at the periphery of the mass), absence of lobular shape (defined as <2 or more, deep lobulations), lack of extensive peritumoral oedema (extensive defined as >18 mm), and a defined capsule. These findings are reliable for a schwannoma, if two or more coexist.
The management for asymptomatic schwannomas is conservative. Symptomatic lesions can be marginally excised (ideally done under a general anaesthetic due to being exquisitely painful) however there is a risk of sensory paraesthesia or the development of neuropathic pain. Recurrence is uncommon but excision is associated with motor dysfunction.
Schwannomas are typically eccentric, arising between the nerve and the nerve sheath whereas neurofibromas arise from the nerve itself. The patient may exhibit the cutaneous stigmata of neurofibromatosis type 1 (Café-au-lait spots, freckling in the axilla and groin, and ocular Leish nodules). Neurofibromas which involve the nerve itself require sacrifice of the nerve and as such can result in paraesthesia, motor dysfunction or neuropathic pain.
At surgery, the entering and exiting nerve is identified and protected and the sheath is opened. The neurofibroma is present within the nerve along its distribution and greyish in colour, oval and encapsulated. If the symptoms are pain, simply decompressing the nerve may help. (Fig. 4)
Fig. 4(a) MRI coronal slice of plantar fibroma, 4 (b) Surgical photograph of plantar fibroma.
Plantar fibromas are benign desmoid tumour of the plantar fascia caused by the proliferation of mature fibroblasts. These lesions can be solitary or multiple and are often locally aggressive. There are numerous types of plantar fibroma; solitary fibroma, plantar fibromatosis –(multiple lesions are associated with Ledderhose disease) and can be associated with Dupuytren's diasthesis.
Plantar fibromatosis is ten-times more common in males than females, and most often seen in middle age Caucasians. Plantar fibromatosis can be locally aggressive, invading the neurovascular and tendinous structures, making complete excision difficult. However, unlike Dupuytren's disease, these nodules do not contract. Palpation of the lesions often elicits pain and reveals a firm, flattened nodule often on the anteromedial portion of the foot. Solitary fibromas are often located on the medial arch.
The sonographic presentation of plantar fibromatosis (Ledderhose disease) includes typically single (rarely multiple) iso-hypoechoic, well-demarcated, nodular thickenings of the plantar fascia, with no calcifications or fluid collection. Doppler ultrasound shows no vascular flow inside the lesion
MRI signal is low intensity on T1 and T2, the extent of these lesions can be viewed, aiding surgical planning for the more aggressive disease.
Fibromas will not regress without intervention. Accommodative shoe-wear with custom made orthoses to offload the fibroma can be used. Solitary fibromas can be managed with a peri-lesional corticosteroid injection, however this reduces the pain only and will not alter the size of the lesion. Pain is the main indication for removal but recurrence rates can be high, especially for the fibromatosis group (up to 60%).
More aggressive resection can reduce the risk of recurrence but increases the risk of post-operative complications and can result in increased discomfort on walking. (Fig. 5)
Fig. 5MRI T2 weighted image of large nodular tenosynovial giant cell tumour.
At surgery; these are well defined firm whitish nodules.
6. Tenosynovial giant cell tumours (TGCT)
Tenosynovial giant cell tumours are a rare, benign proliferation of cells arising from the synovium, tendon sheath, or bursae. TGCT can be solitary (nodular) or diffuse in nature. The diffuse type have a higher recurrence rate and tend to be more locally aggressive locally.
The diffuse type was previously referred to as pigmented villonodular synovitis (PVNS) and the nodular form as Giant Cell Tumour of Tendon Sheath. TGCT have a reported incidence of 44 per 100,000 for nodular type and 12 per 100,000 for diffuse type. The recurrence rate is reported at 9.8% for nodular and 19.1% for diffuse.
Nodular (nTGCT): The nodular form of the disease is not as locally destructive as the diffuse form. Clinically, this presents with a small nodule or firm tissue which may be connected via a stalk (these are pedunculated lesions).
Radiographs are often unhelpful for the solitary nodular form but can be used to exclude other causes of joint pain. USS as a dynamic examination tool, has a role in distinguishing a pedunculated from a superficial mass and is often hypoechoic. MRI elicits a low signal intensity on T1 and T2 due to the deposition of haemosiderin within synovial tissue.
Nodular TGCTs should be managed with a marginal excision. At surgery, these tumours are well circumscribed. The extra-articular TGCT may have a stalk. They are often multilobulated, light in colour and have a smooth surface. Intra-articular ones are readily amenable to excision through a limited arthrotomy. Arthroscopy is best avoided as this can convert a nodular tumour to the diffuse form.
Diffuse Type (dt-TGCT): Diffuse tenosynovial giant cell tumour are a proliferation of synovial villi and nodules and can be monoarticular or polyarticular. The underlying condition is benign but aggressive leading to joint erosion. The incidence of local recurrence is higher than the nodular form at 30–50%, with men and women equally affected.
The symptoms are joint pain, stiffness (pseudo-locking), effusion, and recurrent atraumatic haemoarthrosis (because the hypervascular villi bleed). If left untreated dt-TGCT can cause significant intra-articular destruction and degenerative arthritis.
Plain radiographs may not clearly indicate the early disease process but secondary signs may be due to mass effect and erosions with bone reparative processes showing sclerosis. Computerised Tomography (CT) shows synovial thickening with mildly increased attenuation because of the iron containing haemosiderin. MRI returns low signal intensity on T1 and T2 weighted images. MRI with contrast will enhance the lesions as they are hypervascular (with the diffuse disease form enhancing more than the local nodular form).
Non-operative management can be undertaken if the lesion is asymptomatic but there is risk of progression. Marginal excision of the tumour with radical synovectomy is the currently recommended surgical management. There is promising early clinical trial evidence of the efficacy of Pexidartinib in dt-TGCT.
At surgery the tumour has a brownish colour with a macroscopic pigmented appearance secondary to the haemosiderin deposition within the tissues. The synovium is hypertrophied and this generally involves the whole of the surrounding synovium. (Fig. 6) Recurrence rates are 55% (51%–58%) at 5 years, and 40% (35%–45%) at 10 years.
Synovial chondromatosis is a benign condition with multiple loose bodies of osteochondral origin contained within the synovial joint. This commonly monoarticular condition affects patients in the fourth to fifth decade of life. There is a increased incidence in men over women.
Patients present with reduced range of movement and a joint effusion due to the proliferation of the synovium. Other symptoms include pain, stiffness, locking and if the loose body is large enough. Large loose bodies can be palpable. Although osteochrondral bodies are intra-articular, they represent a soft tissue lesion.
These lesions can be chondral initially and therefore difficult to distinguish on plain radiographs, which classically show a fine stippled calcification within the joint. Effusion and erosions are seen in the later stages. MRI is helpful in early detection, especially in the chondral stage. They appear as intraarticular rim enhancing lesions which may contain a central fatty marrow and a peripheral intermediate signal on T2.
Non-operative management is appropriate if the patient has minimal symptoms. Operative treatment is removal of the loose bodies, most easily achieved with an open synovectomy. Recurrence rate is reported at 12.5%.
The synovium macroscopically appears multilobulated with whitish hyaline cartilage nodules (commonly spherical in shape) measuring between 1 and 2 mm and several centimetres. It is these nodules that can detach and cause the loose bodies. (Fig. 7)
Fig. 7(a) Surgical photograph of intra-operative chondromatosis, (b) Clinical photograph post excision.
It is essential to rule out Soft Tissue Sarcomas (STS) when evaluating any foot and ankle mass. Whilst the majority of standard red flag signs (painful, deep, and growing) still apply, it is essential to be mindful that the mean size of acral sarcomas is much smaller than that axial ones.
STS can mimic benign lesions and therefore it is important to manage all foot and ankle swellings as a potentially aggressive lesions until proven otherwise.
A high index of clinical suspicion, and appropriate imaging with USS and MRI are critical in the diagnosis and evaluation of synovial sarcoma. The STS are most commonly mistaken for a ganglion cyst due to the propensity of fat saturated sequence use in the MRI of limbs by musculoskeletal radiologists. Careful evaluation of T1 weighted imaging can demonstrate iso-intensity of the mass to muscle suggesting a solid lesion. T2 weighted imaging can demonstrate atypical stranding and a myxoid signal intensity which is atypical for a simple ganglion cyst. Contrast enhancement can demonstrate the presence of a solid lesion.
Suspicion of a STS in the foot and ankle should trigger referral to a specialist centre for investigation and management.
(c) and (d) deep dorsal lesion diagnosed as a ganglion. Both lesions are small (less than 2 cm) and were treated by excision biopsy proving to be synovial sarcomata.
Synovial sarcoma comprises of 8%–10% of all soft tissue sarcomas, representing approximately 6% of all soft tissue tumours in the foot and ankle, and is the most common sarcoma of the foot.
The majority of soft tissue lesions within the foot and ankle are benign in nature. Clinical signs and symptoms along with targeted imaging can aid diagnosis. We have reviewed the salient points for the most common soft tissue lesions and highlighted the management for each. The importance of distinguishing benign tumours from soft tissue sarcomas early will guide management of this patient cohort.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Acknowledgements
The authors want to extend our thanks to the patients who allowed for these photographs to be use for educational purposes.
References
Kirby EJ, Shereff MJ, Lewis MM. Soft-tissue Tumors and Tumor-like Lesions of the Foot. An Analysis of Eighty-Three Cases. (n.d).
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