The foot and ankle are relatively common sites for tumors. The lack of muscle coverage in this region results in a tendency toward early presentation1. Foot and ankle tumors are more likely to be benign than malignant: one large series from a general foot and ankle clinic reported that 16% of all tumors were malignant2. Even in a tertiary referral center, we have found that most tumors of the foot and ankle are benign. Osteochondroma is the commonest benign bone lesion and xanthomatous and giant cell-rich tumors are the most frequently encountered benign soft tissue lesions3.
Benign and malignant tumors in the foot and ankle can be clinically indistinguishable and therefore there is a risk of incomplete surgical excision if the preoperative planning is poor4. If there is a suspicion of a malignant bone or soft tissue tumor from the history, examination, and basic imaging, then prompt referral to a regional musculoskeletal tumor service is advised, so that appropriate imaging, biopsy, and staging can be performed and, once accurate diagnosis had been obtained, definitive management5.
This chapter focuses on the commonly encountered benign foot and ankle tumors and their management.
Tumors in the foot and ankle can be difficult to accurately diagnose clinically. In one series only 58 out of 101 tumors in the foot were accurately diagnosed prior to surgery6. A thorough patient history and clinical examination are important and can help guide imaging. A malignant tumor may be painless and very small, thus these features should never be used alone to distinguish between benign and malignant lesions.
Often the patient will present early with a painless mass that interferes with footwear or walking. If there is a history of pain, the timing, duration, severity, and response to analgesia are important. Osteoid osteomas classically present with nocturnal pain, relieved by NSAIDs.
Although foot and ankle tumors tend to be primary in origin, it is important to determine whether there has been previous exposure to ionizing radiation or chemicals, both of which predispose to development of a soft tissue sarcoma. As tumors grow, pressure effects on nerves may lead to numbness. Tendon subluxation, tendon rupture, and loss of joint motion are also occasional presentations. A family history may reveal a familial tendency to tumors, such as multiple hereditary exostoses.
Trauma and infection, especially after foreign travel, should be considered in the differential diagnosis. Rarely, tumors develop as the result of chronic infection.
The site, size, and consistency of the tumor should be recorded. Fluctuance, transillumination, and a smooth defined surface suggest a ganglion, which may move with tendon motion. Any tethering, either deeply or to skin, should be noted. The examination of a foot or ankle tumor should also include an assessment of joint, tendon, and neurovascular function.
Weightbearing AP and lateral radiographs with an oblique view are obtained7 for both osseous and non-osseous lesions. Tumor location can be an important predictor of the histological type, for example chondroblastomas have a tendency to be epiphyseal/subarticular. The radiolucency or radio-opacity of an osseous lesion is noted. Osseous lesions can be described in terms of their “zone of transition.” This defines the interface between normal and abnormal bone. A lesion with a narrow zone of transition often shows “geographic” bone destruction and in many circumstances is benign. When the transition between normal and abnormal bone is less obvious or “wide,” the growth rate is likely to be greater, which indicates a more aggressive, possibly malignant, bone tumor8. It is also necessary to establish what effect the osseous lesion is having on the bone: is there erosion or “scalloping” of the endosteum, or pathological fracture? Analyze what the bone is doing in response to the lesion: is there periosteal new bone formation? If there is a geographic lesion with surrounding sclerosis as the only major host-bone reaction, the lesion is more likely to be slow growing and benign. Calcification in the soft tissues may be observed on x-rays of soft tissue lesions; extra-osseous extension (and most bone lesions) should be further evaluated with MRI scanning.
Ultrasound is the investigation of choice for differentiating solid from cystic soft tissue tumors. It also shows the vascularity of masses and is particularly useful for superficial lesions. It is ideal for investigating suspected lipomas, hemangiomas, and nerve sheath tumors. It showed a sensitivity of 94.1% and a specificity of 99.7% in identifying malignant, superficial musculoskeletal tumors in a large case series9. Ultrasound is a dynamic investigation and allows an appreciation of the tumor relationship to the surrounding tendons and nerves. It can also guide the aspiration of simple ganglia.
Deep and solid soft tissue lesions should be further evaluated with MRI scanning, which gives useful information about the presence or absence of marrow infiltration, multifocal tumors, for example “skip” lesions in osteosarcoma, and the extent of the “reactive zone” around the tumor. The extent of this reactive zone is especially useful in planning resection margins and identifying very “active” tumors, such as osteoid osteomas.
Computed tomography scanning remains an alternative cross-sectional imaging technique, which provides greater osseous detail, but less tissue-specific information, and less accurate evaluation of extra-osseous extent and neurovascular involvement. It is occasionally needed for local tumor staging if there are contraindications to MRI. CT angiography can also help reveal vascular encasement by a tumor.
Technetium bone scanning assesses bone turnover and blood supply, and can be useful for osteoid osteomas and bone metastases. SPECT-CT can add further anatomic detail, which is typically lacking in standard SPECT.
The foot and ankle are unique in having a great number of articular surfaces, which serve as barriers to tumor extension. The epiphyses, however, are relatively thin with multiple vascular channels through which tumor extension into the soft tissues is possible. Within the forefoot, fascial compartments between the rays provide a relative barrier to tumor extension, although the lack of thick soft tissue coverage means bone tumors can easily spread into the soft tissues10.
Most tumors in bone and soft tissue require confirmation of histological diagnosis prior to surgery, and in our unit the vast majority undergo imaging-guided needle biopsy. On the relatively infrequent occasions when imaging appearances are diagnostic, excision biopsy will be performed and is sufficient treatment for benign or low-grade malignant lesions, aiming to remove the tumor and its reactive zone, avoiding potential tumor seeding to uninvolved tissue. However, poorly planned and executed biopsy may complicate subsequent limb-salvage surgery and therefore it is recommended that biopsy be performed in the tumor center, or the excision by the surgeon ultimately tasked with resecting and managing the tumor11.
The approach to a potentially malignant lesion requires meticulous planning at a multidisciplinary musculoskeletal tumor center. The planning involves a tumor surgeon, musculoskeletal radiologist, pathologist, and oncologist.
Staging provides the basis for further surgical and oncological management. Staging is determined by the clinical, radiological, and histological features. For benign tumors of bone and soft tissue the staging system is12:
Surgical margins are defined as follows13:
The planned choice of margin depends on:
the staging of the tumor
whether or not it involves neurovascular structures or joint surfaces
the anticipated functional deficit after tumor removal.
For soft tissue benign stage 1 and 2 lesions, a wide excision is ideal, providing this will not affect functional outcome, in which case marginal excision may be acceptable. For benign stage 3 lesions, a wide surgical margin is obligatory. If there is extensive involvement of a single metatarsal or toe, a ray amputation may be preferred10.
Benign Soft Tissue Tumors of the Foot and Ankle
Plantar Fibromas and Fibromatosis
These lesions are superficial fibroblastic proliferations – other common examples are Dupuytren’s and Peyronie’s disease. The fibromatoses include isolated plantar fibroma, juvenile aponeurotic fibroma, and deep (desmoid-type) fibromatosis14. Characteristically occurring along the medial border of the plantar fascia, either singly or multiply, they can be debilitating and painful due to their location under the medial longitudinal arch (Figure 17.1). They often become noticeable in adolescence, grow to 2 to 3 cm and then become indolent. Calcification within a more dorsally located fibroma is suggestive of a juvenile aponeurotic fibroma15. Desmoid-type fibromatosis can also occur in the foot and may be larger and more aggressive, with infiltration into the dermis, between the web-spaces and neurovascular structures. In those lesions that demonstrate rapid growth, a soft tissue sarcoma is the differential diagnosis and so the lesion should always be biopsied.
Figure 17.1 Plantar fibroma, M/31y. (a) Sagittal T1 and (b) STIR MRI of right foot. Small, elliptical mass arising from the plantar fascia at TMT joint level (white arrows). The mass is hypointense on fluid-sensitive sequences, in keeping with its fibrous content. (c) Coronal T2 MRI: lobular hypointense fibroma (white arrow) arises from the central band of the fascia (black arrow).
The mainstay of treatment of plantar fibromas should be non-operative with footwear modification, offloading insoles, and patient reassurance. There are isolated reports of success with intralesional steroid injections16.
Surgery should be reserved for painful lesions unresponsive to orthotic management or, occasionally, when there is evidence of neurovascular infiltration by aggressive fibromatosis. Wide excision of the fibromatosis and its adjacent fascia is recommended, paying close attention so that the skin incision is situated away from the loadbearing area of the foot17. Despite this, attempted surgical resection may be incomplete, leading to rapid local recurrence with additional scarring and pain. Dermofasciectomy with skin grafting has been performed for recurrent plantar fibromatosis, particularly when there is skin infiltration, but painful neuroma formation seems to be more disabling than the original fibromatosis18.
These rare tumors arise from a neuroarterial structure called the “glomus body,” which is thought to be responsible for thermoregulatory control, particularly in the digits19. They present as bright-red to bluish masses, and can occur beneath the nail. They are usually too small to palpate but are very painful, and especially tender to cold and direct pressure20. Radiographs may demonstrate a distal phalangeal bone erosion, but MRI is the investigation of choice. MRI shows a well circumscribed mass with a low signal on T1- and a high signal on T2-weighted images21. Treatment is by excision, occasionally necessitating subungual resection. Subungual melanoma should always be considered and the lesion should always be sent for histological analysis.
Many adult hemangiomas are more accurately classified as “malformations,” rather than neoplasms. The malformations arise from congenitally dysplastic vessels, often unappreciated at birth, which slowly enlarge and can cause pain and limb overgrowth and do not regress. The malformations may be high flow (arteriovenous malformations and fistulae) or low flow (venous, lymphatic, capillary)22.
Infantile hemangioma is a separate entity presenting in the first few weeks of life as a slowly enlarging benign vascular tumor, which then regresses.
These tumors can occur anywhere in the body, but are rare in the foot and ankle. In one review of 83 soft tissue tumors of the foot, only one hemangioma was found23. They can be associated with gigantism of the limb and equinus deformities of the ankle if they are intramuscular24.
Multiple hemangiomata should alert one to the possibility of an accompanying syndrome (Klippel–Trenaunay, Sturge–Weber, Maffucci, Proteus). Plain x-rays may show a soft tissue mass with multiple phleboliths. Ultrasound demonstrates low or no flow on Doppler, with tumor compressibility useful to help differentiate from sarcomas. On MRI, hyperintense vascular channels interspersed between the solid soft tissue matrix are seen on T2-weighted images, with fluid–fluid levels in areas of static blood and signal voids due to phleboliths25.
Hemangiomata in isolation do not usually require surgical excision and are best managed with compression hosiery and camouflage make-up of the skin for cosmesis. The infiltrative nature of hemangiomata means they are difficult to separate from normal tissue, often prompting sacrifice of uninvolved tissue and risking incomplete excision. The literature is made up of case reports of successful resection of hemangiomata in the foot26–27. If surgical excision is contemplated for a large hemangioma, preoperative embolization may be required in high flow-lesions. Ethanol sclerotherapy has been used with some success to reduce pain and induce lesion involution in symptomatic malformations. Sclerotherapy would be advocated over surgery in most cases28.
Neurilemmoma and Neurofibroma
Neurilemmomas are also known as schwannomas. They are benign tumors of the peripheral nerve sheath. The lesions are usually solitary, well-encapsulated, and situated on the surface of the nerve. In contrast, a neurofibroma is a spindle cell tumor, which is more permeative and less easy to separate from the surrounding normal nerve fibers. It can be solitary or multiple in association with neurofibromatosis29. Neurilemmoma in the foot is rare. In one series of 303 neurilemmomas, none were located in the foot30.
The usual mode of presentation is of a painful nodule with a positive Tinel sign in the nerve distribution. Neural tumors affecting the tibial or plantar nerves around or distal to the tarsal tunnel can mimic plantar fasciitis. The patient presents with deep-seated heel pain without the palpable nodule or Tinel sign, and this can be easily misdiagnosed as plantar fasciitis31.
Magnetic resonance imaging is the investigation of choice for neurilemmomas and neurofibromas. MRI can help differentiate a neurofibroma within the nerve from a neurilemmoma on the nerve surface, compressing and displacing the nerve32. The MRI scan may also demonstrate the target-pattern of central fibrocollagenous (Antoni A) and peripheral myxoid (Antoni B) tissue characteristic of a benign nerve sheath tumor (Figure 17.2)33.
Figure 17.2 (a) Neurofibroma, F/38y. Sagittal PD MRI shows an elliptical intraneural mass in the tibial nerve proximal to the ankle (arrow). (b) Axial PD fat saturated MRI shows the mass (arrow) has a slightly eccentric relation to the nerve and a faint target sign (relatively low signal center, hyperintense periphery), suggesting a benign nerve sheath tumor.
Neurilemmomas are well encapsulated and can usually be removed from the nerve without damaging the adjacent nerve fibers. Local recurrence and malignant transformation are rare. In contrast, the lack of a clear plane of dissection around a neurofibroma means the affected section of nerve often requires sacrificing.
Although lipomas are one of the most commonly encountered benign soft tissue tumors, they are relatively rare in the foot owing to the scarcity of adipose tissue. In a large series of 67 000 tumors and other lesions of the foot, lipomas only constituted 0.24%34.
Lipomas are subclassified depending on their morphological features and non-lipomatous elements. Thus there are conventional lipomas, fibrolipomas, angiolipomas, spindle-cell lipomas, myelolipomas, and pleomorphic lipomas. Conventional lipomas are soft, mobile, and painless and are treated by simple marginal excision35. Angiolipomas are frequently painful. Atypical presentations of lipoma variants include pain from tibial nerve compression within the tarsal tunnel36.
Subcutaneous lipomas are usually easy to diagnose. Deeper intramuscular, parosteal, or intraosseous lipomas often require imaging evaluation. Plain radiography may show large lipomas as areas of relative radiolucency compared to the surrounding tissue. Calcification is present in about 11% of lipomas. Ultrasound and MRI scanning are usually diagnostic. On MRI, incomplete suppression of signal on fat-suppressed sequences, enhancing thick nodular septa, and focal non-lipomatous areas should raise the possibility of an atypical lipomatous tumor/lipoma-like well-differentiated liposarcoma, but there is overlap in the imaging of these benign and malignant fatty lesions37.
Giant cells are found in many bone and soft tissue lesions. Tenosynovial giant cell tumor (giant cell tumor of the tendon sheath) arises in synovial spaces (joint, bursa, or tendon sheath) and is classified as either localized (also known as nodular tenosynovitis or giant cell tumor of the tendon sheath – localized type, GCT-LT) or diffuse (also known as pigmented villonodular synovitis/tenosynovitis or diffuse-type giant cell tumor – GCT-DT)38.
Giant cell tumor of the tendon sheath typically presents with a focal, slow-growing, painless nodular swelling in the foot, adjacent to a small joint or tendon. It can produce pain as a result of compressive effects. GCT-DT develops from the synovial lining of joints and typically presents with painful or painless swelling and recurrent hemarthrosis.
Plain radiographs of GCT only rarely show soft tissue calcification adjacent to a small joint or tendon but may show bone erosion. MRI evaluation characteristically identifies a hemosiderin-laden soft tissue matrix of low signal intensity on T2-weighted images (exaggerated on T2 gradient echo (T2*) images), helping to differentiate tenosynovial GCT from other soft tissue tumors. Biopsy of both localized and diffuse GCT is advised: periarticular malignancies such as synovial, clear cell, and epithelioid sarcoma can certainly mimic localized GCT. Wide excision is the treatment of choice as intralesional resection leads to a high rate of local recurrence39.
GCT-DT (Figure 17.3) arises from the synovial lining of joints but may be intra- or extra-articular. Joint aspiration of a brownish, hemosiderin-laden fluid is diagnostic, but imaging with plain radiography and MRI is necessary to establish whether the recurrent hemarthroses have caused joint erosions and the extent of the disease. GCT-DT can extend for long distances along tendon sheaths. Meticulous surgical planning is advised – ideally in a multidisciplinary setting since the recurrence rates with incomplete excision of the disease are high.
Figure 17.3 Tenosynovial giant cell tumor/pigmented villonodular synovitis, F/56y. (a) Sagittal and (b) axial proton density (PD) MRI of the left hindfoot showing a hypointense tumor in the posterior subtalar joint (arrows), anteriorly extending into the sinus tarsi. There is deep erosion of the calcaneum (*).
Although arthroscopic synovectomy is occasionally possible, the extent of disease often precludes complete excision. Therefore arthrotomy, if necessary using multiple approaches, is the treatment of choice. Radiosynovectomy with yttrium-90 has been utilized as an adjunct after subtotal surgical synovectomy in the knee. In the ankle it has resulted in severe complications, including full thickness skin necrosis, and is therefore not recommended40.
Synovial chondromatosis is a process of chondral metaplasia within the synovium producing multiple cartilaginous bodies, which become detached and lie loose in the ankle joint. They can cause pain, locking, instability, or a palpable mass.
Plain radiographs may show multiple smooth, spherical loose bodies if they are calcified. MRI characteristically shows the loose bodies as low signal on T1-weighted images and high signal on T2-weighted images with multiple signal voids of calcification41. Occasionally, MRI shows a chondral mass with no significant punctate mineralization, arising from a joint or within a tendon sheath (Figure 17.4). Radiographs are useful to show mineralization in these cases.
Figure 17.4 (a) Synovial chondromatosis in the left great toe, F/56y. Oblique radiograph shows pressure erosion of the plantar surface of the proximal phalanx distally (*) and calcified masses on the dorsal and plantar aspects of the toe, arising from the interphalangeal joint (arrows). (b) Sagittal T1 MRI shows a lobular hyperintense mass arising from the dorsal aspect of the interphalangeal joint. (c) Axial PD fat-saturated MRI shows the mass, which is homogeneously hyperintense, on both the dorsal and plantar aspects of the joint.
Synovectomy and retrieval of the loose cartilaginous bodies is often required as longstanding chondromatosis can lead to osteoarthritis, necessitating arthrodesis or joint replacement. Synovial chondromatosis in the foot and ankle is rare, with only small cohorts reported in the literature. Chondrosarcomatous differentiation has been reported and it is suggested that in cases with rapid recurrence, especially with significant erosive joint destruction, repeat biopsy should be performed42.
Ganglia are well-encapsulated fluid-filled lesions arising as a result of mucoid degeneration of tendon sheaths and joint capsules. Treatment is reserved for ganglia producing pressure or mechanical symptoms, as otherwise they tend to be painless. The cystic nature of the lesion should be confirmed with USS prior to intervention. At aspiration, a gelatinous clear fluid is obtained and this helps confirm the diagnosis. MRI scanning (Figure 17.5) is helpful to find the origin of the lesion and plan excision, the aspiration track being excised with the tumor. Marginal excision is the treatment of choice for symptomatic ganglia, ensuring excision of a portion of the degenerate joint capsule or tendon sheath. The patient should be warned that the recurrence rate of foot and ankle ganglia is reported as being up to 43%, which is higher than in the wrist43.
Figure 17.5 Ganglion, F/73y. (a) Coronal T1 MRI of the left forefoot shows an elongated fluid signal mass (*) adjacent to the third metatarsal (3). (b) Axial T1 and (c) T2 MRI demonstrate the ganglion (*) abutting the third and fourth metatarsals.
Most common in the tendo Achillis, xanthomas are pathognomic of familial hypercholesterolemia. They manifest clinically as discrete fusiform swellings within the tendon (Figure 17.6) and can be painful, but are usually painless. Xanthomas are infiltrative and difficult to dissect free from the tendon substance, necessitating reconstructive procedures rather than simple debulking. Management includes medical referral for investigation and management of hypercholesterolemia44.
Figure 17.6 Tendon xanthoma, F/62y. (a) Axial T1 and (b) PD fat-saturated MRI showing a speckled/reticulated appearance of the tendo Achillis. Sagittal scans showed fusiform enlargement but little internal signal change. (c) Longitudinal ultrasound image shows a lobular area of low reflectivity, bulging the superficial aspect of the tendon (*), blending with more normal tendon fibers (arrows).
Infection should always be entertained in the differential diagnosis of a patient with a mass, particularly when accompanied by a history of an inoculating foreign body. Not uncommonly, bacterial infections such as tuberculosis and more indolent fungal infections such as Madura foot, particularly in patients from overseas or with a history of travel, are encountered and the relevant investigations including blood tests, imaging, and even biopsy should be undertaken45.
Benign Bone Tumors
The surgical treatment of benign bone tumors follows the same principles as those of benign soft tissue tumors. Stage 1 and 2 lesions are most often treated with intralesional curettage. The resulting bone defects are managed with bone graft or, in the case of large voids left by giant cell tumors, polymethyl methacrylate cement. Cement augmentation not only provides structural support to the surrounding native bone, but the heat generated during polymerization also sterilizes the reactive zone. It may be appropriate to treat some stage 2 and 3 lesions with wide surgical resection. When the tumor involves the articular surfaces it is better to arthrodese the joint to prevent late local recurrence46.
Solitary lesions rarely occur in the foot and ankle. They arise from the paraphyseal region of growing long bones. They develop as a consequence of the aberrant growth of cells from the proliferative zone of the physis, and hence appear during childhood and cease growing when the physis fuses. They characteristically have an osseous stalk directed away from the physeal plate with a thin (<2 cm) cartilage cap. The normal metaphyseal bone is said to “flow into” the osseous stalk47. Osteochondromas are benign, but the cartilage cap undergoes malignant transformation into a chondrosarcoma in approximately 1% of non-syndromic lesions. This presents with an increase in size and pain after physeal closure. Benign lesions can, however, cause symptoms as a result of the local mechanical irritation of tendons and nerves. Pain is an indication for excision.
Hereditary multiple exostoses or diaphyseal aclasis is an autosomal dominant disorder with a greater incidence of malignant transformation, in which the exostoses are larger and the metaphyses are widened and dysplastic. In this condition the exostoses can cause growth arrest and potentially ankle deformity, which may require corrective surgery48.