Pseudotumoural soft tissue lesions of the foot and ankle: a pictorial review

Intermetatarsal bursa/bursitis

Inflammation of the intermetatarsal bursae is a known cause of metatarsalgia. The intermetatarsal bursa is a naturally occurring synovium-lined cavity between the metatarsal heads containing a small amount of lubricating fluid, located immediately dorsal to the deep transverse metatarsal ligament. This bursa is normally present in the first through fourth intermetatarsal space [16]. Studies in asymptomatic volunteers have shown that demonstration of a small amount of fluid in the bursae of the first through third intermetatarsal space on MR imaging cannot be considered a sign of bursitis. A transverse diameter greater than 3 mm of these bursae has been proposed as a cut-off value for suggesting intermetatarsal bursitis. However, demonstration of fluid in the fourth intermetatarsal space always appears to be connected to clinical symptoms [12].

Although the exact pathogenesis of intermetatarsal bursitis is unclear, it is thought to be the result of chronic mechanical compression and irritation between the metatarsal heads, similar to the suspected pathogenesis of Morton’s neuroma [17]. It is thus unsurprising that these disorders are frequently associated (Fig. 5).

MR imaging demonstrates a well-defined fluid collection at a typical location between the metatarsal heads with low SI on T1-WI and high SI on T2-WI. Subtle peripheral enhancement is seen following intravenous administration of gadolinium contrast medium [18].

Adventitious bursitis

Adventitious bursae may develop in adulthood at sites where subcutaneous tissue is exposed to high pressure and friction [19]. Under these conditions formation of a subcutaneous adventitious bursa starts as a coalescence of pre-existing small spaces in loose connective tissue. The walls progressively become differentiated from the adjacent connective tissue and a well-defined fluid-filled cavity is formed, which is lined by synovium-like columnar cells [20]. Unlike permanent bursae, which serve to reduce friction between skin and adjacent bony protuberances (e.g. over the olecranon), they lack a mesothelial lining [4].

In the foot and ankle, adventitious bursae may develop almost anywhere, usually adjacent to bony prominences. The most common locations are over the medial aspect of the first metatarsal head and plantar to the metatarsal heads.

MR imaging (Fig. 6) demonstrates an ill-defined lesion in the subcutaneous fat which is of low SI on T1-WI, and high SI on T2-WI, consistent with the enclosed fluid, often with intralesional string- or band-like structures with low SI [21, 22].

Although plantar fat pad alterations under the first and fifth metatarsal heads are a common incidental finding on MR imaging in asymptomatic volunteers, these are smaller than those in symptomatic individuals. These asymptomatic alterations are of low SI on T1-WI and T2-WI, and are believed to represent reactive fibrous tissue induced by mechanical stress (see “Miscellaneous disorders—Diabetic callus”) [22].

Epidermoid cyst

An epidermoid cyst results from the proliferation of keratin-producing epidermal cells within a circumscribed space of the dermis. Histologically, an epidermoid cyst appears as a keratin-containing cyst lined with surface epidermis. The source of this epidermis is nearly always the infundibulum of a hair follicle [23].

Epidermoid cysts are, therefore, most common in the hair-bearing parts of the body, such as the scalp, face, trunk and scrotum. Occurrence in the extremities is less frequent, and thought to be due to traumatic implantation of epidermal tissue in the dermis. As a consequence, the most common location in the foot is at the plantar or medial aspect of the first metatarsal head [24].

On ultrasound, an uncomplicated epidermoid cyst displays as an ovoid or spherical hypoechoic mass containing variable echogenic foci with posterior acoustic enhancement, without colour Doppler signals (Fig. 7). The scattered echogenic reflectors in the cyst are related to the lamellation of keratin debris. A ruptured epidermal cyst, however, may have lobulated contours and show colour Doppler signals, mimicking a solid mass [25].

On MR imaging (Fig. 8), an unruptured epidermoid cyst appears as a unilocular well-defined lesion, isointense or slightly hyperintense to muscle on T1-WI. Signal intensity on T2-WI is high, although irregular areas of low SI, reflecting the presence of dependent debris, are frequently noted [24, 26]. After intravenous administration of gadolinium contrast medium epidermoid cysts display faint wall enhancement [27].

If an epidermoid cyst ruptures, the cyst contents are released into the dermis, with formation of a keratin granuloma [28]. Ruptured epidermoid cysts tend to present as multilocular lesions, with enhancement of the cyst wall due to fibrosis and giant-cell reaction to keratin, possibly mimicking malignancy [29].

Granuloma annulare

Granuloma annulare (GA) represents a group of benign dermatoses of unknown aetiology which include cutaneous (localised, generalised or perforating) and subcutaneous types. This last type is referred to as subcutaneous granuloma annulare (SGA), occasionally also referred to as pseudorheumatoid nodule.

Histologically, SGA consists of a palisading granuloma with a small area of central necrosis [30].

The SGA is most common in otherwise healthy children between the ages of 2 and 5 years, but it can occur at any time from infancy to young adulthood [31].

Typically, it presents as a painless, immobile, solitary mass on the lower extremities, with normal overlying skin. Other, less frequent locations are upper extremities, buttocks, face and scalp. SGA will spontaneously regress without treatment and does not require biopsy or diagnostic imaging in clinically apparent cases. However, because of the subcutaneous location, clinical diagnosis can be difficult for non-dermatological specialists [32], which can result in a request for diagnostic imaging of a non-specific soft tissue mass.

Typically, GA appears on MR images (Fig. 9) as an ill-defined lesion in the subcutaneous fat that does not extend to the deep tissues or fascia, isointense to muscle on T1-WI, and heterogeneous, predominantly hyperintense on T2-WI [33]. Degree and pattern of contrast enhancement are variable [34].

The most important clues for the radiologist to suggest SGA on MR imaging are the patient’s age, location limited to subcutaneous tissue, indistinct lesion margins, and the previously mentioned signal characteristics on MR imaging. However, as imaging characteristics are non-specific, making the correct diagnosis in the absence of cutaneous abnormalities requires a biopsy.

Rheumatoid nodules

Rheumatoid nodules are granulomatous lesions with central areas of necrosis that occur in about 20% of patients with rheumatoid arthritis and less frequently in patients with rheumatic fever, systemic lupus erythematosus and ankylosing spondylitis [35].

These nodules can occur in the subcutaneous tissues in areas susceptible to trauma and in tissue overlying bursae, joints, tendons or ligaments. They are most common along the extensor side of the upper extremity. Less frequently they occur in the foot, typically at pressure points such as the heel pad and under the metatarsal heads.

MR imaging characteristics of rheumatoid nodules are non-specific (Fig. 10). They appear most commonly as ill-defined nodular lesions isointense to muscle on T1-WI with heterogeneous intermediate-to-high SI on T2-WI. On contrast-enhanced MR images, rheumatoid nodules display varying patterns of contrast enhancement, ranging from homogeneous enhancement in solid lesions with no central necrotic areas to heterogeneous increased SI or faint peripheral enhancement in nodules with less or more advanced central necrosis [36, 37].

Diagnosis can be suggested if associated MR signs of inflammatory joint and bone disease are present (e.g. erosions, joint destruction, bone marrow oedema).

Tenosynovitis

Tenosynovitis involves inflammation of the tendon sheath. The aetiology may include acute and chronic trauma, infection or inflammatory or metabolic disease. Excessive fluid accumulation in the tendon sheath can sometimes lead to a mass-like presentation [38].

Ultrasound examination reveals an effusion in the tendon sheath, along with thickening of the synovial lining. Doppler signals in the synovium and the adjacent soft tissues may be increased [39].

MR imaging demonstrates a fluid collection in the tendon sheath with high SI on T2-WI and low SI on T1-WI (Fig. 11). An abnormal thickening of the tendon sheath with enhancement after intravenous administration of gadolinium contrast medium may be seen. Associated signal alterations in the tendon indicate concomitant tendinosis [40].

Abscess

The primary cause of the development of an abscess in the foot is the infection of ulcers, which occur most frequently in diabetic patients. An abscess often communicates with sinus tracts that extend to bones, joints, tendon sheaths or skin ulceration. Clinically, associated cellulitis will be apparent. On MR imaging (Fig. 12), an abscess appears as a focal area isointense or hypo-intense to muscle on T1-WI, with fluid-equivalent (high) SI on T2-WI. There is thick rim enhancement after intravenous administration of gadolinium contrast medium [41]. Typically, there is evidence of adjacent soft tissue oedema. In diabetic patients, abscess formation is frequently associated with osteomyelitis and its characteristic bone marrow signal alterations [42].

Gout

Tophaceous gout is a pseudotumoural process that usually manifests in later stages of gout arthritis. A gouty tophus is an amorphous or crystalline mass of urate with a surrounding layer of inflammatory tissue [43]. Lesions typically occur at the metatarsophalangeal or interphalangeal joints [44]. In rare cases extensive tophaceous gout can result in multiple soft tissue masses with destruction of multiple bones of the foot [45].

Clinical history, laboratory examination and radiographic findings will usually steer the clinician to the correct diagnosis of tophaceous gout, but occasionally a gouty tophus can mimic a neoplastic or infectious process, for which the referring clinician requests imaging studies.

Gouty tophi, like rheumatoid nodules, display non-specific signal characteristics on MR imaging (Fig. 13): tophi typically are isointense to muscle on T1-WI, whereas the signal on T2-WI varies from being of homogeneous or heterogeneous high intensity to near-homogeneous low intensity [46]. This variability in SI on T2-WI could be due to differences in the amount of crystalline urate deposition, as well as the degree of calcification and fibrotic reaction.

On contrast-enhanced MR images, gouty tophi typically display a moderate to marked heterogeneous or near-homogeneous enhancement [47].

Tumoural calcinosis

Tumoral calcinosis is a disease characterised by lobular, calcified soft tissue masses that occur in periarticular locations, especially the hip, elbow, shoulder, foot and wrist. The masses are often multiple and bilateral [48].

Tumoural calcinosis is usually classified as primary (idiopathic or familial) or secondary. The primary form typically presents with soft tissue masses in the first or second decade of life, whereas the secondary form occurs mainly in patients with chronic renal failure, and has been attributed to secondary hyperparathyroidism [49].

On radiographs, tumoural calcinosis appears as multilobulated calcific deposits in a periarticular distribution, usually along the extensor surfaces of joints. The calcified masses may have a dense appearance or may contain fluid-calcium levels, known as the “sedimentation sign” [50]. Occasionally, pressure erosion on the underlying bone is seen [51].

MR examination is not required to make the diagnosis. If it is performed, tumoural calcinosis appears as a pseudotumour in periarticular regions with large septated regions of variable SI on both T1-WI and T2-WI [52].

Other metabolic disorders

Amyloidosis in rare cases results in the formation of pseudotumoural lesions at the foot and ankle. Amyloid deposition within the joint capsules or in the dermis can occasionally present as a soft tissue mass [53, 54]. Characteristically, the amyloid has a low SI on T1-WI, and a low to intermediate SI on T2-WI [55].

Tophaceous pseudogout caused by periarticular tumoural calcium pyrophosphate dehydrate (CPPD) or calcium hydroxyapatite (HA) deposition can occasionally mimic soft tissue or skeletal lesions [56]. On conventional radiography, the tophaceous lesions demonstrate calcification in a more granular and delicate pattern than the lobular-appearing calcification of tumoural calcinosis. On MR imaging, signal intensities of pseudogouty tophi can vary from low to intermediate on T1-WI and from low to high on T2-WI. Often areas of low signal are seen, corresponding to calcifications [57].

Diabetic callus

Soft tissue callus is a superficial soft tissue thickening that forms in response to mechanical pressure. Although benign and fairly common in the adult population, it occurs more often and builds up faster on the feet of diabetic patients. It develops in areas of abnormal weight-bearing and friction caused by diabetic foot deformities, and can eventually lead to ulceration and deeper infection.

Callus is commonly seen in the subcutaneous fat plantar to the first and fifth metatarsal heads.

On MR imaging, callus typically displays as an area in the plantar fat with low SI on T1-WI and low to intermediate SI on T2-WI (Fig. 14) [58]. After IV administration of gadolinium contrast medium low-level enhancement may be observed in and around callus [59].

Stress reaction and fracture

Post-traumatic bone abnormalities such as stress reactions and fractures may mimic a tumoural lesion of the ankle or foot. A clinical history of a professional sporting activity or daily activity with increased stress on foot and/or ankle may aid in the diagnosis of such lesions.

Stress reaction in soft tissues around the ankle presenting as a tender mass has been described in figure skaters and professional snow boarders at the level of the shoe rim [60]. On MR imaging, these lesions appear as ill-defined mass-like regions iso-intense to muscle on T1-WI, with increased SI on T2-WI and enhancement after intravenous gadolinium contrast medium administration.

Occasionally, marked callus formation secondary to a (stress) fracture or metatarsal osteochondrosis may mimic an osseous and soft tissue mass of the foot [61]. A linear signal alteration in the bone marrow with low SI on T1-WI and high SI on T2-WI, which becomes more obvious with contrast enhancement and fat suppression, should lead to the diagnosis of stress fracture [62]. Periosteal callus formation associated with flattening of the head of the second metatarsal bone in an adolescent patient indicates a Freiberg’s infraction (Fig. 15).

Myositis ossificans

Myositis ossificans (MO) is a benign condition of heterotopic bone formation, which can mimic soft tissue malignancy. It is very rarely seen in the foot and ankle region [63, 64]. Late-stage lesions display a zonal ossification pattern making diagnosis possible with conventional radiography [65]. Detailed discussion of the imaging features of MO is beyond the scope of this short review.

Plantar fibromatosis (Ledderhose disease)

The aetiology of plantar fibromatosis is still a matter of debate, but it is thought to represent a benign fibroblastic tumour, rather than a pseudotumour. Patients present clinically with painless, fixed, subcutaneous nodules on the sole of the foot, along the course of the plantar aponeurosis.

On ultrasound examination and MR imaging, plantar fibromatosis typically appears as a poorly defined mass in the deep aponeurosis in the medial aspect of the foot (Fig. 16). On both T1-WI and T2-WI the lesions are iso-intense or hypo-intense to skeletal muscle. Enhancement after IV gadolinium contrast medium administration is variable [66].

As plantar fibromatosis is considered to of a tumoural nature, detailed discussion of this entity and its imaging features is beyond the scope of this review.