Normal muscle variants

These can present as a clinically palpable mass. They have presumably been present for years but for some reason the patient has only just noticed the swelling. One of the commonest sites is at the ankle where an accessory soleus muscle may mimic a soft tissue mass. Knowledge of the normal anatomy will assist in making the diagnosis along with the observation that the area of interest has identical signal to adjacent muscle on all MR sequences. The palmar aspect of the wrist is another area where anomalous developments in the flexor tendons may sometimes result in compression of the ulnar or median nerves in their fibro-osseous tunnels.

Muscle tears

Symptomatic muscle tears are not usually a problem since there is typically a history of trauma and painful onset of swelling. Clinically there is therefore no suggestion that this is a soft tissue mass. However some patients, particularly in an older age group, present with a painless swelling with no clear history of any single incident to account for it. Characteristically this is seen in the anterior thigh. The reason is spontaneous rupture of rectus femoris muscle. The proximal muscle can contract unopposed with a resulting soft tissue mass palpable in the thigh. In cases of clinical doubt an MRI scan will confirm that the swelling is muscle and demonstrate the defect at the site of rupture (see Fig. 18.29, p. 414).

Haematoma

An acute tissue haematoma, whether from injury or post-surgery, should not pose a problem for clinical diagnosis. Difficulty can occur when a large deep haematoma does not resolve and enters a chronic phase that may mimic the swelling from a recurrent lesion. The MRI scan should aid diagnosis showing a peripheral dark rim from the presence of iron-containing haemoglobin breakdown products (Fig. 10.1).

Fig. 10.1 A and B T2 weighted axial and sagittal MR scans of a chronic haematoma in the calf. There is a well-circumscribed mass which is of high signal in the posterior compartment. The rim is very low (dark) signal indicating haemosiderin deposition seen with breakdown of blood.

Aneurysm

Occasionally an aneurysm originating from a deep vessel may mimic a soft tissue tumour, but careful clinical examination will normally identify arterial pulsation. Imaging with MR and digital arteriography can confirm the nature of the lesion and the vessel of origin.

Synovial cysts and ganglia

Ganglia are of unknown origin situated typically around joints and contain mucoid tissue. Trauma and synovial herniation are suggested as causative factors. They do not usually have a demonstrable connection with the underlying joint. They are seen most commonly around the wrist and ankle, but also occur near the knee arising from the proximal tibio fibular joint (Fig. 10.2).

Fig. 10.2 A and B T2 weighted axial and sagittal MR scans showing a large ganglion arising from the proximal tibio-fibular joint. The high signal of the well-circumscribed mass in the anterior compartment suggests a fluid content. The sagittal image demonstrates the long thin proximal extension from the joint.

Synovial cysts are a synovial lined outpouching which connects with the underlying joint space.

These therefore occur around joints most commonly and are typically seen especially in the knee. Popliteal (Baker’s) cysts (Fig. 18.32A, p. 418) and cysts associated with meniscal tears can present as palpable soft tissue masses. Ultrasound is the easiest method of confirming that these are cystic and not solid lesions.

Myositis ossificans (heterotopic ossification)

This is an uncommon but troublesome lesion because of its apparent malignant behaviour. It presents as a painful and tender lump in a limb muscle, with or without a history of recent trauma to the site. Imaging is crucial to obtain the correct diagnosis and MRI and ultrasound are the most useful in the very early stages as they can detect the presence of calcification in the lesion which is the key to diagnosis (Fig. 10.3A). Later imaging with CT scanning will reveal a diagnostic rim of ossification at the periphery of the well-circumscribed lesion. Plain radiographs after several weeks will show some calcification in the muscle which should indicate the correct diagnosis (Fig. 10.3B). However, in some patients where the early changes are not recognised these appearances may be misinterpreted and may lead to a presumed diagnosis of soft tissue osteosarcoma. This assumption may be reinforced if a biopsy is performed, as this will show a very active lesion containing immature mesenchymal cells with atypical nuclei and osteoid formation. No active treatment is required, other than careful observation, as the lesion will resolve in 2–3 months.

Fig. 10.3 A T2 weighted axial MR scan of the arm with myositis ossificans in the biceps muscle. Inside the mass of high signal there is a ring of low signal representing calcification. MR or ultrasound will detect calcification long before it is visible on plain films. B Radiograph of humerus in the same patient as Fig. 10.3A. This was obtained several weeks after the MR scan and the calcification within the muscle is now evident and a diagnosis of myositis ossificans is most likely.

Benign peripheral nerve sheath tumours

Tumours arising from nerves are of two types: schwannomas and neurofibromas.

Neurofibroma

These are a little commoner and present as a soft, circumscribed, rounded, and slightly tender swelling in the skin or deeper tissues. They are more often associated with neurological symptoms. Pathologically they are not separate from the nerve which can be seen entering and leaving the tumour. Thus resection of the tumour requires sacrifice of the involved nerve.

Histologically the tumour is composed of cellular fibrous tissue arranged in whorls. The tumour may be solitary; but in the condition known as multiple neurofibromatosis (von Recklinghausen’s disease) (p. 70) numerous tumours are associated with pigmented areas in the skin. A neurofibroma growing within the spinal canal is an important cause of compression of the spinal cord or cauda equina.

Imaging with MR scanning can identify the nature of the lesion and sometimes its neural origin (Fig. 10.4).

Fig. 10.4 Coronal T2 weighted MR scan of a schwannoma in the medial aspect of the arm, just proximal to the elbow. The high signal in the round lesion suggests nerve tissue and this extends as a ‘tail’ at the superior and inferior ends making it likely that this is most likely a nerve sheath tumour.

Lipoma

A lipoma is a common tumour that may arise in almost any part of the body. It usually occurs in the subcutaneous tissues, but may also develop more deeply as an intramuscular lesion. It forms a soft, often large, lobulated mass enclosed within a thin capsule. It consists of fat, usually with little connective tissue stroma. When the lesion is deep to fascia and of a large size it may be difficult to differentiate on clinical grounds from malignant liposarcoma. Imaging by MRI may be diagnostic of lipoma if it shows a homogeneous white lesion on the T1 weighted image with the same density as the subcutaneous and intramedullary fat (Fig. 10.5). However where there is less than 75% of fat in the lesion, or if the septation is nodular rather than linear, there is a possibility of malignancy and a biopsy is required prior to surgical excision.

Fig. 10.5 Axial T1 weighted scan of thigh showing a large mass deep to the quadriceps muscles wrapped around the femur. This high signal, which is the same as subcutaneous and marrow fat, confirms that this is a lipoma.

Treatment, if required, is by local excision and the specimen when removed must always be sent for histological examination.

Haemangioma

Haemangioma is a benign tumour of blood vessels, usually present at birth. A capillary haemangioma forms a dark red, irregular, slightly raised blotch on the skin (‘port wine stain’). It is usually congenital. A cavernous haemangioma is composed of widely dilated vascular channels with intervening connective tissue. It forms a localised or diffuse tumour within the skin, subcutaneous tissue, or muscle. A characteristic feature of diagnostic importance is that the tumour is compressible and may vary in size from day to day. It may cause symptoms from its cosmetic appearance, or from the complication of thrombosis, which may be associated with local pain and swelling. When a cavernous haemangioma occurs in a massive form in a limb it may lead to marked increase in growth of the limb as a whole (elephantiasis).

Imaging by MR scanning shows characteristic clusters of abnormal dilated blood vessels giving a variegated serpiginous appearance.

Treatment. Superficial capillary haemangiomata fall within the province of the plastic surgeon or dermatologist. Some are amenable to laser ablation therapy. A localised deep cavernous haemangioma is usually amenable to excision. Extensive diffuse tumours cannot be eradicated, but improvement may follow transcutaneous arterial embolisation. In the worst cases amputation may be required.

Musculo-aponeurotic fibromatosis (desmoid tumour)

This rare tumour occurs mostly in young adults as a slowly growing hard swelling in the musculo-aponeurotic tissues, particularly in the trunk and shoulder regions. When it involves the abdominal wall or arises intra-abdominally it is known as an abdominal desmoid tumour. It is important because of its histological resemblance to malignant sarcoma and the characteristic infiltrative growth locally. Imaging by MRI scans will show a variable degree of heterogeneity (Fig. 10.6), which is not diagnostic, but will show the extent of the lesion and any involvement of neuro-vascular structures. Biopsy and histological examination will differentiate it from sarcoma and fortunately it has a more benign course and never metastasises. However, there is a high 30–40% risk of local recurrence, even after radical wide-margin excision, suggesting that it may arise from multicentric foci in the same limb.

Fig. 10.6 Coronal T1 weighted scan of right hip affected by fibromatosis. There is an ill-defined mass of mostly low (dark) signal overlying the greater trochanter and infiltrating the gluteal muscle. The low signal indicates either calcification, or as in this case dense fibrous tissue. This suggests fibromatosis, but a biopsy is still required for definitive diagnosis.

Malignant fibrous histiocytoma

This tumour is composed predominantly of histiocytic-type cells, but may also include spindle cells resembling fibroblasts. It was formerly included under the diagnosis of fibrosarcoma. It is now regarded as the commonest malignant soft-tissue tumour in older adults. It has a poor prognosis, with local recurrence or metastasis to the lungs in more than half the patients.

Liposarcoma

This is the second commonest of the soft-tissue sarcomas, usually occurring in the deep tissues. It grows as a lobulated mass, usually in the buttock or thigh, and may often attain an enormous size (Fig. 10.8). It has a wide range of behaviour, depending upon its histological appearance and the amount of myxomatous content. Tumours showing predominantly pleomorphic and round cells have a poor prognosis, but the presence of myxoid tissue improves the outlook.

Fig. 10.8 Magnetic resonance image of large low-grade liposarcoma in the buttock, showing typical lobulated appearance of the soft tissue mass.

Imaging with MRI scans is not diagnostic. These lesions do not contain macroscopic fat despite their name. MR scanning is generally non-specific and there is no fat signal from most of these lesions. Treatment is by radical excision or amputation, with the possible addition of adjuvant radiotherapy or chemotherapy in selected cases. Recurrence or metastasis is generally slow.

Synovial sarcoma (malignant synovioma)

This highly malignant but rare tumour was once thought to arise from the synovial lining of joints or tendon sheaths because of its proximity to these structures. Pathologically it forms a solid, whitish, fleshy mass with a slow but insidious invasion of the soft tissues. It is most common in young and middle-aged adults. It has a poor prognosis, with early pulmonary metastases or spread to local lymph nodes. Histologically the tumour is composed of masses of fusiform cells, but the picture is characterised by the formation of spaces or clefts lined by cuboidal cells which suggest the formation of a synovial cavity – hence the name given to this tumour. Treatment is by wide excision if this is practicable, with pre-operative and post-operative radiotherapy. When the tumour involves a joint amputation offers the best hope of survival.

Rhabdomyosarcoma

This rare variety of soft-tissue sarcoma which arises from skeletal muscle occurs mainly in children or young adults. It affects particularly the trunk or lower limbs. It forms a rapidly growing mass. Histologically it is characterised by cells showing longitudinal and cross striations typical of primitive myoblasts. It metastasises early, mainly to the lungs.

Bursitis

Inflammation may occur in a normally situated bursa or in an adventitious bursa. It may arise from mechanical irritation or from bacterial infection.

Irritative bursitis

This is caused by excessive pressure or friction, occasionally by a gouty deposit. There is a mild inflammatory reaction in the wall of the bursa, and there is usually an effusion of clear fluid within the sac. Examples are the common ‘bunion’ that forms over a prominent metatarsal head in hallux valgus, prepatellar bursitis or ‘housemaid’s knee’, olecranon bursitis (sometimes caused by gout), and subacromial bursitis.

Treatment. In many cases the inflammation subsides with rest if continued pressure or friction is prevented. If the sac is distended the fluid may be aspirated, and the instillation of hydrocortisone through the aspiration needle may help to prevent recurrence. In resistant cases cure can be effected only by operative excision of the bursa.

Infective bursitis

There may be acute inflammation from infection by an organism of the pyogenic group, or chronic inflammation as in tuberculous bursitis. Examples of acute pyogenic bursitis are infected bunion, infected prepatellar bursitis, and infected olecranon bursitis. A bursa that is sometimes affected by tuberculosis is the trochanteric bursa.

Treatment. Treatment of acute suppurative bursitis is by surgical drainage and antibacterial drugs. In chronic bursitis excision of the bursa is required.

Tenosynovitis

The term tenosynovitis implies inflammation of the thin synovial lining of a tendon sheath as distinct from its outer fibrous sheath. Like bursitis, tenosynovitis may be caused by mechanical irritation or by bacterial infection.

Irritative (frictional) tenosynovitis and peritendinitis

This is caused by excessive friction from over-use. The synovial sheath is mildly inflamed and there is an exudate of watery fluid within it, visible on ultrasound scanning (Fig. 10.9). A similar traumatic inflammation may affect the flimsy paratenon surrounding those tendons that are devoid of synovial sheaths. This is termed paratendinitis and is a common problem around the wrist and hand. The controversial condition known as repetitive stress syndrome comes into this category (p. 327).

Fig. 10.9 Sagittal ultrasound scan showing tenosynovitis of the wrist. The arrow indicates a flexor tendon and the dark area above this represents excessive fluid within the tendon sheath.

Infective tenosynovitis

Bacterial infection of a tendon sheath may be acute or chronic. Acute infective (suppurative) tenosynovitis is caused by an organism of the pyogenic group. There is an acute inflammatory reaction in the wall of the sheath, with a purulent exudate from it. It is an uncommon condition, but it is well recognised in the flexor tendon sheaths in the hand (p. 316).

In chronic bacterial tenosynovitis, also an uncommon lesion in Western countries, the infection is often tuberculous. The synovial wall is much thickened and there is a fibrinous exudate. The flexor sheaths of the forearm and hand are the usual sites (compound palmar ganglion, p. 317).

Tenovaginitis

In tenovaginitis there is a mild chronic inflammation or thickening of the fibrous wall of a tendon sheath as distinct from the synovial lining. The cause is unknown: it is not due to bacterial infection. The only common sites are the mouths of the fibrous flexor sheaths in the fingers or thumb (‘trigger’ finger, p. 329), and the sheaths of the extensor pollicis brevis and abductor pollicis longus tendons at the radial side of the wrist (de Quervain’s syndrome, p. 328).