AAOS Comprehensive Orthopaedic Review

Section 4 - Orthopaedic Oncology and Systemic Disease

Chapter 40. Benign Soft-Tissue Tumors and Reactive Lesions

I. Lipoma

A. Definition and demographics

 

1. Lipoma is a benign tumor of adipose tissue.

 

2. Slightly more common in men than in women

 

3. Occurs primarily in patients 40 to 60 years of age

 

4. Superficial/subcutaneous lesions are common; deep lesions are uncommon.

 

5. Hibernomas are tumors of brown fat; they occur in slightly younger patients (20 to 40 years of age).

 

B. Genetics/etiology

 

1. White (common) versus brown fat (usually in hibernating animals or human infants)

 

2. Lipomas occur when white fat accumulates in inactive people.

 

3. Chromosomal abnormalities have been described.

 

C. Clinical presentation

 

1. Soft, painless, mobile mass characterizes the common superficial variety.

 

2. 5% to 8% of patients with superficial lipomas have multiple lesions.

 

3. Superficial lipomas are common in the upper back, the shoulders, the arms, the buttocks, and the proximal thighs.

 

4. Deep lipomas are usually intramuscular, fixed, and painless and can be large.

 

5. Deep lesions are found frequently in the thigh, shoulder, and calf.

 

6. Most are stable after an initial period of growth.

 

D. Imaging appearance

 

1. Plain radiographs are not helpful; may see a radiolucency in deep lipomas

 

2. CT scan: appearance of subcutaneous fat

 

3. Magnetic resonance imaging

 

a. Bright on T1-weighted images, moderate on T2-weighted images (

Figure, A1 and B)

 

b. Lipomas will image exactly as fat on all sequences (suppress with fat-suppressed images).

 

c. They have a homogenous appearance, although minor linear streaking may occur.

 

d. Hibernomas have increased signal on T1-weighted images, but not always the same appearance as fat.

 

4. Occasionally, lipomas contain calcific deposits, bone.

 

5. Usually classic in MRI appearance and do not require a biopsy

 

E. Pathology

 

1. Gross appearance of lipoma is soft, lobular, white or yellow, with a capsule; hibernoma is red-brown in color because of profusion of mitochondria and more extensive vascularity than lipoma.

 

2. Histology reveals mature fat cells with moderate vascularity (Figure 1, C).

 

3. Occasionally note focal calcium deposits, cartilage, bone

 

4. Histologic variants include spindle cell lipoma, pleomorphic lipoma, angiolipoma—all benign but can be confused histologically with malignant lesions.

 

F. Treatment/outcome

 

1. Observation or local excision (can do excisional biopsy with marginal margin if imaging studies clearly document a lipoma).

 

2. Local recurrence is <5% if removed.

 

3. Malignant transformation is not clinically relevant; few cases reported.

 

G. Atypical lipoma

 

1. Usually very large, deep tumors

 

[Figure 1. Intramuscular lipoma. Axial T1 fat-suppressed (A) and T2 fat-suppressed (B) MRIs of the right thigh reveal a well-circumscribed lesion with the same signal as the subcutaneous fat. Note that the lesion suppresses on the fat-suppressed images. This is classic for an intramuscular lipoma. C, The histologic appearance is of mature fat cells without atypia. A loose fibrous capsule is visible.]

[

Figure 2. Atypical lipoma. A, Axial MRI reveals an extensive intramuscular lipomatous lesion infiltrating the posterior thigh musculature. Note the extensive stranding within the lesion. From this appearance, an intramuscular lipoma cannot be differentiated from an atypical lipoma. B, The histologic appearance of this atypical lipoma is more cellular than a classic lipoma.]

2. May look identical to classic lipomas or may have increased stranding on MRI (Figure 2, A and B)

 

3. Histology shows greater cellularity than classic lipoma (Figure 2, C).

 

4. Often called atypical lipoma in extremities and well-differentiated liposarcoma in the retroperitoneum

 

5. Treatment is marginal excision; often not differentiated from classic lipoma until after excision (based on histology).

 

6. Higher chance of local recurrence (50% at 10 years) compared with lipoma, but does not metastasize



II. Intramuscular Hemangioma

A. Definition and demographics

 

1. Intramuscular hemangioma is a benign vascular neoplasm occurring in the deep tissues.

 

[

Figure 3. Intramuscular hemangiomas. A, Phleboliths seen in the lateral arm on this plain radiograph suggest a hemangioma. B, Axial T2-weighted MRI of the thigh reveals a poorly circumscribed soft-tissue lesion with both fatty and vascular features within the muscle, consistent with a hemangioma. C, Histologic view of this capillary hemangioma shows large blood-filled spaces but no cellular atypia.]

2. Accounts for <1% of all benign vascular tumors

 

3. Males and females affected equally

 

4. Usually seen in patients <30 years of age

 

5. Often confused with other vascular malformations (arteriovenous malformations, cavernous hemangioma, angiomatosis, vascular ectasias, etc)

 

B. Genetics/etiology

 

1. Vascular malformations are caused by errors in morphogenesis affecting any segment of the vascular system.

 

2. Possible hormonal modulation

 

3. 20% associated with a history of trauma (no causal relationship)

 

C. Clinical presentation

 

1. Lesions are usually deep in the lower extremities but can involve any muscle.

 

2. Variable growth; often fluctuates with activity

 

3. Pain is variable and can increase with activity.

 

4. Usually no overlying skin lesions or bruits

 

5. Usually isolated lesions, but a rare form called diffuse hemangioma manifests in childhood and involves a limb extensively

 

D. Imaging appearance

 

1. Plain radiographs may reveal phleboliths or calcifications within the lesion (Figure 3, A).

 

2. May erode the adjacent bone

 

3. MRI reveals increased signal on T1- and T2-weighted images (Figure 3, B).

 

4. Focal low-signal areas are due to blood flow or calcifications.

 

5. Often ill-defined or described as a bag of worms; can appear infiltrative within the muscle

 

6. Frequently mistaken for a malignant soft-tissue tumor

 

E. Pathology

 

1. Gross appearance varies depending on whether the lesion is the capillary type or the cavernous type (capillary more common).

 

2. Color varies from red to tan to yellow.

 

3. Histology shows capillary-sized vessels with large nuclei (Figure 3, C).

 

4. Well-developed vascular lumens, infiltration of muscle fibers

 

5. No significant cellular pleomorphism

 

6. Cavernous type composed of large vessels with a large degree of adipose tissue

 

7. Differential diagnosis includes angiosarcoma.

 

F. Treatment/outcome

 

1. Most should be treated with observation, anti-inflammatory medications, compressive sleeves.

 

2. Many are amenable to interventional radiology techniques of embolization or sclerotherapy to decrease size of lesion or symptoms.

 

[

Figure 4. Neurilemoma of the pelvis. Axial T1-weighted (A) and T2-weighted (B) MRIs reveal a large soft-tissue mass (arrows) that has low signal intensity on T1 sequences and high signal intensity on T2 sequences. It would enhance after gadolinium administration. C, Low-power histologic view reveals the compact spindle cell areas (Antoni A) of a neurilemoma. Note the palisading nuclei and verocay bodies. D, Within the same tumor, there are areas of loosely arranged cells within a haphazard collagenous stroma (Antoni B). Antoni B areas contain numerous blood vessels.]

3. Surgical excision carries a high risk of local recurrence.

 

4. No incidence of malignant transformation



III. Neurilemoma (Schwannoma)

A. Definition and demographics

 

1. Neurilemoma (schwannoma) is an encapsulated benign soft-tissue tumor composed of Schwann cells.

 

2. Commonly discovered in patients 20 to 50 years of age (may also occur in older patients)

 

3. Affects males and females equally

 

4. Can affect any motor or sensory nerve

 

5. More common than neurofibroma

 

B. Genetics/etiology—Often associated with mutations affecting the NF2 gene.

 

C. Clinical presentation

 

1. Usually asymptomatic; sometimes causes pain with stretch or activity

 

2. Occurs frequently on the flexor surfaces of the extremities as well as the head/neck

 

3. Pelvic lesions can become quite large.

 

4. May change in size given frequent occurrence of cystic degeneration

 

5. Multiple lesions occur rarely.

 

6. Positive Tinel sign may be present.

 

D. Imaging appearance

 

1. Low signal intensity on T1-weighted MRI, high signal intensity on T2-weighted MRI (Figure 4, A and B)

 

2. Diffusely enhanced signal with gadolinium administration

 

[

Figure 5. Coronal T2-weighted MRI of the wrist reveals a small, oval soft-tissue mass in continuity with a nerve, consistent with a neurilemoma.]

3. In sagittal or coronal images, may see in continuity with affected nerve (Figure 5)

 

4. Difficult to differentiate neurilemoma and neurofibroma

 

E. Pathology

 

1. Gross appearance is a well-encapsulated lesion, gray-tan in color (

Figure 6).

 

2. Grows eccentrically from the nerve

 

3. Histology shows alternating areas of compact spindle cells (Antoni A) (Figure 4, C) and loosely arranged cells with large vessels (Antoni B) (Figure 4, D).

 

4. Verocay bodies are composed of two rows of aligned nuclei in a palisading formation, and their appearance is pathognomonic.

 

5. Strongly uniform positive staining for S100 antibody

 

F. Treatment/outcome

 

1. Observation or marginal/intralesional excision with nerve fiber preservation as symptoms dictate

 

2. Small risk of sensory deficits or long-standing palsies after dissection

 

3. Extremely rare risk of malignant degeneration

 

[Figure 6. Gross appearance of a bisected neurilemoma (schwannoma) reveals cystic degeneration of the well-encapsulated lesion.]

IV. Neurofibroma

A. Definition and demographics

 

1. Neurofibroma is a benign neural tumor involving multiple cell types.

 

2. Occurs in patients 20 to 40 years of age (younger when associated with neurofibromatosis)

 

3. Males and females are affected equally.

 

B. Genetics/etiology

 

1. Most neurofibromas arise sporadically.

 

2. Neurofibromatosis type 1 is an autosomal dominant syndrome characterized by multiple neurofibromas.

 

3. NF1—abnormal chromosome 17 (1 in 4,000 births)

 

4. NF2—abnormal chromosome 22 (1 in 40,000 births)

 

C. Clinical presentation

 

1. Can affect any nerve; may be cutaneous or plexiform (infiltrative)

 

2. Most are asymptomatic, but sometimes neurologic symptoms are present.

 

3. Tumors are slow-growing.

 

4. May have positive Tinel sign

 

5. National Institutes of Health criteria for neurofibromatosis:

 

• More than 6 cafe-au-lait spots

 

• Lisch nodule (melanocyte hamartoma affecting the iris)

 

• Axillary freckling

 

• Two neurofibromas or one plexiform neurofibroma

 

• Optic glioma

 

• Bone scalloping

 

• Relative with NF1 disease

 

[

Figure 7. Neurofibroma of the elbow. A, Axial T2-weighted MRI of the elbow in a 26-year-old man with neurofibromatosis reveals an area of high signal intensity (arrow) consistent with a neurofibroma within the volar forearm muscles. The same lesion would be dark on T1-weighted sequences. B, Low-power histology reveals a cellular lesion with a wavy or storiform appearance. C, Higher power reveals elongated cells with dark nuclei and no atypia.]

6. Rapid enlargement of a neurofibroma suggests malignant transformation.

 

D. Imaging appearance

 

1. Varies in size; usually a fusiform expansion of the nerve

 

2. MRI—Low signal intensity on T1-weighted sequences, high on T2-weighted sequences (Figure 7, A).

 

3. Dumbbell-shaped lesion that can expand a neural foramen

 

4. More common than neurilemoma to have target sign (peripheral T2-hyperintensity and low signal center)

 

5. Bone changes include penciling of the ribs, sharp vertebral end plates, tibial pseudarthrosis, nonossifying fibromas, or scoliosis.

 

6. Plexiform neurofibroma has extensive signal on MRI scan; infiltrative

 

E. Pathology

 

1. Gross appearance is fusiform expansion of the nerve, usually unencapsulated.

 

2. Histology contains interlacing bundles of elongated cells with wavy, dark nuclei (Figure 7, B and C).

 

3. Cells are associated with wirelike collagen fibrils.

 

4. Sometimes the cells are arranged in fascicles or a storiform pattern.

 

5. Mixed cell population of Schwann cells, mast cells, lymphocytes

 

6. Stroma may have a myxoid appearance.

 

7. S100 staining is variable.

 

F. Treatment/outcome

 

1. Observe if asymptomatic

 

2. Surgical excision can leave significant nerve deficit and may require grafting.

 

3. 5% of patients with neurofibromatosis develop malignant transformation of a lesion (often a plexiform neurofibroma).

 

4. Malignant transformation of a solitary lesion is rare.



V. Nodular Fasciitis

A. Definition and demographics

 

1. Nodular fasciitis is a self-limited reactive process often mistaken for a fibrous neoplasm.

 

2. Most common in adults 20 to 40 years of age

 

3. Males and females affected equally

 

4. Most common fibrous soft-tissue lesion

 

B. Genetics/etiology—Reactive rather than neoplastic process.

 

C. Clinical presentation

 

1. Rapid growth of a nodule over 1 to 2 weeks

 

[

Figure 8. Nodular fasciitis. A, Low-power histology reveals a highly cellular lesion with a nodular pattern. B, Higher power shows regular plump fibroblasts with vessels, erythrocytes, and lipid macrophages consistent with nodular fasciitis.]

2. 50% of patients have pain and/or tenderness.

 

3. Lesion is usually 1 to 2 cm in size.

 

4. Commonly occurs on volar forearm, back, chest wall, head/neck

 

5. Occurs as a solitary lesion

 

D. Imaging appearance

 

1. MRI shows nodularity, extension along fascial planes, and avid enhancement with gadolinium.

 

2. Usually small in size

 

3. Occurs superficially (most common), intramuscularly, or along the superficial fascial planes

 

E. Pathology

 

1. Grossly nodular without a surrounding capsule

 

2. Histology is cellular with numerous mitotic figures (Figure 8, A).

 

3. Cells are plump, regular fibroblasts arranged in short bundles or fascicles (Figure 8, B).

 

4. Additional cells include lymphoid cells, erythrocytes, giant cells, and lipid macrophages.

 

F. Treatment/outcome

 

1. Local resection with marginal or intralesional margin has a low risk of local recurrence.

 

2. No risk of malignant transformation

 

3. Reports of resolution of lesion after needle biopsy



VI. Intramuscular Myxoma

A. Definition and demographics

 

1. Intramuscular myxoma is a benign, nonaggressive myxomatous soft-tissue tumor.

 

2. Occurs in adults 40 to 70 years of age

 

3. Male to female ratio = 1:2

 

B. Clinical presentation

 

1. Usually presents as a painless mass

 

2. Pain/tenderness in ~20% of patients

 

3. Possible numbness or paresthesias in patients with large lesions

 

4. Usually solitary

 

5. Most commonly located in the thigh, buttocks, shoulder, and upper arm

 

6. Multiple intramuscular myxomas associated with fibrous dysplasia (Mazabraud syndrome)

 

C. Imaging appearance

 

1. MRI appearance is homogenous.

 

2. Low signal intensity (lower than muscle) on T1-weighted sequences, high on T2-weighted sequences (

Figure 9, A and B)

 

3. Located within the muscle groups; usually 5 to 10 cm in size

 

4. In Mazabraud syndrome, fibrous dysplasia develops at a young age and the myxomas occur later in the same general anatomic area.

 

D. Pathology

 

1. Grossly lobular and gelatinous with cyst-like spaces (Figure 9, C)

 

2. Histology shows minimal cellularity with cells suspended in abundant mucoid material (Figure 9, D).

 

3. Loose network of reticulin fibers

 

4. No atypia and only sparse vascularity

 

[Figure 9. Intramuscular myxoma. Axial T1-weighted (A) and T2-weighted (B) MRIs in a 52-year-old woman with Mazabraud syndrome show a large soft-tissue lesion (arrows) along the anterior aspect of the right hip consistent with an intramuscular myxoma. It has lower signal intensity than muscle on T1-weighted images and is bright on T2-weighted images. C, Gross appearance of a bisected intramuscular myxoma reveals a white, gelatinous surface. D, The histologic appearance reveals a paucicellular lesion with extensive reticulin fibers and a mucoid stroma.]

5. "Cellular myxoma" has increased cellularity and can be mistaken for a malignant myxoid neoplasm.

 

E. Treatment/outcome

 

1. Marginal excision is the preferred treatment.

 

2. Very rarely recurs locally and does not metastasize



VII. Desmoid Tumor (Extra-abdominal Fibromatosis)

A. Definition and demographics

 

1. Desmoid tumor is a benign, locally aggressive fibrous neoplasm with a high risk of local recurrence.

 

2. Common tumor; approximately 900 cases/year in United States

 

3. Occurs in young individuals (15 to 40 years of age)

 

4. Slight female predominance

 

5. Desmoid tumors occur within a family of fibromatoses that also includes superficial lesions in the palmar and plantar fascia (Dupuytren contracture, Ledderhose disease).

 

B. Genetics/etiology

 

1. Can have mutations in the adenomatous polyposis coli gene (more common in patients with familial polyposis)

 

2. Cytogenetic abnormalities include trisomy of chromosomes 8 or 20.

 

C. Clinical presentation

 

1. Usually present with painless mass

 

2. Rock hard, fixed, and deep on examination

 

3. Most commonly occurs in the shoulder, chest wall/back, thigh

 

4. >50% are extra-abdominal; 50% are intra-abdominal (pelvis, mesentery).

 

5. Occasionally multicentric; usually the subsequent lesion is more proximal in the same limb.

 

D. Imaging appearance

 

1. Typical MRI appearance: low signal intensity on T1-weighted sequences, low to medium signal intensity on T2-weighted sequences (

Figure 10, A and B)

 

2. Gadolinium administration enhances appearance.

 

3. Infiltrative within the muscles; usually 5 to 10 cm in size

 

4. May have adjacent osseous changes (erosion)

 

E. Pathology

 

1. Gross characteristics: gritty, white, poorly encapsulated

 

2. Histology: bland fibroblasts with abundant collagen (Figure 10, C and D)

 

3. Uniform spindle cells with elongated nuclei and only occasional mitoses

 

4. Moderate vascularity

 

5. Sweeping bundles of collagen less defined than fibrosarcoma

 

6. Often infiltrates into adjacent tissues

 

F. Treatment/outcome

 

1. Treat similar to sarcoma, with wide resection

 

2. High risk of local recurrence given infiltrative pattern

 

3. Difficult to differentiate recurrent tumor from scar tissue

 

[Figure 10. Desmoid tumors. Axial T1-weighted (A) and short tau inversion recovery (STIR) (B) MRIs of the right shoulder of a 58-year-old woman reveal a desmoid tumor (arrows). The STIR sequence is fluid-sensitive and reveals findings similar to those found on a fat-sensitive T2 image. There is low signal noted on both images. C, Low-power histologic view reveals sweeping bundles of collagen. D,Higher power demonstrates bland, elongated, fibrous cells without atypia.]

4. External beam radiation (up to 60 Gy) is often used for recurrent lesions.

 

5. Low-dose chemotherapy ± radiation is used for inoperable lesions.

 

6. Unusual natural history: hard-to-predict behavior, occasional spontaneous regression

 

7. Treatment should not be worse than the disease; avoid amputation.

 

8. No risk of metastasis or malignant transformation unless related to radiation



VIII. Elastofibroma

A. Definition and demographics

 

1. Elastofibroma is an unusual, tumorlike reactive process that frequently occurs between the scapula and chest wall.

 

2. Occurs in patients 60 to 80 years of age

 

3. More common in females than males

 

B. Genetics/etiology

 

1. High familial incidence

 

2. Often occurs after repeated trauma

 

C. Clinical presentation

 

1. Usually asymptomatic; found in ~17% of elderly people at autopsy

 

2. Snapping scapula on examination

 

3. Firm, deep lesion

 

4. Occurs almost exclusively in the soft tissues between the tip of the scapula and the chest wall (rare in deltoid, greater trochanter)

 

5. Bilateral in 10% of cases (often noted on chest CT scans)

 

D. Imaging appearance

 

1. Ill-defined lesion with appearance of muscle on CT scan

 

[

Figure 11. Elastofibroma. A, Axial MRI of the chest in a 73-year-old woman reveals bilateral soft-tissue masses between the inferior tip of the scapula and the underlying chest wall consistent with elastofibromas (arrows). B, High-power histology reveals the beaded appearance of the elastic fibers admixed with the extensive collagen fibers. The elastin stain highlights the elastic fibers throughout the lesion. Note the extensive vascularity.]

2. MRI: mixed low and high signal intensity on both T1- and T2-weighted sequences (Figure 11, A)

 

E. Pathology

 

1. Gross appearance is gray with cystic degeneration, 5 to 10 cm in length.

 

2. Histology shows elastic fibers having a beaded appearance with characteristic staining for elastin (Figure 11, B).

 

3. Equal proportion of intertwined collagen fibers

 

F. Treatment/outcome

 

1. Observation if asymptomatic

 

2. Simple excision is curative.

 

3. No risk of malignant transformation



IX. Glomus Tumor

A. Definition and demographics

 

1. Glomus tumor is a rare, benign tumor of the normal glomus body usually occurring in the subungual region.

 

2. Extremely rare

 

3. Occurs in patients 20 to 40 years of age

 

4. Males and females are affected equally (except with subungual tumors, in which the male to female ratio = 1:3)

 

B. Clinical presentation

 

1. Small (<1 cm) red-blue nodule in the subungual region or other deep dermal layers in the extremities

 

2. More difficult to see color in subungual region; may have ridging of the nail or discoloration of the nail bed

 

3. Characteristic triad of symptoms includes paroxysmal pain, cold insensitivity, and localized tenderness.

 

4. Frequent delay in diagnosis

 

5. Less common locations include the palm, wrist, forearm, and foot.

 

6. Multiple in 10% of cases

 

[

Figure 12. Low-power histology of a glomus tumor reveals small, rounded cells with dark nuclei in well-defined clumps. On higher power, the glomus cells and the admixed capillaries would be present within a myxoid stroma.]

C. Imaging appearance

 

1. MRI is the best imaging modality to identify glomus tumors.

 

2. MRI sensitivity 90% to 100%, specificity 50%

 

3. Low signal intensity on T1-weighted sequences, high on T2-weighted sequences

 

4. Plain radiographs not very helpful in diagnosis, but can show a scalloped osteolytic defect with a sclerotic border in the distal phalanx

 

D. Pathology

 

1. Gross: small, red-blue nodule

 

2. Histology shows a well-defined lesion of small vessels surrounded by glomus cells in a hyaline or myxoid stroma (Figure 12).

 

3. A glomus cell is a uniform, round cell with a prominent nucleus and eosinophilic cytoplasm.

 

4. Periodic acid-Schiff stain gives a chicken-wire appearance to the matrix between cells.

 

E. Treatment/outcome

 

1. Marginal excision is curative

 

2. Extremely rare reports of malignant glomus tumors



X. Synovial Chondromatosis

A. Definition and demographics

 

1. Synovial chondromatosis is a metaplastic proliferation of hyaline cartilage nodules in the synovial membrane.

 

2. Occurs in patients 30 to 50 years of age

 

3. Male to female ratio = 2:1

 

B. Genetics/etiology

 

1. Generally thought to be a metaplastic condition

 

2. Occasional chromosome 6 abnormalities have been identified.

 

C. Clinical presentation

 

1. Symptoms include joint pain, clicking, and limited range of motion.

 

2. Pain worse with activity

 

3. May have warmth, erythema, or tenderness, depending on location

 

4. Slow progression of symptoms over years

 

5. Most common in the hip and knee, followed by shoulder and elbow

 

6. Occasionally occurs in the bursa overlying an osteochondroma

 

D. Imaging appearance

 

1. Plain radiographs show variable appearance depending on early or late disease.

 

2. Initially the cartilage nodules are not visible, except on MRI.

 

3. Over time, the nodules calcify, then undergo endochondral ossification (

Figure 13, A and B).

 

4. The densities are smooth, well-defined, and remain in the confines of the synovial membrane.

 

[Figure 13. Synovial chondromatoses. A, AP radiograph of the right hip in a 46-year-old male with synovial chondromatosis demonstrates discrete calcifications superior and inferior to the femoral head, both within and external to the hip capsule. B, Sagittal MRI of the knee in a 33-year-old man with extensive synovial osteochondromatosis. The patient had multiple ossified nodules within the joint and required open synovectomies of the anterior and posterior compartments of the knee. C, Low-power histology of synovial osteochondromatosis reveals discrete hyaline cartilage nodules. D, Higher power reveals increased cellularity with occasional binucleate cells.]

5. Process may erode cartilage and underlying bone.

 

6. CT scan can define the intra-articular loose bodies.

 

7. MRI shows lobular appearance with signal dropout consistent with calcification.

 

E. Pathology

 

1. Gross: may be hundreds of osteocartilaginous loose bodies within an affected joint

 

2. Histology shows discrete hyaline cartilage nodules in various phases of calcification or ossification (Figure 13, C).

 

3. Ossification starts on the periphery of the nodules.

 

4. Cellular appearance of chondrocytes includes mild atypia, binucleate cells, and occasional mitoses (more cellular atypia than allowed in an intramedullary benign cartilage tumor) (Figure 13, D).

 

F. Treatment/outcome

 

1. Open or arthroscopic synovectomy

 

2. Less than adequate removal of nodules increases risk for local recurrence.

 

3. Natural history is self-limited, but the process can damage the joint.



XI. Pigmented Villonodular Synovitis

A. Definition and demographics

 

1. Pigmented villonodular synovitis (PVNS) is a proliferative synovial process characterized by mono-nuclear stromal cells, hemorrhage, histiocytes, and giant cells.

 

2. Most commonly affects patients 30 to 50 years of age, occasionally teenagers

 

3. Males and females affected equally

 

4. Occurs in focal or diffuse forms

 

5. Can be intra-articular or extra-articular (giant cell tumor of tendon sheath)

 

B. Genetics/etiology

 

1. 50% of patients report an earlier traumatic incident

 

2. Reactive rather than neoplastic

 

C. Clinical presentation

 

1. Pain, swelling, effusion, erythema, and decreased joint range of motion with diffuse joint involvement

 

2. Mechanical joint symptoms with focal involvement

 

3. Most commonly affects the knee (80%), followed by hip, shoulder, ankle

 

4. Extra-articular form (giant cell tumor of tendon sheath) usually affects the hand/wrist with a small, painless, superficial soft-tissue nodule.

 

D. Imaging appearance

 

1. Well-defined erosions on both sides of a joint noted on plain radiographs signify advanced, diffuse disease (

Figure 14, A).

 

2. MRI reveals either a focal low-signal nodule within a joint or a diffuse process with low signal intensity (due to hemosiderin deposits) on T1-and T2-weighted images (Figure 14, B and C).

 

3. MRI shows presence of fat signal within the lesion.

 

4. MRI may reveal extra-articular extension of the process.

 

5. Differential diagnosis includes reactive or inflammatory synovitis, hemophilia, or synovial chondromatosis.

 

E. Pathology

 

1. Gross appearance: reddish-brown stained synovium with extensive papillary projections (Figure 14, D).

 

2. Histology: diagnostic mononuclear stromal cell infiltrate within the synovium (Figure 14, E)

 

3. Cells are round with a large nucleus and eosinophilic cytoplasm.

 

4. Hemosiderin-laden macrophages, multinucleated giant cells, and foam cells present; not required for diagnosis

 

5. Mitotic figures relatively common

 

F. Treatment/outcome

 

1. Treatment is arthroscopic or open removal of a focal PVNS lesion (arthroscopic preferred).

 

2. Diffuse form requires aggressive total synovectomy using arthroscopic, open, or combined arthroscopic and open techniques.

 

3. Often an anterior arthroscopic synovectomy is combined with open posterior removal of any extra-articular disease.

 

4. High local recurrence rate suggests frequent incomplete synovectomy.

 

5. Total joint arthroplasty is indicated for advanced disease with secondary degenerative changes.

 

6. Occasionally, external beam radiation is used following multiple local recurrences.

 

7. Giant cell tumor of tendon sheath is treated with marginal excision.

 

[Figure 14. Examples of pigmented villonodular synovitis. A, AP radiograph of the left hip in a patient with PVNS reveals osteolytic lesions with sclerotic rims on both sides of the joint. Sagittal T1-weighted (B) and T2-weighted (C) images of a knee show extensive disease in the posterior aspect of the knee. The lesion is both intra-articular and extra-articular. There is dark signal on both of the images as a result of the hemosiderin deposits within the synovium. D, Gross appearance of the reddish-brown synovial fronds seen in PVNS. E, Histologic examination reveals a cellular infiltrate within the synovium with multinucleated giant cells and faint hemosiderin. The round, mononuclear stromal cells are the key to the diagnosis.]

XII. Myositis Ossificans

A. Definition and demographics

 

1. Myositis ossificans is a reactive process characterized by a well-circumscribed proliferation of fibroblasts, cartilage, and bone within a muscle (or, rarely, within a nerve, tendon, or fat).

 

2. Occurs in young, active individuals (most common in individuals 15 to 35 years of age)

 

3. Occurs in males more than females

 

B. Genetics/etiology—Almost always a posttraumatic condition.

 

C. Clinical presentation

 

1. Pain, tenderness, swelling, decreased range of motion usually within days of an injury

 

2. Mass increases in size over several months (usually 3 to 6 cm)

 

3. Growth stops and mass becomes firm

 

4. Commonly occurs in the quadriceps, brachialis, gluteal muscles

 

D. Imaging appearance

 

1. Mineralization begins 3 weeks after injury.

 

2. Initially, irregular, fluffy densities in the soft tissues are noted on plain radiographs (

Figure 15, A).

 

3. There may be adjacent periosteal reaction in the bone.

 

4. Rim enhancement is seen on MRI with gadolinium within the first 3 weeks.

 

[Figure 15. Examples of myositis ossificans. A, Lateral radiograph of the knee obtained 4 weeks after a football injury to the posterior thigh in a 19-year-old man reveals fluffy calcifications (arrow) in the posterior thigh musculature consistent with early myositis ossificans. B, CT scan of the thigh in a patient with traumatic myositis ossificans reveals the calcified outline of the lesion (arrows) with more mature tissue on the periphery. C, Histologic view of a myositis ossificans lesion reveals a zonal pattern, with more mature bone toward the periphery (white arrow) and looser fibrous tissue toward the center (black arrow).]

5. With time and maturation, a zoning pattern occurs, with increased peripheral mineralization and a radiolucent center.

 

6. CT scan defines the ossified lesion (looks like an eggshell) (Figure 15, B).

 

7. Differential diagnosis includes extraskeletal or parosteal osteosarcoma (more ossified in the center with peripheral lucencies).

 

E. Pathology

 

1. Grossly immature tissue in center of lesion with mature bone around outer edge

 

2. Histology reveals a zonal pattern (Figure 15, C).

 

a. Periphery—mature lamellar bone

 

b. Intermediate—poorly defined trabeculae with osteoblasts, fibroblasts, and large ectatic blood vessels

 

c. Center—immature, loose, fibrous tissue with moderate pleomorphism and mitoses

 

3. Skeletal muscle can be entrapped in the periphery of the lesion.

 

4. No cytologic atypia

 

5. Differential diagnosis includes extraskeletal osteosarcoma (periphery is least ossified and cells show extreme pleomorphism).

 

F. Treatment/outcome

 

1. Myositis ossificans is a self-limited process, so observation and physical therapy to maintain motion are indicated.

 

2. Repeat radiographs to confirm maturation and stability of the lesion

 

3. Excision only when lesion is mature (~6 to 12 months) and if symptomatic; excision at initial stages predisposes to local recurrence

 

4. Often, the size of the mass decreases after 1 year.



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Top Testing Facts

1. Lipomas should image the same as fat on all MRI sequences.

 

2. Intramuscular hemangiomas or other vascular malformations are best treated nonsurgically.

 

3. NF1 involves an abnormal chromosome 17, and 5% of patients will develop malignant transformation of a lesion.

 

4. Neurilemomas have Antoni A (cellular) and Antoni B (myxoid) areas on histology.

 

5. Desmoid tumors are one of the few benign soft-tissue lesions to require a wide resection.

 

6. Radiation is often part of the treatment regimen for recurrent desmoid tumors.

 

7. Elastofibromas commonly occur between the scapula and chest wall; they stain with elastin.

 

8. Glomus tumors usually occur in a subungual location.

 

9. Synovial chondromatosis and PVNS require a complete synovectomy to achieve local control.

 

10. The imaging and histologic appearance of myositis ossificans shows a zonal pattern with increased peripheral mineralization and immature tissue in the center.