Arborescent lipoma of the joint

MRI diagnostics of lipoma arborescens

Lipoma arborescens of the knee joint (Lipoma arborescens) is a rare yet highly notable condition. When analyzing magnetic resonance images in the sagittal projection on T1-weighted images (T1WI), a specific picture is visualized. This refers to the so-called frond-like synovial hyperplasia (frond-like synovial hyperplasia).

The preferred localization for this process is the suprapatellar pouch (suprapatellar pouch). The main diagnostic criterion that allows for a confident diagnosis lies in the signal characteristics. T1WI reveals a high signal intensity that is completely identical to normal adipose tissue. It is this fat pattern within the overgrown synovial villi that gives the radiologist grounds for the diagnosis.

Differential diagnosis: muscular and osseous sarcoidosis

In clinical practice, it is important to be able to differentiate various lesions of the musculoskeletal system. One such disease is sarcoidosis (sarcoidosis).

When muscle tissue is involved, MRI visualizes nodular sarcoid infiltration. It is characterized by a strictly specific pattern: a low-signal area on all pulse sequences is determined in the center of the nodule (which morphologically corresponds to an area of fibrosis), while a high-intensity signal zone forms on the periphery on T2WI.

Osseous sarcoidosis occurs in only 5–10% of patients. Clinically, it is often asymptomatic and extremely rare to detect in the absence of typical lung or skin lesions. The preferred localization is the phalanges of the hands and feet. The radiographic presentation of the osseous form is diverse:

• Most common form: manifests as coarsening of the trabecular pattern. The border between the cortical layer and the medullary cavity loses its distinctness. As a result, radiographs (especially of the distal phalanges of the toes) form a characteristic «lace-like» (lace-like) or reticular (reticular) pattern.
• Cystic form: characterized by localized lytic (cystic) lesions with sharply demarcated borders. The sizes of such lesions vary, and they may be detected, for example, in the knee joint region.
• Rare form: so-called mutilating sarcoidosis (mutilating sarcoidosis), leading to severe deformities.

Hypertrophic pulmonary osteoarthropathy

Another important syndrome requiring attention in joint and bone imaging is hypertrophic pulmonary osteoarthropathy (hypertrophic pulmonary osteoarthropathy). This term describes the classical clinical and radiological triad. It is important to remember that far from every patient will present with all three components simultaneously.

The triad includes:

1. Periosteal new bone formation (periosteal new bone formation).
2. Widening of the terminal phalanges of the fingers, known as finger clubbing (finger clubbing).
3. Synovitis (synovitis). If synovial inflammation presents as the initial symptom, the clinical picture may erroneously resemble rheumatoid arthritis.

The secondary form of the syndrome is most commonly associated with severe pulmonary diseases, primarily bronchogenic carcinoma (bronchogenic carcinoma). Additionally, the triad may accompany inflammatory bowel disease (inflammatory bowel disease) and biliary cirrhosis (biliary cirrhosis).

Radiologically, periostitis first involves the diaphyses of the tubular bones and then spreads to the metaphyses. The radius, ulna, tibia, and fibula are most frequently affected. The femur, humerus, metacarpals, metatarsals, and phalanges are less commonly involved. With a prolonged chronic course of periostitis, the cortical layer of the bones significantly thickens.

The primary form of osteoarthropathy is unrelated to underlying pathologies and is termed pachydermoperiostosis (pachydermoperiostosis). Epidemiologically, it is most characteristic of males of African descent and is clinically accompanied by pronounced skin thickening.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Morphological basis of villous synovial hyperplasia.
• Cystic and mutilating forms of bone sarcoidosis.
• Primary hypertrophic osteoarthropathy (pachydermoperiostosis).
• Radiological evolution of periostitis in chronic courses.

Other topics in the "Musculoskeletal System" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.