• Joint pain, limitation of motion or instability, due to dysfunctional articulating surfaces, loss of integrity of muscles/tendons around joint or their innervation, or mechanical properties of cartilaginous or bony extracellular matrix
• When specimen is received, disease is usually advanced, making precise cause difficult to determine

Carpometacarpal joint

• If etiology is not known, examine synovial fluid before biopsy to detect TB or other granulomatous lesions

"Fibrillation"

Loose body

• Cartilage has irregular surface with pitting and loss of cartilage
• Subchondral bone shows eburnation (polishing due to friction of bone against bone in joint), fractures
• Bony spurs (osteophytes), loose bodies (detached cartilage or cartilage/bone within joint space with necrotic calcified centers, may become attached to synovial membrane, revascularize and convert to viable bone)

"Osteophytes"

• Findings are related to either injury or reparative change

• Death of chondrocytes (no visible nuclei) or necrotic chondrocytes, marked irregularity and thickening of tidemark (indicates disturbed calcification) or duplication of tidemark
• Surface of cartilage may be intact
• Diminution of basophilic staining due to proteoglycan depletion
• Vertical and horizontal clefts within cartilage matrix extending from articular surface
• If rapid injury, synovium often contains pieces of bone or cartilage and chronic inflammatory cells
• Loose bodies may have concentric rings of calcification and may grow into enormous size
• Endochondral ossification can occur in loose bodies
• Subchondral bone has superficial bone necrosis, microfractures, replacement of bone by solid or cystic fibromyxomatous tissue

• Chondrocyte proliferation within damaged cartilage or from underlying bone and periphery of joint
• Bone / joint proliferative cartilage is cellular fibrocartilage, more coarse and disorganized with polarized light
• Usually marked osteoblastic activity, new bone formation, thickening of superficial trabeculae
• Marked synovial cell hyperplasia with multilayering or papillary folds, often containing hemosiderin (evidence of bleeding)
• Polarized microscopy demonstrates discontinuity between collagen network of repair cartilage and preexisting cartilage

Sheep with cylindrical metal implant

Pentachrome-stained knee joint of rabbit

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