PA32.1-7 | Bone & Soft Tissue — Glossary

Secondary amyloidosis complicating chronic RA; serum amyloid A (SAA) protein is deposited as amyloid fibrils in kidney, liver, and spleen; presents with nephrotic syndrome and progressive renal failure

Enzyme produced by osteoblasts (and liver, placenta); elevated serum ALP in bone disease reflects osteoblast hyperactivity; markedly elevated in Paget disease (can be 10–20× normal) and in bone metastases; normal in multiple myeloma (helpful differentiating point)

Serum enzyme markedly elevated in Paget disease (reflecting intense osteoblastic activity); highest levels of any bone disease; used to monitor treatment response to bisphosphonates

Anti-citrullinated protein/peptide antibody; specificity ~95% for RA; can precede clinical onset by years; marker of erosive disease

Histologic pattern in synovial sarcoma: two distinct components — epithelial-like cells forming glandular spaces and malignant spindle cells; both are derived from the same SYT-SSX-rearranged clone.

A bone metastasis that stimulates osteoblast activity, producing dense bone formation; appears as radiodense (white) foci on X-ray; classic for prostatic carcinoma.

Bony osteophytes at proximal interphalangeal joints in OA; often accompanied by Heberden's nodes at DIP joints

Subacute walled-off lytic cavity in cancellous bone (classic site: distal tibia metaphysis), lined by granulation tissue, caused by S. aureus; appears as a well-defined lytic lesion with sclerotic rim on X-ray

Clusters of proliferating chondrocytes seen microscopically in OA; represent a reparative response to cartilage damage; a pathognomonic histological feature of OA

Multiple large, rounded, well-defined pulmonary nodules on chest X-ray or CT, typically from hematogenous spread of sarcomas (especially osteosarcoma) or renal cell carcinoma.

Cell surface glycoprotein showing strong diffuse membranous immunoreactivity in Ewing sarcoma; used in IHC panel; not entirely specific (also in T-cell lymphoblastic lymphoma) but highly sensitive

Transverse pathological fracture through Pagetic bone; occurs because the sclerotic Pagetic bone is brittle (despite bulk); most common in the femur; distinct from the spiral/oblique fractures of normal bone

The cell of hyaline cartilage; malignant transformation produces chondrosarcoma; marked nuclear atypia (pleomorphism, binucleation) distinguishes chondrosarcoma from enchondroma.

Elongated, blunt-ended (fusiform) nuclei characteristic of smooth muscle cells and leiomyosarcoma; the nuclei resemble cigars in cross-section; contrasted with tapered, wavy nuclei of schwannoma.

Post-translational modification converting arginine to citrulline in proteins, particularly in lung tissue (triggered by smoking); generates neoantigens targeted by anti-CCP antibodies in RA

Opening in the involucrum through which pus and bone debris are discharged; connects to the skin surface via a sinus tract (Latin: sewer)

A triangular shadow on X-ray formed when a rapidly growing bone lesion elevates the periosteum; the periosteum deposits new bone at the elevated edges, creating a triangular opacity at the tumor margins. Not specific to osteosarcoma.

Triangular area of periosteal elevation seen on X-ray in osteosarcoma (and other aggressive bone lesions) where tumour lifts the periosteum away from the cortex; not pathognomonic but raises suspicion for aggressive lesion

Abscess in TB osteomyelitis lacking the cardinal signs of acute inflammation (heat, redness, tenderness); reflects granulomatous (not acute suppurative) inflammation; tracks along fascial planes as a paraspinal or psoas abscess

Radiological appearance of the osteosclerotic (late) phase of Paget disease: patchy, irregular sclerotic densities superimposed on an enlarged skull vault, resembling wisps of cotton wool

A fully human monoclonal antibody targeting RANKL (receptor activator of NF-κB ligand); used clinically for unresectable or recurrent giant cell tumor of bone and for bone metastasis prevention; blocks osteoclast activation.

The shaft (tubular middle portion) of a long bone; the site predilected by Ewing sarcoma.

Polishing of exposed subchondral bone to an ivory-like glistening surface in OA, occurring after complete cartilage loss through bone-on-bone contact

Erosion of the inner cortical surface (endosteum) by an expanding intramedullary lesion; >2/3 cortical thickness scalloping in an adult enchondroma suggests malignant transformation to chondrosarcoma.

The end of a long bone, covered by articular cartilage; the preferred site of giant cell tumor of bone.

The fusion oncogene produced by t(11;22)(q24;q12) translocation in ~85% of Ewing sarcomas; encodes an aberrant transcription factor driving uncontrolled cell proliferation.

Earliest morphological change in OA — vertical cracks developing in the superficial layer of articular cartilage due to proteoglycan and collagen loss

Fédération Nationale des Centres de Lutte Contre le Cancer system for grading soft tissue sarcomas (grades 1–3) based on tumor differentiation, mitotic count, and presence of necrosis; higher grade correlates with worse prognosis.

Sharp angular kyphosis (hunchback) at the site of vertebral collapse in Pott disease, caused by anterior wedging of one or more vertebral bodies by the destructive granulomatous process

Hard bony swellings (osteophytes) at distal interphalangeal joints in OA; occur at DIP joints (contrast Bouchard's nodes at PIP joints)

Bone infection arising from bacteremia seeding the bone via blood, classically targeting the metaphysis of long bones in children due to slow-flow sinusoidal vascular loops

Characteristic arrangement of spindle cells in fibrosarcoma: intersecting bundles of fibroblasts arranged in a V-shaped, chevron, or fishbone configuration.

Cardiac failure caused by sustained elevated cardiac output, as seen in Paget disease where hypervascular bone acts as a large arteriovenous shunt; presents with warm extremities, bounding pulse, and high pulse pressure (opposite of typical low-output failure)

MHC class-II allele strongly associated with seropositive RA; presents citrullinated self-peptides to CD4+ T cells in the pathogenesis

Isocitrate dehydrogenase 1 and 2 mutations; found in ~50% of central chondrosarcomas and in gliomas; produce the oncometabolite 2-hydroxyglutarate; targetable by IDH inhibitors.

Shell of reactive new bone formed by the stripped periosteum surrounding a sequestrum in chronic osteomyelitis; histologically composed of woven bone initially

A malignant adipocyte precursor with multiple cytoplasmic lipid vacuoles that indent and scallop the hyperchromatic nucleus; the diagnostic hallmark of liposarcoma.

A bone metastasis that destroys bone by activating osteoclasts; appears as a radiolucent 'punched-out' defect on X-ray; typical of breast, lung, kidney, and thyroid carcinomas.

Pink amorphous unmineralised bone matrix produced directly by pleomorphic malignant tumour cells in osteosarcoma; the defining diagnostic feature distinguishing osteosarcoma from other malignant bone tumours

Squamous cell carcinoma (SCC) arising in a chronically inflamed sinus tract of osteomyelitis; latency 20–40 years; aggressive, early metastasis; represents malignant transformation of chronic wound

The flared zone between the shaft (diaphysis) and growth plate of a long bone; site of rich sinusoidal vasculature and preferential bacterial lodgment in childhood hematogenous osteomyelitis

Needle-shaped crystals of urate that deposit in joints and soft tissues in gout; show negative birefringence under polarised light with a red compensator

Pathognomonic histological feature of Paget disease: irregular lamellar bone fragments pieced together with numerous prominent basophilic cement lines (reversal lines), resembling a mosaic or crazy paving; result of repeated asynchronous resorption-deposition cycles

Histological hallmark of Paget disease of bone — haphazard irregular cement lines in lamellar bone giving a jigsaw puzzle or crazy-paving appearance, reflecting repeated cycles of resorption and disorganised formation

Autosomal dominant condition caused by mutations in EXT1 or EXT2 genes; hundreds of osteochondromas develop throughout the skeleton; ~1–5% risk of malignant transformation to chondrosarcoma.

Optical property of MSU crystals: appear yellow when oriented parallel to the slow axis of the compensator and blue when perpendicular — opposite to CPPD crystals (which are positively birefringent)

Intracellular multiprotein complex activated by MSU crystals in neutrophils and macrophages; cleaves pro-IL-1β to active IL-1β, initiating the explosive inflammatory response in acute gout

Laminated (layered) periosteal new bone deposition around a diaphyseal lesion, classically seen in Ewing sarcoma; the periosteum repeatedly lifts and re-deposits bone in concentric rings.

Mononuclear bone-forming cell of mesenchymal origin; secretes osteoid; the cell of origin for osteosarcoma.

Large, multinucleated bone-resorbing cell derived from monocyte-macrophage lineage; activated by RANKL signaling; the predominant cell type in giant cell tumor (reactive giant cells) and the effector of osteolysis in metastatic bone disease.

Unmineralised organic bone matrix, composed mainly of type I collagen, produced by osteoblasts before it is mineralised into mature bone; its accumulation (without mineralisation) characterises osteomalacia

Infection of bone and bone marrow, classified by route (hematogenous/contiguous/direct), organism (pyogenic/TB), and duration (acute/subacute/chronic)

Bony spur forming at joint margins in OA as a reparative response to cartilage loss and mechanical instability; absent in RA

Radiological appearance of the osteolytic phase of Paget disease in the skull: a sharply demarcated area of lytic bone destruction in the calvarium, representing the advancing osteoclastic front

Focal disorder of bone remodeling characterised by osteoclast hyperactivation (driven by SQSTM1 mutations and/or paramyxovirus) producing enlarged, structurally disorganised bone prone to fracture, nerve compression, and sarcomatous transformation

Destructive proliferative tissue in RA consisting of synovial fibroblasts, inflammatory cells, and granulation tissue that spreads over and erodes articular cartilage and subchondral bone

A fracture occurring through abnormal bone — bone weakened by tumor (primary or metastatic), infection, or metabolic disease — with minimal or no trauma.

Acute gout affecting the first metatarsophalangeal joint; the most common initial presentation of gout in ~50% of cases

Tuberculous osteomyelitis of the vertebral column (named after Percivall Pott, 1779); most common form of skeletal TB in India; characterised by thoracolumbar involvement, disc destruction, cold abscess, and gibbus deformity

Crystal-induced arthritis caused by calcium pyrophosphate dihydrate crystals; rhomboid/rod-shaped, positively birefringent (blue when parallel to slow axis); commonly affects knee in elderly; distinct from gout

Receptor Activator of Nuclear Factor κB Ligand; expressed by activated synovial fibroblasts and T cells in RA; binds RANK on osteoclast precursors to drive osteoclast differentiation and periarticular bone erosion

Tumor suppressor gene on chromosome 13q14 encoding the pRb protein, a key G1/S cell-cycle checkpoint regulator; biallelic loss is a central event in osteosarcoma pathogenesis.

The diagnostic cell of rhabdomyosarcoma; an elongated ('strap cell') or round cell with deeply eosinophilic cytoplasm (abundant myosin/actin); may show cross-striations in well-differentiated tumors; positive for desmin, myogenin, and myoD1.

IgM antibody directed against the Fc portion of IgG; present in 70–80% of RA but also found in other autoimmune diseases and normal elderly; not specific for RA

Extra-articular manifestation of RA; firm subcutaneous nodule showing central fibrinoid necrosis surrounded by palisading macrophages and peripheral lymphocytic infiltrate; common at extensor surfaces

Systemic amyloidosis caused by chronic inflammation (e.g., chronic osteomyelitis sinus), in which serum amyloid A (SAA) overproduced during sustained acute-phase response deposits as AA amyloid fibrils in kidney, liver, and spleen

Osteosarcoma arising in pre-existing Paget disease of bone; occurs in ~1% of polyostotic cases; affects elderly patients (7th–8th decade); highly aggressive with poor prognosis; presents as sudden pain increase, soft tissue mass, and new destructive lesion

Joint space infection; a complication of osteomyelitis when pus breaches the joint capsule; most dangerous at the hip (where the femoral neck metaphysis is intracapsular) and in infants (transphyseal vessels allow direct spread)

Fragment of devitalised (dead) cortical bone within an osteomyelitis focus; appears as a dense white fragment on X-ray; cannot be resorbed and acts as a permanent nidus for chronic infection

Epithelialised channel from chronic osteomyelitis focus through soft tissue to the skin surface, chronically discharging pus; a risk factor for Marjolin ulcer (SCC) after decades

A descriptive histologic category for tumors composed of small cells with high nuclear-to-cytoplasm ratio, round nuclei, and scant cytoplasm that stain blue on H&E; includes Ewing sarcoma, neuroblastoma, lymphoma (Burkitt's), rhabdomyosarcoma, and medulloblastoma.

Radiologic sign in giant cell tumor: multiple rounded radiolucencies separated by thin bony septa in an eccentrically placed epiphyseal lesion, resembling soap bubbles.

Scaffold protein in the NF-κB signalling pathway; mutations in SQSTM1 make osteoclasts hyperresponsive to RANKL, causing excessive osteoclast activation; mutated in 30–50% of familial Paget disease

Radiologic appearance caused by tumor bone growing radially outward through the elevated periosteum in osteosarcoma, producing bony spicules that radiate from the cortex.

Reciprocal chromosomal translocation in Ewing sarcoma creating EWSR1-FLI1 fusion gene; present in ~85% of Ewing sarcomas; acts as an aberrant transcription factor driving tumour proliferation

Transducin-like enhancer of split 1; a highly sensitive immunohistochemical marker for synovial sarcoma; nuclear positivity helps distinguish synovial sarcoma from other spindle cell sarcomas.

Nodular deposit of monosodium urate crystals surrounded by a granulomatous reaction (foreign-body giant cells, macrophages) in chronic tophaceous gout; may occur in soft tissues, ear cartilage, bursae

Renal damage in chronic gout caused by MSU crystal deposition in the medullary interstitium, leading to tubular obstruction, interstitial inflammation, and progressive renal failure

Immature bone with randomly oriented collagen fibres, produced rapidly in response to injury or pathological stimulation (fracture callus, osteosarcoma, Paget disease active phase); mechanically inferior to lamellar bone; abnormal in adults outside healing

Enzyme catalysing the final two steps of purine catabolism — converting hypoxanthine to xanthine and xanthine to uric acid; target of allopurinol (competitive inhibitor) and febuxostat (non-competitive inhibitor) in gout management