Bone, Joint And Soft Tissue Pathology
On this page
Direct answer
Bone and joint pathology spans congenital and metabolic bone disease, infection, primary bone tumours with their age-site-pattern rules, the arthritides and soft tissue sarcomas. Osteoporosis is reduced bone mass with normal mineralisation at DEXA T-score −2.5 or below; osteomalacia and rickets are defective mineralisation from vitamin D deficiency; hyperparathyroidism produces brown tumours and osteitis fibrosa cystica. Osteosarcoma of the adolescent metaphysis around the knee with sunburst reaction contrasts with Ewing sarcoma of the childhood diaphysis showing small round blue cells; metastases, not primary tumours, are the commonest bone malignancies of adults.
What you must remember
- Congenital: achondroplasia from FGFR3 — short limbs, normal trunk, frontal bossing, the commonest skeletal dysplasia; osteogenesis imperfecta from type I collagen defects — multiple fractures and blue sclerae; osteopetrosis from failed osteoclast resorption — dense marble bones with anaemia and cranial nerve palsies.
- Metabolic: osteoporosis type I postmenopausal trabecular (vertebral crush, Colles) and type II senile cortical (hip) with normal biochemistry; osteomalacia-rickets with Looser zones, rachitic rosary and bow legs, vitamin D deficiency being widespread in Indian studies; Pott spinal tuberculosis with gibbus and cold abscess remains common.
- Osteomyelitis: Staphylococcus aureus commonest, metaphysis of long bones in children; sickle cell disease favours Salmonella; complications of sequestrum (dead bone) with involucrum (new bone) and chronic sinus.
- Benign tumours: osteochondroma commonest, metaphyseal cartilage-capped; giant cell tumour of the epiphysis after plate closure with soap-bubble lucency; osteoid osteoma with night pain relieved by aspirin and a radiolucent nidus.
- Malignant: osteosarcoma — adolescent metaphysis about the knee, osteoid production, Codman triangle and sunburst reaction, lung metastases, also after Paget disease or radiation; Ewing sarcoma — childhood diaphysis, small round blue cells, t(11;22), onion-peel periostitis, fever mimicking infection. Metastases dominate in adults — breast, lung, prostate, kidney, thyroid — prostate classically osteoblastic; myeloma and neuroblastoma lead in children.
- Arthritis: osteoarthritis — degenerative, distal interphalangeal nodes, eburnation, osteophytes; rheumatoid — symmetric small joints, morning stiffness, pannus, anti-CCP; gout — needle crystals strongly negatively birefringent, first metatarsophalangeal podagra, tophi; calcium pyrophosphate — rhomboid weakly positive crystals with chondrocalcinosis; seronegative spondyloarthropathies with HLA-B27.
- Soft tissue: lipoma commonest benign adult tumour; liposarcoma and undifferentiated pleomorphic sarcoma lead adult sarcomas, spreading haematogenously to lung; rhabdomyosarcoma is the commonest paediatric sarcoma; desmoid fibromatosis is locally aggressive but never metastasises; Kaposi sarcoma from HHV-8 in HIV.
Common confusion
Osteosarcoma versus Ewing: adolescent metaphysis with osteoid and Codman triangle against child diaphysis with blue cells and onion-peel layers. Osteoarthritis versus rheumatoid: degenerative distal-interphalangeal disease with osteophytes against inflammatory symmetric small-joint disease with pannus. Osteoporosis separates from osteomalacia on biochemistry and Looser zones; gout crystals are needle-shaped and strongly negative, pyrophosphate rhomboid and weakly positive.
Exam-focused takeaway
FMGE asks tumour identification from age, bone segment and radiographic pattern, and arthritis from joint distribution plus crystal or antibody. Brown tumour, sequestrum-involucrum, Pott gibbus, sunburst and onion-peel reactions are quoted verbatim in stems; keep tuberculosis and vitamin D deficiency ready as Indian associations.
Frequently asked questions
Which tumour affects the adolescent metaphysis around the knee?
Osteosarcoma, producing malignant osteoid with sunburst periosteal reaction and Codman triangle, metastasising early to lung; chemotherapy plus limb-salvage surgery is standard.
What is a brown tumour?
An osteoclast-rich haemorrhagic lesion of hyperparathyroidism within osteitis fibrosa cystica; controlling parathyroid hormone heals it and separates it from primary bone tumours.
How do osteoporosis and osteomalacia differ?
Osteoporosis is reduced mass with normal mineralisation and normal calcium-phosphate biochemistry; osteomalacia is deficient mineralisation with bone pain and Looser zones from vitamin D deficiency. DEXA defines osteoporosis at −2.5 or less.
What is a sequestrum?
Devitalised bone within new involucrum in chronic osteomyelitis, acting as a persistent foreign body needing debridement and culture-directed antibiotics.