Osteoarthritis Pathology
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Direct answer
Cartilage has no blood vessels and no nerves — which is why osteoarthritis stays painless until subchondral bone and synovium are drawn into the disease. OA is a whole-organ failure of the synovial joint, not passive "wear and tear": mechanical stress and inflammatory cytokines (IL-1-beta, TNF-alpha) drive chondrocytes and their matrix metalloproteinases to degrade type II collagen and aggrecan faster than they are rebuilt, while subchondral bone stiffens and remodels, marginal osteophytes form, and low-grade synovitis advances alongside. Knees dominate Indian practice, where lifelong squatting and cross-legged sitting load the patellofemoral joint; hands follow with the classic nodal pattern. Radiology shows the tetrad of joint-space narrowing, osteophytes, subchondral sclerosis and subchondral cysts, graded 0 to 4 by Kellgren and Lawrence with osteophytes as the anchor.
What you must remember
- The radiological tetrad and its grade: joint-space narrowing, marginal osteophytes, subchondral sclerosis, subchondral cysts (geodes); Kellgren-Lawrence grade 4 is end-stage with complete joint-space loss — definite OA needs at least a grade 2 osteophyte.
- Primary versus secondary: primary is age- and load-related (knees, hips, spine, first carpometacarpal joint, distal interphalangeal nodes); secondary follows trauma, developmental dysplasia, or metabolic diseases — haemochromatosis (second-third metacarpophalangeal), ochronosis, acromegaly, crystal deposition.
- Cartilage events in order: surface fibrillation and softening, chondrocyte cluster proliferation, aggrecan loss (loss of safranin-O staining), vertical clefts, then eburnation of exposed bone with a polished, ivory-like surface.
- Clinical pattern that scores: Heberden nodes at the distal and Bouchard nodes at the proximal interphalangeal joints, first CMC squaring, crepitus, bony tenderness, morning stiffness under 30 minutes, worsening with use through the day — the mirror image of rheumatoid stiffness.
- Synovial fluid signature: non-inflammatory — white cell count under 2,000 per microlitre, high viscosity, good mucin clot — separating it from RA (inflammatory) and septic arthritis (purulent).
- Risk factors for the knee: obesity is the strongest modifiable factor (each unit of BMI multiplies load), female sex, age, prior meniscectomy or trauma, and deep-flexion occupational and lifestyle postures — squatting and cross-legged sitting being the cited Indian contributors.
- Management skeleton: education, quadriceps strengthening and weight loss as the core; topical then oral NSAIDs for pain; intra-articular steroid for flares (short-lived); total knee replacement for end-stage disease — Indian joint-replacement volumes having risen steeply as the modality became affordable.
An OA knee versus a RA knee
A 58-year-old woman has a swollen, painful right knee. Two diseases could write this page, and the bedside separates them in minutes. The OA knee is bony-hard, cool or barely warm, with coarse crepitus, varus deformity in established disease, and pain worst after activity; her hands show Heberden nodes, and the stiffness eases within minutes of moving. The RA knee is boggy and warm with a thickened synovial membrane, morning stiffness lasting over an hour, and pain worst at rest and in the small hours; the hands show metacarpophalangeal and proximal interphalangeal soft swelling with ulnar deviation in advanced disease. Aspiration completes the split: OA fluid is viscous with a count under 2,000 cells, RA fluid inflammatory in the thousands to tens of thousands, septic fluid purulent above 50,000 with neutrophil predominance. Add rheumatoid factor, anti-CCP and a radiograph — OA narrows the joint asymmetrically with osteophytes, RA erodes bone with periarticular osteopenia — and the pair is fully resolved. This comparison, reasoned rather than listed, is the answer examiners wait for.
Where students slip
Calling OA degenerative "wear and tear" alone is out of date — chondrocytes actively degrade their matrix under cytokine and mechanotransductive signalling, and anti-catabolic therapies are researched on that basis. Students confuse Heberden (DIP) with Bouchard (PIP) nodes, and forget that primary OA spares the metacarpophalangeal joints, wrists, elbows and ankles — involvement there demands a secondary cause. Erosive hand OA mimics RA but stays nodal and MCP-sparing. Finally, Kellgren-Lawrence grading uses osteophytes as its decisive criterion — the detail that turns a viva answer into a distinction.
Frequently asked questions
Which four radiological features characterise osteoarthritis?
Joint-space narrowing, marginal osteophytes, subchondral sclerosis and subchondral cysts — the Kellgren-Lawrence tetrad.
What are Heberden and Bouchard nodes?
Bony proliferations at the distal (Heberden) and proximal (Bouchard) interphalangeal joints — the nodal phenotype of hand osteoarthritis.
What does osteoarthritic synovial fluid show?
A non-inflammatory picture — white cells under 2,000 per microlitre, high viscosity and a good mucin clot.
Why does osteoarthritis spare the metacarpophalangeal joints?
Primary OA follows load-bearing and intrinsic joint stress; MCP wrists, elbows and ankles are spared, and their involvement signals secondary OA.
Which metabolic diseases cause secondary osteoarthritis?
Haemochromatosis (especially second and third MCP joints), ochronosis, acromegaly and crystal deposition diseases.