Concept
Osteoarthritis is a disease of the whole synovial joint rather than isolated loss of articular cartilage. Changes occur in:
- articular cartilage
- subchondral bone
- synovium
- meniscus or labrum where present
- ligaments and periarticular tissues
Cartilage changes
Normal cartilage homeostasis depends on a balance between synthesis and degradation of extracellular matrix.
In osteoarthritis:
- chondrocyte phenotype changes
- matrix-degrading enzymes increase
- aggrecan and collagen network are progressively lost
- water distribution changes
- fibrillation and fissuring develop
- cartilage thickness eventually decreases
Once the collagen framework is substantially disrupted, restoration of normal architecture is limited.
Early matrix disturbance includes loss of proteoglycan organisation, altered water content and collagen network damage. Chondrocytes initially attempt repair but may later develop a catabolic phenotype with increased production of matrix-degrading enzymes and inflammatory mediators.
Surface fibrillation progresses to deeper fissuring and eventual full-thickness loss. Because adult articular cartilage has poor intrinsic repair capacity, structural damage can become self-perpetuating once the collagen framework is substantially disrupted.
Subchondral bone
Subchondral bone responds to altered loading with:
- increased turnover
- sclerosis
- cyst formation
- remodelling of the osteochondral unit
These changes can both result from and further alter joint mechanics.
Subchondral bone remodels in response to altered load. Findings can include:
- sclerosis
- cyst formation
- microdamage
- altered trabecular architecture
- changes at the osteochondral junction
The subchondral plate and cartilage act as a mechanical unit. Increased bone stiffness may alter the stresses experienced by remaining cartilage, while cartilage loss increases focal load on underlying bone.
Osteophytes
Osteophytes form at joint margins through an endochondral ossification process. They are part of the joint's biological response to altered mechanics and instability.
Osteophytes form at joint margins through endochondral ossification. They may increase the apparent surface area and stability of a degenerating joint but can also contribute to loss of movement, impingement and deformity.
Synovium
Low-grade synovitis may be present and contributes inflammatory mediators that influence cartilage and pain pathways.
Risk factors
Important contributors include:
- age
- previous joint injury
- malalignment
- obesity and increased joint load
- instability
- abnormal joint morphology
- genetic susceptibility
- occupational or sporting exposure in some settings
Major risk domains include:
- age
- previous injury
- malalignment
- obesity and metabolic factors
- occupational or repetitive loading
- abnormal joint morphology
- instability
- genetics
- muscle weakness
Post-traumatic osteoarthritis illustrates the interaction between cartilage injury, incongruity, altered mechanics and inflammation.
Ageing is not the same as osteoarthritis
Ageing cartilage may show:
- lower cellular activity
- altered matrix composition
- reduced repair capacity
Osteoarthritis includes progressive structural joint disease with characteristic cartilage, bone and synovial responses.
Pain
Articular cartilage itself is aneural. Pain can arise from:
- subchondral bone
- synovium
- capsule
- ligaments
- periarticular muscles and tendons
- marrow lesions and mechanical overload
This helps explain why cartilage loss on imaging does not perfectly predict pain severity.
Clinical principle
Treatment addresses symptoms, mechanics and joint environment rather than simply "replacing worn cartilage". Modifiable contributors such as load, alignment, instability and muscle function should be considered alongside pharmacological or surgical options.
Viva framework
A strong answer describes osteoarthritis as whole-joint failure, then covers cartilage matrix breakdown, subchondral bone response, osteophytes, synovitis and mechanical risk factors.
Osteoarthritis as organ failure of the joint
Osteoarthritis is better understood as failure of the whole synovial joint than as isolated loss of articular cartilage. The pathological process involves:
- articular cartilage
- subchondral bone
- synovium
- menisci or labrum where present
- ligaments and capsule
- periarticular muscle
- osteophyte formation at joint margins
Mechanical factors and biological responses continually interact.
Synovium and pain
Synovitis may accompany osteoarthritis and contributes inflammatory mediators and effusion. Pain cannot be attributed directly to cartilage because cartilage is aneural. Potential pain generators include:
- synovium
- subchondral bone
- capsule and ligaments
- osteophyte impingement
- periarticular muscle
- meniscal or labral pathology
- central pain processing in chronic disease
This explains why radiographic severity and symptoms correlate imperfectly.
Why alignment matters
A mechanically abnormal limb can concentrate load within one compartment. Varus alignment increases the medial knee load, while valgus alignment increases lateral compartment loading. Corrective osteotomy is therefore not simply a pain operation; it changes the mechanical environment by redistributing load.
Treatment rationale
Non-operative treatment addresses symptoms and modifiable contributors through education, activity modification, weight management where relevant, exercise and analgesic strategies. Joint-preserving surgery aims to correct a mechanical driver or focal lesion before end-stage disease. Arthroplasty replaces the damaged articulating surfaces when symptoms and structural disease justify it.
FRCS synthesis
If asked about pathogenesis, do not give a one-way sequence of “cartilage loss → bone-on-bone”. Present osteoarthritis as a feedback loop between matrix failure, abnormal load distribution, subchondral adaptation, synovial response and progressive joint dysfunction.