Definition
No accepted definition
Chronic joint disorder in which there is progressive softening and disintegration of articular cartilage
- accompanied by new growth of cartilage and bone at the joint margins
- these changes secondary to mechanical failure of hyaline cartilage
Essentially no inflammatory component
Classification
Primary
Idiopathic
- cartilage degenerates in all as time goes by
- high incidence of cartilage wear & OA in 7th decade
Secondary
Traumatic
- fracture / meniscectomy / instability / limb malalignment
Infection
Tumour
- PVNS / synovial chondromatosis / lipoma arborescens
Inflammation
- RA / spondyloarthopathy / CT disorders (SLE, sarcoidosis, scleroderma)
Metabolic
- gout / pseudogout / haemochromatosis / onchronosis
Neuromuscular
- charcot
Endocrine
- acromegaly / Paget's
Development
- SUFE / DDH / Perthes/ skeletal dysplasias
Epidemiology
Prevalence
- rises steeply with age
- 15% at 40
- 75% at 70
- > 50% have symptoms
Cause
1. Abnormal forces on normal cartilage
Force = Load / Unit area
Increased Load - obesity
Decreased contact area - subluxation / ankle diastasis
2. Normal forces on abnormal cartilage
Age
- cartilage more stiff / less strong & elastic
- hypocellular
- decreased water content
Increased Stiffness
- ochronosis / CPPD / HA deposition
Increased softness
- chronic Inflammation
Pathology
Cardinal Features
- cartilage disintegration
- subchondral cysts
- subchondral sclerosis
- osteophyte formation
- capsular fibrosis
1. Collagen network damaged
- disorganised & loosened
2. Loss of Proteoglycan
- leach from matrix
- decreased Chondroitin : Keratin
3. Increase of water
- as a result of above the water content increases
4. Cartilage swells
- less stiff and more prone to damage
- increased permeability
5. Chondrocyte damage
- IL1 released from synovium & chondrocytes
- IL1 important mediator of metalloproteinases
- collagenase breaks down collagen
6. Cartilage damage
- attempt at repair
- hypermetabolic state
7. Subchondral Bone
- increased force transmitted to bone
- result is increased mechanical strain on overlying cartilage
- precipitates cartilage degeneration
8. Subchondral Cysts
- ? caused by stress fractures / focal AVN / synovial fluid pumps through cracks
9. Osteophytes
- result of piezoelectric forces from abnormal stress
- increase surface area of joint
Patterns
Monoarticular or Pauciarticular OA
Pain & dysfunction in 1 or 2 of the large weight bearing joints
Secondar OA
- OA single joint from previous problem
- most common is knee post meniscectomy
Hip OA
- secondary to mild dysplasia
Polyarticular OA
Usually middle-aged woman
Hands
- pain, swelling & stiffness in fingers
- characteristic knobbly appearance of IPJ
- from osteophytes & ST swelling
Often affects knees
- medial & PFJ compartments
Also
- base of thumb
- MTPJ of hallux
- facet joints
Rapidly Destructive OA
Rapidly progressive loss of joint space
- usually affects hip