OA

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