· clinical and radiological diagnosis
· characterised by articular cartilage damage
· clinical features - pain, stiffness and deformity
· radiological features - loss of joint space, osteophytes, subarticular sclerosis, cyst formation
Primary
· idiopathic
Secondary
Traumatic
· articular fracture
· meniscectomy
· instability
· limb malalignment
· hypermobility
Developmental
· slipped femoral epiphysis
· epiphyseal dysplasia
· Perthes’ disease
· congenital dislocation of hip
Metabolic
· ochronosis
· haemochromatosis
· crystal deposition
Inflammatory
· inflammatory arthridites
· crystal arthropathies
· septic arthritis
Neuropathic
· Charcot’s joints
Vascular
· osteonecrosis
Endocrine
· acromegaly
Specific
Ochronosis
· enzyme deficiency
· leads to accumulation of homogentistic acid
· deposited in connective tissue
· excreted in urine
· urine turns dark on standing (alkaptonuria)
· cartilage and other connective tissues stained grey
· leads to early articular cartilage degeneration
· two categories of cause
· abnormal forces on normal cartilage
· normal forces on abnormal cartilage
Abnormal forces
· force = load per unit area
Increased load
· both magnitude and frequency
· example is occupational degeneration
· footballer’s knees
· ballet dancer’s ankles
Decreased contact area
· leads to excessive stress concentrations
· increased stress
· predisposes to failure
· reduces probability of fluid film lubrication and causes contact of asperities
· examples are
· acetabular dysplasia
· intra-articular fracture
· meniscectomy
· ligament rupture
Abnormal cartilage
Physiological senenscence
· ability of cartilage to withstand compression decreases with age
· cell changes
· increased size
· decreased mitosis
· increased degradative enzyme concentration
· matrix changes
· decreased water content
· increased protein content
· decreased PG content
· altered PG ratio (chondroitin:keratin decreases)
· decreased PG half-life
· structural changes
· fibrillation of superficial layers
· cartilage softening
Increased stiffness
· caused by conditions such as onchronosis
Increased softness
· caused by chronic inflammation
Generalised susceptibility
Obesity
· closely associated with knee OA
· 7x risk from lowest 1/5 to highest 1/5 of body weight distribution
· less strongly associated with hip OA
· not associated with hand OA
· maybe metabolic rather than mechanical reason
Heredity
· increased risk for polyarthritic form in families
Osteoporosis
· negative effect (ie. decreased incidence of OA with osteoporosis)
· increased bone density may accelerate cartilage damage
Mechanical factors
Trauma
· knee trauma related to OA
· cruciate ligament damage
· meniscal tears
· fractures may alter mechanical function and predispose to OA
· femoral shaft and hip OA
· tibia and ankle OA
Occupation
· repetitive use of particular joints predisposes to OA
· association of hand OA with handedness
· repetitive knee use shown to lead to knee OA
Leisure activity
· in high level sporting activity, combination of joint trauma and repetitive use
· no increase in knee OA with recreational runners
· principally mechanical rather than inflammatory
· earliest change is disruption of collagen network
· leads to
· increased water content and swelling of cartilage
· increased proteoglycan leaching from matrix
· decreased lubrication ability
· initial response is increased PG synthesis
· new PGs have decreased ability to form aggregations
· PG loss outstrips synthesis and synthesis decreases with time
· result is PG depletion
· biomechanical effect is decreased stiffness of cartilage
· results in development of cartilage fissures on surface
· chondrocyte and synoviocyte damage causes enzyme release and further damage
· cells produce cytokines (esp. IL1 and TNFa)
· synthesis of metalloproteases and prostaglandins
Sclerosis
· increased force transmitted to subchondral bone
· causes increased thickness and sclerosis (Wolff’s law)
· result is increased mechanical strain on overlying cartilage
· precipitates cartilage degeneration
Cysts
· cysts form in subchondral bone
· may be due to
· stress disintegration of trabeculae
· focal areas of osteonecrosis
· forceful pumping of synovial fluid through cracks in bone
· bony structures at margins of joint
· articular cartilage at periphery of joint becomes hypertrophic
· fibrocartilage develops
· enchondral ossification of fibrocartilage occurs
· result is osteophyte formation
· increase surface area of joint
· cardinal features are
· cartilage destruction
· subchondral cyst formation
· subchondral sclerosis
· osteophyte formation
· capsular fibrosis
· starts at most heavily loaded part
· cartilage becomes softened and fibrillated
· surface fibrillations propagated as fissures to subchondral bone
· fragmentation occurs and articular cartilage lost
· subchondral bone exposed
· bone becomes eburnated
· separated fragments may become digested
· may remain free as loose bodies
· may grow larger due to proliferation of cells on surface
Histology
· clefts in surface, mostly vertical
· increased cellularity with clusters of chondrocytes
· duplication and irregularity of tidemark
· subchondral plate dense and thickened
Histology
· marked osteoblastic activity
· increased vascularity
· usually near greatest articular cartilage loss
· may extend into metaphysis
Histology
· margins sclerotic
· filled with thick gelatinous material
· bony outgrowths on joint margin
Histology
· cortex thickned and sclerotic
· prominent cement lines and vascular spaces
· covered with hyaline cartilage
· capsule markedly thickened
· may be adherent
Histology
· minimal activity
· may be focal inflammatory infiltrate and vascularisation
· hyalinisation and amyloid
· debris in synovial fluid incites phagocytosis by synovial cells
· membrane becomes hypertrophic
· breakdown products evoke inflammatory response
· membrane becomes thickened by inflammatory infiltrate
· synovial response much less than in RA
· rises steeply with age
· 15% at age 40
· 75% at age 70
· less than 50% have symptoms
· involves
· DIP joint of hand
· base of thumb
· knee
· hip
· facet joints
· three patterns of occurrence
· knee and hand disease
· isolated hip disease
· polyarthritic disease