· in situ death of a segment of bone due to ischaemia
· multifactorial heterogenous group of disorders that lead to common final pathway of bone death
· femoral head
· humeral head
· scaphoid
· lunate
· talus
· femoral condyles
· capitellum
Traumatic
· fractures
· dislocations
Nontraumatic
· corticosteroid use
· alcohol abuse
· sickle cell disease
· dysbarism
· Gaucher’s disease
· radiation
· due to ischaemia of bone
· numerous theories
· are mutual rather than exclusive
· fracture
· dislocation
· slipped upper femoral epiphysis
· post-treatment of CDH
· dysbarism (nitrogen bubbles)
· corticosteroids (fat emboli)
· alcohol (fat emboli)
· sickle cell disease (abnormal red cells)
· radiation injury
· vasculitis (SLE)
· Gaucher’s disease (bloated macrophages)
· corticosteroids (enlarged marrow fat cells)
· alcohol (enlarged marrow fat cells)
· post-infection (inflammatory response)
Death
· some ischaemic event occurs
· segment of bone dies
· marrow cells die within 6-12 hrs
· osteocytes die within 12-48 hrs
Revascularisation
· capillaries and mesenchymal cells grow in from abutting live marrow
· grow into dead marrow spaces
· mesenchymal cells differentiate
Repair
· macrophages remove dead fat and cellular debris
· dead bone resorbed by osteoclasts
· new bone laid down on surface of dead trabeculae by osteoblasts
· if healing incomplete, dead bone replaced by fibrous tissue and granulation tissue
· most commonly affects
· anterosuperolateral portion of femoral head
· central dome of humeral head
· femoral condyles
· talus
· scaphoid
· lunate
· capitellum
Stage 1 - before onset of structural failure and collapse
· patient symptom-free
· joint contour normal
Pathology
· articular cartilage normal
· beneath articular cartilage is variable depth of
· opaque yellow marrow
· bone trabeculae devoid of osteocytes
· dead tissue separated from underlying living bone by
· dense fibrous layer
· vascular granulation tissue
· in border zone, trabeculae broadened by formation of new bone on surface
Radiology
· death of bone does not alter its radiological density
· if area immobilised
· living bone becomes porotic due to osteoclastic bone resorption
· dead bone appears relatively denser
· after a period
· cystic areas appear within lesion due to osteoclastic resorption
· irregular mottled areas of increased density appear at junction of living and dead bone due to new trabecular bone
· coalesce to form compact sclerotic line
Stage 2 - structural failure and collapse
· marked by sudden onset of pain
Pathology
· due to incomplete healing response
· occurs in 90%
· resorption > formation
· trabecular microfractures occur in dead bone
· leads to weakening and fracture
1. subchondral fracture
· immediately below subchondral plate
· produces thin articular sequestrum (eggshell)
· joint contour initially appears normal
· wrinkle then appears at dead margin
· then cracks and fissures appear
· escape of bony detritus through crack into joint leads to synovitis
· fibrocartilage may form on undersurface of sequestrum
· sequestrum may become reattached by fibrocartilage
· sequestrum may be ground away
· fibrocartilage may be exposed to form new joint surface
2. deep fracture
· trabecular fracture may occur more deeply
· at junction between living and dead bone
· large sequestrum sinks into femoral head
· uncollapsed margin may produce ring-like ridge
Radiology
· area of dead bone may appear dense due to
· compression of dead trabeculae
· calcification of dead marrow or debris
1. subchondral fracture
· trabecular fracture appears as lucent crescentic line running parallel to joint surface
· ‘crescent sign’
· in hip, best seen on frog-leg lateral
2. deep fracture
· no crescent sign
· articular step seen beneath acetabular margin and at fovea
Stage 3 - development of secondary arthritis
Pathology
· joint incongruity leads to loss of articular cartilage
· osteophytes form
· asymptomatic
· occurs with
· dysbarism
· haemoglobinopathies
· Gaucher’s disease (occas.)
· most commonly affects
· lower femoral shaft
· upper tibial shaft
· upper humeral shaft
· extent varies
· small foci
· large areas involving endosteal cortex
Pathology
· dead marrow is yellow and opaque
· surrounded by dense collagen layer
· collagen may be calcified
· bone trabeculae at margin may be broadened by repair
· if endosteal cortex involved, cortical width increased
Radiology
· main change is calcification
· if occurs in collagen layer, is wavy line of increased density
· ‘coil of smoke’ sign
· endosteal cortex may be thickened
· may be difficult to distinguish from
· calcified enchondroma
· bone island
· early changes
· mottling
· sclerotic line at junction between dead and living bone
· late changes
· crescent sign
· segmental collapse
· end-stage changes
· osteoarthritis
· until osteoarthritis develops, joint space maintained
· initially absent or decreased uptake seen due to avascularity
· does not absolutely predict necrosis as revascularisation may occur
· later increased uptake seen due to repair
· does not absolutely predict good outcome as revascularisation may not be adequate or permanent
· non-specific
· cold areas may be due to metastatic deposits
· hot areas may be due to any cause of increased bone vascularity or formation
· most useful to detect avascularity following acute femoral neck fracture or hip dislocation
· not useful early
· can be used in later stages to assess
· extent of subchondral fracture
· flattening and collapse of articular surface
· most sensitive and specific method
· proximal femur most extensively studied
Changes
Normal
· on T1 image
· normal medullary cavity emits strong signal (white) from hydrogen-rich fatty marrow
· normal cortex and subchondral plate emit weak signal (black)
Necrosis
· T1 shows low-signal line
· corresponds to sclerotic line on x-ray
· takes weeks to develop
· area cupped by low-signal band shows varying amount of decreased signal signifying dead bone
· T2 shows characeristic double line
· outer low-signal line related to thickened trabeculae
· inner high-signal line related to granulation tissue
Advantages
· able to identify necrosis in high-risk patients in pre-radiological state
· accurately reveals site and extent of bone death
· three-phase invasive investigation developed by Ficat
· no longer routinely used if MRI available
Intraosseous pressure
· intertrochanteric cannula inserted
· bone marrow pressure measured
· abnormal if
· baseline pressure > 30 mm Hg
· pressure > 30 mm Hg 5 min after injection of 5 ml isotonic saline
· oxygen saturation of blood from cannula > 85%
Intramedullary venogram
· 10 ml contrast injection
· abnormal if
· injection difficult and painful
· diaphyseal stasis or reflux seen at 15 min
Biopsy
· trephine from greater trochanter to within 5 mm of articular cartilage
· abnormal if
· marrow necrosis
· medullary and trabecular necrosis
· necrosis with marrow fibrosis and appositional new bone