Osteonecrosis

Osteonecrosis refers to cell death within bones that occurs from a lack of circulation rather than from disease. The structural components are unaffected. The cells include osteocytes, and usually the marrow contents.

Related Terms

Osteonecrosis is preferred over other terms avascular necrosis, ischaemic necrosis, and aseptic necrosis because it is more accurate histolo-pathologically and doesn't imply any specific aetiology. Historically, ischaemic and avascular necrosis was reserved for subchondral (epiphyseal) osteonecrosis, while bone infarct was referred to as medullary (metaphyseal) osteonecrosis. Osteonecrosis is a more general and inclusive term.

Pathogenesis

Osteonecrotic bone is not avascular as the vessels are still present. However in all mechanisms of osteonecrosis there is compromise of the circulation. There are four possible mechanisms of compromise

Mechanical disruption of the vessels: From fracture or dislocation, or from atraumatic events such as stress or fatigue fractures
Occlusion of the arterial vessels: From thrombosis, embolism, circulating fat, nitrogen bubbles, or abnormally shaped cells (sickle cell crises).
Injury to or pressure on the arterial wall: From damage within the wall due to vasculitis or radiation injury; from damage within the vessel from release of materials causing angiospasm; or from external pressure or chemical reaction on the wall in extravasated blood, fat, or cellular elements in the marrow cavity.
Occlusion to the venous outflow vessels: From venous pressure exceeded arterial pressure, may be mitigated by collateral circulation if sufficient.

Aetiology

The cause of osteonecrosis in most cases is not completely understood. The most common causes of nontraumatic osteonecrosis are idiopathic, and in association with ethanol and corticosteroids. The mechanisms is unknown.

Fracture or dislocation: The most common traumatic injuries leading to osteonecrosis are fractured femoral neck, dislocation of the femoral head, displaced fracture of the scaphoid, displaced fracture of the talar neck, and a 4-part fracture of the humeral head. There are clinically significant secondary complications in the form of collapse of the subchondral bone and adjacent articular surface.
Infection: In osteomyelitis and pyarthrosis there is a combination of increased intramedullary pressure and arterial occlusion.
Gaucher's disease: The marrow cavity is packed with Gaucher's cells which are macrophages filled with cerebroside. This leads to direct occlusion of intraosseous arteries.
Sickle cell disease: The marrow cavity is packed with sickle shaped red blood cells. This also leads to direct occlusion of intraosseous arteries.
Decompression sickness (caisson disease): There is likely vascular occlusion by nitrogen bubbles coming out of solution with a rapid drop in barometric pressure
Radiation: This occurs from radiation damage to the capillaries
Ethanol
Corticosteroid administration
Hyperlipidaemia
Pancreatitis
Idiopathic osteonecrosis: One theory is intraosseous hypertension with excessive pressure within the medullary space resulting in occlusion of intraosseous vessels.

Eponymous Names for Specific Sites

Ahlback disease: medial femoral condyle, i.e. SONK
Brailsford disease: head of the radius
Buchman disease: iliac crest
Burns disease: distal ulna
Caffey disease: entire carpus or intercondylar spines of the tibia
Dias disease: trochlea of the talus
Dietrich disease: head of metacarpals
Freiberg infraction: head of the second metatarsal
Friedrich disease: medial clavicle
Hass disease: humeral head
Iselin disease: base of 5th metatarsal
Kienböck disease: lunate
Köhler disease: patella or navicular (children)
Kümmell disease: vertebral body
Legg-Calvé-Perthes disease: femoral head
Mandl disease: greater trochanter
Mauclaire disease: metacarpal heads
Milch disease: ischial apophysis
Mueller-Weiss disease: navicular (adult)
Panner disease: capitellum of the humerus
Pierson disease: symphysis pubis
Preiser disease: scaphoid
Sever disease: calcaneal epiphysis
Siffert-Arkin disease: distal tibia
Thiemann disease: base of phalanges