Osteonecrosis refers to infarction (ischemic necrosis) of bone and marrow cells. A diverse set of conditions predisposes to bone ischemia, including vascular injury (e.g., trauma, vasculitis), drugs (e.g., corticosteroids), systemic disease (sickle cell crisis), and radiation. In about 25% of cases, the cause is unknown. The three suspected mechanisms causing osteonecrosis are mechanical disruption of vessels, thrombotic occlusion, and extravascular compression. Osteonecrosis has a wide age range but peaks in the 30s to 50s and accounts for about 10% of hip replacements in the United States.
MORPHOLOGYRegardlessofetiology,medullaryinfarctsaregeographicandinvolvethetrabecularboneandmarrow.Thecortexisusuallynotaffectedbecauseofitscollateralbloodflow.Insubchondralinfarcts,atriangularorwedge-shapedsegmentoftissuethathasthesubchondralboneplateasitsbaseundergoesnecrosis.Theoverlyingarticularcartilageremainsviable,asitcanaccessnutrientsthatarepresentinsynovialfluid.Microscopically,deadboneisrecognizedbyemptylacunaesurroundedbynecroticadipocytesthatfrequentlyrupture.Inthehealingresponse,osteoclastsresorbthenecrotictrabeculae.Trabeculaethatremainactasscaffoldingforthedepositionofnewboneinaprocessknownascreepingsubstitution.Insubchondralinfarcts,thepaceofthissubstitutionistooslowtobeeffective,sothereiscollapseofthenecroticboneanddistortion,fracture,andevensloughingofthearticularcartilage(
Fig.21.13
Fig.21.13Femoralheadwithasubchondral,wedge-shapedpaleyellowareaofosteonecrosis(arrow).Thespacebetweentheoverlyingarticularcartilageandboneiscausedbytrabecularcompressionfractureswithoutrepair.
The symptoms depend on the location and extent of infarct. Typically, subchondral infarcts cause pain that is initially associated only with activity but then becomes constant. Subchondral infarcts often collapse and may lead to severe, secondary osteoarthritis. Treatment ranges from conservative measures (limited weight bearing, immobilization) to surgery.
Osteomyelitis denotes inflammation of bone and marrow, virtually always secondary to infection. Osteomyelitis may be a complication of any systemic infection but frequently manifests as a primary solitary focus of disease. All types of organisms, including viruses, parasites, fungi, and bacteria, can produce osteomyelitis, but infections caused by certain pyogenic bacteria and mycobacteria are the most common.
Pyogenic osteomyelitis is almost always caused by bacteria and rarely by fungi. Organisms may reach the bone by (1) hematogenous spread, (2) extension from a contiguous site, and (3) direct implantation after compound fractures or orthopedic procedures. In otherwise healthy children, most osteomyelitis is hematogenous in origin and develops in the long bones. In adults, however, osteomyelitis more often occurs as a complication of open fractures, surgical procedures, and infections of the feet in diabetics.