Avascular Necrosis (Osteonecrosis)

Avascular necrosis, also called osteonecrosis, is the death of bone tissue caused by interruption of its blood supply. When it affects subarticular bone, particularly the femoral head, the weakened bone can fracture and collapse beneath the cartilage, leading to joint incongruity and secondary degenerative arthritis. It is a distinctive arthropathy because the primary failure is ischaemic injury to bone rather than to cartilage or synovium.

Definition

Osteonecrosis (avascular necrosis) is death of bone and marrow resulting from impaired or interrupted blood supply; in joints it characteristically affects subchondral bone, where structural failure can lead to articular collapse and secondary osteoarthritis.

Scope

This topic covers the causes and mechanisms of osteonecrosis: the disruption of blood supply that initiates bone death, the structural consequences of subchondral collapse, and the imaging features by which the process is recognised. It is framed as a reference and educational entry on disease mechanism, not as management guidance.

Core questions

• What disrupts the blood supply to bone and causes it to die?
• How does subchondral bone death progress to articular collapse?
• Why does osteonecrosis preferentially affect sites such as the femoral head?

Key concepts

• Interruption of bone blood supply (ischaemia)
• Traumatic versus nontraumatic osteonecrosis
• Risk factors: corticosteroids, alcohol, sickle cell disease
• Subchondral fracture and femoral head collapse
• Vulnerable vascular anatomy of the femoral head
• Staging by imaging (radiograph, MRI)

Mechanisms

Osteonecrosis begins when the blood supply to a segment of bone is interrupted, depriving osteocytes and marrow of oxygen and causing cell death. The interruption may be traumatic, as when a femoral neck fracture or dislocation disrupts the precarious blood supply of the femoral head, or nontraumatic, associated with factors such as corticosteroid use, excess alcohol, and conditions including sickle cell disease that impair perfusion (Mont, 2006). In weight-bearing subarticular bone the dead segment loses mechanical strength; a subchondral fracture can develop, after which the overlying articular surface flattens and collapses, producing joint incongruity and secondary osteoarthritis. Because changes precede radiographic findings, magnetic resonance imaging is central to recognising early disease and to staging its progression (Pierce, 2015).

Clinical relevance

Osteonecrosis is clinically important because, untreated, subchondral collapse of a major weight-bearing joint such as the hip can lead to disabling secondary arthritis, often in relatively young patients. This entry describes the disease process for reference and education and is not a basis for individual diagnosis or treatment.

Epidemiology

Nontraumatic osteonecrosis of the femoral head characteristically affects adults in their thirties to fifties and is strongly associated with corticosteroid exposure and alcohol use; traumatic osteonecrosis follows injuries that disrupt femoral head perfusion, and the femoral head is the most commonly involved and most studied site (Mont, 2006).

Seminal works

• mont-2006
• pierce-2015

Frequently asked questions

What causes avascular necrosis of bone?

It is caused by interruption of the bone's blood supply, which may be traumatic (for example after a femoral neck fracture) or nontraumatic (associated with factors such as corticosteroid use, alcohol, and sickle cell disease).

Why is the femoral head so often affected?

The femoral head has a precarious blood supply that is easily disrupted, and as a weight-bearing surface the dead subchondral bone is prone to fracture and collapse, leading to secondary arthritis.