Bite sign | Radiology Signs

Answer:

The bite sign is a sharply demarcated, gouged-out depression of the anterosuperior femoral head contour, as though a segment had been bitten out of an otherwise spherical head. It represents segmental collapse of the necrotic subchondral bone after a subchondral fracture has separated it from viable bone, and it therefore marks the transition from a structurally intact head to an incongruent one. It is a later finding than the crescent sign and defines flattening, ARCO stage 3B onward, with stage 4 reached once secondary acetabular degenerative change appears. Like the crescent sign it is a frog-leg lateral or sagittal reformat finding, because the collapsed segment is anterosuperior and is projected en face on the anteroposterior view, where only a subtle break in the cortical arc may be visible; recent comparative work confirms that anteroposterior radiography understages collapse and that lateral projections or CT are needed. Quantifying the depression matters, since depression greater than 2 mm and involvement of more than 30 per cent of the weight-bearing surface predict progression and steer management toward arthroplasty rather than joint preservation. The differential is a subchondral insufficiency fracture with articular collapse in the elderly, and post-traumatic or Perthes deformity, both distinguished by history and by the absence of a demarcating necrotic interface on MRI.

Why is it called so?

The focal defect in the round femoral head contour looks like a bite taken out of a piece of fruit.

Pathophysiology

Once a subchondral fatigue fracture has isolated the necrotic segment, the dead bone can no longer transmit load. Continued weight-bearing crushes the trabeculae of the segment, and the overlying cartilage, still viable because it is nourished by synovial fluid, sinks with it, producing a step-off rather than a defect. The anterosuperior location follows the distribution of the retinacular arterial supply and of peak contact stress. Collapse changes joint congruity, concentrates load on the acetabular rim and drives rapid secondary osteoarthritis, which is why radiographic collapse and not marrow signal change is the threshold that decides surgical management.

Alternative names: Gouged-out appearance; segmental collapse of the femoral head

Other associated named signs: Crescent sign, the double line sign on MRI, and sagging rope sign of healed Perthes disease

References

1. Kramer J, Scheurecker G, Scheurecker A, Stöger A, Huber A, Hofmann S. [Femoral head necrosis]. Radiologe. 2009;49(5):410-8. PMID: 19296066.