MRI finds it before the radiograph: a serpiginous subchondral band with the double-line sign on T2 — then measure the extent of the head involved and look for subchondral collapse, because collapse decides joint-preserving versus replacement surgery.
Orient first
- Risk factors: steroids, alcohol, sickle cell disease, trauma (femoral neck fracture), and post-COVID steroid exposure — often bilateral, so image both hips.
- The necrotic segment is usually anterosuperior, in the weight-bearing dome.
- Staging systems (Ficat–Arlet, ARCO) turn on whether there is subchondral collapse (crescent sign, flattening) — verify the version in use.
Acquire the study
- Both hips: coronal T1, coronal STIR, sagittal and axial fat-saturated T2 or PD.
The manoeuvre
- Coronal T1: a low-signal serpiginous band outlining the necrotic segment of the femoral head.
- T2 or STIR: the double-line sign (inner high-signal and outer low-signal line) at the reactive interface.
- Extent: estimate the proportion of the weight-bearing head involved on coronal and sagittal planes.
- Collapse: subchondral fracture line, loss of head sphericity, and articular step.
- Marrow oedema and effusion — oedema often accompanies collapse and pain.
- The other hip: silent contralateral disease is common.
What confirms it
- A subchondral band enclosing a segment of the femoral head, with or without collapse, on MRI.
What licenses you to exclude it
- A normal MRI of both hips excludes avascular necrosis; a normal radiograph does not.
The classic misread
- Calling transient osteoporosis (diffuse oedema without a subchondral band) avascular necrosis.