Avascular necrosis (AVN), also known as osteonecrosis, is ischemic necrosis of bone tissue due to disrupted blood supply. It most commonly affects the femoral head (80%+). Corticosteroid use, alcohol abuse, sickle cell disease, radiotherapy, and trauma (femoral neck fracture) are the main risk factors. MRI is the gold standard for early diagnosis, with the 'double-line sign' on T2-weighted images being pathognomonic — the outer hypointense line (sclerotic bone) and inner hyperintense line (granulation tissue) represent the reactive interface between necrotic and viable bone. Ficat/ARCO staging systems determine disease stage and treatment strategy. Core decompression is performed in early stages (before subchondral collapse), while total hip replacement is needed in advanced stages.
Age Range
20-60
Peak Age
40
Gender
Male predominant
Prevalence
Common
AVN is an ischemic process beginning with disruption of bone vascular supply. The femoral head is particularly vulnerable because: (1) the medial femoral circumflex artery is nearly the sole blood supply — terminal artery anatomy limits collateral compensation, (2) the subchondral region has the lowest vascular density, (3) intracapsular location is susceptible to increased intracapsular pressure. Ischemia → osteocyte death (within 6-12 hours) → structural integrity of bone matrix is initially preserved but problems arise when the reparative process begins: osteocytes in the necrotic area are resorbed and new bone formation begins but this 'creeping substitution' process leads to structural weakness → subchondral fracture → bone collapse. The pathophysiological correlate of the double-line sign on MRI: outer hypointense line = reactive sclerotic bone (newly formed sclerotic bone barrier, low signal on T2 because dense trabecular bone has short T2), inner hyperintense line = granulation tissue and hypervascular reactive zone (edema, vascular proliferation, free water → long T2). This double line represents the biologically active boundary between necrotic and viable bone.
Two parallel lines between necrotic and viable bone in the subchondral region of the femoral head on T2-weighted images: outer low signal (sclerotic bone barrier) + inner high signal (granulation tissue and reactive hypervascular zone). Pathognomonic finding for AVN with nearly 100% specificity.
On T2-weighted images, a characteristic pattern consisting of two parallel lines in the femoral head subchondral region: the outer (peripheral) low signal line represents reactive sclerotic bone, the inner (central) high signal line represents granulation tissue and hypervascular reactive zone. This double line is the reactive interface between necrotic and viable bone. Pathognomonic for AVN with 80% sensitivity and nearly 100% specificity. The double line is typically anterosuperiorly located in the subchondral bone.
Report Sentence
A double-line sign is seen in the subchondral region of the femoral head on T2-weighted images, pathognomonic for avascular necrosis.
On T1-weighted images, a well-defined, geographic (band-like) low signal area is seen in the anterosuperior subchondral region of the femoral head. This area represents the necrotic bone zone. While normal bone marrow shows high T1 signal (fat), signal drops in the necrotic area due to fat loss. T1 may be the earliest MRI finding of AVN — T1 abnormality can be detected while T2 changes are not yet apparent in ARCO Stage I. The size of the necrotic area directly correlates with prognosis: >30% femoral head involvement indicates poor prognosis.
Report Sentence
A geographic low signal area is seen in the anterosuperior subchondral region of the femoral head on T1, consistent with avascular necrosis.
The subchondral crescent sign appears as a crescent-shaped high signal area in the subchondral bone — representing the subchondral fracture and fluid/gas accumulation along the fracture line. It shows low signal on T1 and high signal on T2/STIR. On conventional radiography, this sign appears as a thin radiolucent band between subchondral bone and cartilage. It is the cardinal finding of ARCO Stage III and indicates that subchondral collapse has begun — at this stage, core decompression success rate decreases.
Report Sentence
A subchondral crescent sign is noted in the femoral head, consistent with subchondral fracture and ARCO Stage III avascular necrosis.
On T2-weighted and STIR images, diffuse bone marrow edema surrounding the necrotic area is seen — reflecting the reactive inflammatory process and reperfusion injury. Bone marrow edema directly correlates with pain: patients are typically symptomatic when edema is present. The edema pattern is usually peripheral to the necrotic zone and extends toward the femoral neck. On post-contrast images, the edema area shows enhancement (inflammation and vascular permeability).
Report Sentence
Hyperintense bone marrow edema is seen on STIR in the femoral head and neck surrounding the necrotic area, consistent with an active process.
On CT, a sclerotic band (asterisk sign) and subchondral fracture line (crescent sign) are visible in the femoral head subchondral region. The sclerotic band represents reactive bone formation at the necrotic-viable bone interface. In advanced stages, femoral head contour disruption (collapse) and secondary osteoarthritis findings (joint space narrowing, osteophytes) are seen. CT evaluates subchondral fracture line and amount of bone collapse better than MRI.
Report Sentence
A sclerotic band and subchondral fracture line are seen in the subchondral region of the femoral head on CT, consistent with avascular necrosis.
Criteria
Normal radiograph and CT; bone marrow signal abnormality on MRI. No subchondral changes.
Distinct Features
Geographic low signal on T1, double-line sign may or may not be present on T2. Reversible potential exists — ideal stage for core decompression.
Criteria
Sclerotic changes on radiograph; no subchondral fracture or collapse. Double-line sign typical on MRI.
Distinct Features
Femoral head contour preserved. Sclerotic band visible on radiograph. Core decompression may still be effective.
Criteria
Stage III: Subchondral fracture (crescent sign), femoral head collapse <2mm or >2mm. Stage IV: Secondary osteoarthritis, joint space narrowing.
Distinct Features
Core decompression usually insufficient in Stage III → osteotomy or replacement. Total hip replacement is standard in Stage IV.
Distinguishing Feature
Transient osteoporosis shows diffuse bone marrow edema WITHOUT subchondral changes (no double-line, no subchondral fracture), while AVN shows characteristic subchondral necrotic zone and double-line sign
Distinguishing Feature
Stress fracture shows a linear low signal fracture line on T1 with surrounding marrow edema, while AVN shows geographic subchondral necrotic zone and double-line sign
Distinguishing Feature
Bone metastasis shows multifocal involvement, irregular margins, and diffusion restriction, while AVN has specific geographic necrotic zone in subchondral location; double-line sign is absent in metastasis
Urgency
urgentManagement
surgicalBiopsy
Not NeededFollow-up
3-monthAVN treatment is determined by stage. In early stages (ARCO I-II), core decompression is performed — a large channel is drilled into the necrotic area to reduce pressure and promote revascularization. In Stage III, osteotomy or replacement is evaluated based on collapse degree. In Stage IV, total hip replacement is standard. Because bilateral involvement rate is 40-80%, the contralateral hip should also be screened with MRI. Risk factors should be modified. Early diagnosis is critical for prognosis — intervention before subchondral collapse increases joint preservation chances.
AVN requires early diagnosis and intervention. In early stages (Ficat I-II), core decompression may prevent collapse. In advanced stages (Ficat III-IV), total hip replacement is necessary. Bilateral involvement occurs in 50-80% — contralateral hip must be evaluated.