Avascular Necrosis (AVN) Hip — Ficat & ARCO

Avascular necrosis (AVN) of the femoral head — also termed osteonecrosis — results from interruption of the blood supply to the femoral head, leading to death of bone and marrow cells, subchondral fracture, and ultimately articular collapse. It is a devastating condition disproportionately affecting working-age adults (peak 30–50 years), and bilateral involvement occurs in up to 80% of cases depending on aetiology. Early diagnosis and stage-appropriate treatment are essential to prevent or delay femoral head collapse and the need for arthroplasty.

• Aetiology: the most common causes are corticosteroid use (most common non-traumatic cause — dose-dependent; mechanism is fat cell hypertrophy causing intraosseous pressure rise and ischaemia + direct osteocyte toxicity + fat emboli), alcohol excess, and trauma (displaced femoral neck fracture, hip dislocation — damage to the retinacular vessels); other causes include sickle cell disease (vascular occlusion), systemic lupus erythematosus, diving/dysbaric osteonecrosis (nitrogen bubble formation), Gaucher disease, radiation, coagulopathy
• Pathophysiology: intraosseous ischaemia → osteocyte and marrow cell death → structural bone weakening → repetitive loading leads to subchondral fracture (crescent sign) → articular surface collapse → secondary osteoarthritis of the hip
• The anterosuperior segment of the femoral head is the most commonly and severely affected area — it bears the greatest mechanical load and has the most tenuous blood supply (terminal branches of the lateral epiphyseal vessels); the degree of involvement of this weight-bearing segment determines the prognosis

The Ficat and Arlet classification (1980) is the original and most historically important staging system for femoral head AVN, based on plain radiographic and clinical findings.

The ARCO (Association Research Circulation Osseous) classification is the most widely used modern staging system, incorporating MRI findings and quantifying the extent of femoral head involvement. It supersedes Ficat for clinical decision-making.

• Double-line sign on MRI: pathognomonic of AVN of the femoral head; seen on T2-weighted MRI; the inner bright (high signal) line represents reactive granulation tissue (hyperaemia); the outer dark (low signal) line represents the sclerotic margin of the necrotic zone; this sign at the interface between living and dead bone is specific for AVN and distinguishes it from other causes of hip pain
• Crescent sign: a subchondral lucency visible on the frog-leg lateral view of the hip; represents separation of the dead subchondral bone from the overlying articular cartilage (subchondral fracture); marks the transition from Stage II to Stage III ARCO; its appearance indicates that femoral head collapse is imminent or has begun; best seen on frog-leg lateral view, also visible on CT

• MRI: the gold standard investigation for early AVN (Stages 0–II); sensitivity and specificity approaching 99%; the double-line sign is pathognomonic; MRI also quantifies the size of the necrotic lesion (as a percentage of femoral head volume) and its location (anterosuperior = worst prognosis); mandatory bilateral MRI — up to 80% bilateral involvement
• Plain radiographs (AP pelvis and frog-leg lateral): normal in early disease; sclerosis and cysts in Stage II; crescent sign in Stage III; collapse in Stage IV; essential for staging and planning; always include a frog-leg lateral to assess for the crescent sign
• CT: excellent for quantifying the degree of femoral head collapse and the extent of subchondral fracture; useful pre-operatively for surgical planning; identifies the crescent sign reliably
• Bone scintigraphy (technetium-99m): historically used; "cold-in-hot" pattern (cold photopenic area within an area of increased uptake); superseded by MRI for diagnosis
• Blood tests: FBC, ESR, CRP (exclude infection and inflammatory arthritis); thrombophilia screen; fasting lipids; coagulation screen where haematological aetiology is suspected

• Core decompression: the primary joint-preserving intervention for pre-collapse AVN (ARCO I–II, selected Stage III); multiple drill holes or a single core tract decompresses the elevated intraosseous pressure, promotes revascularisation, and may halt disease progression; success rates of approximately 70–80% in Stage I, 60–70% in Stage II, significantly lower in Stage III; can be supplemented with bone grafting (structural allograft, vascularised fibular graft) to provide mechanical support
• Vascularised fibular graft (FVFG): a vascularised segment of the fibula is harvested with its peroneal vessel pedicle and inserted into the core decompression channel; provides both mechanical support and vascular supply to the necrotic zone; the most biologically effective joint-preserving procedure; technically demanding microsurgical procedure; results superior to simple core decompression in Stage II–III in specialist centres

• Necrotic volume and prognosis: the size and location of the necrotic lesion on MRI is the most important prognostic factor for progression to collapse; lesions involving <15% of the femoral head volume (small lesion) have an excellent prognosis with or without treatment; lesions >30% (large lesion) involving the weight-bearing anterosuperior segment have a very high rate of progression to collapse; the Kerboul combined necrotic angle (>200° = large lesion, high risk) is a validated quantitative MRI measure used in treatment decisions
• Steroid-induced AVN — dose and risk: the risk of AVN increases with cumulative steroid dose and with pulse high-dose intravenous steroids; a single short course of oral steroids has minimal risk; patients on prolonged corticosteroids (e.g., post-transplant, rheumatological disease, haematological malignancy) should be counselled about AVN risk; early MRI should be performed in steroid-treated patients presenting with hip pain
• THA in AVN — specific technical considerations: AVN patients tend to be younger than primary OA patients; ceramic-on-ceramic bearing surfaces are preferred (maximise longevity, avoid polyethylene wear in a young high-demand patient); uncemented fixation preferred (press-fit acetabular shell + cementless femoral stem); the femoral head in late AVN is often collapsed and irregular — correct femoral head centre restoration is important; the acetabulum is usually spared until very late (Stage V) — acetabular bone stock is generally normal
• Sugioka transtrochanteric rotational osteotomy: a technically demanding procedure in which the femoral head is rotated to bring the posterior non-necrotic cartilage into the weight-bearing position; can delay or avoid THA in selected young patients with anterosuperior AVN and an intact posterior segment; requires careful pre-operative MRI assessment to confirm adequate viable posterior cartilage; results are technique-sensitive and operator-dependent

• MRI double-line sign: pathognomonic of AVN; T2-weighted; inner high signal (granulation) + outer low signal (sclerosis); ARCO Stage I–II; bilateral MRI mandatory (up to 80% bilateral)
• Crescent sign: subchondral lucency on frog-leg lateral view; subchondral fracture; ARCO Stage III; collapse is imminent
• Most common non-traumatic cause: corticosteroid use; mechanism — fat cell hypertrophy + intraosseous pressure rise + direct osteocyte toxicity; dose-dependent
• ARCO I–II (pre-collapse): core decompression ± grafting; joint-preserving; ~70–80% success in Stage I; lower in Stage III
• Necrotic volume >30% of femoral head + anterosuperior location = high risk of collapse; Kerboul angle >200° = large lesion; THA likely required regardless of joint-preserving attempt
• Vascularised fibular graft: most effective joint-preserving procedure; provides mechanical support AND vascular supply; technically demanding; specialist centres
• ARCO IV–V (collapse/OA): THA; ceramic-on-ceramic preferred in young AVN patients; uncemented fixation; acetabulum usually spared until late
• Ficat vs ARCO: Ficat I–IV (plain X-ray based); ARCO 0–V (MRI incorporated, lesion quantification); ARCO is the modern standard
• Anterosuperior segment: most commonly affected; highest mechanical load; most tenuous blood supply; most important for prognosis
• Bilateral disease: up to 80% bilateral involvement; always image both hips; stagger treatment if both require intervention