Hip AVN: Ficat & ARCO Staging Explained on MRI

Why Staging Matters

AVN follows a predictable but highly variable course. In the earliest stages, the necrotic segment of bone is still structurally intact — it has not collapsed, and the joint surface is still smooth. Joint-preserving procedures such as core decompression, vascularized bone grafting, or protected weight-bearing can slow or halt progression and allow patients to keep their own hip for decades. Once the femoral head collapses and the cartilage is destroyed, joint preservation is no longer realistic, and total hip arthroplasty becomes the definitive treatment. Staging makes this distinction precise and reproducible so that surgeons and patients can plan together.

Ficat Stage I: Pre-Radiographic Disease

In Ficat Stage I, plain X-rays appear completely normal, yet the patient typically has hip or groin pain and the necrotic process is already underway. MRI is the only imaging modality that reliably detects this stage. The characteristic MRI findings are bone marrow edema — seen as increased signal on T2-weighted fat-suppressed sequences — and the double-line sign on T2-weighted images. The double-line sign consists of an inner hyperintense rim of granulation tissue surrounded by a hypointense band of reactive sclerosis; when present, it is pathognomonic for AVN, meaning no other condition produces this exact pattern. Identifying Stage I disease is critical because it represents the best window for joint-preserving treatment.

Ficat Stage II: Radiographic Changes Without Collapse

Ficat Stage II is defined by visible changes on plain X-ray — including areas of sclerosis, cyst formation, or mixed density within the femoral head — while the overall spherical contour of the femoral head remains intact and there is no collapse. On MRI, the necrotic segment is clearly demarcated and typically shows low T1 signal and a well-defined reactive interface. The size of the necrotic lesion matters enormously at this stage: small lesions in the medial femoral head carry a low risk of collapse, while large lesions occupying more than one-third of the femoral head surface are at high risk of progressing regardless of treatment.

Ficat Stage III: Subchondral Collapse and the Crescent Sign

Stage III marks a pivotal and often irreversible threshold: the subchondral bone beneath the dead segment fractures, producing the crescent sign — a thin radiolucent line paralleling the articular surface on X-ray or a subchondral fracture line on MRI. The articular cartilage itself may still be intact at this stage, but the structural support beneath it has failed. Once the crescent sign is visible, collapse of the femoral head typically follows within weeks to months without intervention. The amount of depression (measured in millimetres on coronal MRI) guides surgical decisions: small collapse under 2 mm may still be approached with joint-preserving surgery; collapse greater than 4 mm generally predicts a poor result with anything other than total hip arthroplasty.

Ficat Stage IV: Acetabular Involvement and End-Stage Arthritis

In Ficat Stage IV, the collapsed and flattened femoral head begins to damage the acetabular cartilage. X-rays show joint space narrowing, acetabular sclerosis, and often osteophyte formation — the full picture of secondary osteoarthritis. MRI confirms the extent of cartilage loss on both the femoral and acetabular sides. At this stage, the patient experiences severe pain, markedly limited range of motion, and significant functional disability. Total hip replacement is the standard treatment and reliably restores function and eliminates pain.

ARCO 1–4: The Modern Equivalent

The Association Research Circulation Osseous (ARCO) classification was developed to standardise AVN staging internationally and to add lesion-size substaging that the Ficat system lacks. ARCO 1 corresponds to Ficat Stage I — MRI-detectable disease with normal X-rays — and is further subdivided by lesion size into 1A (less than 15% of femoral head), 1B (15–30%), and 1C (greater than 30%). ARCO 2 corresponds to Ficat Stage II, with visible X-ray changes but no collapse. ARCO 3 corresponds to Ficat Stage III, defined by subchondral fracture and crescent sign, with subclassification by the degree of depression in millimetres. ARCO 4 corresponds to Ficat Stage IV with acetabular involvement. Most major orthopaedic centres now use ARCO staging because the lesion-size information directly informs treatment strategy: the same stage may carry a very different prognosis depending on whether the lesion is small or large.

Treatment by Stage

For Stage I and early Stage II (small lesions, ARCO 1A–2A), the approach is joint preservation. Protected weight-bearing reduces mechanical loading on the at-risk segment. Bisphosphonates, by reducing bone turnover, may slow the expansion of the necrotic area, though evidence is stronger for symptom relief than for halting progression. Core decompression — drilling one or more channels into the necrotic segment through the femoral neck — decompresses the elevated intraosseous pressure and creates a channel for revascularisation. Adding concentrated bone marrow aspirate (stem cells) to the decompression tunnel has shown improved outcomes in several prospective trials, particularly in Stage I and Stage IIA disease.

For Stage IIb–III with larger lesions and early collapse, vascularised fibular grafting — where a segment of the patient's own fibula with its blood supply is inserted into the femoral head to provide structural support and new vascularity — offers the best chance of preserving the native joint. Free vascularised iliac crest grafts are an alternative. These procedures are technically demanding and performed at specialist centres. When collapse exceeds 4 mm or the patient is older with lower functional demands, total hip arthroplasty is often a more reliable choice even at Stage III.

For Stage IV, total hip arthroplasty is the definitive treatment. Modern implants provide excellent pain relief and restore function to near-normal in most patients. Younger patients with AVN who undergo hip replacement should discuss implant choice and activity restrictions with their surgeon, as the goal is a prosthesis that will last many decades. For more on how AVN appears on MRI and what the AI analysis looks for, see the AVN condition page. For general guidance on reading a hip MRI report, see our hip MRI reading guide.

Key Takeaways

• MRI detects AVN years before X-ray changes — Ficat I and ARCO 1 are invisible on plain radiographs
• The double-line sign on T2-weighted MRI is pathognomonic for AVN and is the key Stage I finding
• The crescent sign marks subchondral fracture (Stage III) and signals that collapse is imminent without intervention
• Lesion size within each stage strongly predicts collapse risk — ARCO substaging (A, B, C) encodes this information
• Stage I–IIa disease with small lesions is best treated with core decompression ± stem cells to preserve the native joint
• Stage IV disease with acetabular involvement requires total hip arthroplasty, which reliably restores function and eliminates pain

Frequently Asked Questions