Crescent Sign

1. High-Yield Overview & Pathognomonic Essence

• Morphological Hallmark: The crescent sign presents as a thin, well-demarcated curvilinear radiolucent line running parallel and immediately subjacent to the cortical bone margin of the femoral head.
• Pathognomonic Significance: It represents an in situ subchondral fatigue microfracture through mechanically compromised necrotic cancellous trabeculae in avascular necrosis (osteonecrosis of the femoral head, ONFH), occurring before gross flattening or articular collapse.
• Critical Surgical Watershed: The appearance of the crescent sign marks the definitive transition from pre-collapse (Ficat Stage II / ARCO Stage 2) to structural collapse (Ficat Stage III / ARCO Stage 3), representing a prognostic inflection point where joint-preserving core decompression outcomes precipitously decline, prompting consideration of structural osteotomy or total hip arthroplasty.

2. Diagnostic Sign Summary Matrix

3. Clinical Case Imaging & Standardized Figures

4. Pathophysiology & Biomechanical Mechanism

The crescent sign reflects a distinct biomechanical and histopathological failure mode resulting from progressive vascular disruption within the femoral head. Terminal arterial blood supply to the epiphysis depends primarily on the lateral epiphyseal branches arising from the deep branch of the medial femoral circumflex artery (MFCA). Following ischemic insult—triggered by corticosteroid exposure, chronic alcohol consumption, sickle cell hemoglobinopathy, trauma, or idiopathic microvascular occlusion—coagulative necrosis develops in both hematopoietic marrow and structural osteocytes throughout the anterosuperior epiphyseal wedge.

In response to necrosis, a peripheral reparative response initiates from adjacent viable cancellous bone. Capillary neurovascular buds and undifferentiated mesenchymal stem cells invade the necrotic perimeter, initiating “creeping substitution.” In this repair front, osteoclastic bone resorption outpaces osteoblastic bone deposition. Consequently, the trabeculae bordering the necrotic segment undergo transient structural thinning and microarchitectural porosity.

Under repetitive physiologic cyclical weight-bearing (reaching 3 to 4 times body weight during single-leg stance), high contact shear stresses concentrate directly across the unsupported, osteopenic repair front. The necrotic, brittle trabeculae suffer mechanical fatigue microfractures that coalesce into a contiguous transverse subchondral cleavage plane. This fracture line detaches the overlying osteochondral shell—consisting of intact articular cartilage supported by a thin sliver of viable subchondral bone—from the underlying dead cancellous spongiosa.

The newly formed subchondral space fills with synovial fluid, nitrogen gas liberated under transient negative intra-articular pressure (a true micro-vacuum phenomenon), or friable fibrovascular tissue. On tangential radiographic projections, this radiolucent cleft projects as the characteristic curvilinear “crescent line.” Once this mechanical cleavage develops, the overlying osteochondral fragment loses foundational structural support, inevitably predisposing the femoral head to progressive macroscopic articular collapse and secondary osteoarthritic degradation.

5. Modality-Specific Imaging Findings

Plain Radiography

On anteroposterior (AP) radiographs of the pelvis and dedicated hip, the crescent sign appears as a smooth, curvilinear, lucent fracture line 1 to 2 mm in thickness, positioned immediately beneath the dense convex subchondral bony plate in the anterosuperior quadrant of the femoral head. Importantly, the adjacent acetabular joint space remains completely preserved, and femoral head sphericity is maintained in early presentations (ARCO Stage 3A).

The frog-leg lateral (Lauenstein) projection represents the single most sensitive plain radiographic projection for identifying the crescent sign. Because avascular necrosis characteristically involves the anterior and superior weight-bearing surfaces, standard AP views frequently suffer from superimposition of dense reactive sclerotic margins. The frog-leg projection rotates the anterosuperior dome into true profile, demonstrating subtle subchondral fractures that are invisible on standard AP films.

Computed Tomography (CT)

Multiplanar thin-slice (≤1 mm) computed tomography represents the definitive imaging modality for confirming the crescent sign, demonstrating 97.3% sensitivity. Coronal and sagittal reconstructions readily detect subtle subchondral cortical disruptions, intraosseous gas (negative Hounsfield attenuation between -900 and -1000 HU), or fluid collections (+20 to +40 HU) within the cleavage plane.

CT imaging accurately quantifies the exact depth of articular depression, establishing the threshold between ARCO Stage 3A (<2 mm depression) and ARCO Stage 3B (≥2 mm depression). Furthermore, CT cross-sections determine the modified Kerboul combined necrotic angle (the sum of the necrotic arc angles measured on mid-coronal and mid-sagittal reconstructions), providing an objective prognostic metric where combined angles greater than 200 degrees correlate with rapid articular collapse.

Magnetic Resonance Imaging & Ultrasound

On MRI, the necrotic zone is delineated by a low-signal serpentine margin on T1-weighted sequences. On T2-weighted and short tau inversion recovery (STIR) sequences, the classic “double line sign” is identifiable at the boundary of viable and necrotic bone, consisting of an outer hyperintense rim (hypervascular granulation tissue) and an inner hypointense rim (sclerotic bone). When a subchondral fracture develops, high-signal T2 fluid or low-signal fibrous tissue tracks along the subchondral cleavage line, matching the location of the radiographic crescent sign.

Point-of-care musculoskeletal ultrasound demonstrates secondary reactive capsular distension and joint effusion, characterized by an anterior capsule-to-femoral neck distance greater than 7 mm or an asymmetry greater than 1 mm compared to the contralateral asymptomatic hip. In advanced cases, dynamic real-time ultrasound reveals cortical step-offs and contour step deformities during gentle passive rotational maneuvers.

6. Diagnostic Utility, Sensitivity & Specificity

The crescent sign demonstrates virtually 100% specificity for avascular necrosis of the femoral head in adult patients. Its presence confirms that structural fatigue failure of the cancellous bone arch has occurred. In retrospective and prospective validation series, the diagnostic accuracy parameters across imaging modalities reflect distinct technical capabilities:

• Plain Radiographic Sensitivity: 65% to 70% on standard AP pelvic views, which increases to 80% to 85% when high-quality frog-leg lateral radiographs are systematically evaluated.
• Computed Tomography Sensitivity: 97.3% sensitivity and 98% specificity for detecting early subchondral fractures and cortical breaches, making multiplanar CT the definitive structural imaging test.
• Magnetic Resonance Imaging Utility: Demonstrates >99% sensitivity for the diagnosis of osteonecrosis overall (identifying pre-collapse Stage I and Stage II lesions), while thin-slice CT remains superior for measuring precise cortical depression depth and subchondral bony fracture fragments.
• Operator Dependency: Visualization of the crescent line on plain radiographs is highly dependent on radiographic tube alignment and tangential beam projection. Rotation of the hip can obscure the thin radiolucency within dense surrounding sclerotic repair rims.

7. Differential Diagnosis & Radiographic Mimics

8. Clinical Significance & Management Implications

The emergence of the crescent sign serves as the pivotal fork in orthopaedic management algorithms for osteonecrosis of the femoral head, separating joint preservation from arthroplasty pathways:

• Failure of Core Decompression Alone: In pre-collapse osteonecrosis (Ficat I and II / ARCO 1 and 2), core decompression with or without bone marrow aspirate concentrate (BMAC) achieves clinical success rates of 75% to 85%. Once a crescent sign appears (Ficat III / ARCO 3), structural trabecular collapse has already occurred, and simple decompression yields poor long-term femoral head preservation rates (<30% to 40%).
• Candidate Selection for Joint-Preserving Osteotomies: In young, active patients (<45 years) with ARCO Stage 3A disease (<2 mm depression) and a substantial uninvolved posterior and medial articular surface (modified Kerboul angle <200 degrees), transtrochanteric rotational osteotomies (Sugioka anterior or posterior rotational osteotomy) or free vascularized fibular grafting (FVFG) can rotate viable, non-fractured cartilage into the principal weight-bearing zone, halting progressive collapse.
• Total Hip Arthroplasty (THA) Decision Threshold: In patients demonstrating an extensive crescent arc involving greater than 30% of the articular surface (Steinberg Stage IIIC), articular depression of 2 mm or greater (ARCO Stage 3B), or secondary acetabular cartilage wear, total hip arthroplasty is the definitive treatment of choice, providing reliable pain eradication and exceptional implant survivorship (>95% at 15 years with modern highly cross-linked polyethylene and ceramic bearings).

9. Master Class Pro-Tip

Master Class Pro-Tip: Never order or rely solely on standard anteroposterior (AP) pelvis views when screening for early femoral head collapse. The frog-leg lateral (Lauenstein) projection is mandatory: because ischemic osteonecrosis predominantly afflicts the anterosuperior weight-bearing dome, the frog-leg view projects this specific anatomic quadrant in tangential profile, unmasking subtle, occult subchondral crescent lines that are completely obscured on AP views by surrounding dense sclerotic bone. If the plain radiograph is equivocal in a high-risk patient (such as those receiving high-dose systemic corticosteroid therapy or individuals with sickle cell disease), proceed immediately to high-resolution thin-slice multiplanar CT to definitively confirm or exclude an ARCO Stage 3A subchondral fracture before attempting joint-preserving core decompression.

Master Class Radiological Pearl: Tangential Profile Imaging of Subchondral Collapse

10. Evidence-Based Synthesis

11. References

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