This composite educational image illustrates the phenotypic hallmarks of Legg-Calvé-Perthes disease (Perthes disease) in a rabbit model compared to a healthy control. Panel A presents a gross clinical photograph of excised proximal femurs. The Perthes model (left) shows a misshapen, flattened femoral head with a pale, opaque surface and loss of normal articular gloss. In contrast, the healthy control (right) displays a smooth, spherical, and glistening femoral head. Panel B shows corresponding X-ray diagnostic images. The Perthes disease specimen (left) exhibits a loss of spherical structure, heterogeneous bone density (sclerosis and lucency), and focal collapse of the femoral head epiphysis. The healthy control (right) demonstrates uniform bone density and a well-preserved, rounded epiphysis. This visual comparison highlights the osteonecrotic changes, structural deformity, and cartilage thickening characteristic of ischemic necrosis of the femoral head, modeling the clinical progression observed in pediatric patients. The images serve as a comparative reference for orthopedic pathology and experimental models of avascular necrosis.

**Modality:** Plain radiography (X-ray) of the hip. **Anatomical Region:** Proximal femur and acetabulum. **Observed Pathology:** Advanced stage of Legg-Calvé-Perthes disease or severe avascular necrosis (AVN) of the femoral head. **Visual Features:** * **Morphology:** Significant collapse and severe flattening of the femoral head epiphysis, resulting in a characteristic "hatchet-like" deformity. * **Articular Surface:** There is a pronounced eccentric depression of the lateral articular surface. * **Alignment:** Evidence of superolateral subluxation of the femoral head relative to the acetabulum. * **Bone Density:** Marked sclerosis within the femoral head, interspersed with subchondral lucencies and defects, indicating ongoing bone remodeling and necrosis. * **Joint Space:** Notable joint space narrowing and incongruity between the femoral head and the acetabular roof. **Diagnostic Features:** The combination of severe epiphyseal flattening, subchondral collapse, and lateral subluxation are hallmark signs of end-stage ischemic necrosis and joint secondary degeneration. This appearance is crucial for classifying disease severity using the Waldenström or Herring systems.

**Modality:** Anterior-posterior (AP) pelvic radiograph. **Anatomy:** Pediatric pelvis including the bilateral proximal femurs, hips, and pelvic ring. **Pathology:** Bilateral Legg-Calvé-Perthes disease (LCPD) or avascular necrosis of the femoral heads. **Visual Features:** - **Right Hip:** Significant fragmentation and flattening of the femoral capital epiphysis. Increased radiodensity (sclerosis) of the epiphysis is visible, indicating the necrotic phase. There is visible widening of the joint space and metaphyseal radiolucency. - **Left Hip:** Subtle subchondral lucency (crescent sign) and mild flattening of the superior femoral head surface, consistent with early articular collapse. - **General:** The triradiate cartilage is open, confirming a pediatric skeleton. The acetabular morphology appears preserved at this stage. **Diagnostic Features:** Asymmetrical involvement of the bilateral femoral heads characterized by collapse, sclerosis, and fragmentation. These findings are diagnostic of osteochondrosis of the capital femoral epiphysis. **Retrieval Keywords:** AP Pelvis X-ray, Legg-Calvé-Perthes disease, femoral head collapse, avascular necrosis, pediatric orthopedics, fragmentation phase, subchondral lucency.

**Imaging Modality:** Anteroposterior (AP) pelvic radiograph (X-ray). **Anatomical Region:** Pediatric pelvis and bilateral hip joints. **Observed Pathology:** Bilateral Legg-Calvé-Perthes Disease (LCPD), characterized by idiopathic avascular necrosis of the proximal femoral epiphyses. **Characteristic Visual Features:** * **Femoral Heads:** Significant bilateral flattening (coxa plana), fragmentation, and increased radiopacity (sclerosis) of the capital femoral epiphyses. * **Symmetry:** The findings are asymmetrical, with more pronounced collapse and fragmentation observed on the right femoral head compared to the left. * **Joint Space:** Preservation of the joint space is noted, though the right femoral head shows lateral subluxation/extrusion relative to the acetabulum. * **Femoral Necks:** Relative widening and shortening of the bilateral femoral necks (coxa magna) are visible, consistent with remodeling secondary to epiphyseal ischemia. **Diagnostic Differentiating Features:** The combination of epiphyseal flattening, fragmentation, and sclerosis in a skeletally immature patient is pathognomonic for the fragmentation or re-ossification stages of Legg-Calvé-Perthes disease. The absence of destructive articular changes helps differentiate this from septic arthritis or inflammatory arthropathies.