A 52-year-old woman with a history of rheumatoid arthritis presents with progressive groin pain and limited hip abduction. Coronal CT pelvis is performed. The structure marked **C** (acetabular teardrop) is noted to be disrupted and the femoral head has migrated medially into the pelvis. Which of the following best describes the underlying pathology and its primary mechanism?

Explanation

## Why Protrusio Acetabuli is right Protrusio acetabuli is characterized by chronic medial migration of the femoral head through the acetabulum into the pelvis, resulting in disruption of the acetabular teardrop (the medial wall radiographic shadow on coronal CT). The clinical context of rheumatoid arthritis is a classic predisposing condition (along with Paget disease, osteomalacia, and osteogenesis imperfecta). The disruption of the structure marked **C** (acetabular teardrop) indicates compromise of the medial acetabular wall and quadrilateral plate, which is the pathognomonic finding in protrusio. This is a chronic, progressive process, not an acute injury. ## Why each distractor is wrong - **Posterior hip dislocation with posterior wall fracture**: While posterior dislocation can disrupt acetabular anatomy, it is an acute high-energy injury (dashboard injury) and presents with the femoral head appearing smaller and superior on AP radiographs, not medially migrated into the pelvis. The clinical context of RA with progressive symptoms points to a chronic process, not acute trauma. - **Femoroacetabular impingement with cam-type deformity**: FAI is a structural abnormality of the femoral head-neck junction or acetabular over-coverage that predisposes to labral tears and early arthritis in young adults. It does not cause medial migration of the femoral head through the acetabulum or disruption of the acetabular teardrop. FAI is assessed by alpha angle on radial MRI, not by teardrop disruption on CT. - **Central dislocation from acetabular fracture**: Central dislocation is an acute injury from high-energy lateral force and is associated with acute trauma history. Although it does involve medial displacement of the femoral head, it is not the chronic progressive process seen in RA, and the clinical presentation would be acute with severe trauma history, not gradual progressive groin pain over time. **High-Yield:** Protrusio acetabuli = chronic medial femoral head migration through acetabulum (disrupts teardrop on CT) in RA, Paget, osteomalacia, OI; central dislocation = acute trauma with femoral head pushed medially. [cite:Maheshwari Orthopedics 10e Ch 28-30; Sutton Radiology 7e]