A 17-year-old girl comes to the office for evaluation of pain in her right hip and groin. She first noticed the pain approximately 3 months ago during a scuba class. The patient initially felt an intermittent ache in the right hip and groin that she attributed to carrying heavy scuba gear. She recently started competitive rowing, and the pain has become more intense and constant. She sometimes experiences a "clicking" sensation in the right hip. The patient has had no swelling or stiffness of the knees, feet, hands, or elbows. However, she now has difficulty exercising due to the pain; ibuprofen and ice provide no relief. The patient was adopted at age 18 months from a developing country and has no known medical problems. She lives with her parents and a cat. BMI is 20 kg/m². The patient is afebrile and vital signs are normal. The left side of the pelvis drops when she bears weight on the right leg. She stands on her right toes with her left foot flat on the ground. During supine examination, the right hip has decreased abduction. Which of the following is the most likely cause of this patient's condition?

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Explanation: Developmental dysplasia of the hip (DDH) is characterized by abnormal acetabular development resulting in a shallow hip socket and inadequate support of the femoral head. When DDH is identified and treated in infancy, the prognosis is excellent as >95% of patients have reduction of a dislocated hip. However, DDH may be missed in the newborn period if classic symptoms and signs (eg, hip clunk, asymmetric leg creases) are absent or undiagnosed. asymmetric leg creases Children with DDH may initially be asymptomatic until they are weight-bearing. However, due to posterior positioning of the femoral head, adolescents and young adults may have a leg-length discrepancy and gait abnormalities such as toe-walking on the affected side or a Trendelenburg gait. In addition, abnormal traction on the dysplastic hip joint accelerates wear on the cartilage, resulting in premature joint degeneration and osteoarthritis in adolescents and young adults. Young patients may develop activity-related pain in the front hip and groin, as seen in this patient. Trendelenburg gait (Choice B) Juvenile idiopathic arthritis is an autoinflammatory disease characterized by fever, joint pain, and rash. This diagnosis is unlikely in this patient who has isolated hip pain and a Trendelenburg gait. (Choice C) Osteoid osteoma is a benign tumor most commonly occurring in the proximal femur in adolescents. The typical presentation is significant pain at night rather than with activity. (Choice D) Focal pain is expected with chronic osteomyelitis, but the absence of fever, erythema, and swelling makes infection less likely. (Choice E) Legg-Calvé-Perthes disease is characterized by avascular necrosis of the femoral head. Idiopathic interruption of blood supply can result in limp and/or hip pain, typically in children age 5-7. This patient's age and leg-length discrepancy are not typical of this disease. Legg-Calvé-Perthes disease Educational objective: Abnormal acetabular development is characterized by a shallow hip socket with inadequate support of the femoral head. The misaligned and dysplastic hip undergoes accelerated cartilage wearing, resulting in chronic, activity-related hip pain and osteoarthritis in adolescents and young adults. References: Diagnosing and managing hip problems in childhood. (http://www.ncbi.nlm.nih.gov/pubmed/23905285) Developmental dysplasia of the hip. (http://www.ncbi.nlm.nih.gov/pubmed/21808709) Hip dysplasia in the young adult caused by residual childhood and adolescent-onset dysplasia. (http://www.ncbi.nlm.nih.gov/pubmed/27613709)

A 17-Year-Old Girl Comes to the Office for Evaluation of Pain