The underlying cause of Legg-Calve-Perthes disease is not fully understood. The vast majority of cases are idiopathic and sporadic, occurring in children with no clear family history. The mechanism is thought to involve a temporary disruption of the blood supply to the developing femoral head epiphysis, but what triggers the disruption in any individual child is usually unknown. Risk factors include male sex, lower birth weight, shorter stature relative to peers, and, in some studies, tendencies toward abnormal blood clotting (thrombophilia). A small minority of cases cluster within families; in a limited number of these familial presentations, variants in the COL2A1 gene on chromosome 12 have been described, and this is the only gene currently listed in this entry. Familial clustering is uncommon. Care is typically directed by a pediatric orthopedic surgeon, with referral to hematology if a thrombophilia evaluation is indicated.

Diagnosis begins with a clinical evaluation that includes a history of the limp, observation of gait, and an examination of hip range of motion. Plain radiographs of the pelvis and hips are the standard initial imaging study. Depending on the stage, X-rays can show characteristic findings such as increased density (sclerosis) of part of the femoral head, a subchondral fracture line, fragmentation, and later reossification. Magnetic resonance imaging is more sensitive than X-ray for detecting early disease and helps define the extent of femoral head involvement. Bone scintigraphy is used in selected cases. The differential diagnosis includes transient synovitis, septic arthritis, slipped capital femoral epiphysis, multiple epiphyseal dysplasia, sickle cell disease with bone infarction, and steroid-induced avascular necrosis; these are distinguished through clinical context, laboratory testing where appropriate, and imaging.

There is no medication that cures Legg-Calve-Perthes disease. Management is directed at keeping the hip mobile while the femoral head heals and at keeping the femoral head contained within the acetabulum so that it can remodel into a more spherical shape. No specific drugs are listed as approved treatments in this entry, so treatment categories are described in generic terms. Common non-surgical components include activity modification with periods of protected weight bearing, physical therapy aimed at maintaining hip range of motion, and pain management with general anti-inflammatory measures. Bracing has been used in selected cases, though its role varies by center. For children with significant femoral head involvement, particularly older children whose remodeling capacity is more limited, surgical containment procedures such as femoral or pelvic osteotomies may be considered. Care is generally multidisciplinary. Treatment decisions should be individualized in consultation with the patient's healthcare team.

The long-term outlook for Legg-Calve-Perthes disease is highly variable and depends largely on the age at which the condition begins and the extent of femoral head involvement. Younger children tend to fare better, because their hips have more remaining growth and remodeling capacity to reshape the femoral head. Many children eventually achieve good long-term hip function, although some are advised to limit certain high-impact activities during the healing phase. A subset of patients develop residual hip irregularity that contributes to early-onset hip osteoarthritis in adulthood, and some may eventually require additional orthopedic care, including joint-preserving surgery or hip replacement later in life. Because outcomes vary widely, ongoing follow-up with the orthopedic team during childhood and adolescence is important.

Active clinical research in Legg-Calve-Perthes disease is modest and largely academic. Ongoing studies focus on refining containment surgical techniques, evaluating bracing and physical therapy protocols, and identifying clinical and imaging predictors of long-term outcome. There is also interest in biologic adjuncts intended to support femoral head healing during the active phase. Existing trials use a mix of medical device, procedural, and other interventional approaches; gene therapy trials are not currently part of the active landscape for this condition. The most up-to-date list of studies can be found on ClinicalTrials.gov by searching for 'Legg-Calve-Perthes disease.'