The anterior inferior iliac spine (AIIS) is the bony projection of the iliac wing immediately below the anterior superior iliac spine, providing the direct head origin of the rectus femoris muscle. It is separated from the acetabular rim by the iliopectineal eminence. Extra-articular hip impingement from a prominent AIIS impinging against the femoral neck in deep flexion (subspine impingement) is a recognised cause of anterior hip pain in young athletes.
AIIS avulsion fractures occur in adolescents during explosive hip flexion against a contracting quadriceps (kicking, sprinting), producing sudden anterior hip pain and a visible avulsion on pelvic radiograph. Conservative management for small avulsions; surgical fixation for widely displaced fragments. Subspine impingement from a prominent AIIS (high-riding type) is identified on hip arthroscopy as the AIIS contacting the femoral neck in deep flexion — arthroscopic reduction of the AIIS prominence (subspine decompression) relieves impingement.
Explosive rectus femoris contraction during kicking or sprinting avulses the AIIS from the iliac wing in the skeletally immature athlete whose apophysis is unfused, producing anterior hip pain and a visible bony fragment below the ASIS on radiograph; most avulsions heal conservatively with 6-8 weeks protected weight-bearing and physiotherapy.
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