Explanation
Avascular necrosis (AVN) is the most devastating complication of an unstable SCFE and its incidence is significantly correlated with the timing of reduction and decompression.

Clinical Context: According to the Loder classification, an unstable SCFE is defined by the patient's inability to ambulate, even with the assistance of crutches. The vascular supply to the epiphysis (primarily the lateral epiphyseal vessels from the medial circumflex femoral artery) is severely compromised either by kinking/tearing or by capsular tamponade from an intracapsular hematoma. Urgent surgical intervention—typically within 24 hours via urgent capsular decompression and in-situ pinning or an open capital realignment (e.g., modified Dunn procedure)—is critical. Delayed treatment significantly increases the AVN risk, which can reach up to 47% in unstable slips.

References:
1. Loder RT, et al. 'Acute slipped capital femoral epiphysis: the importance of physeal stability.' J Bone Joint Surg Am. 1993.
2. Tachdjian's Pediatric Orthopaedics, 6th Edition, Elsevier.