37 patients (38 hips) underwent sub-capital osteotomy for slipped upper femoral epiphysis (SUFE) between 1980 and 1999. All slips were severe, and 28 (74%) were unstable. Patients were followed-up at a mean 6. 9 years (range 2. 2–20 years) to identify the relationship between the timing of surgery and complications.

Stable slips underwent urgent elective operations. Unstable slips, admitted as emergencies, were operated upon following two different protocols. 17 cases underwent the Dunn procedure on the next available list at a mean 1. 7 days after admission. 21 cases underwent the Fish procedure after a mean 22. 2 days of bedrest on “slings and springs”.

Of 23 patients (24 hips) who suffered no complications and for whom on Iowa Flip Score was available, the means score was 96. 5 (range 91–100). There were 6 cases of Avascular Nervosis (AVN) (16%), all occurring after unstable slips, and 4 cases of chondrolysis (10%).

Amongst unstable slips, shorter periods of pre-operative bedrest were associated with a higher incidence of avascular necrosis (AVN) (p< 0. 025). Direct comparison of the two surgical procedures showed no statistically significance difference in the incidence of AVN.

Sub-capital osteotomy is valid treatment for severe SUFE. More than 20 days of pre-operative bedrest decreased the incidence of AVN in unstable SUFE