To evaluate changes in muscle length when retracting the extraocular rectus muscle with muscle hooks during strabismus surgery.

Forty-four rectus muscles of 42 patients consecutively resected in two hospitals (A, B) were included in this study. After isolation of the rectus muscle, the length of the muscle was recorded and the portion to be resected was marked using gentian violet stain on the tip of calipers. After the rectus muscle was retracted with two muscle hooks in either direction, its length was measured again with the calipers.

The length of the rectus muscle was not changed by retraction in 25 of 44 muscles (56.8%). The length of the muscle was changed by 0.5 mm in 13 muscles (29.5%) and by 1mm in six muscles (13.6%). Changes of rectus muscle length over 0.5 mm were observed in 15 of 27 muscles of patients treated at hospital A (55.5%) and four of 17 muscles of patients treated at hospital B (23.5%). The results for the two hospitals were significantly different ( p =0.037).

Small errors in measurement can occur when muscle resections are performed during strabismus surgery. Institutional or clinician related factors may explain variations in error rates between hospitals.