To identify the muscle weakening effect and the change of muscle tension according to degree of superior rectus Z-myotomy in rabbits.

After dissection of superior rectus in 15 rabbits (30 eyes), marking was done on 10 mm apart from muscle insertion. Then, Z-myotomy was done on 2 mm and 8 mm apart from muscle insertion which are two different positions on apposite sides. 25%, 50% and 75% Z-myotomy were defined as group 1 (10 eyes), group 2 (10 eyes) and group 3 (10 eyes). After Z-myotomy, the change of muscle lengthening between muscle insertion and marking, and muscle tension were measured. At 4 weeks after Z-myotomy, all parameters were re-measured.

After Z-myotomy all markings moved posteriorly, and showed result as 10.83 ± 0.13 mm in group 1, 11.02 ± 0.17 mm in group 2 and 12.01 ± 0.23 mm in group 3 respectively. Muscle tension result showed as 3.98 ± 0.22 mm in group 1, 3.54 ± 0.18 mm in group 2 and 2.87 ± 0.25 mm in group 3. In comparison of three groups, group 2 and group 3 showed the significant results ( p < 0.05). At 4 weeks after Z-myotomy the markings showed result as 10.55 ± 0.14 mm, 10.85 ± 0.20 mm, 11.91 ± 0.14 mm respectively, and in group 2 and group 3 the significant changes were seen ( p < 0.05). Muscle tensions were 4.01 ± 0.31 mm, 3.88 ± 0.53 mm, 3.12 ± 0.42 mm respectively. There were significant results in group 2 and group 3 ( p < 0.05).

75% Z-myotomy of superior rectus showed significant results in muscle weakening and decreasing muscle tension.