To experimentally investigate the effect for muscle weakness after superior rectus Z-myotomy and histological changes.

Superior rectus muscle fibers of rabbits (16 rabbits, 32 eyes) were cut transversely with scissors across 75% of the muscle in two different positions on opposite sides. In group 1 (16 eyes), myotomies were performed at 2 and 7 mm from the muscle insertion (5 mm gap) and in group 2, performed at 2 and 5 mm (3 mm gap). The change of mark, eyeball position, and muscle tension after myotomy and 4 weeks postoperatively was evaluated, the location of the mark was examined, and muscle tissue biopsy was performed.

After Z-myotomy, the marks of the two groups moved significantly posteriorly from insertion within groups ( p <0.05), with no significant differences between groups ( p =0.469). Eyeball positions of the two groups moved significantly inferiorly ( p <0.05); the amount of position change of group 1 was greater than group 2 ( p <0.05). When the globe was pulled in opposite directions for muscle action, the degree of change decreased with significant difference within groups ( p <0.05), but there were no significant differences between groups ( p =0.32).

Z-myotomy of the superior rectus muscle affected the recession of eyeball position and weakened the muscle action. Muscle weakening affected by the different gaps between myotomies did not show consistent results.