This is a retrospective analysis of 60 patients who underwent posterior approach ptosis surgery utilising a Muller’s muscle resection. The study compares those who underwent a fixed resection length of 7mm of Muller’s muscle with those having a variable resection length based on a 4:1 ratio of resection length to desired elevation. There were 30 eyes in both groups, which had no significant difference in age, sex, the degree of preoperative ptosis or brow height. There was no overall difference in the average amount of resection, but the variable group ranged from 4.5 to 9mm. Interestingly there was no difference either in the average final margin reflex distance to the upper lid (MRD1) or the change in MRD1. Regression analysis failed to show a correlation between amount of tissue resected and change in MRD1 in either group. However, there was a positive correlation between the degree of pre-operative ptosis and the change in MRD1. The authors comment that the results throw into question the mechanism behind Muller’s muscle resection, with the data implying this is not a simple mechanical effect as the change in lid height was independent of the resection length. They speculate on other possible mechanisms, suggesting that the changes in afferent and efferent activity, including proprioception and visual input might play a part. Although the mechanism remains obscure, the conclusion that a variable 4:1 resection length and a fixed 7mm resection achieve the same results seems valid.