DYNAMIC MAGNETIC-RESONANCE-IMAGING OF THE LEVATOR PALPEBRAE SUPERIORIS MUSCLE

The relationship between upper lid elevation (h) and shortening (s) of the levator palpebrae superioris muscle (LPS) has an influence on the dose-response relationship in ptosis surgery. In order to investigate the relationship between upper lid elevation and shortening of the LP S, parasagittal T-1-weighted MR images of the orbit were obtained in h ealthy subjects with the eye in down-and upgaze. The position of the u pper lid margin and the length of the LPS were measured in the images in down-and upgaze and the amount of h and s was determined. For a mea n (+/- SD) vertical upper lid elevation of 15 +/- 1 mm, the mean (+/- SD) shortening of the LPS muscle was 21 +/- 3 mm (n = 4). The mean rat io of h:s was calculated to be 1:1.4, which means that the levator mus cle must contract by 1.4 cm in order to achieve a lid elevation of 1 c m. Therefore, the force of the LPS which is necessary to lift the uppe r eyelid can be smaller than the lid-closing force. This strongly sugg ests a physiological mechanism which reduces the muscle force necessar y for lifting the upper eyelid.