EXTRAOCULAR MUSCLES: VARIATION IN THEIR ANATOMY, LENGTH AND CROSS-SECTIONAL DIAMETER

Background: The extraocular muscles (EOMs) bring about eye movement and studies exist which measure EOM length, cross-sectional diameter and volume. Knowledge of the normal values is crucial for determining when an EOM becomes pathological. The aim of this study was to dissect the orbit and measure the length and cross-sectional diameter of the EOMs. Methods and Materials: Eighteen orbits from 9 formalin fixed cadavers (4 male, 5 female), age range 70-95, were dissected. The length of the EOM was measured with a digital caliper, the halfway point of the EOM found and the cross-sectional diameter measured. Length and cross-sectional diameter measurements from the left and right orbits were compared. The correlation between age and EOM length and age and EOM cross-sectional diameter was assessed. The association between gender and EOM length and gender and EOM was analysed. Any anatomical variation in the EOMs dissected would be noted. Results: Mean (±SD) lengths in numerical order were: levator palpebrae superioris, 42.8±4.6mm, superior oblique, 39.2±4.5mm, medial rectus, 38.5±3.1mm, lateral rectus, 38.4±2.4mm, superior rectus, 38.2±4.1mm, inferior rectus, 37.2±2.4mm and inferior oblique, 22.5±4.4mm. Mean (±SD) cross-sectional diameters in numerical order were: medial rectus, 7.9±1.2mm, lateral rectus, 6.7±1.4mm, superior rectus,6.5±1.3mm, inferior oblique, 6.5±0.9mm, inferior rectus, 6.2±0.9mm, levator palpebrae superioris, 6.0±1.1mm and superior oblique 4.3±1.1mm. There was no significant difference between left and right sides for length and cross-sectional diameter. There was also no association between age and length and age and cross-sectional diameter. There was no association between gender and length and gender and cross-sectional diameter. Discussion: This study presents normative measurements for EOM length and cross-sectional diameter. One anatomical variation was found: a thin muscle belly passing medially and originating from the same point as the LPS. This is estimated to occur in 8-15% of cases. Although no anatomical variations in the rectus muscles were observed this is likely due to their much lower frequency.