Inflection in Inactive Lateral Rectus Muscle: Evidence Suggesting Focal Mechanical Effects of Connective Tissues

¹ Jules Stein Eye Institute, UCLA, 100 Stein Plaza, Los Angeles, California, 90095-7002, United States

^* To whom correspondence should be addressed. E-mail: jld{at}ucla.edu.

	Abstract

Purpose: We studied paths of inactive lateral rectus (LR) muscles to investigate putative roles of orbital fat and intrinsic muscle stiffness suggested to be alternatives to connective tissue pulleys as determinants of pulling direction. Methods: We performed surface coil orbital magnetic resonance imaging in axial planes in adult humans: 7 with chronic, unilateral LR paralysis; three with non-paralytic, concomitant esotropia of similar angle; and 15 normal controls. Fixation was controlled using targets placed at a broad range of horizontal positions. Results: Paralyzed LRs exhibited marked atrophy compared with functional contralateral LRs and LRs of orthotropic and esotropic subjects without LR paralysis. The normal LR exhibited a gradual 18.8 ± 4.5° (mean ± SD) lateral inflection 14.4 ± 2.6 mm posterior to globe center, bowing the LR away from the orbital center. The paralyzed LR exhibited a significantly (p < 0.002) larger and typically more discrete 29.2 ± 8.8° lateral inflection similar to that observed in concomitant esotropia in maximal adduction. Average position of this inflection was 11 - 14 mm posterior to globe center in all three subject groups, but in LR palsy only the inflection of the paralyzed LR depended upon horizontal gaze, being 0.17 mm further posterior per degree of abduction (linear fit R = 0.85). Behavior of the paralyzed LR inflection is consistent with LR pulley anatomy. Conclusions: Sharper lateral inflection in the flaccid than tense LR seems inconsistent with intrinsic muscle stiffness or diffuse orbital fat pressure, but suggests influence of discrete connective tissues.

Key Words: eye movements, extraocular muscle, oculomotor mechanics, strabismus, magnetic resonance imaging, pulley