Identification of factors limiting the action of the horizontal recti muscles. A guide to the avoidance of gross limitation of adduction or abduction after squint surgery.

Limitation of the action of the medial rectus muscle will cause limitation of adduction of the eye just as limitation of the lateral rectus will cause limitation of abduction. Factors limiting the action of the medial rectus are: (i) loss of ability of the muscle to contract beyond a certain proportion of its length; (ii) limitation of relaxation of the antagonist lateral rectus and fascial connections; (iii) innervational factors seen most obviously in Duane's syndrome, but also possibly active with various vertical anomalies; and (iv) loss of mechanical advantage or torque as the eye rotates inward. Using A scan the last factor may be assessed for a particular eye. There will be no loss of torque until the muscle passes a tangential point where its line of action is tangential to the globe, but torque will then be lost progressively until it becomes negligible as the null point is approached. At the null point the muscle has no torque, being perpendicular to the globe, and it marks the point where the oculomotor axis joining the centre of rotation of the eye and the apex of the orbit intersects the globe. The first three factors may be assessed clinically by measuring the adduction of the eye in degrees, and for 50 normal patients (100 eyes) the mean was 44 degrees, standard deviation 3 degrees. The tangential point for an average eye lies 54 degrees behind the medial rectus muscle so for normal adduction there is no loss of torque in full adduction.(ABSTRACT TRUNCATED AT 250 WORDS)