A quantitative analysis of extraocular muscle cooperation and squint.

The mechanical properties of human extraocular muscles have recently been described. This allows one to use the force-balance equation of mechanics to solve for the complete mechanical state of all six extraocular muscles and the passive orbital tissues in any eye position. This includes each muscle's force, length, innervation, and unit action vector, which describes how its force is distributed to act horizontally, vertically, and in torsion. Most important, this calculation method allows one to predict the tropias that occur in an eye subject to any imaginable form of peripheral pathology. It also permitts, by calculation, an estimation of the correction that might be expected from any sort of muscle surgery. The tight lateral rectus syndrome is used to illustrate the application of the method in diagnosis and surgery. It also offers, for the first time, a quantitative estimate of the multitude of ways in which muscles can interact and interfere with each other when they hold the globe. Two of the results are interesting: because muscles have different lengths and sizes, their innervational participation in a movement can appear to be quite different than their mechanical participation. From an innervational standpoint, the vertical recti and obliques participate equally in vertical gaze. Muscles interfere with each other a good deal and necessitate changes of innervation to counteract these cross-couplings. This causes unexpected dependencies of innervation on eye position and leads, for example, to the fact that the superior rectus innervation in up gaze is just as large in adduction as in abduction.