The influence of pulleys on the quantitative characteristics of medial rectus muscle recessions: the torque vector model.

Purpose: To develop a new pulley-based torque vector mathematical model for medial rectus muscle recessions and compare it based on known clinical characteristics, to the currently accepted nonpulley length-tension model.

Methods: The following quantitative characteristics of the results of bilateral medial rectus muscle recessions were chosen for study to see whether the new torque vector model or the classic length-tension model would better predict these characteristics: (1) larger bilateral medial rectus muscle recessions produce more effect per millimeter, with the dose-response curve approximating an exponential shape; (2) the exact location of the preplaced medial rectus muscle suture prior to muscle disinsertion in recessions has minimal effect on the postoperative ocular alignment; and (3) medial rectus muscle recessions of more than eight mm are likely to produce an early consecutive exotropia.

Results: Based on the documented location of the medial rectus muscle pulley, the change in the torque vector per millimeter of medial rectus muscle recession was calculated and shown to have an exponential shape. For all three of the quantitative characteristics chosen, the torque vector model appears to better predict the results of medial rectus muscle recessions when compared with the length-tension model.

Conclusions: Many quantitative characteristics of medial rectus muscle recessions are better explained by the torque vector model, instead of the classical length-tension model, and support the presence, location, and function of the medial rectus muscle pulley. This new understanding of ocular motility mechanics may influence surgical technique and introduce new surgical considerations for correction of ocular motility disorders.