Abstract
Abstract: :
Purpose: The effect of an extraocular muscle recession has been classically described as a linear reduction in muscle tension based on its related movement down length–tension (LT) curves. However, with the discovery of fibromuscular "pulleys" affecting the extraocular muscle paths, new mathematical geometric considerations can be made. If the pulleys are the functional origin of the extraocular muscles, then the permanent change in ocular alignment after bilateral medial rectus recession (MROU) may be due to a change in the directional pull or torque vector (TV) at the globe surface rather than being due to the classical idea that the recessed medial recti (MR) stay down their LT curves. Methods: The following five well–documented quantitative characteristics of the results of MROU were chosen for study to see whether the TV model or the LT model would better predict these charateristics: (1) larger MROU produce more effect/mm, with the dose–response curve approximating an exponential shape; (2) in multivariate analysis the head circumference (which would correlate with the length of the MR), was insignificant; (3) successful MROU performed early in childhood remain successful throughout life in most patients; (4) high myopes with esotropia require larger MROU, even though the length of the MR is not longer in high myopes; and (5) MROU of more than 8mm are likely to produce an early consecutive exotropia. Using analytical geometry techniques drawn from mechanical engineering, the effect per millimeter of recession on the original medial rectus torque vector from the globe can be calculated. Results: Based on the location of the MR pulley, the change in the TV per millimeter of MR recession was calculated and is shown as an exponential function in the figure shown below. For all five of the quantitatitve characteristics chosen, the TV model predicts the results of the MROU much more closely than the LT model. Brightly colored, animated graphics were produced to render these analyses easily understandable at the intuitive level. Conclusions: Quantitative characteristics of MROU can be explained by a change in the TV and confirm the presence, location, and function of the MR pulley.
Keywords: strabismus: treatment • extraocular muscles: structure • strabismus