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Qi Wei, Joseph L Demer; Biomechanical Modeling of Actively Controlled Rectus Extraocular Muscle (EOM) Pulleys. Invest. Ophthalmol. Vis. Sci. 2021;62(8):2606.
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Active Pulley Hypothesis posits that Listing’s Law (LL) is implemented mechanically by connective tissue pulleys actively positioned by the rectus EOMs (Demer et al., IOVS, 42:1280-1290, 2000). A computational model is needed to clarify this scheme. We developed a new biomechanical model of active horizontal rectus pulleys and examined its behavior.
A previously developed 3-D neuro-biomechanical model of the orbit (Wei et al., Prog. Biophys. Mol. Biol., 103:2-3:273-283, 2010) was augmented to realistically simulate pulley mechanics. The orbital (OL) and global (GL) layers of the horizontal rectus EOMs were modeled as separate strands. The pulley sleeve was modeled as a tube suspended by elastic strands and receiving the OL insertion. Stiffnesses and orientations of pulley suspensions were determined empirically to limit EOM side-slip while allowing anteroposterior pulley travel. Independent neural drives of the OL greater than GL were assumed. The model was refined in secondary gazes by incremental iterations to implement realistic behavior using the simplest mechanical configuration and neural control strategy.
Quantitatively realistic behavior required both insertion of each OL on its pulley, and differential control of OL and GL tensions. Actively-controlled pulleys stabilized medial (MR) and lateral rectus (LR) paths during vertical ductions. From 30o supra- to 30o infraduction, LR and MR pulleys shifted vertically by less than 1 mm from central position, realistically demonstrating anterior path inflections ~half the gaze angle as necessary for LL. During progressive rotation from 30o add- to 30o abduction, simulated LR and MR insertional forces agreed quantitatively with corresponding empirical measurements (Collins et al., IOVS, 20(5):652-664, 1981). Predicted innervations of horizontal rectus GL and OL, as well as of cyclovertical EOMs, were consistent with previous empirical and computational work.
A novel bilayer biomechanical model realistically implements mechanical behavior of actively-controlled pulleys in secondary gazes that is quantitatively consistent with available EOM force and path data in humans. This simulation, which is consistent with LL, suggests that horizontal rectus pulleys must be under influence of OLs differently innervated from GLs, since physiological behavior could not be implemented without these features.
This is a 2021 ARVO Annual Meeting abstract.
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