EOM Pulleys and Sequelae: A Critical Review

Abstract

PURPOSE. The discovery of extraocular muscle (EOM) pulleys resolved long-standing issues in oculomotor physiology, revived interest in EOM function generally, and led to several new theories. We describe the pulley concept of Miller and Demer (M-D Pulleys) and briefly review evidence, distinguishing this well-supported notion from the Active Pulley Hypothesis (APH) and the EOM Compartments hypothesis, and critically reviewing the methodologies and evidence on which the latter are based.METHODS. We analyze evidence on mechanical independence of individual EOM fibers, implications of nerve tracing for functional independence of EOM layers and compartments, validity of image-based methods of assessing EOM contraction, and data analysis issues.CONCLUSIONS. M-D Pulleys are well-supported by several lines of evidence from several labs. The APH, which predicts relative movements of EOM lamina sufficient to alter muscle actions, has been effectively disproved. The width-wise articulations of EOM Compartments, in contrast, might produce significant contractile oculorotary force gradients across muscle tendons, although existing evidence is unconvincing. We suggest how this hypothesis could be effectively tested.

Instead of pulley bands, does retrobulbar fat keep the eye muscle bellies in place and thereby act like a pulley?

“…The pulley effect has been analyzed by Miller with a model of eye motility that has three degrees of freedom. 1,2 The model eyeball rotates about its center that is assumed to be fixed in the orbit. That was the case in David Robinson's model, 9 in Miller's version of the model 28 and in my version of the model.…”

“…The recent review by Joel Miller 1 of Extraocular Muscle Pulleys with a critical appraisal of (i) the Pulley Concept of Miller and Demer, 2 (ii) the Active Pulley Hypothesis by Demer 3 and (iii) the Extraocular Muscle Compartments Hypothesis by Demer 4 explains the subtle differences between these Pulley Hypotheses in great detail. The impression is raised that, notwithstanding the differences between Active and Passive Pulleys, all agree that eye muscles are kept in place by connective-tissue bands between the rectus muscles and the orbital wall, thereby exerting pulley action.…”

“…The impression is raised that, notwithstanding the differences between Active and Passive Pulleys, all agree that eye muscles are kept in place by connective-tissue bands between the rectus muscles and the orbital wall, thereby exerting pulley action. In the review by Joel Miller 1 , 63% of the references originate from the debating authors themselves, the review disregards alternative explanations for pulley action and makes it seem as if the history of pulleys began in 1989.…”

Author Response: Letter to the Editor of IOVS From Joseph L. Demer and Robert A. Clark Regarding Joel M. Miller, “EOM Pulleys and Sequelae: A Critical Review”

“…The Demer laboratory infers muscle contraction from MRI data. Muscle volume in a region of interest (ROI) centered on the point of maximum cross-section (MaxCS) in such images would be a reasonable measure of muscle contraction, 11 but the method they use, posterior partial volume (PPV), being both complexly contaminated and vulnerable to bias, is not as follows:…”

“…To forestall further confusion and obfuscation, I've urged that the fundamental and well-supported EOM pulley concept be referred to as "longitudinally-dragged pulleys" to highlight its passive mechanics, or "M-D pulleys," to reference its developers. 11…”