Evidence supporting extraocular muscle pulleys: refuting the platygean view of extraocular muscle mechanics

Abstract

Background: Late in the 20th century, it was recognized that connective tissue structures in the orbit influence the paths of the extraocular muscles and constitute their functional origins. Targeted investigations of these connective tissue "pulleys" led to the formulation of the active pulley hypothesis, which proposes that pulling directions of the rectus extraocular muscles are actively controlled via connective tissues.

Purpose: This review rebuts a series of criticisms of the active pulley hypothesis published by Jampel, and Jampel and Shi, in which these authors have disputed the existence and function of the pulleys.

Methods: This article reviews published evidence for the existence of orbital pulleys, the active pulley hypothesis, and physiological tests of the active pulley hypothesis. Magnetic resonance imaging in a living subject and histological examination of a human cadaver directly illustrate the relationship of pulleys to extraocular muscles.

Results: Strong scientific evidence is cited that supports the existence of orbital pulleys and their role in ocular motility. The criticisms of the hypothesis have ignored mathematical truisms and strong scientific evidence.

Conclusions: Actively control led orbital pulleys play a fundamental role in ocular motility. Pulleys profoundly influence the neural commands required to control eye movements and binocular alignment. Familiarity with the anatomy and physiology of the pulleys is requisite for a rational approach to diagnosing and treating strabismus using emerging methods. Conversely, approaches that deny or ignore the pulleys risk the sorts of errors that arise in geography and navigation from incorrect assumptions such as those of a flat ("platygean") earth.

Fig. 1

Gadodiamide-enhanced, T1 weighted axial magnetic resonance image (MRI) of a human left orbit in 2 mm thickness planes at 390 micron resolution at right, and 312 micron resolution at left, demonstrating the medial rectus (MR) pulley. Clearly seen at higher resolution at left, but not so evident at lower resolution, the MR pulley is evident as a less vascular sleeve around the enhancing, more vascular muscle. LR – lateral rectus muscle. ON – optic nerve. (With permission from IOVS5).

Fig. 2

Gadodiamide enhanced, multiplanar T1 MRI of left orbit of normal human in adduction and abduction. Images were originally acquired in 18 contiguous 2 mm thick quasi-coronal planes perpendicular to the long axis of the orbit at 312 micron resolution. Using three-dimensional technique, the image stacks for each gaze direction were then interactively reconstructed in the planes indicated to produce the axial images shown. The horizontal rectus pulleys are recognized as dark rings In each case, the coronal images that include the medial rectus (MR) and lateral rectus (LR) pulleys are directly displayed for each gaze direction, and the corresponding location on the axial reconstruction is marked with a horizontal white line. Note that the MR pulley shifts posteriorly in adduction, and anteriorly in abduction. The LR pulley shifts anteriorly in adduction, and posteriorly in abduction.

Fig. 3

Quasi-coronal histological micrograph of a whole, serially sectioned right human orbit stained with van Gieson’s elastin stain, taken 11.40 mm posterior to the anterior corneal surface so as to intersect the rectus pulleys. Pulleys are seen as rings of connective tissue encircling the muscle bellies. At higher power, dense black deposits of elastin may be seen at the points where pulleys inflect the muscle paths in eccentric gaze positions. The method also stains the artificially thinned orbital bones black. IO – inferior oblique muscle. IR – inferior rectus muscle. LPS – levator palpebrae superioris muscle. LR – lateral rectus muscle. MR – medial rectus muscle. SR – superior rectus muscle. Note that the LPS lacks an encircling pulley.