Three Dimensional Reconstruction and Detailed Structural Analysis of the Extraocular Muscle-Pulley Interface

Abstract

Abstract: : Purpose: Each of the rectus extraocular muscles (EOM) is comprised of two functionally distinct layers. The global (GL) inserts in the sclera and the orbital (OL) layer terminates in a connective tissue ring (rectus pulley) suspended from the bony orbit. According to the ‘active pulley hypothesis’ this pulley is actively translocated along the A-P axis of the GL and defines the functional origin of each EOM. The underlying tissue mechanics of the relative laminar movement of the OL and GL are not fully understood. The detailed reconstruction aims at a better understanding the interplay of the pulley and the independently contracting layers. Methods: A computer aided 3D reconstruction from serial sections was used to define the overall architecture of a rat inferior rectus EOM and the pulley. Immunohistochemistry, light and electron microscopy was used to study structural details of the OL, the pulley insertion site (or muscle-pulley interface) and the suspending ligament. Morphometric analysis was used to validate structural findings. Results: Most of the OL fibers insert into the pulley and the layer show a bifurcated shape as the outer portions extend further anterior than the central portion. A part of the OL extends for a considerable distance as a thin layer anterior to the insertion site, and the proportion of multiple innervated fibers in this region is increased. The connective tissue at the global aspect of the EOM is attached to the GL. Conclusion: Light microscopic and ultrastructural details of the pulley-forming connective tissue in the EOM clearly support the concept of a functional pulley that is actively translocated by a contraction of the OL. We provide structural details of the laminar organization that explains the ability of the two layers to contract independently as a relative, linear movement , while simultaneously ensuring the necessary mechanical bond for path inflection. In the suggested model the connective tissue between the two layers and the orbital multiple innervated fibers are predicted to play a mechanical role for pulley function.

Keywords: extraocular muscles: structure • eye movements