Reply to the Letter to Editor by J.L. Demer regarding: Van den Bedem, Schutte, van der Helm, and Simonsz: Mechanical properties and functional importance of pulley bands or ‘Faisseaux Tendineux’ (Vision Research 45: 2710–2714, 2005)

Dear Editor, We appreciate the careful analysis Demer has provided of our recent paper describing the first measurements of the mechanical properties of pulley bands. We wish to initially delineate the areas of agreement, before responding to the criticism of Demer in detail. The extraocular muscle (EOM) bellies do not displace sideways in eye movements out of the plane of the muscle, this is accomplished by pulleys. The APH proposes rectus pulleys that consist of encircling collagen rings, stiffened by elastin and smooth muscle. These pulley rings are suspended anteriorly from the orbital rim by bands or 'slings' that mainly consist of collagen. In our original article , we have called these bands 'pulley bands'. In the APH, these bands are portrayed as springs (Demer et al., 2000, Fig. 10). The APH proposes that shifts of the pulley locations are generated by contractile activity of the orbital layers of EOMs acting against pulley band elasticity. Existence of the pulley bands has been beyond dispute for almost two centuries (Tenon, 1816). However, opinions divide on whether stabilization of the EOM bellies is assigned, in part, to the pulleys and pulley bands. The EOM bellies can be kept in place by external forces on the muscles: via connections to the orbital wall, to the globe, to other muscles (via the intermuscular membrane) (Simonsz et al., 1985), by the supportive action of the fat (Schutte, van den Bedem, van Keu-len, van der Helm, & Simonsz, 2006) and by bending stiffness of the muscles themselves. Demer's criticism focuses primarily on the quality of the force–length measurements of the pulley bands, and on the question of what kept the muscle bellies in place in our patient with the extremely shallow orbits, due to Crouzon's syndrome. We agree with Demer that the measurements were done with limited accuracy and that it was not easy to mark the anterior border of the cutoff periorbit at the point where the pulley band originated from the orbit. On the other hand, our measurements of the mechanical properties of these bands are the only measurements to date, and were performed on human specimens that were removed from the body five minutes prior to testing. Despite the fact that our study was exploratory in nature, the finding that the pulley bands have leash-like properties makes it difficult to conceive that the pulley bands play an important role in stabilizing EOM bellies. …