Abstract
Purpose: :
The active pulley hypothesis (APH) proposes that rectus pulleys receive insertions from orbital layers (OLs) of extraocular muscles (EOMs) to shift pulleys anteroposteriorly (AP) so as to maintain constant distance from pulleys to scleral insertions. Magnetic resonance imaging (MRI) before and after recession and resection of rectus EOMs has not detected AP shifts in pulleys after surgery. We computationally simulated horizontal rectus EOMs and pulleys to determine if the lack of surgical effect on pulley positions is consistent with known orbital anatomy and reasonable forces.
Methods: :
To account for greater resistance to transverse than AP displacement, we modeled each rectus pulley as an encircling ring sliding within an elastic sling coupled via bands to the orbital wall and adjacent pulleys. EOM paths were determined from human MRI. EOMs were modeled as an OL terminating on the pulley, and a global layer (GL) consisting of one parallel string terminating on the pulley, and another passing through the pulley to terminate on sclera. Dual–string parameterization of GL permitted exploration of effects of possible GL force coupling to the pulley. Tissues suspending pulleys were modeled as elasticities scaled based on data of Kono et al. IOVS 43:2923, 2002. The globe was free to translate, but its posterior displacement was resisted by a lumped spring representing Tenon’s fascia and orbital fat.
Results: :
Reasonable parameterization of connective tissue stiffnesses and total EOM forces yielded translational globe stability, as well as coordinated AP movements of pulleys and realistic EOM paths during horizontal gaze shifts of up to 30°. Consistent with metabolic and structural properties, forces were higher in the OL than GL of each EOM over the active gaze range. Recessions up to 6 mm and resections of up to 8 mm of rectus tendons altered AP pulley positions by <0.6 mm with no GL force coupling to the pulley, and <0.8 mm even with 20% GL force coupling to the pulley.
Conclusions: :
A model implementing horizontal rectus pulleys as rings within sleeves produces realistic active pulley shifts due to the substantial force in OL even with some GL force coupling to the pulley. Recess–resect surgery has little effect on pulley position because pulleys have stiff passive suspensions. Modeling supports the APH proposition that rectus pulleys are actively translated by forces exerted mainly by the OLs.
Keywords: strabismus: treatment • extraocular muscles: structure • eye movements