Isometric force development in human horizontal eye muscles and pulleys during saccadic eye movements

The connective tissue elements forming the check ligaments and portals of the human eye muscles have recently been ascribed with a pulley function. Active positioning of the pulleys over orbital layer contraction during eye movements has been suggested. Other studies have instead demonstrated fibrous tissue connections between all parts of the muscle and the pulleys. We aimed to compare the isometric force developed at the muscle tendon and at the pulleys of the horizontal eye muscles, and to investigate which eye muscle structures might exert force on the pulleys.

Isometric force development was recorded from the lateral and medial rectus muscles in six patients operated for strabismus under topical anaesthesia. Two strain gauge probes were used, each attached with 5-0 silk sutures either to the muscle tendon or to the pulley. The eye muscles were activated by horizontal saccadic eye movements in steps from 30 degrees in the off-direction to 30 degrees in the on-direction of the muscles.

The forces developed at the tendon and pulley were almost identical with respect to amplitude and other parameters. No differences were found in forces developed at the pulleys of the medial and lateral rectus muscles.

The results support the presence of fibrous tissue connections between all eye muscle fibres and pulley structures, rather than orbital fibre control of the pulley.