Mechanical Behavior Of Extraocular Muscles And The Corresponding Modeling In The Eye Movement

Modeling in eye movement is useful for the clinical diagnosis and treatment of some eye diseases, e.g., strabismus, vertigo, and nystagmus etc. Eye movement is controlled by extraocular muscles, which are the essential factor in the corresponding modeling. There are six extraocular muscles in human orbital tissues, i.e., lateral rectus, medial rectus, superior rectus, inferior rectus, superior oblique and inferior oblique.The objective of this work is to investigate the mechanical behavior of extraocular muscles and modeling the corresponding eye movement model. This investigation was mainly performed using modeling in mathematics and mechanics, experimental determination of mammals, numerical simulation. The main contents contain three sections, i.e., initial tension forces of extraocular muscle in the primary position, passive tensile tests performed on the mammal specimens, and the eye movement model.In the previous models, the primary position is usually settled as the initial position of eye movement. Eye suspension in the primary position depends on the contributions from six extraocular muscles. The initial forces of extraocular muscles should be known before modeling eye movement model. Based on the previous investigations, e.g., the coordination of extraocular muscles and the experimentally passive tensile data of human extraocular muscles, the corresponding initial forces were estimated using the theory of mechanical equilibrium and the method of mathematical optimization. This calculation model was established on the popularly modern viewpoint of orbital motility, i.e., eye movement mainly depends on the mechanical effect rather than the innervation. Therefore, this modeling weakened the factor of innervation.The passive behavior of mammal extraocular muscles was determined using the experiments in vitro. Several fox extraocular muscles, whose eye is located anteriorly like human beings, were obtained from a local farm. The specimens of extraocular muscles were performed the uniaxial tensile test in laboratory. The corresponding experimental data were analyzed using Ogden hyperelastic model. Using the average values of the hyperelastic parameters fitted, a finite element model for extraocular muscle was established to simulate the uniaxial tensile test. Compared with the experimental data, there was no statistical difference between the simulation errors and analysis errors. In addition, considering different mammals have different eye locations, the difference of three different extraocular muscles, fox and pig as well as sheep, was investigated using the same experimental protocol.Based on the active pulley hypothesis proposed by the previous investigators, an active pulley model, which was used to describe the eye movement in the horizontal plane, was modeling in mathematics and mechanics. Little knows the experimental data of the active behavior of extraocular muscles due to the difficulty from anatomy. Therefore, in modeling, a mathematical optimum method was added into the mechanical equilibrium to solve the force values of extraocular muscles controlling eye movement. In addition, compared with the non pulley model, the simulation results from active pulley model show that active pulley can maintain the mechanical advantage of medial rectus when eye adduction in the large degrees. This result demonstrates the viewpoint of modern orbital motility, i.e., pulley is the functional origin of extraocular muscles.