Mechanical considerations in myopia: relative effects of accommodation, convergence, intraocular pressure, and the extraocular muscles.

Axial myopia is a common ocular problem characterized by the fact that the posterior sclera has apparently stretched out of shape over a period of several years. The debate persists as to whether myopia is an inherited or acquired disorder. During the last few years, several new laboratory techniques have surfaced which can create large amounts of myopia in normal experimental animals. In an attempt to find a rational mechanical explanation common to both human and laboratory myopia, this report examines the stresses experienced by the posterior sclera as a result of accommodation, convergence, vitreous pressure, and the extraocular muscles. The conclusion is that convergence and, more generally, the tension in the extraocular muscles are mechanically more important than accommodation because of the sizable increase in vitreous pressure. The oblique muscles, because of their attachment sites at the back of the globe near the optic nerve entrance port, have the capability of producing local stress concentrations which may be very important in understanding pathological myopia. Suggestions are offered for future experiments in terms of muscle surgery, the use of prisms, diagnostic techniques, and animal models.