Part 1 introduced The Eye As A Joint, a concept which I began considering in earnest about 15 years ago. My thinking on it was reinforced in a book on The Sclera and Systemic Disorders a few years later. In it the authors write (p.15): “It has long been recognized that conditions which affect the joints also affect the sclera. Superficially there is little resemblance between the structure of the joint and that of the sclera except that both consist of collagenous tissue and are acted upon by muscles. However close inspection reveals many similarities. It is not too far-fetched to suggest that the eye is a high specialized form of joint which is used for seeing.”
The global view that we’re taking (pun intended) of the eye as a ball-in-socket joint space is completed of course by considering not only the scleral properties of the eyeball, but the orbital contents and orbital soft tissue biomechanics.
Here is a crucial point. It is often said that extraocular muscles (EOMs) are about 150 times as strong as they need to be for the purpose of moving the eye or load bearing. However the forces through which EOMs adjust are much more intricate than simple load bearing. As an example (according to science facts from the Library of Congress), in an hour of reading a book, the eyes execute nearly 10,000 coordinated movements. Ask not why EOMs fatigue, but why they don’t fatigue more often!
Last week I blogged about an incredible resource, Encyclopledia of the Eye, and volume 4 contains a superb entry on Orbital Soft Tissue Biomechanics by Schutte et al that underscores the complexities of behavior within the orbital joint space. Here are a series of pertinent quotes from that entry:
- Most engineering materials, like steel, show a liner relation (i.e. proportional) between stress and strain, provided the deformations remain sufficiently small … Most biological soft tissues, however, show a more complex relation between stress and strain and usually do not have a linear relation … In addition to the nonlinear elasticity, the orbital tissues show viscoelastic behavior, and are anisotropic and inhomogeneous.
- The EOMs, orbital fat, and the eye mechanically interact and exert forces on each other at every location where they contact .
- The six EOMs are structurally and functionally unique among the cross-striated muscles. For proper function, each muscle needs fatigue resistance, and force and velocity development. At optimum length, that is, the lengthen at which the muscle develops maximum force, the EOMs are the fastest among the cross-striated muscles.
- The future of modeling the mechanical eye muscle behavior lies in developing physically nonlinear continuum models.
- The trochlea is a fibro-cartilage pulley that is attached to the medial wall of the orbit. A synovial membrane lines the inner surface of the trochlea.
- The large range of motion is partly is partly facilitate by sliding at the inter ace between the sclera and Tenon’s capsule, the connective tissue that surrounds the globe and the EOMs The interface is lubricated, and the eye is able to rotate within … the insertions of the eye muscles rotate with the eye and exert force on the orbital fat.
- At the level of the posterior pole of the eye, connective tissue bands (often referred to as pulley bands, or pulley slings) are connected to the rectus muscles and course anteriorly to the orbital wall.
In the figure above, the geometry of the eye, the optic nerve, and the six EOMs have been subdivided into a large number of tetrahedra for the purpose of computer modeling the globe and its contents. As an aside, this 3D model of orbital biomechanics, the EOMs attached to the eyeball on the left, and the orbital fat encompassing the eyeball as a shock absorber on your right, bear resemblance to spherical bodies known as geodesic domes, popularized by Buckminster Fuller. (We could refer to this as being in the category of the buckminsterfullerene or buckyballs; or perhaps – in a nod to The Ohio State – “Buckeyeballs”.)