Measurement of the Mechanical Characteristics of the Fluid–Interface Layer Between Sclera and Tenon

Abstract

 

Purpose:

 

The fat immediately behind the eye follows approx. 50% of eye rotation (Schoemaker et al., ARVO 2004). Presumably, a fluid–interface layer facilitates the required sliding. To incorporate this layer in our Delft Finite–element Model of Orbital Mechanics (Schutte et al., Vision Res., accepted for publ.), we measured its mechanical characteristics.

 

Methods:

 

A pendulum axis equipped with a perilimbal suction ring was applied to the eye of a pig, approx. 18 minutes post mortem. The measurements were conducted on a total of four eyes (two pigs). The eyes were rotated about the visual axis, avoiding large movement of the optic nerve. The amplitude of the pendulum decreased over time because of the internal friction of the device, friction between muscles, sclera and Tenon’s capsule and deformation of the fat. The measurement was repeated after detachment of the eye muscles from the globe and, as a control, detached from the eye to determine the internal friction of the device.

 

Results:

 

The time of oscillation was shorter and the amplitude decreased faster when the suction ring was connected to the eye (figure left) as compared to the control measurement (figure right). The time of oscillation decreased together with amplitude, especially when the muscles were still attached to the eye. Distinction was made between Coulomb’s friction and viscous friction, based on the linear or logarithmic decrease of amplitude.

 

Conclusions:

 

The fact that the time of oscillation decreased together with amplitude indicates an increasing coefficient of dampening at lower speed, similar to the decrease of viscosity found at higher speeds of deformation (shear thinning) in orbital fat.  

 

Keywords: eye movements • orbit • strabismus: etiology