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S. Schutte, F. van Keulen, C. P. Botha, F. C. T. van der Helm, H. J. Simonsz; Friction in the Orbit. Invest. Ophthalmol. Vis. Sci. 2009;50(13):660.
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The human eye is suspended inside the orbit. The interaction between pressure and tensile forces in the soft tissues keeps the eye in place, while a large range of rotational motion is enabled. The eye, the extraocular muscles (EOMs), orbital fat and the optic nerve mechanically interact and exert forces on each other at every location where they touch. The large range of motion of the eye is facilitated by sliding at interfaces between tissues. It is currently unknown what the magnitude of friction is that occurs at these interface layers and how that relates to the energy lost in viscous deformations of the fat and other tissues.
The main sliding areas in the orbit are (1) between the sclera and the orbital fat, (2) between the EOMs and the orbital fat and (3) between the EOMs and the orbital wall. The total amount of lost energy in eye rotations was measured post mortem in pigs by passively rotating the eye around the visual axis with a pendulum. This energy was lost in tissue viscosity and friction at the interface layers. The Delft Finite Element Model of Orbital Biomechanics, combined with the material properties of the orbital fat (G’=250-500 Pa and G’’=80-170 Pa;Schoemaker et al. 2006) and deformations of the tissues found by MRI imaging and optical flow analysis, were used to distinguish between energy lost in friction and energy lost in viscosity.
The amount of energy that is lost in eye movements due to friction is small(<10%) compared to the energy lost in viscosity of the tissues. Sliding in the human body is usually facilitated by a synovial-fluid like substance (mainly consisting of glycosaminoglycans and glycoproteins) which is known to cause low friction in joints.
Sliding plays an important role in the orbit and friction at the interface layers is low. Finite-element modeling enabled estimation of the amount of energy lost in friction.
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