Changes in ocular biomechanics following vitreoretinal surgery and its effect on intraocular pressure measurement using different modalities.

BACKGROUND

Pars plana vitrectomy (PPV) is one of the main surgical methods for management of posterior segment pathologies.Commonly used intraocular tamponadeagents following PPV include either ringer lactate (RL)/ balanced salt solution (BSS) or sterile air, or gases like sulfur hexafluoride (SF6), or perfluoropropane (C3F8) or silicone oil (SO). RL/BSS/sterile air are used for tamponade in simple PPV surgeries like nucleus drop, cortical drop, intraocular lens (IOL) drop, vitreous hemorrhage etc. Intraocular gas as tamponading agent is

used following PPV in macular hole, epiretinal membrane (ERM) or uncomplicated rhegmatogenous retinal detachment (RRD). Silicone oil is used in complex retinal surgeries including RRD associated with proliferative vitreoretinopathy (PVR), giant retinal tears, diabetic macular tractional RD or RD associated with ocular trauma.Scleral buckles are used for external support in simple phakic retinal detachments while encircling scleral bands are used in conjunction with PPV and SO tamponade in complex RD associated

with PVR and/or choroidal detachment. PPV surgeries and scleral buckles or encircling scleral bands can cause significant changes in the axial length of the eyeball, induce refractive errors and also cause alteration in the corneal biomechanics. Viscoelastic materials exhibit characteristics of both, elasticity as well as viscosity, resulting in energy

dissipation when stress is applied. This is known as hysteresis. The cornea has viscoelastic property, giving it the quality of hysteresis. Currently two devices are commercially available for measurement of the corneal biomechanical properties–the Ocular Response Analyser (ORA) and the Corvis ST. The ORA is based on noncontact tonometer technology, which uses a jet of air to applanate the cornea and an electro-optical system to measure the applanation pressure.

RELEVANCE

In recent years, there has been growing interest in the ocular biomechanical properties that influence intraocular pressure measurements and are independent risk factors

for glaucoma. Vitreoretinal surgery by virtue of its effect on the entire globe, affects ocular biomechanics. However, there is very sparse literature on this. Also, the effect of intraocular tamponade and external buckling on ocular biomechanics merits further evaluation.

NOVELTY

A study detecting the changes in ocular biomechanics following vitreoretinal surgery and the effect of tamponade (silicone oil injection, gas injection) and scleral buckle has not been done before and herein lies the novelty of our study.

OBJECTIVES

A. Among patients undergoing vitreoretinal surgery,

1. To compare corneal hysteresis pre-operatively and post-operatively at 1 week, 1 month and 3 months.

2. To evaluate the effect of tamponade (ringer lactate, sterile air, silicone oil, gas) and scleral buckle or band on corneal hysteresis.

B. Among patients undergoing vitreoretinal surgery,

1. To compare intraocular pressure measurement by NCT, GAT, iCare and Tonopen

2. To assess the influence of change in corneal hysteresis on IOP measurement by NCT, GAT, iCare and Tonopen following vitreoretinal surgery.