Measuring Corneal Biomechanical Properties in Keratoconic Eyes Undergoing Crosslinking
Corneal collagen crosslinking is a relatively new treatment to stop the progression of keratectasia in patients with keratoconus. It works by strengthening the stromal collagen fibrillae of the cornea and is easy to perform and relatively inexpensive; however, long-term side effects are generally unknown at this time.
Recently, my colleagues and I studied the effects of corneal collagen crosslinking on corneal hysteresis, corneal resistance factor, and intraocular pressure (IOP) using the Ocular Response Analyzer (ORA; Reichert Ophthalmic Instruments, Inc., Depew, New York; Figure 1). This dynamic test uses bidirectional applanation to measure the biomechanical properties of the cornea, aiding in the diagnosis and management of ocular tissue diseases.1-2
GEOMETRIC, DYNAMIC CORNEAL PROPERTIES
The cornea is a complex viscoelastic system. Its geometric properties include curvature, thickness, and elevation; dynamic properties include static resistance (ie, elasticity) and viscous damping (ie, viscosity). Muller et al3 found that the stability of the corneal shape is determined by the architecture of the most anterior portion of the corneal stroma (100–120 µm). More recently, Gatinel et al4 concluded that the creation of a stromal flap modifies the biomechanical properties of the cornea, including a reduction in corneal hysteresis.
The ORA measures a parameter called corneal hysteresis, an indicator of the viscous damping in the cornea—its ability to absorb and dissipate energy. Previous studies have confirmed that patients with keratoconus have significantly lower corneal hysteresis than healthy patients.5-7
Additionally, the ORA measures another parameter called the corneal resistance factor, an indicator of the cornea's overall resistance, which is mostly determined by its elastic properties. The ORA can also supply two IOP measurements: corneal-compensated IOP (IOPcc), a new parameter that is less affected by corneal properties, and Goldmann-correlated IOP (IOPg), a standard measurement for IOP.
STUDY DESIGN, RESULTS
A total of 24 eyes with progressive stage 2 or 3 keratoconus that were scheduled to undergo corneal collagen crosslinking (see Description of the Corneal Crosslinking Process for more information) were included in our corneal evaluation. We used the ORA at baseline, after deepithelialization, at the end of the crosslinking procedure, and at 1, 3, 6, and 12 months postoperatively to determine the biomechanical properties of the crosslinked keratoconic corneas. A control group consisting of the patients' 24 contralateral eyes was also studied.
Scheimpflug photography (Pentacam; Oculus Optikgerte GmbH, Wetzlar, Germany) was used on all eyes in both groups at baseline and 6 and 12 months. Before corneal crosslinking, the average pupil center thickness was 470 µm, and the thinnest point was 455 µm. The average chamber volume was 196 mm³, and the total corneal volume was 56.17 mm³. Twelve months after crosslinking, the average pupil center thickness was 455 µm, the thinnest point was 431 µm, the chamber volume was 196 mm³, and the total corneal volume was 55.69 mm³. The anterior chamber depth remained similar (3.34 mm before crosslinking vs 3.35 mm 12 months after crosslinking).
Our intraoperative evaluation showed no significant changes in corneal hysteresis and corneal resistance factor after epithelial removal; however, the difference was statistically significant soon after corneal collagen crosslinking. No significant changes in corneal hysteresis or corneal resistance factor were seen at 12 months. No changes in IOPcc or IOPg were seen at this time either, although crosslinking did induce a significant reduction in corneal thickness and total corneal volume.
Therefore, we discovered a significant correlation between corneal thickness, corneal hysteresis, and corneal resistance factor at baseline and 12 months after collagen crosslinking in keratoconic eyes. No correlation between corneal thickness, corneal hysteresis, and corneal resistance factor was observed soon after the crosslinking procedure.
In summary, the ORA facilitated the measurement of the effects of corneal collagen crosslinking in keratoconic eyes. Although no change in corneal hysteresis or corneal resistance factor was noted after epithelium removal, these factors significantly increased soon after corneal crosslinking was performed. That significance could not be shown at 6 and 12 months. At 12 months, corneal crosslinking did not induce any change in IOP as measured by the ORA. Crosslinking did induce a significant reduction in corneal thickness; however, this level tends to increase between 6 and 12 months postoperative.
The central corneal thickness, total corneal volume, corneal hysteresis, and corneal resistance factor are all related parameters that help to study the biomechanical properties of the cornea before and after crosslinking.
Elena Albé, MD, is a Consultant in the Department of Ophthalmology, Cornea Service, the Istituto Clinico Humanitas Ophthalmology Clinic, Milan, Italy. Dr. Albé states that she no financial interest in the products or companies mentioned. She may be reached at E-mail: elena.albe@humanitas.it.
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