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    Progress in corneal biomechanics

    Corneal biomechanical measurements help in screening for keratectasia risk

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    Refractive surgery has motivated tremendous progress in many aspects of ophthalmology over the past decades. The development of new diagnostic instruments, such as wavefront aberrometers and 3-D corneal tomography systems, has been determined by a continuous need for improvement in safety and efficacy. It has been accepted for a long time that the biomechanical properties of the cornea individually influence treatment response, thereby undermining the predictability of refractive procedures. In addition, these properties are modified in different ways by distinct corneal procedures, affecting the measurement of IOP and also possibly affecting corneal stability postoperatively.

    Ectasia


    Table 1: Calculated variables reported by original tonometer software.
    Progressive ectasia (keratectasia) is a rare but very severe complication of LASIK that is caused by the weakening of corneal biomechanical stability by lamellar keratotomy and stromal ablation.1 Classic predisposing factors for ectasia are well described and are based mostly on abnormal anterior curvature-based corneal topography, a too-thin residual stromal bed (calculated from ablation depth and central corneal thickness [CCT]), and young age.2 Post-LASIK corneal ectasia, however, has been reported in patients without preoperative evidence of risk factors based on current technologies.3 For example, the Ectasia Risk Score System,4 which represents a significant improvement over previously used screening strategies, has a false-negative rate of 8% when preoperative data from cases in which ectasia developed after LASIK are retrospectively analysed.

    The need for more advanced understanding of corneal biomechanical properties is definitively highlighted by the ectasia mysteries.

    Corneal biomechanics

    Corneal biomechanics also has the potential to improve outcomes in refractive surgery, not to mention the ability to measure correctly the IOP that is relevant for every general ophthalmologist. Therefore, the ability to measure and understand biomechanical properties in vivo has long been sought by refractive surgeons. The January 2005 (Volume 31, Issue 1) special issue on corneal biomechanics of the Journal of Cataract & Refractive Surgery, guest edited by Cynthia Roberts, PhD, introduced the concept of biomechanical customisation in refractive surgery.5

    Until the commercial introduction of the non-contact applanation tonometer at the 2005 European Society of Cataract and Refractive Surgeons annual meeting in Lisbon, Portugal, however, corneal biomechanical evaluations were limited to in vitro studies in the laboratory and to virtual mathematical corneal finite element models. The device was designed as an improvement in non-contact tonometry (NCT) with the goal of providing a more accurate measurement of IOP through the understanding of corneal properties. The advantage over standard NCT was based on a precisely metered collimated air pulse and a quantitative electro-optical system that monitors the deformation of the cornea through the corneal reflex of an infrared light.

    During the tonometer measurement, the air pressure forces the cornea to deform inward, passing applanation into a slight concavity until the air pump shuts off and the cornea gradually recovers its normal configuration, passing through a second applanation state. The applanation events are registered by a peak on the corneal reflex signal that monitors the entire process. The peak pressure of the air pulse is controlled according to the first applanation event, when an internal command on the instrument shuts off the air pump. Thus, two independent values are obtained for the pressure of the air pulse at the moments of the inward (P1) and outward (P2) applanation events.

    Because of energy absorption, or corneal damping related to its viscoelasticity, the inward and outward applanation events are delayed, resulting in two different pressures. The pressure measurements (P1 and P2) were the basis for all calculated variables reported by the original tonometer software (Table 1). The difference between the two pressures is called corneal hysteresis (CH), a new concept when introduced to the ophthalmology community. The term "hysteresis" is derived from an ancient Greek word meaning "lagging behind."

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