Cynthia J. Roberts

Influence of corneal biomechanical properties on intraocular pressure measurement Quantitative analysis

Purpose: To understand and quantify intraocular pressure (IOP) measurement errors introduced by corneal variables during applanation tonometry using a cornea biomechanical model. Setting: Department of Ophthalmology, Biomedical Engineering Center, The Ohio State University, Columbus, Ohio, USA. Methods: The model assumed an overall resultant pressure that was based on the summation of the applanation pressure, the true IOP, and the surface tension caused by the tear film to determine the final deformation of the corneal apex during IOP measurement. Corneal resistance was varied according to the corneas biomechanical properties, thickness, and curvature, and the effect of each variable on the accuracy of IOP tonometry readings was examined quantitatively. Results: The model demonstrated that tonometry readings do not always reflect true IOP values. They deviate when corneal thickness, curvature, or biomechanical properties vary from normal values. Based on the model, predicted IOP readings have a 2.87 mm Hg range resulting from the variation in the corneal thickness in the normal population and a 1.76 mm Hg range from the variation in the corneal radius of curvature. Considering that Youngs modulus of the corneal varies from 0.1 to 0.9 MPa in the normal population, the model predicts tonometry IOP readings will have a range of 17.26 mm Hg because of the variation in this corneal biomechanical parameter alone. Conclusions: The simulation based on the model demonstrated quantitatively that variations in each corneal variable cause errors in tonometry IOP readings. The simulation results indicate that differences in corneal biomechanics across individuals may have greater impact on IOP measurement errors than corneal thickness or curvature.

Computational modelInfluence of corneal biomechanical properties on intraocular pressure measurement: Quantitative analysis

Purpose: To understand and quantify intraocular pressure (IOP) measurement errors introduced by corneal variables during applanation tonometry using a cornea biomechanical model. Setting: Department of Ophthalmology, Biomedical Engineering Center, The Ohio State University, Columbus, Ohio, USA. Methods: The model assumed an overall resultant pressure that was based on the summation of the applanation pressure, the true IOP, and the surface tension caused by the tear film to determine the final deformation of the corneal apex during IOP measurement. Corneal resistance was varied according to the corneas biomechanical properties, thickness, and curvature, and the effect of each variable on the accuracy of IOP tonometry readings was examined quantitatively. Results: The model demonstrated that tonometry readings do not always reflect true IOP values. They deviate when corneal thickness, curvature, or biomechanical properties vary from normal values. Based on the model, predicted IOP readings have a 2.87 mm Hg range resulting from the variation in the corneal thickness in the normal population and a 1.76 mm Hg range from the variation in the corneal radius of curvature. Considering that Youngs modulus of the corneal varies from 0.1 to 0.9 MPa in the normal population, the model predicts tonometry IOP readings will have a range of 17.26 mm Hg because of the variation in this corneal biomechanical parameter alone. Conclusions: The simulation based on the model demonstrated quantitatively that variations in each corneal variable cause errors in tonometry IOP readings. The simulation results indicate that differences in corneal biomechanics across individuals may have greater impact on IOP measurement errors than corneal thickness or curvature.

The Cornea is Not a Piece of Plastic

he future of laser refractive surgery is exciting with the potential for ever-improved postoperative visual performance. In the past, the operative goal has been 20/20 uncorrected visual acuity with zero residual refractive error. Criteria for a successful procedure are no longer 20/40 or better with ±1.00 diopter (D) of residual refractive error, because neither surgeons nor patients are satisfied with such gross measurements of visual performance. Hence, considerable research effort is being devoted to develop customized procedures for each patient. The new goal is 20/10 uncorrected visual acuity with aberration-free postoperative vision. How can these lofty goals be accomplished? First, lasers have been improved with the development of scanning, small spot systems, as opposed to broad-beam systems. Scanning systems have brought customized procedures into the realm of feasibility. Second, more comprehensive and sophisticated input data can be used to guide the laser based on individual patient measurements, as opposed to the simple refractive sphere and cylinder. Two types of approaches are currently being pursued—wavefront-guided and topography-guided procedures. Early results are promising, yet neither approach has demonstrated consistently superior results to non-guided procedures in controlled, scientific studies. Is a piece of the perfect vision puzzle still missing? Is there an additional, complimentary approach to customization that has yet to emerge? Are we leaping into the future ahead of our understanding of how these procedures may be implemented to be optimally successful? Munnerlyn and colleagues 1 gave us an elegant analysis of how corneal surface shape may be altered to correct both myopia and hyperopia. His geometric approach can be thought of as a “shapesubtraction” model, where tissue of an appropriate profile is simply “subtracted” using an excimer laser to produce the desired surface curvature. Essentially, the cornea is treated like a piece of plastic to be sculpted into the ideal surface shape. The Munnerlyn approach, combined with empirical experience, has been relatively successful in correcting spherical and cylindrical errors for the majority of patients treated to date. However, only 50% to 85% achieve 20/20, despite greater than 90% of patients being “satisfied.” In addition, significant postoperative optical aberrations are produced using conventional algorithms 2-4 , encouraging the development of aberration-reducing ablation profiles.

Correlations between corneal hysteresis, intraocular pressure, and corneal central pachymetry

PURPOSE: To analyze the correlation between corneal hysteresis (CH) measured with the Ocular Response Analyzer (ORA, Reichert) and ultrasonic corneal central thickness (CCT US) and intraocular pressure measured with Goldmann applanation tonometry (IOP GA). SETTING: Bordeaux 2 University, Ophthalmology Department, Bordeaux, France. METHODS: This study comprised 498 eyes of 258 patients. Corneal hysteresis, corneal resistance factor (CRF), and IOP corneal‐compensated (IOPcc) were provided by the ORA device; CCT US and IOP GA were also measured in each eye. The study population was divided into 5 groups: normal (n = 122), glaucoma (n = 159), keratoconus (n = 88), laser in situ keratomileusis (LASIK) (n = 78), and photorefractive keratectomy (n = 39). The Pearson correlation was used for statistical analysis. RESULTS: Corneal hysteresis was not strongly correlated with IOP or CCT US. The mean CH in the LASIK (8.87 mm Hg) and keratoconus (8.34 mm Hg) groups was lower than in the glaucoma (9.48 mm Hg) and normal (10.26 mm Hg) groups. The lower the CH, the lower its correlation with IOPcc and IOP GA. A CH higher than the CRF was significantly associated with the keratoconus and post‐LASIK groups. CONCLUSIONS: Corneal hysteresis, a new corneal parameter, had a moderate dependence on IOP and CCT US. Weaker corneas could be screened with ORA parameters, and low CH could be considered a risk factor for underestimation of IOP. The CCT US should continue to be considered a useful parameter.

Effect of Acute Biomechanical Changes on Corneal Curvature After Photokeratectomy

PURPOSE Unintended hyperopic shift is a common yet poorly understood complication of phototherapeutic keratectomy (PTK) that raises fundamental questions about the etiology of corneal curvature change in PRK and LASIK. We investigated the relative contributions of ablation profile and peripheral stromal thickening to intraoperative PTK-induced central flattening, and propose a biomechanical model of the acute corneal response to central ablation. METHODS Fourteen de-epithelialized eye bank globes from seven donors underwent either broadbeam ablation (approximately 100-microm depth, no programmed dioptric change) or sham photoablation in paired-control fashion. Peripheral stromal thickness changes and the pattern of thickness loss across each ablation zone were evaluated by optical section image analysis as predictors of acute corneal flattening. RESULTS Relative to sham ablation, keratectomy caused significant anterior corneal flattening (-6.3+/-3.2 D, P = .002). Concomitant peripheral stromal thickening (+57+/-43 microm, P = .01) was a significant predictor of acute hyperopic shift (r = 0.68, P = .047). Ablation pattern bias did not consistently favor hyperopia and was a poor lone predictor of hyperopic shift. CONCLUSIONS Unintended keratectomy-induced hyperopic shift is replicable in a human donor model and is associated with significant thickening of the unablated peripheral stroma. This biomechanical response may have a considerable impact on early refractive outcomes in PTK, PRK, and LASIK.

A Viscoelastic Biomechanical Model of the Cornea Describing the Effect of Viscosity and Elasticity on Hysteresis

PURPOSE To develop a method for evaluating viscosity and elasticity of the cornea and to examine the effect that both properties have on hysteresis. METHODS A three-component spring and dashpot model was created in Simulink in Matlab to represent the purely elastic and viscoelastic behavior of the cornea during a measurement using device called an ocular response analyzer (ORA). Values for elasticity and viscosity were varied while sinusoidal stress was applied to the model. The simulated stresses were used to determine how hysteresis is affected by the individual components of elasticity, viscosity, and maximum stress. To validate the model, high-speed photography was used to measure induced strain in a corneal phantom during ORA measurement. This measured strain was compared with the strains simulated by the model. RESULTS When the spring in the viscoelastic portion of the model was stiffened, hysteresis decreased. When the spring in the purely elastic element was stiffened, hysteresis increased. If both springs were stiffened together, hysteresis peaked strongly as a function of the viscosity of the viscoelastic element. Below the peak value, lower elasticity was associated with higher hysteresis. Above the peak value, higher elasticity was associated with higher hysteresis. In addition, hysteresis increased as the air maximum pressure was increased. Measurements from phantom corresponded to predictions from the model. CONCLUSIONS A viscoelastic model is presented to illustrate how changing viscosity and elasticity may affect hysteresis. Low hysteresis can be associated with either high elasticity or low elasticity, depending on the viscosity, a finding consistent with clinical reports.

Biomechanics of the cornea and wavefront-guided laser refractive surgery.

PURPOSE Wavefront-guided laser refractive surgery induces postoperative optical aberrations that are not explained by the ablation profile. METHODS A conceptual model is presented for a potential mechanism of surgically-induced corneal shape change that is dependent on structural modification, rather than ablation profile. RESULTS Examples are provided from clinical and basic studies that are consistent with the proposed model. CONCLUSION Corneal biomechanical response to ablative surgery may significantly affect outcomes, and should be taken into account when planning customized procedures.

Screening of Forme Fruste Keratoconus with the Ocular Response Analyzer

PURPOSE To evaluate the performance of the Ocular Response Analyzer (ORA) in the screening of forme fruste keratoconus (FFKc). METHODS A retrospective comparative study was conducted involving 180 eyes. ORA preoperative data were analyzed for 125 normal control eyes (64 patients) undergoing laser in situ keratomileusis (LASIK) without corneal ectasia after 24 months of follow-up and 55 case eyes with unilateral keratoconus from a database (BCVA of 1.0, KISA index <60%). All eyes were matched in four groups of central corneal thickness (CCT): group 1, <500 microm; group 2, 500 to 539 microm; group 3, 540 to 579 microm; and group 4 >580 microm. Corneal hysteresis (CH), the corneal resistance factor (CRF), the air pressure curve, and the infrared signal were compared between FFKc and normal eyes in each group. RESULTS The mean CH was 9.1 +/- 1.8 mm Hg for FFKc and 10.3 +/- 1.9 mm Hg for control eyes (P < 0.001), and the mean CRF was 9.2 +/- 1.8 and 11.1 +/- 2 mm Hg (P < 0.001), respectively. Sensitivity in each group was as follows: group 1, CH < 9.5 mm Hg (91%) and CRF < 9.5 mm Hg (81%); group 2, CH < 10.5 mm Hg (91%) and CRF < 10 mm Hg (87%); group 3, CH < 11.5 mm Hg (79%) and CRF < 11 mm Hg (74%); group 4 had two cases of FFKc, and the difference was not significant. Air pressure levels at inward and outward applanation and the maximum air pressure level were significantly lower and shorter in time in FFKc (P < 0.001), whereas the shape of the infrared signal was more variable. CONCLUSIONS The ORA provides additional information in the screening of FFKc, with an accurate analysis of the corneal biomechanical properties according to CCT, air pressure, and infrared curves.

Corneal Biomechanics and Biomaterials

The ability to clearly observe ones environment in the visible spectrum provides a tremendous evolutionary advantage in most of the worlds habitats. The complex optical processing system that has evolved in higher vertebrate animals gathers, focuses, detects, transduces, and interprets incoming visible light. The cornea resides at the front end of this imaging system, where it provides a clear optical aperture, substantial refractive power, and the structural stability required to protect the fragile intraocular components. Nature has resolved these simultaneous design requirements through an exceedingly clever manipulation of common extracellular-matrix structural materials (e.g., collagen and proteoglycans). In this review, we (a) examine the biophysical and optical roles of the cornea, (b) discuss increasingly popular approaches to altering its natural refractive properties with an emphasis on biomechanics, and (c) investigate the fast-rising science of corneal replacement via synthetic biomaterials. We close by considering relevant open problems that would benefit from the increased attention of bioengineers.

Biomechanical characteristics of the ectatic cornea

&NA; The ocular response analyzer (ORA) (Reichert, Inc.) was used in the case of a middle‐aged man who developed unilateral corneal ectasia after bilateral laser in situ keratomileusis (LASIK). The preoperative refraction was similar in the 2 eyes. Post‐LASIK ectasia was central in the left eye; topography was oblate in the right eye. The ORA values consisted of the mean of 4 measurements. Corneal hysteresis and corneal resistance factor were almost equal in the ectatic eye and the nonectatic eye. However, significant between‐eye differences in the morphology of the signals were noted, most prominently in the lower amplitude of the applanation peaks in the ectatic eye. The shape of the applanation signal yielded important information in addition to corneal hysteresis and corneal resistance factor.