A Corneal Biomechanical Study Measured with a Scheimpflug Dynamic Analyser in Soft Contact Lens Wearers

Citation: López-Muñoz, A.; López-Castaño, I.; Torres-Parejo, Ú.; García-Romera, M.-C. A Corneal Biomechanical Study Measured with a Scheimpflug Dynamic Analyser in Soft Contact Lens Wearers. Life 2023, 13, 2313. https://doi.org/10.3390/ life13122313 Academic Editor: Gerhard A. Holzapfel Received: 10 October 2023 Revised: 1 December 2023 Accepted: 6 December 2023 Published: 8 December 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). life Article A Corneal Biomechanical Study Measured with a Scheimpflug Dynamic Analyser in Soft Contact Lens Wearers Alfredo López-Muñoz 1,2,* , Isabel López-Castaño 1,Úrsula Torres-Parejo 3and Marta-C. García-Romera 1 1Department of Physics of Condensed Matter, Optics Area, Vision Sciences Research Group (CIVIUS), Pharmacy School, University of Seville, 41009 Seville, Spain; [email protected] (I.L.-C.); [email protected] (M.-C.G.-R.) 2Research & Development Department (Miranza Virgen de Luján®), Ophthalmology Center, 41011 Seville, Spain 3Department of Statistics and Operations Research, University of Grenade, 18071 Grenade, Spain; [email protected] *Correspondence: [email protected]; Tel.: +34-639-388-598 Abstract: The aim of this study was to evaluate the biomechanical changes in the cornea after wearing soft contact lenses (CLs) in healthy myopic patients measured with a Corvis ST ® (CST, Oculus Optikgeräte GmbH, Wetzlar, Germany) analyser. This prospective, cross-sectional, single-centre study was performed on twenty-two Caucasian patients aged between 19 and 24 years (2 0.64 ±1.21 year s) range. Five device-specific biomechanical parameters, the central corneal thickness (CCT), and biomechanically corrected intraocular pressure (bIOP) were measured prior to fitting and one month after CL wear. Differences between the means of the deflection amplitude ratio (DA Ratio) and the standard deviation of the DA Ratio (SD DA Ratio) pre- and post-CL wear were found to be significant (pvalue = 0.002 in both cases). Significant differences were found between pre- and post-CL wear values in CCT (pvalue = 0.013). For all other biomechanical measures, no significant differences were observed before and after treatment. A significant association was found between changes in bIOP and classification according to changes in Int. Radius (pvalue = 0.047) and SSI (pvalue = 0.026) standard deviations. The corneal biomechanical indices provided by CST demonstrate that the fitting of soft CLs is a safe optical compensation method for the stability of corneal stiffness. No significant differences were found pre- and post-CL wear in the assessment of bIOP. Keywords: myopia; soft contact lenses; corneal biomechanics; Scheimpflug technology; Corvis ST® 1. Introduction The shape of the cornea is a determining factor in ocular refraction but is itself determined by its biomechanical properties. The cornea must be soft enough to expand into the spherical hemisphere but rigid enough to hold its shape and resist intraocular pressure (IOP) [1]. Biological properties, such as healing responses and biomechanics, are essential in determining and maintaining corneal transparency, as well as geometric and optical properties [2]. Most biomechanical studies to date have focused on the stroma, which constitutes 90% of the total thickness of the cornea and is generally considered to be the primary supporting layer of the cornea. Studies have demonstrated the complex nature of the stroma and the regular diameter and spacing of the collagen fibrils, as well as their influence on corneal transparency and biomechanical behaviour [3]. Notably, the cornea maintains a delicate and complex balance between stiffness, strength, elasticity, and overall strength to withstand internal and external forces that constantly compress it, distort its shape, or threaten its integrity [ 4 ]. This is measured by corneal biomechanics, which has emerged as a research and development topic in modern ophthalmology due to its many potential applications [5]. Life 2023,13, 2313. https://doi.org/10.3390/life13122313 https://www.mdpi.com/journal/life Life 2023,13, 2313 2 of 14 Biomechanics is commonly defined as the application of mechanics to biology. However, the term is better described as an extension and development of mechanics with the goal of understanding physiology and physiopathology, as well as the diagnosis and treatment of injuries and diseases due to the intricate and diversified behaviour of biological structures and materials [6]. Corneal biomechanics is the study of the structure of the cornea by defining the physical and mathematical principles that can predict the dynamic response of the cornea to physiological and/or pathological conditions through behavioural patterns or model definitions of the corneal tissue. Corneal biomechanics is the science that deals with the balance and deformation of tissue subjected to any force. It studies the function and structure of the cornea and forms a basis for predicting its dynamic response in physiological and pathological conditions [7]. The capability to measure the biomechanical properties of the cornea in vivo is of great clinical importance as it helps to improve many treatment and management procedures that mechanically interact with or affect the eye. Examples include measuring IOP for effective glaucoma management [ 8 ]; planning refractive surgery [ 9 ]; determining keratoconus risk [ 10 ]; and optimising various protocols for collagen cross-linking treatments or evaluation [ 11 ], including preoperative evaluations for the retreatment of refractive surgery. The mechanical interaction between the lens and the anterior segment is not currently considered in the selection of intracorneal ring implants or even in the design of soft contact lenses [ 12 ]. Additionally, corneal biomechanical analysis has been suggested as a potentially relevant factor in orthokeratology (OK), but the role of corneal biomechanical properties in predicting the correction obtained with this refractive compensation option is unclear [6]. Interest in the use of biomechanical principles in the cornea has increased significantly in recent years with the aim of better understanding corneal behaviour and improving the safety and efficacy of various ocular treatments or refractive techniques [13]. An in vivo evaluation of corneal parameters is essential to understand corneal behaviour under physical stress. However, in clinical practice, it is not easy to accurately evaluate the behaviour of the cornea under stress and use the results to estimate some mechanical properties of the cornea [ 13 ]. However, until recently, the evaluation of the biomechanical properties of the cornea was limited to ex vivo laboratory studies and mathematical models of the cornea [14]. There are still a limited number of techniques developed and tested to characterise corneal biomechanics with potential application in clinical practice. Two instruments are currently available to characterise the biomechanical properties of the cornea in clinical settings, namely the Ocular Response Analyser (ORA; Reichert Inc., Depew, NY, USA) and the Corvis Scheimpflug Technology (CST, Oculus Optikgeräte GmbH, Wetzlar, Germany), based on the measurement of corneal deformation using the Scheimpflug technique. Both have unique parameters that describe corneal biomechanics, but their relationship to standard mechanical properties is unknown and is not associated with a specific biomechanical model. Therefore, there is inconsistency in the definition of some fundamental biomechanical parameters, such as viscosity or elasticity, to characterise the biomechanical properties of the cornea. As a result, comparative analysis between studies using different technologies is difficult [15]. The ORA was presented as the first equipment to assess the biomechanical behaviour of the cornea in vivo at the 2005 ESCRS meeting (Lisbon, Portugal) [ 16 ]. The ORA is a modified non-contact tonometer (NCT) initially designed to provide more accurate IOP measurements through corneal biomechanical compensation. It analyses the behaviour of the cornea during bidirectional applanation induced by an air jet and generates estimates of corneal hysteresis (CH) and the corneal resistance factor (CRF) along with a set of 36 waveform-derived parameters [ 14 ]. The ORA combines an air puff with an infrared transmitter and receiver. This device can evaluate corneal deformation only indirectly based on infrared signals [17]. Life 2023,13, 2313 3 of 14 The photoelectric coherence detection system monitors the curvature of the cornea with a central diameter of 3.0 mm during a 20 ms measurement [ 18 ]. In this system, the maximum air pressure generated varies from the first stabilisation event. The maximum pressure of the ORA is adjusted from test to test so that eyes with early initial loading and typically low IOP receive a lower maximum pressure, and eyes with high IOP receive a larger maximum pressure bladder [ 6 ]. The measurement involves automatic alignment with the top of the cornea and triggers the air puff. The measurement takes approximately 25 ms. The cornea deforms due to air pressure (internal phase), and the first flattening occurs when the pressure is recorded (P1). The cornea takes a concave shape, until the air pressure decreases, allowing the cornea to gradually return to its normal shape. During the exit phase, it undergoes a second delamination state, where the pressure (P2) is recorded again. Both abrasion events are recognised by peaks in the corneal reflex signal corresponding to two independent pressure values in the air puff pressure profile. These pressure measurements (P1 and P2) form the basis of the first-generation variants reported from the original ORA program [16]. The average of the two applanation pressures was correlated with Goldmann tonometry results in an internal study with the aim of providing (linear) calibration coefficients for reporting intraocular pressure and CH in millimetres of mercury (mmHg). The procedure has been described [18]. The CST, the analytical tool used in this study, was later introduced as NCT. This device uses an ultrahigh-speed (UHS) Scheimpflug camera to monitor the corneal response to air pressure pulses and uses the acquired image sequences to estimate IOP and strain response parameters [14]. The CST has been commercially available since 2011 and is based on UHS dynamic Scheimpflug imaging technology. It measures IOP, central corneal thickness (CCT), and corneal biomechanical parameters by directly observing and imaging corneal deformation in response to a standard puff of air in real time [ 19 ]. The instrument is ergonomically designed with an adjustable head control and chin rest. The patient is positioned comfortably with the chin and forehead positioned appropriately. The patient is asked to focus on the central red LED (light-emitting diode). A front camera with a keratometer-type projection system is installed to focus and align the corneal apex. The test is programmed to trigger automatically when synchronisation with the first corneal Purkinje reflex is achieved. Manual triggering is also possible [2]. The UHS Scheimpflug camera uses more than 4300 frames per second to monitor the corneal response to a collimated puff of air measured in a fixed profile with a symmetrical configuration and a fixed maximum internal pump pressure of 25 kPa. UHS Scheimpflug cameras are equipped with blue light LEDs (455 nm, no UV) and cover 8.5 mm horizontally with a slit. The exposure time is 30 ms, and 140 digital images can be acquired. Each image has 576 pixels [2]. In addition to IOP, corrected IOP (corrected IOP based on the Dresden correction table), and CCT, the following parameters are measured: time to the first (A1) and second (A2) lamination (time to reach the first and second lamination, respectively); A1 and A2 lengths (the length of the segment flatness in the Scheimpflug image during the first and second flattening); A1 and A2 velocities (the velocity of corneal movement in inner and outer flattening); and features of the highest concavity, including time to reach maximum stress (hours), strain amplitude (DA), and distance between the peak point of the curvature (PD) and the radius of the curvature [19]. The development of the new biomechanical principles of ocular structure is an emerging area of research in optometry and ophthalmology. This is a challenge that must be met in order to create more appropriate in vivo biomechanical models of the cornea and to define appropriate predictive models of corneal behaviour. These tools will allow the clinician to predict the clinical outcomes of various ocular treatments before they are performed, thus allowing for their optimisation [20]. Life 2023,13, 2313 4 of 14 On the other hand, the adaptation of contact lenses (CLs) on the ocular surface causes a multitude of physical corneal changes, modifying its curvature and tear quality. From the first hours of wear, corneal alterations are recorded that lead to the appearance of allergic, infectious, anatomical, and metabolic phenomena resulting in discomfort or discomfort for the wearer, with infection being the most serious complication [ 21 ]. However, there are limited data on corneal biomechanical changes after the daily wear of soft CLs. There are several reports of changes in corneal topography [ 22 ], changes in anterior corneal topography [23], and central corneal oedema [24] after CL use. A possible mechanism for changes in biomechanical properties has been attributed to corneal stromal oedema after CL use, which increases the distance between collagen fibrils and affects the biomechanical function of the cornea [ 25 ]. Furthermore, another possible hypothesis regarding the repolarisation of corneal tissue and the resulting changes in corneal biomechanical behaviour may be related to local alterations in inflammatory cytokines and chemokines after CL use [26]. The scientific literature contains several studies on the characterisation of corneal biomechanics in different study designs, with investigations using both the ORA and the CST [ 6 ]. Regarding corneal biomechanics related to the wearing of soft CLs, we found studies utilising the ORA, but to our knowledge, there are limited reports about the relationship between the use of soft CLs and corneal biomechanics using the CST [27]. Therefore, we consider it of utmost importance to understand the structural and biomechanical changes in the cornea after wearing CLs in our research of hydrophilic material because this may have important clinical implications, especially for patients whose properties are already altered before use, as in the case of pathological corneas, as well as for the development of new CLs with different uses or applications. Thus, the need for this project arises. 2. Materials and Methods 2.1. Design This prospective, cross-sectional, single-centre study was conducted between February 2023 and May 2023 at the facilities of the Faculty of Pharmacy (Department of Optics and Optometry) of the University of Seville, Spain. The study complied with the standards established by the Declaration of Helsinki and the Andalusian Ethics Committee Council. After explaining the nature of the study, informed consent was obtained from each subject. 2.2. Subjects Twenty-two Caucasian patients belonging to the student community of the University of Seville were recruited. To avoid any bias, only one eye from each participant was randomly selected for inclusion in this study [ 28 ]. Inclusion criteria were as follows: (1) 18 years of age; (2) habitual CL wearer; (3) simple or compound myopic refractive error (with astigmatism); (4) cessation of CL wear for at least 7–21 days among hydrophilic and rigid gas permeable lens wearers, respectively, with special topography follow-up in case of corneal moulding until disappearance or topographic stabilisation; and (5) the acceptance of participation in the study and ability to understand the informed consent and the subsequent signature of the consent form. Exclusion criteria were as follows: (1) collagen or autoimmune diseases; (2) being pregnant; (3) previous intraocular or corneal surgery; (4) ocular pathologies; (5) corneal dystrophies and degenerations; (6) dry eye syndrome; (7) persistent epithelial defects; (8) history of herpetic corneal ulcer; (9) central corneal leukoma; (10) topographic map compatible with subclinical keratoconus or other corneal ectatic disorder; and (11) current antiglaucomatous or hypotonic treatment. 2.3. Contact Lenses The lens used by the subjects were Lens 55 ® UV (Ocufilcon D4 (19) (55%); Servilens (Granada, Spain), https://www.lens55.com/documents/catalogo_Servilens_V20.pdf, ac- Life 2023,13, 2313 5 of 14 cessed on 1 January 2023), a hydrogel lens with a thin rim design, performing well in terms of comfort and parameters. CL prescriptions were determined based on the refraction of the spectacle distance and modified, if necessary, by taking into account the vertex distance. The lenses were fitted binocularly, and the same type of lens was used in each eye. Each pair of lenses was worn for 10 min to settle on the eye before starting the measurements and then worn for approximately half an hour during the assessment of distance visual acuity. After the settling period, the CL fit was checked and confirmed to be acceptable (<1.0 mm of decentration, <1.0 mm of movement) before proceeding with the assessment [ 29 ]. Thus, the wearers’ initial response to the CLs was assessed. 2.4. Procedure Once the informed consent form had been signed, each study subject underwent a pre-visit in which, in addition to collecting the relevant data from the clinical history, they underwent a complete ophthalmological examination with the following diagnostic tests: optically corrected visual acuity (BCVA) according to the Snellen decimal scale; subjective and objective refraction without cycloplegia; corneal topography and axial length measurement with Pentacam AXL ® , a device with a single rotation Scheimpflug camera with version 6.08r19 software produced by Oculus Optikgeräte in Wetzlar, Germany; corneal biomechanical analysis using Scheimpflug technology via the CST analyser (Oculus Optikgeräte GmbH, Wetzlar, Germany); and a slit-lamp anterior pole examination. Another visit was scheduled for CL fitting and a final examination one month after CL wear, at which time the biomechanical parameters were remeasured. All measurements with the CST were performed by the same technicians and recorded with automatic release to ensure the absence of examiner dependence. If the examination quality (QS) box showed any type of alteration, this was identified in the device software; to be considered an optimal image, the quality factor had to be higher than 95% (this figure may be lower due to the presence of artifacts in the image, eye blinking, or insufficient eye opening by the patient); otherwise, the necessary correction was made, and the acquisition was repeated. Only CST exams with an “OK” quality rating were included in the analysis, excluding alignment errors. Flicker errors were also excluded. In all cases, 3 measurements were taken per patient, and the mean was calculated. The variables included in this study are the most reproducible parameters of the CST, as collected in the Biomechanical Comparison Display (Figure 1): (1) the deflection amplitude ratio (DA Ratio): This is the ratio between the central corneal deflection and the average of two points located at 1.0 mm (DA Ratio 1) or 2.0 mm (DA Ratio 2) either side of the centre. Stiffer corneas would have a lower DA Ratio because the centre of the cornea and the cornea at 1.0 or 2.0 mm deflect at the same time, whereas a higher DA Ratio indicates that the central cornea deflects more than the average of the other two points, corresponding to softer tissue; (2) the Ambrósio horizontal relational thickness (ARTh): Corneal thickness is measured using the horizontal Scheimpflug image. This allows the rate of increase in corneal thickness from the apex to the nasal and temporal sides to be calculated. The characterisation of the thickness profile allows for the calculation of the Ambrósio relational thickness across the horizontal meridian, which is a relative simplification of the tomographic relational thickness calculations also provided by the Pentacam; (3) the stiffness parameter-A1 (SP-A1): This is defined as the pressure at first flattening, which is the difference between the air bubble pressure at the corneal surface and the bIOP, divided by the DA Ratio. It is determined from the displacement of the apex from rest to the first flattening. This value has been clinically proven to be useful in assessing KC with the highest sensitivity and specificity of any of the parameter values. Higher values indicate stiffer corneas; (4) integrated radius (Int. Radius): This is a dynamic corneal deformation response parameter representing the reciprocal of the radius at the state of maximum corneal concavity. A larger concave radius is associated with greater resistance to deformation, i.e., a stiffer cornea. The larger the integrated inverse radius Life 2023,13, 2313 6 of 14 and the maximum inverse radius, the lower the resistance to deformation and the lower the corneal stiffness; and (5) the stress–strain index (SSI): This is a new parameter for estimating the material stiffness of corneal tissue that is independent of IOP and corneal geometry. The stress–strain curve describes the elastic properties of the cornea. The curves are shifted to the right if the cornea is soft and to the left if it is stiff. The SSI index describes the position of the curve. A value of 1 indicates average elasticity, a value less than 1 indicates softer behaviour, and a value greater than 1 indicates stiffer than average behaviour. The secondary variables were (1) the central corneal thickness (CCT) and (2) the biomechanically corrected intraocular pressure (bIOP). None of the variables have units, and moreover, they do not have standardised values. However, the standard deviation (SD) PRE–POST was compared, and depending on the variable, the change was qualified as not significant, softer/stiffer, or thinner/thicker (Table 1). All measurements with the CST were taken by the same technicians and captured via automatic release to ensure no examiner dependence. To be considered an optimal image, the quality factor had to be higher than 95%. Other parameters that were measured, not included as study variables, were the spherical equivalent (SE), with a mean value of − 3.63 D ± 0.41 D ( − 9.25 D, −0.75 D ); the axial length of the eyeball (AXL), with a mean value of 25.16 mm ± 0.17 mm (2 4.82 m m, 25.50 mm); and the mean keratometry (Km), calculated as the average of K1 and K2 within the 3 mm central optical zone, with a mean result of 7.80 mm ± 0.49 mm (7.28 mm, 8.25 m m). In all cases, 3 measurements were obtained per patient, and the mean value was used. Life2023,13,23136of14   ues.Highervaluesindicatestiffercorneas;(4)integratedradius(Int.Radius):Thisisa dynamiccornealdeformationresponseparameterrepresentingthereciprocaloftheradiusatthestateofmaximumcornealconcavity.Alargerconcaveradiusisassociated withgreaterresistancetodeformation,i.e.,astiffercornea.Thelargertheintegratedinverseradiusandthemaximuminverseradius,thelowertheresistancetodeformation andthelowerthecornealstiffness;and(5)thestress‒strainindex(SSI):Thisisanew parameterforestimatingthematerialstiffnessofcornealtissuethatisindependentof IOPandcornealgeometry.Thestress–straincurvedescribestheelasticpropertiesofthe cornea.Thecurvesareshiftedtotherightifthecorneaissoftandtotheleftifitisstiff. TheSSIindexdescribesthepositionofthecurve.Avalueof1indicatesaverageelasticity, avaluelessthan1indicatessofterbehaviour,andavaluegreaterthan1indicatesstiffer thanaveragebehaviour.Thesecondaryvariableswere(1)thecentralcornealthickness (CCT)and(2)thebiomechanicallycorrectedintraocularpressure(bIOP).Noneofthe variableshaveunits,andmoreover,theydonothavestandardisedvalues.However,the standarddeviation(SD)PRE–POSTwascompared,anddependingonthevariable,the changewasqualifiedasnotsignificant,softer/stiffer,orthinner/thicker(Table1).All measurementswiththeCSTweretakenbythesametechniciansandcapturedviaautomaticreleasetoensurenoexaminerdependence.Tobeconsideredanoptimalimage,the qualityfactorhadtobehigherthan95%.Otherparametersthatweremeasured,notincludedasstudyvariables,werethesphericalequivalent(SE),withameanvalueof−3.63 D±0.41D(−9.25D,−0.75D);theaxiallengthoftheeyeball(AXL),withameanvalueof 25.16mm±0.17mm(24.82mm,25.50mm);andthemeankeratometry(Km),calculated astheaverageofK1andK2withinthe3mmcentralopticalzone,withameanresultof 7.80mm±0.49mm(7.28mm,8.25mm).Inallcases,3measurementswereobtainedper patient,andthemeanvaluewasused.   Figure1.Comparisonpre‐andpost‐contactlenswearofbiomechanicalCorvisST®parameters.DA Ratio=deflectionamplituderatio;Integr.Radius=integratedradius;ARTh=Ambrósiohorizontal relationalthickness;SP‐A1=stiffnessparameter‐A1;SSI=stress‒strainindex. Table1.Classificationofthechangeinstandarddeviationofbiomechanicalparameterspost–pre (B‐A)oftheBiomechanicalComparisonDisplaysoftware version1.6r2554. B‐ANotSignificantB‐AStifferB‐ASofter DARatio±1.0>−1.0>+1.0 Int.Radius±0.7>−0.7>+0.7 Figure 1. Comparison pre- and post-contact lens wear of biomechanical Corvis ST ® parameters. DA Ratio = deflection amplitude ratio; Integr. Radius = integrated radius; ARTh = Ambrósio horizontal relational thickness; SP-A1 = stiffness parameter-A1; SSI = stress–strain index. 2.5. Data Analysis The collected data were reviewed for consistency and correctness. Subsequently, statistical analysis was carried out using SPSS software for Windows version 22.0 (IBM, SPSS, Inc., Chicago, IL, USA). The normality of both the variables that record the biomechanical values and those that record the standard deviations of these values was checked using the Shapiro–Wilk test. For the variables that followed a normal distribution, the t-test for related samples was employed. For those variables for which it was not possible to assume normality, non-parametric techniques were used, specifically the Wilcoxon test. For all statistical tests, a pvalue < 0.05 was considered statistically significant. The statistical analysis of the collected data under the guidelines of the clinical protocol allowed us to Life 2023,13, 2313 7 of 14 draw and establish the conclusions of the study, thus quantifying the biomechanical impact on the use of soft CLs. Table 1. Classification of the change in standard deviation of biomechanical parameters post–pre (B-A) of the Biomechanical Comparison Display software version 1.6r2554. B-A Not Significant B-A Stiffer B-A Softer DA Ratio ±1.0 >−1.0 >+1.0 Int. Radius ±0.7 >−0.7 >+0.7 SP-A1 ±0.8 >−0.8 >+0.8 SSI ±0.4 >−0.4 >+0.4 B-A Not Significant B-A Thicker B-A Thinner ARTh ±0.3 >−0.3 >+0.3 DA Ratio = deflection amplitude ratio; Int. Radius = integrated radius; SP-A1 = stiffness parameter-A1; SSI = stress–strain index; ARTh = Ambrósio horizontal relational thickness. 3. Results In total, 22 eyes (6 right (27.3%) and 16 left (72.7%)) of 22 patients (mean age, 2 0.64 ±1.21 yea rs, range 19–24 years) were included. There were 7 males (31.8%) and 15 females (68.2%). Descriptive data for biomechanical measurements pre- and post-CL wear are presented in Table 2. Table 2. Descriptive analysis of biomechanical data, bIOP and CCT, pre- and post-contact lens wear. Variables Mean ±SD Mean ±SD DA Ratio Pre 4.15 ±0.32 SD_DA Ratio Pre −0.27 ±0.78 Post 4.02 ±0.29 Post −0.60 ±0.68 Int. Radius Pre 7.57 ±1.05 SD_Int. Ratius Pre −0.43 ±0.93 Post 7.38 ±0.83 Post −0.59 ±0.75 ARTh Pre 521.14 ±81.60 SD_ARTh Pre 0.33 ±0.66 Post 528.77 ±106.75 Post 0.29 ±0.85 SP-A1 Pre 112.77 ±15.50 SD_SP-A1 Pre −0.04 ±0.82 Post 108.82 ±12.69 Post 0.16 ±0.69 SSI Pre 0.97 ±0.14 SD_SSI Pre 0.20 ±0.58 Post 0.95 ±0.14 Post 0.17 ±0.66 bIOP Pre 16.14 ±2.67 CCT Pre 556.00 ±91.38 Post 15.61 ±1.85 Post 545.23 ±21.75 DA Ratio = deflection amplitude ratio; Int. Radius = integrated radius; ARTh = Ambrósio horizontal relational thickness; SP-A1 = stiffness parameter-A1; SSI = stress–strain index; bIOP = biomechanically corrected intraocular pressure; CCT = central corneal thickness; SD = standard deviation. Differences between the means of the DA Ratio and the SD DA Ratio pre- and post-CL wear (Figure 2) were found to be significant according to Student’s t-test for the related samples (pvalue = 0.002 in both cases). Similarly, significant differences were found between pre- and post-CL wear values in CCT according to the Wilcoxon test (pv alue = 0. 013), indicating corneal thinning. For all other biomechanical measures, no significant differences were observed before and after treatment. Table 3presents the summary of the changes, showing the 95% confidence intervals for the mean SD difference (pre–post) at normality assumptions and the classification value for these changes, according to which the patients are classified as shown in histograms in Figure 3. The biomechanical changes correlating significantly through Pearson’s r correlation coefficient were changes in the DA Ratio and ARTh (r = 0.442, p= 0.039), changes in the Int. Radius and SP-A1 (r = 0.475, p= 0. 025), and changes in SP-A1 and SSI (r = 0.434, p= 0.044), showing direct linear associations. Similarly, an inverse linear correlation was found between the spherical equivalent (SE) and axial Life 2023,13, 2313 8 of 14 length (AXL) (r = − 0.633, p= 0.002). No significant associations were found for any variable according to age, sex, or eye (right or left). A significant association was found according to the chi-square test between changes in bIOP and classification according to changes in the Int. Radius (pvalue = 0.047) and SSI (pvalue = 0.026) standard deviations. Life2023,13,23138of14   ARThPre521.14±81.60SD_ARThPre0.33±0.66 Post528.77±106.75Post0.29±0.85 SP‐A1Pre112.77±15.50SD_SP‐A1Pre−0.04±0.82 Post108.82±12.69Post0.16±0.69 SSIPre0.97±0.14SD_SSIPre0.20±0.58 Post0.95±0.14Post0.17±0.66 bIOPPre16.14±2.67CCTPre556.00±91.38 Post15.61±1.85Post545.23±21.75 DARatio=deflectionamplituderatio;Int.Radius=integratedradius;ARTh=Ambrósiohorizontal relationalthickness;SP‐A1=stiffnessparameter‐A1;SSI=stress‒strainindex;bIOP=biomechanicallycorrectedintraocularpressure;CCT=centralcornealthickness;SD=standarddeviation.  (a)(b) Figure2.(a)DifferencesbetweenthemeanoftheDARatioplotpre‐andpost‐contactlenswear;(b) DifferencesbetweenthemeanoftheSD_DARatioplotpre‐andpost‐contactlenswear. Table3.Summaryofchangesincornealbiomechanicalparameters,bIOPandCCT,beforeandafter contactlenswearandclassification. Variablesp‐ValorIC(μpre–μpost)ResultsClassification SD_DARatio 0.002(01361;0.5184)pre>post|post–pre|<1 SD_Int.Radius0.1NonormalityNosig.|post–pre|<0.7 SD_SP‐A10.129(−0.46355;0.06355)Nosig.|post–pre|<0.8 SD_SSI0.779(−0.20054;0.26418)Nosig.|post–pre|<0.4 SD_ARTh0.986NonormalityNosig.|post–pre|<0.3 b IOP0.135NonormalityNosig. CCT0.013NonormalityPre<post DARatio=deflectionamplituderatio;Int.Radius=integratedradius;SP‐A1=stiffnessparameter‐A1;SSI=stress‒strainindex;ARTh=Ambrósiohorizontalrelationalthickness;bIOP=biomechanicallycorrectedintraocularpressure;CCT=centralcornealthickness;SD=standarddeviation. Figure 2. ( a ) Differences between the mean of the DA Ratio plot pre- and post-contact lens wear; (b) Differences between the mean of the SD_DA Ratio plot pre- and post-contact lens wear. Table 3. Summary of changes in corneal biomechanical parameters, bIOP and CCT, before and after contact lens wear and classification. Variables p-Value IC (µpre–µpost) Results Classification SD_DA Ratio 0.002 (01361;0.5184) pre > post |post–pre| < 1 SD_Int. Radius 0.1 No normality No sig. |post–pre| < 0.7 SD_SP-A1 0.129 (−0.46355; 0.06355) No sig. |post–pre| < 0.8 SD_SSI 0.779 (−0.20054; 0.26418) No sig. |post–pre| < 0.4 SD_ARTh 0.986 No normality No sig. |post–pre| < 0.3 bIOP 0.135 No normality No sig. CCT 0.013 No normality Pre < post DA Ratio = deflection amplitude ratio; Int. Radius = integrated radius; SP-A1 = stiffness parameter-A1; S SI = s tr ess–strai n index; ARTh = Ambrósio horizontal relational thickness; bIOP = biomechanically corrected intraocular pressure; CCT = central corneal thickness; SD = standard deviation. Life2023,13,23139of14    (a)(b)  (c)(d)   (e) Figure3.Histogramsofclassificationaccordingtochangesindifferentbiomechanicalmeasures pre‐andpost‐contactlenswear:(a)SD_DARatio;(b)SD_Int_Ratius;(c)SD_SP_A1;(d)SD_ARTh; (e)SD_SSI. 4.Discussion Moststudiesanalysingchangesinbiomechanicalproperties,especiallyafterdifferentcornealrefractiveprocedures,havebeenperformedwiththeORA,asitwasthe firsttobeavailable.Itsclinicalintroductionwasextremelyimportantbecauseitwasthe firsttimethatthebiomechanicalresponseofthecorneatoaperturbationcouldbe measuredinvivousingapuffofairtodeformit.However,basicmisconceptionshave beenperpetuated,obscuringtheinterpretationoftheresults,includingthedesireto biomechanicallycharacterisethecorneawithasinglenumberthatmayanswerclinical questionsaboutcornealstiffnessorbasiccornealweakness[20]. Figure 3. Cont. Life 2023,13, 2313 9 of 14 Life2023,13,23139of14    (a)(b)  (c)(d)   (e) Figure3.Histogramsofclassificationaccordingtochangesindifferentbiomechanicalmeasures pre‐andpost‐contactlenswear:(a)SD_DARatio;(b)SD_Int_Ratius;(c)SD_SP_A1;(d)SD_ARTh; (e)SD_SSI. 4.Discussion Moststudiesanalysingchangesinbiomechanicalproperties,especiallyafterdifferentcornealrefractiveprocedures,havebeenperformedwiththeORA,asitwasthe firsttobeavailable.Itsclinicalintroductionwasextremelyimportantbecauseitwasthe firsttimethatthebiomechanicalresponseofthecorneatoaperturbationcouldbe measuredinvivousingapuffofairtodeformit.However,basicmisconceptionshave beenperpetuated,obscuringtheinterpretationoftheresults,includingthedesireto biomechanicallycharacterisethecorneawithasinglenumberthatmayanswerclinical questionsaboutcornealstiffnessorbasiccornealweakness[20]. Figure 3. Histograms of classification according to changes in different biomechanical measures pre- and post-contact lens wear: ( a ) SD_DA Ratio; ( b ) SD_Int_Ratius; ( c ) SD_SP_A1; ( d ) SD_ARTh; (e) SD_SSI. 4. Discussion Most studies analysing changes in biomechanical properties, especially after different corneal refractive procedures, have been performed with the ORA, as it was the first to be available. Its clinical introduction was extremely important because it was the first time that the biomechanical response of the cornea to a perturbation could be measured in vivo using a puff of air to deform it. However, basic misconceptions have been perpetuated, obscuring the interpretation of the results, including the desire to biomechanically characterise the cornea with a single number that may answer clinical questions about corneal stiffness or basic corneal weakness [20]. Similarly, when the CST became available, there was a new wave of studies, from the first in 2014, in which Hassan et al. compared the results of PRK and LASIK techniques [ 30 ], until 2017, when the same team discussed the effects of FEMTOLASIK and PRK [31]. Interest in corneal biomechanics was spreading as the instrument evolved, providing new parameters. Thus, authors such as Yang et al. decided to compare these new parameters in healthy eyes undergoing LASIK surgery, patients with post-LASIK ectasia, and patients with keratoconus [32]. Regarding CL wear, most studies are related to OK and carried out with the ORA. This is the case for Chen et al. [ 33 ], who determined an alteration in biomechanical properties such as a lower corneal resistance factor (CRF) as the duration of orthokeratology lens wear increased. On the other hand, Manuel González-Méijome et al. found a faster recovery effect in less resistant corneas, correlating corneal hysteresis (CH) with changes