Detection of Keratoconus With a New Biomechanical Index

Keratoconus Detection in High-Astigmatism Pediatric Patients: Optimal Pentacam Indices and Cutoff Points

PURPOSE

To assess the accuracy of various Pentacam indices in distinguishing keratoconus (KC) in pediatric patients with high astigmatism and to establish appropriate cutoff values.

METHODS

This prospective multicenter cross-sectional study included 312 eyes from 167 patients aged 6 to 18 years (mean age, 13.1 ± 3.2 years) evaluated with Oculus Pentacam HR. Patients were categorized into 4 groups: KC, forme fruste keratoconus, astigmatism greater than 2 diopters (Cyl2D), and control. A subgroup of Cyl2D comprised patients with astigmatism greater than 4 diopters (Cyl4D). Twenty-three Pentacam indices were analyzed, and receiver operating characteristic curves determined optimal cutoff points, sensitivity, and specificity.

RESULTS

The best indices for distinguishing KC from Cyl2D were high-order aberration root mean square of the anterior corneal surface (area under the receiver operating characteristic curve 0.987), Belin/Ambrosio enhanced ectasia total derivation (0.971), index of vertical asymmetry (0.971), average pachymetric progression index (0.962), maximum Ambrosio relational thickness (0.960), posterior elevation (0.952), and anterior elevation (0.948). The accuracy of these indices was highest in the control group and lowest in the Cyl4D group. Area under the receiver operating characteristic curve was significantly lower for fruste keratoconus than KC. Optimal cutoff values were higher for astigmatic patients than for those with no refractive error.

CONCLUSIONS

Most indices effectively distinguished between KC and normal pediatric patients. For individuals with high astigmatism, we suggest focusing on specific indices such as high-order aberration root mean square, Belin/Ambrosio enhanced ectasia total derivation value, maximum Ambrosio relational thickness, average pachymetric progression index, and index of vertical asymmetry. Optimal cutoff points for these patients were higher than those for nonastigmatic children and differed from adult populations.

Altered Corneal Biomechanics According to the Biomechanical E-Staging in Pellucid Marginal Degeneration

PURPOSE

The purpose of this study was to investigate corneal biomechanics in pellucid marginal degeneration (PMD) compared with healthy controls using Corvis ST (Oculus, Germany) by using the new biomechanical E-staging (based on the Corvis Biomechanical Factor, the linearized Corvis Biomechanical Index) together with tomographic parameters.

METHODS

Corneal biomechanical and topographic data of 75 eyes of 75 patients with PMD and 75 eyes of 75 age-matched and sex-matched healthy controls were investigated. Topographic parameters (K1, K2, Kmax, central corneal thickness (CCT), and Belin/Ambrósio Deviation Index (BAD-D) were evaluated in dependence of and correlated with the biomechanically defined E-stages. Biomechanical parameters were also recorded for the 2 groups.

RESULTS

Patients with PMD showed higher K2, Kmax, BAD-D, and Corvis Biomechanical Factor values and a lower CCT compared with healthy controls ( P < 0.001). The E-stage was positively correlated with K1, K2, Kmax, BAD-D, and intraocular pressure difference and negatively correlated with CCT. Stage-dependent analysis revealed a significant increase in K1, K2, Kmax ( P < 0.001), and BAD-D ( P = 0.041) in stage E3 compared with E0 and a significant decrease in stage E2 in CCT ( P = 0.009) compared with E0.

CONCLUSIONS

This study showed that patients with PMD may have a reduced corneal stiffness compared with healthy controls which worsens with increasing E-stage. Significant changes in topographic parameters were observed at stage E2 for CCT and at stage E3 for K1, K2, Kmax, and BAD-D when compared with stage E0.

Keratoconus

Keratoconus is a progressive eye disorder primarily affecting individuals in adolescence and early adulthood. The ectatic changes in the cornea cause thinning and cone-like steepening leading to irregular astigmatism and reduced vision. Keratoconus is a complex disorder with a multifaceted aetiology and pathogenesis, including genetic, environmental, biomechanical and cellular factors. Environmental factors, such as eye rubbing, UV light exposure and contact lens wearing, are associated with disease progression. On the cellular level, a complex interplay of hormonal changes, alterations in enzymatic activity that modify extracellular membrane stiffness, and changes in biochemical and biomechanical signalling pathways disrupt collagen cross-linking within the stroma, contributing to structural integrity loss and distortion of normal corneal anatomy. Clinically, keratoconus is diagnosed through clinical examination and corneal imaging. Advanced imaging platforms have improved the detection of keratoconus, facilitating early diagnosis and monitoring of disease progression. Treatment strategies for keratoconus are tailored to disease severity and progression. In early stages, vision correction with glasses or soft contact lenses may suffice. As the condition advances, rigid gas-permeable contact lenses or scleral lenses are prescribed. Corneal cross-linking has emerged as a pivotal treatment aimed at halting the progression of corneal ectasia. In patients with keratoconus with scarring or contact lens intolerance, surgical interventions are performed.

Pediatric keratoconus

Keratoconus is a common pediatric corneal disease, leading to vision impairment and amblyopia. Compared to its adult counterpart, pediatric keratoconus has an advanced presentation, rapid progression, higher incidence of complications such as corneal hydrops, and greater potential impact on the quality of life. It typically manifests during puberty and can evolve rapidly to more severe stages if left untreated. This rapid progression underscores the importance of early diagnosis through regular screening in pediatric populations and vigilant monitoring of pediatric keratoconus suspects. Concomitant ocular allergies, ocular anomalies, systemic diseases (e.g. syndromes), and poor compliance with contact lenses might impede prompt intervention and frequently postpone rehabilitation. Corneal collagen crosslinking is a crucial intervention in the management of pediatric keratoconus because it strengthens the corneal microstructure and halts the disease progression. When conservative measures fail, keratoplasty remains a viable option with generally favorable outcomes, though with unique challenges in post-operative care, including concerns related to sutures, long-term graft survival and need for repeated examinations under anesthesia. A multidisciplinary approach involving ophthalmologists, optometrists, pediatricians, and other healthcare professionals, focusing on early diagnosis and timely intervention, is essential for the comprehensive management of pediatric keratoconus and to mitigate its impact on children's lives.

Comparison of a Scheimpflug imaging with other screening indices in diagnosing keratoconus and keratoconus suspect

Keratoconus (KC) is an irreversible blinding eye disease; therefore, early screening of KC suspects (KCS) is crucial for protecting patients' quality of life. Scheimpflug imaging is a commonly used screening device in clinical practice. We aimed to evaluate the diagnostic ability of a Scheimpflug imaging device (Scansys) for KC and KCS and compared it with other Scheimpflug-based devices (Pentacam and Corvis ST). This prospective case-control study included 107 normal eyes, 72 KCS, and 57 KC. Scansys screening index Keratoconus probability (KCP) showed excellent performance in diagnosing KC at a cutoff value of 16.4 (area under the receiver operating characteristic [AUROC] = 1.000), with 100% sensitivity and 98.11% specificity. KCP had a better KCS diagnostic ability at a cutoff value of 8.9 (AUROC = 0.813) than Corvis biomechanical index (CBI, AUROC = 0.764), reaching 67.61% sensitivity and 85.85% specificity. Pentacam screening index Belin/Ambrósio enhanced ectasia display deviation (BAD-D) showed the best performance with 92.96% sensitivity and 89.62% specificity at a cutoff value of 1.525 (AUROC = 0.970) in diagnosing KCS. Scansys provides accurate KCP parameters in diagnosing KC; however, the efficiency of diagnosing KCS should be further optimized.

The Increase in Corneal Stiffness After Accelerated Corneal Cross-Linking in Progressive Keratoconus Using Different Methods of Epithelial Debridement

Purpose

The purpose of this study was to investigate corneal stiffening after epithelium-off accelerated corneal cross-linking (CXL; 9 mW/cm²) in progressive keratoconus (KC) with different methods of epithelial debridement.

Methods

This was a retrospective, interventional, and non-randomized study. In group 1, the epithelium was removed using a hockey knife (N = 45). In group 2 (N = 39) and group 3 (N = 22), the epithelial thickness was measured by optical coherence tomography (OCT) and the epithelium was ablated by excimer laser, but, in group 3, stromal ablation was performed additionally to correct high order aberrations (HOAs). Corneal biomechanics (integrated invers radius [IIR], stress-strain index [SSI]) and corneal tomography (thinnest corneal thickness [TCT]) were assessed with Corvis ST and Pentacam prior to and 1 month after CXL.

Results

Corneal tomography did not differ among the groups preoperatively (P > 0.05). TCT decreased significantly in all groups after surgery (all P < 0.05). Nonetheless, corneal biomechanical stiffening was found in all three groups indicated by a decreased IIR and an increased SSI (all P < 0.05). For group 3, the HOA improved significantly (P < 0.001). Among the groups, there were no significant differences in changes of biomechanical parameters, but TCT was significantly reduced after laser ablation.

Conclusions

Corneal stiffening after CXL is independent from epithelial removal. In particular, despite the removal of stromal tissue to correct HOA, a stiffening effect was achieved in keratoconic corneas, even it was less pronounced compared to mechanical epithelial removal. The reduction in HOA indicates a possible improvement in visual acuity.

Translation Relevance

Cross-linking stiffens the keratoconus independent of epithelial debridement technique and may compensate minor stromal laser ablation.

Evaluation of a New Diabetes Mellitus Index Based on Measurements Using the Scheimpflug Analyzer Corvis ST

PURPOSE

Chronic hyperglycemia causes changes in corneal biomechanics that can be measured with the Scheimpflug Analyzer Corvis ST. The diagnostic reliability of the new diabetes mellitus (DM) index developed based on this should be evaluated.

METHODS

In a prospective cross-sectional study, the index was initially developed using data from 81 patients with DM and 75 healthy subjects based on logistic regression analysis. The reliability of the DM index was subsequently assessed using data from another 61 patients and 37 healthy individuals. In addition, the dependence of the DM index on indicators of disease severity was analyzed.

RESULTS

The index initially achieved a sensitivity of 79% and specificity of 80% with a cutoff value of 0.58. The evaluation showed a sensitivity of 67% and specificity of 76% with an optimized cutoff of 0.51 (area under the curve = 0.737, P < 0.001). The DM index correlated weakly with the severity of diabetic retinopathy (r = 0.209, P = 0.014). It was increased in the presence of diabetic maculopathy (P = 0.037) and in type 1 DM compared with patients with type 2 disease (P = 0.039).

CONCLUSIONS

In this first evaluation, the new DM index achieved sufficiently good sensitivity and specificity and was weakly associated with disease-specific factors. With further improvements, it could complement the diagnostic options in DM with a simple, rapid, and noninvasive assessment method.

Age-related analysis of corneal biomechanical parameters in healthy Chinese individuals

To report the correlation between corneal biomechanical parameters and age in healthy Chinese individuals. This cross-sectional study was conducted on 864 eyes of 543 healthy participants. A comprehensive ophthalmic examination and corneal biomechanics examination using Corneal Visualization Scheimpflug Technology (Corvis ST) were conducted. Based on age, all participants were further divided into five age groups (n) as follows: group A, 11-20 years (105); group B, 21-30 years (112); group C, 31-40 years (113); group D, 41-50 years (100); and group E, > 50 years (113). Using Corvis ST, we examined 35 corneal biomechanical parameters and compared them across the different age groups. Spearman's correlation coefficients and stepwise multivariate linear regression models were used to investigate whether the corneal biomechanical parameters were related to demographic and ocular characteristics. A correlation analysis between the left and right eyes revealed that 13 parameters were significantly associated with eye differences. Among the 35 corneal biomechanical parameters, 28 exhibited significant differences across the age groups, with stiffness parameter at applanation 1(SPA1) showing an upward trend after the age of 30 and stress-strain index (SSI) demonstrates a statistically significant upward trend when comparing the five age groups in the study. Additionally, Spearman's correlation analysis and stepwise multivariate linear regression analysis revealed that 11 corneal biomechanical parameters were positively correlated with age and 10 were negatively correlated with age. Corvis biomechanical index (CBI) was significantly negatively correlated with intraocular pressure (IOP) and central corneal thickness (CCT), SSI was significantly positively correlated with age and IOP, and SPA1 were positively correlated with IOP and CCT. In conclusion, most corneal biomechanical parameters showed a significant correlation with age, with corneal stiffness progressively increasing alongside advancing age, IOP, or CCT.

Corneal biomechanical properties and potential influencing factors in varying degrees of myopia

To compare the corneal biomechanical parameters measured by Corvis ST in subjects with varying degrees of myopia. And the factors that may affect corneal biomechanical properties were also investigated. Participants in this prospective cross-sectional study were classified into three groups according to spherical equivalent (SE) and axial length (AL): Non-myopia (NM, SE > - 0.50 D and AL < 26 mm), Mild-to-moderate myopia (MM, - 6.00 D < SE ≤ - 0.50 D and AL < 26 mm), high myopia (HM, SE ≤ - 6.00 D or AL ≥ 26 mm). Ten corneal biomechanical parameters were finally included. Linear mixed-effects model accounting for using both eyes in the same participant was carried out to evaluate how the corneal biomechanical parameter was influenced by varying degrees of myopia after adjusting for biomechanically corrected intraocular pressure (bIOP) and central corneal thickness (CCT). Further, multiple linear regression was performed to explore the correlation between corneal biomechanical parameter and SE, AL, bIOP or CCT. A total of 304 eyes from 224 healthy myopic subjects were recorded. There were 95 eyes with NM, 122 eyes with MM, and 87 eyes with HM. After adjusting for bIOP and CCT, eyes with high myopia showed shorter highest concavity time (HC-time, p = 0.025), greater peak distance (PD, p = 0.001), greater deflection amplitude (DA-Max, p = 0.002), smaller whole eye movement (WEM-Max, p < 0.001) and reduced stiffness parameter (SP-A1, p < 0.001). Multiple regression analysis showed that five parameters (HC-time, p < 0.001; PD, p < 0.001; DA-Max, p = 0.001; WEM-Max, p < 0.001; and SP-A1, p < 0.001) of Corvis ST were significantly correlated with AL, and one parameter (Corvis biomechanical index, p = 0.016) has significant relationship with SE. With the increase of myopia, significant changes in several corneal biomechanical parameters indicated a progressive decrease in corneal stiffness, independent of bIOP and CCT. Corneal biomechanical parameters may be predictors of scleral mechanical strength in high myopia, which has certain application value in clinical management of myopia.

Scheimpflug Tomographic Indices for Classifying Normal, Down Syndrome and Clinical Keratoconus in Pediatric Patients

The study aimed to evaluate the precision of different Pentacam indices in diagnosing keratoconus (KC) in pediatric patients with and without Down Syndrome (DS) and determine suitable cutoff values. This prospective multicenter cross-sectional study evaluated 216 eyes of 131 patients aged 6-18 years (mean age 12.5 ± 3.2 years) using Pentacam. Patients were categorized into four groups: KC, forme fruste keratoconus (FK), DS, and control, excluding DS patients with topographic KC. Receiver operating characteristic curves were generated to determine the optimal cutoff points and compare the accuracy in identifying KC and FK in patients with and without DS. In DS patients, corneal morphology resembled KC features. The most effective indices for distinguishing KC in DS patients were the average pachymetric progression index (AUC = 0.961), higher-order aberration of the anterior cornea (AUC = 0.953), anterior elevation (AUC = 0.946), posterior elevation (AUC = 0.947), index of vertical asymmetry (AUC = 0.943), and Belin/Ambrosio enhanced ectasia total derivation value (AUC = 0.941). None of the indices showed good accuracy for distinguishing FK in DS patients. The thresholds of these indices differed significantly from non-DS patients. The results highlighted the need for DS-specific cutoff values to avoid false-positive or false-negative diagnoses in this population.

Refractive surgical approaches to keratoconus: A systematic review and network meta-analysis

Advancements in diagnostic methods and surgical techniques for keratoconus (KC) have increased non-invasive treatment options. Successful surgical planning for KC involves a combination of clinical science, empirical evidence, and surgical expertise. Assessment of disease progression is crucial, and halting the progression should be the focus if it is progressive. While surgeons used to rely on experience alone to decide the surgical method, comparing the network of primary factors, such as visual acuity, across studies can help them choose the most appropriate treatments for each patient and achieve optimal outcomes. Meticulous tabulation methods facilitate interpretation, highlighting the importance of selecting the correct surgical and rehabilitation approach based on each patient's condition and stage of the disease. We detail the outcomes of a comprehensive network meta-analysis comparing the effectiveness of various combined therapeutic refractive treatments for KC at identical stages of the disease, spanning 4 distinct follow-up intervals. Additionally, the comprehensive analysis suggests that for corneas with optimal best corrected visual acuity (BCVA) preoperatively (classified as regular), combining phakic intraocular lenses with intracorneal ring segments (ICRS) and corneal cross-linking (CXL) could offer the best therapeutic approach provided the disease stage does not exceed stage 3. For irregular corneas, although initial follow-ups show a significant difference in BCVA with surface ablation, longer-term follow-ups recommend combining surface ablation with ICRS and CXL, especially at higher stages.

Combined corneal biomechanical and tomographical indices in subclinical and forme fruste keratoconus

PURPOSE

Evaluation of combined corneal tomographic and biomechanical parameters in subclinical/forme fruste keratoconus (ScKC/FFKC).

DESIGN

Cross-sectional observational case-control study.

METHODS INCLUSION CRITERIA

Thirty-one eyes with ScKC (fellow eye of KC with any one sign: keratometry >47 diopters, cylinder >1.5 D, central corneal thickness <500 µm, with/without abnormal topography) or FFKC (fellow eye of KC with normal topography and slit lamp examination) >13 years (cases) and 44 eyes of age-matched 22 healthy subjects (controls).

EXCLUSION CRITERIA

Clinically diagnosed KC, presence of corneal scars, and prior ocular surgery eyes.

STUDY PARAMETERS

Sixteen Pentacam, 15 Corvis ST, and five Sirius parameters were analyzed using paired sample t -test, and a subsample found to be significantly different was used in receiver operating characteristic curve analysis. The Youden index was calculated, and Pearson's correlation analysis was done.

RESULTS

Five Pentacam, three Corvis ST, and two Sirius parameters had an area under curve (AUC) >0.75. Tomographic and biomechanical index (TBI) (cutoff 0.59, 95% specificity, 77% sensitivity), Belin Ambrosio enhanced ecstasia display (cutoff 1.8, 81% specificity, 80% sensitivity), and symmetry index of posterior corneal curvature (cutoff 0.16, 97% specificity, 67% sensitivity) best identified early KC. TBI strongly correlated with maximum Pentacam parameters in both cases and controls. Corvis biomechanical index strongly correlated only in cases, and SP-A1-SD weakly correlated in cases.

CONCLUSION

Upon combined analysis, the average sensitivity and specificity, respectively, of top three parameters (according to AUC) from Pentacam and Corvis ST were 74.1% and 95.4% for posterior elevation and TBI.

TRIAL REGISTRATION

The trial was registered in Clinical Trial Registry of India on January 28, 2022. The Trial Registration Number is REF/2022/01/050638.

Crosslinking with UV-A and riboflavin in progressive keratoconus: From laboratory to clinical practice - Developments over 25 years

Changes in the biomechanical and biochemical properties of the human cornea play an important role in the pathogenesis of ectatic diseases. A number of conditions in primarily acquired (keratoconus or pellucid marginal degeneration) or secondarily induced (iatrogenic keratectasia after refractive laser surgeries) ectatic disorders lead to decreased biomechanical stability. Corneal collagen cross-linking (CXL) represents a technique to slow or even halt the progression of ectatic pathologies. In this procedure, riboflavin is applied in combination with ultraviolet A radiation. This interaction induces the production of reactive oxygen species, which leads to the formation of additional covalent bonds between collagen molecules and subsequent biomechanical corneal strengthening. This procedure is so far the only method that partially interferes etiopathogenetically in the treatment of ectatic diseases that slows or stops the process of corneal destabilization, otherwise leading to the need for corneal transplantation. Besides, CXL process increases markedly resistance of collagenous matrix against digesting enzymes supporting its use in the treatment of corneal ulcers. Since the discovery of this therapeutic procedure and the first laboratory experiments, which confirmed the validity of this method, and the first clinical studies that proved the effectiveness and safety of the technique, it has been spread and adopted worldwide, even with further modifications. Making use of the Bunsen-Roscoe photochemical law it was possible to shorten the duration of this procedure in accelerated CXL and thus improve the clinical workflow and patient compliance while maintaining the efficacy and safety of the procedure. The indication spectrum of CXL can be further expanded by combining it with other vision-enhancing procedures such as individualized topographically-guided excimer ablation. Complementing both techniques will allow a patient with a biomechanically stable cornea to regularize it and improve visual acuity without the need for tissue transplantation, leading to a long-term improvement in quality of life.

The KERATO Biomechanics Study 1: A Comparative Evaluation Using Brillouin Microscopy and Dynamic Scheimpflug Imaging

PURPOSE

To assess the corneal biomechanical properties in normal individuals and patients with keratoconus using the Brillouin optical scanning system (Intelon Optics) (BOSS) and compare them with ultra-high-speed Scheimpflug imaging (Corvis ST; Oculus Optikgeräte GmbH).

METHODS

Sixty eyes from 60 patients (30 normal and 30 keratoconus) were included in this prospective, single-center, comparative, non-interventional study. Corneal biomechanics were evaluated using the Corvis ST and the BOSS. With the BOSS, each corneal image was acquired three times, measuring 10 locations within an 8-mm diameter. Parameters extracted included mean, maximum, and minimum Brillouin shift. These 10 points were also grouped into superior, central, and inferior regions. BOSS repeatability was assessed using the coefficient of repeatability and coefficient of variation. Furthermore, normal individuals and patients with keratoconus were compared using the Corvis ST and BOSS.

RESULTS

The BOSS exhibited good repeatability, with coefficient of repeatability ranging from 0.098 to 0.138 GHz for single points in normal individuals and 0.096 to 0.149 GHz for patients with keratoconus. Statistical analysis revealed significant differences between normal individuals and patients with keratoconus, indicating softer corneas in keratoconus, observed with both the Corvis ST and BOSS. Specifically, the BOSS showed significant differences in mean, inferior, and superior mean, maximum, and minimum Brillouin frequency shift (all P < .05), whereas the Corvis ST displayed highly significant differences in stiffness parameter at first applanation, stress strain index, deformation amplitude ratio, and inverse integrated radius (all P < .001).

CONCLUSIONS

Corneal biomechanical measurements proved highly repeatable and effectively demonstrated significant differences between normal individuals and patients with keratoconus using both the BOSS and the Corvis ST. [J Refract Surg. 2024;40(8):e569-e578.].

BCLA CLEAR Presbyopia: Management with corneal techniques

Corneal techniques for enhancing near and intermediate vision to correct presbyopia include surgical and contact lens treatment modalities. Broad approaches used independently or in combination include correcting one eye for distant and the other for near or intermediate vision, (termed monovision or mini-monovision depending on the degree of anisometropia) and/or extending the eye's depth of focus [1]. This report reviews the evidence for the treatment profile, safety, and efficacy of the current range of corneal techniques for managing presbyopia. The visual needs and expectations of the patient, their ocular characteristics, and prior history of surgery are critical considerations for patient selection and preoperative evaluation. Contraindications to refractive surgery include unstable refraction, corneal abnormalities, inadequate corneal thickness for the proposed ablation depth, ocular and systemic co-morbidities, uncontrolled mental health issues and unrealistic patient expectations. Laser refractive options for monovision include surface/stromal ablation techniques and keratorefractive lenticule extraction. Alteration of spherical aberration and multifocal ablation profiles are the primary means for increasing ocular depth of focus, using surface and non-surface laser refractive techniques. Corneal inlays use either small aperture optics to increase depth of field or modify the anterior corneal curvature to induce corneal multifocality. In presbyopia correction by conductive keratoplasty, radiofrequency energy is applied to the mid-peripheral corneal stroma, leading to mid-peripheral corneal shrinkage and central corneal steepening. Hyperopic orthokeratology lens fitting can induce spherical aberration and correct some level of presbyopia. Postoperative management, and consideration of potential complications, varies according to technique applied and the time to restore corneal stability, but a minimum of 3 months of follow-up is recommended after corneal refractive procedures. Ongoing follow-up is important in orthokeratology and longer-term follow-up may be required in the event of late complications following corneal inlay surgery.

Impacts and Correlations on Corneal Biomechanics, Corneal Optical Density and Intraocular Pressure after Cataract Surgery

The study aimed to investigate the extended effects and interrelations of corneal biomechanics, corneal optical density (COD), corneal thickness (CT), and intraocular pressure (IOP) following cataract surgery. Sixteen eyes were analyzed prospectively. The Corneal Visualization Scheimpflug Technology (Corvis ST) device assessed corneal biomechanics, while the Pentacam AxL® (Pentacam) measured COD and CT. Postoperative data were collected around six months after surgery, with a subgroup analysis of data at nine months. The Pearson correlation was used to examine the relationship between surgical-induced changes in corneal biomechanics and COD. At six months, significant postoperative differences were observed in various biomechanical indices, including uncorrected IOP (IOPuct) and biomechanics-corrected IOP (bIOP). However, many indices lost statistical significance by the nine-month mark, suggesting the reversibility of postoperative corneal changes. Postoperative COD increased at the anterior layer of the 2-6 mm annulus and incision site. The changes in COD correlated with certain biomechanical indices, including maximal (Max) deformative amplitude (DA) and stiffness parameter (SP). In conclusion, despite significant immediate postoperative changes, corneal biomechanics, COD, and IOP experienced a gradual recovery process following cataract surgery. Clinicians should maintain vigilance for any unusual changes during the short-term observation period to detect abnormalities early.

Systemic and Ocular Associations of Keratoconus

Introduction

Keratoconus (KC) is the most prevalent corneal ectasia in the world and its pathogenesis is influenced by both ocular and systemic factors. This review explores the multifaceted associations between keratoconus and systemic health conditions, ocular characteristics, and various other environmental/exogenous factors, aiming to illuminate how these relationships influence the pathophysiology of the disease.

Areas Covered

This review will summarize the fundamental attributes of KC, review and discuss the systemic and ocular association of KC including molecular biomarkers, and provide an organized overview of the parallel alterations occurring within various biological pathways in KC.

Expert Opinion

Despite the substantial volume of research on keratoconus, the precise etiology of the disease remains elusive. Further studies are necessary to deepen our understanding of this intricate disorder and improve its management.

Keratectasia severity staging and progression assessment based on the biomechanical E-staging

Until recently, corneal topography has been the gold standard in detecting keratectasia and monitoring its progression. The recently introduced ABCD tomographic keratoconus staging system focuses on anterior ("A") and posterior ("B") radius of curvature, thinnest corneal thickness ("C"), best-corrected visual acuity with spectacles ("D") and is supplemented with the introduction of the biomechanical E-staging (BEST, "E"). The need for biomechanical staging arose from the fact of altered biomechanical characteristics of keratectasia in comparison to healthy corneas. Ectatic corneas usually exhibit a biomechanical weakening and greater deformation than healthy corneas when exposed to a biomechanical stressor such as a standardized air puff indentation as provided by the Corvis ST® (CST, Oculus, Wetzlar, Germany). The BEST is based on the linear term of the Corvis Biomechanical Index (CBI) and provides a biomechanical keratoconus severity staging and progression assessment within the CST software. This review traces the development of the BEST as an addition to the tomographic ABCD staging system and highlights its strengths and limitations when applied in daily practice for the detection, monitoring and progression assessment in keratectasia.

The Utilization of Brillouin Microscopy in Corneal Diagnostics: A Systematic Review

Corneal biomechanical data has been used since 2005 to screen for keratoconus and corneal ectasia by corneal specialists. Older technology uses force applanation techniques over a 3 mm area in the central cornea, making it highly dependent on extraneous variables and unable to calculate the elasticity of the tissue. Brillouin microscopy is a newer method that uses a natural shift in the frequency of light as it passes through a material. This frequency shift can be used to estimate the viscoelasticity of the tissue. The advantage of Brillouin microscopy is that it can create a full three-dimensional (3D) map of the entire cornea without direct contact. A literature search was conducted using the databases PubMed, Google Scholar, and Ovid regarding the applications of Brillouin microscopy in corneal diagnostics. A final total of 16 articles was included describing the various ex vivo and in vivo studies conducted using Brillouin microscopy. Applications of this technology spanned from keratoconus diagnosis to post-corneal refractive surgery evaluation. All studies evaluated corneal biomechanics and other corneal properties through the quantification of Brillouin frequency shifts. Many of the studies found that this diagnostic device is capable of detecting subtle changes in corneal thickness and biomechanics in keratoconic corneas at a high level of specificity and sensitivity. However, limitations of Brillouin microscopy may include the duration of time required for use and fluctuations in accuracy depending on the corneal hydration state. Future technology seems to be geared toward a combination of optical coherence tomography (OCT) and Brillouin microscopy, using OCT as a three-dimensional pupil-tracking modality. Further research and understanding of the technology involved will lead to better care of patients in the field of ophthalmology.

Biomechanical changes following corneal crosslinking in keratoconus patients

PURPOSE

To evaluate the biomechanical and tomographic outcomes of keratoconus patients up to four years after corneal crosslinking (CXL).

METHODS

In this longitudinal retrospective-prospective single-center case series, the preoperative tomographic and biomechanical results from 200 keratoconus eyes of 161 patients undergoing CXL were compared to follow-up examinations at three-months, six-months, one-year, two-years, three-years, and four-years after CXL. Primary outcomes included the Corvis Biomechanical Factor (CBiF) and five biomechanical response parameters obtained from the Corvis ST. Tomographically, the Belin-Ambrósio deviation index (BAD-D) and the maximal keratometry (Kmax) measured by the Pentacam were analyzed. Additionally, Corvis E-staging, the thinnest corneal thickness (TCT), and the best-corrected visual acuity (BCVA) were obtained. Primary outcomes were compared using a paired t-test.

RESULTS

The CBiF decreased significantly at the six-month (p < 0.001) and one-year (p < 0.001) follow-ups when compared to preoperative values. E-staging behaved accordingly to the CBiF. Within the two- to four-year follow-ups, the biomechanical outcomes showed no significant differences when compared to preoperative. Tomographically, the BAD-D increased significantly during the first year after CXL with a maximum at six-months (p < 0.001), while Kmax decreased significantly (p < 0.001) and continuously up to four years after CXL. The TCT was lower at all postoperative follow-up visits compared to preoperative, and the BCVA improved.

CONCLUSION

In the first year after CXL, there was a temporary progression in both the biomechanical CBiF and E-staging, as well as in the tomographic analysis. CXL contributes to the stabilization of both the tomographic and biomechanical properties of the cornea up to four years postoperatively.

Implantation of Intracorneal Ring Segments in Keratectasia: Effects on Corneal Biomechanics in 112 Eyes

PURPOSE

The aim of this study was to analyze changes in corneal biomechanical properties after implantation of intracorneal ring segments (ICRSs) in keratectasia.

METHODS

This retrospective single-center study included 112 patient eyes that underwent femtosecond laser-assisted ICRS implantation (Intacs SK; Addition Technology Inc, Des Plaines, IL) for keratectasia. Biomechanical analysis was performed using the Ocular Response Analyzer (ORA; Reichert Inc, Depew, NY), with determination of corneal resistance factor, corneal hysteresis, and Keratoconus Match Index, as well as by Corvis ST (OCULUS, Wetzlar, Germany), with determination of stiffness parameter A1, Ambrosio relational thickness to the horizontal profile (Arth), integrated radius, deformation amplitude ratio, and stress-strain index as well as Corvis Biomechanical Index and Tomographic Biomechanical Index. Data collection was performed preoperatively and 6 months postoperatively for ORA and Corvis ST and additionally after 1 and 2 years for ORA.

RESULTS

The corneal resistance factor decreased significantly postoperatively (5.8 ± 1.7 mm Hg) compared with preoperatively (6.75 ± 3.7 mm Hg; P = 0.021) and increased again during follow-up (6.2 ± 1.9 mm Hg; P = 0.024), without regaining preoperative values. Corneal hysteresis and Keratoconus Match Index did not change significantly. Stiffness parameter A1 ( P = 0.045) increased significantly after ICRS implantation and Arth decreased significantly from 181 ± 85 to 150 ± 92 ( P = 0.016). However, there was no significant postoperative change for others Corvis parameters.

CONCLUSIONS

Corneal biomechanical properties showed inconsistent changes after ICRS implantation. Classical corneal biomechanical parameters (using single central air-puff tonometers) do not seem to be suitable for follow-up after ICRS implantation.

Correlation between Corneal Volume and Corneal Biomechanics and Corneal Volume Significance in Staging and Diagnosing Keratoconus

Purpose

To investigate the relationship between corneal volume (CV) at different zones and corneal biomechanics in keratoconus (KC) along with the significance of CV in diagnosing and staging KC.

Methods

This prospective clinical study included 456 keratoconic eyes (Group B) and 198 normal eyes (Group A). Using the topographic KC classification method, Group B was divided into subgroups based on severity (mild, moderate, and severe). The CVs of the 3 mm, 5 mm, and 7 mm zones and biomechanical parameters were obtained by Pentacam and Corvis ST. The diagnostic utility of multirange CVs at different disease stages and severity was determined using a receiver operating characteristic (ROC) curve analysis.

Results

The CV of the 7-mm zone had the strongest correlation with A1V, A2T, PD, DA ratio max (2 mm), DA ratio max (1 mm), ARTh, integrated radius, SPA1, and CBI (p < 0.01). The CVs of the Group B subgroups were significantly lower than those of Group A for each diameter range (p < 0.05). There were significant differences between the severe, mild, and moderate subgroups for the 3 mm zone (p < 0.05, all). The 3 mm zone CV exhibited better diagnostic ability in each group for distinguishing KC from the normal cornea (Groups A vs. B: area under the ROC curve (AUC) = 0.926, Groups A vs. B1: AUC = 0.894, Groups A vs. B2: AUC = 0.925, Groups A vs. B3: AUC = 0.953).

Conclusion

The CV significantly decreased in keratoconic eyes. Progressive thinning in the 3 mm zone may be a valuable measurement for detecting and staging KC. Combining the CV examination with corneal biomechanical information may effectively enhance the ability to detect KC.

Descemet's membrane endothelial keratoplasty is the predominant keratoplasty procedure in Germany since 2016: a report of the DOG-section cornea and its keratoplasty registry

BACKGROUND/AIMS

This retrospective multicentric panel study provides absolute numbers, types of and indications for corneal transplantation in Germany from 2011 to 2021 and sets them into the international context.

METHODS

A questionnaire was sent to the 104 German ophthalmologic surgery departments and 93 (89%) provided their data.

RESULTS

The number of reported keratoplasties more than doubled from 2011 (n=4474) to 2021 (n=8998). Lamellar keratoplasties (49% posterior (n=2883), 4% anterior (n=231)) surpassed penetrating keratoplasty (PKP, 47%, n=2721) for the first time in 2014. Since 2016, Descemet's membrane endothelial keratoplasty (DMEK) has become the predominant keratoplasty procedure in Germany. Its number increased by 1.5-fold from 3850 (2016) to 5812 (2021). Main indications in 2021 were Fuchs' endothelial corneal dystrophy (FECD, 43%), pseudophakic corneal decompensation (12%), repeated keratoplasty (11%), infections (7%), keratoconus (6%) and corneal scarring (4%, others: 9%). The PKP percentage decreased from 70.2% in 2011 (n=3141) to 31.7% in 2021 (n=2853). Descemet's stripping (automated) endothelial keratoplasties (DSAEKs) decreased to 1% in 2021 (n=74). 98.6% of all posterior lamellar keratoplasties were DMEKs in Germany in 2021. The number of deep anterior lamellar keratoplasties (DALKs) remained comparable from 2011 (n=269) to 2021 (n=251, 2.8%).

CONCLUSION

Main indications for corneal transplantation in Germany (2021) were FECD and pseudophakic corneal decompensation. DMEK is by far the predominant corneal transplantation procedure since 2016 followed by PKP, whose absolute number decreased only slightly during the decade from 2011 to 2021. DALK proportions remain low, but stable, whereas DSAEK decreased annually and plays a minor role in Germany.

TRIAL REGISTRATION NUMBER

NCT03381794.

Multi-modal imaging for the detection of early keratoconus: a narrative review

Keratoconus is a common progressive corneal disorder that can be associated with significant ocular morbidity. Various corneal imaging techniques have been used for the diagnosis of established cases. However, in the early stages of the disease, which include subclinical keratoconus and forme fruste keratoconus, detection of such cases can be challenging. The importance of detecting such cases is very important because early intervention can halt disease progression, improve visual outcomes and prevent postrefractive surgery ectasia associated with performing corneal refractive procedures in such patients. This narrative review aimed to examine several established and evolving imaging techniques for the detection of early cases of keratoconus. The utilization of combinations of these techniques may further increase their diagnostic ability.

Evaluation of dynamic corneal response parameters and the biomechanical E-staging after Intacs® SK implantation in keratoconus

PURPOSE

This retrospective longitudinal study evaluated the biomechanical E-staging in KC corneas before and after intracorneal ring segment (ICRS) implantation (Intacs® SK, Addition Technology, Illinois, United States).

METHODS

Biomechanical E-staging for ectatic corneal diseases was applied retrospectively on 49 KC corneas of 41 patients who underwent ICRS implantation. The main outcome parameters included the Corvis Biomechanical Factor (CBiF, the linearized Corvis Biomechanical Index and the biomechanical parameters included), the resulting biomechanical E-staging, the stress-strain index, thinnest corneal thickness (TCT), maximal anterior keratometry (Kmax), and the anterior radius of curvature (ARC). They were evaluated at 1.9 ± 1.1 months preoperatively and postoperatively after 2.8 ± 0.7, 5.8 ± 1.0, and 10.6 ± 2.3 months.

RESULTS

The CBiF decreased (4.9 ± 0.5 | 4.7 ± 0.5, P = 0.0013), and the E-staging increased significantly (2.8 ± 0.8 | 3.1 ± 0.9, P = 0.0012, paired t-test) from preoperatively to the first postoperative follow-up. The difference remained significant after 6 months; however, there was no more difference after 11 months. TCT was stable, whereas Kmax and ARC significantly decreased after ICRS implantation (TCT: 464 ± 49, 470 ± 51, 467 ± 38, 461 ± 48; Kmax: 56.3 ± 4.5, 54.7 ± 4.5, 54.2 ± 4.8, 54.1 ± 4.3; ARC: 51.5 ± 3.4, 48.3 ± 3.8, 48.6 ± 3.0, 48.6 ± 3.2 preoperatively and 3, 6, and 11 months postoperatively, respectively). Besides Kmax and ARC, Ambrósio's relational thickness to the horizontal profile (ARTh) was the only parameter that was significantly lower than preoperatively at any follow-up (P ≤ 0.0024, Wilcoxon matched-pairs test).

CONCLUSION

Intacs® SK implantation results in an increasing biomechanical E-staging in the first postoperative months with stabilization near preoperative values after 1 year. Significantly lower ARTh values at any follow-up document the ICRS effect and contribute to a slightly higher postoperative biomechanical E-staging value.

Establishing Standardization Guidelines For Finite-Element Optomechanical Simulations of Refractive Laser Surgeries: An Application to Photorefractive Keratectomy

Purpose

Computational models can help clinicians plan surgeries by accounting for factors such as mechanical imbalances or testing different surgical techniques beforehand. Different levels of modeling complexity are found in the literature, and it is still not clear what aspects should be included to obtain accurate results in finite-element (FE) corneal models. This work presents a methodology to narrow down minimal requirements of modeling features to report clinical data for a refractive intervention such as PRK.

Methods

A pipeline to create FE models of a refractive surgery is presented: It tests different geometries, boundary conditions, loading, and mesh size on the optomechanical simulation output. The mechanical model for the corneal tissue accounts for the collagen fiber distribution in human corneas. Both mechanical and optical outcome are analyzed for the different models. Finally, the methodology is applied to five patient-specific models to ensure accuracy.

Results

To simulate the postsurgical corneal optomechanics, our results suggest that the most precise outcome is obtained with patient-specific models with a 100 µm mesh size, sliding boundary condition at the limbus, and intraocular pressure enforced as a distributed load.

Conclusions

A methodology for laser surgery simulation has been developed that is able to reproduce the optical target of the laser intervention while also analyzing the mechanical outcome.

Translational Relevance

The lack of standardization in modeling refractive interventions leads to different simulation strategies, making difficult to compare them against other publications. This work establishes the standardization guidelines to be followed when performing optomechanical simulations of refractive interventions.

Corneal Morphological and Biomechanical Changes in Thyroid-Associated Ophthalmopathy

PURPOSE

This study aimed to evaluate corneal morphological and biomechanical changes in patients with thyroid-associated ophthalmopathy (TAO) and their correlations with activity and severity.

METHODS

Patients diagnosed with TAO were recruited and divided into groups by activity and severity. All subjects underwent a complete ophthalmic examination, including magnetic resonance imaging. Corneal topography was measured using a Pentacam device, and biomechanical parameters were obtained using a CorVis ST tonometer. Correlations among the corneal parameters, clinical activity score, and NOSPECS score were analyzed. Areas under the receiver operating characteristic curves were calculated to evaluate the diagnostic accuracy of corneal changes for active and severe TAO.

RESULTS

Fifty-three eyes with TAO and 16 healthy eyes were enrolled in our study. The back elevation, CorVis biomechanical index, tomographic and biomechanical index, stiffness parameter at the first applanation, deviation from normality in back elevation, relational thickness, and overall deviation from normality were significantly increased in patients with TAO (all P <0.05), whereas the smallest corneal thickness, maximum Ambrósio relational thickness, and deformation amplitude (DA) ratio were significantly decreased (all P <0.05). The clinical activity score was strongly positively correlated with back elevation (γ = 0.515, P <0.001). The NOSPECS score was strongly positively correlated with relational thickness and tomographic and biomechanical index (γ = 0.429 and 0.515, P <0.001) and negatively correlated with maximum Ambrósio relational thickness (γ = -0.53, P <0.001). Moreover, maximum Ambrósio relational thickness and the Ambrósio relational thickness through the horizontal meridian showed desirable diagnostic capacity in distinguishing mild TAO from moderate-severe TAO (areas under the receiver operating characteristic curve, 0.799 and 0.769).

CONCLUSIONS

Corneal morphological and biomechanical changes were found in patients with TAO, which might be related to the presence of inflammation. Measurements of corneal morphological and biomechanical parameters could serve as references in evaluating TAO.

Effect of Intraorbital Mechanical Compression on Retinal Microvascular Perfusion in Quiescent Thyroid-Associated Ophthalmopathy Based on Ocular Biomechanics Measured by Corvis ST

INTRODUCTION

To analyze the correlation between orbital compliance and retinal vessel density (VD) based on dynamic Scheimpflug analyzer (Corvis ST) and optical coherence tomographic angiography (OCT-A).

METHODS

In this prospective observational study, 65 eyes of 44 patients with thyroid-associated ophthalmopathy (TAO) in quiescent stage were included (15 males and 29 females). The whole eye movement (WEM) was detected by Corvis ST. The superficial capillary plexus VD (SCP-VD) and deep capillary plexus VD (DCP-VD) were obtained by scanning the 3 × 3 mm area around the fovea using OCT-A, while the peripapillary vessel density (ppVD) was obtained by scanning the 4.5 × 4.5 mm area around the optic disk. Covariances including biomechanically corrected intraocular pressure (bIOP), axial length, age and gender were adjusted during data analysis.

RESULTS

The mean WEM of the participants was 0.235 ± 0.066 mm. The mean SCP-VD and DCP-VD in whole image were 46.20% ± 3.77% and 50.51% ± 3.96%; the mean whole pp-VD was 49.75% ± 2.01%. WEM was positively correlated with SCP-VD (r = 0.327, p = 0.01) and the whole pp-VD (r = 0.394, p < 0.01) after adjusting by gender, axial length (AL), age and bIOP, but it was not significantly correlated with DCP-VD (r = 0.072 p = 0.581).

CONCLUSION

Increase in orbital pressure might reduce retinal microvascular perfusion. Our data suggest orbital mechanical compression may be an important cause of retinal VD changes in quiescent patients with TAO.

Diagnosis of Forme Fruste Keratoconus Using Corvis ST Sequences with Digital Image Correlation and Machine Learning

PURPOSE

This study aimed to employ the incremental digital image correlation (DIC) method to obtain displacement and strain field data of the cornea from Corvis ST (CVS) sequences and access the performance of embedding these biomechanical data with machine learning models to distinguish forme fruste keratoconus (FFKC) from normal corneas.

METHODS

100 subjects were categorized into normal (N = 50) and FFKC (N = 50) groups. Image sequences depicting the horizontal cross-section of the human cornea under air puff were captured using the Corvis ST tonometer. The high-speed evolution of full-field corneal displacement, strain, velocity, and strain rate was reconstructed utilizing the incremental DIC approach. Maximum (max-) and average (ave-) values of full-field displacement V, shear strain γxy, velocity VR, and shear strain rate γxyR were determined over time, generating eight evolution curves denoting max-V, max-γxy, max-VR, max-γxyR, ave-V, ave-γxy, ave-VR, and ave-γxyR, respectively. These evolution data were inputted into two machine learning (ML) models, specifically Naïve Bayes (NB) and Random Forest (RF) models, which were subsequently employed to construct a voting classifier. The performance of the models in diagnosing FFKC from normal corneas was compared to existing CVS parameters.

RESULTS

The Normal group and the FFKC group each included 50 eyes. The FFKC group did not differ from healthy controls for age (p = 0.26) and gender (p = 0.36) at baseline, but they had significantly lower bIOP (p < 0.001) and thinner central cornea thickness (CCT) (p < 0.001). The results demonstrated that the proposed voting ensemble model yielded the highest performance with an AUC of 1.00, followed by the RF model with an AUC of 0.99. Radius and A2 Time emerged as the best-performing CVS parameters with AUC values of 0.948 and 0.938, respectively. Nonetheless, no existing Corvis ST parameters outperformed the ML models. A progressive enhancement in performance of the ML models was observed with incremental time points during the corneal deformation.

CONCLUSION

This study represents the first instance where displacement and strain data following incremental DIC analysis of Corvis ST images were integrated with machine learning models to effectively differentiate FFKC corneas from normal ones, achieving superior accuracy compared to existing CVS parameters. Considering biomechanical responses of the inner cornea and their temporal pattern changes may significantly improve the early detection of keratoconus.

Longitudinal Analysis of Corneal Biomechanics of Suspect Keratoconus: A Prospective Case-Control Study

BACKGROUND

To evaluate the corneal biomechanics of stable keratoconus suspects (Stable-KCS) at 1-year follow-up and compare them with those of subclinical keratoconus (SKC).

METHODS

This prospective case-control study included the eyes of 144 patients. Biomechanical and tomographic parameters were recorded (Corvis ST and Pentacam). Patients without clinical signs of keratoconus in both eyes but suspicious tomography findings were included in the Stable-KCS group (n = 72). Longitudinal follow-up was used to evaluate Stable-KCS changes. Unilateral keratoconus contralateral eyes with suspicious tomography were included in the SKC group (n = 72). T-tests and non-parametric tests were used for comparison. Multivariate general linear models were used to adjust for confounding factors for further analysis. Receiver operating characteristic (ROC) curves were used to analyze the distinguishability.

RESULTS

The biomechanical and tomographic parameters of Stable-KCS showed no progression during the follow-up time (13.19 ± 2.41 months, p > 0.05). Fifteen biomechanical parameters and the Stress-Strain Index (SSI) differed between the two groups (p < 0.016). The A1 dArc length showed the strongest distinguishing ability (area under the ROC = 0.888) between Stable-KCS and SKC, with 90.28% sensitivity and 77.78% specificity at the cut-off value of -0.0175.

CONCLUSIONS

The A1 dArc length could distinguish between Stable-KCS and SKC, indicating the need to focus on changes in the A1 dArc length for keratoconus suspects during the follow-up period. Although both have abnormalities on tomography, the corneal biomechanics and SSI of Stable-KCS were stronger than those of SKC, which may explain the lack of progression of Stable-KCS.

Biometric and corneal characteristics in marfan syndrome with ectopia lentis

PURPOSE

To describe the biometric and corneal characteristics of patients with Marfan Syndrome (MFS) and ectopia lentis.

STUDY DESIGN

Observational, descriptive, prospective study. Subjects Individuals with MFS with ectopia lentis (EL).

METHODS

Fourty-four eyes of 23 patients underwent Scheimpflug analysis using the Pentacam (Oculus, Wetzlar, Germany), axial length (AL) using the IOL master 700 (Carl Zeiss AG, Oberkochen, Germany), endothelial cell count (ECC) using the CEM-350 (NIDEK, Maihama, Japan) and corneal biomechanics evaluation with the Ocular Response Analyzer: ORA (Reichert Ophthalmic Instruments, Buffalo, New York, USA) and Corvis (Oculus, Wetzlar, Germany). Statistical analysis was performed using IBM SPSS Statistics 25.0.

RESULTS

The direction of lens subluxation was most frequently supero-nasal 40.9% (18/44). Mean keratometry (Km) was 40.22±1.76 Diopters (D); mean corneal astigmatism was 1.68±0.83 D; total corneal aberrometric root mean square (RMS) was 2.237±0.795μm; higher-order aberrations (HOAs) RMS were 0.576±0.272μm; mean AL was 25.63±3.65mm; mean ECC was 3315±459cell/mm2; mean CBI was 0.13±0.24, mean TBI was 0.31±0.25, mean posterior elevation was 4.3±4.5μm; mean total corneal densitometry was 16.0±2.14 grayscale units (GSU).

CONCLUSION

Increased axial length, flatter and thicker corneas with higher regular astigmatism, normal densitometry, normal corneal biomechanical indices and normal posterior elevation were observed in Marfan patients with EL.

CorNet: Autonomous feature learning in raw Corvis ST data for keratoconus diagnosis via residual CNN approach

PURPOSE

To ascertain whether the integration of raw Corvis ST data with an end-to-end CNN can enhance the diagnosis of keratoconus (KC).

METHOD

The Corvis ST is a non-contact device for in vivo measurement of corneal biomechanics. The CorNet was trained and validated on a dataset consisting of 1786 Corvis ST raw data from 1112 normal eyes and 674 KC eyes. Each raw data consists of the anterior and posterior corneal surface elevation during air-puff induced dynamic deformation. The architecture of CorNet utilizes four ResNet-inspired convolutional structures that employ 1 × 1 convolution in identity mapping. Gradient-weighted Class Activation Mapping (Grad-CAM) was adopted to visualize the attention allocation to diagnostic areas. Discriminative performance was assessed using metrics including the AUC of ROC curve, sensitivity, specificity, precision, accuracy, and F1 score.

RESULTS

CorNet demonstrated outstanding performance in distinguishing KC from normal eyes, achieving an AUC of 0.971 (sensitivity: 92.49%, specificity: 91.54%) in the validation set, outperforming the best existing Corvis ST parameters, namely the Corvis Biomechanical Index (CBI) with an AUC of 0.947, and its updated version for Chinese populations (cCBI) with an AUC of 0.963. Though the ROC curve analysis showed no significant difference between CorNet and cCBI (p = 0.295), it indicated a notable difference between CorNet and CBI (p = 0.011). The Grad-CAM visualizations highlighted the significance of corneal deformation data during the loading phase rather than the unloading phase for KC diagnosis.

CONCLUSION

This study proposed an end-to-end CNN approach utilizing raw biomechanical data by Corvis ST for KC detection, showing effectiveness comparable to or surpassing existing parameters provided by Corvis ST. The CorNet, autonomously learning comprehensive temporal and spatial features, demonstrated a promising performance for advancing KC diagnosis in ophthalmology.

Improving early detection of keratoconus by Non Contact Tonometry. A computational study and new biomarkers proposal

Keratoconus is a progressive ocular disorder affecting the corneal tissue, leading to irregular astigmatism and decreased visual acuity. The architectural organization of corneal tissue is altered in keratoconus, however, data from ex vivo testing of biomechanical properties of keratoconic corneas are limited and it is unclear how their results relate to true mechanical properties in vivo. This study explores the mechanical properties of keratoconic corneas through numerical simulations of non-contact tonometry (NCT) reproducing the clinical test of the Corvis ST device. Three sensitivity analyses were conducted to assess the impact of corneal material properties, size, and location of the pathological area on NCT results. Additionally, novel asymmetry-based indices were proposed to better characterize corneal deformations and improve the diagnosis of keratoconus. Our results show that the weakening of corneal material properties leads to increased deformation amplitude and altered biomechanical response. Furthermore, asymmetry indices offer valuable information for locating the pathological tissue. These findings suggest that adjusting the Corvis ST operation, such as a camera rotation, could enhance keratoconus detection and provide insights into the relative position of the affected area. Future research could explore the application of these indices in detecting early-stage keratoconus and assessing the fellow eye's risk for developing the pathology.

Intraocular lens power calculation: angle κ and ocular biomechanics

PURPOSE

To study the effect of ocular biomechanics on the prediction error of intraocular lens (IOL) power calculation.

SETTING

Centro Hospitalar Universitário do Porto, Porto, Portugal.

DESIGN

Prospective longitudinal study.

METHODS

This study included 67 subjects. Before cataract surgery subjects underwent biometry with IOLMaster 700 and biomechanical analysis with Corvis Scheimpflug technology. The targeted spherical equivalent was calculated with SRK-T and Barrett Universal II. Associations between prediction error (PE), absolute prediction error (AE), and biometric and biomechanical parameters were performed with stepwise multivariate linear correlation analysis.

RESULTS

Using the SRKT formula, there was association between PE and Corvis Biomechanical Index (CBI, B = -0.531, P = .011) and between AE and the horizontal offset between the center of the pupil and the visual axis (angle κ, B = -0.274, P = .007). Considering the Barret Universal II formula, PE was independently associated with anterior chamber depth ( B = -0.279, P = .021) and CBI ( B = -0.520, P = .013) and AE was associated with angle κ ( B = -0.370, P = .007).

CONCLUSIONS

A large angle κ may reduce the predictability of IOL power calculation. Ocular biomechanics likely influence the refractive outcomes after IOL implantation. This study showed that eyes with softer corneal biomechanics had more myopic PE. This may relate to anteriorization of the effective lens position. Dynamic measurements may be the way to progress into future formulas.

Comparison of different corneal imaging modalities using artificial intelligence for diagnosis of keratoconus: a systematic review and meta-analysis

PURPOSE

This review was designed to compare different corneal imaging modalities using artificial intelligence (AI) for the diagnosis of keratoconus (KCN), subclinical KCN (SKCN), and forme fruste KCN (FFKCN).

METHODS

A comprehensive systematic search was conducted in scientific databases, including Web of Science, PubMed, Scopus, and Google Scholar based on the PRISMA statement. Two independent reviewers assessed all potential publications on AI and KCN up to March 2022. The Critical Appraisal Skills Program (CASP) 11-item checklist was used to evaluate the validity of the studies. Eligible articles were categorized into three groups (KCN, SKCN, and FFKCN) and included in the meta-analysis. The pooled estimate of accuracy (PEA) was calculated for all selected articles.

RESULTS

The initial search yielded 575 relevant publications, of which 36 met the CASP quality criteria and were included in the analysis. Qualitative assessment showed that Scheimpflug and Placido combined with biomechanical and wavefront evaluations improved KCN detection (PEA, 99.2, and 99.0, respectively). The Scheimpflug system (92.25 PEA, 95% CI, 94.76-97.51) and a combination of Scheimpflug and Placido (96.44 PEA, 95% CI, 93.13-98.19) had the highest diagnostic accuracy for the detection of SKCN and FFKCN, respectively. The meta-analysis outcomes showed no significant difference between the CASP score and accuracy of the publications (all P > 0.05).

CONCLUSIONS

Simultaneous Scheimpflug and Placido corneal imaging methods provide high diagnostic accuracy for early detection of keratoconus. The use of AI models improves the discrimination of keratoconic eyes from normal corneas.

Corneal biomechanics in early diagnosis of keratoconus using artificial intelligence

Keratoconus is a blinding eye disease that affects activities of daily living; therefore, early diagnosis is crucial. Great efforts have been made toward an early diagnosis of keratoconus. Recent studies have shown that corneal biomechanics is associated with the occurrence and progression of keratoconus. Hence, detecting changes in corneal biomechanics may provide a novel strategy for early diagnosis. However, an early keratoconus diagnosis remains challenging due to the subtle and localized nature of its lesions. Artificial intelligence has been used to help address this problem. Herein, we reviewed the literature regarding three aspects of keratoconus (keratoconus, early keratoconus, and keratoconus grading) based on corneal biomechanical properties using artificial intelligence. Furthermore, we summarized the current research progress, limitations, and possible prospects.

Characterization of Corneal Biomechanics Using CORVIS ST Device in Different Grades of Myopia in a Sample of Middle Eastern Ethnicity

Purpose

To characterize corneal biomechanical properties using the CORVIS-ST device in myopic individuals.

Methods

This prospective cross-sectional study included patients with myopia. Our study included 154 eyes of 154 myopic patients aged between 18 and 40 years, with stable refraction for at least 2 years. A full ophthalmological examination and corneal tomography were performed using a Pentacam HR device. Corneal biomechanical parameters were assessed using the CORVIS-ST device in mild, moderate, severe, and extreme myopia groups.

Results

Statistically significant differences were observed in the DA ratio (p = 0.033), SP-A (p=0.009), CBI (p=0.041), SSI (p=0.000), and Peak distance (p = 0.032). In correlation with different Corvis ST biomechanical variables, SE was found to be correlated with DA ratio(r=-0.191, p=0.018), SP-A(r=0.199, p=0.013) and SSI(r=-0.336, p=0.000), while in multiple regression analysis, SE was found to be independently correlated with SSI and peak distance(p=0.036,0.038 respectively) while the grade of myopia was found to be independently correlated with SP-A(p=0.034).

Conclusion

SSI, Peak distance, and SP-A were independently related to SE and myopia grade, confirming the hypothesis that eyes with higher myopia are more deformable and less stress resistant.

Corneal biomechanics and diagnostics: a review

PURPOSE

Corneal biomechanics is an emerging field and the interest into physical and biological interrelations in the anterior part of the eye has significantly increased during the past years. There are many factors that determine corneal biomechanics such as hormonal fluctuations, hydration and environmental factors. Other factors that can affect the corneas are the age, the intraocular pressure and the central corneal thickness. The purpose of this review is to evaluate the factors affecting corneal biomechanics and the recent advancements in non-destructive, in vivo measurement techniques for early detection and improved management of corneal diseases.

METHODS

Until recently, corneal biomechanics could not be directly assessed in humans and were instead inferred from geometrical cornea analysis and ex vivo biomechanical testing. The current research has made strides in studying and creating non-destructive and contactless techniques to measure the biomechanical properties of the cornea in vivo.

RESULTS

Research has indicated that altered corneal biomechanics contribute to diseases such as keratoconus and glaucoma. The identification of pathological corneas through the new measurement techniques is imperative for preventing postoperative complications.

CONCLUSIONS

Identification of pathological corneas is crucial for the prevention of postoperative complications. Therefore, a better understanding of corneal biomechanics will lead to earlier diagnosis of ectatic disorders, improve current refractive surgeries and allow for a better postoperative treatment.

Corneal Biomechanical Properties to Predict Prognosis of Abnormal Tomographic Corneas: A Prospective Cohort Study

PURPOSE

To analyze the corneal biomechanical properties in patients with abnormal corneal tomography (ACT) and predict their stability using the biomechanical stability index (BSI).

DESIGN

Prospective cohort study.

METHODS

Setting: Multicenter study.

STUDY POPULATION

This study included 385 eyes of 278 patients with stable ACT (n = 70), subclinical keratoconus (SKC, n = 65), keratoconus (n = 65), normal controls (NL, n = 142). Forty-three eyes with first-visit ACT were included in a separate cohort (follow-up ACT group).

OBSERVATION PROCEDURE

Tomographical and biomechanical parameters (Pentacam and Corvis ST) were recorded.

MAIN OUTCOME MEASURES

Nonparametric tests were used for comparison. Logistic regression was employed to introduce BSI to separate stable ACT and SKC accurately. An independent dataset of 43 first-visit ACT eyes was followed up for 1 year to validate BSI's accuracy and positive and negative predictive values (PPV, NPV).

RESULTS

The tomographical and biomechanical parameters in patients with Stable ACT remained stable over the follow-up period (12.73 ± 2.57 months, P > .05). Stable ACT had 12/14 biomechanical parameters different (P < .05) from SKC but not different from NL (P > .05). With a cut-off value of 0.585, BSI demonstrated the strongest ability to distinguish between stable ACT and SKC (area under the receiver operating characteristic curve = 0.991), with 93.85% sensitivity and 97.14% specificity. During the 1-year follow-up of 43 eyes (follow-up ACT group), 30 remained stable. The accuracy, PPV, and NPV of the BSI were 95.35%, 100%, and 93.75%, respectively.

CONCLUSIONS

Biomechanical properties of patients with stable abnormal tomography corneas were stronger than SKC and close to normal corneas, which may explain the reason for tomographic stability. The BSI may be useful for predicting disease progression in patients with ACT and the possible management of corneal cross-linking at the first visit.

Keratoconus Detection-based on Dynamic Corneal Deformation Videos Using Deep Learning

Objective

To assess the performance of convolutional neural networks (CNNs) for automated detection of keratoconus (KC) in standalone Scheimpflug-based dynamic corneal deformation videos.

Design

Retrospective cohort study.

Participants

We retrospectively analyzed datasets with records of 734 nonconsecutive, refractive surgery candidates, and patients with unilateral or bilateral KC.

Methods

We first developed a video preprocessing pipeline to translate dynamic corneal deformation videos into 3-dimensional pseudoimage representations and then trained a CNN to directly identify KC from pseudoimages. We calculated the model's KC probability score cut-off and evaluated the performance by subjective and objective accuracy metrics using 2 independent datasets.

Main Outcome Measures

Area under the receiver operating characteristics curve (AUC), accuracy, specificity, sensitivity, and KC probability score.

Results

The model accuracy on the test subset was 0.89 with AUC of 0.94. Based on the external validation dataset, the AUC and accuracy of the CNN model for detecting KC were 0.93 and 0.88, respectively.

Conclusions

Our deep learning-based approach was highly sensitive and specific in separating normal from keratoconic eyes using dynamic corneal deformation videos at levels that may prove useful in clinical practice.

Financial Disclosures

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Subclinical Keratoconus Detection and Characterization Using Motion-Tracking Brillouin Microscopy

PURPOSE

To characterize focal biomechanical alterations in subclinical keratoconus (SKC) using motion-tracking (MT) Brillouin microscopy and evaluate the ability of MT Brillouin metrics to differentiate eyes with SKC from normal control eyes.

DESIGN

Prospective cross-sectional study.

PARTICIPANTS

Thirty eyes from 30 patients were evaluated, including 15 eyes from 15 bilaterally normal patients and 15 eyes with SKC from 15 patients.

METHODS

All patients underwent Scheimpflug tomography and MT Brillouin microscopy using a custom-built device. Mean and minimum MT Brillouin values within the anterior plateau region and anterior 150 μm were generated. Scheimpflug metrics evaluated included inferior-superior (IS) value, maximum keratometry (Kmax), thinnest corneal thickness, asymmetry indices, Belin/Ambrosio display total deviation, and Ambrosio relational thickness. Receiver operating characteristic (ROC) curves were generated for all Scheimpflug and MT Brillouin metrics evaluated to determine the area under the ROC curve (AUC), sensitivity, and specificity for each variable.

MAIN OUTCOME MEASURES

Discriminative performance based on AUC, sensitivity, and specificity.

RESULTS

No significant differences were found between groups for age, sex, manifest refraction spherical equivalent, corrected distance visual acuity, Kmax, or KISA% index. Among Scheimpflug metrics, significant differences were found between groups for thinnest corneal thickness (556 μm vs. 522 μm; P < 0.001), IS value (0.29 diopter [D] vs. 1.05 D; P < 0.001), index of vertical asymmetry (IVA; 0.10 vs. 0.19; P < 0.001), and keratoconus index (1.01 vs. 1.05; P < 0.001), and no significant differences were found for any other Scheimpflug metric. Among MT Brillouin metrics, clear differences were found between control eyes and eyes with SKC for mean plateau (5.71 GHz vs. 5.68 GHz; P < 0.0001), minimum plateau (5.69 GHz vs. 5.65 GHz; P < 0.0001), mean anterior 150 μm (5.72 GHz vs. 5.68 GHz; P < 0.0001), and minimum anterior 150 μm (5.70 GHz vs. 5.66 GHz; P < 0.001). All MT Brillouin plateau and anterior 150 μm mean and minimum metrics fully differentiated groups (AUC, 1.0 for each), whereas the best performing Scheimpflug metrics were keratoconus index (AUC, 0.91), IS value (AUC, 0.89), and IVA (AUC, 0.88).

CONCLUSIONS

Motion-tracking Brillouin microscopy metrics effectively characterize focal corneal biomechanical alterations in eyes with SKC and clearly differentiated these eyes from control eyes, including eyes that were not differentiated accurately using Scheimpflug metrics.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Enhanced morphological assessment based on interocular asymmetry analysis for keratoconus detection

PURPOSE

To clarify the interocular asymmetry of corneal morphological descriptors and evaluate its discriminant ability of keratoconus (KC).

METHODS

This retrospective study recruited 344 normal participants and 290 KC patients, randomized to training and validation datasets. Interocular correlation and agreement were evaluated on 44 corneal morphological descriptors derived from Schiempflug tomography. Logistic regression models were constructed using binocular data and of which diagnostic performance was evaluated using the area under receiver operating characteristics curve (AUC), net reclassification improvement (NRI), and integrated discrimination improvement (IDI).

RESULTS

Interocular agreement of corneal descriptors is better in the normal than in KC except for dimensions of cornea and anterior chamber. The interocular asymmetry increases along with the severity of KC. Interocular asymmetry in maximum anterior keratometry, mean anterior keratometry and higher-order aberrations of anterior surface show high AUC above 0.950. Binocular logistic regression index reaches an AUC of 0.963 with high specificity (95.2%) and brings gain to monocular parameters in distinguishing the normal eyes from KC (NRI = 0.080 (0.042 ~ 0.118), P < 0.001) and IDI = 0.071 (0.049 ~ 0.092), P < 0.001). Interocular asymmetry benefits even more in subclinical keratoconus (SKC) detection reflected by NRI (0.4784 (0.2703-0.6865), P < 0.001) and IDI (0.2680 (0.1495-0.3866), P < 0.001) measures.

CONCLUSION

Interocular asymmetry is a well-characterized feature of KC and related to the severity. It is feasible to apply the interocular asymmetry in diagnosis of KC and SKC as a replenishment of monocular parameters and in progression tracking.

Accuracy of an Air-Puff Dynamic Tonometry Biomarker to Discriminate the Corneal Biomechanical Response in Patients With Keratoconus

PURPOSE

The aim of this study was to assess accuracy of the mean corneal stiffness ( kc , N/m) parameter to discriminate between patients with keratoconus and age-matched healthy subjects.

METHODS

Dynamic Scheimpflug imaging tonometry was performed with Corvis ST (Oculus Optikgeräte GmbH, Germany) in patients with keratoconus (n = 24; study group) and age-matched healthy subjects (n = 32; control). An image processing algorithm was developed to analyze the video sequence of the Corvis ST air-puff event and to determine the geometric and temporal parameters that correlated with the corneal tissue biomechanical properties. A modified 3-element viscoelastic model was used to derive the kc parameter, which represented the corneal tissue resistance to deformation under load. Receiver operating characteristic curves were used to assess the overall diagnostic performance for determining the area under the curve, sensitivity, and specificity of the kc in assessing the corneal tissue deformation to the Corvis ST air-puff event in keratoconus and control eyes. The Corvis Biomechanical Index ( CBI ) was analyzed for external validation.

RESULTS

The kc parameter was significantly different between keratoconus and controls ( P < 0.001), ranging from 24.9 ±3.0 to 34.2 ±3.5 N/m, respectively. It was highly correlated with CBI (r = -0.69; P < 0.001); however, the kc parameter had greater specificity (94%) than CBI (75%), whereas the 2 biomarkers had similar area under the curve (0.98 vs. 0.94) and sensitivity (96% vs. 92%) in predicting the occurrence of keratoconus.

CONCLUSIONS

The kc parameter extracted by video processing analysis of dynamic Scheimpflug tonometry data was highly accurate in discriminating patients with clinically manifest keratoconus compared with controls.

Scheimpflug-Based Corneal Biomechanical Analysis As A Predictor of Glaucoma in Eyes With High Myopia

Purpose

To address if corneal biomechanical behavior has a predictive value for the presence of glaucomatous optical neuropathy in eyes with high myopia.

Patients and Methods

This observational cross-sectional study included 209 eyes from 108 consecutive patients, divided into four groups: high myopia and primary open-angle glaucoma (POAG) - HMG, n = 53; high myopia without POAG - HMNG, n = 53; non-myopic with POAG - POAG, n = 50; non-myopic and non-POAG- NMNG, n = 53. Biomechanical assessment was made through a Scheimpflug-camera-based technology. Receiver operating characteristic curves were made for the discrimination between groups. Multivariable logistic regression models were performed to address the predictive value of corneal biomechanics for the presence of glaucoma.

Results

Areas Under the Receiver Operating Characteristic (AUROCs) above 0.6 were found in 6 parameters applied to discriminate between HMG and HMNG and six parameters to discriminate between POAG and NMNG. The biomechanical models with the highest power of prediction for the presence of glaucoma included 5 parameters with an AUROC of 0.947 for eyes with high myopia and 6 parameters with an AUROC of 0.857 for non-myopic eyes. In the final model, including all eyes, and adjusted for the presence of high myopia, the highest power of prediction for the presence of glaucoma was achieved including eight biomechanical parameters, with an AUROC of 0.917.

Conclusion

Corneal biomechanics demonstrated differences in eyes with glaucoma and mainly in myopic eyes. A biomechanical model based on multivariable logistic regression analysis and adjusted for high myopia was built, with an overall probability of 91.7% for the correct prediction of glaucomatous damage.

Evaluation of changes in corneal biomechanics after orthokeratology using Corvis ST

PURPOSE

To investigate the alterations in corneal biomechanical metrics induced by orthokeratology (ortho-k) using Corvis ST and to determine the factors influencing these changes.

METHOD

A prospective observational study was conducted to analyze various Corvis ST parameters in 32 children with low to moderate myopia who successfully underwent ortho-k lens fitting. Corneal biomechanical measurements via Corvis ST were acquired at six distinct time points: baseline (pre) and 2 h (pos2h), 6 h (pos6h), and 10 h (pos10h) following the removal of the first overnight wear ortho-k, one week (pos1w) and one month (pos1m) subsequent to the initiation of ortho-k.

RESULT

Significant differences were observed in Corvis ST Biomechanical parameters DAR2, IIR, CBI, and cCBI post ortho-k intervention. The integration of covariates (CCT, SimK, and bIOP) mitigated the differences in DAR2, IIR, and cCBI, but not in CBI. Initially, the stiffness parameter at first applanation, SP-A1, did not demonstrate significant variations, but after adjusting for covariates, noticeable differences over time were observed. The Stress-Strain Indeces, SSIv1 and SSIv2, did not manifest considerable changes over time, irrespective of the adjustment for covariates. No significant disparities were identified among different ortho-k lens brands.

CONCLUSION

Corneal biomechanics remained consistent throughout the one-month period of ortho-k lens wear. The observed changes in Corvis ST parameters subsequent ortho-k are primarily attributable to alterations in corneal pachymetry and morphology, rather than actual alterations in corneal biomechanics. The stability of corneal biomechanics post ortho-k treatment suggests the safety of this approach for adolescents from a corneal biomechanics perspective.

Performance of Corvis ST Parameters Including Updated Stress-Strain Index in Differentiating Between Normal, Forme-Fruste, Subclinical, and Clinical Keratoconic Eyes

PURPOSE

This study seeks to evaluate the ability of the updated stress strain index (SSIv2) and other Corvis ST biomechanical parameters in distinguishing between keratoconus at different disease stages and normal eyes.

DESIGN

Diagnostic accuracy analysis to distinguish disease stages.

METHODS

1084 eyes were included and divided into groups of normal (199 eyes), forme fruste keratoconus (FFKC, 194 eyes), subclinical keratoconus (SKC, 113 eyes), mild clinical keratoconus (CKC-Ⅰ, 175 eyes), moderate clinical keratoconus (CKC-Ⅱ, 204 eyes), and severe clinical keratoconus (CKC-Ⅲ, 199 eyes). Each eye was subjected to a Corvis ST examination to determine the central corneal thickness (CCT), biomechanically corrected intraocular pressure (bIOP), SSIv2 (updated stress-strain index), and other 8 Corvis parameters including the stress-strain index (SSIv1), stiffness parameter at first applanation (SP-A1), first applanation time (A1T), Ambrósio relational thickness to the horizontal profile (ARTh), integrated inverse radius (IIR), maximum deformation amplitude (DAM), ratio between deformation amplitude at the apex and at 2 mm nasal and temporal (DARatio2), and Corvis biomechanical index (CBI). The sensitivity and specificity of these parameters in diagnosing keratoconus were analyzed through receiver operating characteristic curves.

RESULTS

Before and after correction for CCT and bIOP, SSIv2 and ARTh were significantly higher and IIR and CBI were significantly lower in the normal group than in the FFKC group, SKC group and the 3 CKC groups (all P < .05). There were also significant correlations between the values of SSIv2, ARTh, IIR, CBI, and the CKC severity (all P < .05). AUC of SSIv2 was significantly higher than all other Corvis parameters in distinguishing normal eyes from FFKC, followed by IIR, ARTh and CBI.

CONCLUSION

Corvis ST's updated stress-strain index, SSIv2, demonstrated superior performance in differentiating between normal and keratoconic corneas, and between corneas with different keratoconus stages. Similar, but less pronounced, performance was demonstrated by the IIR, ARTh and CBI.

Keratoconus cone location influences ocular biomechanical parameters measured by the Ocular Response Analyzer

BACKGROUND

Keratoconus is characterized by asymmetry in the biomechanical properties of the cornea, with focal weakness in the area of cone formation. We tested the hypothesis that centrally-measured biomechanical parameters differ between corneas with peripheral cones and corneas with central cones.

METHODS

Fifty participants with keratoconus were prospectively recruited. The mean ± standard deviation age was 38 ± 13 years. Axial and tangential corneal topography were analyzed in both eyes, if eligible. Cones in the central 3 mm of the cornea were considered central, and cones outside the central 3 mm were considered peripheral. Each eye was then measured with the Ocular Response Analyzer (ORA) tonometer. T-tests compared differences in ORA-generated waveform parameters between cohorts.

RESULTS

Seventy-eight eyes were analyzed. According to the axial topography maps, 37 eyes had central cones and 41 eyes had peripheral cones. According to the tangential topography maps, 53 eyes had central cones, and 25 eyes had peripheral cones. For the axial-topography algorithm, wave score (WS) was significantly higher in peripheral cones than central cones (inter-cohort difference = 1.27 ± 1.87). Peripheral cones had a significantly higher area of first peak, p1area (1047 ± 1346), area of second peak, p2area (1130 ± 1478), height of first peak, h1 (102 ± 147), and height of second peak, h2 (102 ± 127), than central cones. Corneal hysteresis (CH), width of the first peak, w1, and width of the second peak, w2, did not significantly differ between cohorts. There were similar results for the tangential-topography algorithm, with a significant difference between the cohorts for p1area (855 ± 1389), p2area (860 ± 1531), h1 (81.7 ± 151), and h2 (92.1 ± 131).

CONCLUSIONS

Cone location affects the biomechanical response parameters measured under central loading of the cornea. The ORA delivers its air puff to the central cornea, so the fact that h1 and h2 and that p1area and p2area were smaller in the central cone cohort than in the peripheral cone cohort suggests that corneas with central cones are softer or more compliant centrally than corneas with peripheral cones, which is consistent with the location of the pathology. This result is evidence that corneal weakening in keratoconus is focal in nature and is consistent with localized disruption of lamellar orientation.

Biomechanical changes in keratoconus after customized stromal augmentation

PURPOSE

To verify corneal biomechanical changes, poststromal augmentation using myopic small-incision lenticule extraction's (SMILEs) lenticules in advanced keratoconus (KCN) through Corvis ST (Oculus, Wetzlar, Germany).

MATERIALS AND METHODS

A clinical trial enrolled 22 advanced KCN patients. We implanted lenticules exceeding 100 μ according to a nomogram and evaluated biomechanical factors through Corvis ST at 3-, 6-, and 24-month postimplantation. We examined parameters during the first applanation (A1), second applanation (A2), highest concavity (HC)/max concavity events, and Vinciguerra screening parameters, as recently established criteria derived from the ideal blend of diverse biomechanical and ocular factors and formulated through the application of logistic regression. Regression analyses explored relationships with age, mean keratometry value, thickness, sphere, cylinder, and best-corrected visual acuity.

RESULTS

Patients were well matched for age, intraocular pressure, and central corneal thickness (CCT). The mean spherical equivalent decreased from -13.48 ± 2.86 Diopters (D) to -8.59 ± 2.17 D (P < 0.007), and mean keratometry decreased from 54.68 ± 2.77 D to 51.95 ± 2.21 D (P < 0.006). Significant increases were observed in HC time (HCT), Radius-central curvature radius at the HC state-, peak distance (PD) during HC state, CCT, first applanation time, and stiffness parameter (A1T and SP-A1), whereas HC deformation amplitude, maximum deformation amplitude ratio at 2 mm, Corvis Biomechanical Index (CBI), integrated radius (IR), second applanation deformation amplitude (A2DA), first applanation velocity and deflection amplitude (A1V and A1DeflA) significantly decreased postlenticule implantation. Multivariable regression revealed age positively correlated with SP-A1 (P = 0.003) and negatively with HC delta Arc length (P = 0.007). Mean K positively correlated with CCT (P = 0.05) and negatively with CBI (P = 0.032). Best-corrected visual acuity positively correlated with HCT (P = 0.044), and the cylinder positively correlated with PD (P = 0.05) and CCT (P = 0.05) whereas negatively with IR (P = 0.025).

CONCLUSIONS

Stromal augmentation using myopic SMILE lenticules induces significant corneal biomechanical changes in KCN.

Refractive associations with corneal biomechanical properties among young adults: a population-based Corvis ST study

PURPOSE

To assess the associations of corneal biomechanical properties as measured by the Corvis ST with refractive errors and ocular biometry in an unselected sample of young adults.

METHODS

A total of 1645 healthy university students underwent corneal biomechanical parameters measurement by the Corvis ST. The refractive status of the participants was measured using an autorefractor without cycloplegia. Ocular biometric parameters were measured using the IOL Master.

RESULTS

After adjusting for the effect of age, sex, biomechanical-corrected intraocular pressure and central corneal thickness, axial length was significantly associated with A1 velocity (A1v, β = -10.47), A2 velocity (A2v, β = 4.66), A2 deflection amplitude (A2DeflA, β = -6.02), HC deflection amplitude (HC-DeflA, β = 5.95), HC peak distance (HC-PD, β = 2.57), deformation amplitude ratio max (DA Rmax, β = -0.36), Ambrósio's relational thickness to the horizontal profile (ARTh, β = 0.002). For axial length / corneal radius ratio, only A1v (β = -2.01), A1 deflection amplitude (A1DeflA, β = 2.30), HC-DeflA (β = 1.49), HC-PD (β = -0.21), DA Rmax (β = 0.07), stress-strain index (SSI, β = -0.29), ARTh (β < 0.001) were significant associates. A1v (β = 23.18), HC-DeflA (β = -15.36), HC-PD (β = 1.27), DA Rmax (β = -0.66), SSI (β = 3.53), ARTh (β = -0.02) were significantly associated with spherical equivalent.

CONCLUSION

Myopic eyes were more likely to have more deformable corneas and corneas in high myopia were easier to deform and were even softer compared with those in the mild/moderate myopia.