Changes in biomechanically corrected intraocular pressure and dynamic corneal response parameters before and after transepithelial photorefractive keratectomy and femtosecond laser-assisted laser in situ keratomileusis

Full range of vision and visual quality after mini-monovision FS-LASIK in high myopic patients with presbyopia

BACKGROUND

To evaluate clinical outcomes and visual quality 3 months after mini-monovision (spherical equivalent [SE] between - 0.25 D and - 0.50 D) femtosecond laser-assisted in situ keratomileusis (FS-LASIK) for correction of high myopia in patients with presbyopia.

METHODS

Patients who had mini-monovision FS-LASIK for high myopia (SE < -6.0 D) and aged between 40 and 50 years were included. At the 3-month postoperative visit, we evaluated full range of visual acuity; defocus curve; optical quality; accommodation function, contrast sensitivity and stereopsis. Binocular tests were done twice, once in mini-monovision condition and once with the residual myopia in the non-dominant eye corrected. Subjective visual quality was evaluated with questionnaire postoperatively with mini-monovision correction.

RESULTS

Clinical data of 31 cases were analyzed. The average patient age was 42.58 ± 3.06 years. At the 3-month follow-up, the mean uncorrected binocular visual acuity at distance, intermediate, and near was - 0.11 ± 0.07, -0.06 ± 0.10, and 0.04 ± 0.11 logMAR separately. In comparison, patients with binocular full distance correction achieved better uncorrected distance visual acuity (UDVA), and they achieved superior uncorrected near visual acuity (UNVA, P = 0.04) with mini-monovision correction. FS-LASIK induced significant increases in higher-order aberrations (HOAs) (P < 0.001). For accommodative function, only the negative relative accommodation (NRA) improved significantly after surgery (P < 0.001). A slight decrease in contrast sensitivity was observed at low spatial frequency with mini-monovision correction (P < 0.05). Questionnaire demonstrated high satisfaction with near vision and visual quality.

CONCLUSION

FS-LASIK with mini-monovision (SE between - 0.25 D and - 0.50 D) appeared to be safe and effective in treating high myopia combined with presbyopia to get satisfying visual quality at distant and at near.

A New Biomechanical Deformation Response Parameter: Change in Central Corneal Thickness During Air Puff Induced Corneal Deformation

PURPOSE

To investigate the percent change in central corneal thickness (%ΔCCT) during air-puff-induced deformation as an indicator of corneal biomechanical response.

METHODS

Forty ex vivo human eyes from forty donors were imaged using the CorVis ST at experimentally controlled intraocular pressure (IOP) of 10, 20, 30, and 40 mmHg, followed by uniaxial strip testing to calculate tensile modulus. The CorVis ST research software tracked the anterior and posterior cornea edges and determined the dynamic corneal response (DCR) parameters. Eyes were excluded if image quality or posterior tracking issues were present. Custom algorithms were used to calculate CCT during deformation using a ray-tracing method to correct for Scheimpflug and optical distortion within each image. Correlation and stepwise regression analyses between the shape-related DCR parameters and %ΔCCT were conducted. A mixed model analysis was performed to test the effect of IOP and the strongest significant predictors of the stepwise regression on %ΔCCT. The significance threshold was set to p < 0.05.

RESULTS

Thirty eyes were ultimately analyzed and CCT increased significantly from the pre-deformation state to the highest concavity state at each IOP level (p < 0.001). IOP and multiple shape DCRs were found to be significantly related to %ΔCCT (p < 0.0001). The strongest predictor of %ΔCCT was integrated inverse radius (IIR) (p < 0.0001; partial R2 = 0.4772) with no other parameter having a partial R2 value greater than 0.04. The mixed model analysis showed that IIR was the sole predictor (p = 0.0098) and IOP was no longer significant as a single predictor. However, the interaction of IIR with IOP (p = 0.0023) had a significant effect on %ΔCCT.

CONCLUSION

Percent change in CCT is influenced by corneal stiffness as indicated by the significant relationship with IIR. The %ΔCCT may be a potential biomarker for determining differences in corneal deformation response with corneal diseases.

Corneal stress‒strain index in relation to retinal nerve fibre layer thickness among healthy young adults

BACKGROUND/OBJECTIVES

To determine the relationship between corneal stress-strain index (SSI) and retinal nerve fibre layer (RNFL) thickness.

SUBJECTS/METHODS

1645 healthy university students from a university-based study contributed to the analysis. The RNFL thickness was measured by high-definition optical coherence tomography (HD-OCT), axial length (AL) was measured by IOL Master, and corneal biomechanics including SSI, biomechanical corrected intraocular pressure (bIOP), and central corneal thickness (CCT) were measured by Corvis ST. Multivariate linear regression was performed to evaluate the relationship between the SSI and RNFL thickness after adjusting for potential covariates.

RESULTS

The mean age of the participants was 19.0 ± 0.9 years, and 1132 (68.8%) were women. Lower SSI was significantly associated with thinner RNFL thickness ( β =8.601, 95% confidence interval [CI] 2.999-14.203, P  = 0.003) after adjusting for age, CCT, bIOP, and AL. No significant association between SSI and RNFL was found in men, while the association was significant in women in the fully adjusted model. The association was significant in the nonhigh myopic group ( P for trend = 0.021) but not in the highly myopic group. Eyes with greater bIOP and lower SSI had significantly thinner RNFL thickness.

CONCLUSIONS

Eyes with lower SSI had thinner RNFL thickness after adjusting for potential covariates, especially those with higher bIOP. Our findings add novel evidence of the relationship between corneal biomechanics and retinal ganglion cell damage.

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.

Effect of corneal cross-linking on biomechanical changes following transepithelial photorefractive keratectomy and femtosecond laser-assisted LASIK

Purpose: To evaluate the change in corneal biomechanics in patients with postoperative ectasia risk when combining two common laser vision correction procedures (tPRK and FS-LASIK) with cross-linking (in tPRK Xtra and FS-LASIK Xtra). Methods: The study included 143 eyes of 143 myopic, astigmatic patients that were divided into non-cross-linked refractive surgery groups (non-Xtra groups, tPRK and FS-LASIK) and cross-linked groups (Xtra groups, tPRK Xtra and FS-LASIK Xtra) according to an ectasia risk scoring system. The eyes were subjected to measurements including the stress-strain index (SSI), the stiffness parameter at first applanation (SP-A1), the integrated inverse radius (IIR), the deformation amplitude at apex (DA), and the ratio of deformation amplitude between apex and 2 mm from apex (DARatio2mm). The measurements were taken preoperatively and at 1, 3, and 6 months postoperatively (pos1m, pos3m, and pos6m). Posterior demarcation line depth from the endothelium (PDLD) and from the ablation surface (DLA) were recorded at pos1m. Results: SP-A1 significantly decreased, while IIR, deformation amplitude, and DARatio2mm increased significantly postoperatively in all four groups (p < 0.01)-all denoting stiffness decreases. In the FS-LASIK group, the changes in IIR, DA, and DARatio2mm were 32.7 ± 15.1%, 12.9 ± 7.1%, and 27.2 ± 12.0% respectively, which were significantly higher (p < 0.05) compared to 20.1 ± 12.8%, 6.4 ± 8.2%, and 19.7 ± 10.4% in the FS-LASIK Xtra group. In the tPRK group, the change in IIR was 27.3 ± 15.5%, significantly larger than 16.9 ± 13.4% in the tPRK Xtra group. The changes of SSI were minimal in the tPRK (-1.5 ± 21.7%, p = 1.000), tPRK Xtra (8.4 ± 17.9%, p = 0.053), and FS-LASIK Xtra (5.6 ± 12.7%, p = 0.634) groups, but was significant in the FS-LASIK group (-12.1 ± 7.9%, p < 0.01). After correcting for baseline biomechanical metrics, preoperative bIOP and the change in central corneal thickness (△CCT) from pre to pos6m, the changes in the IIR in both FS-LASIK and tPRK groups, as well as DA, DARatio2mm and SSI in the FS-LASIK group remained statistically greater than their corresponding Xtra groups (all p < 0.05). Most importantly, after correcting for these covariates, the changes in DARatio2mm in the FS-LASIK Xtra became statistically smaller than in the tPRK Xtra (p = 0.017). Conclusion: The statistical analysis results indicate that tPRK Xtra and FS-LASIK Xtra effectively reduced the biomechanical losses caused by refractive surgery (tPRK and FS-LASIK). The decrease in corneal overall stiffness was greater in FS-LASIK than in tPRK, and the biomechanical enhancement of CXL was also higher following LASIK than after tPRK.

[Effect of refractive surgery on Maklakov tonometry results]

PURPOSE

The study investigates the influence of changes in keratometric parameters after refractive surgery on the results of Maklakov tonometry.

MATERIAL AND METHODS

The study examined a total of 61 people (121 eyes). The patients were divided into a control group with no history of surgery (16 people, 31 eyes), a LASIK group (13 people, 26 eyes), a femtosecond-assisted LASIK (FS-LASIK) group (16 people, 32 eyes), and a photorefractive keratectomy (PRK) group (16 people, 32 eyes). The patients underwent standard examination, keratometry (Km), Maklakov tonometry with a 10 g weight, and elastotonometry with 5, 7.5, and 15 g weights.

RESULTS

In the LASIK group, the indentation diameter with 5 and 7.5 g weights correlated with Km in the central and near-paracentral zone (r=0.3-0.5). Tonometry with a 10 g weight did not correlate with anything. Tonometry with a 15 g weight inversely correlated with Km in the paracentral points (4 mm) of the strong meridian (r= -0.5 ... -0.7). In the FS-LASIK group, a significant inverse correlation with Km was observed only for the indentation diameter with a 10 g weight in the paracentral (3-4 mm) zone (r= -0.4 ... -0.5). In the PRK group, weak (r<0.4) correlations were found between Km and the indentation diameter of the 7.5 and 10 g weights for the central zone (1-2 mm). No significant correlations were found for 5 and 15 g weights.

In the control group, there were practically no correlations for 5 and 7.5 g weights. The indentation diameter of the 10 g weight evenly correlated with Km at all points (r= -0.38 ... -0.60), the indentation of the 15 g weight correlated mainly with the curvature of the horizontal meridian (r= -0.37 ... -0.49).

CONCLUSION

Tonometry readings with the 10 g weight are the most dependent on Km in different groups, and the readings with the 5 g weight are the least dependent. LASIK is characterized by the largest scatter of dependencies for weights of different masses, FS-LASIK - by the smallest. Tonometry readings with the 5 g weight correlated with Km only in the LASIK group, and this was the only direct correlation. Considering the inverse nature of most correlations, higher Km may be associated with an overestimation of tonometry results, and lower Km - with its underestimation.

Comparison of Corneal Biomechanics Treated With Femtosecond Laser-Assisted In Situ Keratomileusis and Small-Incision Lenticule Extraction by New Corneal Biomechanical Parameters of Corvis ST II

PURPOSE

The aim of this study was to compare corneal biomechanics treated with femtosecond laser-assisted in situ keratomileusis (FS-LASIK) and small-incision lenticule extraction (SMILE) for myopia and astigmatism using the new corneal biomechanical parameters of Corvis ST II.

METHODS

This was a prospective nonrandomized controlled study. Patients treated with FS-LASIK or SMILE between January 2018 and July 2018 were included. Corvis ST II was performed to measure corneal biomechanical parameters, including deformation amplitude ratio 2.0 mm (DA ratio 2.0 mm), integrated inverse radius (Integr Radius), stiffness parameter at first applanation (SP-A1), and Ambrosio relational thickness through the horizontal meridian (ARTh), preoperatively, 1 month postoperatively, and 6 months postoperatively. Pentacam pachymetry was used to assess the reduction in pachymetry.

RESULTS

Forty-five eyes underwent FS-LASIK, and 45 eyes underwent SMILE. The new parameters obtained by Corvis ST II between preoperative and postoperative measurements showed significant changes after FS-LASIK or SMILE (all P < 0.001). Postoperative SP-A1 significantly decreased in the 2 groups (108.88 ± 14.47-73.32 ± 13.2 in FS-LASIK and 105.79 ± 17.68-73.91 ± 14.81 in SMILE). Eyes with equal preoperative pachymetry, intraocular pressure, and spherical equivalents showed no significant differences in these new parameters measured using Corvis ST II ( all P > 0.05) between the 2 groups. The prediction of the laser platform overestimated the measured pachymetry reduction in the SMILE group (111.93 ± 15.18 μm vs. 87.16 ± 15.47 μm).

CONCLUSIONS

New corneal biomechanical parameters measured using Corvis ST II showed no significant differences between FS-LASIK and SMILE in eyes with homogeneous preoperative parameters. The laser software platform may have overestimated the actual corneal reduction in the eyes treated with SMILE.

Short- and mid-term changes in CORVIS ST parameters in successful, adult orthokeratology patients

CLINICAL RELEVANCE

The changes in various biomechanical and tomographic characteristics of the cornea associated with orthokeratology may allow us to identify potential mid- and long-term structural alterations, resulting in a better understanding of the governing mechanisms of this procedure and in its optimisation.

BACKGROUND

The study aimed at describing short and mid-term changes in CORVIS ST® parameters and indices in orthokeratology (ortho-k), and their diurnal variations.

METHODS

A prospective observational study was designed in which several CORVIS ST® parameters of 75 new adult participants successfully fitted with overnight ortho-k Seefree® (Conóptica - Hecht Contactlinsen) contact lenses were explored. Measurements were conducted in baseline (BL) conditions and in the morning and evening at the one-night (1 NM/1NT), one-week (1WM/1 WT) and 3-month (3 MM/3MT) follow-up visits.

RESULTS

Statistically significant differences were found in DARatio_2 mm, IntRad, ARTh, CBI and TBI following overnight ortho-k, when compared with BL values, with most values reaching stability at 1WM or reverting to BL values at 3 MM. The ARTh and CBI parameters showed some of the most significant temporal variations (both p < 0.001), probably reflecting the encountered differences in central corneal thickness between BL and 1WM (p = 0.010) and between BL and 3 MM (p = 0.016). In general, corneal rigidity was higher in the morning at all follow-up visits, and decreased during the day. No statistically significant changes in adjusted intraocular pressure values were found.

CONCLUSION

Ortho-k in adults may be considered a safe procedure in terms of short and mid-term changes in CORVIS ST® parameters. The observed alterations in most of the parameters provided by the Corvis ST® probably responded to the well-described changes in corneal pachymetry and tomography, rather than to actual alterations in corneal rigidity.

Accuracy of tomographic and biomechanical parameters in detecting unilateral post-LASIK keratoectasia and fellow eyes

Background: Patients with unilateral post-LASIK keratectasia (KE) have clinical ectasia in one eye but not in the fellow eye. As serious complications, these cases are rarely reported but are worth investigating. This study aimed to explore the characteristics of unilateral KE and the accuracy of corneal tomographic and biomechanical parameters to detect KE and distinguish fellow eyes from control eyes. Methods: The study analyzed 23 KE eyes, 23 KE fellow eyes, and 48 normal eyes from age- and sex-matched patients who had undergone LASIK. The Kruskal-Wallis test and further paired comparisons were performed to compare the clinical measurements of the three groups. The receiver operating characteristic curve was used to evaluate the ability to distinguish KE and fellow eyes from the control eyes. Binary logistic regression with the forward stepwise method was performed to produce a combined index, and the DeLong test was used to compare the discriminability difference of the parameters. Results: Males accounted for 69.6% of patients with unilateral KE. The duration between corneal surgery and the onset of ectasia ranged from 4 months to 18 years, with a median time of 10 years. The KE fellow eye had a higher posterior evaluation (PE) value than the control eyes (5 vs. 2, p = 0.035). Diagnostic tests showed that PE, posterior radius of curvature (3 mm), anterior evaluation (FE), and Corvis biomechanical index-laser vision correction (CBI-LVC) were sensitive indicators for distinguishing KE in the control eyes. The ability of PE to detect the KE fellow eye from the control eye was 0.745 (0.628 and 0.841), with 73.91% sensitivity and 68.75% specificity at a cut-off value of 3. The ability of a combined index, constructed using PE and FE, to distinguish fellow eyes of KE from controls was 0.831 (0.723 and 0.909), which was higher than that of PE and FE individually (p < 0.05). Conclusion: The fellow eyes of patients with unilateral KE had significantly higher PE values than control eyes, and a combination of PE and FE enhanced this differentiation in a Chinese population. More attention should be paid to the long-term follow-up of patients after LASIK and to be wary of the occurrence of early KE.

Development and validation of an intuitive biomechanics-based method for intraocular pressure measurement: a modal analysis approach

BACKGROUND

Current intraocular pressure (IOP) measurements based on non-contact tonometry are derived from statistics-driven equations and lack biomechanical significance, which often leads to under-estimation in post-refractive surgery cornea. This study aims to introduce and validate modal analysis-derived intraocular pressure (mIOP) as a novel method generated through Legendre-based modal decomposition of the anterior corneal contour; it provides an accurate and intuitive IOP measurement from an energy-based perspective.

METHODS

This retrospective study included 680 patients. Healthy participants were divided into reference (n = 385) and validation (n = 142) datasets, and the others underwent either femtosecond-assisted laser in situ keratomileusis (FS-LASIK, n = 58) or transepithelial photorefractive keratectomy (TPRK, n = 55). Corneal curvature of the right eyes was extracted from raw serial cross-sectional images of the cornea generated by Corvis ST, a noncontact tonometer with a high-speed Scheimpflug-camera. Legendre expansion was then applied to the corneal curvature to obtain the modal profiles (i.e., temporal changes of the coefficient for each basis polynomial [modes]). Using the reference dataset, feature selection on the modal profiles generated a final mIOP model consisting of a single parameter: total area under curve (frames 1-140) divided by the area under curve of the rising phase (frames 24-40) in the fourth mode, i.e. the M₄ ratio. Validation was performed in both the healthy validation and postoperative datasets. IOP-Corvis, pachymetry-corrected IOP, biomechanically corrected IOP, and mIOP values were compared. For the FS-LASIK and TPRK groups, pairwise postoperative IOP changes were analyzed through repeated measures analysis of variance, and agreement was examined through Bland-Altman analysis. Using a finite element analysis based three-dimensional model of the human cornea, we further compared the M₄ ratio with the true intraocular pressure within the physiological range.

RESULTS

The M₄ ratio-based mIOP demonstrated weak to negligible association with age, radius of corneal curvature, and central corneal thickness (CCT) in all validation analyses, and performed comparably with biomechanically corrected IOP (bIOP) in the refractive surgery groups. Both remained nearly constant postoperatively and were not influenced by CCT changes. Additionally, M₄ ratio accurately represented true intraocular pressure in the in silico model.

CONCLUSIONS

mIOP is a reliable IOP measurement in healthy and postrefractive surgery populations. This energy-based, ratio-derived approach effectively filters out pathological, rotational, misaligned movements and serves as an interpatient self-calibration index. Modal analysis of corneal deformation dynamics provides novel insights into regional corneal responses against pressure loading.

Corneal Biomechanics After SMILE, Femtosecond-Assisted LASIK, and Photorefractive Keratectomy: A Matched Comparison Study

Purpose

To evaluate the change in corneal stiffness after small incision lenticule extraction (SMILE), femtosecond laser-assisted in situ keratomileusis (FS-LASIK), and photorefractive keratectomy (PRK).

Methods

Age, gender, spherical equivalent, and central corneal thickness (CCT)-matched cases undergoing SMILE with a 120-µ cap, FS-LASIK with a 110-µ flap, and PRK were enrolled. One-year change in the stress-strain index, stiffness parameter at first applanation, integrated inverse radius, deformation amplitude ratio at 2 mm, and deformation amplitude ratio at 1 mm were compared between the surgical groups by linear mixed-effect models.

Results

Within each surgical group, 120 eyes completed 1 year of follow-up. The residual stromal bed (RSB) thickness and (RSB/CCTpostop) were 348.1 ± 35.0 (0.74), 375.4 ± 31.0 (0.77) and 426.7 ± 2 µm (0.88) after SMILE, FS-LASIK, and PRK, respectively. The 1-year change in all biomechanical indices was significant, except the stress-strain index with PRK (P = 0.884). The change in all indices with SMILE were significantly greater than with FS-LASIK and with PRK (all P < 0.01), except the deformation amplitude ratio at 1 mm change between SMILE and FS-LASIK (P = 0.075). The changes in all indices with FS-LASIK were significantly greater than with PRK (all P < 0.05).

Conclusions

Although SMILE preserves the greatest amount of anterior cornea with a cap thickness of 120 µ, this also produces the smallest RSB and the greatest decrease in stiffness. Thus, the RSB is shown to be the predominant determinant of stiffness decreases, rather than the preserved anterior cornea. We recommend using a thinner cap to achieve a thicker RSB and a lesser decrease in the corneal stiffness in the SMILE procedure.

Translational Relevance

After refractive surgery, RSB is predominant determinant of stiffness decreases, rather than the preserved anterior cornea.

Changes in aberrations and biomechanics after femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK) in eyes with high astigmatism: a retrospective case control study

BACKGROUND

To compare the 6-month changes in aberration and biomechanics after femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK) for high astigmatism.

METHODS

In this retrospective case control study, 47 eyes with high astigmatism (≥ 2.5 D, HA group) and 47 eyes with low astigmatism (≤ 1.0 D, LA group) underwent FS-LASIK. Preoperative and follow-up examinations included visual outcomes, higher order aberrations (HOAs) and biomechanics. Biomechanical parameters include a deformation amplitude ratio of 2 mm (DA ratio 2 mm), integrated inverse radius (IIR), stiffness parameter at first applanation (SP-A1), and ambrosio relational thickness through the horizontal meridian (ARTh).

RESULTS

Six months postoperatively, there was no significant difference in the efficacy and safety index (both P > 0.05) between the two groups, but the cylinder was higher in the HA group. The HOAs increased significantly after surgery in both groups (all P < 0.05). Six months postoperatively, the changes in spherical aberration and HOAs were larger in the HA group (both P < 0.005), but there was no significant difference between the changes in coma (P > 0.05). Significant decreases in SP-A1 and ARTh and significant increases in the IIR and DA ratio of 2 mm (all P < 0.05) were observed after surgery in both groups. The changes in the DA ratio 2 mm, IIR, SP-A1, and ARTh were not significantly different between the groups.

CONCLUSION

FS-LASIK had relative comparable efficacy and safety in correcting high and low myopic astigmatism, with higher astigmatic under-correction in eyes with high astigmatism. High astigmatism in eyes after FS-LASIK could introduce larger corneal aberrations, but the impact on corneal stiffness was the same as that in eyes with low astigmatism.

Corneal Biomechanics Losses Caused by Refractive Surgery

Recent advances, specifically in the understanding of the biomechanical properties of the cornea and its response to diseases and surgical interventions, have significantly improved the safety and surgical outcomes of corneal refractive surgery, whose popularity and demand continue to grow worldwide. However, iatrogenic keratectasia resulting from the deterioration in corneal biomechanics caused by surgical interventions, although rare, remains a global concern. On one hand, in vivo biomechanical evaluation, enabled by clinical imaging systems such as the ORA and the Corvis ST, has significantly improved the risk profiling of patients for iatrogenic keratectasia. That is despite the fact the biomechanical metrics provided by these systems are considered indicators of the cornea's overall stiffness rather than its intrinsic material properties. On the other hand, new surgical modalities including SMILE were introduced to offer superior biomechanical performance to LASIK, but this superiority could not be proven clinically, creating more myths than answers. The literature also includes sound evidence that tPRK provided the highest preservation of corneal biomechanics when compared to both LASIK and SMILE. The aim of this review is twofold; to discuss the importance of corneal biomechanical evaluation prior to refractive surgery, and to assess the current understanding of cornea's biomechanical deterioration caused by mainstream corneal refractive surgeries. The review has led to an observation that new imaging techniques, parameters and evaluation systems may be needed to reflect the true advantages of specific refractive techniques and when these advantages are significant enough to offer better protection against post-surgery complications.

Clinical Ocular Biomechanics: Where Are We after 20 Years of Progress?

Purpose: Ocular biomechanics is an assessment of the response of the structures of the eye to forces that may lead to disease development and progression, or influence the response to surgical intervention. The goals of this review are (1) to introduce basic biomechanical principles and terminology, (2) to provide perspective on the progress made in the clinical study and assessment of ocular biomechanics, and (3) to highlight critical studies conducted in keratoconus, laser refractive surgery, and glaucoma in order to aid interpretation of biomechanical parameters in the laboratory and in the clinic.Methods: A literature review was first conducted of basic biomechanical studies related to ocular tissue. The subsequent review of ocular biomechanical studies was limited to those focusing on keratoconus, laser refractive surgery, or glaucoma using the only two commercially available devices that allow rapid assessment of biomechanical response in the clinic.Results: Foundational studies on ocular biomechanics used a combination of computer modeling and destructive forces on ex-vivo tissues. The knowledge gained from these studies could not be directly translated to clinical research and practice until the introduction of non-contact tonometers that quantified the deformation response of the cornea to an air puff, which represents a non-destructive, clinically appropriate load. The corneal response includes a contribution from the sclera which may limit corneal deformation. Two commercial devices are available, the Ocular Response Analyzer which produces viscoelastic parameters with a customized load for each eye, and the Corvis ST which produces elastic parameters with a consistent load for every eye. Neither device produces the classic biomechanical properties reported in basic studies, but rather biomechanical deformation response parameters which require careful interpretation.Conclusions: Research using clinical tools has enriched our understanding of how ocular disease alters ocular biomechanics, as well as how ocular biomechanics may influence the pathophysiology of ocular disease and response to surgical intervention.

Introduction and Clinical Validation of an Updated Biomechanically Corrected Intraocular Pressure bIOP (v2)

PURPOSE

To improve the stability of the Corvis ST biomechanically-corrected intraocular pressure measurements (bIOP) after refractive surgery and its independence of corneal biomechanics.

METHODS

A parametric study was carried out using numerical models simulating the behavior of the eye globe under the effects of IOP and Corvis ST external air pressure and used to develop a new algorithm for bIOP; bIOP(v2). It was tested on 528 healthy participants to evaluate correlations with CCT and age. Its ability to compensate for the geometrical changes was tested in 60 LASIK and 80 SMILE patients with six months follow up. The uncorrected Corvis ST IOP (CVS-IOP) and the two versions of biomechanically corrected IOP; bIOP(v1) and bIOP(v2), were compared.

RESULTS

In the healthy dataset, bIOP(v2) had weak and non-significant correlation with both CCT (R = -0.048, p = .266) and age (R = 0.085, p = .052). For bIOP(v1), the correlation was non-significant with CCT (R = -0.064, p = .139) but significant with age (R = -0.124, p < .05). In both LASIK and SMILE groups, the median change in bIOP(v2) following surgery was below 1 mmHg at follow-up stages and the interquartile range was smaller than both bIOP(v1) and CVS-IOP.

CONCLUSION

The bIOP(v2) algorithm performs better than bIOP(v1) and CVS-IOP in terms of correlation with CCT and age. The bIOP(v2) also demonstrated the smallest variation after LASIK and SMILE refractive surgeries indicating improved ability to compensate for geometrical changes.

Clinical Evaluation of Corneal Biomechanics following Laser Refractive Surgery in Myopic Eyes: A Review of the Literature

The role of corneal biomechanics in laser vision correction (LVC) is currently being raised in the assessment of postoperative corneal ectasia risk. The aim of the paper was to evaluate the changes in corneal biomechanics after LVC procedures based on a systematic review of current studies. The results of a search of the literature in the PubMed, Science Direct, Google Scholar, and Web of Science databases were selected for final consideration according to the PRISMA 2020 flow diagram. Included in our review were 17 prospective clinical studies, with at least 6 months of follow-up time. Corneal biomechanical properties were assessed by Ocular Response Analyzer (ORA), or Corvis ST. The results of the study revealed the highest corneal biomechanics reduction after laser in situ keratomileusis (LASIK) followed by small incision lenticule extraction (SMILE) and surface procedures, such as photorefractive keratectomy (PRK) or laser-assisted sub-epithelial keratectomy (LASEK). In SMILE procedure treatment planning, the use of thicker caps preserves the corneal biomechanics. Similarly, reduction of flap thickness in LASIK surgery maintains the corneal biomechanical strength. Future prospective clinical trials with standardization of the study groups and surgical parameters are needed to confirm the results of the current review.

Changes in Corneal Biomechanical Properties After Small-Incision Lenticule Extraction and Photorefractive Keratectomy, Using a Noncontact Tonometer

PURPOSE

The aim of this study was to evaluate and compare early corneal biomechanical changes after small-incision lenticule extraction (SMILE) and photorefractive keratectomy (PRK).

METHODS

The study comprised 74 patients eligible for refractive surgery, equally allocated to PRK (37 patients) and SMILE (37 patients). Corneal biomechanical properties were recorded and compared between the 2 groups at preoperatively and 3 months after surgery using a dynamic ultra-high-speed Scheimpflug camera equipped with a noncontact tonometer.

RESULTS

Both procedures significantly affected corneal biomechanical properties at 3 months after surgery. Patients in the PRK group showed significantly better results for deformation amplitude ratio (DA ratio) ( P = 0.03), maximum inverse radius (InvRadMax) ( P = 0.02), and A2 time ( P = 0.03). The mean changes in DA ratio, HC radius, InvRadMax, and Ambrosio relational thickness were significantly higher in the SMILE group in comparison with those of the PRK group (all, P < 0.05). In both groups, change in CCT was significantly correlated with changes in DA ratio and InvRadMax ( P < 0.05).

CONCLUSIONSS

Both SMILE and PRK refractive surgeries significantly altered corneal biomechanical properties but the changes were more prominent after SMILE.

Comparison of Three Tonometers in Measuring Intraocular Pressure in Eyes That Underwent Myopic Laser in situ Keratomileusis and Photorefractive Keratectomy

Objective

Methods

Results

Conclusion

A stochastic approach to estimate intraocular pressure and dynamic corneal responses of the cornea

IntraOcular Pressure (IOP) is one of the most informative factors for monitoring the eye-health. This is usually measured by tonometers. However, the outputs of the tonometers depend on the physical and geometrical properties of the cornea. Therefore, the common practice is to develop a numerical model to generate some correction factors. The main challenge here is the accuracy and efficiency of a numerical model in predicting the IOP and Dynamic Corneal Response (DCR) of each patient. This study addresses this issue by developing a two-step surrogate model based on adaptive sparse Polynomial Chaos Expansion (PCE) for fast and accurate prediction of the IOP. In this regard, first, an FE model of the cornea has been developed to predict the DCR parameters. This FE model has been replaced with a PCE-based surrogate model to speed up the simulation step. The uncertainties in the geometry and material model of the cornea have been propagated through the surrogate model to estimate the distributions of the DCR parameters. In the second step, the combination of DCR parameters and the input parameters provide a proper parameter space for developing an efficient data-driven PCE model to predict the IOP. Moreover, sensitivity analysis by using PCE-based Sobol indices has been performed. The results demonstrate the accuracy and efficiency of the proposed method in predicting the IOP. Sensitivity analysis revealed that IOP measurement was influenced mostly by deflection amplitude and applanation time. The analysis indicates the importance of the interactions between the parameters.

Biomechanical Effects of tPRK, FS-LASIK, and SMILE on the Cornea

Purpose: The objective of this study is to evaluate the in vivo corneal biomechanical response to three laser refractive surgeries.

Methods: Two hundred and twenty-seven patients who submitted to transepithelial photorefractive keratectomy (tPRK), femtosecond laser-assisted in-situ keratomileusis (FS-LASIK), or small-incision lenticule extraction (SMILE) were included in this study. All cases were examined with the Corvis ST preoperatively (up to 3 months) and postoperatively at 1, 3, and 6 months, and the differences in the main device parameters were assessed. The three groups were matched in age, gender ratio, corneal thickness, refractive error corrections, optical zone diameter, and intraocular pressure. They were also matched in the preoperative biomechanical metrics provided by the Corvis ST including stiffness parameter at first applanation (SP-A1), integrated inverse radius (IIR), deformation amplitude (DA), and deformation amplitude 2 mm away from apex and the apical deformation (DARatio2mm).

Results: The results demonstrated a significant decrease post-operation in SP-A1 and significant increases in IIR, DA, and DARatio2mm (p < 0.05), all of which indicated reductions in overall corneal stiffness. Inter-procedure comparisons provided evidence that the smallest overall stiffness reduction was in the tPRK group, followed by the SMILE, and then the FS-LASIK group (p < 0.05). These results remained valid after correction for the change in CCT between pre and 6 months post-operation and for the percentage tissue altered. In all three surgery groups, higher degrees of refractive correction resulted in larger overall stiffness losses based on most of the biomechanical metrics.

Conclusion: The corneal biomechanical response to the three surgery procedures varied significantly. With similar corneal thickness loss, the reductions in overall corneal stiffness were the highest in FS-LASIK and the lowest in tPRK.

Intraocular pressure after myopic laser refractive surgery measured with a new Goldmann convex prism: correlations with GAT and ORA

Background

The purpose of this study is to describe measurements using a newly developed modified Goldmann convex tonometer (CT) 1 year after myopic laser refractive surgery. Intraocular pressure (IOP) measurements were compared with IOP values obtained by Goldmann applanation tonometer (GAT), and Ocular Response Analyzer (ORA).

Methods

Prospective double-masked study performed on thirty eyes of thirty patients that underwent laser in situ keratomileusis (LASIK; n = 19) or photorefractive keratectomy (PRK; n = 11). IOP was measured before and 3 and 12 months after surgery. Intraclass correlation coefficient (ICC) and Bland-Altman plot were calculated to assess the agreement between GAT, CT, IOPg (Goldmann-correlated IOP) and IOPcc (corneal-compensated IOP) from ORA.

Results

Twelve months after LASIK, IOP measured with CT showed the best correlation with IOP measured with GAT before surgery (GATpre) (ICC = 0.886, 95% CI: 0.703–0.956) (15.60 ± 3.27 vs 15.80 ± 3.22; p < 0.000). However, a moderate correlation was found for IOP measured with IOPcc and CT 12 months after LASIK (ICC = 0.568, 95% CI: − 0.185 – 0.843) (15.80 ± 3.22 vs 12.87 ± 2.77; p < 0.004). Twelve months after PRK, CT showed a weak correlation (ICC = − 0.266, 95% CI: − 3.896 – 0.663), compared to GATpre (17.30 ± 3.47 vs 16.01 ± 1.45; p < 0.642), as well as poor correlation (ICC = 0.256, 95% CI: − 0.332 – 0.719) with IOPcc (17.30 ± 3.47 vs 13.38 ± 1.65; p < 0.182).

Conclusions

Twelve months after LASIK, IOP measured with CT strongly correlated with GAT before surgery and could therefore provide an alternative method for measuring IOP after this surgery. More studies regarding this new convex prism are needed to assess its accuracy.

Changes in Corneal Biomechanical Properties in PRK Followed by Two Accelerated CXL Energy Doses in Rabbit Eyes

PURPOSE

To evaluate whether photorefractive keratectomy (PRK) combined with the two commonly delivered energy doses in accelerated corneal cross-linking (A-CXL) could help the cornea maintain its preoperative stiffness level.

METHODS

A total of 72 corneas of 36 healthy white Japanese rabbits were randomly divided into four equal groups. The groups included an untreated control group and three that had undergone PRK. After tissue ablation, one of the latter three groups (PRK group) was left untreated, whereas the other two were exposed to riboflavin (0.22% concentration by volume) and ultraviolet-A (370 nm) with the same irradiation (30 mW/cm2) but different CXL energy doses of 1.8 J/cm2 (PXL group) and 2.7 J/cm2 (PXH group). Dynamic Scheimpflug analyzer (Corvis ST; Oculus Optikgeräte GmbH) measurements of stiffness parameter at first applanation (SP-A1), Stress-Strain Index (SSI), and other dynamic corneal response parameters were taken 3 days preoperatively and 1 month postoperatively. Subsequently, ex vivo inflation testing was performed and the tangent modulus of each specimen was estimated using an inverse analysis process.

RESULTS

In comparison to the control group, the tangent modulus at a stress of 10 kPa decreased by 8.9% in the PRK group and increased by 10.6% and 22.4% in the PXL and PXH groups, respectively. SP-A1 decreased postoperatively in the PRK group (P < .05), indicating an overall stiffness reduction of -7.4, -3.5, and -5.3 mm Hg/mm in PRK, PXL, and PXH groups, respectively. The material stiffness parameter SSI remained almost unchanged in the PRK group (P = .989), increased slightly in the PXL group (8.3%, P = .077), and increased significantly in the PXH group (11.1%) (P < .05).

CONCLUSIONS

Biomechanical deterioration following PRK was significant and could not be fully compensated for by ACXL with either 1.8 or 2.7 J/cm2 doses. The increased value of corneal overall stiffness was higher in A-CXL with 2.7 J/cm2 energy than with 1.8 J/cm2 energy. [J Refract Surg. 2021;37(12):853-860.].

Time-varying regularity of changes in biomechanical properties of the corneas after removal of anterior corneal tissue

Background

The corneal biomechanical properties with the prolongation of time after corneal refractive surgery are important for providing a mechanical basis for the occurrence of clinical phenomena such as iatrogenic keratectasia and refractive regression. The aim of this study was to explore the changes of corneal elastic modulus, and stress relaxation properties from the 6-month follow-up observations of rabbits after a removal of anterior corneal tissue in simulation to corneal refractive surgery.

Methods

The anterior corneal tissue, 6 mm in diameter and 30–50% of the original corneal thickness, the left eye of the rabbit was removed, and the right eye was kept as the control. The rabbits were normally raised and nursed for 6 months, during which corneal morphology data, and both of corneal hysteresis (CH) and corneal resistance factor (CRF) were gathered. Uniaxial tensile tests of corneal strips were performed at months 1, 3, and 6 from 7 animals, and corneal collagen fibrils were observed at months 1, 3, and 6 from 1 rabbit, respectively.

Results

Compared with the control group, there were statistical differences in the curvature radius at week 2 and month 3, and both CH and CRF at months 1, 2, and 6 in experiment group; there were statistical differences in elastic modulus at 1, 3, and month 6, and stress relaxation degree at month 3 in experiment group. The differences in corneal elastic modulus, stress relaxation degree and the total number of collagen fibrils between experiment and control groups varied gradually with time, and showed significant changes at the 3rd month after the treatment.

Conclusions

Corneas after a removal of anterior corneal tissue undergo dynamic changes in corneal morphology and biomechanical properties. The first 3 months after treatment could be a critical period. The variation of corneal biomechanical properties is worth considering in predicting corneal deformation after a removal of anterior corneal tissue.

Changes in Intraocular Pressure and Factors that Influence such Changes after FS-LASIK and SMILE

Purpose: We compared the intraocular pressure (IOP) changes and the relationships thereof with corneal biomechanics after small incision lenticule extraction (SMILE) and femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK).Methods: A total of 234 eyes of 117 patients who underwent FS-LASIK surgery and 244 eyes of 122 patients who underwent SMILE surgery were included in this retrospective study. Data were collected preoperatively, and at 1, 3, and 6 months postoperatively, including IOP measured via non-contract tonometry, central corneal thickness (CCT), and mean keratometry (Km), flat keratometry (Kf), and steep keratometry (Ks) data obtained using a dual Scheimpflug analyzer. Between-group differences were compared. Simple linear regression analysis was used to determine the relationship between changes in the IOP and the various parameters.Results: The IOP changes were 5.41 ± 2.28 mmHg in the FS-LASIK group and 4.77 ± 2.38 mmHg in the SMILE group, thus significantly different (p = 0.004). The IOP did not change significantly from 1 to 6 months after FS-LASIK but increased significantly from 12.07 ± 1.77 mmHg at 3 months postoperatively to 12.77 ± 1.89 mmHg at 6 months postoperatively after SMILE (p = 0.002). All of the preoperative spherical equivalent and IOP, and changes in the Km, Kf, Ks, and CCT, were significantly associated with the IOP changes in both groups. The preoperative IOP evidenced the strongest relationship with the IOP changes in both groups (R2 = 0.414 in the FS-LASIK group, R2 = 0.292 in the SMILE group).Conclusions: The IOP fell significantly after FS-LASIK and SMILE surgery, significantly more so after FS-LASIK than SMILE. The preoperative IOP exhibited the greatest influence on the IOP decreases after both FS-LASIK and SMILE.

Clinical Outcomes of Laser Asymmetric Keratectomy to Manage Postoperative Adverse Effects–A Retrospective Clinical Trial

Background:

Laser asymmetric keratectomy reduces the regional asymmetry of corneal thickness.

Objective:

We aimed to describe the clinical outcomes of laser asymmetric keratectomy keratectomy with laser refractive surgery performed to resolve the adverse effects following ophthalmic surgeries.

Methods:

We compared the preoperative and postoperative outcomes and complaints of blurring after performing laser asymmetric keratectomy with laser refractive surgery in 24 eyes of 16 patients with a deviation sum in corneal thickness in four directions >80 µm. Laser asymmetric keratectomy with laser refractive surgery, with full integration of the Vision Up software, was used to analyze the corneal thickness deviation, employed selective laser ablation to create central symmetry on the thicker cornea to reduce regional asymmetry of corneal thickness, simultaneously correcting the refractive power and myopic shift. The pre-and postoperative clinical and topographic findings were analyzed.

Results:

The patients’ age was 37.57±22.30 (range, 23–65) years. The follow-up period was 16.56±3.23 months. The spherical equivalent (p=0.026), sphere (p=0.022), uncorrected distance visual acuity (LogMAR, p=0.045), blurring score (p=0.000), central corneal thickness (p=0.024), sum of deviations in corneal thickness in four directions (p=0.02), distance between the maximum posterior elevation and visual axis (p=0.04), and kappa angle (p=0.031), significantly decreased postoperatively. The efficiency and safety indices were 0.96±0.11 and 1.00, respectively. There was no myopic regression or blurred vision postoperatively.

Conclusion:

Performing laser asymmetric keratectomy with laser refractive surgery improved corneal symmetry and visual acuity and reduced blurring.

The effect of central corneal thickness on Goldmann tonometry: a retrospective study

PURPOSE

To determine the relation between central corneal thickness and intraocular pressure.

PATIENTS AND METHODS

This retrospective study investigated 112 eyes from 56 individuals who underwent photorefractive keratectomy in a single private medical center between May 2018 and September 2019. Intraocular pressure readings were obtained with Goldmann applanation tonometry, and central corneal thickness measurements were evaluated preoperatively. All the examinations were repeated at 3 and 6 months postoperative.

RESULTS

At 3 and 6 months postoperative, the mean intraocular pressure was only slightly reduced from baseline (mean reduction of 0.6 ± 2.0 mmHg, P < 0.001 and 0.73 ± 2.14 mmHg, P < 0.001, respectively). The change in intraocular pressure following photorefractive keratectomy was not clinically significant, and this change was not correlated with postoperative central corneal thickness at 3 months (p = 0.620, r = 0.047).

CONCLUSION

This study showed that the change in intraocular pressure following photorefractive keratectomy was not clinically significant, and ruled out a correlation in this context between the change in central corneal thickness and the delta intraocular pressure. Our results might question the axiom between central corneal thickness and intraocular pressure and may thus challenge the current clinical setting for evaluating glaucoma.

Measurement of In Vivo Biomechanical Changes Attributable to Epithelial Removal in Keratoconus Using a Noncontact Tonometer

PURPOSE

To compare the biomechanical properties of the cornea after epithelial removal in eyes with keratoconus undergoing corneal cross-linking.

METHODS

Prospective interventional case series at a university hospital tertiary referral center. Corneal biomechanical properties were measured in patients with keratoconus undergoing corneal cross-linking, immediately before and after epithelial debridement by using a dynamic ultrahigh-speed Scheimpflug camera equipped with a noncontact tonometer.

RESULTS

The study comprised 45 eyes of 45 patients with a mean age of 19.6 ± 4.9 years (range 14-34). The cornea was found to be 23.7 ± 15.7 μm thinner after epithelial removal (P < 0.01). Corneal stiffness was reduced after epithelial removal as demonstrated by a significant decrease of parameters such as stiffness parameter A1 (12.31, P < 0.01), stiffness parameter-highest concavity (2.25, P < 0.01), A1 length (0.13 mm, P = 0.04), highest concavity radius of curvature (0.26 mm, P = 0.01), highest concavity time (0.22 ms, P = 0.04) and an increase in A1 velocity (-0.01 m/s, P = 0.01), A1 deformation amplitude (-0.03 mm, P ≤ 0.01), A1 deflection length (-0.32 mm, P < 0.01), A2 deformation amplitude (-0.03 mm, P = 0.01), and A2 deflection length (-1.00 mm, P < 0.01). There were no significant differences in biomechanical intraocular pressure (0.15 mm Hg, P = 0.78), deformation amplitude (0.03, P = 0.54), maximum inverse radius (-0.01 mm, P = 0.57), and whole eye movement length (-0.02 mm, P = 0.12).

CONCLUSIONS

Dynamic ultrahigh-speed Scheimpflug camera equipped with a noncontact tonometer offers an alternative method for in vivo measurements of the epithelial layer's contribution to corneal biomechanical properties. Our results suggest that corneal epithelium may play a more significant role in corneal biomechanical properties in patients with keratoconus than previously described.

Postoperative corneal biomechanics and influencing factors during femtosecond-assisted laser in situ keratomileusis (FS-LASIK) and laser-assisted subepithelial keratomileusis (LASEK) for high myopia

The purpose of this study is to compare the postoperative corneal biomechanics and assess the influence factors after femtosecond-assisted laser in situ keratomileusis (FS-LASIK) and laser-assisted subepithelial keratomileusis (LASEK) for high myopia. In this retrospective study, patients who completed 1-year follow-up were included. The corneal biomechanical parameters, including deformation amplitude ratio 2.0 mm (DA ratio 2.0 mm), integrated inverse radius (IntInv Rad), stiffness parameter at first applanation (SP-A1), and Ambrosio relational thickness through the horizontal meridian (ARTh), were measured with Corvis STII. We also investigated the relationship between these biomechanics and preoperative or intraoperative variables. Thirty eyes had FS-LASIK and 30 eyes had LASEK. The changes in DA ratio 2.0 mm, IntInv Rad, and SP-A1 after surgery were significantly smaller in the LASEK group than in the FS-LASIK group, while the change in the ARTh was not significantly different between groups. No significant differences were detected in the mean values of postoperative DA ratio 2.0 mm, IntInv Rad, and SP-A1 between LASEK and FS-LASIK, while significant difference was detected in the mean value of postoperative ARTh. There was a significant correlation between the resident stromal bed thickness and the postoperative DA ratio 2.0 mm, IntInv Rad, or SP-A1. The postoperative ARTh has shown significant correlation with postoperative central corneal thickness and the amount of myopic correction. The effect of LASEK on corneal biomechanics was smaller than FS-LASIK when the same central corneal thickness was consumed. LASEK may be performed with a lower risk of postoperative corneal ectasia than FS-LASIK.

A Novel Approach to Enhancement Linked Laser Asymmetric Keratectomy Using Semi-Cylindrical Ablation Pattern in Patients with Myopic Regression After Laser Refractive Surgery

Purpose

We aimed to introduce a new technique to reduce regional asymmetry of corneal thickness by assessing its effectiveness in four patients with myopic regression after laser refractive surgery (LRS).

Patients and Methods

Four patients (four eyes) with myopic regression after LRS were included in this study. A new technique of enhancement with laser epithelial keratomileusis-linked laser asymmetric keratectomy using semi-cylindrical ablation pattern (E-LAK-SCAP) with full integration of the Vision-Up software for analyzing the corneal thickness deviation can be used to create central symmetry by blocking laser ablation on the thin cornea. It reduces the regional asymmetry of the corneal thickness, thus improving corneal symmetry and correcting the refractive power and myopic shift due to E-LAK-SCAP. We measured refraction, visual acuity, intraocular pressure (IOP), central corneal thickness (CCT), corneal irregularities in the 3.0mm, and 5.0 zones on Orbscan maps, the sum of corneal thickness deviations in four directions (SUM), distance between the maximum posterior elevation (best-fit-sphere [BFS]) and the visual axis (DISTANCE), and angle kappa before and after LRS and E-LAK-SCAP. Blurring scores were measured before and after E-LAK-SCAP.

Results

The uncorrected far visual acuity (LogMAR) increased after LRS and E-LAK-SCAP. SUM (µm) increased after LRS in three cases, but decreased in all four cases after E-LAK-SCAP. DISTANCE increased after LRS, but decreased after E-LAK-SCAP. The spherical equivalent, CCT, decreased after LRS and E-LAK-SCAP. Blurring scores decreased after E-LAK-SCAP, and angle kappa was similar before and after LRS, but decreased after E-LAK-SCAP. IOP was similar before and after both LRS and E-LAK-SCAP.

Conclusion

E-LAK-SCAP improved corneal symmetry by reducing the SUM and DISTANCE, showing good postoperative visual acuity, and blurring was reduced postoperatively. There was no myopic regression in the one-year postoperative period.

Comparison of outcomes of laser refractive surgery (LRS) alone and LRS with laser asymmetric keratectomy in patients with myopia: A retrospective study

To compare and analyze the postoperative 1-year outcomes of laser refractive surgery (LRS) alone vs LRS with laser asymmetric keratectomy (LAK), in patients with myopia, for preventing and resolving LRS complications.This retrospective study compared the preoperative and 1-year postoperative outcomes between the control and comparison groups using a sum of deviations in corneal thickness in 4 directions >80 μm. The control group included 41 patients with myopia (41 eyes) who underwent LRS. The comparison group included 33 patients (33 eyes) who received LAK-linked LRS. Age, spherical equivalent (SE), sphere, cylinder, uncorrected distance visual acuity (UDVA), pupil size, kappa angle, central corneal thickness, corneal irregularity in the 3.0 mm zone on Orbscan maps (SUM), distance between the maximum posterior elevation (best-fit-sphere) and the visual axis (DISTANCE), postoperative blurring scores, frequency of postoperative myopic regression, and efficiency index were compared.Preoperative age (P = .198), SE (P = .686), sphere (P = .562), cylinder (P = .883), UDVA (P = .139), pupil size (P = .162), kappa angle (P = .807), central corneal thickness (P = .738), corneal irregularity (P = .826), SUM (P = .774), and DISTANCE (P = .716) were similar between the 2 groups. The 1-year postoperative SE (P = .024), sphere (P = .022), corneal irregularity (P = .033), SUM (P = .000), DISTANCE (P = .04), blurring scores (P = .000), and frequency of postoperative myopic regression (P = .004) were significantly decreased in the comparison group compared to the control group. UDVA (P = .014) and the efficiency index (P = .035) were higher in the comparison group.LAK with LRS improved corneal symmetry by reducing the SUM and DISTANCE. UDVA and efficiency index were also improved and blurring and myopic regression were reduced postoperatively.

Excimer laser tissue interactions in the cornea

Excimer lasers induces significant changes to corneal structure and corneal biomechanics. The aim of this paper is to describe all laser-tissue interactions which are relevant for clinical practice, particularly, we will focus on laser ablations profiles, causes of regression and haze and prevention of those. At last the manuscript will describe the impact on corneal biomechanics of different Laser Vision Corrections techniques.

Agreement of Corrected Intraocular Pressure Values Between Corvis ST and Pentacam in Patients With Keratoconus, Subclinical Keratoconus, and Normal Cornea

PURPOSE

To analyze the agreement of corrected intraocular pressure (IOP) values between Corvis ST (ΔIOP1) and Pentacam (ΔIOP2) in patients with keratoconus (KC), subclinical KC (sub-KC), and normal cornea.

METHODS

In total, 235 eyes were divided into KC, sub-KC, and control groups. Differences in ΔIOP1 (biomechanically corrected IOP minus uncorrected IOP) and ΔIOP2 (central corneal thickness-corrected amounts of IOP) were analyzed within and among groups. Topographical and biomechanical differences were compared among the 3 groups. Factors affecting differences between ΔIOP1 and ΔIOP2 were analyzed. Agreement analysis of ΔIOP2 and ΔIOP1 was performed by Bland-Altman plots for all 3 groups.

RESULTS

Mean ΔIOP1 was highest in the KC group (1.23 ± 0.84 mm Hg), followed by sub-KC and control groups (all P < 0.05). Deformation amplitude ratio at 2 mm (DA-2 mm), integrated radius, stiffness parameter at first applanation, and Corvis biomechanical index values significantly differed between sub-KC and control groups. The differences between ΔIOP1 and ΔIOP2 were affected by stiffness parameter at first applanation, after adjusting for central corneal thickness and age, in all 3 groups. The lowest agreement between ΔIOP2 and ΔIOP1 was observed in the KC group (mean difference: 1.90 mm Hg; 95% limit of agreement ranged from -0.2 to 3.9 mm Hg).

CONCLUSIONS

Among the 3 groups in this study, the KC group exhibited the worst consistency between ΔIOP2 and ΔIOP1. For the sub-KC and control groups, corrected IOP values derived by Pentacam were similar to Corvis ST. Ophthalmologists should carefully consider the mechanical properties of eyes with KC during IOP management.

Changes in Intraocular Pressure after Transepithelial Photorefractive Keratectomy and Femtosecond Laser In Situ Keratomileusis

PURPOSE

To investigate the changes in intraocular pressure (IOP) and biomechanically corrected IOP (bIOP) in patients undergoing transepithelial photorefractive keratectomy (TPRK) and femtosecond laser in situ keratomileusis (FS-LASIK) and to determine the effects of preoperative biomechanical factors on IOP and bIOP changes after FS-LASIK and TPRK.

DESIGN

A retrospective comparative study.

METHODS

We retrospectively investigated the IOP and corneal biomechanical changes in 93 eyes undergoing FS-LASIK and 104 eyes undergoing TPRK in a clinical setting. Preoperative and postoperative data on ophthalmic and Corvis ST examinations, in vivo Young's modulus, and noncontact tonometry were analyzed. Marginal linear regression models with generalized estimating equations were used for intragroup and intergroup comparisons of IOP and bIOP changes.

RESULTS

In the univariate model, IOP reduction after FS-LASIK was 2.49 mmHg higher than that after TPRK. In addition, bIOP reduction after FS-LASIK was 1.85 mmHg higher than that after TPRK. In the multiple regression model, we revealed that IOP reduction after FS-LASIK was 1.75 mmHg higher than that after TPRK. Additionally, bIOP reduction after FS-LASIK was 1.64 mmHg higher than that after TPRK. Postoperative changes in bIOP were less than those in IOP. In addition, Young's modulus and CBI had no significant effect on postoperative IOP and bIOP changes. We establish a biomechanically predictive model using the available data to predict postoperative IOP and bIOP changes after TPRK and FS-LASIK.

CONCLUSIONS

Reductions in IOP and bIOP after FS-LASIK were 1.75 mmHg and 1.64 mmHg, respectively, more than those after TPRK, after adjustment for confounders. We revealed that the type of refractive surgery and peak distance (PD) were significant predictors of postoperative IOP and bIOP changes. By contrast, depth of ablation showed a significant effect on only IOP changes.

Changes in ocular biomechanics after treatment for active Graves' orbitopathy

PURPOSE

To evaluate the changes in ocular biomechanical properties in active moderate-to-severe Graves' orbitopathy (GO) after intravenous glucocorticoids (IVGCs), and to clarify correlations between clinical findings and ocular biomechanical properties.

METHODS

A prospective study. A total of 20 consecutive GO patients and 20 age- and sex-matched healthy control subjects were included. GO was diagnosed on the basis of the recommendation by the European Group on Graves' Orbitopathy (EUGOGO), and disease activity was assessed by the clinical activity score (CAS). Patients were assigned to receive once weekly IVGCs (0.5 g, then 0.25 g, 6 weeks each). All participants received a full ophthalmic examination and biomechanical evaluation was performed with dynamic Scheimpflug analyzer (Corvis ST) at baseline and 12th weeks after therapy.

RESULTS

The biomechanically corrected intraocular pressure (bIOP) in GO patients was significantly higher than that in healthy subjects. In contrast, the whole eye movement (WEM) in GO patients was significantly lower than in healthy subjects after adjusting for bIOP. The CAS, NOSPECS score, and exophthalmos were significantly positively correlated with the bIOP and negatively correlated with the WEM after adjusting for bIOP, CCT and age. The WEM significantly increased, whereas bIOP significantly decreased after IVGCs (P < 0.001, P = 0.001 respectively). The overall response rate at the 12th week was 85% (17 of 20).

CONCLUSIONS

The changes of ocular biomechanical properties measured by Corvis ST were an objective indicator of inflammatory activity and severity of GO. Combining CAS and ocular biomechanical properties could better evaluate the therapeutic outcome of active moderate-to-severe GO.

Advances in Imaging Technology of Anterior Segment of the Eye

Advances in imaging technology and computer science have allowed the development of newer assessment of the anterior segment, including Corvis ST, Brillouin microscopy, ultrahigh-resolution optical coherence tomography, and artificial intelligence. They enable accurate and precise assessment of structural and biomechanical alterations associated with anterior segment disorders. This review will focus on these 4 new techniques, and a brief overview of these modalities will be introduced. The authors will also discuss the recent advances in research regarding these techniques and potential application of these techniques in clinical practice. Many studies on these modalities have reported promising results, indicating the potential for more detailed comprehensive understanding of the anterior segment tissues.

Association between Corneal Stiffness Parameter at the First Applanation and Keratoconus Severity

Objective

The study aimed to evaluate the character of corneal stiffness parameter at the first applanation (SP-A1) in normal and keratoconus eyes and explore the association between SP-A1 and keratoconus severity indicators.

Methods

A total of 351 normal and 351 keratoconus eyes were included in the current study. Keratoconus was diagnosed according to the corneal topography map and slit-lamp examination. The severity of keratoconus was classified to mild (steep keratometry (Ks) < 48D), moderate (48 ≤ Ks < 55D), and severe (Ks ≥ 55D). The SP-A1 was measured using the Corvis ST software. The correlation analyses and receiver operating characteristic (ROC) curve were performed in the current analysis.

Results

The SP-A1 values of keratoconus were lower than that of normal eyes (72.11 (57.02, 83.08) mmHg/mm vs 110.89 (100.45, 122.47) mmHg/mm, P < 0.001). With the severity of keratoconus increasing, the SP-A1 decreased and the value of SP-A1 was 79.54 (70.30, 90.93) mmHg/mm, 65.11 (53.14, 77.46) mmHg/mm, and 47.59 (37.50, 62.14) mmHg/mm in mild, moderate, and severe keratoconus eyes, respectively (P < 0.001). The negative association between SP-A1 and Ks was found in mild, moderate, and severe keratoconus eyes (r _mild = -0.171, r _moderate = -0.317, r _severe = -0.288, all P < 0.05). A positive association between SP-A1 and the thinnest corneal thickness (TCT) was found in all eyes (r_normal = 0.687, r _mild = 0.519, r _moderate = 0.488, r _severe = 0.382, all P < 0.05). SP-A1 was found to be statistically positively associated with intraocular pressure (IOP), biomechanical corrected IOP (bIOP), time from the initiation of air puff until the first applanation (A1T), corneal velocity at the second applanation (A2V), and negatively associated with deformation amplitude (DA), peak distance (PD), corneal velocity at the first applanation (A1V), time from the initiation of air puff until the second applanation (A2T), and DA Ratio Max [2 mm] both in normal and keratoconus eyes (all P < 0.05). The ROC analysis indicated that the AUC (95% CI) of SP-A1 was 0.952 (0.934-0.967) and 0.930 (0.904-0.951) in detecting keratoconus eyes and mild keratoconus eyes from normal eyes, respectively.

Conclusions

The SP-A1 value decreased while the keratoconus severity increased. It was lower in keratoconus than that in normal eyes and could be helpful in identifying keratoconus eyes from normal eyes. Further researches would be warranted to expand the clinical utility of SP-A1.

Similar Biomechanical Cross-linking Effect After SMILE and PRK in Human Corneas in an Ex Vivo Model for Postoperative Ectasia

PURPOSE

To evaluate the biomechanical effect of corneal cross-linking (CXL) in paired human corneas following small incision lenticule extraction (SMILE) or photorefractive keratectomy (PRK) in an ex vivo model for postoperative ectasia.

METHODS

Twenty-six paired human corneas preserved in tissue culture medium were equally divided into two groups: right and left corneas were treated with PRK and SMILE, respectively. Corneal thickness was measured in all eyes before surgery. Corneas were stretched using an extensometer with two cycles of up to 9 N (570 kPA stress), followed by accelerated CXL with irradiance of 9 mW/cm² for 10 minutes (fluence 5.4 J/cm²) in both groups. The elastic modulus was evaluated using two-dimensional stress-strain extensometry.

RESULTS

Following accelerated CXL, the ectatic cornea model showed a mean effective elastic modulus of 17.2 ± 5.3 MPa after PRK and 14.1 ± 5.0 MPa after SMILE. Although the elastic modulus in corneas previously subjected to PRK was higher, there was no significant biomechanical difference between the two groups (P = .093).

CONCLUSIONS

Under similar conditions, both experimental groups (PRK followed by CXL and SMILE followed by CXL) were characterized by similar biomechanical stability as measured experimentally on ex vivo human fellow corneas. The data suggest that, in the event of postoperative ectasia, the biomechanical improvement achieved by CXL may be similar after PRK and SMILE. [J Refract Surg. 2020;36(1):49-54].

A Review on Different Tonometers for Intraocular Pressure Measurement After Photorefractive Keratectomy or Small Incision Lenticule Extraction

This paper aims to review different tonometers for intraocular pressure measurement in eyes after photorefractive keratectomy or small incision lenticule extraction. An Entrez Pubmed search was performed on July 19th, 2020. There were 32 eligible articles investigated in the use of tonometers or biomechanical properties of cornea in post photorefractive keratectomy eyes and 11 articles investigated in post small incision lenticule extraction eyes. Corvis ST and dynamic contour tonometry were found to be accurate for intraocular pressure measurement in post photorefractive keratectomy eyes. For post small incision lenticule extraction eyes, Corvis ST was found to be more accurate than other tonometers. Other tonometers such as Goldmann applanation tonometer, noncontact tonometry, Tonopen, ocular response analyzer can also be used in post photorefractive keratectomy or small incision lenticule extraction eyes in clinical practice; however, it is important for ophthalmologists to take note of the chances of discrepancies.

Comparison of corneal biomechanical changes after LASIK and F-SMILE with CorVis ST

PURPOSE

To evaluate changes in corneal biomechanics after LASIK and F-SMILE.

SETTING

Elite Eye Center, Tanta, Egypt.

DESIGN

Prospective, randomized, unmasked, interventional comparative case series.

PATIENTS AND METHODS

A total of 60 eyes were equally divided into two groups; group A: received LASIK and group B: received F-SMILE. Pentacam and CorVis ST parameters were compared before and 6 months after procedures. P ⩽ 0.05 was used for significance of results.

RESULTS

bIOP decreased by 0.762 ± 1.211 mmHg in group A (p = 0.092), and by 2.52 ± 1.389 mmHg in group B (p < 0.001) and the difference between groups became significant (p = 0.001). A1 increased significantly in group A only (p = 0.036); while A2 decreased insignificantly in both groups. DAR increased significantly in both groups (p < 0.001, p = 0.022), but the difference between groups remained insignificant (p = 0.461). IR increased significantly in group A only (p < 0.001) and the difference between groups became significant (p = 0.026). ARTH decreased in both groups (p = 0.245, p = 0.695) and the difference remained insignificant (p = 0.928). SP-A1 decreased in group A by 8.89 ± 1.38 mmHg/mm (p = 0.637), and by 32.6 ± 4.39 mmHg/mm in group B (p < 0.001) and the postoperative difference between groups was significant (p = 0.013). CBI increased significantly in group A (p < 0.001), but insignificantly in group B (p = 0.098) and postoperative difference between groups was highly significant (p < 0.001). A significant correlation was found between change in CCT and corresponding changes in DAR, IR, and ARTH after surgery in both groups.

CONCLUSION

The significant differences between groups postoperatively as regards bIOP (p = 0.001), IR (p = 0.026), SP-A1 (p = 0.013), and CBI (p < 0.001) indicate stiffer corneas after F-SMILE and suggest less influence on corneal biomechanics than LASIK.

Corneal Biomechanical Properties after Small Incision Lenticule Extraction Surgery on Thin Cornea

PURPOSE

To explore the biomechanical changes in thin corneas after the small incision lenticule extraction (SMILE) surgery.

METHODS

This prospective survey screened patients scheduled for the SMILE surgery from November 2017 to March 2018. Patients with thin corneas (central corneal thickness [CCT] ≤500 μm) and those with normal corneal thickness (CCT > 500 μm) were enrolled. Corneal biomechanics were examined by the ocular response analyser and Corvis ST, preoperatively and at 1 day, 3 weeks, and 3 months postoperatively.

RESULTS

Twenty-seven patients (46 eyes) with mean spherical equivalent of -5.1 ± 1.7 D were assigned to the thin cornea group, and 28 (45 eyes) cases with mean spherical equation of -5.3 ± 1.8 D were assigned to the control group. Safety and efficacy did not differ between the two age- and refraction-matched groups (safety index, 1.15 ± 0.14 vs 1.14 ± 0.17 [P = .7]; efficacy index, 1.11 ± 0.13 vs 1.16 ± 0.22 [P = .2]). All biomechanical parameters changed significantly after SMILE. The thin cornea group showed less decrease in the second applanation time (A2 Time), stiffness parameters at first applanation (SP-A1). Pooling data from two groups, lower CCT was correlated with less A2 Time change (r = 0.37, P < .01) and less SP-A1 change (r = -0.33, P < .01). Less bIOP change was correlated with higher residual stromal thickness index and thicker CCT (P = .003, R² = 0.12).

CONCLUSION

Over a short-term observation period, less corneal biomechanic deterioration may have contributed to the safety of SMILE on thin cornea.

Comparison between Surgical Outcomes of LASIK with and without Laser Asymmetric Keratectomy to Avoid Conventional Laser Refractive Surgery Adverse Effects

This study compared one-year postoperative outcomes of laser refractive surgery combined with laser asymmetric keratectomy (LAK) and laser in situ keratomileusis (LASIK)for myopia correction in middle-aged patients (aged 40–49 years) with a total corneal thickness deviation (summed across four directions) ≥ 80 microns. The control group (n = 26; 52 eyes) underwent LASIK; the comparison group (n = 26; 52 eyes) underwent combined laser refractive surgery and LAK. Age, spherical equivalence, uncorrected visual acuity (near and far), corneal irregularity on the Orbscan map, sum of corneal thickness deviations in four directions, corneal thickness distribution, distance between the maximum posterior elevation (best-fit sphere; BFS) and visual axis, and postoperative blurring scores were analysed retrospectively between the groups. Both groups had similar preoperative findings. Postoperatively, the sum of corneal thickness deviations in four directions (p = 0.000), distance between maximum posterior elevation (BFS) and visual axis (p = 0.003),blurring score (p = 0.001), and corneal irregularity in the 3.0 and 5.0 mm zones on the Orbscan map (p = 0.033 and p < 0.0001, respectively) were significantly lower in the comparison group (p = 0.000). LAK reduced total corneal thickness deviation, improved corneal symmetry, and reduced blurring scores significantly, one-year postoperatively. LAK could resolve shortcomings of LASIK, producing better surgical outcomes.

Effectiveness of 4 tonometers in measuring IOP after femtosecond laser-assisted LASIK, SMILE, and transepithelial photorefractive keratectomy

PURPOSE

To test the performance of 4 tonometers in estimating intraocular pressure (IOP) after 3 forms of refractive surgery.

SETTING

Eye Hospital, Wenzhou Medical University, China.

DESIGN

Prospective case series.

METHODS

Patients matched for preoperative age, corneal thickness, and myopic correction enrolled for femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK), small-incision lenticule extraction (SMILE), or transepithelial photorefractive keratectomy (tPRK) were included in the study. For each patient, 4 measurements of IOP were obtained preoperatively and 3 months postoperatively, using the Goldmann applanation tonometer (GAT-IOP), the Dynamic Contour Tonometer (DCT-IOP), corneal-compensated IOP (IOPcc) from the Ocular Response Analyzer, and biomechanically corrected IOP (bIOP) from the Corvis ST. Overall corneal stiffness was also estimated based on the stiffness parameter (SP-A1) provided by the Corvis ST.

RESULTS

The study included 144 eyes of 144 patients. Among the 3 procedures, the smallest variances between preoperative and postoperative IOP estimates and SP-A1 values were observed with the tPRK, followed by SMILE and FS-LASIK. In the tPRK group, no significant differences were observed in both bIOP (-0.18 ± 1.63 mm Hg) and DCT-IOP (-.64 ± 2.34 mm Hg), whereas they were larger and significant in GAT-IOP (-1.78 ± 2.29 mm Hg) and IOPcc (-2.77 ± 1.84 mm Hg). In FS-LASIK and SMILE groups, although there were similar significant reductions in IOP postoperatively, these reductions were still lower in bIOP and DCT-IOP than those in GAT-IOP and IOPcc.

CONCLUSIONS

The bIOP and DCT-IOP were the least affected IOP estimates between the 3 refractive surgery procedures considered. It was evident that tPRK produced significantly smaller reductions in IOP readings than did FS-LASIK and SMILE.

Intraocular pressure changes and corneal biomechanics after hyperopic small-incision lenticule extraction

Background

We aimed to compare the intraocular pressure (IOP) measurements by a dynamic Scheimpflug analyzer (Corvis ST), a non-contact tonometer, and an ocular response analyzer after hyperopic small-incision lenticule extraction (SMILE).

Methods

Thirteen patients who underwent hyperopic SMILE in one eye were enrolled prospectively. IOP and corneal biomechanical parameters were measured preoperatively and at 1 week, 1 month, and 3 months postoperatively with a non-contact tonometer (IOP_NCT), Corvis ST (biomechanical corrected IOP [bIOP]), and ocular response analyzer (Goldmann-correlated intraocular pressure [IOPg] and cornea compensated IOP [IOPcc]). A linear mixed model was used to compare the IOPs and biomechanical values among methods at each time point.

Results

IOP_NCT, IOPg, and IOPcc dropped significantly after surgery, with the amplitude being 3.15 ± 0.48 mmHg, 5.49 ± 0.94 mmHg, and 4.34 ± 0.97 mmHg, respectively, at the last follow-up visit. IOP_NCT decreased by 0.11 ± 0.06 mmHg per μm of excised central corneal thickness. bIOP did not change significantly after surgery. Preoperatively, no difference was found among the four measurements (P > 0.05). Postoperatively, IOP_NCT and bIOP were higher than IOPg and IOPcc. bIOP was independent of cornea thickness at last follow-up visit, whereas it correlated significantly with corneal biomechanics similar to the other three IOP values.

Conclusion

bIOP is a relative accurate measure of IOP after hyperopic SMILE.

Evaluation of new Corvis ST parameters in normal, Post-LASIK, Post-LASIK keratectasia and keratoconus eyes

The aim of this study was to evaluate the distribution of new Corneal Visualisation Scheimpflug Technology (Corvis ST) parameters in normal, Post-laser in situ keratomileusis (LASIK), Post-LASIK keratectasia (KE) and keratoconus (KC) eyes, and explore the diagnostic ability of these parameters in distinguishing KE from LASIK eyes. Twenty-three normal eyes, 23 LASIK eyes, 23 KE eyes and 23 KC eyes were recruited in this study. The following new Corvis ST parameters were measured: Max Inverse Radius, deformation amplitude (DA) Ratio Max [2 mm], Pachy Slope, DA Ratio Max [1 mm], Ambrosio’s relational thickness horizontal (ARTh), Integrated Radius, stiffness parameter at first applanation (SP-A1) and Corvis biomechanical index (CBI). The general linear model, linear regression model, relation analysis and receiver operating characteristic (ROC) curve were performed. The Max Inverse Radius, DA Ratio Max [2 mm], Pachy Slope, DA Ratio Max [1 mm], Integrated Radius and CBI in LASIK eyes, KE eyes and KC eyes were higher than in normal eyes, while the ARTh and SP-A1 parameters were lower than in normal eyes. The KE eyes had higher Max Inverse Radius, DA Ratio Max [2 mm], Pachy Slope, DA Ratio Max [1 mm], Integrated Radius, and lower SP-A1 value than LASIK eyes (all P < 0.05). The central corneal thickness was related to the Pachy Slope (r = −0.485), ARTh (r = −0.766), SP-A1 (r = 0.618) in KE eyes (all P < 0.05). The area under the ROC curve of Integrated Radius, Max Inverse Radius, DA Ratio Max [2 mm] and SP-A1 were above 0.800 in identifying KE from LASIK eyes. Thus, the new Corvis ST parameters were different between LASIK and KE eyes, suggesting that they might be helpful in distinguishing KE eyes from LASIK eyes. However, a further multi-center and large sample study is necessary to confirm these findings.

Evaluation of Biomechanically Corrected Intraocular Pressure Measurements in Keratoconus and Forme Fruste Keratoconus

INTRODUCTION

Although biomechanically corrected intraocular pressure (bIOP) is available, the effectiveness of intraocular pressure (IOP) correction in keratoconus and forme fruste keratoconus (FFK) eyes has not been investigated.

OBJECTIVE

Evaluation of bIOP measurements in eyes with keratoconus and FFK.

METHODS

Forty-two eyes in 21 patients with keratoconus in one eye and FFK in the fellow eye were examined (KC/FFK group; mean age 24.62 ± 8.6 years; 16 males and 5 females). The control group consisted of 62 eyes in 31 unaffected subjects (mean age 26.26 ± 3.64 years; 15 males and 16 females). The bIOP was determined using a Scheimpflug-based tonometer (Corvis Scheimpflug Technology [Corvis ST®]) after measuring the IOP with a conventional non-contact tonometer (NIOP). The agreement between NIOP and bIOP values was examined using the Bland-Altman plot. The difference between NIOP and bIOP (bIOP correction amount) was compared between keratoconus and FFK eyes.

RESULTS

In the control group, there were no significant differences between right and left eyes in both NIOP and bIOP values (p = 0.975 and p = 0.224, respectively). In the KC/FFK group, NIOP values were significantly lower in the keratoconus eyes (9.93 ± 1.96 mm Hg) than in the FFK eyes (12.23 ± 3.03 mm Hg; p = 0.0003). There was no significant difference in bIOP values between the right and left eyes of the KC/FFK group (p = 0.168). The bIOP correction amount was significantly increased in keratoconus eyes (3.58 ± 2.12 mm Hg) compared to in FFK eyes (1.80 ± 3.32 mm Hg; p = 0.011).

CONCLUSIONS

For eyes with keratoconus and FFK, the bIOP method is effective to adjust IOP measurements based on corneal biomechanical properties.

Effectiveness of the Goldmann Applanation Tonometer, the Dynamic Contour Tonometer, the Ocular Response Analyzer and the Corvis ST in Measuring Intraocular Pressure following FS-LASIK

Purpose: To test the performance of the four tonometers in providing IOP measurements that were free of the effects of corneal biomechanics changes caused by refractive surgery.Methods: Four tonometers were employed to provide IOP measurements for 65 participants who accepted Femtosecond laser-assisted LASIK (FS-LASIK). The measurements included GAT-IOP by the Goldmann Applanation Tonometer, DCT-IOP by the Dynamic Contour Tonometer, Goldmann-correlated IOP (ORA-IOPg) and corneal-compensated IOP (ORA-IOPcc) by the Ocular Response Analyzer, and uncorrected IOP (CVS-IOP) and biomechanically corrected IOP (CVS-bIOP) by the Corvis ST. Statistical analyses were performed to assess the association of the differences in IOP caused by FS-LASIK with central corneal thickness (CCT), mean corneal curvature (Km), age, refractive error correction (REC), optical zone diameter (OZD), ablation zone diameter (AZD), residual stromal bed thickness (RSB) and RSB ratio (RSB/CCT). Multiple linear regression models were constructed to explore factors influencing IOP changes.Results: All four tonometers exhibited significant differences between IOP measurements taken pre and post-surgery except for CVS-bIOP in the low to moderate myopia group (t = 1.602, p = .12). CVS-bIOP, followed by DCT-IOP, provided the best agreement between pre and post-FS-LASIK measurements with the lowest differences in IOP and the narrowest limits of agreement. The pre-post IOP differences were also significantly associated with the reduction in CCT in only GAT-IOP, ORA-IOPg, and CVS-IOP. CVS-bIOP and ORA-IOPcc were the only measurements that were not correlated with CCT, Km or age both before and after FS-LASIK.Conclusions: The biomechanically corrected bIOP from the Corvis ST provided post-FS-LASIK measurements that were in closest agreement with those obtained before surgery. In comparison, GAT-IOP, ORA-IOPg, ORA-IOPcc, and CVS-IOP appeared to be more influenced by the changes in corneal biomechanics caused by FS-LASIK.

Biomechanically-Corrected Intraocular Pressure Compared To Pressure Measured With Commonly Used Tonometers In Normal Subjects

PURPOSE

To compare the biomechanically-corrected intraocular pressure (bIOP) measured by the Corvis ST (Oculus, Wetzlar, Germany) with IOP measurements made by other commonly used tonometers; and to test the correlations between IOP measures and central corneal thickness.

METHODS

One randomly-selected eye from each of 94 healthy subjects was assessed. The bIOP was determined by the CorVis ST and compared with the IOP measurements made by standard Goldmann Applanation Tonometer (GAT: Haag-Streit AG, Bern, Switzerland), the Icare (Icare Finland Oy, Vantaa, Finland), and the Ocular Response Analyzer (ORA-IOPcc: Reichert, New York, USA). Corneal thickness was assessed by the Oculus Pentacam. The correlation between bIOP and the other devices and between CCT were assessed using the Pearson correlation test or Spearman's rho test accordingly to the distribution of these values. The Bland-Altman method and intraclass correlation coefficients (ICC) were used to assess the agreement of bIOP results with IOP obtained with other techniques. The limits of agreement (LoA) were determined as the mean difference ±1.96 SD of the mean differences. In all tests, the significance level was considered to be 0.05.

RESULTS

Mean and SD of the bIOP were 16.11±1.66 mmHg. Significant differences were found between the bIOP and other IOP measurements (GAT, 3.02±2.60 mmHg, p<0.001, Icare, 1.51±2.95 mmHg, p<0.001, IOPcc, 1.09±1.96 mmHg, p<0.001). The lowest and highest mean differences in IOP were with the IOPcc and GAT, respectively. Interestingly, there were no significant differences in bIOP, GAT-IOP and ORA-IOPcc between the eyes with thin or thick corneal thicknesses, with Icare-IOP being the only exception (p<0.001).

CONCLUSION

The Corvis bIOP has a higher correlation with the IOPcc by ORA, which are also compensated for the effects of corneal biomechanics and have less association with corneal thickness relative to the uncorrected GAT and Icare measurements.