New parameters for evaluating corneal biomechanics and intraocular pressure after small-incision lenticule extraction by Scheimpflug-based dynamic tonometry

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.

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.

Changes in corneal biomechanical parameters in keratoconus eyes with various severities after corneal cross-linking (CXL): A comparative study

OBJECTIVES

To compare changes in corneal biomechanical parameters one year after corneal cross-linking (CXL) in keratoconus (KCN) eyes of different severities.

METHODS

Seventy-five eyes with mild, moderate, and severe grades of KCN (n = 24, 31, and 20 eyes, respectively) that were treated with CXL, based upon the standard Dresden protocol, were included. The corneal biomechanical assessment was performed using Corvis ST and Ocular Response Analyzer (ORA). Changes in Corvis's dynamic corneal response (DCR) parameters and ORA's derived parameters (corneal hysteresis (CH), and corneal resistance factor (CRF)) were assessed whilst the corneal thickness and intraocular pressure were considered as covariates.

RESULTS

There was no statistically significant difference in the corneal biomechanical parameters obtained using both devices after surgery separately in different KCN grades, except for the deformation amplitude (DA) in the severe KCN group (P = 0.017). Changes in the classic parameters of the highest concavity phase of Corvis ST (peak distance, radius, and DA) were more positive and in the newer parameters (integrated inverse radius (IIR), deformation amplitude ratio (DAR)) more negative in the severe group compared to the other groups. Also, the mean change in CH (P = 0.710), and CRF (P = 0.565), showed a negative shift in higher grades of KCN; however, there was no significant difference in the mean changes of all parameters between different groups. (P > 0.05).

CONCLUSIONS

Similar changes in the Corvis ST and ORA parameters in mild, moderate, and severe KCN indicate biomechanical stability and the effective role of CXL in stopping the progressive nature of keratoconus in eyes of varying severities one year after CXL.

Effect of eye rubbing on corneal biomechanical properties in myopia and emmetropia

Purpose: To investigate short-term changes in corneal biomechanical properties caused by eye rubbing in myopia and emmetropia and compare the different responses between the two groups. Methods: This was a prospective observational study of 57 eyes of 57 healthy subjects aged 45 years and younger. The participants were divided into myopia and emmetropia groups. All the subjects underwent eye rubbing by the same investigator using the same technique. Biomechanical parameters were recorded using the Corvis ST device before and after 1 min of eye rubbing. One week later, all the participants underwent the test again. Statistical methods were employed to compare the differences between the data from before and after the 1 min of eye rubbing and demonstrate the different responses of the two groups. Results: After 1 min of eye rubbing, smaller SP-A1 (p < 0.001), higher deformation and deflection amplitudes (p < 0.001, p = 0.012), higher peak distances (p < 0.001), earlier A1 times (p < 0.001), faster velocities (p < 0.001), and lower maximum inverse radii (p = 0.004) were observed. According to the automatic linear modeling analysis, the refractive states (B = -5.236, p = 0.010) and biomechanically corrected intraocular pressure (bIOP) (B = 0.196, p = 0.016) had influenced a decrease in the stiffness parameter at the first applanation (SP-A1). The central corneal thickness (CCT) had decreased only in the myopia group (p = 0.039). The change of SP-A1 in amplitude was larger in the myopia group than in the emmetropia group (p < 0.001). All the parameters returned to the baseline level 1 week later. Conclusion: Eye rubbing appears to alter corneal biomechanical properties temporarily and make the cornea softer, especially for myopic young patients.

Differences of Corneal Biomechanics Among Thin Normal Cornea, Forme-Fruste Keratoconus, and Cornea After SMILE

Background: To compare the corneal biomechanics of thin normal cornea (TNC) with thinnest corneal thickness (TCT) (≤500 µm), forme-fruste keratoconus (FFKC) and cornea after small incision lenticule extraction (Post-SMILE) had their central corneal thickness (CCT) matched by Corneal Visualization Scheimpflug Technology (Corvis ST).

Methods: CCT were matched in 23 eyes with FFKC, 23 eyes by SMILE in 3 months post-operatively, and 23 TNC eyes. The differences in corneal biomechanics by Corvis ST among the three groups were compared.

Results: There was no significant difference in CCT among the three groups, and the biomechanically corrected intraocular pressure (bIOP) did not differ significantly among the three groups (all p &gt; 0.05). There were significant differences in most DCR parameters between pre- and post-operatively (all p &lt; 0.05). Compared with TNC, the values of corneal deflection amplitude during the first applanation (A1DA), length at the first applanation (A1L), corneal deflection amplitude during the second applanation (A2DA), and maximum deformation amplitude (DA) decreased in 3 months after SMILE (all p &lt; 0.05), these values increased in the FFKC (all p &lt; 0.05).

Conclusion: The majority of the DCR parameters were different among the three groups. The parameters A1DA, A1L, A2DA, and DA may be different between TNC and Post-SMILE, TNC and FFKC, and Post-SMILE and FFKC.

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.

Long-Term Clinical Outcomes of Small-Incision Femtosecond Laser-Assisted Intracorneal Concave Lenticule Implantation in Patients with Keratoconus

Purpose

The purpose of this study was to evaluate the long-term prognosis of small-incision femtosecond laser-assisted intracorneal concave lenticule implantation (SFII) in correction of human keratoconus.

Methods

This was a prospective study for 11 patients who received SFII after being diagnosed as progressive keratoconus based on the Amsler–Krumeich classification system. Clinical assessment was performed for all the patients prior to and postsurgically at different time points for 5 years. These included uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), biomechanically corrected intraocular pressure (bIOP), corneal topography, anterior segment optical coherence tomography (AS-OCT), confocal microscopy, and biomechanical assessment with Corvis ST.

Results

Comparison of preoperative and 60-month postoperative UDVA and CDVA (P_60months=0.081 and 0.001, respectively), all eyes showed an improvement in CDVA. Corneal topography showed no significant changes in corneal anterior K1, K2, posterior K1, K2, posterior elevation, or corneal densitometry compared with preoperative levels (P > 0.05). Corvis ST showed that central corneal thickness (CCT) and stiffness at applanation 1 (SP-A1) were significantly greater 1 week postsurgically when compared to the baseline (P < 0.05) and remained stable thereafter. The lenticule under the AS-OCT remained transparent throughout the entire postsurgical period. Under confocal microscopy, corneal edema and an increase in cell activation and reflectivity were observed at the lenticule-stromal interface within 1 week postoperatively. These reactions gradually subsided with time within 6 months.

Conclusion

SFII is an effective procedure to prevent the progression of keratoconus due to its minimal invasiveness and capability of maintaining a steady biometry of the cornea.

One-Year Follow-Up of Corneal Biomechanical Changes After Accelerated Transepithelial Corneal Cross-Linking in Pediatric Patients With Progressive Keratoconus

Aims: This study aimed to investigate the corneal biomechanical changes and topographic outcomes of accelerated transepithelial corneal cross-linking (ATE-CXL) in pediatric progressive keratoconus.

Methods: In this prospective longitudinal study, 31 eyes of 28 pediatric patients with keratoconus (21 boys and 7 girls; mean age, 14.35 ± 2.68 years) undergoing ATE-CXL (epithelium-on procedure with 45 mW/cm² for 320 s) were included. Corvis ST was used to measure dynamic corneal response parameters at baseline and at 12 month after ATE-CXL. Corneal keratometry and corneal thickness were measured using Pentacam pre-operatively and 1, 6, and 12 month post-operatively.

Results: No serious complications occurred during or after ATE-CXL. The maximum keratometry values were 60.10 ± 7.51 D pre-operatively and 61.42 ± 8.92, 61.17 ± 7.96, and 60.02 ± 7.58 D at 1, 6, and 12 month after ATE-CXL (P > 0.05), respectively. Corneal thickness remained stable during the 12-month follow-up (P > 0.05). At post-operative 12 month, first applanation time (P < 0.001), first applanation length (P = 0.004), second applanation velocity (P = 0.014), highest concavity time (P = 0.022), and radius of curvature at highest concavity (P = 0.031) increased significantly. The value of stiffness parameter at first applanation was significantly increased from 57.70 ± 27.57 pre-operatively to 63.36 ± 27.09 at 12 months after ATE-CXL (P = 0.018).

Conclusions: ATE-CXL is safe and effective in stabilizing the progression of pediatric keratoconus. Changes in corneal biomechanical response consistent with stiffening following ATE-CXL were observed in pediatric patients with keratoconus.

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.

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 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.

Corneal Biomechanical Properties in Varying Severities of Myopia

Purpose: To investigate corneal biomechanical response parameters in varying degrees of myopia and their correlation with corneal geometrical parameters and axial length.

Methods: In this prospective cross-sectional study, 172 eyes of 172 subjects, the severity degree of myopia was categorized into mild, moderate, severe, and extreme myopia. Cycloplegic refraction, corneal tomography using Pentacam HR, corneal biomechanical assessment using Corvis ST and Ocular Response Analyser (ORA), and ocular biometry using IOLMaster 700 were performed for all subjects. A general linear model was used to compare biomechanical parameters in various degrees of myopia, while central corneal thickness (CCT) and biomechanically corrected intraocular pressure (bIOP) were considered as covariates. Multiple linear regression was used to investigate the relationship between corneal biomechanical parameters with spherical equivalent (SE), axial length (AXL), bIOP, mean keratometry (Mean KR), and CCT.

Results: Corneal biomechanical parameters assessed by Corvis ST that showed significant differences among the groups were second applanation length (AL2, p = 0.035), highest concavity radius (HCR, p < 0.001), deformation amplitude (DA, p < 0.001), peak distance (PD, p = 0.022), integrated inverse radius (IR, p < 0.001) and DA ratio (DAR, p = 0.004), while there were no significant differences in the means of pressure-derived parameters of ORA between groups. Multiple regression analysis showed all parameters of Corvis ST have significant relationships with level of myopia (SE, AXL, Mean KR), except AL1 and AL2. Significant biomechanical parameters showed progressive reduction in corneal stiffness with increasing myopia (either with greater negative SE or greater AXL), independent of IOP and CCT. Also, corneal hysteresis (CH) or ability to dissipate energy from the ORA decreased with increasing level of myopia.

Conclusions: Dynamic corneal response assessed by Corvis ST shows evidence of biomechanical changes consistent with decreasing stiffness with increasing levels of myopia in multiple parameters. The strongest correlations were with highest concavity parameters where the sclera influence is maximal.

Relationship between corneal stiffness parameters and lamina cribrosa curvature in normal tension glaucoma

PURPOSE

To evaluate the relationship between corneal biomechanical parameters and lamina cribrosa (LC) curvature in normal tension glaucoma (NTG).

METHODS

95 eyes of 56 NTG patients were enrolled in this prospective, observational study. Corneal biomechanical parameters, including stiffness parameters at applanation 1 (SP-A1), deformation amplitude ratio (DA ratio), inverse concave radius and biomechanically corrected intraocular pressure estimate (bIOP), were captured using the Corneal Visualization Scheimpflug Technology instrument (Corvis-ST). LC curvature was evaluated by mean adjusted LC curvature index (maLCCI) averaged by the measurements on 12 radial B-scan images obtained using swept-source optical coherence tomography (SS-OCT). Linear mixed models were constructed to assess the relationship between corneal biomechanical parameters and LC curvature.

RESULTS

The mean age of participants was 51.04 ± 13.74 years (range, 24-82 years). The SP-A1 and maLCCI were 93.50 ± 13.82 mm Hg/mm and 7.57 ± 1.58, respectively. In univariate and multivariate analysis, SP-A1 (p < 0.001 and p = 0.001) and age (p = 0.010 and p = 0.024) were both significantly associated with maLCCI. The LC curvature increased with softer cornea demonstrated by lower SP-A1 and younger eyes. There was no statistical significance interaction between SP-A1 and age (p = 0.194).

CONCLUSIONS

The greater posterior LC curvature was associated with lower corneal stiffness parameters and younger eyes in NTG patients.

CLINICAL TRIAL REGISTRATION

Chinese Clinical Trial Registry, ChiCTR1900021465.

Influence of Cap Thickness on Corneal Curvature and Corneal Biomechanics After SMILE: A Prospective, Contralateral Eye Study

PURPOSE

To evaluate the impact of cap thickness for small incision lenticule extraction (SMILE) on changes in corneal curvature and biomechanics.

METHODS

One hundred eyes (50 patients) were enrolled in this prospective contralateral eye study. The difference in manifest refraction spherical equivalent (MRSE) in the same patient was less than 0.50 diopters. SMILE was performed with a randomized cap thickness of 110 µm in one eye and 140 µm in the other eye. MRSE, uncorrected distance visual acuity (UDVA), and corneal curvature and biomechanics were evaluated. The anterior and posterior surfaces of the corneal curvature (mean keratometry [Km] values 2, 4, and 6 mm from the pupil center) were measured by Pentacam HR (Oculus Optikgeräte, Wetzlar, Germany) and changes in corneal biomechanics at 6 months postoperatively by Scheimpflug technology.

RESULTS

There was no significant between-group difference in UDVA or MRSE postoperatively. Postoperative changes in Km at the anterior surface (ΔKm-ant) in the 4-mm zone were significantly higher in the 110-µm group than in the 140-µm group at 1 day and 1, 3, and 6 months postoperatively (P = .043, .045, .003, and .049, respectively); at 3 months, the ΔKm-ant in the 6-mm zone was higher in the 110-µm group (P = .035). The changes in second appla-nation time, deformation amplitude, and integrated radius were significantly less in the 110-µm group (P = .031, .049, and < .001, respectively).

CONCLUSIONS

A thicker corneal cap caused less change in anterior surface curvature after SMILE for moderate or low myopia, with no significant difference in UDVA and MRSE. [J Refract Surg. 2020;36(2):82-88.].

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.

Age distribution and associated factors of cornea biomechanical parameter stress-strain index in Chinese healthy population

Background

To investigate the new cornea biomechanical parameter stress-strain index (SSI) in Chinese healthy people and the factors associated with SSI.

Methods

A total of 175 eyes from 175 participants were included in this study. Axial length was measured with the Lenstar LS-900. Pentacam measured curvature of the cornea and anterior chamber volume (ACV). Cornea biomechanical properties assessments were performed by corneal visualization Scheimpflug technology (Corvis ST). Student’s t-test, one-way ANOVA, partial least square linear regression (PLSLR) and linear mixed effects (LME) model were used in the statistical analysis.

Results

The mean (±SD) SSI was 1.14 ± 0.22 (range, 0.66–1.78) in all subjects and affected by age significantly after age of 35 (P < 0.05). In LME models, SSI was significantly associated with age (β = 0.526, P < 0.001), axial length (AL) (β = − 0.541, P < 0.001), intraocular pressure (IOP) (β = 0.326, P < 0.001) and steepest radius of anterior corneal curvature (RsF) (β = 0.229, P < 0.001) but not with ACV, biomechanical corrected intraocular pressure (bIOP), flattest radius of anterior corneal curvature (RfF) or central corneal thickness (CCT) (P > 0.05 for each).

Conclusions

SSI increased with age after the age of 35. In addition to age, SSI was positively correlated with RsF and IOP, while negatively correlated with AL.

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 of Biomechanically Corrected Intraocular Pressure Obtained by Corvis ST and Goldmann Applanation Tonometry in Patients With Open-angle Glaucoma and Ocular Hypertension

PRECIS

Biomechanically corrected intraocular pressure (bIOP) measurements provided by the new Corvis ST (CST) were higher than measurements obtained with Goldman applanation tonometry (GAT) in eyes with ocular hypertension (OHT) or open-angle glaucoma (OAG).

PURPOSE

To compare bIOP obtained with a new version of CST with GAT measurements in patients with OAG and OHT, and to identify factors affecting IOP measurement differences between these methods.

METHODS

A total of 122 eyes with OAG or OHT were enrolled. Sixty eyes were treated with prostaglandin analogs (PGA) and 62 eyes with selective laser trabeculoplasty (SLT). IOP was measured with CST, followed by GAT. Central corneal thickness (CCT) was measured with ultrasound pachymetry. Measurements obtained with these 2 approaches were compared.

RESULTS

The overall mean IOP was 15.2±3.0 mm Hg and 14.1±3.2 mm Hg with the CST and GAT, respectively (r=0.74; P<0.0001). There was a 1.17 mm Hg bias between tonometers, with 95% limits of agreement of -2.66 to 5.01 mm Hg. According to multivariate regression analysis, differences between bIOP and GAT-IOP were associated with CCT (P=0.001) and age (P=0.007) in the PGA group, but only with CCT in the SLT group (P=0.002). bIOP and GAT-IOP values were influenced by age (PGA: P=0.014; SLT: P=0.006) and CCT (PGA: P=0.007; SLT: P=0.032), respectively.

CONCLUSIONS

BIOP values were higher and less affected by CCT than GAT-IOP values in eyes with OAG and OHT. However, these measurements may not be interchangeable in the clinic.

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.

A new method for detecting the outer corneal contour in images from an ultra-fast Scheimpflug camera

Background

The Corvis^® ST tonometer is an innovative device which, by combining a classic non-contact tonometer with an ultra-fast Scheimpflug camera, provides a number of parameters allowing for the assessment of corneal biomechanics. The acquired biomechanical parameters improve medical diagnosis of selected eye diseases. One of the key elements in biomechanical measurements is the correct corneal contour detection, which is the basis for further calculations. The presented study deals with the problem of outer corneal edge detection based on a series of images from the afore-mentioned device. Corneal contour detection is the first and extremely important stage in the acquisition and analysis of corneal dynamic parameters.

Result

A total of 15,400 images from the Corvis^® ST tonometer acquired from 110 patients undergoing routine ophthalmologic examinations were analysed. A method of outer corneal edge detection on the basis of a series of images from the Corvis^® ST was proposed. The method was compared with known and commonly used edge detectors: Sobel, Roberts, and Canny operators, as well as others, known from the literature. The analysis was carried out in MATLAB^® version 9.0.0.341360 (R2016a) with the Image Processing Toolbox (version 9.4) and the Neural Network Toolbox (version 9.0). The method presented in this paper provided the smallest values of the mean error (0.16%), stability (standard deviation 0.19%) and resistance to noise, characteristic for Corvis^® ST tonometry tests, compared to the methods known from the literature. The errors were 5.78 ± 9.19%, 3.43 ± 6.21%, and 1.26 ± 3.11% for the Roberts, Sobel, and Canny methods, respectively.

Conclusions

The proposed new method for detecting the outer corneal contour increases the accuracy of intraocular pressure measurements. It can be used to analyse dynamic parameters of the cornea.

Novel dynamic corneal response parameters in a practice use: a critical review

Background

Non-contact tonometers based on the method using air puff and Scheimpflug’s fast camera are one of the latest devices allowing the measurement of intraocular pressure and additional biomechanical parameters of the cornea. Biomechanical features significantly affect changes in intraocular pressure values, as well as their changes, may indicate the possibility of corneal ectasia. This work presents the latest and already known biomechanical parameters available in the new offered software. The authors focused on their practical application and the diagnostic credibility indicated in the literature.

Discussion

An overview of available literature indicates the importance of new dynamic corneal parameters. The latest parameters developed on the basis of biomechanics analysis of corneal deformation process, available in non-contact tonometers using Scheimpflug’s fast camera, are used in the evaluation of laser refractive surgery procedures, e.g. LASIK procedure. In addition, the assessment of changes in biomechanically corrected intraocular pressure confirms its independence from changes in the corneal biomechanics which may allow an intraocular pressure real assessment. The newly developed Corvis Biomechanical Index combined with the corneal tomography and topography assessment is an important aid in the classification of patients with keratoconus.

Conclusion

New parameters characterising corneal deformation, including Corvis Biomechanical Index and biomechanical compensated intraocular pressure, significantly extend the diagnostic capabilities of this device and may be helpful in assessing corneal diseases of the eye. Nevertheless, further research is needed to confirm their diagnostic pertinence.

Long-term Evaluation of Corneal Biomechanical Properties After Corneal Cross-linking for Keratoconus: A 4-Year Longitudinal Study

PURPOSE

To compare the long-term changes in corneal biomechanics, topography, and tomography before and 4 years after corneal cross-linking (CXL) with the Dresden protocol and correlate these changes with visual acuity.

METHODS

In this longitudinal study, 18 eyes of 18 patients with progressive keratoconus who were treated with CXL were included. All patients received a standard ophthalmological examination and were examined by Placido disc-based topography, Scheimpflug tomography, and biomechanical assessments with the Corvis ST (OCULUS Optikgeräte GmbH, Wetzlar, Germany) and Ocular Response Analyzer (ORA; Reichert Ophthalmic Instruments, Buffalo, NY) before and 4 years after CXL. The main outcome measures were dynamic corneal response (DCR) parameters obtained from the Corvis ST, corneal hysteresis (CH), corneal resistance factor (CRF), visual acuity, refraction, corneal curvature, and corneal thickness.

RESULTS

There were no significant differences in mean visual acuity, refraction, intraocular pressure, corneal topography, corneal astigmatism in both corneal surfaces, maximum keratometry, corneal thickness at apical and thinnest points, thickness profile indices, corneal volume, and specular microscopy before and 4 years after CXL (P > .05). Significant changes were observed in many DCR parameters, including radius at highest concavity and integrated inverse radius, both of which were consistent with stiffening. The CH and CRF values after CXL were not statistically significant. The new parameters using the Corvis ST include integrated inverse concave radius, which showed a significant decrease 1.07 ± 0.93 mm^-1, consistent with stiffening. The corneal stiffness parameter at the first applanation, Ambrósio's Relational Thickness to the horizontal profile, deformation amplitude ratio, and Corvis Biomechanical Index as a combined biomechanical screening parameter did not show significant changes.

CONCLUSIONS

CXL is a minimally invasive treatment option to prevent keratoconus progression over 4 years. Pressure-derived biomechanical parameters obtained from the ORA did not show any change following CXL at 4 years of follow-up, whereas the Corvis ST DCR parameters detected changes in corneal biomechanical properties. [J Refract Surg. 2018;34(12):849-856.].

Biomechanical changes in the cornea following cataract surgery: A prospective assessment with the Corneal Visualisation Scheimpflug Technology

IMPORTANCE

Intraocular pressure (IOP) is often reduced following cataract surgery. Postoperative changes in corneal stiffness are likely to be at least partly responsible for any reduction in IOP measured with applanation tonometry.

BACKGROUND

To determine the effect of cataract surgery and corneal incision size on corneal biomechanics.

DESIGN

Prospective randomized trial.

PARTICIPANTS

One hundred prospectively enrolled patients qualifying for cataract surgery.

METHODS

Participants were randomized to clear corneal incisions with a 2.20 or 2.85 mm keratome. Corneal Visualisation Scheimpflug Technology (Corvis-ST) tonometry and dynamic corneal response measurements were obtained preoperatively, and 3 mo postoperatively. Multiple regression analysis was completed using R software.

MAIN OUTCOME MEASURES

Corvis-ST biomechanical parameters.

RESULTS

Ninety-three eyes of 93 patients were included in the final analysis. Mean Corvis-ST biomechanically corrected IOP decreased by 3.63 mmHg postoperatively (95% confidence interval = 2.97-4.35, P ≤ 0.01), and central pachymetry increased by 6.96 μm (4.33-9.59, P ≤ 0.01). Independent of IOP and pachymetry changes, mean (±SE) corneal first applanation stiffness parameter reduced by 9.761 ± 3.729 (P = 0.01) postoperatively. First applanation velocity increased by 0.007 ± 0.002 ms, second applanation velocity increased by 0.012 ± 0.004 ms (P ≤ 0.01), the first applanation deformation amplitude increased by 0.008 ± 0.002 mm (P ≤ 0.01) and the deflection amplitude at highest concavity increased by 0.030 ± 0.069 (P ≤ 0.01). There were no significant differences between different incision size groups.

CONCLUSIONS AND RELEVANCE

Corneal stiffness is reduced 3 mo following cataract surgery and is associated with falsely low IOP measurements. This finding may be important for glaucoma patients and in particular when assessing the effectivity of minimally invasive glaucoma surgery devices.

Changes in Corneal Biomechanics and Intraocular Pressure Following Cataract Surgery

PURPOSE

To investigate the effects of cataract surgery on corneal biomechanics and intraocular pressure (IOP) measured with the updated Corvis ST tonometer (CST).

DESIGN

Prospective, interventional case series study.

METHODS

This study included 39 eyes of 39 cataract patients. CST measurements were performed at presurgery (Pre) as well as 1 week (1W), 1 month (1M), and 3 months (3M) postsurgery. The following CST parameters were recorded: deformation amplitude max (DA max), DA ratio max 1 mm and 2 mm, integrated radius, stiffness parameter at applanation 1 (SP A1), Ambrosio relational thickness to the horizontal profile (ARTh), Corvis biomechanical index (CBI), central corneal thickness (CCT), noncorrected intraocular pressure (IOPnct), and biomechanically corrected IOP (bIOP). IOP was also measured with Goldmann applanation tonometry and the noncontact tonometer CT-90A. All measurements were compared at each period using the linear mixed model, with and without adjustment for bIOP and CCT.

RESULTS

All IOP measurements decreased over time (P < .01). CCT was increased at 1W and 3M (P < .01), whereas ARTh was decreased at 1W and 1M (P < .01), but returned to its Pre level at 3M. DA max and Integrated radius were increased at 3M (P < .01), whereas SP A1 was decreased at 3M (P < .01). CBI was increased at 1W (P < .01), but returned to its Pre level at 1M.

CONCLUSIONS

IOP and Corneal biomechanical properties are changed after cataract surgery. In particular, SP A1 decreases while DA max and integrated radius increase, even at 3M, suggesting a less stiff cornea.

An Experimental Study of Femto-Laser in Assisting Xenograft Acellular Cornea Matrix Lens Transplantation

Background

The aim of this study was to evaluate the feasibility of using a femto-laser in assisting xenograft cornea matrix lens transplantation in correcting ametropia, along with evaluating the effectiveness and predictability of this procedure.

Material/Methods

A corneal matrix pouch was prepared on the right eyes on 8 healthy New Zealand rabbits by a femto-laser that was also employed to perform small incision lenticule extraction (SMILE) on 8 bovine cornea matrix lenses (+6D). A lens was treated acellular and implanted into a right rabbit cornea matrix pouch. Surface inflammation was observed at 1, 2, 4, 8, 12, and 24 weeks after surgery. Anterior ocular segment optical coherence tomography (OCT), corneal topography, retinoscopy, and cornea endothelial cell enumeration were performed.

Results

All the surgeries were successfully performed without any complications. The hyperopia condition of the rabbit eyes transformed into myopia status at an early stage and gradually developed hyperopia. Diopter at 24 weeks after surgery was 1/3 of that before surgery. Central corneal thickness stabilized at 4 weeks after surgery. Anterior segment OCT showed a clear lens edge at early post-operative stage, and blurred edge at 24 weeks later, indicating gradual fusion with the rabbit corneal matrix.

Conclusions

Femto-laser assisted xenograft corneal matrix lens transplantation is safe and effective in correcting ametropia, with satisfactory predictability, thus providing novel choice for correcting ametropia.

Corneal biomechanics after laser refractive surgery: Unmasking differences between techniques

The hypothesis that small-incision lenticule extraction provides better preservation of corneal biomechanics than previous laser refractive techniques has led to a growth in the interest in clinical and experimental research in this field. This hypothesis is based on the fact that corneal layers with greater stiffness are preserved with this new technique. However, this hypothesis is controversial because clinical research has shown a great disparity in the outcomes. In this review, we performed an in-depth analysis of the factors that might affect corneal biomechanics in laser refractive surgery procedures from a macrostructural to a microstructural viewpoint. New advances in algorithms with current devices or the introduction of new devices might help unmask the possible advantages of small-incision lenticule extraction in corneal biomechanics.