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

A novel corneal indentation device for comparison of corneal tangent modulus before and after FS-LASIK in vivo

BACKGROUND

Corneal refractive laser surgery is widely used to correct myopia and astigmatism due to its safety and effectiveness. However, postoperative changes in corneal biomechanics, such as corneal ectasia, can occur, necessitating a deeper understanding of these changes. Finite Element Analysis has shown promise in predicting surgical outcomes based on corneal biomechanics. Devices like the Ocular Response Analyser (ORA) and Corvis ST provide noninvasive ways to measure corneal biomechanics, aiding in the assessment of corneal behavior post-surgery. Young's modulus and tangent modulus are crucial parameters for describing corneal elasticity, but there is limited data on the changes in tangent modulus following Femtosecond Laser-Assisted LASIK (FS-LASIK) in humans. This study aimed to investigate the effect of FS-LASIK on the corneal tangent modulus using a novel corneal indentation device (CID). The study sought to explore changes in corneal tangent modulus after FS-LASIK, taking into account central corneal thickness (CCT) and corneal radius, to enhance our understanding of the biomechanical changes induced by this surgical procedure.

RESULTS

Sixty-six patients (66 eyes) underwent FS-LASIK, resulting in significant changes in CCT, corneal radius, and Goldmann intraocular pressure (GAT IOP) 6 months post-surgery (△CCT = - 88 ± 31 µm, △corneal radius = 0.81 ± 0.30 mm, △GAT IOP = - 3.2 ± 2.4 mmHg, p < 0.001) 6 months after surgery. However, corneal stiffness did not significantly change (△ = - 0.002 ± 0.011, p < 0.2). The corneal tangent modulus showed a significant increase post-surgery (△ = 0.263 ± 0.146, p < 0.001), exhibiting a negative correlation with CCT (r = - 0.68, P < 0.001) and a positive correlation with corneal radius (r = 0.71, P < 0.001). For each 1 mm increase in corneal radius, there was a 0.23 MPa increase in corneal modulus, and for every 100 µm reduction in corneal thickness, there was a 0.14 MPa increase in corneal modulus.

CONCLUSIONS

The corneal tangent modulus, influenced by corneal radius and CCT, increased significantly following FS-LASIK. This study highlights the biomechanical changes induced by FS-LASIK, with implications for understanding corneal behavior post-surgery and its potential impact on patient outcomes.

Intraocular Pressure Before and After Corneal Refractive Surgery: A Prospective Comparison of Corvis ST and Ocular Response Analyzer

PRCIS

The study showed that Corvis ST's biomechanical intraocular pressure (bIOP) and ocular response analyzer's (ORA) cornea-compensated intraocular pressure (IOPcc) do not agree well, before or after photorefractive keratectomy (PRK), and may not be used interchangeably. bIOP remained unchanged after PRK.

OBJECTIVE

To evaluate the agreement between the biomechanically corrected intraocular pressure (bIOP) measured by the Corvis ST and the IOPcc measured by the ORA before and after PRK.

PATIENTS AND METHODS

In this prospective interventional study, a total of 53 patients (53 eyes) were included. Measurements were acquired using both the Corvis ST and ORA devices before and 3 months post-PRK. The agreement between the 2 devices was evaluated using limits of agreement (LoA) and Bland-Altman plots.

RESULTS

The participants had a mean age of 29.6 ± 5.21 years (range: 21 to 40), with 41 (77.4%) of them being females. After the surgery, the average change in intraocular pressure (IOP) was 0.3 ± 1.7 mm Hg for bIOP and -1.6 ± 4.0 mm Hg for IOPcc. The corresponding 95% LoA were -3.5 to 4.2 mm Hg and -9.5 to 6.3 mm Hg, respectively. The 95% LoA between bIOP and IOPcc after PRK was -2.3 to 8.5 mm Hg. Notably, the bIOP values were higher for IOPs <20 mm Hg and lower for IOPs >20 mm Hg compared with IOPcc.

CONCLUSIONS

The findings indicate a weak agreement between the Corvis ST-bIOP and the ORA-IOPcc both before and after PRK. These devices may not be used interchangeably for IOP measurement. bIOP exhibited less variation compared with the IOPcc, suggesting that the bIOP may be a better option for IOP reading after PRK.

Femtosecond-laser-assisted Small Incision Lenticule Extraction to Treat Refractive Error: A Narrative Review

With the approval of the Federal Drug Agency of the USA in 2018, femtosecond laser-assisted small incision lenticule extraction (SMILE) became an officially available option in several countries including Saudi Arabia to treat myopia and astigmatism. Intense marketing by industries has placed both clients and corneal surgeons into a dilemma of the plus and minuses of this option. Although several reviews compare SMILE to other conventional methods, especially laser in situ keratomileusis, this narrative review perhaps for the first time offers a detailed summary of this procedure and compares two sets of published literature, by authors from China and other countries. The efficacy and safety of SMILE seem to outnumber the complications and high cost of the investment. Its long-term usefulness for treating high myopia and hyperopia is still a matter of research.

Impact of Small Incision Lenticule Extraction for High Myopia on Intraocular Pressure Measurements With Pneumotonometry

PURPOSE

To evaluate the medium-term impact of small incision lenticule extraction (SMILE) on air-puff tonometry (APT) and how it may be corrected.

METHODS

In this controlled prospective study, 69 eyes from 69 patients treated with SMILE for high myopia (mean: -7.22 diopters) were included. Central corneal thickness (CCT), spherical equivalent refraction (SEQ), corneal power, and intraocular pressure (IOP) (assessed by APT) were measured before and 3 months after surgery.

RESULTS

The measured IOP decreased significantly after SMILE surgery (mean: -6.43 mm Hg), indicating a significant underestimation of the true IOP by APT. There was a significant correlation between change in SEQ, CCT, and apparent IOP. Correcting the apparent IOP measurement by applying a multiple regression correction did not provide significantly better estimates compared to adding 6.4 mm Hg to the measurement.

CONCLUSIONS

This study showed that previous SMILE surgery should be considered when interpreting APT measurements, because the measurements underestimate the true IOP. Multiple regressions cannot estimate the true IOP with certainty. For screening purposes, adding 6.4 mm Hg to the apparent IOP of a patient treated for high myopia may be sufficient. [J Refract Surg. 2023;39(9):606-611.].

Agreement of ocular response analyzer cornea compensated IOP with corvis ST biomechanical IOP following Femtosecond Laser-assisted LASIK

OBJECTIVES

To compare intraocular pressure (IOP) measurement by ORA-IOPcc and Corvis-bIOP after femtosecond laser-assisted LASIK (FS-LASIK).

METHODS

In this prospective cohort study, 56 eyes from 56 consecutive patients scheduled for FS-LASIK were enrolled. All patients had IOP measurement with ORA and Corvis ST by two blinded independent expert examiners. IOP examinations were conducted between 8 and 11 A.M. Data were collected at baseline and 3 months after FS-LASIK.

RESULTS

The mean age of the participants was 29.1 ± 6.3 years, and 42 (75%) were female. The average of central corneal thickness (CCT) decreased from 537 ± 23 µm at baseline to 458 ± 31 µm after FS-LASIK. The mean postoperative change of IOP was 0.0 ± 2.1 for bIOP and -2.5 ± 3.2 mmHg for IOPcc. The corresponding 95% limits of agreement (LoA) was -4.1 to 4.1 mmHg and -3.8 to 8.8 mmHg, respectively. Both methods showed no significant correlation between ∆IOP and ∆CCT. The 95% LoA between bIOP and IOPcc after FS-LASIK was -4.8 to 9.1 mmHg.

CONCLUSIONS

Compared to the ORA-IOPcc, the Corvis-bIOP showed less variation after FS-LASIK and might be a more appropriate choice for measuring IOP in this condition. The agreement of bIOP vs. IOPcc after FS-LASIK is below the clinically acceptable level, and the two methods could not be regarded as interchangeable.

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.

Predictive models for IOPs measured with NCT, GAT, and ORA among patients undergoing SMILE

Purpose: To develop predictive models for the intraocular pressure (IOP) of patients undergoing small incision lenticule extraction (SMILE) procedures, measured with a noncontact tonometer (NCT), Goldmann applanation tonometry (GAT), and an ocular response analyzer (ORA). Methods: In this prospective study, a total of 104 eyes (-6.23 ± 2.06 diopters) of 52 patients (24.38 ± 4.76 years) undergoing SMILE procedures were included. The intraocular pressure was measured (IOPNCT with NCT, IOPGAT with GAT, and IOPcc and IOPg with ORA) before surgery and at postoperative 6 months. Information on age, preoperative and attempted spherical equivalent (SE), ablation depth, preoperative values and postoperative changes in central corneal thickness (CCT), K1, K2, Km, corneal hysteresis (CH) and corneal resistance factor (CRF) values was collected in order to predict IOPs. Results: All surgeries were uneventful. At postoperative 6 months, the efficacy and safety index were 1.04 ± 0.15 and 1.08 ± 0.18, respectively. Significant decreases were detected in postoperative IOPNCT, IOPGAT, IOPcc, and IOPg compared to preoperative values (all p < 0.001). No relationship was found between any IOP and ablation depth, attempted SE, and preoperative SE, as well as CCTdifference (all p > 0.05). Predictive models for IOPs were constructed to predict preoperative values, and R 2 values were 67.5% (IOPNCT), 64.5% (IOPGAT), 78.7% (IOPcc), and 82.0% (IOPg). The prediction band of IOPNCT and IOPGAT was 7.4-15.1 mmHg and 8-16 mmHg, respectively. Conclusion: Predictive models for IOP measurements after SMILE procedures can be helpful in clinical practice.

Comparison of different correction formulas and measurement methods for the accurate determination of intraocular pressure after SMILE and FS-LASIK surgery

Background

To compare the accuracy of non-contact tonometry, Pentacam, and corneal visualization Scheimpflug technology (Corvis ST) for the measurement of intraocular pressure (IOP) after small incision lenticule extraction (SMILE) or femtosecond laser-assisted in situ keratomileusis (FS-LASIK) surgery.

Methods

A total of 49 patients (98 eyes) undergoing FS-LASIK or SMILE surgery at West China Hospital, Sichuan University from January to March 2021 were enrolled in this prospective, comparative, self-controlled study. IOP values were measured with non-contact tonometer, Pentacam, and Corvis ST before surgery and 1 month after surgery. Pentacam-derived postoperative IOP values were corrected using five correction formulas (Ehlers, Shah, Dresden, Orssengo-Pye, and Kohlhaas), while Corvis ST-derived values were corrected using a single formula.

Results

In the SMILE group, no significant differences were observed between the IOP values corrected with the Shah formula before and after surgery (t = 0.17, P = 0.869), whereas postoperative IOP values corrected with the other formulas were significantly different from the corresponding preoperative measurements (P < 0.05). In the FS-LASIK group, postoperative IOP values corrected with the Ehlers, Shah, or Corvis ST formulas were significantly different from the corresponding preoperative IOP measurements (P < 0.05), but no significant differences were observed between pre- and postoperative IOP values corrected with the Dresden (t =  − 0.08, P = 0.941), Orssengo-Pye (t =  − 0.52, P = 0.604), or Kohlhaas (t = 1.22, P = 0.231) formulas.

Conclusions

Pentacam’s Shah correction formula seemed to be the most appropriate method for accurately measuring postoperative IOP in patients undergoing SMILE surgery, while the Dresden, Orssengo-Pye, and Kohlhaas correction formulas of Pentacam were identified as the most reliable methods for estimating IOP in patients after FS-LASIK surgery.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12886-022-02620-7.

Evaluation of corneal biomechanical behavior in vivo for healthy and keratoconic eyes using the stress-strain index

PURPOSE

To evaluate the characteristics of corneal material properties in healthy individuals and keratoconic patients using the stress-strain index (SSI).

SETTING

Vincieye Clinic in Milan, Italy, and Instituto de Olhos Renato Ambrósio in Rio de Janeiro, Brazil.

DESIGN

Retrospective observational cross-sectional study.

METHODS

Records of 1221 patients were divided into 3 groups: healthy corneas (n = 728), bilateral keratoconus (KC, n = 388), and very asymmetric ectasia (VAE, n = 105) when patients presented with clinical ectasia in 1 eye and normal topography (VAE-NT) in the fellow eye. All patients were examined with Pentacam HR and Corvis ST. Severity of KC cases was stratified according to the Pentacam topographic KC classification. The SSI distribution across the different groups and its correlation with age, biomechanically corrected intraocular pressure (bIOP), and central corneal thickness (CCT) were assessed.

RESULTS

A statistically significant difference between healthy individuals and each of the keratoconic groups ( P < .001) was observed, and a progressive reduction in the SSI was observed across the groups. A significant correlation was observed between the SSI and age in all groups ( P < .010) but KC severe subgroup ( P = .361). No correlation between the SSI and bIOP and CCT was observed in all KC subgroups and VAE-NT groups ( P > .050). Among healthy eyes, there was only a mild correlation between the SSI and bIOP ( R = 0.12, P = .002) and CCT ( R = 0.13, P = .001).

CONCLUSIONS

This study estimates the in vivo corneal material properties in healthy individuals and patients with KC using a new method. The SSI showed a progressive deterioration within the advance in disease stages while being relatively independent of bIOP and CCT but positively correlated with age.

Effect of prostaglandin analogues on the biomechanical corneal properties in patients with open-angle glaucoma and ocular hypertension measured with dynamic scheimpflug analyzer

PURPOSE

The aim of the study is to evaluate the effect of topical prostaglandin (PG) treatment on the corneal biomechanical properties in treatment-naïve patients with either primary open-angle glaucoma (POAG) or ocular hypertension (OHT) using the Corvis ST device.

METHODS

This is an observational study. We analyzed the Corvis ST dynamic corneal response parameters of our database using the newest software available. Thirty-four eyes of 34 patients were included. They were all newly diagnosed and treatment-naïve. Patients were evaluated at baseline and after 6 months of treatment with prostaglandin analogues. Ultrasound pachymetry, Optical Coherence Tomography (OCT) and a 24-2 visual field test were performed in baseline visit. Goldman Applanation Tonometry (GAT-IOP) and Corvis ST dynamic corneal response parameters were registered at baseline and at the 6-month visit.

RESULTS

After 6 months of treatment, the IOP decrease (Δ) values obtained with the different tonometers were ΔGAT -6.5 ± 3.7, ΔIOPnct -4.4 ± 5.7 and ΔbIOP -3.8 ± 5.4. The differences between ΔGAT vs ΔIOPnct, ΔGAT vs ΔbIOP, and ΔIOPnct vs ΔbIOP, were statistically significant (p < 0.05 for all comparisons). Statistically significant lower values of the stress-strain index (SSI) (1.77 ± 0.3 at baseline vs 1.54 ± 0.27 at the 6-month visit) were found (p = 0.0002).

CONCLUSION

The SSI provided by the Corvis ST seems to decrease significantly after topical prostaglandin therapy. We believe that our results support the hypothesis that topical PG therapy does decrease the corneal stiffness and thus, that the ocular hypotensive effect of these drugs is overestimated if GAT is used for IOP measurement.

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

Objective

Methods

Results

Conclusion

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 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.

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.