Corneal Biomechanics Losses Caused by Refractive Surgery

Multimodal corneal imaging before refractive cataract surgery

PURPOSE OF REVIEW

To prospectively review the advances in diagnostics for refractive cataract surgery, focusing on multimodal corneal analysis.

RECENT FINDINGS

The article explores the considerations related to planning refractive cataract surgery, emphasizing its potential to enhance patients' vision performance and quality of life. This review explores preoperative analysis in cataract surgery, focusing on assessing the cornea and ocular surface using multimodal diagnostics, including Placido disk-based corneal topography, Scheimpflug tomography OCT (optical coherence tomography), and biomechanical assessment. Wavefront technology, gene therapy, and artificial intelligence are also relevant in enhancing surgical precision and outcomes. It highlights the importance of preoperative thorough evaluations and technological advancements in ophthalmology.

SUMMARY

The rapid evolution and advancement of technology enable excellent refractive outcomes in most cases following cataract surgery. Consideration of appropriate preoperative factors is essential for achieving the desired postoperative outcome.

Effect of Belin-Ambrósio deviation index on 2-year refractive outcomes of PRK

PURPOSE

To assess the effect of preoperative Belin-Ambrósio deviation (BAD-D) index on 2-year visual, refractive, and tomographic outcomes, as well as the efficacy and safety of photorefractive keratectomy (PRK) surgery.

SETTING

Eye clinic, Mashhad, Iran.

DESIGN

Prospective cohort study.

METHODS

This study included 66 patients (66 eyes) who underwent PRK surgery, with a minimum follow-up period of 2 years. Participants were divided into 2 groups: preoperative BAD-D ≥1.60 (high BAD-D) and preoperative BAD-D <1.60 (low BAD-D). Preoperative and postoperative visual, refractive, and tomographic parameters were evaluated, and the efficacy and safety of the procedure were compared between groups.

RESULTS

66 patients with a mean age of 35.50 ± 8.21 years (range 22 to 55 years) were included. Postoperatively, the mean spherical equivalent (SE) of refractive error was +0.32 ± 0.65 diopters (D) in the high BAD-D group and +0.18 ± 0.66 D in the low BAD-D group ( P = .40). In addition, at 2 years of follow-up, the mean uncorrected distance visual acuity was 0.98 ± 0.07 in the high BAD-D group and 0.97 ± 0.08 in the low BAD-D group ( P = .905). Among the postoperative tomographic parameters, front elevation thickness, maximum Ambrósio relational thickness, astigmatism, and central corneal thickness were significantly different between the 2 groups (all, P < .05). At the 2-year follow-up, the mean Safety Index was 1.02 ± 0.04, and 1.01 ± 0.04 in high and low BAD-D groups, respectively ( P = .37), and the mean Efficacy Index was 0.99 ± 0.07 and 0.98 ± 0.06 in high and low BAD-D groups, respectively ( P = .40).

CONCLUSIONS

The preoperative BAD-D index does not predict postoperative visual, refractive, and tomographic outcomes in patients with low-to-moderate myopia. However, in patients with normal preoperative BAD-D values, higher agreement was expected between the attempted and achieved SE.

Research progress on measurement methods and clinical applications of corneal elastic modulus

Various corneal diseases are strongly associated with corneal biomechanical characteristics, and early measurement of patients' corneal biomechanics can be utilized in their diagnosis and treatment. Measurement methods for corneal biomechanical characteristics are classified into ex vivo and in vivo. Some of these methods can directly measure certain corneal biomechanical parameters, while others require indirect calculation through alternative methods. However, due to diversities in measurement techniques and environmental conditions, significant differences may exist in the corneal mechanical properties measured by these two methods. Therefore, comprehensive research on current measurement methods and the exploration of novel measurement techniques may have great clinical significance. The corneal elastic modulus, a critical indicator in corneal biomechanics, reflects the cornea's ability to return to its initial shape after undergoing stress. This review aims to provide a comprehensive summary of the corneal elastic modulus, which is a critical biomechanical parameter, and discuss its direct, indirect, and potential measurement methods and clinical applications.

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

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

Comparison of errors in ablation depth calculation after myopic femtosecond laser in situ keratomileusis in patients with different degrees of myopia: a prospective study

BACKGROUND

To investigate the difference between the predicted preoperative corneal ablation depth and the measured ablation depth for femtosecond laser in situ keratomileusis (FS-LASIK) in patients with different degrees of myopia, and to analyze the source of the difference.

METHODS

A total of 55 patients (109 eyes) were included in this study. Multiple logistics regression was applied to analyze the sources affecting postoperative refractive outcomes. The difference between the preoperative predicted corneal ablation depth and the 1-day postoperative ablation depth in patients with different degrees of myopia was explored using linear regression. Corneal biomechanical parameters influencing error in ablation depth calculation were examined using multiple linear regression.

RESULTS

One hundred and nine eyes were divided into low to moderate myopia (55 eyes, myopia of 6 D or less), high myopia (45 eyes, myopia ranging from 6 D to a maximum of 9 D), and very high myopia group (9 eyes, myopia greater than 9 D) based on preoperative refractive error (spherical equivalent). Postoperative visual outcomes were comparable among the three groups of patients, with no significant difference in uncorrected visual acuity (UCVA). We did find notable disparities in spherical equivalent (SE) and central corneal thickness (CCT) in patients with different degrees of myopia at 1 day postoperatively (all p < 0.001). Logistic regression analysis showed that error in ablation depth calculation was an independent risk factor for refractive outcomes one day after surgery (OR = 1.689, 95% CI: 1.366 - 2.089). There was a substantial discrepancy in error in ablation depth calculation at 1 day postoperatively between the three groups. The measured ablation depth of the laser platform was lower than the predicted ablation depth in the low to moderate myopia and very high myopia groups, but the opposite was true in the high myopia group. Pre-operative SE (p < 0.001) and corneal front minimum radius of curvature (Front Rmin) (p = 0.007) obviously influenced the error in ablation depth calculation.

CONCLUSIONS

Error in ablation depth calculation values vary significantly between patients with different degrees of myopia and correlate highly with preoperative SE and Front Rmin. At the same time, the available evidence suggests that error in ablation depth calculation is an influential factor in postoperative refractive status, so it is imperative to control error in ablation depth calculation.

Ocular surface inflammation and ectatic corneal disorders

PURPOSE OF REVIEW

Inflammatory mediators are a focus of recent corneal ectasia (CE) research and are a profound, modifiable contributor to CE in general and keratoconus (KC) in particular, opening a path to explore new methods of control. As advanced imaging technology and expanded population screening allow for earlier detection, the possibility of early intervention can profoundly change the prognosis of CE.

RECENT FINDINGS

Significant increases in the inflammatory mediators and immune components have been observed in the cornea, tear fluid, and blood of ectasia patients, while inflammation dampeners such as vitamin D and their receptors are reduced. Atopy and allergy have a strong association with KC, known to increase itch factors and stimulate eye rubbing, a risk factor in ectasia pathogenesis. Management of atopy or allergic conditions and topical anti-inflammatories has helped stabilize CE disease.

SUMMARY

Strategies such as monitoring inflammatory factors and using immune or inflammatory modulators, including managing subclinical inflammation, may be clinically beneficial in stabilizing the disease and improving outcomes. The detected factors are biomarkers, but as yet unproven to be sensitive or specific enough to be considered biomarkers for early detection of CE. The establishment of such biomarkers could improve the therapeutic outcome.

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.