A review of corneal biomechanics: Mechanisms for measurement and the implications for refractive surgery

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

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

Orthokeratology effect on the corneoscleral profile: Beyond the bull's eye

PURPOSE

To assess the impact of 3 months of orthokeratology (ortho-k) contact lenses (CLs) for myopia correction on the corneoscleral profile, as changes in scleral geometry could serve as indirect evidence of alteration in the corneal biomechanical properties.

METHODS

Twenty subjects (40 eyes) were recruited to wear ortho-k lenses overnight; however, after discontinuation (two CL fractures, one under-correction and two non-serious adverse events), 16 subjects (31 eyes) finished a 3-month follow-up. Corneoscleral topographies were acquired using the Eye Surface Profiler (ESP) system before and after 3 months of lens wear. Steep (SimKs) and flat (SimKf) simulated keratometry and scleral sagittal height measurements for 13-, 14- and 15-mm chord lengths were automatically calculated by the ESP software. Additionally, sagittal height and slope were calculated in polar format from 21 radii (0-10 mm from the corneal apex) at 12 angles (0-330°). Linear mixed models were fitted to determine the differences between visits.

RESULTS

SimKs and SimKf were increased significantly (p ≤ 0.02). The sagittal height in polar format increased significantly (p = 0.046) at a radius of 2.5 mm for 150°, 180°, 210° and 240° orientations and at a radius of 3.0 mm for 210°. Additionally, the slope in polar format significantly decreased (p ≤ 0.04) at radii ranges of 0.0-0.5, 0.5-1.0 and 1.0-1.5 mm for multiple angles and at a radii range of 5.0-5.5 mm for 90°. It also increased significantly (p ≤ 0.045) at a radii range of 1.5-2.0 mm for 30° and at radii ranges of 2.0-2.5, 2.5-3.0 and 3.0-3.5 mm for multiple angles. No significant changes were found for any parameter measured from the scleral area.

CONCLUSIONS

Three months of overnight ortho-k lens wear changed the central and mid-peripheral corneal geometry as expected, maintaining the peripheral cornea and the surrounding sclera stability.

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

Purpose

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

Methods

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

Results

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

Conclusion

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

Bibliometric and visualized analysis of myopic corneal refractive surgery research: from 1979 to 2022

Background

Myopic corneal refractive surgery is one of the most prevalent ophthalmic procedures for correcting ametropia. This study aimed to perform a bibliometric analysis of research in the field of corneal refractive surgery over the past 40 years in order to describe the current international status and to identify most influential factors, while highlighting research hotspots.

Methods

A bibliometric analysis based on the Web of Science Core Collection (WoSCC) was used to analyze the publication trends in research related to myopic corneal refractive surgery. VOSviewer v.1.6.10 was used to construct the knowledge map in order to visualize the publications, distribution of countries, international collaborations, author productivity, source journals, cited references, keywords, and research hotspots in this field.

Results

A total of 4,680 publications on myopic corneal refractive surgery published between 1979 and 2022 were retrieved. The United States has published the most papers, with Emory University contributing to the most citations. The Journal of Cataract and Refractive Surgery published the greatest number of articles, and the top 10 cited references mainly focused on outcomes and wound healing in refractive surgery. Previous research emphasized "radial keratotomy (RK)" and excimer laser-associated operation methods. The keywords containing femtosecond (FS) laser associated with "small incision lenticule extraction (SMILE)" and its "safety" had higher burst strength, indicating a shift of operation methods and coinciding with the global trends in refractive surgery. The document citation network was clustered into five groups: (1) outcomes of refractive surgery: (2) preoperative examinations for refractive surgery were as follows: (3) complications of myopic corneal refractive surgery; (4) corneal wound healing and cytobiology research related to photorefractive laser keratotomy; and (5) biomechanics of myopic corneal refractive surgery.

Conclusion

The bibliometric analysis in this study may provide scholars with valuable to information and help them better understand the global trends in myopic corneal refractive surgery research frontiers. Two stages of rapid development occurred around 1991 and 2013, shortly after the innovation of PRK and SMILE surgical techniques. The most cited articles mainly focused on corneal wound healing, clinical outcomes, ocular aberration, corneal ectasia, and corneal topography, representing the safety of the new techniques.

Characterization of the Immediate and Delayed Biomechanical Response to UV-A Crosslinking of Human Corneas

PURPOSE

Keratoconus leads to visual deterioration due to irregular astigmatism and corneal thinning. Riboflavin-based corneal UV-A crosslinking (CXL) induces novel intramolecular and intermolecular links resulting in corneal tissue stiffening, thereby halting disease progression. The purpose of this study was to analyze the immediate and delayed biomechanical responses of human donor corneas to CXL.

METHODS

CXL was performed according to the Dresden protocol to corneas not suitable for transplantation. Biomechanical properties were subsequently monitored by measuring the Young modulus using nanoindentation. The immediate tissue response was determined after 0, 1, 15, and 30 minutes of irradiation. Delayed biomechanical effects were investigated with follow-up measurements immediately and 1, 3, and 7 days after CXL.

RESULTS

Young's modulus indicated a linear trend in direct response to increasing irradiation times (mean values: total 61.31 kPa [SD 25.53], 0 minutes 48.82 kPa [SD 19.73], 1 minute 53.44 kPa [SD 25.95], 15 minutes 63.56 kPa [SD 20.99], and 30 minutes 76.76 kPa [SD 24.92]). The linear mixed model for the elastic response of corneal tissue was 49.82 kPa + (0.91 kPa/min × time [minutes]); P < 0.001. The follow-up measurements showed no significant delayed changes in the Young modulus (mean values: total 55,28 kPa [SD 15.95], immediately after CXL 56,83 kPa [SD 18.74], day 1 50.28 kPa [SD 14.15], day 3 57.08 kPa [SD 14.98], and day 7 56.83 kPa [SD 15.07]).

CONCLUSIONS

This study suggests a linear increase of corneal Young modulus as a function of CXL timing. No significant short-term delayed biomechanical changes posttreatment were observed.

Dynamic Ocular Response to Mechanical Loading: The Role of Viscoelasticity in Energy Dissipation by the Cornea

We have used vibrational optical coherence tomography (VOCT) to measure the resonant frequency, elastic modulus, and loss modulus of components of the anterior segment of pig eyes in vitro. Such basic biomechanical properties of the cornea have been shown to be abnormal not only in diseases of the anterior segment but also in posterior segment diseases as well. This information is needed to better understand corneal biomechanics in health and disease and to be able to diagnose the early stages of corneal pathologies. Results of dynamic viscoelastic studies on whole pig eyes and isolated corneas indicate that at low strain rates (30 Hz or less), the viscous loss modulus is as high as 0.6 times the elastic modulus for both whole eyes and corneas. This large viscous loss is similar to that of skin, which has been hypothesized to be dependent upon the physical association of proteoglycans with collagenous fibers. The energy dissipation properties of the cornea provide a mechanism to dissipate energy associated with blunt trauma, thereby preventing delamination and failure. The cornea possesses the ability to store impact energy and transmit excess energy to the posterior segment of the eye through its serial connection to the limbus and sclera. In this manner, the viscoelastic properties of the cornea, in concert with that of the posterior segment of the pig eye, function to prevent mechanical failure of the primary focusing element of the eye. Results of resonant frequency studies suggest that the 100-120 Hz and 150-160 Hz resonant frequency peaks reside in the anterior segment of the cornea since the removal of the anterior segment of the cornea decreases the peak heights at these resonant frequencies. These results suggest that there is more than one collagen fibril network found in the anterior portion of the cornea that provides structural integrity to prevent corneal delamination and that VOCT may be useful clinically to diagnose corneal diseases.

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.

[Direct assessment of changes in biomechanical properties of the cornea after ReLEx SMILE and FemtoLASIK surgeries]

The aim of this study was to evaluate corneal biomechanical properties in myopic patients after ReLEx SMILE and FemtoLASIK surgeries using the Corvis ST analyzer.

MATERIAL AND METHODS

The SMILE group comprised 23 patients (46 eyes) with spherical refraction -3.8±1.8 diopters (D), the FemtoLASIK group included 18 patients (36 eyes) with spherical refraction -3.5±1.3 D. Analysis of the biomechanical properties of the cornea was carried out using the CORVIS ST device (Oculus, Germany) before and seven days after corneal refractive surgery.

RESULTS

In the SMILE group, with intraoperative decrease in corneal thickness of 91.43±19.43 µm, a significant increase in the following parameters was observed: deformation coefficient (DA ratio; p=0.0001), peak distance (PD; p=0.02) and inverse concave radius (ICR; p=0.003); a decrease in the stiffness parameter at first applanation (SP-A1; p=0.0001), Corvis biomechanical index (CBI; p=0.0001), intraocular pressure (IOP; p=0.001). In the FemtoLASIK group, with intraoperative decrease in corneal thickness of 75.33±32.3 µm, we observed a significant increase in DA ratio (p=0.0002), PD (p=0.04), ICR (p=0.0002), a decrease in SP-A1 (p<0.0001), IOP values (p=0.0003). In comparison with the FemtoLASIK group, the deformation amplitude DA changed significantly less in the SMILE group (p=0.04). In the FemtoLASIK group, in comparison with the SMILE group, the DA ratio (p=0.0009) and SP-A1 (p=0.0003) significantly increased. Intraoperative corneal thickness change correlates with ICR both in SMILE (R=0.52) and in FemtoLASIK (R=0.65).

CONCLUSION

Corneal biomechanical properties determined with CORVIS ST in eyes with mild to moderate myopia change to a lesser extent after ReLEx SMILE compared to FemtoLASIK.

Corneal Biomechanics - an Emerging Ocular Property with a Significant Impact

Corneal biomechanical properties reflect the capacity of the cornea to respond to applied mechanical forces. They are an increasingly important domain in ocular pathology, correlated to the diagnosis and evolution of eye diseases such as refractive errors, glaucoma or corneal ectasias. Refractive errors constitute a significant etiology of decreased vision worldwide, with a particular impact in children. Myopic eyes significantly differ from emmetropic eyes in terms of morphology and biomechanics, with differences being reported in both adults and children. In the latter, corneal hysteresis (CH) and the corneal resistance factor (CRF) are significantly lower in myopic individuals, and both biomechanical parameters correlate with the central corneal thickness and axial length. Glaucoma is a progressive optic neuropathy that leads to thinning of the nerve fiber layer and specific visual field loss, in which intraocular pressure (IOP) is an important risk factor. There is an inverse correlation between IOP and CH - a low hysteresis is associated with a high IOP. Furthermore, CH is on average lower in primary open angle glaucoma (POAG) compared to ocular hypertension (OHT) for the same IOP. Significant correlations between CH and the thickness of the ganglion cell layer (GCL) and retinal nerve fiber layer (RNFL), in both POAG and OHT, have been described. Keratoconus is the most frequent corneal ectasia, which leads to a progressive thinning and protruding of the cornea. Biomechanical parameters are severely affected in keratoconus - usually, both CH and CRF are lower compared to normal eyes. The biomechanical behavior of the cornea modulates the evolution of several ocular pathologies. As research is ongoing, more data will enable us to apply this knowledge in diagnosing disease more efficiently and targeting the right treatment for the right patient, including refractive surgery.

VisuMax Flap 2.0: a flap plus technique to reduce incidence of an opaque bubble layer in femtosecond laser–assisted LASIK

Purpose

To evaluate the incidence of an opaque bubble layer (OBL) in femtosecond laser–assisted in situ keratomileusis (FS-LASIK) flaps created with VisuMax Flap 2.0 as a result of a modification in the parameters of the flap programming.

Methods

This retrospective study was comprised of 1400 eyes of 715 patients who received FS-LASIK surgery. OBLs were measured and reported as a percentage of the flap area to identify the incidence and extent. Flap creation, which is a modification technique, was performed with 8.1-mm flap diameters plus 0.3-mm enlarged interlamellar photodisruption (group Flap 2.0). The same flap diameters without extra photodisruption as the previous standard setting were also implemented (group Flap 1.0). The preoperative measurements, including sphere, cylinder, keratometry, and intraoperative characteristics such as flap size and thickness, were documented. Possible risk factors for the occurrence of OBLs were investigated in this study.

Results

The incidence of an OBL was reduced when using the Flap 2.0 program (31.4%) compared to the Flap 1.0 program (63.7%). The area of hard and soft OBLs created by the Flap 2.0 program is smaller than those created by the Flap 1.0 program (P = 0.007 and P < 0.001). Multivariate logistic regression indicated that a thinner flap (P = 0.038) and a higher sphere (P = 0.001) affected the chance of hard OBLs occurring.

Conclusion

The VisuMax Flap 2.0 program promotes gas venting by enlarging the interlamellar photodisruption size. The incidence and extent of OBLs appear to be reduced significantly when the Flap 2.0 program is applied.

Co-axial acoustic-based optical coherence vibrometry probe for the quantification of resonance frequency modes in ocular tissue

We present a co-axial acoustic-based optical coherence vibrometry probe (CoA-OCV) for vibro-acoustic resonance quantification in biological tissues. Sample vibrations were stimulated via a loudspeaker, and pre-compensation was used to calibrate the acoustic spectrum. Sample vibrations were measured via phase-sensitive swept-source optical coherence tomography (OCT). Resonance frequencies of corneal phantoms were measured at varying intraocular pressures (IOP), and dependencies on Young´s Modulus (E), phantom thickness and IOP were observed. Cycling IOP revealed hysteresis. For E = 0.3 MPa, resonance frequencies increased with IOP at a rate of 3.9, 3.7 and 3.5 Hz/mmHg for varied thicknesses and 1.7, 2.5 and 2.8 Hz/mmHg for E = 0.16 MPa. Resonance frequencies increased with thickness at a rate of 0.25 Hz/µm for E = 0.3 MPa, and 0.40 Hz/µm for E = 0.16 MPa. E showed the most predominant impact in the shift of the resonance frequencies. Full width at half maximum (FWHM) of the resonance modes increased with increasing thickness and decreased with increasing E. Only thickness and E contributed to the variance of FWHM. In rabbit corneas, resonance frequencies of 360-460 Hz were observed. The results of the current study demonstrate the feasibility of CoA-OCV for use in future OCT-V studies.

A Critical Assessment of Friedenwald’s Technique for Estimating the Coefficient of Rigidity of the Cornea

Purpose

To determine if Friedenwald's technique for estimating the coefficient of corneal rigidity (Ko, units mmHg/μL), could differentiate between the cornea in keratoconus, normal eyes, and after crosslinking (CXL).

Methods

Two operators (1 and 2) independently measured Ko in three groups (keratoconus, normal, and post-CXL corneas), and repeated the procedure in some where their care remained unchanged and others after routine CXL (>28 days postop, epi-off treatment, 3.0 mW/cm², 30 min). The data were subsequently used to quantify interoperator error, test-retest/intersessional reliability for estimation of Ko, the significance of intergroup differences, and the effect of CXL on Ko.

Results

The major findings were: (i) Ko values were not normally distributed; (ii) mean (±sd, 95% CI) interoperator error was -0.002 (±0.019, −0.006 to 0.003, n = 95) and the limit of agreement between the operators was ±0.039; (iii) RMS differences in the intersessional estimation of Ko values were 0.011 (operator 1) and 0.012 (operator 2); (iv) intergroup differences in Ko were not significant (p > 0.05); (v) intersessional change in Ko (y) was linearly related to Ko estimated (x) at 1^st session (for operator 2 y = 1.187x−0.021, r = 0.755, n = 16, p < 0.01); and (vi) change in Ko (y₁) after CXL was linearly related to Ko (x₁) at preop (for operator 2 y₁ = 0.880x₁−0.016, r = 0.935, n = 20, p < 0.01).

Conclusion

Friedenwald's technique for estimating the Ko is prone to substantial interoperator error and intersessional differences. According to the technique, the change in Ko following CXL is on par with the expected intersessional change observed in controls.

Biomechanical Evaluation of Decellularized and Crosslinked Corneal Implants Manufactured From Porcine Corneas as a Treatment Option for Advanced Keratoconus

Currently corneal transplantation is the main treatment for late-stage keratoconus; however, transplantation procedures are accompanied by significant risk of post-surgical complications; this in addition to supply limitations imposed by a worldwide shortage of human donor corneas, has driven the development of alternative therapies. One such therapy is the use of corneal implants derived from porcine corneas (Xenia^®, Gebauer Medizintechnik GmbH, Neuhausen, DE). In contrast to human donor tissue, these implants can be produced on demand and due to the processes used pose no risks for host-immune rejection. Their use has already been demonstrated clinically in patients for preventing the progression of topographic changes in keratoconus whilst improving visual acuity. The implants are derived from natural tissue and not standardised synthetic material, whilst this likely reduces the risk of issues with bio-incompatibility, there is inevitably variability in their intrinsic mechanical properties which requires investigation. Here, speckle interferometry is employed to examine the biomechanical properties, in response to physiologically representative forces, of native porcine corneal tissue prior to processing and after a proprietary 4-stage process involving decellularization, washing, compression and crosslinking. The control lenticules had an average Young’s modulus (E) of 11.11 MPa (range 8.39–13.41 MPa), following processing average E of the lenticules increased by 127% over that of the unprocessed tissue to 25.23 MPa (range 18.32–32.9 MPa). The variability in E of the lenticules increased significantly after processing suggesting variability in the propensity of the native tissue to processing. In summary, it is possible to produce thin (<90 µm) lenticules from porcine corneas with enhanced stiffness that are effective for treating late-stage keratoconus. Due to the observed variability in the responses of lenticules to processing, interferometry could be a useful technique for ensuring quality control in commercial production via biomechanical screening.

Diagnosis of Subclinical Keratoconus with a Combined Model of Biomechanical and Topographic Parameters

This study sought to develop a diagnostic model with aberrometry and biomechanical variables for subclinical keratoconus. The design was a cross-sectional study. The topographic data were obtained with a rotating Scheimpflug camera (Pentacam HR), and biomechanical data were obtained with Corvis ST. The study included 81 eyes distributed in 61 healthy corneas and 20 subclinical keratoconus (SCKC), defined as eyes with suspicious topographic findings, normal slit-lamp examination, and a manifestation of keratoconus. Analyses of the topographic and biomechanical data were performed, and a classifying model of SCKC was elaborated. The model for the diagnosis of SCKC includes posterior coma to 90°, Ambrósio's Relational Thickness in the horizontal profile (ARTh), and velocity when the air pulse is off (A2 velocity). The sensitivity was 89.5%, specificity 96.7%, accuracy 94.9%, and precision 89.5%. The area under the curve (AUC) of the receiver operating characteristic (ROC) curve for the model was 0.951. Diagnosis of subclinical keratoconus depends on the aberrometry variable posterior coma to 90° and the biomechanical variables A2 velocity and ARTh.

Interferometric Ex Vivo Evaluation of the Spatial Changes to Corneal Biomechanics Introduced by Topographic CXL: A Pilot Study

PURPOSE

To determine the efficacy of interferometry for examining the spatial changes to the corneal biomechanical response to simulated intraocular pressure (IOP) fluctuations that occur after corneal cross-linking (CXL) applied in different topographic locations.

METHODS

Displacement speckle pattern interferometry (DSPI) was used to measure the total anterior surface displacement of human and porcine corneas in response to pressure variations up to 1 mm Hg from a baseline pressure of 16.5 mm Hg, both before and after CXL treatment, which was applied in isolated topographic locations (10-minute riboflavin soak [VibeX-Xtra; Avedro, Inc], 8-minute ultraviolet-A exposure at 15 mW/cm²). Alterations to biomechanics were evaluated by directly comparing the responses before and after treatment for each cornea.

RESULTS

Before CXL, the corneal response to loading indicated spatial variability in mechanical properties. CXL treatments had a variable effect on the corneal response to loading dependent on the location of treatment, with reductions in regional displacement of up to 80% in response to a given pressure increase.

CONCLUSIONS

Selectively cross-linking in different topographic locations introduces position-specific changes to mechanical properties that could potentially be used to alter the refractive power of the cornea. Changes to the biomechanics of the cornea after CXL are complex and appear to vary significantly depending on treatment location and initial biomechanics. Hence, further investigations are required on a larger number of corneas to allow the development of customized treatment protocols. In this study, laser interferometry was demonstrated to be an effective and valuable tool to achieve this. [J Refract Surg. 2021;37(4):263-273.].