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Dr.Ritika Dalal, D14793, Dr.Pooja Khamar, Dr.Rohit Shetty

Introduction

The anterior stroma of the cornea has an interweaving network of collagen fibers around the collagen lamellae and this, progressively becomes less through the depth of the stroma.1 These structural features could be responsible for the greater tensile strength of the anterior stroma as compared to the posterior stroma.2Procedures like laser-assisted in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) alter the anterior stroma. In PRK, less of the anterior stroma is ablated in compared to LASIK. Small incision lenticule extraction (SMILE), leaves more of the anterior stroma intact compared to LASIK and PRK. Using theoretical models, it was concluded that corneas undergoing SMILE could be biomechanically strongercompared to LASIK and PRK postoperatively.3,4

Using dynamic air-puff applanation, studiesindicated a better biomechanical result after PRK than after LASIK.5,6Another study reported similar outcomes betweenPRK and LASIK.3Recent clinical datademonstrated equivalence between SMILE and LASIK with respect to biomechanical changes in the cornea after surgery.8-10Further, Corvis-ST was an improved device over the Ocular response analyzer since it had a highly repeatable results and quantified the mechanical deformation of the cornea. Theoretical models showed that LASIK caused a greater increase in mechanical stress in the residual stromal bed than SMILE.4Therefore, this study investigated simulated air-puff applanation on LASIK, PRK and SMILE finite element models, using the pressure profile generated by Corvis-ST (OCULUS OptikgerateGmbh, Germany).11

Methods

This was a simulation study using finite element modeling.The method for the finite element model generation is briefly described here from another recent study.113-D geometry of a patient cornea was created from Pentacam. The Pentacam provided Cartesian coordinates of the anterior and posterior corneal surface. Epithelium thickness of the cornea was measured with RTVue. Finite element mesh was created with 8-noded linear hexahedral elements. An anisotropic, hyperelastic, fiber dependent material model with material incompressibility was chosen.1 The material model accounted for both the arrangement of fibers in the central cornea and periphery.

This was a retrospective analysis of data. The study was approved by ethics committee of the Narayana Nethralaya Eye Hospital, Bangalore, India.All the surgeries were performed by a single experienced surgeon under topical anesthesia. LASIK was performed with the WaveLight FS200 femtosecond laser and WaveLight EX500 excimer laser platform (Alcon Laboratories, Ft Worth, USA). A flap (9.0 mm diameter, 110 µm thickness) was created. The flap was lifted and excimer abalation was performed. SMILE was performed with the VisuMax femtosecond laser system (Carl ZiessMeditec AG, Germany). Cap thickness was 110 µm. Lenticule and cap diameter was 6.0 mm and 7.8 mm, respectively. These flap and gap geometrical details were incorporated in the 3-D finite element model. After creating the lenticule it was dissected and extracted manually. After the surgery, one drop of antibiotic was instilled in both eyes.

All PRK procedures weredone under topical aneasthesiaunder strict aseptic conditions. The epithelium was manually scraped in the central 8 mm diameter zone before ablation. Wavefront optimized PRK was performed with WaveLight Allegretto EX-500 laser (Alcon) using a optical zone of 6mm. A bandage contact lens (Ciba Vision, Duluth, USA) was applied after the surgery.Routine postoperative regimen was followed for all eyes. All eyes underwent Corvis-ST measurement before the surgery and 3 months after surgery.

Corvis ST and Pentacam HR data (12 eyes per surgery) derived preop corneal material properties and refractive correction were used to predict post-op corneal biomechanics using FEM algoritghm. Predicted corneal biomechanics was then compared to in-vivo post-op peak of corneal biomechanics and corneal biomechanics derived corneal stiffness to assess repeatability.

Results

Overall,prediction of peak deformation amplitude and deflection amplitude was best in PRK eyes (Inter Class Correlation(ICC) ~0.9), which could be due to absence of any cut in PRK. However, the difference between in vivo preoperative and postoperative peak deformation and deflection amplitude were similar between all the eye groups.Further, PRK eyeshadthe least decrease in magnitude of Kc (mean) and Kc (constant) after surgery, e.g., in vivomean Kc (mean) and mean Kc (constant) decreased by 6.35 and 5.04 N/m only. In contrast, LASIK and SMILE caused a greater decrease in stiffness.

Overall, PRK eyes had the best agreement (ICC ~ 0.9 and above) between in vivo postoperative measurement and predicted postoperative value.The waveforms were derived by averaging the in vivodeformation amplitude waveforms of all the eyes at respective time points for a given treatment(LASIK, SMILE or PRK).Linear regression analyses were performed between the in vivo and predicted postoperative variables for all the 36 eyes. The correlation coefficients (r) were 0.95, 0.94, 0.87 and 0.90 for Kc (mean), Kc (constant), peak deformation amplitude and peak deflection amplitude, respectively, for all eyes (n=36). The slopes of the linear regressions were 0.93, 0.91, 0.81 and 0.70, respectively (p<0.001 for all).

In vivo post op and predicted post op corneal stiffness was 86.1N/m (81-91.3N/m) and 87.9N/m (82-93.9N/m) for SMILE, 91.3N/m (84.5-98.1 N/m) and 91.6N/m (85.3-97.8N/m) for LASIK and 98.3N/m (86.6-110.1N/m) and 95N/m (83.1-107N/m) for PRK. The ICC was 0.95, 0.95 and 0.97 respectively for SMILE, LASIK and PRK. ICC of peak corneal biomechanics was greater than 0.81 for all.

Discussion

SMILE leaves most of the anterior stroma intact, which seems to be the stiffest region of the stroma. Hence, SMILE caused the least biomechanical changes in the cornea in the theoretical models.3,4This study focused exclusively on expected deformation response of the cornea after simulated LASIK, PRK and SMILE coupled with air-puff applanation. The key outcomes from this study were the following:

  • When a cohort of eyes was measured with Corvis-ST before and after surgery, PRK eyes had the least decrease in stiffness parameters. Also, LASIK and SMILE caused a much greater decrease in stiffness parameters. However, the change in in vivo peak deformation and deflection amplitude was similar between the groups.
  • PRK eyes had the best agreement between in vivo and predicted postoperative value of stiffness, peak deformation amplitude and peak deflection amplitude. SMILE and LASIK eyes also had excellent agreement for stiffness parameters. Flap or cap could have reduced the level of agreement between in vivo and predicted postoperative value of peak deformation amplitude and peak deflection amplitude in LASIK and SMILE eyes.

Clinical results of studies comparing LASIK and PRK with the Corvis-ST generally indicate a stiffer biomechanical response after PRK than predicted by the model.5-7 This indicates the importance of greater degree of fibrotic scars and haze formation in PRK than the others,12 which could have resulted in some biomechanical compensation to removal of the stiffest region of the stroma.Comparative studies between LASIK and SMILE using the Corvis-ST indicated similar biomechanical changes.8-10 This study provided an explanation to these observations and demonstrated the limitation in using device deformation variables such as peak deformation amplitude to compare SMILE and LASIK. Our inverse simulation method of comparing post-operative outcomes may yield better segregation of biomechanical responses after SMILE and LASIK.Future inverse models could benefit with a continuum mechanics approach to simulate thebiological stiffening effectafter PRK, though this would be a challenging task.

Another limitation was that a reduced eye model was implemented since patient specific geometrical data on globe, muscles and fat weren’t available in routine clinic. The postoperative epithelium thickness is no longer as uniform as preoperative thickness and this resulted inan approximation to the true thickness of the postoperative stroma. Most current OCT devices limit epithelium thickness reports to the central 6 mm cornea only. Thus, the data was insufficient for inclusion in patient specific simulations, where the corneal diameter was significantly greater. These limitations could also explain the differencebetweenpostoperative corneal stiffness derived from in vivo deformation amplitude and the same estimated from simulation results.

To conclude, this novel endeavor shows great promise in predicting post-op biomechanics, thus would be an asset for refractive surgery planning.

References

  1. Reinstein DZ, Archer TJ, Randleman JB. Mathematical model to compare the relative tensile strength of the cornea after PRK, LASIK, and small incision lenticule extraction.J Refract Surg. 2013;29:454-60.
  2. Sinha Roy A, Dupps WJ Jr, Roberts CJ. Comparison of biomechanical effects of small-incision lenticule extraction and laser in situ keratomileusis: finite-element analysis. J Cataract Refract Surg. 2014;40:971-80.
  3. Kamiya K, Shimizu K, Ohmoto F. Comparison of the changes in corneal biomechanical properties after photorefractive keratectomy and laser in situ keratomileusis. Cornea. 2009;28:765-9.
  4. Hashemi H, Asgari S, Mortazavi M, Ghaffari R. Evaluation of Corneal Biomechanics After Excimer Laser Corneal Refractive Surgery in High Myopic Patients Using Dynamic Scheimpflug Technology. Eye Contact Lens. 2016; In press.
  5. Hassan Z, Modis L Jr, Szalai E, Berta A, Nemeth G. Examination of ocular biomechanics with a new Scheimpflug technology after corneal refractive surgery. Cont Lens Anterior Eye. 2014; 37: 337-41.
  6. Pedersen IB, Bak-Nielsen S, Vestergaard AH, Ivarsen A, Hjortdal J. Corneal biomechanical properties after LASIK, ReLEx flex, and ReLEx smile by Scheimpflug-based dynamic tonometry. Graefes Arch ClinExpOphthalmol. 2014; 252: 1329-35.
  7. Shen Y, Chen Z, Knorz MC, Li M, Zhao J, Zhou X. Comparison of corneal deformation parameters after SMILE, LASEK, and femtosecond laser-assisted LASIK.J Refract Surg. 2014; 30: 310-8.
  8. Sefat SM, Wiltfang R, Bechmann M, Mayer WJ, Kampik A, Kook D. Evaluation of Changes in Human Corneas After Femtosecond Laser-Assisted LASIK and Small-Incision Lenticule Extraction (SMILE) Using Non-Contact Tonometry and Ultra-High-Speed Camera (Corvis ST). Curr Eye Res. 2016; 41: 917-22.
  9. Sinha Roy A, Kurian M, Matalia H, Shetty R. Air-puff associated quantification of non-linear biomechanical properties of the human cornea in vivo. J MechBehav Biomed Mater. 2015;48:173-82.
  10. Mrochen M, Donitzky C, Wüllner C, Löffler J. Wavefront-optimized ablation profiles: theoretical background. J Cataract Refract Surg. 2004;30:775-85.
  11. Shroff R, Francis M, Pahuja N, Veeboy L, Shetty R, Sinha Roy A. Quantitative Evaluation of Microdistortions in Bowman’s Layer and Corneal Deformation after Small Incision Lenticule Extraction. Transl Vis Sci Technol. 2016;5:12.
  12. Shetty R, Francis M, Shroff R, Pahuja N, Khamar P, Girrish M, Nuijts RMMA, Sinha Roy A. Corneal Biomechanical Changes and Tissue Remodeling After SMILE and LASIK. Invest Ophthalmol Vis Sci. 2017;58:5703-5712.

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