Biomechanical Changes After Flap-Based Vs Lenticule-Based Refractive Procedures: Implications For Refractive Outcomes And Ectasia Risk

Several surgical options exist for refractive correction including PRK, LASIK and SMILE. Controversies remain regarding which is the best and safest procedure for a given patient, in addition to the optimal settings to be used for each of the different procedures (treatment depth, incision angle) with decisions relying on the preferences and experience of the surgeon, guided by available patient data. Here we assess the micro-structural changes that occur to corneal collagen after LASIK and SMILE, and evaluate effects on corneal biomechanical response to intra-ocular pressure (IOP) changes; with the aims of providing a basis for surgical optimisation in terms of refractive outcomes; and safety through maintenance of biomechanical integrity.

Imaging and data analysis was conducted by Abby Wilson and Maryame Boutkhil at UCL Mechanical Engineering and UCL Institute of Child Health (ICH), London, UK. All surgeries were performed by Dan Reinstein at the London Vision Clinic, London, UK

Second harmonic generation (SHG) microscopy was used to image fresh intact porcine corneas (n = 8), mounted in a custom artificial anterior chamber, pre and post-surgery whilst subjected to IOP variations from 10mmHg to 30mmHg (10mmHg intervals). Forward-SHG images were acquired, using the Zeiss LSM980 microscope with a 20x objective, over the full corneal depth. Depth-dependent structure and structural features were confirmed consistent across all corneas prior to surgery. Corneas underwent the following procedures standardised at - 3.00D: LASIK (depth = 110µm (n = 2), 220µm (n = 2)), SMILE (depth = 110µm (n = 2), 220µm (n = 2)). Collagen crimp was defined via waviness ratio (wr = length of crimped fibre / straight line distance).

In all corneas, pre-treatment, stromal collagen showed crimp from a depth of 110μm, with crimp period decreasing with depth, and extending under IOP increases (wr = 1.24 ± 0.04 (10mmHg) vs 1.10 ± 0.03 at (30mmHg), p < 0.05). Post-treatment, in LASIK corneas, collagen in the flap became highly crimped, but straight in the 240μm region under the flap, with only deeper lamellae maintaining crimp. On IOP increase, extension of the crimp period was observed only in the deeper residual stromal bed (RSB), not in the flap. In SMILE, distribution of crimp was consistent with that of native corneas, with the crimp period extending on pressure increase in both the cap and RSB (wr = 1.132 ± 0.01 (10mmHg) vs 1.048 ± 0.02 (30mmHg), p < 0.05).

Surgical procedures alter the stress-strain distribution in the cornea. Post-LASIK, strain is concentrated in the RSB only, leading to increased tension in the collagen fibres in this region contributing to topographic flattening. In SMILE, normal collagen architecture is better preserved with strain distributed across both the cap and RSB under IOP variations. These differences may explain the increased prevalence of ectasia after LASIK, and should be considered during procedure selection, and may assist in the optimisation of SMILE treatment algorithms. Further, the biomechanical advantages of keyhole methods should be considered for lenticule addition procedures for the treatment of Keratoconus and presbyopia.