Comparison of biomechanical effects of small-incision lenticule extraction and laser in situ keratomileusis: Finite-element analysis

Abstract

PURPOSE: To theoretically compare the corneal stress distribution of laser in situ keratomileusis (LASIK) with the stress distribution of small-incision lenticule extraction. SETTING: Cleveland Clinic Cole Institute, Cleveland, and The Ohio State University, Columbus, Ohio, USA. DESIGN: Computational modeling study. METHODS: A finite-element anisotropic collagen fiber–dependent model of myopic surgery using patient-specific corneal geometry was constructed for LASIK, small-incision lenticule extraction, and a geometry analog model with unaltered material properties from preoperative but with postoperative geometry including thickness. Surgical parameters, magnitude of myopic correction, LASIK flap thickness, and lenticule depth in small-incision lenticule extraction were varied. Two sets of models, 1 with uniform and 1 with depth-dependent material properties, were constructed. RESULTS: Stress distribution between small-incision lenticule extraction simulations and the geometry analog model were similar. In contrast, LASIK consistently reduced stress in the flap and increased stress in the residual stromal bed (RSB) compared with the geometry analog model. An increase in flap thickness or lenticule depth resulted in a greater increase in RSB stress in the LASIK model than in the small-incision lenticule extraction model. CONCLUSIONS: Small-incision lenticule extraction may present less biomechanical risk to the residual bed of susceptible corneas than comparable corrections involving LASIK flaps. Deeper corrections in the stroma may be possible in small-incision lenticule extraction without added risk for ectasia.

Small-incision lenticule extraction

“…Assessment of corneal stress differences between LASIK and small-incision lenticule extraction through use of an ancillary mathematical formula determined that small-incision lenticule extraction conferred a protective effect on the residual stromal bed (RSB) compared with LASIK. 77 Diminished cumulative tension on the RSB is thought to potentially offer advantages such as avoiding refractive instability due to anterior corneal shift as well as the incidence of postoperative ectasia. Reinstein et al 78 evaluated postoperative stromal thickness variation after small-incision lenticule extraction and found a stromal augmentation of 8 mm compared with the lenticule depth measurement originally indicated by the femtosecond platform software.…”

Refractive Correction and Biomechanical Strength Following SMILE With a 110- or 160-μm Cap Thickness, Evaluated Ex Vivo by Inflation Test

“…3 The intrastromal lenticule can be created in the desired corneal depth and leaves the anterior stromal layer (the cap) intact. Mathematical 4 and computational modelling analyses 5,6 suggest that SMILE may be better at preserving the biomechanical integrity than other flapbased laser refractive procedures such as laser-assisted in situ keratomileusis (LASIK) and femtosecond lenticule extraction (FLEX). In vivo biomechanical studies evaluating the corneal deformation response with noncontact tonometry have been inconsistent, and SMILE has been equal or superior to LASIK.…”

Bilateral ectasia after femtosecond laser–assisted small-incision lenticule extraction

“…Characteristics of this technique would make it conceivable to consider small-incision lenticule extraction as minimally altering corneal biomechanics, as recently reported in the literature. 4,5 This supposed advantage of small-incision lenticule extraction is believed to rely on the absence of the flap lamellar cut that strongly weakens the biomechanical integrity of the cornea and also that refractive lenticule dissection preserves the integrity of the most anterior stromal lamellae, which are known to provide the greatest corneal biomechanical strength. 6 However, this principle can be applied most likely to normal corneas and not to keratoconic corneas, in which the structure of the corneal stroma is pathological.…”