The effect of flap thickness on corneal biomechanics after myopic laser in situ keratomileusis using the M-2 microkeratome

Abstract

PurposeThe purpose of this study was to determine the effect of flap thickness on corneal biomechanics after myopic laser in situ keratomileusis (LASIK).MethodsThis is a prospective controlled non-randomized, institutional study. Patients underwent either epi-LASIK with mitomycin (advanced surface ablation [ASA]), thin-flap LASIK (90 µm head), or thick-flap LASIK (130 µm head). In ASA, the Moria Epi-K hydroseparator was used. LASIK flaps were created using the Moria M-2 mechanical microkeratome. The corneal hysteresis (CH) and corneal resistance factor (CRF) were measured preoperatively and 3 months after surgery, using the Ocular Response Analyzer®.ResultsTen patients (19 eyes) underwent ASA, 11 patients (16 eyes) underwent thin-flap LASIK, and 11 patients (16 eyes) underwent thick-flap LASIK. The mean preoperative CH was 10.47±0.88, 10.52±1.4, and 11.28±1.4 mmHg (p=0.043), respectively, decreasing after surgery by 1.75±1.02, 1.66±1.00, and 2.62±1.03 mmHg (p=0.017). The mean reduction of CH per micron of central corneal ablation was 0.031, 0.023, and 0.049 mmHg/µm (p=0.005). Mean preoperative CRF was 10.11±1.28, 10.34±1.87, and 10.62±1.76 mmHg (p=0.66), decreasing after surgery by 2.33±1.35, 2.77±1.03, and 2.92±1.10 mmHg (p=0.308). The mean reduction of CRF per micron of central corneal ablation was 0.039, 0.040, and 0.051 mmHg/µm (p=0.112).ConclusionThick-flap LASIK caused a greater reduction of CH and CRF than thin-flap LASIK and ASA, although this was statistically significant only for CH. ASA and thin-flap LASIK were found to be biomechanically similar.

Corneal biomechanical changes after myopic and hyperopic laser vision correction

“…In addition, the study revealed that vertical side cuts cause more decrease in corneal strength than horizontal incisions [4]. Similar results were also obtained by other authors in the numerous clinical in vivo studies, confirming that corneal biomechanical parameters such as CH and CRF decrease significantly with the increase of LASIK-flap thickness [13,21,[27][28][29]. Corneal biomechanical changes after myopic and hyperopic laser vision correction Z. Pniakowska, P. Jurowski, J. Wierzbowska…”

Short term changes in corneal stress-strain index and other corneal biomechanical parameters post-laser in situ keratomileusis

“…Corneal biomechanical weakening increases with an increase in the flap thickness and ablation depth. [ 30 31 32 33 ] A thin flap (SBK) allows a thicker residual stromal bed, less damage to the nerves, and induces less biomechanical weakening of the cornea. Thus, it is more relevant to have a LASIK procedure which is tissue saving and inducing the least amount of corneal weakening.…”

“…Although it has been reported that microkeratome tends to over-cut the flap by 10–15 microns on average. [ 30 34 35 ] With this consideration, the present study could be useful to extrapolate biomechanical changes in the eyes with the 110-microns flap surgeries. Also, with the advent of Femtosecond procedures opting for thinner flaps and caps being a trend, the results of the present study would be applicable to clinical practice.…”

Optical Coherence Tomography and Densitometry in Assessing the Effect of Corneal Collagen Cross-Linking Upon Photorefractive Ablation with Riboflavin

“…Photorefractive ablation as a second step of LASIK provides even more profound decrease in corneal strength. The higher is ametropia degree, the greater the decrease in corneal strength is [3][4][5].…”