Corneal nerves are responsible for the perceptions of touch, pain, and temperature and regulate the blink reflex and tear production and secretion.1,2 They release various trophic factors, including neurotrophins, neuropeptides, and neurotransmitters, which play a significant role in physiological homeostatic processes, as well as modulation of epithelial integrity, proliferation, and wound healing on the ocular surface.3 Cornea laser refractive surgery is one of the most performed surgical procedures; it alters the shape and refractive power of the cornea by surgically removing corneal tissue.4,5 However, the corneal nerve plexus is damaged to varying degrees because of the corneal incision and laser ablation or photodisruption.6 Corneal denervation leads to a postoperative neuropathic ocular surface, with the clinical manifestations ranging from decreased tear secretion and volume, decreased tear film stability, increased corneal and conjunctival staining, and decrease or loss of corneal sensitivity to corneal epithelial erosions with neurotrophic epitheliopathy or keratopathy in severe cases.7

Factors that may influence the denervation and regeneration of corneal nerves after refractive surgeries include surgical procedure, age, the flap/ablation parameters, the location of hinge, flap or cap depth, and the refractive power of correction.8 Based on the Munnerlyn formula, the ablation depth in laser in situ keratomileusis (LASIK) or the extracted lenticule thickness is directly proportional to the refractive power to be corrected, and therefore, it is conceivable that the stromal volume removal and resultant-affected corneal nerves are greater in high myopic correction given the same diameter of the optical zone.9 Moreover, the stromal tissue devoid of keratocytes is greater in high myopic correction, which may lead to a greater extent of impairment of biological and nutritional cues from keratocytes for nerve regeneration.10

Zhao et al. demonstrated that nerve recovery in patients with high myopia was slower than that in patients with low-moderate myopia after LASIK.11 In 29.6% and 44.4% of the eyes with high myopia, 1 or 2 subbasal nerve fibers were detectable in an in vivo confocal microscopy (IVCM) micrograph 7 days and 6 months after the surgery, respectively, whereas the figures were 38.5% and 64.8%, respectively, in the eyes with low-moderate myopia. The results showed that the recovery of nerve fibers required an extended period in patients with higher myopia after LASIK. The authors concluded that higher myopic treatment, which is associated with deeper ablation depth and more removal of tissue, caused increased nerve depletion after LASIK. This might further delay the nerve recovery of patients with high myopia after LASIK. Similarly, our group previously showed that compared with low-moderate treatment, high myopic small-incision lenticule extraction (SMILE) (corrected spherical equivalent [SE] >−6.0 diopters [D]) resulted in significantly greater reduction in corneal nerve fiber density (CNFD) at 1 week, 1 month, and 6 months.12 In a separate randomized controlled, paired-eye, cross-sectional study from our group, we demonstrated that most eyes that had undergone LASIK, with low-moderate myopia, had equivalent or better CNFD, corneal nerve branch density (CNBD), and corneal nerve fiber total branch density (CTBD) recovery compared with SMILE at 5.5 years postoperatively.10 These data suggest that the corrected refractive power plays a role in postoperative corneal nerve depletion and restoration. However, the detailed relationship between the extent of corneal denervation, corneal nerve regeneration, and the corrected refractive power has not been elucidated.

In this study, we aimed to investigate the corneal denervation and subsequent regeneration in relation to the corrected refractive power after SMILE and LASIK. We also evaluated the impact of the different refractive power corrected on the postoperative ocular surface status.

METHODS Study Population

This prospective study recruited a total of 44 patients (88 eyes) who had SMILE and LASIK at Singapore National Eye Center between July 2020 and January 2022. The study was conducted in accordance with the tenets of the Declaration of Helsinki, and approval for the study was granted by the institutional review board of SingHealth, Singapore (2020/2050/A). We collected patients' characteristics, including preoperative spherical power, cylindrical power, and preoperative manifest refractive spherical equivalent (MRSE). Patients were divided into low-moderate (MRSE <−6.00 D) and high myopia (MRSE >−6.00 D) groups. The intraoperative parameters, including the cap/flap thickness, and optical zone/ablation zone were also recorded. Due to the fact that the study was not in a randomized controlled trial design, propensity score matching was performed to take into account the factors that may affect baseline corneal nerve status included patient's age, the cap/flap thickness, and optical zone/ablation zone. The propensity score is a scalar summary of all measured pretreatment characteristics (potential confounders).13 The propensity score e(X) is the conditional probability of receiving a certain treatment, vs a comparator, given the measured pretreatment characteristics, X, denoted as e(X) = pr(Z = 1∣∣X), where Z = 1 in the SMILE group and Z = 0 in the LASIK group.14

LASIK and SMILE Procedures

For the LASIK procedure, a superiorly hinged, 100 to 130 μm thick flap was created using the VisuMax femtosecond laser (VisuMax, Carl Zeiss Meditec AG), and the flap diameter was 8.0 ± 0.2 mm (range 7.5 to 8.1 mm). The mean optical zone was 6.5 ± 0.1 mm (range 6.5 to 7.0 mm), and the mean treatment zone was 8.0 ± 0.8 mm (range 6.7 to 9.0 mm). Excimer ablation was performed with the WaveLight EX500 excimer laser (Alcon Laboratories, Inc.) after lifting the flap with a spatula. The flap was carefully repositioned after ablation, and a bandage contact lens was placed.

For the SMILE procedure, suction fixation was applied after the eye was centered and docked with an S-sized curved interface cone. The anterior cap with the anterior surface of the lenticule exceeding the posterior lenticule diameter by 0.5 mm was formed, and a 2.1 mm vertical circumferential incision was placed at 120 degrees with the femtosecond laser.15 The cap thickness was 100 to 130 μm, the mean optical zone was 6.4 ± 0.1 mm (range 6.0 to 6.5 mm), and the mean treatment zone was 6.6 ± 0.1 mm (range 6.2 to 6.7 mm). The dissection of the anterior plane and the posterior border was performed by inserting a SMILE dissector (ASICO LLC) through the incision after suction release. The lenticule was then grasped and removed through the incision using an EndoGlide microforceps (AngioTech/Network Medical Products).

All the procedures were performed under topical anesthesia by the same refractive surgeon (J.S.M.). All the patients received the same postoperative regimen, consisting of topical preservative-free dexamethasone (Maxidex, Alcon Laboratories, Inc.) and moxifloxacin (Vigamox, Alcon Laboratories, Inc.), 3 hourly for a week and then 4 times a day for 2 weeks. Artificial tears (Tears Naturale Free, Alcon Laboratories, Inc.) were also prescribed, with the frequency of 4 times daily for the first 2 months, and were applied if needed afterward.

IVCM and Image Analysis

IVCM (Heidelberg Retina Tomography III, Rostock Cornea Module, Heidelberg Engineering GmbH) was used for the evaluation of the subbasal nerve plexus preoperatively and at 1 month, 3 months, 6 months, and 12 months postoperatively. Patients were asked to fixate on a light source, and the subbasal nerve plexus of the central cornea and 4 quadrants of peripheral areas, which were 3 mm away from the corneal apex, were recorded with a field of view of 400 × 400 mm2. For the image analysis, 5 most representative and best-focused images of subbasal nerves were selected for each area, and each nerve (main truck or branched nerve) was selected only once. These 25 micrographs selected for each eye were analyzed using ACCMetrics software (University of Manchester) for the following nerve metrics: CNFD (the number of fibers/mm2, each frame area = 0.16033 mm2), CNBD (the number of branch points on the main fibers/mm2), corneal nerve fiber length (CNFL; the total length of fibers in mm/mm2), CTBD (the total number of branch points/mm2), corneal nerve fiber area (CNFA; the total nerve fiber area in mm2/mm2), corneal nerve fiber width (CNFW; mean nerve fiber width in mm/mm2), and nerve fiber fractal dimension (CFracDim). CFracDim represents the spatial loss of nerves, and the higher CFracDim value corresponds to the more evenly distribution of the corneal nerve fiber.16,17 We used 1-month nerve data to assess the relationship between the postoperative corneal denervation status and refractive power because 1-month data were less interfered by subsequent nerve regeneration activity. We expressed the percentage of nerve reduction=1−1−month resultspreoperative results.

Clinical Assessments

Ocular surface assessments contained the Schirmer test (without anesthesia, mm/5 minutes), ocular surface fluorescein staining (Oxford score; 0: absent, 5: severe), corneal fluorescein staining (National Eye Institute scale; 0: minimal, 15: maximal), tear break-up time (TBUT), and corneal sensitivity (Cochet-Bonnet aesthesiometer; Luneau Ophthalmologia; 0 to 6 cm for each quadrant and central cornea, 0 to 30 cm for the entire cornea), as described previously.18 Three measurements in the same visit were taken for all the assessments, and the mean was used for analysis. All patients received assessment preoperatively and at 1 month, 3 months, 6 months, and 12 months postoperatively.

Statistical Analysis

The sample size was calculated using the results of the first 5 patients and CNFL as the primary outcome because CNFL has been shown the most reliable parameter.16 The CNFL reduction was 43.2% ± 13.2% and 26.8% ± 8.5% in high myopic SMILE and low-moderate myopic SMILE, respectively, and was 47.9% ± 6.2% and 34.1% ± 6.0% in high myopic LASIK and low-moderate myopic LASIK, respectively. Considering a statistical power of 80% and a significance level of 5%, a sample size of 9 and 5 eyes was required to confirm the differences between the high myopic and low-moderate myopic groups for SMILE and LASIK, respectively. The comparisons between the low-moderate and high myopia groups or between SMILE and LASIK eyes were performed using an independent t test. The data of both eyes were used for statistical analysis. The Pearson correlation was used to screen and visualize the parameters. A linear mixed model was further applied to analyze the relationship between the nerve parameters and corrected MRSE, as well as the association between the nerve variables and clinical assessments, to take into account the correlation between both eyes and repeated measures. A P value less than 0.05 was considered statistically significant.

RESULTS Patient Characteristics

In the propensity score matching analysis, there was no significant difference in the probability between the SMILE and LASIK groups (P = .159). The mean age of patients was 29.2 ± 5.0 years and 31.4 ± 5.7 years for the SMILE and LASIK groups, respectively (P = .190). Among them, 68.1% of patients were female. Among the 88 eyes in our study, 24 eyes had SMILE treatment for their low-moderate myopia (−4.14 ± 1.14 D), 36 eyes had LASIK treatment for their low-moderate myopia (−4.26 ± 0.88 D), 21 eyes had SMILE treatment for their high myopia (−8.80 ± 1.96 D), and 7 eyes had LASIK for their high myopia (−6.48 ± 0.37 D).

Impact of Corrected Refractive Power on Corneal Denervation in SMILE

The high myopic SMILE, compared with the low-moderate myopic SMILE, presented with a significantly greater extent of corneal denervation postoperatively in the aspect of CNFA reduction (48.6% ± 6.2% vs 20.9% ± 4.9%, P = .003, respectively) and CFracDim reduction (68.1% ± 1.6% vs 32.7% ± 0.4%, P = .049, respectively; Table 1). There was a significant and negative correlation between the corrected MRSE and the reduction in CNFD (r = −0.66, P < .001), CNBD (r = −0.47, P = .007), CNFL (r = −0.61, P = .002), CNFA (r = −0.55, P = .001), and CFracDim (r = −0.38, P = .032), suggesting that greater corrected MRSE resulted in more significant reduction in corneal nerve metrics. Figure 1 illustrates the impacts of the corrected MRSE on corneal nerve metrics in SMILE with quadratic regression plots.

Table 1. - Reduction in each nerve parameter sorted by surgery and refractive power Parameter Low-moderate myopic SMILE (%) High myopic SMILE (%) P valuea Low-moderate myopic LASIK (%) High myopic LASIK (%) P valueb P valuec P valued 1 mo  CNFD 48.1 ± 10.0 55.4 ± 11.5 .640 85.4 ± 5.4 81.6 ± 9.6 .744 .001* .177  CNBD 48.1 ± 12.2 72.6 ± 9.6 .128 94.4 ± 2.4 91.4 ± 8.6 .635 <.001* .246  CNFL 34.7 ± 5.2 46.9 ± 8.4 .241 61.9 ± 4.0 64.7 ± 4.4 .732 <.001* .194  CTBD 39.1 ± 11.5 56.4 ± 7.2 .208 57.8 ± 5.2 74.6 ± 8.4 .132 .100 .156  CNFA 20.9 ± 4.9 48.6 ± 6.2 .003* 39.6 ± 5.1 48.4 ± 8.6 .422 .023* .989  CNFW −3.5 ± 2.7 −8.5 ± 4.2 .346 −16.2 ± 2.3 −31.4 ± 7.9 .018* .002* .014*  CFracDim 3.3 ± 0.4 6.8 ± 1.6 .049* 7.3 ± 1.0 7.5 ± 1.0 .906 .010* .777 3 mo  CNFD 36.3 ± 17.2 22.4 ± 13.0 .540 75.5 ± 5.7 86.8 ± 6.6 .453 .010* .021*  CNBD 14.7 ± 49.9 12.4 ± 23.3 .970 84.0 ± 5.4 100 .259 .043* .144  CNFL 35.3 ± 10.6 30.7 ± 8.1 .741 62.7 ± 4.7 75.0 ± 5.5 .325 .004* .015*  CTBD 25.3 ± 23.9 36.8 ± 14.8 .697 70.1 ± 5.3 67.1 ± 17.1 .840 .012* .057  CNFA 20.0 ± 13.8 33.1 ± 9.2 .455 47.9 ± 4.7 64.6 ± 9.5 .193 .011* .078  CNFW −2.7 ± 5.0 1.5 ± 1.9 .471 −7.5 ± 1.7 −27.8 ± 9.3 .001* .259 .024*  CFracDim 5.2 ± 1.6 32.0 ± 17.6 .127 9.4 ± 1.2 12.9 ± 1.8 .263 .037* .261 6 mo  CNFD 48.9 ± 7.4 46.9 ± 7.4 .887 71.5 ± 5.3 90.8 ± 9.2 .192 .001* .015*  CNBD 51.3 ± 11.0 57.3 ± 17.0 .760 78.8 ± 5.7 88.1 ± 11.9 .557 .043* .045*  CNFL 41.2 ± 5.2 41.3 ± 8.3 .995 54.3 ± 3.1 66.7 ± 8.9 .166 .002* .010*  CTBD 38.2 ± 16.1 49.5 ± 10.5 .614 64.8 ± 5.4 42.7 ± 21.9 .177 .015* .270  CNFA 32.7 ± 6.6 36.2 ± 6.1 .723 38.0 ± 4.0 48.8 ± 14.8 .354 .021* .078  CNFW −1.1 ± 2.2 −2.8 ± 2.5 .620 −28.5 ± 6.8 −9.3 ± 2.3 .007* .022* .022*  CFracDim 5.6 ± 0.9 5.1 ± 1.6 .774 7.1 ± 0.7 10.8 ± 1.9 .064 .013* .512 12 mo  CNFD 29.4 ± 7.6 32.3 ± 6.4 .781 62.2 ± 4.6 80.7 ± 19.3 .189 <.001* .007*  CNBD 41.8 ± 17.0 49.6 ± 8.9 .658 72.4 ± 4.1 88.5 ± 11.5 .170 <.001* .261  CNFL 29.4 ± 4.5 34.5 ± 5.7 .503 46.8 ± 3.3 54.3 ± 7.1 .421 .001* .026*  CTBD 41.5 ± 18.9 39.6 ± 8.3 .912 49.4 ± 5.2 71.6 ± 14.2 .139 .585 .126  CNFA 36.0 ± 9.9 27.3 ± 5.3 .403 29.7 ± 4.1 45.4 ± 13.0 .190 .488 .026*  CNFW 1.7 ± 2.8 −1.5 ± 1.2 .233 −5.9 ± 1.6 −30.2 ± 14.3 .001* .017* .004*  CFracDim 4.3 ± 1.0 2.9 ± 0.6 .248 6.2 ± 0.8 7.3 ± 0.7 .620 .006* .007*

CFracDim = corneal nerve fiber fractal dimension; CNBD = corneal nerve branch density; CNFA = corneal nerve fiber area; CNFD = corneal nerve fiber density; CNFL = corneal nerve fiber length; CNFW = corneal nerve fiber width; CTBD = corneal nerve fiber total branch density

*Statistically significant

aP values comparing low-moderate myopic SMILE vs high myopic SMILE

bP values comparing low-moderate myopic LASIK vs high myopic LASIK

cP values comparing low-moderate myopic SMILE vs low-moderate myopic LASIK

dP values comparing high myopic SMILE vs high myopic LASIK

Figure 1.:

Regression plots showing the correlation between the corrected MRSE and the reduction in CNFD (A), CNBD (B), CNFL (C), CNFA (D), and CFracDim (E) in SMILE. Red line indicates the quadratic fitted line. CFracDim = corneal nerve fiber fractal dimension; CNBD = corneal nerve branch density; CNFA = corneal nerve fiber area; CNFD = corneal nerve fiber density; CNFL = corneal nerve fiber length; MRSE = manifest refraction spherical equivalent

Impact of the Corrected Refractive Power on Corneal Denervation in LASIK

Compared with low-moderate myopic LASIK, high myopic LASIK led to a significant increase in CNFW at all the timepoints (P = .018, P = .001, P = .007, and P = .001; Table 1). The corrected MRSE presented a significant and negative correlation with the reduction in CNBD, CTBD, and CNFA (r = −0.37, P = .015; r = −0.42, P = .005; and r = −0.41, P = .006, respectively), suggesting that greater corrected MRSE resulted in more significant reduction in corneal nerve metrics. There was also a significant correlation between the corrected MRSE and CNFW (r = 0.43, P = .004) in a positive direction. Figure 2 demonstrates the relationship between the corrected MRSE and corneal nerve parameters after LASIK.

Figure 2.:

Regression plots showing the correlation between the corrected MRSE and the reduction in CNBD (A), CTBD (B), CNFA (C), and CNFW (D) in LASIK. Red line indicates the quadratic fitted line. CNBD = corneal nerve branch density; CNFA = corneal nerve fiber area; CNFW = corneal nerve fiber width; CTBD = corneal nerve fiber total branch density; MRSE = manifest refraction spherical equivalent

Comparison of Corneal Denervation in SMILE and LASIK

CNBD and CNFA were included for the regression analysis to compare the corneal denervation in SMILE vs LASIK because these 2 parameters are the common significant parameters associated with the corrected MRSE for both surgical types. In the mixed linear model analysis with the adjustment of the cap/flap thickness and optical zone/ablation zone, we found that when the corrected MRSE increased 1 D, the CNBD reduced 4.9% after SMILE (R2 = 0.220, β0 = 30.1%, P = .007), whereas it was 6.1% after LASIK (R2 = 0.140, β0 = 63.4%, P = .015). Similarly, with the increase in every diopter in the corrected MRSE, the CNFA reduced 4.0% after SMILE (R2 = 0.304, β0 = 10.5%, P = .001) but 9.1% after LASIK (R2 = 0.172, β0 = 1.7%, P = .006) (Figure 3).

Figure 3.:

Linear mixed model analysis demonstrating the relationship between the preoperative MRSE and the reduction in CNBD (A) or CNFA (B) in SMILE and LASIK. Blue line represents the quadratic fitted line in SMILE, and red line represents the quadratic fitted line in LASIK. CNBD = corneal nerve branch density; CNFA = corneal nerve fiber area; MRSE = manifest refraction spherical equivalent

For low-moderate myopic correction, post-SMILE patients had significantly less reduction in CNFD, CNBD, CNFL, and CFracDim than post-LASIK patients at all timepoints (all P values < 0.05). LASIK led to significantly greater reduction in CTBD at 3 and 6 months (P = .012, P = .015), as well as in CNFA at 1, 3, and 6 months (P = .023, P = .011, and P = .021, respectively). There was also a significant difference in the postoperative changes in CNFW between SMILE and LASIK at postoperative 1, 6, and 12 months (P = .002, P = .022, and P = .017, respectively; Table 1).

For high myopic correction, LASIK resulted in significantly greater changes in CNFW at 1 month (P = .014). Post-SMILE patients had significantly less reduction in CNFD, CNFL, and CNFW than post-LASIK patients at 3 months (P = .021, P = .015, and P = .024, respectively). There was also a significant difference in the changes of CNFD, CNBD, CNFL, and CNFW between 2 groups at 6 months, as well as in the CNFD, CNFL, CNFA, CNFW, and CFracDim at 12 months (all P values < 0.05; Table 1).

Impact of Refractive Power and Corneal Denervation on Ocular Surface Parameters

There was a significant association between postoperative Schirmer test values and the reduction in CNFA (β0 = −8.93; P < .001) and CFracDim (β0 = −59.70; P < .001) after the adjustment of the cap/flap thickness, optical/ablation zone, and MRSE (Figure 4).

Figure 4.:

Relationship between Schirmer test values and the reduction in CNFA (A) and CFracDim (B). Red line indicates the quadratic fitted line. CFracDim = corneal nerve fiber fractal dimension; CNFA = corneal nerve fiber area

In the SMILE group, high myopic treatment led to significantly worse TBUT than low-moderate myopic treatment (P = .049 at 1 month; P < .001 at 6 months). We did not observe this difference in the post-LASIK eyes (Table 2).

Table 2. - Clinical parameters sorted by surgery and refractive power Parameter Low-moderate myopic SMILE High myopic SMILE P valuea Low-moderate myopic LASIK High myopic LASIK P valueb P valuec P valued 1 mo  Schirmer's values 14.6 ± 1.6 12.4 ± 1.9 .839 8.5 ± 1.0 10.2 ± 1.9 .832 .002* .580  Oxford score 0.7 ± 0.1 0.5 ± 0.2 .761 0.7 ± 0.2 0.8 ± 0.5 .431 .003* .240  NEI score 1.3 ± 0.4 1.2 ± 0.3 .237 1.3 ± 0.4 0.8 ± 0.5 .527 .874 .550  TBUT 8.2 ± 0.4 6.5 ± 0.8 <.001* 6.2 ± 0.5 3.8 ± 0.9 .349 .002* .110  Corneal sensitivity 27.7 ± 0.4 28.3 ± 0.4 .406 25.8 ± 0.8 30.0 ± 0.0 .098 .037* .040* 3 mo  Schirmer's values 12.1 ± 2.1