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  5. FP1553 : Effect of cap depth in SMILE on clinical, biomechanical and molecular outcomes

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Dr. SHARON DSOUZA

Dr. POOJA KHAMAR, Dr. ROHIT SHETTY, DR. RITIKA MULLICK


Abstract

AIM- To evaluate the effect of different SMILE cap depth on postoperative visual outcome, corneal nerves, healing and dry eye

METHODS- 100 eyes(50 patients) underwent SMILE with cap depth of 100 &150 µ in contralateral eyes. Visual outcome, epithelial remodelling, biomechanics,nerve changes(IVCM),tear molecular factors were analysed at 1, 3 and 6 months postoperative and compared to presurgery values.

RESULTS – Eyes with 100 µ cap depth showed slower subbasal nerve regeneration than 150µ but recovered by 6 months. No difference noted in epithelial healing and visual outcomes. Transient dry eye noted in both groups Pre and post biomechanics evaluated. Tear molecular factors shows slightly elevated inflammatory factors at 100 µ cap depth

CONCLUSION- Different cap depths in SMILE could influence early and late postsurgical outcomes by its impact on wound healing, inflammation and biomechanics

Full Text  

EFFECT OF CAP DEPTH ON SMILE ON CLINICAL, BIOMECHANICAL AND MOLECULAR OUTCOMES

ABSTRACT

AIM- To evaluate the effect of different SMILE cap depth on postoperative visual outcome, corneal nerves, healing and dry eye

RESULTS – Eyes with 100 µ cap depth showed slower subbasal nerve regeneration than 150µ but recovered by 6 months. No difference noted in epithelial healing and visual outcomes. Transient dry eye noted in both groups Pre and post biomechanics evaluated. Tear molecular factors analysed shows slightly elevated inflammatory factors at 100 µ cap depth

CONCLUSION-
Different cap depths in SMILE could influence early and late postsurgical outcomes by its impact on wound healing, inflammation and biomechanics

Introduction

Small incision lenticule extraction (SMILE) by the VisuMax femtosecond system (Carl Zeiss Meditec AG,Jena, Germany) is a flapless minimally invasive laser vision correction (LVC) for the treatment of myopia and myopic astigmatism. The stability and safety of the procedure has been studied in different studies and has been compared to other forms of LVC.1,2 An advantage of the laser platform is the ability to customize treatment parameters like the cap depth and morphology.3 The quality of vision, stability of refractive error correction, wound healing, dry eye incidence and corneal biomechanics are important parameters to study outcomes of any LVC. Changes in the epithelial thickness and wound healing after SMILE may have a bearing on this.9 Higher order aberrations like spherical aberrations were also shown to be less with deeper cap of 150 microns in another study in the early postoperative period.10 The incidence of dry eye post SMILE surgery has been shown to be less compared to LASIK in certain studies,13,14 but still needs further evaluation.

15 Measuring the tear inflammatory factors give us a good indication of the causes of inflammation, irritation and pain in the eye and need for modifying therapy if required.18 These measurements reveal how the depth of ablation affects nerve related parameters and if the ablation depth increases the chances of postsurgical dry eye and inflammation. Biomechanical changes post SMILE surgery are an important indicator of corneal strength and risk of ectasia. Varying results have been seen in studies comparing LASIK and SMILE with some showing similar results,19,20 and others better results in SMILE.21, The above described observations and literature point out an important aspect of refractive surgery, the cap depth that may affect treatment outcomes. Therefore we have proposed a comprehensive study on eyes undergoing surgery measuring the various imaging biomarkers apart from the clinical and visual parameters as well as outcomes.

Methods

This was a prospective interventional contralateral eye study done at a tertiary care eye hospital. The study was approved by the institutional ethics committee and was conducted as per the declaration of Helsinki. All participants gave prior written informed consent before being included in the study. 100 eyes of 50 patients who underwent SMILE with cap depth of 100 &150 µ in contralateral eyes where included in the study. The study parameters included Visual outcome, epithelial remodelling, biomechanics,nerve changes(IVCM),tear molecular factors were analysed at 1, 3 and 6 months postoperative. These values were compared to presurgery values. Exclusion criteria were – patients not suitable for refractive surgery due to abnormal topography or excess refractive error beyond recommended limit, keratoconus or other corneal ectasia, severe aqueous deficiency dry eye or other chronic ocular surface disorder, corneal scarring, active ocular infection, previous ocular surgery.

Results

All patients were followed up for a period of 6 months postoperatively and their results were compared to preoperative values. At the end of 6 months all eyes included in the study achieved an uncorrected visual acuity of 6/6 or better. No eyes had a loss of lines of vision. No difference in post operative discomfort was noted between the 2 eyes. Eyes with 100 µ cap depth showed slower subbasal nerve regeneration than 150µ but recovered by 6 months. No difference noted in epithelial healing and visual outcomes. Transient dry eye was noted in both groups which recovered completely by 6 months in all patients. Pre and post biomechanics evaluated was also comparable between the groups.

Discussion

Several studies have evaluated the accuracy, precision, and reproducibility of different cap thicknesses in SMILE.4,5 However there is no conclusive evidence to decide on an ideal depth.6 In a contralateral eye study done, no statistically significant differences in the refractive outcome and higher order aberrations were reported between the depths of 100 and 160 mm,7 which was similar to the findings in this study. The depth of the cap has been shown to have a bearing on post operative refractive correction with deeper caps found to have undercorrection in high myopic correction.8 On the other hand, in another study deeper cap thickness were shown to have better recovery, less wound healing response especially in the thicker corneas undergoing SMILE surgery.3

A thicker cap may also cause less stromal nerve damage and therefore less dry eye, 8 however in this study no such difference was noted. Few studies have evaluated the relation of the cap depth with biomechanics of the cornea.23,24 Anatomically, the anterior 1/3 of the stroma has an interwoven arrangement while the posterior 2/3 has a more lamellar arrangement of fibres.25 This could be responsible for the decrease in corneal strength as we go deeper into corneal stroma. No difference was seen in this study

Conclusion

Different cap depths in SMILE could influence early and late postsurgical outcomes by its impact on wound healing, inflammation and biomechanics. However at the depths specified, no significant difference was noted.

REFERENCES

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