Since the first description of small-incision lenticule extraction (SMILE, Carl Zeiss Meditec AG) in 2011 by Sekundo et al.,1 the number of procedures performed to correct myopia and astigmatism with this new technique has steadily increased. The incidence of corneal ectasia after laser in situ keratomileusis (LASIK) has been estimated at 1 in 2500.2 Because small-incision lenticule extraction is a flapless procedure, it has been suggested that the risk for corneal ectasia after small-incision lenticule extraction is lower than after LASIK.3 To date, 4 reports of corneal ectasia have been published.4–8 We report a case of unilateral corneal ectasia 4 years after small-incision lenticule extraction for mild myopia in a patient older than 40 years with preoperative asymmetric astigmatism.
A 43-year-old man had small-incision lenticule extraction in both eyes in 2013. The preoperative refraction was −3.75 −1.25 × 5 in the right eye and −5.25 −0.75 × 180 in the left eye. The corrected distance visual acuity (CDVA) was 20/20 in both eyes. The patient had no family history of keratoconus. Before surgery, the central corneal thickness was 524 μm in the right eye and 517 μm in the left eye, with minimal thickness of 522 μm and 515 μm, respectively, measured with a rotating Scheimpflug camera (Pentacam, Oculus Optikgeräte GmbH) (Figure 1). Preoperative topography analysis showed the presence of asymmetric astigmatism with an elevated area on the inferior anterior corneal surface in both eyes, representing a steepening of more than 2.00 diopters (D) in the right eye and 1.80 D in the left eye. Moreover, Scheimpflug camera images showed an absence of posterior curvature bulging and no correspondence between the steeper anterior curvature and the thinner points of the cornea.
Small-incision lenticule extraction was performed uneventfully with the Visumax femtosecond laser system (Carl Zeiss Meditec AG). The caps were 7.30 mm in diameter and 120 μm thick in both eyes. The diameter of the optical zone was 6.50 mm in both eyes. The maximum and minimum lenticule thicknesses were 94 μm and 15 μm, respectively, in the right eye and 109 μm and 15 μm, respectively, in the left eye, with a residual stromal bed (RSB) of 308 μm in the right eye and 286 μm in the left eye. The immediate postoperative course was uneventful, with an uncorrected distance visual acuity of 20/20 in both eyes at the 1-month examination.
Four years after the initial small-incision lenticule extraction, the patient reported vision loss in the right eye. The CDVA was 20/40 with −1.75 −1.50 × 60 in the right eye and 20/20 with −0.25 −0.25 × 65 in the left eye. The corneal thickness at the thinnest point, determined by Scheimpflug pachymetry, was 407 μm in and 418 μm, respectively. Ectasia was clearly visible in the right eye, which showed posterior bulging and an inferior steepening of more than 8.00 D. Figure 2 shows topographies with ectasia present. Standard corneal crosslinking (CXL) with a 3 mW treatment for 30 minutes was performed in the patient's right eye. After 6 months, the topographies remained stable (Figures 3 and 4).
To our knowledge, this is the fifth report of corneal ectasia after small-incision lenticule extraction and the first report of a patient older than 40 years.4–8 Two previous reports4,5 were of cases of ectasia after small-incision lenticule extraction with preoperative signs of forme fruste keratoconus; however, ectasia has also been observed in patients with normal preoperative data.6 Mattila and Holopainen7 reported a case of bilateral ectasia after small-incision lenticule extraction in which 1 eye had signs of keratoconus before surgery, whereas the preoperative data for the contralateral eye were normal.
Retrospectively, a diagnosis of forme fruste keratoconus was discussed. The preoperative data showed the presence of asymmetric astigmatism without the posterior elevation of the cornea that is considered to be an early indicator of keratoconus. The optimum cutoff point for posterior corneal elevation for distinguishing forme fruste keratoconus from normal corneas on Scheimpflug analysis is 29 μm or 16 μm on scanning-slit corneal topography analysis (Orbscan IIz, Bausch & Lomb, Inc.).2,9
The patient presented with a clear case of ectasia of the right eye, which showed posterior bulging and inferior steepening of more than 8.00 D. In contrast, postoperative Scheimpflug images in the left eye showed an absence of features typical of ectasia with no posterior bulging. However, the left eye has required frequent topographic monitoring so that CXL can be performed if ectasia develops.
Several studies have reported ectasia after LASIK in patients with and without predisposing factors.2 The cornea is a complex anisotropic composite with nonlinear elastic and viscoelastic properties. The anterior cornea and peripheral cornea are the strongest parts of this structure because of their higher levels of intralamellar collagen branching. The posterior and central parts of the cornea are weaker. Small-incision lenticule extraction is a flapless procedure and preserves the collagen networks of the anterior stroma that account for 60% of the total corneal tensile strength.8,10 It has therefore been suggested that this procedure damages the biomechanical properties of the cornea to a lesser extent than femtosecond laser–assisted LASIK.11 A finite-element model for theoretical comparison of the distribution of corneal stress after LASIK and small-incision lenticule extraction treatments has been used by Sinha Roy et al.12 According to this model, stress increases on the posterior cornea and decreases on the anterior cornea after LASIK. In contrast, the distribution of stress after small-incision lenticule extraction simulation more closely resembles the geometric analog model. It has been suggested that changes in the distribution of stress after LASIK increase the risk for corneal ectasia to a greater extent than small-incision lenticule extraction.13
The biomechanical properties of the cornea can be estimated in vivo by measuring corneal hysteresis (CH) and the corneal resistance factor (CRF) with a biomechanical waveform analyzer (Ocular Response Analyzer, Reichert Technologies). Wu et al.11 compared the change in CH and CRF values between small-incision lenticule extraction and femtosecond laser–assisted LASIK. They found a significantly lower decrease in CH and the CRF 6 months after small-incision lenticule extraction than 6 months after LASIK. Another study also reported a smaller decrease in CH and CRF values after small-incision lenticule extraction than after LASIK, but only in patients with myopia greater than 6.00 D.14
Thus, practitioners who recognize the improved biomechanics of small-incision lenticule extraction have proposed extracting a deeper lenticule and leaving a lower safety threshold RSB given the preserved integrity of the stronger anterior stroma.3,8 Moshirfar et al.8 recently proposed a modified percentage tissue altered formula that accounts for the difference in anterior corneal lamellae disruption between LASIK and small-incision lenticule extraction. More than 750 000 cases of small-incision lenticule extraction have been performed worldwide, and 4 reports of corneal ectasia have been published. Thus, the literature lacks data to assess the risk for ectasia. The authors stated that more data from cases that develop ectasia after small-incision lenticule extraction are needed to fully delineate the association between the percentage tissue altered and the risk for postoperative ectasia and that an accurate calculation of the percentage tissue altered must be developed using these data.8
Age is a risk factor for ectasia after refractive surgery. The stiffness of the human cornea increases with age.14 According to Randleman et al.,15 an age younger than 30 years is associated with a higher risk for ectasia. The ages of patients developing ectasia after small-incision lenticule extraction reported in previous studies range from 19 to 33 years.4–8 This case report indicates that ectasia can occur after small-incision lenticule extraction in patients older than 40 years (43 years in this case) with asymmetric astigmatism. Considering that preoperative asymmetric astigmatism could already be considered a sign of disease, further studies should examine the patient characteristics that predispose individuals to develop ectasia after small-incision lenticule extraction and compare those with characteristics of similar patients who developed ectasia after LASIK. For now, preoperative screening for patient selection having small-incision lenticule extraction should be as restrictive as that for LASIK correction.8
Sekundo W, Kunert KS, Blum M. Small incision corneal refractive surgery using the small incision lenticule extraction (SMILE) procedure for the correction of myopia and myopic astigmatism: results of a 6 month prospective study. Br J Ophthalmol
Randleman JB, Russell B, Ward MA, Thompson KP, Stulting RD. Risk factors and prognosis for corneal ectasia after LASIK. Ophthalmology
Reinstein DZ, Archer TJ, Randleman JB. (2013). Mathematical model to compare the relative tensile strength of the cornea after PRK, LASIK, and small incision lenticule extraction. J Refract Surg, 29
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El-Naggar MT. Bilateral ectasia after femtosecond laser–assisted small-incision lenticule extraction. J Cataract Refract Surg
Wang Y, Cui C, Li Z, Tao X, Zhang C, Zhang X, Mu G. Corneal ectasia 6.5 months after small-incision lenticule extraction. J Cataract Refract Surg
Sachdev G, Sachdev MS, Sachdev R, Gupta H. Unilateral corneal ectasia following small-incision lenticule extraction. J Cataract Refract Surg
Mattila JS, Holopainen JM. Bilateral ectasia after femtosecond laser-assisted small incision lenticule extraction (SMILE). J Refract Surg
Moshirfar M, Albarracin JC, Desautels JD, Birdsong OC, Linn SH, Hoopes PC Sr. (2017). Ectasia following small-incision lenticule extraction (SMILE): a review of the literature. Clin Ophthalmol, 11
, 1683-1688, Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5608083/pdf/opth-11-1683.pdf
De Sanctis U, Loiacono C, Richiardi L, Turco D, Mutani B, Grignolo FM. Sensitivity and specificity of posterior corneal elevation measured by Pentacam in discriminating keratoconus/subclinical keratoconus. Ophthalmology
Wu D, Wang Y, Zhang L, Wei S, Tang X. Corneal biomechanical effects: small-incision lenticule extraction versus femtosecond laser–assisted laser in situ keratomileusis. J Cataract Refract Surg
Sinha Roy A, Dupps WJ Jr, Roberts CJ. Comparison of biomechanical effects of small-incision lenticule extraction and laser in situ keratomileusis: finite-element analysis. J Cataract Refract Surg
Kamiya K, Shimizu K, Igarashi A, Kobashi H, Sato N, Ishii R. Intraindividual comparison of changes in corneal biomechanical parameters after femtosecond lenticule extraction and small-incision lenticule extraction. J Cataract Refract Surg
Knox Cartwright NE, Tyrer JR, Marshall J. (2011). Age-related differences in the elasticity of the human cornea. Invest Ophthalmol Vis Sci, 52
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Randleman JB, Woodward M, Lynn MJ, Stulting RD. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology
Disclosures:None of the authors has a financial or proprietary interest in any material or method mentioned.