HISTOLOGY OF THE HUMAN EYE : An Atlas and Textbook

PURPOSE: To develop a mathematical model to estimate the relative differences in postoperative stromal tensile strength following photorefractive keratectomy (PRK), LASIK, and small incision lenticule extraction (SMILE).

http://www.choeye.co.kr/webi_board/upFile/mathematical_model_to_compare_the_reletive_tensile_strength_of_the_cornea_after_PRK_Lasik_Smile.pdf

Mathematical model to compare the relative tensile strength of the cornea after PRK, LASIK, and small incision lenticule extraction.

Changes in the biomechanical properties of the human cornea play an important role in the pathogenesis of corneal ectatic diseases. Biomechanical investigation shows significant differences between human ectatic corneas and normal corneas, including decreased stiffness and reduction of collagen crosslinks in the ectatic cornea. Induction of crosslinks is a well-established procedure in polymer chemistry to increase the elastic modulus of materials. Crosslinking (CXL) in connective tissue can occur during aging and as a side effect of diabetes mellitus. CXL has been used medically to increase stability and reduce the biodegradation of collagen-based biomaterials for bioprostheses. CXL of the cornea using riboflavin and UVA light with a wavelength of 370 nm and a dosage of 5.4 J/cm² is a new approach that increases the mechanical and biochemical stability of stromal tissue. This technique combines the principles of CXL (chemical and nonenzymatic) and the biochemical mechanisms of photo-oxidative CXL with riboflavin as a photosensitizer. In this review, the enrichment of riboflavin in the stroma by standard (epi-off) and transepithelial (epi-on) CXL is discussed. The theoretical and experimental measurements of the absorption of UV light explain the stronger CXL effect in the anterior stroma and its importance for the prevention of damage to the endothelial cells. UV devices are described. Changes of the physical properties after CXL, as well as the cellular changes, are discussed. From these basic investigations, treatment parameters for effective and safe CXL are identified.

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Corneal Crosslinking with Riboflavin and Ultraviolet A. I. Principles

United States Multicenter Clinical Trial of Corneal Collagen Crosslinking for Keratoconus Treatment

This review summarizes the current status of the small incision lenticule extraction (SMILE) procedure. Following the early work by Sekundo et al. and Shah et al., SMILE has become increasingly popular. The accuracy of the creation of the lenticule with the VisuMax femtosecond laser (Carl Zeiss Meditec) has been verified using very high-frequency (VHF) digital ultrasound and optical coherence tomography (OCT). Visual and refractive outcomes have been shown to be similar to those achieved with laser in situ keratomileusis (LASIK), notably in a large population reported by Hjortdal, Vestergaard et al. Safety in terms of the change in corrected distance visual acuity (CDVA) has also been shown to be similar to LASIK. It was expected that there would be less postoperative dry eye after SMILE compared to LASIK because the anterior stroma is disturbed only by the small incision, meaning that the anterior corneal nerves should be less affected. A number of studies have demonstrated a lower reduction and faster recovery of corneal sensation after SMILE than LASIK. Some studies have also used confocal microscopy to demonstrate a lower decrease in subbasal nerve fiber density after SMILE than LASIK. The potential biomechanical advantages of SMILE have been modeled by Reinstein et al. based on the non-linearity of tensile strength through the stroma. Studies have reported a similar change in Ocular Response Analyzer (Reichert) parameters after SMILE and LASIK, however, these have previously been shown to be unreliable as a representation of corneal biomechanics. Retreatment options after SMILE are discussed. Tissue addition applications of the SMILE procedure are also discussed including the potential for cryo-preservation of the lenticule for later reimplantation (Mohamed-Noriega, Angunawela, Lim et al.), and a new procedure referred to as endokeratophakia in which a myopic SMILE lenticule is implanted into a hyperopic patient (Pradhan et al.). Finally, studies reporting microdistortions in Bowman’s layer and corneal wound healing responses are also described.

/pdf/small-incision-lenticule-extraction-smile-history-3479neaqlp.pdf

Small incision lenticule extraction (SMILE) history, fundamentals of a new refractive surgery technique and clinical outcomes.

PURPOSE: The goal of this study was to determine age-related variation in the elasticity of the human cornea using nondestructive means METHODS: Organ cultured human corneoscleral buttons were studied Changes in strain were measured with a radial shearing speckle pattern interferometer after an increase in intraocular pressure from 150 to 155 mm Hg Changes in central corneal displacement were calculated by integration, and a bulk corneal Young's modulus was derived by mathematical analysis RESULTS: Fifty corneas, including 17 pairs, were studied Donors were aged between 24 and 102 years (mean, 731); 29 (58%) specimens were from male donors and 21 from female donors Young's modulus of the cornea increased with age, with the line of best fit indicating an approximate doubling from 027 MPa at age 20 years (95% confidence interval, 022-031) to 052 (050-054) MPa at age 100 years (R² = 070) CONCLUSIONS: The stiffness of the human cornea increases by a factor of approximately two between the ages of 20 and 100 years This variation is relevant to the algorithms used to predict the response to incisional and ablative refractive surgery and will also affect the formulas used to calculate intraocular pressure by applanation

Age-related differences in the elasticity of the human cornea.

Purpose To study the corneal biomechanical effects of varying LASIK flap depth and side cut angulations and evaluate the relative contribution of the lamellar and side cuts using a femtosecond laser and radial shearing speckle pattern interferometry (RSSPI). Methods Forty-two organ-cultured human corneas were divided into a control group and three investigative groups, each undergoing different incision types at both 90- and 160-μm depth using a femtosecond laser. In the first group, typical LASIK flaps were created; in the second group, only the bed was cut (delamination); and in the third group, side cuts alone were affected. Corneal strain was measured using RSSPI before and after treatment following an increase in hydrostatic pressure from 15.0 to 15.5 mmHg and again after 1 week of incubation in culture medium. Results The flap group demonstrated a weakening of strength related to the depth of cut, with strain increasing by 9% and 32% at 90 and 160 μm, respectively. Similar changes, 9% and 33%, were observed following execution of side cuts to the same depths. By contrast, strain increase following delamination showed no relationship with depth, increasing by 5% in both instances. When the side cut angle was made more acute, strain decreased with a 2% strain increase being measured after a 90-μm, 150° side cut was created. No significant changes occurred during the period of organ culture. Conclusions Vertical side cuts through corneal lamellae rather than horizontal delamination incisions contribute to the loss of structural integrity during LASIK flap creation. Angulating side cuts such that the stromal diameter of the flap exceeds its epithelial diameter can decrease this effect.

Effects of Variation in Depth and Side Cut Angulations in LASIK and Thin-flap LASIK Using a Femtosecond Laser: A Biomechanical Study

PURPOSE To quantify the magnitude of the stiffening effect of corneal cross-linking (CXL) by studying intact human corneas exposed to physiological pressure transients. METHODS Nine organ-cultured human corneas mounted in artificial anterior chambers were studied. A radial shearing speckle pattern interferometer was used to measure changes in corneal strain following an increase in artificial anterior chamber pressure from 15.0 to 15.5 mmHg before and after treatment. Measurements were taken from all corneas with their epithelium intact before all underwent epithelial debridement. Three specimens were used as controls and did not receive any further treatment; three underwent riboflavin/ultraviolet A (UVA) CXL using 30 minutes of 370-nm irradiation at 3 mW/cm2 following epithelial removal and saturation with 0.1% riboflavin; and three were fixed with the cross-linking agent 2.5% glutaraldehyde. Strain measurements were repeated after these treatments. Young's moduli of individual corneas were calculated by mathematical analysis. RESULTS Mean donor age was 81.7 years. Before treatment, the mean Young's moduli of the control, riboflavin/UVA CXL, and glutaraldehyde-fixed corneas did not differ significantly: 0.46±0.03, 0.48±0.03, and 0.47±0.03 MPa, respectively. Following treatment these values changed to 0.46±0.2, 2.06±0.22, and 3.48±0.41 MPa, respectively. In proportional terms, this was equivalent to an increase in corneal Young's modulus by a factor of 4.3 (P<.05) following riboflavin/UVA CXL and 7.3 (P<.05) after glutaraldehyde fixation. CONCLUSIONS Riboflavin-UVA CXL increases the stiffness of organ-cultured corneas by a factor of more than four. This finding quantifies the efficacy of CXL in a physiologic configuration.

Nathaniel E Knox Cartwright

Papers

Age-related differences in the elasticity of the human cornea.

Effects of Variation in Depth and Side Cut Angulations in LASIK and Thin-flap LASIK Using a Femtosecond Laser: A Biomechanical Study

In vitro quantification of the stiffening effect of corneal cross-linking in the human cornea using radial shearing speckle pattern interferometry.