Faculty Bibliography

PURPOSE/OBJECTIVE:To evaluate if three dentin treatments improved mechanical properties of demineralized dentin. METHODS:Dentin slices were demineralized and treated with a universal adhesive, Scotchbond Universal (SBU), a cross-linker, Riboflavin (RF), and a calcium phosphate-based product, Teethmate (TM). The groups (n= 8 per group) were: Group 1: SBU, Group 2: RF + SBU, Group 3: RF + TM + SBU. Tensile tests were performed; stress/strain curves and E modulus were calculated. Differences between groups were assessed by one-way ANOVA and Duncan post hoc test. RESULTS:At high strains, no significant differences in E moduli were observed between dentin specimens treated only with SBU and those treated with RF + SBU. A significantly higher E modulus was observed in dentin specimens treated with RF + TM + SBU. In the presence of an adhesive system, crosslinking collagen with RF and TM addition significantly improved mechanical properties of dentin. CLINICAL SIGNIFICANCE/CONCLUSIONS:Restitution of mineral content into dentin, in addition to collagen strengthening, may significantly improve mechanical properties of previously demineralized dentin, when covered by an adhesive system in a reasonable clinical timeframe.

PURPOSE:To assess whether full biomechanical stiffening can be achieved with corneal crosslinking (CXL) when applying a reduced ultraviolet (UV) fluence during the standard irradiation time. SETTING:Laboratory of Ocular Cell Biology, Center for Applied Biotechnology and Molecular Medicine, University of Zurich, Zurich, Switzerland. DESIGN:Experimental study. METHODS:). The controls were soaked with riboflavin but not exposed to UV light. Elastic (stress-strain) and viscoelastic (stress-relaxation) 2-dimensional testing was performed with a commercial stress-strain extensometer to quantify the biomechanical stiffening. RESULTS:Corneas crosslinked with low and standard UV irradiances had a significantly higher mean elastic modulus (65.9 MPa ± 15.7 [SD] and 67.1 ± 15.6 MPa, respectively) than controls (52.4 ± 12.3 MPa) (P < .001). Also, the remaining stress after 120 seconds of stress-relaxation was significantly higher after CXL with low and standard UV irradiances (159 ± 21 kPa and 158 ± 25 kPa, respectively) compared with controls (135 ± 20 kPa) (P ≤ .013). No difference was observed in low and standard irradiances between CXL conditions (P = .64). CONCLUSIONS:The UV fluence for CXL might be reduced while maintaining the biomechanical efficacy by using a lower UV irradiance and the same irradiation duration. This might open avenues in the treatment of extremely thin corneas.

PURPOSE:To evaluate the biomechanical stability of ex vivo porcine corneas after femtosecond lenticule extraction (FLEx) and small incision lenticule extraction (SmILE) refractive surgeries. METHODS:Forty-five porcine eyes were equally divided into three groups: Groups 1 and 2 were treated with FLEx and SmILE procedure, respectively. Group 3 served as control. A refractive correction of -14 diopters (D) with a 7-mm zone using either a 160-μm flap (FLEx) or a 160-μm cap (SmILE) was performed. For two-dimensional (2D) elastic and viscoelastic biomechanical characterization, two testing cycles (preconditioning stress-strain curve from 1.27 to 12.5 N, stress-relaxation at 12.5 N during 120 seconds) were conducted. Young's modulus and Prony constants were calculated. RESULTS:At 0.8% of strain, FLEx (370 ± 36 kPa) could resist a significantly lower stress than SmILE (392 ± 19 kPa, P = 0.046) and the control group (402 ± 30 kPa, P = 0.013). Also, FLEx (46.1 ± 4.5 MPa) had a significantly lower Young's modulus than the control group (50.2 ± 3.4 MPa, P = 0.008). The Young's modulus of SmILE (48.6 ± 2.5 MPa) had values situated between untreated corneas and FLEx-treated corneas. When compared to untreated controls, the stress resistance decreased by 8.0% with FLEx and 2.5% with SmILE; Young's modulus decreased by 5.1% with FLEx and 1.04% with SmILE. With a cap-based procedure, both anterior cap and stromal bed carry the intraocular pressure, while in a flap-based procedure, only the stromal bed does. CONCLUSIONS:Compared to flap-based procedures like FLEx, the cap-based technique SmILE can be considered superior in terms of biomechanical stability, when measured experimentally in ex vivo porcine corneas.

PURPOSE/OBJECTIVE:To study the mid-term effects of implanting a single intracorneal ring segment (ICRS) for treatment of mild to moderate progressive keratoconus. METHODS:This retrospective study was conducted on patients with progressive keratoconus treated with ICRS. The mean follow-up time was 15.2 ± 8.8 (range, 7-26) months before and 27.8 ± 7.5 (range, 18-35) months after surgery. Patients received no other treatment during this time. Of the cases studied, 59.1% had inferior keratoconus. RESULTS:Thirty eyes of 30 patients with a mean age of 27.5 ± 4.6 years were included in the study. Corrected distance visual acuity decreased by 1 line during the preoperative follow-up and improved by 2 lines after surgery (P < 0.05). Maximum keratometry (Kmax) of the anterior (2.39 ± 0.75 D) and posterior (0.33 ± 0.32 D) corneas at 3 mm and anterior Kmax at 8 mm (2.39 ± 0.95 D) decreased after treatment (all P < 0.05). The posterior Q value shifted toward a more oblate curvature (0.50 ± 0.52, P = 0.023). Decreases in anterior elevation at the thinnest point (5.22 ± 7.21 μm) and posterior corneal central elevation (8.89 ± 11.26 μm) were significant (both P < 0.05). Corneal thickness at the center (11.11 ± 12.38 μm) and thinnest point (15.89 ± 12.75 μm) increased after treatment (all P < 0.001). Other significant changes included postoperative decreases in the keratoconus index (0.12 ± 0.06), index of height decentration (0.06 ± 0.05), index of surface variance (27.32 ± 13.08), index of vertical asymmetry (0.42 ± 0.21), index of height asymmetry (8.42 ± 20.57), and an increase in the center keratoconus index (0.01 ± 0.02) (P < 0.001). CONCLUSIONS:This study demonstrates that ICRS implantation provides favorable mid-term outcomes in treatment of mild and moderate progressive keratoconus and can improve corneal indices compared with their preoperative states.

PURPOSE:In recent years, the interest in corneal biomechanics has strongly increased. The material properties of the cornea determine its shape and therefore play an important role in corneal ectasia and related pathologies. This review addresses the molecular origin of biomechanical properties, models for their description, methods for their characterisation, techniques for their modification, and computational simulation approaches. RECENT FINDINGS:Recent research has focused on developing non-contact techniques to measure the biomechanical properties in vivo, on determining structural and molecular abnormalities in pathological corneas, on developing and optimising techniques to reinforce the corneal tissue and on the computational simulation of surgical interventions. SUMMARY:A better understanding of corneal biomechanics will help to improve current refractive surgeries, allow an earlier diagnosis of ectatic disorders and a better quantification of treatments aiming at reinforcing the corneal tissue.

PURPOSE/OBJECTIVE:To determine the corneal weakening induced by different re-treatment options after small incision lenticule extraction (SMILE) and investigate the potential of corneal cross-linking (CXL) to reestablish the original corneal stress resistance. METHODS:, 10 min) was performed in two groups with currently common enhancement techniques: following cap-to-flap conversion (-3D LASIK enhancement) and in controls. Biomechanical properties were measured with stress-strain extensometry ranging from 1.27 to 12.5 N. RESULTS:The Re-SMILE and PRK enhancement did not significantly reduce the overall elastic modulus of the cornea compared to controls (24.7 ± 2.23 and 22.7 ± 2.61 versus 23.8 ± 3.35 MPa, P ≥ .176), whereas LASIK enhancement did (22.2 ± 3.37 MPa, P = .048). CXL treatment significantly increased the elastic modulus compared to all non-cross-linked conditions (P ≤.001). Refractive surgery decreased the overall elastic modulus by 7%, whereas CXL increased it by 20%. CONCLUSIONS:In enhancement, the corneal biomechanical integrity is less affected with both Re-SMILE and PRK enhancement. Corneal weakening through laser refractive surgery is small compared to the stiffening effect after CXL. [J Refract Surg. 2017;33(3):193-198.].

PURPOSE/OBJECTIVE:To develop an algorithm to predict the stiffening effect of CXL and to verify the accuracy with results obtained from experimental measurements. METHODS:and irradiation times from 8 to 30 minutes. Stress-relaxation measurements were performed in fresh-enucleated porcine (n = 66) and rabbit (n = 2) eyes directly after treatment, using a load of 0.6 MPa. RESULTS:= 0.9961). CONCLUSIONS:The proposed model, supported by data in porcine, murine, and lapine corneas, suggests a possibility of also predicting the biomechanical CXL efficacy in human corneas. The biomechanical efficacy of CXL may be increased by prolonged UV irradiation at reduced irradiances or by a higher oxygen pressure in the environment. Pulsed CXL does not accelerate CXL or increase its efficacy when compared to standard CXL of the same irradiation duration. This model might be used to calculate customized irradiation settings for high-risk cases, but also topography-guided CXL treatments. [J Refract Surg. 2017;33(2):128-136.].

PURPOSE/OBJECTIVE:To study whether repeated collagen cross-linking (CXL) performed in vivo in mice shows an additive effect on mechanical corneal stiffness. METHODS:). CXL was performed as either a single (1×CXL) or a repeated (2×CXL) treatment. Un-irradiated corneas served as controls. In the 2×CXL group, the procedure was performed on day 1 and day 4 to ensure complete reepithelialization between sessions. Biomechanical analysis was performed on day 7. Corneas were harvested with a small scleral ring and mounted on a customized two-dimensional flap holder. The biomechanical measurement consisted of three parts: (1) pre-conditioned during three cycles from 0.04 to 0.4 N, (2) stress relaxation during 120 seconds following 0.4 N force application, and (3) stress-strain curve until break. RESULTS:After the relaxation period of 120 seconds, highly significant differences (P < .001) were found between the controls and both 1×CXL corneas and 2×CXL corneas. No significant difference (P = .70) was detected between the 1×CXL and 2×CXL groups. The stress remaining after relaxation was 355 ± 25.2 kPa in the control group, 457 ± 34.1 kPa in the 1×CXL group, and 463 ± 22.2 kPa in the 2×CXL group. No significant differences in the stress-strain curves were found between the conditions. CONCLUSIONS:Repeated CXL 3 days after the first procedure does not further increase corneal stiffness in mice in vivo. [J Refract Surg. 2017;33(1):56-60.].