The influence of various substances on the biomechanical behavior of lamina cribrosa and peripapillary sclera. Invest Ophthalmol Vis Sci

PURPOSE. Changes in the biomechanical properties of the lamina cribrosa (LC) and of the peripapillary sclera (ppSc) may play a role in the pathogenesis of glaucoma. The purpose of this study was to assess the influence of glyceraldehyde, methylglyoxal, and collagenase A on the mechanical properties of the LC and ppSc. METHODS. Two strips of 1-mm width were cut from each of 80 porcine eyes and 24 pairs of enucleated human eyes. One strip contained the LC and the other the adjacent superior ppSc. One half of the strips was divided into groups and treated with 0.5 M glyceraldehyde, 0.5 M methylglyoxal, and 0.1% collagenase A. The other strips served as the control. The stress strain relation was measured in the stress range of 0.02 to 6.0 MPa by a biomaterial tester. RESULTS. Stress values at 20% strain of the human LC changed from 1.97 Ϯ 1.48 to 3.40 Ϯ 1.60 MPa after incubation with glyceraldehyde (P ϭ 0.029), from 2.42 Ϯ 2.22 to 5.46 Ϯ 1.91 MPa (P ϭ 0.014) after incubation with methylglyoxal, and from 2.43 Ϯ 1.3 to 1.35 Ϯ 0.19 MPa after incubation with collagenase A. The stress values of human ppSc without glyceraldehyde were 3.40 Ϯ 2.59 and 7.45 Ϯ 4.46 MPa after incubation with glyceraldehyde (P ϭ 0.047), 4.80 Ϯ 3.05 MPa without methylglyoxal and 16.10 Ϯ 5.53 MPa (P ϭ 0.001) after incubation with methylglyoxal, 4.14 Ϯ 2.56 MPa without collagenase A, and 1.97 Ϯ 0.55 MPa after incubation with collagenase A. At a 20% strain, Young's moduli of the untreated LC were in the range of E ϭ 11.8 to 15.6 MPa and E ϭ 28.5 to 36.0 MPa of the untreated ppSc. CONCLUSIONS. Glyceraldehyde and methylglyoxal increase the stiffness of the LC and of the ppSc in human and in porcine eyes. These substances induce changes in the extracellular matrix according to the Maillard reaction as it occurs during the ageing process or in case of high blood glucose levels. Collagenase reduces the stiffness of the tissues. (Invest Ophthalmol Vis Sci. 2005;46:1286 -1290) DOI:10.1167/iovs.04-0978 T he structure of the lamina cribrosa (LC) and of the peripapillary sclera (ppSc) appears particularly relevant to glaucomatous optic nerve damage. Cupping of the optic nerve head is a hallmark of glaucomatous optic nerve damage, and actually precedes field loss. Optic nerve cupping seems to be caused primarily by increased intraocular pressure (IOP) or by a vascular response. However, the optic nerve head reacts differently to equal IOP-levels in different subgroups of patients (e.g., in normal-tension glaucoma or ocular hypertension). 1 These differences in susceptibility to glaucomatous damage may be at least partially caused by different biomechanical properties of the LC and ppSc. These differences may be induced by changes in the extracellular matrix, which consists of elastin, collagen, and proteoglycans. One factor that affects the biomechanical behavior of the optic nerve head tissue is a different elasticity (e.g., in infantile glaucoma optic nerve cupping is reversible after reduction of IOP, whereas it is usually irreversible in adult eyes). 2 Differences in elasticity are probably due to a different degree of collagen and elastin cross-linking. MATERIAL AND METHODS Eighty porcine eyes and 24 pairs of enucleated human eyes (n ϭ 48) were examined. The eyes were cleaned of all extraocular structures. At the equator, the eyes were halved horizontally, and vitreous, retina, and choroid were removed. Two strips of 1-mm width and 8-mm length were cut from each eye. One strip contained the LC Porcine Eyes Each 10 strips of LC and the superior ppSc were incubated in DMEM plus 0.5 M glyceraldehyde for 6 days, DMEM plus 0.5 M methylglyoxal for 6 days, DMEM plus 0.1% glutaraldehyde for 30 minutes, and DMEM plus 0.1% collagenase A for 1 day. For each experiment, 10 untreated LC strips and 10 untreated peripapillary strips were used as the control and were placed in DMEM for the same amount of time. Human Eyes Each eight strips of LC and the superior ppSc (one of each pair) were incubated in DMEM plus 0.5 M glyceraldehyde for 6 days, DMEM plus 0.5 M methylglyoxal for 6 days, and DMEM plus 0.1% collagenase A for 1 day. For each experiment, the strips of the other eye served as the control and were placed in DMEM for the same amount of time. Thickness Measurements The thickness of each specimen was determined with the help of ultrasound pachymetry (Pach-Pen XL; Mentor, Norwell, MA)

A constant-force technique to measure corneal biomechanical changes after collagen cross-linking.

To introduce a constant-force technique for the analysis of corneal biomechanical changes induced after collagen cross-linking (CXL) that is better adapted to the natural loading in the eye than previous methods.For the biomechanical testing, a total of 50 freshly enucleated eyes were obtained and subdivided in groups of 5 eyes each. A Zwicki-Line Testing Machine was used to analyze the strain of 11 mm long and 5 mm wide porcine corneal strips, with and without CXL. Before material testing, the corneal tissues were pre-stressed with 0.02 N until force stabilization. Standard strip extensiometry was performed as control technique. For the constant-force technique, tissue elongation (Δ strain, %) was analyzed for 180 seconds while different constant forces (0.25 N, 0.5 N, 1 N, 5 N) were applied.Using a constant force of 0.5 N, we observed a significant difference in Δstrain between 0.26±0.01% in controls and 0.12±0.03% in the CXL-treated group (p = 0.003) over baseline. Similarly, using a constant force of 1 N, Δstrain was 0.31±0.03% in controls and 0.19±0.02% after CXL treatment (p = 0.008). No significant differences were observed between CXL-treated groups and controls with 0.25 N or 5 N constant forces. Standard stress-strain extensiometry failed to show significant differences between CXL-treated groups and controls at all percentages of strains tested.We propose a constant-force technique to measure corneal biomechanics in a more physiologic way. When compared to standard stress-strain extensiometry, the constant-force technique provides less variability and thus reaches significant results with a lower sample number