Biomechanical Considerations of Patching Material for Posterior Scleral Reinforcement Surgery.

Purpose: To characterize biomechanical properties of genipin-crosslinked human dura mater as reinforcing material for posterior scleral reinforcement (PSR) and to compare it with crosslinked human sclera. Methods: Donor dura mater and sclera were crosslinked in the same optimized genipin solution. Resistance to enzyme degradation for both materials were investigated by exposing the materials to accelerated enzyme degrading. Elastic modulus and tensile strength were measured by biomechanics testing equipment. Crosslinked human dura mater was used as reinforcing patch in PSR on 57 adult pathologic myopic eyes. The patients were followed up for an average 3 years. The main outcome was eye globe axial length change and safety profile of the reinforcing material. Results: Crosslinked dura mater demonstrated similar percentage weight loss to crosslinked sclera when exposed to enzymatic solution. Dura mater has higher density than sclera. The retaining elastic modulus after enzyme exposure was 72.02 MPa for crosslinked dura mater while 53.88 MPa for crosslinked sclera, 34% greater for crosslinked dura mater, P = 0.0186). At the end of 3 years follow-up, the mean globe axis of the surgery eyes was reduced by 1.29 mm (from 30.81 to 29.51 mm, P < 0.0001, paired t-test). Visual acuity (BCVA logMar) improved by 0.10 logMar unit which is an improvement of five letters (P = 0.0184, paired t-test). No material specific complication was noted. Conclusion: Crosslinked human dura mater may be superior to crosslinked human sclera as reinforcing material for PSR to manage progression of high myopia. This material was well tolerated on human eye.

The lymphangiogenic and hemangiogenic privilege of the human sclera.

PURPOSE: Most organs of the human body are supplied with a dense network of blood and lymphatic vessels. However, some tissues are either hypovascular or completely devoid of vessels for proper function, such as the ocular tissues sclera and cornea, cartilage and tendons. Since many pathological conditions are affecting the human sclera, this review is focussing on the lymphangiogenic and hemangiogenic privilege in the human sclera. METHODS: This article gives an overview of the current literature based on a PubMed search as well as observations and experience from clinical practice. RESULTS: The healthy human sclera is the outer covering layer of the eye globe consisting mainly of collagenous extracellular matrix and fibroblasts. Physiologically, the sclera shows only a superficial network of blood vessels and a lack of lymphatic vessels. This vascular privilege is actively regulated by balancing anti- and proangiogenic factors expressed by cells within the sclera. In pathological situations, such as open globe injuries or ciliary body melanomas with extraocular extension, lymphatic vessels can secondarily invade the sclera and the inner eye. This mechanism most likely is important for tumor cell metastasis, wound healing, immunologic defense against intruding microorganism, and autoimmune reactions against intraocular antigens. CONCLUSIONS: The human sclera is characterized by a tightly regulated vascular network that can be compromised in pathological situations, such as injuries or intraocular tumors affecting healing outcomes Therefore, the molecular and cellular mechanisms underlying wound healing following surgical interventions deserve further attention, in order to devise more effective therapeutic strategies.

Inflation experiments and inverse finite element modelling of posterior human sclera.

The complexity of inverse finite element modelling methods used in ocular biomechanics research has significantly increased in recent years in order to produce material parameters that capture microscale tissue behaviour. This study presents a more accessible method for researchers to optimise sclera material parameters for use in finite element studies where macroscale sclera displacements are required. Five human donor sclerae aged between 36 and 72 years were subjected to cycles of internal pressure up to 61 mmHg using a custom-built inflation rig. Displacements were measured using a laser beam and two cameras through a digital image correlation algorithm. Specimen-specific finite element models incorporating regional thickness variation and sclera surface topography were divided into six circumferential regions. An inverse finite element procedure was used to optimise Ogden material parameters for each region. The maximum root mean squared (RMS) error between the numerical and experimental displacements within individual specimens was 17.5 µm. The optimised material parameters indicate a gradual reduction in material stiffness (as measured by the tangent modulus) from the equator to the posterior region at low-stress levels up to 0.005 MPa. The variation in stiffness between adjacent regions became gradually less apparent and statistically insignificant at higher stresses. The study demonstrated how inflation testing combined with inverse modelling could be used to effectively characterise regional material properties capable of reproducing global sclera displacements. The material properties were found to vary between specimens, and it is expected that age could be a contributing factor behind this variation.

Anteroposterior Patterning of Gene Expression in the Human Infant Sclera: Chondrogenic Potential and Wnt Signaling.

Purpose/Aim: We sought to identify the anteroposterior spatial gene expression hierarchy in the human sclera to develop a hypothesis for axial elongation and deformity of the eyeball. MATERIALS AND METHODS: We analyzed the global gene expression of human scleral cells derived from distinct parts of the human infant sclera obtained from surgically enucleated eyes with retinoblastoma, using Affymetrix GeneChip oligonucleotide arrays, and compared, in particular, gene expression levels between the anterior and posterior parts of the sclera. The ages of three donors were 10M, 4M, and 1Y9M. RESULTS: K-means clustering analysis of gene expression revealed that expression levels of cartilage-associated genes such as COLXIA and ACAN increased from the anterior to the posterior part of the sclera. Microarray analyses and RT-PCR data showed that the expression levels of MGP, COLXIA, BMP4, and RARB were significantly higher in the posterior than in the anterior sclera of two independent infant eyes. Conversely, expression levels of WNT2, DKK2, GREM1, and HOXB2 were significantly higher in the anterior sclera. Among several Wnt-family genes examined, WNT2B was found to be expressed at a significantly higher level in the posterior sclera, and the reverse order was observed for WNT2. The results of luciferase reporter assays suggested that a GSK-3ß inhibitor stimulated Wnt/ß-catenin signaling particularly strongly in the posterior sclera. The expression pattern of RARB, a myopia-related gene, was similar in three independent eyes. CONCLUSIONS: Chondrogenic potential was higher and Wnt/ß-catenin signaling was more potently activated by a GSK-3ß inhibitor in the posterior than in the anterior part of the human infant sclera. Although the differences in the gene expression profiles between the anterior and posterior sclera might be involved only in normal growth processes, this anteroposterior hierarchy in the sclera might contribute to disorders involving abnormal elongation and deformity of the eyeball, including myopia.

Form-deprivation myopia induces decreased expression of bone morphogenetic protein-2, 5 in guinea pig sclera.

AIM: To identify the presence of various bone morphogenetic proteins (BMPs) and their receptors in normal sclera of human, rat and guinea pigs, and to determine whether their expression changed with form-deprivation myopia (FDM) in guinea pig sclera. METHODS: The expression of BMPs and BMP receptors were detected using reverse transcription polymerase chain reaction (RT-PCR) and immunofluorescence. Two-week-old guinea pigs were monocularly form-deprived with a translucent lens. After fourteen days induction of FDM, total RNA was isolated and subjected to RT-PCR to examine the changes of BMPs and BMP receptors in tissues from the posterior sclera. Western blotting analysis was used to investigate their changes in protein levels. RESULTS: Human sclera expressed mRNAs for BMP-2, -4, -5, -7, -RIA, -RIB and BMP-RII. Conversely, rat sclera only expressed mRNA for BMP-7 and BMP-RIB, while the expression of BMPs and BMP receptors in guinea pigs were similar to that of humans. Human sclera also expresses BMP-2, -4, -5,-7 in protein level. Fourteen days after the induction of myopia, significant decreased expressions for BMP-2 and BMP-5 in the posterior sclera of FDM-affected eyes (P<0.05 vs internal control eyes). CONCLUSION: Various BMPs were expressed in human and guinea pig sclera. In the posterior sclera, expressions of BMP-2 and BMP-5 significantly decreased in FDM eyes. This finding indicates that various BMPs as components of the scleral cytokines regulating tissue homeostasis and provide evidence that alterations in the expression of BMP-2 and BMP-5 are associated with sclera remodeling during myopia induction.

Effects of Mitomycin C on Scleral Collagen Fibrils According to Atomic Force Microscopy

PURPOSE: To investigate the effects of mitomycin C on the scleral collagen surfaces using atomic force microscopy (AFM). METHODS: Two non-contact mode AFM machines were used to observe changes in the morphological characteristics of human scleral surfaces before and after one, three, and five minutes of 0.02% mitomycin C application. Based on AFM topography and deflection images of the collagen fibril, the morphological characteristics of scleral fibrils including the fibril diameter and D-period were measured using the line profile. RESULTS: The sclera collagen fibril treated with 0.02% mitomycin C for one minute did not show any significant increases in mean fibril diameter (155.04 +/- 17.46 nm) or mean D-periodicity (70.02 +/- 3.33 nm), compared to those of the control group. However, the scleral collagen fibrils treated with 0.02% mitomycin C for three and five minutes showed significant increases in mean fibril diameter (182.33 +/- 16.33 nm, 199.20 +/- 12.40 nm, respectively) and mean D-periodicity (70.27 +/- 13.66 nm, 72.75 +/- 19.32 nm, respectively), compared to those of the control group. CONCLUSIONS: The present study examined the structural changes in the scleral collagen fibrils before and after mitomycin C application according to atomic force microscopy. The results indirectly suggest that three or more minutes of 0.02% mitomycin C application affects the morphology of scleral collagen.