Effects of poly(2-hydroxyethyl methacrylate) and poly(vinyl-pyrrolidone) hydrogel implants on myopic and normal chick sclera

Abstract

There has been generally little attention paid to the utilization of biomaterials as an anti-myopia treatment. The purpose of this study was to investigate whether polymeric hydrogels, either implanted or injected adjacent to the outer scleral surface, slow ocular elongation. White Leghorn (gallus gallus domesticus) chicks were used at 2 weeks of age. Chicks had either (1) strip of poly(2-hydroxyethyl methacrylate) (pHEMA) implanted monocularly against the outer sclera at the posterior pole, or (2) an in situ polymerizing gel [main ingredient: poly(vinyl-pyrrolidone) (PVP)] injected monocularly at the same location. Some of the eyes injected with the polymer were fitted with a diffuser or a −10D lens. In each experiment, ocular lengths were measured at regular intervals by high frequency Ascan ultrasonography, and chicks were sacrificed for histology at staged intervals. No in vivo signs of either orbital or ocular inflammation were observed. The pHEMA implant significantly increased scleral thickness by the third week, and the implant became encapsulated with fibrous tissue. The PVP-injected eyes left otherwise untreated, showed a significant increase in scleral thickness, due to increased chondrocyte proliferation and extracellular matrix deposition. However, there was no effect of the PVP injection on ocular elongation. In eyes wearing optical devices, there was no effect on either scleral thickness or ocular elongation. These results represent "proof of principle" that scleral growth can be manipulated without adverse inflammatory responses. However, since neither approach slowed ocular elongation, additional factors must influence scleral surface area expansion in the avian eye.

Scleral Reinforcement Through Host Tissue Integration with Biomimetic Enzymatically Degradable Semi-Interpenetrating Polymer Network

“…Although these changes contributed an extrafibrous layer to the posterior sclera, they did not retard the rate of elongation of treated eyes compared with the untreated fellow eyes. This lack of effect on ocular elongation is similar to that reported by our group for previously tested HEMA and PVP implants in chick 22 and point to complex interactions between scleral tissue growth and intraocular pressure as determinants of eye enlargement. It is likely that the same edsIPN-induced changes will have different effects on the elongation of mammalian eyes, which lack a rigid inner cartilaginous layer, and also on myopic eyes whose scleras show increased remodeling and subsequent thinning.…”

“…[19][20][21] A subTenon's injection of a polymeric gel formulation comprised mainly of polyvinylpyrrolidone has been used to control myopia progression, but testing has been limited to a Moscow-based small human study and a mechanistic animal study. 20,22 These options have yet to gain wide clinical acceptance because of high toxicity (e.g., glutaraldehyde), lack of complete understanding of the mechanism of action (methylxanthines), limited access to clinical trial data (polyvinylpyrrolidone injections), and for scleroplasty, limited supply of allograft material, its invasive nature, and limited data supporting its long-term efficacy. 23 As an alternative, a minimally invasive scleral reinforcement strategy that takes advantage of the biochemical properties of host sclera was developed.…”

Intact globe inflation testing of changes in scleral mechanics in myopia and recovery

“…In our method, the eye is anchored in place via its limbus during testing, thereby avoiding damage to the sclera. In addition to its application here – as a tool to investigate the contribution of biomechanical changes in the sclera to the development of myopia, this method can be used to obtain insights into the efficacy of new myopia control treatments, including tissue engineering approaches specifically targeting the sclera (Su et al, 2009; Su et al 2010). In this context, the use of additional fiducial markers to divide the posterior eye wall potentially allows the characterization of local, regional differences in scleral deformation and one could envisage applying this technique to better characterize the efficacy of myopia control treatments targeting local ocular segments, and/or suspected of having localized effects.…”

Imaging hydrogel implants in situ

“…50 In another interesting application, ultrasonography was used to measure ocular lengths of chicks that received different ocular hydrogel implants, which affected scleral growth. 51 MicroCT utilizes X-rays for high resolution tomographic imaging. Image contrast comes mainly from materials that are radioopaque; therefore, for medical applications, microCT has primarily been applied to image bone.…”