There are currently few studies evaluating the dissolution properties of prospective artificial corneal skirt devices.21
One of the issues in OOKP surgery is resorption. This can lead to leakage around the optic cylinder which in turn can predispose to microbiological infection and the risk of the resorbed lamina harbouring organisms. In our designed synthetic OOKP device, the optic will be sealed to the haptic with the use of dental cement as in OOKP, thereby reducing the possibilities of an intraocular infection. Ideally our study would have a comparative group consisting of osteodental lamina, even though synthetic HA and osteodental lamina are not totally comparable in all respects. The mineral constituents of synthetic HA and osteodental lamina are comparable. Both of them are made of calcium apatite with the formula Ca(n)(PO4)(n)(OH)(n). Hence they share some similar physical and chemical properties. That is why we and others21
have used HA as control group. Comparison studies with human cadaver osteodental lamina would have been more informative but currently we have no such resources and it will be our future work.
We analysed the inflammatory cellular response and dissolution properties of HA and TiO2
in a simulated inhospitable ocular surface environment. LPS is a component of the cell membrane of gram-negative bacteria. Corneal fibroblasts are sensitive to LPS stimulation and this plays a role in the regulation of cornea inflammation secondary to bacterial infection. LPS has been shown to induce the release of the pro-inflammatory cytokine IL-6 and chemokines G-CSF, monocyte chemotactic protein-1, MIP-1β, IL-8 and intercellular adhesion molecule-1 by corneal fibroblasts.23
Likewise infectious corneal ulcers are associated with the production of pro-inflammatory cytokines, such as IL-1, TNF-α and chemokines such as GM-CSF by resident human corneal fibroblasts.25–27
Such a response in vivo would trigger local infiltration and activation of leucocytes and contribute to localised inflammation. In our studies, we found that LPS induced the production of a wide panel of cytokines/chemokines from cultured human corneal fibroblasts. We found that HA-grown keratocytes were more prone to LPS stimulated cytokines/chemokines secretion than TiO2
(). The reasons that human cornea fibroblasts on TiO2
substrates secreted less cytokine/chemokines than those from HA are not fully understood, but could simply represent the fact that TiO2
presents a more bioinert surface as compared to HA. The difference in cytokine expression we detected was due to the intrinsic properties of the material substrates on the cells. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay excluded the possibility that this difference was due to cellular toxicity caused by LPS on the fibroblasts grown on either biomaterial substrates (supplementary figure). Our results suggest that TiO2
fibroblasts are less responsive to LPS stimulated inflammation and hence may prove effective in reducing the propagation of an acute inflammatory response into a chronic one.
Following ischaemia and inflammation, there is a reduction in tissue pH.12
The reported reduction in pH can vary from 0.1 to 2.0.28
Therefore, to address the issue of implant device stability in an ocular inflammatory milieu, we compared the corrosion resistance properties of material specimens in ATF at a pH value of 6.5 and also at a more acidic pH (pH 5) to simulate an inhospitable ocular surface. We compared the corrosion resistance properties of TiO2
and HA in an electric-polarisation facility. Bioactive HA has a low polarisability and electric conductivity hence it is not possible to compare the electrochemical index of HA directly with TiO2
. Morphological changes after electrochemical treatment were analysed, and compared with HA. TiO2
showed less degradation with electrochemical treatment at both pH 6.5 and pH 5 ATF ( and ). We deduce that this may be due to the passive film formation at the interface between TiO2
Our SEM and AFM data showed that in pH 6.5 ATF, both HA and TiO2 developed some early signs of pitting corrosion formation and an increase in surface roughness ( to ). However, the signs of corrosion in pH 5 ATF were remarkably different as TiO2 showed smaller pits and a gradual increase in surface roughness while HA showed large pit formations and a steep increase in surface roughness ( to ). These observations indicated that, in a milieu of an acidic ocular tissue, TiO2 is more resistant to corrosion as compared with HA.
Chronic inflammation plays an essential role in OOKP lamina degradation.5
Hence, in our study we compared the corrosion resistance properties of HA and TiO2
after long-term incubation with low pH ATF which stimulated a chronic ocular inflammation state. TiO2
showed no significant degradation of surface morphology change while the surface of HA was severely damaged in both pH 6.5 and pH 5 ATF ( and ). The analysis of the elemental compositions showed that ‘Ca’ and ‘P’ were continuously released from HA while in contrast the elemental composition of TiO2
was stable (Supplementary table). Therefore, our data again indicated that TiO2
is more stable to long-term corrosion. TiO2
might not undergo significant resorption in an adverse milieu and thereby may not lead to a leak in the KPro skirt area around the optical cylinder.
In conclusion, our studies involving cytokine array to compare the material mediated cellular reaction to inflammatory stimuli and studies evaluating the morphological changes of material substrates induced by acidic ATF show that TiO2 is a promising skirt material for a fully synthetic OOKP device in terms of corrosion resistance to microbial infection and device stability.