Hydroxyapatite promotes superior keratocyte adhesion and proliferation in comparison with current keratoprosthesis skirt materials

Biological Response of Human Gingival Fibroblasts to Zinc-Doped Hydroxyapatite Designed for Dental Applications-An In Vitro Study

This study aimed to investigate the biological response induced by hydroxyapatite (HAp) and zinc-doped HAp (ZnHAp) in human gingival fibroblasts and to explore their antimicrobial activity. The ZnHAp (with xZn = 0.00 and 0.07) powders, synthesized by the sol-gel method, retained the crystallographic structure of pure HA without any modification. Elemental mapping confirmed the uniform dispersion of zinc ions in the HAp lattice. The size of crystallites was 18.67 ± 2 nm for ZnHAp and 21.54 ± 1 nm for HAp. The average particle size was 19.38 ± 1 nm for ZnHAp and 22.47 ± 1 nm for HAp. Antimicrobial studies indicated an inhibition of bacterial adherence to the inert substrate. In vitro biocompatibility was tested on various doses of HAp and ZnHAp after 24 and 72 h of exposure and revealed that cell viability decreased after 72 h starting with a dose of 31.25 µg/mL. However, cells retained membrane integrity and no inflammatory response was induced. High doses (such as 125 µg/mL) affected cell adhesion and the architecture of F-actin filaments, while in the presence of lower doses (such as 15.625 µg/mL), no modifications were observed. Cell proliferation was inhibited after treatment with HAp and ZnHAp, except the dose of 15.625 µg/mL ZnHAp at 72 h of exposure, when a slight increase was observed, proving an improvement in ZnHAp activity due to Zn doping.

Biomimetic vs. Direct Approach to Deposit Hydroxyapatite on the Surface of Low Melting Point Polymers for Tissue Engineering

Polymers are widely used in many applications in the field of biomedical engineering. Among eclectic selections of polymers, those with low melting temperature (T_m < 200 °C), such as poly(methyl methacrylate), poly(lactic-co-glycolic acid), or polyethylene, are often used in bone, dental, maxillofacial, and corneal tissue engineering as substrates or scaffolds. These polymers, however, are bioinert, have a lack of reactive surface functional groups, and have poor wettability, affecting their ability to promote cellular functions and biointegration with the surrounding tissue. Improving the biointegration can be achieved by depositing hydroxyapatite (HAp) on the polymeric substrates. Conventional thermal spray and vapor phase coating, including the Food and Drug Administration (FDA)-approved plasma spray technique, is not suitable for application on the low T_m polymers due to the high processing temperature, reaching more than 1000 °C. Two non-thermal HAp coating approaches have been described in the literature, namely, the biomimetic deposition and direct nanoparticle immobilization techniques. In the current review, we elaborate on the unique features of each technique, followed by discussing the advantages and disadvantages of each technique to help readers decide on which method is more suitable for their intended applications. Finally, the future perspectives of the non-thermal HAp coating are given in the conclusion.

Surface modification of corneal prosthesis with nano-hydroxyapatite to enhance in vivo biointegration

The majority of clinical corneal prostheses (KPros) adopt a core-skirt configuration. This configuration is favored owing to the optic core (generally a cylindrical, acrylic-based material, such as PMMA), that not only provides a clear window for the patients' vision, but also confers resistance to biodegradability. The surrounding skirt (typically a biological material, such as corneal tissue) allows for host tissue integration. However, due to poor biointegration between the dissimilar core and skirt materials, it results in a weak adhesion at the interface, giving rise to clinical complications, such as bacterial infections in the tissue-PMMA interface and device extrusion. Here, we physically immobilized nano-hydroxyapatite (nHAp) on a PMMA cylinder via a dip-coating technique, to create a bioactive surface that improved biointegration in vivo. We established that the nHAp coating was safe and stable in the rabbit cornea over five weeks. More importantly, we found that apoptotic, wound healing and inflammatory responses to nHAp-coated PMMA were substantially milder than to non-coated PMMA. More mature collagen, similar to the non-operated cornea, was maintained in the corneal stroma adjacent to the nHAp-coated implant edge. However, around the non-coated cylinder, an abundant new and loose connective tissue formed, similar to bone tissue response to bioinert scaffolds. As a result of superior biointegration, tissue adhesion with nHAp-coated PMMA cylinders was also significantly enhanced compared to non-coated cylinders. This study set a precedent for the future application of the nHAp coating on clinical KPros. STATEMENT OF SIGNIFICANCE: Currently, all clinical corneal prostheses utilize as-manufactured, non-surface modified PMMA optic cylinder. The bioinert cylinder, however, has poor biointegration and adhesion with the surrounding biological tissue, which can give rise to postoperative complications, such as microbial invasion in the tissue-PMMA loose interface and PMMA optic cylinder extrusion. In the current study, we showed that surface modification of the PMMA cylinder with bioactive nano-hydroxyapatite (nHAp) significantly enhanced its biointegration with corneal stromal tissue in vivo. The superior biointegration of the nHAp-coated PMMA was signified by a more attenuated corneal wound healing, inflammatory and fibrotic response, and better tissue apposition, as well as a significantly improved corneal stromal tissue adhesion when compared to the non-coated PMMA.

Histopathologic Evaluation of Polymer Supports for Pintucci-type Keratoprostheses: An Animal Study

Purpose

To report histopathological findings for different types of polymers proposed as support for a Pintucci-type keratoprosthesis.

Methods

Six polymers, including three types of polyesters (#1-3), one type of polytetrafluoroethylene (PTFE, #4), polyethylene (#5), and expanded polytetrafluoroethylene (ePTFE, #6) were evaluated. Four samples of each material were placed under the orbicularis oculi muscles of 12 rabbits. After five weeks, the samples were removed and evaluated histopathologically. Fibrovascular tissue ingrowths were investigated in terms of tissue penetration depth into the materials (graded as none, mild, moderate, and intense) and fibrovascular ingrowth area at the ultimate level of tissue penetrance. ImageJ software was used to calculate fibrovascular tissue area between the material fibers, and the mean area values were compared between the materials.

Results

Polyester materials #1 and #3 demonstrated intense fibrovascular tissue penetration with a large fibrovascular ingrowth area; no overt tissue ingrowth was observed into material #6. The mean area of penetrated fibrovascular tissues was significantly different between materials (P < 0.001). Materials #2, #4, and #5 showed moderate fibrovascular tissue ingrowth and the area of presented fibrovascular tissue at the paracentral parts of material #4 was significantly smaller than that of materials #1 (P = 0.02) and #3 (P = 0.01).

Conclusion

Two polyester materials that had relatively large pore sizes demonstrated a deep and large area of fibrovascular ingrowth. Given that material #3 is thicker and more consistent than material #1, the former can be used as the appropriate material for supporting the Pintucci-type keratoprosthesis.

Bioceramics in ophthalmology

The benefits of ceramics in biomedical applications have been universally appreciated as they exhibit an extraordinarily broad set of physico-chemical, mechanical and biological properties which can be properly tailored by acting on their composition, porosity and surface texture to increase their versatility and suitability for targeted healthcare applications. Bioceramics have traditionally been used for the repair of hard tissues, such as bone and teeth, mainly due to their suitable strength for load-bearing applications, wear resistance (especially alumina, zirconia and composites thereof) and, in some cases, bone-bonding ability (calcium orthophosphates and bioactive glasses). Bioceramics have been also applied in other medical areas, like ophthalmic surgery; although their use in such a context has been scientifically documented since the late 1700s, the potential and importance of ceramic ocular implants still seem to be underestimated and an exhaustive, critical assessment is currently lacking in the relevant literature. The present review aims to fill this gap by giving a comprehensive picture of the ceramic-based materials and implants that are currently used in ophthalmology and pointing out the strengths and weaknesses of the existing devices. A prospect for future research is also provided, highlighting the potential of new, smart bioceramics able to carry specific added values which could have a significant impact on the treatment of ocular diseases.

An improved biofunction of Titanium for keratoprosthesis by hydroxyapatite-coating

Titanium framework keratoprosthesis has been commonly used in the severe corneal blindness, but the tissue melting occurred frequently around titanium. Since hydroxyapatite has been approved to possess a good tissue integration characteristic, nanostructured hydroxyapatite was coated on the surface of titanium through the aerosol deposition method. In this study, nanostructured hydroxyapatite coating was characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy, and auger electronic spectrometer. Biological evaluations were performed with rabbit cornea fibroblast in vitro and an animal model in vivo. The outcomes showed the coating had a grain-like surface topography and a good atomic mixed area with substrate. The rabbit cornea fibroblasts appeared a good adhesion on the surface of nanostructured hydroxyapatite in vitro. In the animal model, nanostructured hydroxyapatite-titanium implants were stably retained in the rabbit cornea, and by contrast, the corneal stroma became thinner anterior to the implants in the control. Therefore, our findings proved that nanostructured hydroxyapatite-titanium could not only provide an improved bond for substrate but also enhance the tissue integration with implants in host. As a promising material, nanostructured hydroxyapatite-titanium–based keratoprosthesis prepared by the aerosol deposition method could be utilized for the corneal blindness treatment.

Formation and in vitro biocompatibility of biomimetic hydroxyapatite coatings on chemically treated carbon substrates

Carbon derived materials such as pyrolytic carbon or carbon-carbon composites (CCCs) exhibit excellent mechanical properties making them promising candidates for bone replacement. However, these materials are considered bioinert and not to induce bone formation in vivo. In this study, a two-step chemical surface treatment including etching with HCl/HNO3 solution and subsequent soaking in CaCl2 solution was applied to carbon substrates in order to activate the materials surface towards bioactive behavior. The bioactivity was proven by soaking the samples in simulated body fluid (SBF) and formation of carbonated hydroxyapatite layer (HCA), which indicates the ability of the material to bond to bone in vivo. The materials surface is shown to be functionalized through the chemical etching creating COO(-)Ca(2+) complexes on the surface as confirmed by FTIR and XPS analyses. These ionic complexes provide nucleation sites for HAp precipitation. After similar immersion time in SBF under the condition of local supersaturation the thickness and homogeneity of the HAp layer were found to depend on the chemical pretreatment with HCl/HNO3. Homogenous HAp layers with a thickness ranging from ∼ 6 to ∼ 17 μm were achieved. The proposed bioactivating treatment of carbon stimulates HAp formation in vivo and can be considered an easy biomimetic approach for coating carbon derived materials with bone-like hydroxyapatite. In vitro cell assay with osteosarcoma cells (MG-63) showed increased cell viability (+70%) on HAp coated carbon substrates as compared to uncoated reference while both materials induced ALP expression in MG-63 cells confirming the osteoblastic phenotype.

Surface modification and drug delivery for biointegration

Biointegration refers to the interconnection between a biomedical device and the recipient tissue. In many implant devices, the lack of proper biointegration can cause device failure and potentially serious medical problems. This review summarizes the recent progress in surface chemistry, drug delivery and antifouling methods to improve the biointegration of implants. Much progress has been made as our understanding of biological systems and material properties expands and as new technologies become available. This article addresses methods of enhancing biointegration by means of modifying implant surface chemistry and by drug-delivery approaches.

In vitro effect of a corrosive hostile ocular surface on candidate biomaterials for keratoprosthesis skirt

Aim

Keratoprosthesis (KPro) devices are prone to long-term corrosion and microbiological assault. The authors aimed to compare the inflammatory response and material dissolution properties of two candidate KPro skirt materials, hydroxyapatite (HA) and titania (TiO₂) in a simulated in vitro cornea inflammation environment.

Methods

Lipopolysaccharide-stimulated cytokine secretions were evaluated with human corneal fibroblasts on both HA and TiO₂. Material specimens were subjected to electrochemical and long-term incubation test with artificial tear fluid (ATF) of various acidities. Topography and surface roughness of material discs were analysed by scanning electron microscopy and atomic force microscopy.

Results

There were less cytokines secreted from human corneal fibroblasts seeded on TiO₂ substrates as compared with HA. TiO₂ was more resistant to the corrosion effect caused by acidic ATF in contrast to HA. Moreover, the elemental composition of TiO₂ was more stable than HA after long-term incubation with ATF.

Conclusions

TiO₂ is more resistant to inflammatory degradation and has a higher corrosion resistance as compared with HA, and in this regard may be a suitable material to replace HA as an osteo-odonto-keratoprosthesis skirt. This would reduce resorption rates for KPro surgery.

Osteo-odonto keratoprosthesis: systematic review of surgical outcomes and complication rates

Case series on osteo-odonto keratoprosthesis (OOKP) published in English from 1950-June 2010 were identified in Medline/PubMed. Indications for surgery, visual acuity, anatomical survival, complication and repeat surgery rates were compared among the different studies. Our own case series is a retrospective review of all OOKP surgeries performed in our center from February 2004-July 2011. Eight case series including our own were systematically reviewed. Sample sizes ranged from 4-181 eyes. The most common indications for surgery were severe cases of Stevens-Johnson syndrome and thermal and chemical burns that were unamenable to other forms of surgery or had had previous surgical failure. Anatomical survival rate in all the studies was 87.8% (range 67-100%) at 5 years, and three studies showed survival rates of 81.0% (range 65-98%) at 20 years. Visual acuity was more than 6/18 in 52% (range 46-72%) of the eyes with OOKP surgery. The most common intraoperative complication was vitreous hemorrhage (0-52%) and the most common long-term blinding complication was glaucoma (7-47%). Endophthalmitis rates ranged from 2-8%. The most common repeat surgical procedure was mucosal trimming due to mucosal overgrowth at the optical cylinder and mucosal grafting for extrusion of the OOKP or mucosal ulceration. Of the available biological and synthetic keratoprosthesis, OOKP appears to be an excellent option for the treatment of end-stage corneal diseases.

Hydroxyapatite for keratoprosthesis biointegration

PURPOSE

Integration of keratoprosthesis with the surrounding cornea is very important in preventing bacterial invasion, which may cause ocular injury. Here the authors investigated whether hydroxyapatite (HAp) coating can improve keratoprosthesis (KPro) biointegration, using polymethyl methacrylate (PMMA)--the principal component of the Boston KPro--as a model polymer.

METHODS

HAp coatings were induced on PMMA discs after treatment with concentrated NaOH and coating with poly-dopamine (PDA) or polydopamine and then with 11-mercaptoundecanoic acid (11-MUA). Coatings were characterized chemically (Fourier transform infrared spectroscopy [FTIR], energy dispersive X-ray spectroscopy [EDX]) and morphologically (SEM) and were used as substrates for keratocyte growth in vitro. Cylinders of coated PMMA were implanted in porcine corneas ex vivo for 2 weeks, and the force required to pull them out was measured. The inflammatory reaction to coated discs was assessed in the rabbit cornea in vivo.

RESULTS

FTIR of the coatings showed absorption bands characteristic of phosphate groups, and EDX showed that the Ca/P ratios were close to those of HAp. By SEM, each method resulted in morphologically distinct HAp films; the 11-MUA group had the most uniform coating. The hydroxyapatite coatings caused comparable enhancement of keratocyte proliferation compared with unmodified PMMA surfaces. HAp coating significantly increased the force and work required to pull PMMA cylinders out of porcine corneas ex vivo. HAp coating of implants reduced the inflammatory response around the PMMA implants in vivo.

CONCLUSIONS

These results are encouraging for the potential of HAp-coated surfaces for use in keratoprostheses.

A transcriptomic analysis of the EK1.Br strain of human fibroblastoid keratocytes: the effects of growth, quiescence and senescence

There is a growing need within ocular research for well-defined cellular models of normal corneal biology. To meet this need we created and partially characterised a standard strain of human fibroblastoid keratocytes (EK1.Br) and demonstrated that phenotypic changes occur within these cells with replicative senescence in vitro. Using Affymetrix HG-U133A oligonucleotide arrays, this paper reports both a comprehensive analysis of the transcriptome of EK1.Br in the growing, quiescent and senescent states and a comparison of that transcriptome with those of primary corneal endothelium, lung fibroblasts and dermal fibroblasts grown under identical conditions. Data mining shows (i) that EK1.Br retain the characteristic transcriptional fingerprint of keratocytes in vitro (ii) that this phenotype can be distinguished from those of other 'fibroblasts' by groups of highly differentially expressed genes and (iii) that senescence induces a distinct dedifferentiation phenomenon in EK1.Br. These findings are contextualised into the broader literature on replicative senescence and are supported with a web-accessible and fully searchable public-access database (www.madras.cf.ac.uk/cornea).

Nucleation and growth of apatite on NaOH-treated PEEK, HDPE and UHMWPE for artificial cornea materials

The skirt of an artificial cornea must integrate the implant to the host sclera, a major failure of present devices. Thus, it is highly desirable to encourage the metabolic activity of the cornea by using more bioactive, flexible skirt materials. Here we describe attempts to increase the bioactivity of polyether ether ketone (PEEK), high-density polyethylene (HDPE) and ultra-high molecular weight polyethylene (UHMWPE) films. The effectiveness of different strength NaOH pre-treatments to initiate apatite deposition on PEEK, HDPE and UHMWPE is investigated. We find that exposure of PEEK, HDPE and UHMWPE films to NaOH solutions induces the formation of potential nuclei for apatite (calcium phosphate), from which the growth of an apatite coating is stimulated when subsequently immersing the polymer films in 1.5 strength Simulated Body Fluid (SBF). As immersion time in SBF increases, further nucleation and growth produces a thicker and more compact apatite coating that can be expected to be highly bioactive. Interestingly, the apatite growth is found to also be dependent on both the concentration of NaOH solution and the structure of the polymer surface.

Long-Term Outcome in Ocular Intractable Surface Disease With Seoul-Type Keratoprosthesis

PURPOSE

To evaluate the long-term clinical efficacy of the Seoul-type keratoprosthesis (S-KPro).

METHODS

S-KPros were implanted into 4 unsighted and 5 sighted eyes in 9 patients: 6 patients were diagnosed with Stevens-Johnson syndrome, 2 had chemical burns, and 1 suffered from ocular pemphigoid. The preoperative visual acuity ranged from light perception to hand motion. The average follow-up period was 62.8 months. We evaluated several clinical factors, including visual acuity, visual field, number of additional grafting procedures, number of capsulotomy procedures, and the interval between retinal detachment and skirt exposure.

RESULTS

The S-KPro showed anatomic success for an average of 62.8 months in 66.7% of the eyes. The average visual acuity preservation time was 31.6 months. Localized glaucomatous visual field defect was not found in any of the sighted patients; however, diffuse visual field constriction was observed after long-term follow-up. Average time of skirt exposure and mean number of additional grafting procedures were 12.9 months and 2.44, respectively. Retinal detachments were developed in all the patients at a mean time interval of 2 months after S-KPro exchange.

CONCLUSIONS

S-KPro achieved visual rehabilitation for an average of 31.6 months with long-term anatomic stability in patients with severe intractable ocular surface disease.

Legeais BioKpro III keratoprosthesis implantation: long term results in seven patients

AIMS

The long term results of the Legeais BioKpro III keratoprosthesis are presented for seven patients with severe corneal scarring.

METHODS

The study took place at Moorfields Eye Hospital, London. Patients had either end stage ocular surface disease or corneal opacification after multiple failed graft surgery, with the potential for significant visual improvement. After insertion the device was covered with a conjunctival flap or buccal mucous membrane graft, which was later opened to expose the optic. The outcome measures were vision, complications, and retention of the device.

RESULTS

The BioKpro III was inserted into seven patients with severe corneal scarring: ocular cicatricial pemphigoid, measles keratitis, thermal injury, Stevens-Johnson syndrome, aniridia, chemical injury, and congenital rubella. The follow up was 18-48 months. The keratoprosthesis failed in six, because of extrusion occurring 2-28 months postoperatively. Retroprosthetic membranes occurred in three patients, and endophthalmitis in one. Vision improved from hand movements to 6/12 in the only patient who retained the KPro; however he was troubled by mucus accumulation on the optic.

CONCLUSIONS

The one success has been in a patient with thermal burns. The remaining results have been poor, with the KPro extruding in six of the seven patients.

Three-dimensional Nanohydroxyapatite/Chitosan Scaffolds as Potential Tissue Engineered Periodontal Tissue

The development of suitable three-dimensional scaffold for the maintenance of cellular viability and differentiation is critical for applications in periodontal tissue engineering. In this work, different ratios of porous nanohydroxyapatite/chitosan (HA/chitosan) scaffolds are prepared through a freeze-drying process. These scaffolds are evaluated in vitro by the analysis of microscopic structure, porosity, and cytocompatibility. The expression of type I collagen and alkaline phosphatase (ALP) activity are detected with real-time polymerase chain reaction (RT-PCR). Human periodontal ligament cells (HPLCs) transfected with enhanced green fluorescence protein (EGFP) are seeded onto the scaffolds, and then these scaffolds are implanted subcutaneously into athymic mice. The results indicated that the porosity and pore diameter of the HA/chitosan scaffolds are lower than those of pure chitosan scaffold. The HA/chitosan scaffold containing 1% HA exhibited better cytocompatibility than the pure chitosan scaffold. The expression of type I collagen and ALP are up-regulated in 1% HA/chitosan scaffold. After implanted in vivo, EGFP-transfected HPLCs not only proliferate but also recruit surrounding tissue to grow in the scaffold. The degradation of the scaffold significantly decreased in the presence of HA. This study demonstrated the potential of HA/ chitosan scaffold as a good substrate candidate in periodontal tissue engineering