Five year follow up of biocolonisable microporous fluorocarbon haptic (BIOKOP) keratoprosthesis implantation in patients with high risk of corneal graft failure

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.

Progress in the development of a corneal replacement: keratoprostheses and tissue-engineered corneas

Rapid progress has been made in the past 5 years in the development of corneal replacements. Traditionally they are divided into two categories, keratoprostheses and tissue-engineered corneal equivalents, as replacement tissues are increasingly in demand worldwide. There are currently several different keratoprosthesis models in clinical use around the world. The most popular and most widely publicized is the AlphaCor model, which has enjoyed significant clinical success. However, improvements remain to be made, and the aim of most of the current research is to better understand the interactions between a synthetic material and the surrounding biology on a more fundamental level. This improved understanding will no doubt lead to improvements in current models and to the development of new models in the near future. While tissue-engineered corneal equivalents have been under investigation for considerably less time, there is growing evidence to suggest that a tissue-engineered corneal equivalent comprised of primarily natural materials will exist in the not too distant future. Research groups have reported strong in vitro and in vivo results. The strength of the collagen matrix and its ability to support cell infiltration have been the primary avenues of research. Various collagen crosslinking techniques have been used. Infiltration of three major cells of the cornea has been observed. Most importantly, the ability of these materials to support nerve ingrowth has been demonstrated. While challenges remain with both types of corneal replacements, the considerable progress in the recent past suggests that reliable implants for the treatment of a variety of corneal diseases will be available. This review will provide an overview of recent results, and will provide insight into the future of research on corneal replacements.

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.

Bioactive interpenetrating polymer network hydrogels that support corneal epithelial wound healing

The development and characterization of collagen-coupled poly(ethylene glycol)/poly(acrylic acid) (PEG/PAA) interpenetrating polymer network hydrogels is described. Quantitative amino acid analysis and FITC-labeling of collagen were used to determine the amount and distribution of collagen on the surface of the hydrogels. The bioactivity of the coupled collagen was detected by a conformation-specific antibody and was found to vary with the concentration of collagen reacted to the photochemically functionalized hydrogel surfaces. A wound healing assay based on an organ culture model demonstrated that this bioactive surface supports epithelial wound closure over the hydrogel but at a decreased rate relative to sham wounds. Implantation of the hydrogel into the corneas of live rabbits demonstrated that epithelial cell migration is supported by the material, although the rate of migration and morphology of the epithelium were not normal. The results from the study will be used as a guide toward the optimization of bioactive hydrogels with promise in corneal implant applications such as a corneal onlay and an artificial cornea.

Glucose-permeable interpenetrating polymer network hydrogels for corneal implant applications: a pilot study

Epithelialization of a keratoprosthesis requires that the implant material be sufficiently permeable to glucose. We have developed a poly(ethylene glycol)/poly(acrylic acid) (PEG/PAA) interpenetrating polymer network (IPN) hydrogel that can provide adequate passage of glucose from the aqueous humor to the epithelium in vivo. A series of PEG/PAA IPNs with varying PEG macromonomer molecular weights were synthesized and evaluated through swelling studies to determine their water content and diffusion experiments to assess their permeability to glucose. One of the PEG/PAA hydrogels prepared in this study had a glucose diffusion coefficient nearly identical to that of the human cornea (approximately 2.5 x 10(-6) cm(2)/sec). When implanted intrastromally in rabbit corneas, this hydrogel was retained and well-tolerated in 9 out of 10 cases for a period of 14 days. The retained hydrogels stayed optically clear and the epithelium remained intact and multilayered, indicating that the material facilitated glucose transport from the aqueous humor to the anterior part of the eye. The results from these experiments indicate that PEG/PAA hydrogels are promising candidates for corneal implant applications such as keratoprostheses and intracorneal lenses, and that the PEG/PAA IPN system in general is useful for creating permeable substrates for ophthalmic and other biomedical applications.

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.

Current concepts and techniques in keratoprosthesis

PURPOSE OF REVIEW

Diseases affecting the cornea are a major cause of blindness worldwide, second only to cataract in overall importance, with an estimated 10 to 15 million affected people. Although keratoplasty is by far the most successful transplantation surgery, the outcomes in high-risk adult patients, including those with ocular surface diseases and multiple graft rejections, and in pediatric patients with congenital corneal opacities are disappointing.

RECENT DEVELOPMENTS

Regrettably, no significant clinical developments have been achieved in the field of corneal transplantation since the introduction of steroids for graft rejection. Furthermore, obtaining donor corneal tissues and eye banking, particularly in the developing countries where corneal blindness is most prevalent, are problematic. Although the postoperative complications may be severe and limit the use of currently available devices, keratoprosthesis--artificial corneal implantation--has a role in the management of corneal blindness in carefully selected patients with complex ocular diseases who are at high risk for graft failure.

SUMMARY

This article reviews the recent ophthalmic literature published on the current concepts and techniques of keratoprosthesis surgery.