Synthetic corneal inlays

This review is based on the activities of the Vision Cooperative Research Centre (previously Cooperative Research Centre for Eye Research and Technology) Corneal Implant team from 1991 to 2007. The development of a synthetic polymer of perfluoropolyether (PFPE), meeting essential physical and biological requirements, for use as a corneal inlay is presented. Each inlay was placed in a corneal flap created with a microkeratome and monitored over a two-year period in a rabbit model. The results indicate that the PFPE implant shows excellent biocompatibility and biostability. As a result, a Phase 1 clinical trial is being conducted. Three years post-implantation, the PFPE inlays are exhibiting continued excellent biocompatibility. Corneal inlays made from PFPE are biocompatible with corneal tissue in the long term and offer a safe and biologically-acceptable alternative to other forms of refractive surgery.

The Max Schapero Memorial Award Lecture 2004: Contact Lenses on and in the Cornea, What the Eye Needs

PURPOSE

The aim of this study is to review the advances made in understanding the needs of the cornea and the way in which it responds to contact lens wear and corneal implants.

METHODS

The review is based on personal knowledge and involvement of the author and colleagues from the 1970s to 2005.

RESULTS

Development of silicone hydrogel contact lenses is presented as well as development of synthetic materials for implantation on the corneal surface (corneal onlay).

CONCLUSIONS

The future of vision correction involves developing highly porous and biocompatible lens materials. For contact lenses, a better understanding of the effects of contact lens wear on the ocular surfaces, including the tear film, and development of lens materials with greater bacterial resistance are required. For those who require new solutions to permanent vision correction, corneal onlays (implantable contact lenses) are a minimally invasive and totally reversible procedure that can be removed or replaced as visual needs change over time.

A review of the development of a synthetic corneal onlay for refractive correction

A synthetic corneal onlay, or implantable contact lens, could obviate the need for spectacles or conventional contact lenses in patients who seek convenient, reversible correction of refractive error. Several research groups have attempted to develop such a product in the past but much of the data from these studies remains unpublished due to commercial interests. This article reviews relevant papers and patents in the corneal implant field and discusses our efforts to develop a synthetic corneal onlay using a perfluoropolyether-based polymer.

The use of porcine small intestinal submucosa in ten cases of feline corneal disease

Porcine small intestinal submucosa (SIS) was used as a novel graft material in the management of 10 cases of feline corneal disease. Five cases had stromal ulceration associated with trauma, ocular foreign body and/or suspected infection and required a grafting procedure. Five cases had feline sequestra that were managed by a keratectomy prior to placement of SIS as a graft material. Eight eyes healed with minimal corneal scarring with a very good cosmetic and visual result. One eye with continued aqueous leakage in the immediate postoperative period required a conjunctival pedicle graft to reinforce the SIS graft site. One eye required enucleation 48 h following grafting due to progressive keratomalacia but the SIS material remained intact.

Peptoid-containing collagen mimetics with cell binding activity

Collagen mimetic peptides containing the peptoid residue Nleu (Goodman Bhumralkar, Jefferson, Kwak, Locardi. Biopolymers 1998;47:127-142) were tested for interactions with epithelial cells and fibroblasts. Molecules containing the sequence Gly-Pro-Nleu with a minimum of nine repeats showed cell binding activity. The activity of these molecules appeared to be conformationally sensitive, with the triple-helical form being preferred. When immobilized on a surface, the (Gly-Pro-Nleu)(10)-Gly-Pro-NH(2) sequence stimulated the attachment and growth of corneal epithelial cells and fibroblasts and the migration of epithelial tissue. The peptide sequence KDGEA inhibited cell attachment to the (Gly-Pro-Nleu)(10)-Gly-Pro-NH(2) sequence, suggesting that cell binding to this collagen mimetic involves the alpha2beta1 heterodimer integrin receptor. Interestingly, peptides containing the sequence (GlyNleu-Pro-)(10)-NH(2) did not have cell binding activity. The discovery that triple-helical peptides containing the Gly-Pro-Nleu sequences interact with cells opens up new opportunities in the design of collagen mimetic biomaterials.

Effect of intrastromal corneal ring segment shift on clinical outcome: one year results for low myopia

PURPOSE

To evaluate the effect of intrastromal corneal ring segment (ICRS) shift on clinical outcome.

SETTING

Chu Morvan Ophthalmology Service, Brest, France.

METHODS

Forty-seven eyes with myopia from -1.0 to -5.0 diopters (D) had ICRS surgery according to United States Food and Drug Administration protocol using a 2-segment implant design. Visual acuity, refractive, videotopographic, and biomicroscopic results in eyes that did not have contact between segments (Group A, n = 26) and in eyes that did have contact (Group B, n = 21) were compared.

RESULTS

Soon after surgery, migration of and subsequent contact between the implant segments occurred in 21 eyes (45%). Contact between the inferior ends was most common, occurring in 19 eyes; in the remaining 2 eyes, the superior ends of the implant touched. Superior contact was associated with superficial channels, wound-healing problems, and ICRS explantation. No segment shift was observed after the first postoperative week. No significant differences in visual acuity, qualitative vision, or density of intrastromal deposits were noted between the 2 groups. However, at 6 months there was more induced with-the-rule cylinder in Group B (mean +1.3 D +/- 0.7 [SD]) than in Group A (mean +0.7 +/- 0.5 D). Astigmatism was clinically significant in only 3 cases at 3 months.

CONCLUSIONS

Induced cylinder remains a limitation of the ICRS procedure, and segment contact appears to influence the rate of induced cylinder. Overall, however, the ICRS procedure produces promising results. Research efforts are underway to address induced cylinder and channel deposits associated with ICRS surgery.

Tissue-engineered human bioartificial muscles expressing a foreign recombinant protein for gene therapy

Murine skeletal muscle cells transduced with foreign genes and tissue engineered in vitro into bioartificial muscles (BAMs) are capable of long-term delivery of soluble growth factors when implanted into syngeneic mice (Vandenburgh et al., 1996b). With the goal of developing a therapeutic cell-based protein delivery system for humans, similar genetic tissue-engineering techniques were designed for human skeletal muscle stem cells. Stem cell myoblasts were isolated, cloned, and expanded in vitro from biopsied healthy adult (mean age, 42 +/- 2 years), and elderly congestive heart failure patient (mean age, 76 +/- 1 years) skeletal muscle. Total cell yield varied widely between biopsies (50 to 672 per 100 mg of tissue, N = 10), but was not significantly different between the two patient groups. Percent myoblasts per biopsy (73 +/- 6%), number of myoblast doublings prior to senescence in vitro (37 +/- 2), and myoblast doubling time (27 +/- 1 hr) were also not significantly different between the two patient groups. Fusion kinetics of the myoblasts were similar for the two groups after 20-22 doublings (74 +/- 2% myoblast fusion) when the biopsy samples had been expanded to 1 to 2 billion muscle cells, a number acceptable for human gene therapy use. The myoblasts from the two groups could be equally transduced ex vivo with replication-deficient retroviral expression vectors to secrete 0.5 to 2 microg of a foreign protein (recombinant human growth hormone, rhGH)/10(6) cells/day, and tissue engineered into human BAMs containing parallel arrays of differentiated, postmitotic myofibers. This work suggests that autologous human skeletal myoblasts from a potential patient population can be isolated, genetically modified to secrete foreign proteins, and tissue engineered into implantable living protein secretory devices for therapeutic use.

Effects of surface topography on corneal epithelialization in vivo: a preliminary study

PURPOSE

In this study we investigated the relationship between surface topography and initial epithelialization of synthetic lenticules in vivo.

METHODS

Millipore ultrafiltration membranes of three different pore sizes were used as model lenticule materials. The nominal membrane pore sizes were 0.1 microm, 0.45 microm, and 3 microm; the surface roughness increased in the same order The membranes were coated with a thin layer of collagen I and implanted in a circular pocket of the anterior cornea of adult cats, and were clinically evaluated for the extent of epithelialization and the persistence of epithelial attachment.

RESULTS/CONCLUSIONS

It was demonstrated that a smooth surface appears more attractive for initial epithelialization than a rough surface in vivo.

Mechanism of initial attachment of corneal epithelial cells to polymeric surfaces

The initial attachment of cultured bovine corneal epithelial cells and stromal fibroblasts to two oxygen-containing synthetic polymers was studied. Cultured epithelial cells and stromal fibroblasts were seeded onto two oxygen-containing surfaces: 'tissue culture' polystyrene (TCPS) and a polymer film deposited by RF plasma deposition using a methylmethacrylate monomer (MMA/FEP). To establish the mechanism of cell attachment, the effect of the selective removal of the vitronectin and fibronectin from the serum used in the culture medium was tested. The attachment of cultured epithelial cells during the first 90 min of culture was reduced by 40% (TCPS)-80% (MMA/FEP) as a result of removing vitronectin from the medium. Attachment of these cells to TCPS was reduced by 85-95% when the serum was depleted of both fibronectin and vitronectin. However, depletion of fibronectin reduced cell attachment to TCPS by 20%, whilst on MMA/FEP cell attachment was equivalent, or higher, than that for intact serum. The attachment of cultured corneal stromal fibroblasts was similarly dependent on vitronectin but less dependent on fibronectin. Therefore, for the attachment of both cultured epithelial cells and fibroblasts to oxygen-containing surfaces in the presence of serum, there is a high requirement for serum vitronectin but a lesser requirement for fibronectin. The effects of the establishment of corneal epithelial cells in culture and the site of origin of the cells, were determined. Primary isolates of epithelial cells isolated from the limbal, central or peripheral regions of the cornea were less dependent on vitronectin for initial attachment to TCPS than were these cells after several passages in culture. Furthermore, the primary isolates were dramatically less responsive to vitronectin than the cultured cells. These results indicate that the mechanism of attachment of corneal epithelial cells to TCPS varies with the culture experience of the cells. Cells that are culture neophytes can employe endogenous mechanisms for the initial attachment to TCPS, whereas cells established in culture are dependent on exogenous vitronectin in order to attach.

Keratoprostheses: advancing toward a true artificial cornea

Keratoprosthesis surgery is carried out in very few centers. Elaborate surgical techniques and high complication rates limit the application of currently available keratoprostheses (KPros). However, the clinical need for an alternative to donor tissue has sparked considerable research interest in the development of new KPros. This paper charts the evolution of KPros from the earliest devices to those currently used, describes their drawbacks and discusses the specifications of an ideal device. Recent research focuses upon the use of porous polymers as the skirt component of core-and-skirt KPros in order to obtain improved biological integration of the prosthetic material. Developments in biomaterials technology make a KPro analogous to a donor corneal button an increasingly realistic goal. However, two particular problems still need to be addressed. First, it must be demonstrated that secure long-term fixation that is able to withstand trauma is achievable in a full-thickness artificial cornea. Second, an ideal artificial cornea for a wet eye requires an epithelialized surface, and this has yet to be achieved.

Effects of biologically modified surfaces of synthetic lenticules on corneal epithelialization in vivo

PURPOSE

The present study investigated the association of extracellular matrix (ECM) glycoprotein coatings with initial epithelialization of artificial lenticules in vivo.

METHODS

Collagen I, collagen IV, ECM and fibronectin were individually coated onto the surface of polycarbonate membranes. The membranes were then implanted in the anterior stroma of adult cats and were clinically assessed for rapidity and extent of epithelialization and the persistence of epithelial attachment.

RESULTS/CONCLUSIONS

It was demonstrated that membrane surfaces modified by collagen I, collagen IV and ECM consistently supported initial migration and attachment of corneal epithelial cells and that the surface modified with collagen I performed best.

Tissue engineering

The loss or failure of an organ or tissue is one of the most frequent, devastating, and costly problems in human health care. A new field, tissue engineering, applies the principles of biology and engineering to the development of functional substitutes for damaged tissue. This article discusses the foundations and challenges of this interdisciplinary field and its attempts to provide solutions to tissue creation and repair.