Preliminary evaluation of the use of mussel adhesive protein in experimental epikeratoplasty.

We have used a preliminary formulation of a bioadhesive in an experimental model of epikeratoplasty in rabbits. The adhesive, termed mussel adhesive protein (MAP), is a repeating decapeptide polymer that is the natural adhesive substance produced by the common blue mussel (Mytilus edulis) and is used here in conjunction with an enzymatic cross-linking agent. This study marked the first in vivo use of this material in adhering ophthalmic tissue planes. We used a simplified epikeratoplasty technique highlighted by freehand dissection of donor lenticules and host keratotomies. Approximately 10 microL of a combination of MAP and the cross-linking agent was applied directly to the host cornea and the donor lenticules were secured into the keratotomies using eight interrupted 10-0 nylon sutures. All of the sutures were removed 72 hours postoperatively. Eleven of the 15 animals retained their epikeratoplasty lenticules throughout the entire postoperative period. Four animals had initially intact lenticules that sloughed within the first postoperative week; this sloughing, we believe, was attributable to difficulties in tucking irregularly thickened lenticule edges into the keratotomy. In control animals that had lenticules secured with sutures alone, suture removal at 72 hours consistently produced immediate lenticule sloughing. Clinical examinations and histopathologic studies disclosed no untoward effects of the adhesive on the donor or host corneal tissue. We believe that this preliminary study indicates a potential adjunctive role for MAP in epikeratoplasty.

Surface charge on polymeric implants and calcified tissues: interfacial implications.

Samples of human bone, dentin, and enamel were analyzed through the technique known as thermally stimulated discharge (TSD). It is possible through this technique to detect current flow resulting from appearance or loss of net polarization in a material. Samples of freshly extracted tissue give rise to well-defined TSD current maximal without having been exposed to external electrical potentials. Calculation of activation energies for these currents and their thermal range suggests the involvement of collagen denaturation in the loss of appearance of a net surface charge on bone and dentin surfaces. In the case of enamel samples, TSD current maxima are possibly the result of dipolar alignment in water or biopolymers by surface charges in the mineral phase. Interfacial implications of surface charge were studied through the measurement of adhesive strength in dentin/acrylic polymer joints. Enhancement of joint strength by a factor of two or higher was observed when powder particles of the experimental adhesive carried externally induced surface charge. It is hypothesized that electrostatic coupling between polarization domains on the tissue surface and the setting implant improves wetting and produces stronger interfaces.

Mechanical strength of poly(methyl methacrylate) cement-human bone interfaces.

A device was constructed to test the interfacial strength of PMMA-based bone cement and human cancellous bone under pure tension. Two types of tissue were used in the investigation: (1) formalin-fixed vertebral bone as an in vitro model for weak cancellous bone, and (2) freshly removed metatarsal bone. Tissue--cement joints were allowed to solidify under two different pressures (0.11 and 0.47 MPa), and cement placement time on tissue surfaces was also controlled as a variable. The higher curing pressure only seemed to enhance the strength of interfaces formed with mechanically weak fixed bone but had no significant effect for joints formed with the stronger, freshly extracted tissue. Cement placement time did not have a discernible effect on interfacial strength regardless of the tissue used or the pressure applied during setting. An analysis of fracture morphology by optical microscopy revealed largely cement cohesive failure in some cases and bone or mixed fractures in others. Joints exhibiting mainly cement fracture had the highest interfacial tensile strengths (in the order of 7.5 MPa). Once measured values of tissue porosity were taken into account, the observed joint strength correlated well with cement tensile strength. Based on experimental findings, better stress-dissipating qualities and higher tensile strength are suggested as two important necessary improvements of bone cements based on poly(methyl methacrylate).