Optimal degradation rate for collagen chambers used for regeneration of peripheral nerves over long gaps

The experimental study of peripheral nerve regeneration has depended heavily on the use of a nerve chamber in which the stumps of the transected nerve are inserted. A large variety of chamber fillings and chamber types have been used in an effort to induce a higher quality of regeneration across the gap initially separating the two stumps. In this study we studied the morphology of nerves regenerated across a 15 mm gap following implantation of a series of five chambers. The chambers were fabricated from type I collagen and possessed identical pore volume fractions as well as average pore diameters, but differed in cross-link density continuously along the series. The residual mass of the implanted chambers at 9 weeks was observed to increase continuously with increasing cross-link density along the series, indicating a continuous decrease in degradation rate. The quality of regenerated nerves, determined by the number of large diameter fibers (A-fibers) per nerve, the average diameter of all axons and the ratio of area occupied by axons (N-Ratio), was superior at an intermediate level of chamber degradation rate. The maximal quality of peripheral nerve regeneration corresponded to an optimal degradation rate with an estimated chamber half-life of approximately 2-3 weeks following implantation. A speculative mechanistic explanation of the observed optimum focuses on the hypothetical role of cell and cytokine traffic that may take place through holes in the chamber generated by the degradation process. The data show the presence of a hitherto unreported optimal chamber degradation rate that leads to regenerated nerves of maximum quality.

The effects of tubulation on healing and scar formation after transection of the adult rat spinal cord

PURPOSE

The purpose of this study was to characterize the effects of implantation of a collagen tube on healing and scar formation following transection of tbc adult rat spinal cord.

METHODS

The spinal cords of adult rats were completely transected at the mid-thoracic level. At 30 days after injury, the cellular and extra-cellular components of repair tissue present within tubulated and non-tubulated (control) wounds were compared using qualitative and quantitative histological techniques.

RESULTS

The presence of the tube reduced fibrocollagenous scar invasion into the gap, promoted astrocyte migration, and oriented axonal and connective tissue components of the repair tissue. Tube implants supported the regeneration of a substantial number of myelinated axons. A notable finding was the identification of cells containing a contractile actin isoform in the healing spinal cord.

CONCLUSIONS

The tubulation model allows for the study of spinal cord wound healing and axon elongation in a controlled experimental environment within the tube lumen. Using this model, it will be possible to study manipulation of the healing response by the introduction of exogenous agents within the tube.

Contraction of collagen-glycosaminoglycan matrices by peripheral nerve cells in vitro

The objective of this study was to investigate the contractile behavior of peripheral nerve support cells in collagen-glycosaminoglycan (GAG) matrices in vitro. Contractile fibroblasts (myofibroblasts) are known to participate in wound contraction during healing of selected connective tissues (viz., dermis), but little is known about the activity of non-muscle contractile cells during healing of peripheral nerves. Explants from adult rat sciatic nerves were placed onto collagen-GAG matrix disks and maintained in culture for up to 30 days. Groups of collagen-GAG matrices were tested that differed in average pore diameter and in degree of cross-linking. Cell migration from nerve explants into the matrices was examined, and immunohistochemical staining was used to identify cells expressing a contractile actin isoform (alpha-smooth muscle actin; alpha-SMA) and Schwann cells (S-100). Geometric contraction of matrix disks was quantified every five days as the percent reduction in disk diameter. The amount of contraction of matrix disks was significantly affected by the degree of cross-linking. Cell migration into the matrices and the distribution of cells staining for alpha-SMA or S-100 was not affected by matrix parameters. These studies demonstrate that cells from peripheral nerve explants were capable of adopting a contractile phenotype and causing geometric contraction of matrices in vitro and suggest that contractile processes may be important during nerve wound healing in vivo.

Primary placement of a titanium motility post in a porous polyethylene orbital implant: animal model with quantitative assessment of fibrovascular ingrowth and vascular density

PURPOSE

To determine whether a Medpor porous polyethylene orbital implant, at the time of initial orbital implant surgery, will tolerate the insertion of a titanium screw on the anterior surface of the implant.

METHODS

Twelve New Zealand white rabbits were enucleated and implanted with a porous polyethylene orbital implant. At the time of enucleation, the porous polyethylene orbital implants were drilled, and titanium motility coupling posts were inserted. The motility coupling posts were inserted at two projection heights (2 or 4 mm) and either covered within Tenon capsule/conjunctiva (eight implants) or left exposed (four implants). Rabbits were killed at 6 or 12 weeks. Clinical tissue tolerance, histologic response to the motility coupling post, and vascular density of the porous polyethylene orbital implant were evaluated.

RESULTS

The motility coupling posts were well tolerated, and extrusion or migration of the motility coupling post did not occur. The average percentage cross-sectional area of the implant occupied by fibrovascular tissue at 6 and 12 weeks was 76.3% and 97.5%, respectively. In comparing the vascular density (number of vessels per square millimeter) in the porous polyethylene orbital implant within a 1-mm zone immediately surrounding the motility coupling post, no significant difference between this zone and the vascular density found within its entire corresponding annulus was found at either 6 or 12 weeks.

CONCLUSIONS

During the 6- and 12-week observation periods, all implanted motility coupling posts demonstrated favorable tissue tolerance and stable interfaces with surrounding tissues. The extent of fibrovascular tissue ingrowth and vascular density verify that initial screw insertion does not adversely affect the healing process after porous polyethylene orbital implant implantation. Thus, primary placement of the motility coupling post may obviate the need for a secondary surgical procedure.

Inhibition of conjunctival scarring and contraction by a porous collagen-glycosaminoglycan implant

PURPOSE

To study the healing processes of full-thickness wounds in the adult rabbit conjunctiva after grafting with a porous collagen-glycosaminoglycan (CG) copolymer matrix.

METHODS

A 7-mm trephine was used to produce lesions of the bulbar conjunctiva down to the level of the bare sclera. Full-thickness removal of the conjunctiva and Tenon's capsule created a reproducible wound bed. Wounds either remained ungrafted (control) or were grafted with CG matrix. In previous studies, this CG matrix has induced partial regeneration of the dermis in the human, the swine, and the guinea pig. Healing of the conjunctival epithelium and underlying stroma was evaluated by histology, immunohistochemistry, and measurement of wound contraction kinetics.

RESULTS

By 28 days, ungrafted wounds had closed by contraction (26.4% +/- 5.0% fornix shortening) and the formation of scarlike tissue comprising an aligned array of dense collagen populated with occasional fibroblasts. Grafting of identical defects with CG copolymer matrix resulted in inhibition of wound contraction (6.8% +/- 3.2% fornix shortening) and the formation of a tissue that resembled normal conjunctival stroma, being composed of a loose network of collagen fibers and fibroblasts. Contractile fibroblasts (myofibroblasts) were identified at the edge of both ungrafted and grafted wounds during the period of active contraction. Both ungrafted and grafted wounds were completely re-epithelialized by 28 days.

CONCLUSIONS

Implantation of CG copolymer matrix drastically reduced contraction and promoted the formation of a nearly normal subconjunctival stroma.