Shock/sepsis/trauma/critical carePhotochemical Sealing Improves Outcome Following Peripheral Neurorrhaphy
Introduction
Photochemical tissue bonding (PTB) is a developing tissue repair technique that utilizes a photosensitizing dye and visible laser light to produce an immediate, water-tight seal between tissue surfaces. The dye, without additional proteins or polymers, is applied to the tissue surfaces, which are then brought into contact and irradiated with visible light. While the exact mechanism of action is yet to be elucidated, we believe that light activation of the dye initiates chemical reactions that form covalent bonds between proteins on the tissue surfaces. PTB differs fundamentally from earlier laser tissue welding techniques that used lasers to generate thermal energy, resulting in denaturation of tissue proteins [1]. During the cooling process, the denatured proteins anneal, thus welding adjacent tissues. Laser welding is ultimately limited by the collateral tissue damage produced by heat. In contrast, thermographic studies have demonstrated that, using light and dye parameters that bond the tissue, the temperature in PTB-treated tissues does not rise above 40°C. PTB has been used experimentally in a number of tissue repair models and has been shown to seal corneal incisions in vivo and ex vivo [2, 3, 4], bond transected tendon to produce increased early repair strength [5], seal incisional and excisional skin wounds with healing equivalent to sutured closure [6], and adhere skin grafts [7].
We have also previously demonstrated that PTB can be successfully used in peripheral nerve repair with functional and histological results similar to those obtained with conventional epineural sutures [8]. That study indicated that circumferential sealing of the nerve repair site resulted in excellent restoration of normal neural architecture.
Our understanding of the molecular processes governing nerve regeneration has been greatly improved in recent years [9, 10], and this has fuelled a new approach to nerve repair. Following peripheral nerve injury, the normal intraneural homeostasis is lost [11]. Injury initiates a complex series of events involving the up-regulation of important growth factors from resident cells, e.g., Schwann cells, and from transient cells, e.g., macrophages, in the nerve stumps [12]. Many of these factors have now been well characterized and their specific roles in the regenerative process identified [9, 10]. The precisely timed fluctuation in the regulation of neurotrophic factors and their receptors is likely to play an essential role in the peripheral nervous system's ability to regenerate. With conventional suture approximation, the repair site is not completely sealed and isolated from the surrounding environment, which potentially reduces the concentration of growth factors and their ability to promote survival, regeneration, and restoration of normal nerve function. Many investigators have attempted to manipulate the process by the addition of individual growth factors [10, 13, 14] but precise reenactment of kinetics of the changes in concentration of these factors is not practical. In an alternative approach, tubulization techniques have been described in a bid to confine natural neurotrophic factors at the repair site [15, 16, 17, 18, 19]. While the results of clinical trials have been promising, they have failed to demonstrate a clear advantage over conventional suturing and there is concern regarding the risk of nerve compression within the silicone tubes.
In this study we attempted to contain the growth factors within the repaired nerve by sealing the repair site, thus optimizing the endoneural environment and exploiting the nerve's inherent capacity for regeneration. The concept of nerve wrapping has been advocated both experimentally and clinically. Nerve wraps have been applied as a secondary surgical procedure in cases of chronic and recurrent perineural scarring. Autologous vein wraps have the advantage of being readily available and producing minimal donor site defect [20, 21] and have been shown to be safe and effective in the prevention of perineural scarring and adhesions [22, 23]. More recently successful nerve wrapping with human amnion has been described [24]. Human amniotic membrane induces minimal inflammatory or immunological response [25] and therefore does not contribute to the compression of the nerve.
In this study, we combined PTB with a modified nerve-wrapping technique using autologous vein and human amniotic membrane to seal the peripheral nerve repair site. We tested whether this adjunct to standard neurorrhaphy can improve peripheral nerve regeneration in an acute rat sciatic nerve injury model.
Section snippets
Surgical Procedures
The institutional Subcommittee on Research Animal Care at Massachusetts General Hospital approved all procedures in this study. Forty-eight male Sprague Dawley rats (Charles River Laboratories, Wilmington, MA), weighing 250–350 g were anesthetized with an intraperitoneal injection of pentobarbital sodium (50 mg/kg; Abbott Laboratories, Chicago, IL). A left groin incision was made to expose the femoral vessels. Using an operating microscope (Codman, Randolph, MA), the femoral vein was carefully
Functional Recovery
Animals treated with PTB alone or amnion wrap sealed with PTB (experimental groups 2 and 3, respectively) showed better functional recovery compared to the other groups (Table 1, Fig. 1). The group with PTB alone (group 2) showed greater recovery compared to other groups at wk 8 to 12, and the group with amnion wrap and PTB (group 3) showed greater improvement at wk 6 to 12. At the time of sacrifice (12 wk postoperatively) groups 2 and 3 showed significant functional improvement compared to
Discussion
Our results indicate that sealing the peripheral nerve repair site using a combination of photochemical technology and human amniotic membrane can result in both functional and histological improvements compared to standard neurorrhaphy alone. Amniotic membrane has many attributes that make it appropriate for sealing neurorrhaphy sites in conjunction with PTB. Because amnion is a thin (20-μm) membrane, it can be readily wrapped around a small diameter nerve. It is also translucent, comprising a
Acknowledgments
The authors gratefully acknowledge support from the DOD Medical Free Electron Laser Program F4 9620-01-1-0014 and from CIMIT (Center for Integration of Medicine and Innovative Technology).
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