The nerve gap. Theory and clinical practice.

In peripheral nerve surgery, the term "gap" means the distance between the two stumps of a transected peripheral nerve without further specification. The factors that contribute to the formation of a gap are analyzed in this paper. It becomes clear that the gap formed by a true nerve defect has a different meaning than a gap formed by elastic retraction. The final length of a particular nerve gap in an extremity is decisively influenced by the joint position. Therefore, the question arises regarding how a nerve adapts to the length difference during limb motion, which can be estimated for the median nerve during flexion and extension of the elbow joint with approximately 10 cm in an adult patient. Three mechanisms play an important role: true elongation of the length of the nerve in the relaxed state against elastic forces; movement of the nerve trunk in the longitudinal direction; and increase and decrease of the tissue relaxation at the level of the nerve trunk (relaxed course) and the nerve fibers (change in the undulated course). The efficiency of this mechanism partially depends on the ability of the nerve to move against the surrounding tissue. This ability is provided by the loose connective tissue around the nerve (adventitia, conjunctiva nervorum, perineurium). Only if this movement is possible, traction forces to elongate the nerve are distributed over the whole length of the nerve and are kept minimal for each particular segment. Adhesions of the nerve trunk at the site of repair prevent an equal distribution of forces and cause an unfavorable rise of traction forces at certain segments, according to the anatomic site. True elongation of the nerve, therefore, has only a limited application in overcoming a gap. Alternatives are rerouting, limb-shortening, and nerve-grafting. Today, the most reliable technique is the use of autologous cutaneous nerve segments as free nerve grafts. Advantages and disadvantages of "vascularized" nerve grafts are discussed. The use of neuromatous neurotization to overcome a gap is still in an experimental state.