The Use of Nerve Conduits in the Upper Extremity
The nerve conduit provides a regenerating axon with a means for passage to the distal nerve stump while protecting it from the surrounding environment. The ideal nerve conduit would be biocompatible, be resorbable, possess a basal lamina with preformed guidance channels, contain a reserve of viable Schwann cells and nerve growth factors, and be capable of developing an intrinsic circulation. There would be no foreign body reaction and no graft versus host response. The results of nerve grafting still remain the gold standard by which all new techniques are compared, but the donor site morbidity from the harvest of the graft has led to research into nerve tubulation techniques. The indications continue to evolve as new types of conduits have become available, making it a mainstream alternative for nerve repair and reconstruction. The types of material have included nonabsorbable substances, such as autogenous vein grafts, silicone, and Gore-Tex, as compared with resorbable synthetic tubes made from caprolactone, collagen, or polyglycolic acid (PGA). Good outcomes can be obtained provided that patient selection and the limitations of conduit use are adhered to.
Indications
Reconstruction of small-diameter, noncritical sensory nerves with a gap of less than 3 cm
Digital replantation with nerve gaps of less than 3 cm
In the mangled hand if a primary, tension-free nerve repair cannot be achieved in the acute setting and if the gap is less than 1 to 2 cm
In association with flexor tendon repair in zone 2 to create a tension-free repair and allow early mobilization (relative indication)
For nerve repair after resection of a traumatic neuroma or a neoplasm
As a wrap around a partially injured nerve or after neurolysis of a scarred nerve (relative indication)
Large-diameter mixed sensorimotor nerves should be approached with caution. Mackinnon has noted that because the formula for volume is V = π r 2 l, doubling the radius would require using a conduit that is one-fourth the length to maintain an equal volume and therefore an equal concentration of trophic factors.
Contraindications
Uncertainty about the viability of the nerve ends, especially with avulsion injuries, blast injuries, and gunshot wounds
Local or systemic infection
Inadequate soft-tissue coverage
Nerve gaps greater than 3 cm (relative contraindication)
A pure motor nerve deficit (relative contraindication)
Pathophysiology
After a nerve transection, the distal axon cannot survive without its connection to the cell body and disintegrates (i.e., Wallerian degeneration). The microtubules and neurofilaments of the distal axon undergo proteolysis by a calcium-activated neutral protease, and the normal retrograde transport of neurotrophic factors from the target organ ceases. Endoneurial collagen production from both Schwann cells and fibroblasts increases, causing progressive shrinkage of the distal tubules. The Schwann cells rapidly proliferate, forming columns (the bands of Büngner) that appear to stimulate the direction and magnitude of axonal growth.
The cell body restores nerve continuity by growing a new axon rather than through mitosis. Axons sprout after 24 hours and grow between 1 and 2 mm/day. One axon sends out multiple unmyelinated axon sprouts from the tip of the remaining axon or collateral sprouts from a nearby proximal node of Ranvier. The distal sprout contains the growth cone. This sends out filopodia, which adhere to sticky glycoprotein molecules in the basal lamina of Schwann cells, such as laminin and fibronectin (neurite-promoting factors). The filopodia contain actin, which aids in pulling the growth cone distally. The basal laminas of two abutting Schwann cells form a potential endoneurial tube into which the regenerating axon grows. These axons will deteriorate if a connection with a target organ is not reached. Up to 50 sprouts can advance from one single axon. Initially there are many more nerve fibers crossing a nerve repair than in the parent nerve. Although more than one axon may enter the same endoneurial tube, there is eventual resorption of the multiple sprouts, leaving one dominant axon.
As a general rule, motor endplates degrade at ~ 1% per week. Nerve growth is limited to 1 inch/month or 1 to 1.5 mm/day. By this reckoning, a nerve will have regenerated 12 inches at 1 year, but 50% of the end-plates will be gone. The maximum length that a nerve can grow to restore motor function is hence ~ 13–18 inches due to a critical loss of endplates.
Sensory end organs remain viable, since there is no end plate, and retain the potential for reinnervation. Nerve-grafting a digital nerve defect may provide protective sensation even after many years.
Schwann cell viability plays a large role in nerve regeneration. These cells remove the axonal and myelin debris and produce an immediate source of nerve growth factors, which help to support the proximal stump and aid in directing the advancing growth cone. The laminin and fibronectin in the Schwann cell basal lamina act as a rail for the advanced axon sprouts to grow down. The Schwann cell produces a myelin sheath for the immature axon sprout.
Nerve Biomechanics
A tension-free repair is the goal for any nerve reconstruction. Peripheral nerve is initially easily extensible. It rapidly becomes stiff with further elongation due to the stretching of the connective tissue within the nerve. Elasticity decreases by as much as 50% in nerves in which Wallerian degeneration has occurred. Experimentally, blood flow is reduced by 50% when the nerve is stretched 8% beyond its in vivo length. Complete ischemia occurs at 15%. Suture pullout does not occur until a 17% increase in length. This suggests that ischemia and not disruption of the anastomosis is the limiting factor in acute nerve repairs.
Tension on a nerve repair is one of the main indications for grafts or conduits. The conduit must be long enough to span the nerve gap without tension while the adjacent joints are extended. This is also the position of temporary immobilization. If the limb or digit is immobilized with joint flexion, the repair site will become fixed in this position. When the limb is then mobilized at 8 days, the proximal and distal stumps will be subject to tension even though the conduit was initially long enough.
Relevant Anatomy
Median Nerve
The median nerve arises from the medial and lateral cords of the brachial plexus. It contains the nerve root fibers from C6–T1. It lies lateral to the axillary artery but then crosses medial to it at the level of the coracobrachialis. At the elbow it travels behind the bicipital aponeurosis but in front of the brachialis. At the distal part of the cubital fossa, the motor branches of the median nerve consistently collect into three fascicular groups.
There is an anterior group to the pronator teres (PT) and flexor carpi radialis (FCR); a middle group consisting of motor to the flexor digitorum superficialis (FDS) and hand intrinsics, sensory fibers to the thumb, index and middle fingers; and a posterior group to the anterior interosseous nerve branch. These branch groups can be traced proximally without harm, within the main trunk of the median nerve, for 2.5 to 10 cm.
The nerve and artery pass through the antecubital fossa underneath the lacertus fibrosis and give off branches to the palmaris longus (PL), FCR, FDS, and, rarely, the flexor digitorum profundus (FDP). The nerve then dives between the deep and superficial heads of the PT, to which it supplies 1–4 branches. The fibrous arch of the PT lies 3 to 7.5 cm below the humeral epicondylar line. The fibrous arch of the superficialis arch lies 6.5 cm below the humeral epicondylar line.
The median nerve enters the forearm deep to the fibrous arch of the FDS and is adherent to the undersurface of the FDS muscle until it becomes superficial, 5 cm proximal to the wrist. In the upper third of the forearm, the motor branches usually lie peripherally, typically on the radial and ulnar sides. The nerve emerges beneath the radial side of the muscle belly of the middle finger superficialis, where it is quite superficial and near the PL. It then passes underneath the carpal trans-verse ligament, giving off the recurrent motor branch and sensory branches to the thumb and fingers.
The median nerve at the wrist has ~ 30 fascicles. It has an average diameter of 4–6 mm. The motor recurrent branch often consists of two fascicles, which are situated in a volar position, with the various sensory groups in the radial, ulnar, and dorsal positions.
Topographically, the motor branch is located on the radial-volar aspect 60% of the time, the central-volar aspect 22% and between those two locations in 18%. In 56%, the motor branch passes through a separate distinct fascial tunnel before entering the thenar muscles. The sensory fibers travel within the common digital nerves to the thumb, index and middle fingers, as well as the communicating branch to the 3rd web space.
The common digital at the level of the palm has an average diameter of 3 mm. The proper digital at the level of the fingers has an average diameter of 2 mm. The nerve trifurcates distal to the distal interphalangeal (DIP) joint.
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