Anatomic Principles

Before one can even consider the repair of a peripheral nerve, an understanding of the connective tissue layers, as well as of the fascicular anatomy of a nerve, is important. As seen in Figure 239-1, which contains a diagram of the peripheral nerve architecture and its components, an external epineurial sheath, composed of connective tissue and longitudinal blood vessels, surrounds each peripheral nerve. There is an external epineurium and an internal epineurium. The internal epineurium demarcates fascicles and groups of fascicles within the nerve. Each individual fascicle is surrounded by perineurium. The axons themselves are contained within fascicles, in close association with Schwann cells and the basement membrane that surrounds Schwann cells, the endoneurial basal lamina (also referred to as endoneurial tubes).¹

FIGURE 239-1 Schematic diagram of peripheral nerve architecture and composition. The patterns of fascicular structure of peripheral nerve are illustrated: monofascicular, oligofascicular, and polyfascicular (grouped and ungrouped). The anatomic composition, with emphasis on the connective tissue layers of the nerve (i.e., epineurium [external and internal], perineurium, and endoneurium) are illustrated.

(From Midha R, Lee P, Mackay M. Surgical techniques for peripheral nerve repair. In: Wolfa CE, Resnick DK, eds. Neurosurgical Operative Atlas, 2nd ed. Spine and Peripheral Nerves. New York: Thieme, 2007, pp 402-408.)

Nerves may be generally divided into four basic patterns of intraneural architecture, based on their fascicular structure (see Fig. 239-1).² Nerves containing one large fascicle are termed monofascicular, whereas those containing a few or discrete number of fascicles are oligofascicular. Most nerves contain many fascicles of varying sizes and are termed polyfascicular. The polyfascicular pattern may exist with grouping of fascicles or with a more diffuse (ungrouped) arrangement throughout the cross section of the nerve. The fascicular nature of a nerve changes as it extends from proximal to distal in the extremity. For example, the ulnar nerve is polyfascicular as it comes off the brachial plexus and then generally becomes organized into usually four fascicles at the level of the elbow. These fascicles are further segregated into motor and sensory groupings at the level of the wrist, and finally, the terminal digital branches are monofascicular in the fingers.

The proportion of connective tissue within the nerve varies considerably, from 25% to 85%, across the cross section of the nerve.² In general, there is more connective tissue in the nerve where it crosses the joint. The connective tissue, particularly the perineurium, is the source of the main tensile strength to the nerve. It is also the layer that can take and hold a suture. From a practical viewpoint, the smallest component of nerve that can therefore be repaired using current microsurgical technique is the fascicle.

It must be stressed at the outset that a peripheral nerve repair is not a type of cellular repair but is actually a repair done at the level of the connective tissue to coapt a healthy proximal nerve to a healthy distal nerve stump. This then provides the appropriate anatomic environment so that axons from the proximal stump can regenerate into endoneurial tubes within the distal nerve stump and, hence, be led to end organs to restore function. Note that a nerve graft functions as a conduit, whose axons are destined to undergo wallerian degeneration as soon as it is removed from its harvest site. Thus, the graft provides an endoneurial tube network available to be exploited by regenerating axons from the proximal host nerve stump.³ It also provides viable Schwann cells, as long as the caliber of the nerve graft is not too large. For this reason, small caliber cutaneous nerves are most commonly used as graft material (see later section on donor graft harvesting techniques). The small caliber nerves, when sutured in a series of parallel segments, are in close proximity to tissue fluid and are therefore nourished. They also undergo rapid revascularization and thus remain viable.

Physiologic Principles and Patient Selection for Surgery

Peripheral nerves, once injured, have the potential to regenerate axons and reinnervate end organs, with resulting good functional recovery.⁴ Indeed, this is the case with all minor nerve injuries, such as neurapraxia, in which the axon remains intact. After a nerve injury resulting in axotomy (Sunderland grade II injury or greater), the distal axon undergoes wallerian degeneration. In purely axonotmetic injuries, in which axons are interrupted but the degree of connective tissue damage is minimal, regenerating axons use their existing endoneurial pathways to specifically reinnervate their own precise target end organs, as confirmed in recent experiments using bioengineered fluorescent mice.⁵ Outcome after more severe peripheral nerve injury, however, remains variable and often very poor. Most of these injuries exhibit both a loss of axon continuity and a significant disruption in the internal connective tissue structures. The resulting scarring within the nerve or a frank gap (with lacerating injuries) presents a formidable barrier to regenerating axons, preventing them from effectively innervating the distal nerve stump. These are currently managed with a repair of the divided nerve or, for the usual scenario of longer gaps or scar segments that need to be resected, placement of interposed nerve grafts.

Simplistically, exploration and repair of the peripheral nerve is indicated in clinical situations in which there is either the absence or the lack of expectation that there will be effective spontaneous regeneration. This will be the case in all patients with lacerating nerve injuries and in many of the patients who harbor the more severe injuries in continuity. As a practical rule, nerves known or expected to be sharply lacerated should be explored and repaired primarily and without delay, whereas bluntly lacerated nerves should be repaired after a period of 2 to 4 weeks. Patients with nerve injuries in continuity should be followed for about 3 months with repeat clinical and electrophysiologic evaluation. After 1 to 2 months have elapsed from the time of the trauma, the initial effects of any tissue damage will have resolved, and magnetic resonance neurography may then provide an early view of neuroma formation or of complete discontinuity. In patients failing to demonstrate clinical or electrical evidence of regeneration, the nerve should be explored within 4 to 6 months.⁶ The findings at surgery, including intraoperative electrophysiologic tests (briefly discussed later but in detail elsewhere in this text), determine the fate of the nerve injury (neuroma)–in-continuity.⁷

Operative Principles and Fundamental Techniques

A detailed knowledge of the gross anatomy of the extremities and peripheral nerves is imperative before undertaking nerve exploration and repair. The surgeon must be prepared to expose the nerve well proximal and distal to the area of injury. Appropriate positioning of the limb, padding of pressure points, and wide draping are essential. Special attention to draping of the limb or a different limb for procuring nerve grafts is also required. Because the nerve may need to be stimulated during surgery to evoke muscle contractions, only a short-acting paralyzing agent, given at the induction of anesthesia, should be used. I prefer the use of general anesthesia and do not use tourniquets for these procedures.

Isolation of the nerve itself should be performed using sharp dissection. The surgeon identifies normal nerve proximal and distal to the zone of injury and then works toward the area of injury. In clean lacerating injuries, the area of exposure may be relatively small. However, most injuries leave the nerve in continuity; because these injuries are also explored weeks to months following trauma, there is considerable scar formation and distortion of tissue, necessitating a wide and extensile exposure. Using sharp dissection techniques, the area of injured nerve is circumferentially exposed; that is, an external neurolysis is performed. With this type of circumferential mobilization, the gross anatomic details of the injury are identified. With the aid of an operating microscope, finer anatomic details can be appreciated. If the nerve is in clear discontinuity, nerve repair is necessary. However, most lesions are in-continuity. As demonstrated by Kline and Happel, recording of intraoperative nerve action potentials is useful in assessing these lesions.⁸ Specifically, the presence of a nerve action potential across the lesion argues for the lesion not to be resected (Fig. 239-2A). However, the lack of evidence of spontaneous regeneration (the absence of a nerve action potential) dictates resection of the neuroma and appropriate reconstruction of the resulting nerve injury gap.

FIGURE 239-2 Surgical management of a neuroma–in-continuity. A, Operative photomicrograph of a peroneal nerve injury case, in this situation caused by a constricting ligature and fascia, so that the neuroma appears tapered rather than enlarged. The superficial peroneal (upper part, encircled with Penrose drain) conducted a nerve action potential and underwent neurolysis. There was no demonstrable evidence of regeneration across the deep peroneal branch (no return of clinical function and absence of nerve action potential across the neuroma). The nerve appears attenuated at the site of the lesion, with 6-0 stay sutures placed proximal and distal to the neuroma, which was located just proximal to head of fibula. B, Drawing of a neuroma prepared for resection and then undergoing débridement. Stay or lateral sutures are positioned to orient and topographically align the nerve. The site of initial nerve section is across the middle of the neuroma (solid line in center). The lower schematic diagram demonstrates progressive sectioning until a grossly normal fascicular pattern is observed. C, Operative photomicrograph demonstrating sequential sectioning of a neuroma under microscopic magnification to obtain a fascicular pattern.

(A and B, From Midha R, Lee P, Mackay M. Surgical techniques for peripheral nerve repair. In: Wolfa CE, Resnick DK, eds. Neurosurgical Operative Atlas, 2nd ed. Spine and Peripheral Nerves. New York: Thieme, 2007, pp 402-408. C, Photograph courtesy of Dr. Alan Van Beek.)

The placement of lateral stay sutures using 6-0 monofilament (as illustrated in Fig. 239-2A and B) helps maintain the topographic alignment of the nerve. Under the operating microscope, the surgeon then cuts across the center of the neuroma. Small segments of the nerve are sliced in perfect cross section, using a fresh blade, until a healthy fascicular pattern is identified both at the proximal and at the distal stump⁹ (Fig. 239-2B and C). This step is critical because attempting to appose or graft scarred proximal and distal stumps is a major cause of nerve repair failure. Healthy fascicular tissue is recognized when the epineurium retracts slightly and the endoneurium appears to “pout” or mushroom out of the fascicles (because of positive endoneurial pressure). Fine bleeding from endoneurial microvessels may also be appreciated. This type of adequate débridement invariably leaves some degree of gap between the proximal and the distal stump. If the gap is short and the two ends can be brought together without undue tension, a direct repair is appropriate. One good way to determine the degree of tension present at the suture line is to bring the ends together using the stay epineurial sutures. If this can be performed without suture distraction, a direct repair is appropriate. However, if the ends are under considerable tension and the suture line appears to tear out, a graft repair must be performed.

Several techniques are available to bring the proximal and distal stumps closer together and allow a direct repair. In all situations, proximal and distal mobilization of the nerve for considerable distances should be performed. Thus, tethering forces to adjacent surrounding fascial and subcutaneous tissue are removed, allowing short gap lengths to be overcome. In certain special situations, such as the ulnar nerve at the elbow, the nerve may be transposed, allowing a considerable length to be obtained. Finally, a direct repair can be undertaken with the joint (e.g., the knee joint with a peroneal nerve repair) placed at some degree of flexion. However, this necessitates immobilization of the joint for 3 to 6 weeks in a splint or even a plaster cast before gradual and progressive range of motion is allowed thereafter.