Microsurgical technique

The Peripheral Neuropathy Solution

Peripheral Neuropathy Program By Dr. Randall Labrum

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Millesi [1,2] pioneered the techniques of microsurgical repair of peripheral nerves. Microsurgical techniques are absolutely required for a technically good nerve repair. The operating microscope should be used whenever possible, however surgical loupes (at least x3.5 magnification) are adequate in areas where placement of the microscope is difficult. In challenging locations, the operating microscope can be introduced after the repair to confirm a good-quality repair. The operating microscope clearly has the advantage of superb visualization, although no clear superiority over surgical loupes has been demonstrated in the literature [3]. Regardless, nerve repairs should be performed with either 9-0 or 10-0 nylon, interrupted sutures. Nerve ends are prepared sharply using either a #15 blade on a rigid background, or by using sharp, straight microscissors to achieve a clean cut. The first suture should always be placed intentionally loose, to facilitate optimal alignment of the nerve ends with the remaining sutures. Any outward-pointing fascicles should be gently trimmed so that they are covered by epi-neurium and so that they can properly align themselves within the repair. It is possible to make the repair too tight. This must be avoided because an overly tight coaption can overlap fascicular endings and compromise results. A perfect seal should be avoided in favor of gentle end-to-end contact of endoneurial contents.

Any tension at the nerve repair site must be avoided. As a general rule, any tension greater than that required to overcome an 8-0 nylon suture should be considered too much. Postural maneuvers to decrease tension will cause gapping and scarring at the nerve repair site when the joint is mobilized and may create stiffness of the immobilized joint. Mobilization of the nerve ends proximally and distally for short distances of 1 to 2 cm can provide some relief of tension. Nerves can likely tolerate considerable mobilization without negatively affecting blood flow [4], but extensive mobilization of the nerves should be avoided. It is important to recognize that axo-nal regeneration is more successful across two, tension-free repair sites (ie, with a tension-free nerve graft) than across one repair site that has been performed under tension. Completed nerve repairs are evaluated intraoperatively through the full range of motion to rule out the possibility of tension on the repair in different positions of the adjacent joints. The amount of motion tolerated by the nerve repair is determined intraopera-tively so that early protected range of motion can be safely initiated postoperatively.

Epineurial versus group fascicular repair

Debate continues regarding the optimal technique of microsurgical neural repair. Advances in microsurgery have led to the ability to perform more precise nerve repairs, such as group fascic-ular repairs (perineurial repairs). The pros and cons of epineurial versus fascicular (perineurial) repair have been the subject of much debate. The theoretical advantages of fascicular repair are obvious: proper fascicle-to-fascicle repair should give superior results. However, accomplishing proper fascicular alignment can be quite difficult in clinical practice. The normal topography of the nerve can be easily distorted by trauma, edema, or scar. If fascicular mismatch occurs, then the coapted nerves may be excluded from the opportunity to find their own way to the proper target. In addition, greater manipulation of the fascicles is required to accomplish a fascicular repair and this may lead to increased fibrosis and scarring. Comparisons of the various techniques have yielded conflicting results [5-9]; however, in the only prospective comparison done in humans, no differences were clinically evident when comparing fascicular repair and epineurial repair [10].

Intraoperative methods of fascicular identification

The task of matching fascicles in the proximal and distal stumps remains a significant challenge. Currently there are three techniques available: anatomic, histochemical, and electrophysiologic. Anatomic techniques are most commonly used in clinical practice.

Anatomic

Anatomic techniques consist of extending the dissection proximal and distal to the area of injury. Alignment is facilitated by identifying corresponding longitudinal vessels on both sides, or by using distal branches, or fascicular groupings to determine topography. The median, radial, and ulnar nerves have been extensively studied to identify their motor and sensory topography [11,12].

For example, with respect to the ulnar nerve in the distal forearm, the motor component of the ulnar nerve is located between the sensory component that would innervate the fingers and the sensory component that would innervate the dorsal aspect of the hand. Just after the dorsal cutaneous branch comes off of the ulnar nerve, the fascicular group is located medially and makes up about 40% of the cross-sectional diameter at the ulnar nerve. The sensory fascicular group is located laterally and makes up 60% of the cross-sectional area of the ulnar nerve. With respect to the topography of the radial nerve just after it leaves the posterior cord, the sensory component is superior and the motor component is located inferiorly.

Nerve topography is very precise and can also be identified in an intact, healthy nerve with a simple disposable nerve stimulator. Intraoperative stimulation of a normal nerve provides the opportunity to identify the motor and sensory fascicular group anatomy for future reference.

Histochemical

Histochemical enzyme staining has been used to distinguish motor from sensory nerves. In 1964, Karnovsky and Roots [13] devised a thiocholine staining protocol that identifies the cholinesterases present in motor neurons. Carbonic anhydrase has been used to identify sensory fibers [14]. These techniques are most useful for the proximal stump where enzyme activity will be present indefinitely. In contrast, enzyme staining of the distal stump is limited to 5 days following the injury. Intraoperative histochemical staining requires particular attention to ensure that the specimens are properly oriented. Processing periods of about 1 hour are required and not all surgical facilities are equipped to undertake these studies. Given these limitations, histochemical techniques are not used routinely in clinical practice.

Electrophysiologic awake stimulation

Awake stimulation of the patient with a nerve injury can provide useful information. Nerve exposure is completed with the patient under a short-acting anesthetic or intravenous (IV) regional anesthetic with sedation. Tourniquet time for the dissection is limited to 30 minutes to prevent transient neurapraxia. After a reperfusion period of 10 minutes, fascicles in the proximal stump can be identified using a disposable nerve stimulator or a sterile nerve conduction stimulation electrode. With motor fascicle stimulation, the patient will complain of a dull ache in the extremity. Stimulation of a sensory fascicle at that same level of stimulation will yield a more intense, sharp pain in a specific sensory territory.

Unfortunately, the benefits of electrical mapping are generally limited to the proximal nerve stump. Electrical stimulation of the motor fascicles of the distal stump can elicit a motor contraction; however, this is transient and generally only seen in the initial 72 hours following a nerve injury. For injuries beyond 72 hours, electrical stimulation of the distal stump will not produce any information. In these cases, anatomic techniques for fascicular identification must be used.

Nerve gap management

Nerve gaps that are small and that can be repaired with ''minimal tension'' may be repaired directly. Any significant tension must be managed with another method. Microsurgical methods of reconstructing nerve injuries with a significant nerve gap include nerve grafting, distal nerve transfers, and end-to-side nerve repair.

Nerve grafting

Before grafting, the proximal and distal ends of the injured nerve must be prepared by transversely sectioning the nerve outside of the zone of injury until nerve fascicles are visualized with visible herniation of endoneurium from their cut ends. The defect size is then measured and the donor nerve graft is obtained. The donor nerve graft should be oriented in a reverse fashion from its native position so that regenerating fibers will not be diverted from the distal repair site. Care must be taken to place the grafts in the same sequence proximally and distally to avoid surgically created malalignment. Each graft requires only two or three sutures for repair.

Selection of nerve graft donors is limited by the size of the donor nerves and the functional and aesthetic deficits created by their harvest. The donor nerves available for grafting are typically the sural nerve, the lateral antebrachial cutaneous nerve and the anterior division of the medial antebrachial cutaneous nerve. A vascularized nerve graft may be considered for its ability to provide immediate intraneural perfusion in a poorly vascularized bed, and to reconstruct large nerve gaps [15,16]. Vascularizing the nerve graft is required if the surgeon chooses to use a large-caliber nerve graft, for example a vacular-ized ulnar nerve graft for brachial plexus reconstruction [17]. The vascularized ulnar nerve graft can be used as the donor nerve for a cross-chest C7 nerve transfer. Mixed results have been seen in experimental models [18-21]. Despite its introduction more than 2 decades ago, the role of vas-cularized nerve grafts in clinical practice has not been established.

Sensory nerve grafts are used to reconstruct motor defects because of their relative ease of harvest and low donor site morbidity. However, motor nerve grafts may be much more suitable substrates for regeneration [22,23]. The effects of motor versus sensory grafts on nerve regeneration and functional recovery have recently been studied [24-29]. Sensory nerves have more diverse fiber distributions with smaller endoneurial tubes than motor nerves. They may possess phenotypi-cally distinct Schwann cells that can negatively affect the regeneration of motor neurons down sensory pathways. Nerve grafts of motor origin will support regeneration across a nerve gap more effectively than will sensory nerve grafts. Although sensory nerves are more readily expendable than their motor counterparts, the routine practice of using sensory nerve grafts to reconstruct critical motor nerve defects may warrant reappraisal. Motor nerves to expendable muscles such as the latissimus dorsi, medial or lateral gastrocnemius, vastus lateralis, and gracilis are among the candidate donor nerves that may be harvested with minimal donor morbidity. However, donor site morbidity and available length of nerve tissue may continue to limit the use of motor nerve grafts to reconstruct motor defects. Further investigation is needed to determine whether the benefits of motor nerve grafts are needed. Future strategies may include the use of motor nerve allografts or the development of nerve conduits that contain motor-derived Schwann cells.

Nerve transfers

A nerve transfer recruits redundant or unimportant nerve fascicles from a donor nerve to innervate critical motor or sensory nerves close to their target end-organs. Traditionally, the use of nerve transfers in the upper extremity has been limited to brachial plexus avulsion injuries where no proximal source of nerve is available [30,31]; however, nerve transfers are being increasingly used to reconstruct many proximal nerve injuries. Nerve grafting across the injured segment of a proximal nerve injury has been associated with poor functional outcomes when there is a long distance from the level of the injury to the target muscle [32,33]. In these circumstances, the distance to the target muscle may be too far from the regenerating nerve fibers to provide timely reinnervation. Distal nerve transfers can provide direct coaptation to the injured nerve at a site close to the target muscle, avoiding the long delay for reinnervation.

In 1948, Lurje [34] described the concept of nerve transfers to reconstruct the axillary, supra-scapular, and musculocutaneous nerves. However, with the acceptance of nerve grafting techniques in the 1960s and 1970s, nerve transfers never acquired wide popularity [3]. Today, however, the list of commonly used nerve transfers is rapidly expanding [35-51]. Nerve transfers are being increasingly used to reconstruct many proximal nerve injuries and, at our institutions, nerve transfers are used in preference to long nerve grafts whenever feasible. Many of these transfers can be performed without the need for an interpositional nerve graft and even if a nerve graft is required, it is generally short and can be harvested from the same extremity.

Increased microsurgical skills combined with an improved understanding of nerve topography have greatly enhanced the development of nerve transfer techniques. Sunderland's [52] landmark paper suggesting that there was no constant fascicular pattern in peripheral nerves until quite distal has been challenged. Jabaley [12], Chow [53], and ultimately Brushart [54] have confirmed that, in fact, axons travel in functional units throughout the nerve (Fig. 1). Hence the modern notion of being able to perform internal neuroly-sis over long distances was confirmed. Microsurgi-cal techniques allow the indication of redundant donor fascicles that can be separated surgically and transferred to the recipient nerve. A comprehensive understanding of nerve topography is therefore essential to the peripheral nerve surgeon.

Arthroscopic Brachial Plexus Neurolysis

Fig. 1. Axons travel in functional units throughout the nerve (2) and allow for internal neurolysis of nerves in nerve transfers. Sunderland's description of nerve fascicles (2) has proven to be incorrect. (From Brandt KE, Mackinnon SE. Microsurgical repair of peripheral nerves and nerve grafts. In: Aston SJ, Beasley RW, Throne CHM, editors. Grabb and Smith's Plastic Surgery, 5th edition. Philadelphia: Lippincott Williams & Wilkins, 1997; with permission).

Fig. 1. Axons travel in functional units throughout the nerve (2) and allow for internal neurolysis of nerves in nerve transfers. Sunderland's description of nerve fascicles (2) has proven to be incorrect. (From Brandt KE, Mackinnon SE. Microsurgical repair of peripheral nerves and nerve grafts. In: Aston SJ, Beasley RW, Throne CHM, editors. Grabb and Smith's Plastic Surgery, 5th edition. Philadelphia: Lippincott Williams & Wilkins, 1997; with permission).

Table 1

Nerve tranfers for upper extremity [30-48]

Muscle/nerve deficit

Donor nerve (fascicle)

Elbow flexion

Biceps and brachialis

Biceps alone

Entire musculocutaneous nerve Shoulder abduction Suprascapular nerve Axillary nerve Wrist/digit extension Radial nerve

Pronator

Pronator branches of median nerve Intrinsic hand function Ulnar nerve

Flexor carpi ulnaris portion of ulnar nerve (FCU), flexor carpi radialis portion of median nerve (FCR) FCU, FCR

Medial pectoral nerve (MP), thoracodorsal (TD) nerve, intercostals Distal accessory nerve (DAN)

Triceps branch of radial nerve, MP, TD, intercostals,

Flexor digitorum superficialis (FDS), palmaris longus (PL), branches of median nerve

FDS, PL branches of median nerve

Anterior interosseous nerve (AIN)

Today, the list of commonly used nerve transfers continues to expand and several reconstructive options are available for brachial plexus and proximal upper extremity nerve injuries (Table 1) [35-51]. The example of nerve transfers to restore elbow flexion will be used to illustrate the general role of microsurgery in nerve transfers. In addition, this chapter will review several distal nerve transfers that are being used with increasing frequency, namely, reconstruction of the ulnar nerve with the motor branch of the anterior interosseous nerve (AIN) to the pronator quadratus, and reconstruction of the radial nerve with expendable branches of the median nerve.

The preferred nerve transfer for elbow flexion has been vigorously studied and has evolved over time [35-40]. Currently, a redundant portion of a normal ulnar nerve is used to transfer directly to the biceps branches of the musculocutaneous nerve distally in the arm, and a redundant portion of the median nerve is used to transfer directly to the brachialis branch of the musculocutaneous nerve [35-37].

At the level of the mid arm, adjacent to the biceps and brachialis branches of the musculocu-taneous nerve, a careful internal neurolysis of the ulnar and median nerves is performed. Redundant fascicles to the flexor carpi ulnaris (FCU) in the ulnar nerve and to the flexor carpi radialis (FCR) in the median nerve are located using a nerve stimulator. For the ulnar nerve, these branches are located on the lateral aspect of the nerve, and approximately 20% to 25% of the ulnar nerve is used in this transfer. Careful microsurgical techniques are used to ensure that good intrinsic hand function remains in the ulnar nerve. Similarly for brachialis reconstruction, donor fascicles in the median nerve are selected through an intraneural neurolysis and electrical stimulation of separated fascicles. Expendable motor function is found in the medial fascicles of the median nerve. These fascicles are chosen preferentially and comprise approximately 15% of the median nerve. Microscopic suture of these fascicles to the biceps and brachialis branches is then completed in a tensionfree manner (Fig. 2).

Other distal nerve transfers that are particularly useful to the peripheral nerve surgeon include reconstruction of the ulnar nerve with the motor branch of the AIN to the pronator quad-ratus, and reconstruction of the radial nerve with expendable branches of the median nerve [42,43]. Recovery of intrinsic hand muscles does not usually occur after high injuries to the ulnar nerve because of the long distance from the area of injury to the target muscles requiring reinnervation.

Nerve Reinnervation

Brachialis branch Ulnar Nerve

Fig. 2. Reinnervation of the biceps and brachialis muscle with a double fascicular transfer for elbow flexion. (From Tung TH, Novak CB, Mackinnon SE. Nerve transfers to the biceps and brachialis branches to improve elbow flexion strength after brachial plexus injuries. J Neurosurg 2003;98(2):313—8; with permission).

Brachialis branch Ulnar Nerve

Fig. 2. Reinnervation of the biceps and brachialis muscle with a double fascicular transfer for elbow flexion. (From Tung TH, Novak CB, Mackinnon SE. Nerve transfers to the biceps and brachialis branches to improve elbow flexion strength after brachial plexus injuries. J Neurosurg 2003;98(2):313—8; with permission).

Nerve transfers provide a closer source of motor axons to the target muscles. When the median nerve is intact, the distal branch of the AIN to the pronator quadratus can be transferred to the deep motor branch of the motor nerve. The incision begins over Guyon's canal and extends to the mid-forearm. The deep motor branch of the ulnar nerve is first identified in Guyon's canal and traced proximally. Physical neurolysis of the deep branch of the ulnar nerve is not necessary as the nerve can be neurolysed visually to the level of the distal AIN. The pronator quadratus is then identified as well as the AIN as it enters the pronator quadratus. The AIN is traced distally into the pronator quadratus until it begins to branch. The nerve is then divided just proximal to the branches. Direct repair of the AIN to the motor branch of the ulnar nerve is then accomplished in a tension-free manner that does not require a nerve graft. There are approximately 500 to 600 nerve fibers in the distal AIN to the pronator quadratus and 1200 nerve fibers in the deep motor branches of the ulnar nerve. Thus, the results with this transfer are expected to be "fair to good,'' but never "excellent to outstanding.'' Sensory reconstruction of the ulnar nerve can be accomplished by performing an end-to-side repair of the sensory portion of the ulnar nerve to the ulnar aspect of the median nerve (see end-to-side nerve repair). The digital nerves to the third web space (median nerve) can be used to provide sensation to the ul-nar-innervated digits in an ulnar nerve injury. In complete, high ulnar nerve injuries, this transfer can be accomplished in the mid-forearm, at the same level where the motor nerve transfer of the AIN to the deep motor branch of the ulnar nerve is being performed. The most ulnar fascicles of the median nerve supply sensation to the third web space. The dorsal sensory branch can also be included in the end-to-side repair (Fig. 3). Flexor digitorum profundus function of the ring and small fingers can be reconstructed with tendon transfer to the index and long finger flexor digito-rum profundus tendons.

Orthopedic trauma affects the radial nerve more than any other major nerve. It is estimated that the radial nerve can be injured in up to 12% of humeral shaft fractures. Fortunately, spontaneous recovery within 8 to 16 weeks is frequently seen [55-58]; however, if the radial nerve fails to recover, options for reconstruction include exploration of the nerve injury at the level of the humeral fracture, intraoperative nerve conduction studies, and placement of interpositional nerve

Ain Ulnar Nerve Transfer
Fig. 3. Anterior interosseous nerve (AIN) nerve transfer to deep motor branch of ulnar nerve. End-to-side sensory repair of ulnar digital sensory and dorsal branch to side of median nerve.

grafts across the lesion if it fails to conduct. Other options for a complete radial nerve lesion include distal nerve transfer or tendon transfers.

Donor nerve branches to reconstruct the posterior interosseous nerve can be supplied by the median nerve. The median nerve supplies several sources for nerve transfer to the distal radial nerve including redundant nerve branches to the flexor digitorum superficialis and palmaris longus [44]. Intraoperative nerve stimulation is used to identify redundant branches of the median nerve that can be transferred to the radial nerve. Internal neurolysis of the branches of the median nerve is not required as the median nerve has already branched at this level. It is important to be certain that the branch to the extensor carpi radialis brevis (ECRB) is included in the transfer to ensure adequate wrist extension. In general, the strongest donor nerve seen with intraoperative nerve stimulation should be used to preferentially supply the ECRB as wrist extension provides more function than finger extension. This transfer can be accomplished in the proximal forearm without the need for an interpositional nerve graft. Distal nerve transfer is a useful alternative to tendon transfers in patients with delayed presentation or high proximal nerve injuries or in situations of complete loss of nerve function.

The list of available nerve transfers continues to expand and a complete description of all currently available nerve transfers is beyond the scope of this article. It is clear that our current techniques would not be possible without advanced microsurgical skills and improved understanding of nerve anatomy and repair.

End-to-side nerve repair

End-to-side nerve repair describes the technique of coapting the distal end of an injured nerve to the side of an uninjured donor nerve, either by simple microsurgical attachment without alteration of the donor nerve, or in conjunction with the creation of a surgical incision within the donor nerve (epineurotomy, perineurotomy, neu-rotomy) (Fig. 4). The earliest reports of end-toside nerve repair date back to the late 1800s [59], but the technique was lost until it was reintro-duced by Viterbo in 1992 [60]. Since then, several reports of successful end-to-side nerve repair have been published [61,62].

There are several controversies regarding this technique. One area of controversy has been the source of regenerating axons into the distal nerve following end-to-side nerve repair. There are three possible sources for these regenerating axons, namely, (1) invasion from the transected proximal stump of the injured nerve, (2) terminal sprouting from donor nerve axons that were damaged (intentionally or unintentionally) during nerve preparation, or (3) from de novo collateral (nodal) sprouting from the end-to-side nerve repair site. Confirmation of true collateral sprouting has been suggested by elegant double-labeling studies [63] and is believed to occur from the nodes of Ranvier [64]; however, one must consider the limitations of double-labeling studies before concluding that de novo collateral sprouting occurs in the absence of nerve axonal injury [65].

The use of end-to-side nerve repair in the clinical setting for motor recovery remains controversial. Currently, motor reconstruction in the

Fig. 4. End-to-side nerve repair creates a repair between the distal end of an injured nerve and the side of an uninjured donor nerve with or without some alteration in the donor nerve (epineurotomy, perineurotomy, neurotomy).

absence of available proximal nerve is best handled by deliberate donor nerve injury or nerve-tonerve transfers [66]. We employ its use in sensory nerve reconstruction in circumstances in which distal nerve ends would go without a source of proximal neurons.

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