Injuries involving the upper portion of the brachial plexus are particularly devastating becasue of the involvement of the musculocutaneous nerve. It controls elbow flexion which may be considered the single most important motor function of the upper extremity. This article will review nerve transfer techniques to restore musculocutaneous motor function following brachial plexus injuries.
The basic indication for a nerve transfer in brachial plexus injuries is an injury that preserves suitable donor nerves and is expected to have an inferior outcome from reconstruction by any other means. Nerve transfers have the distinct benefit of involving tissues out of the zone of injury and much closer to the target muscle. This allows for the earlier reinnervation and preservation of target muscle which would otherwise be expected to undergo denervation atrophy within 12 to 18 months following nerve injury. In the past, nerve transfers may have been reserved for injuries untreatable by traditional methods of anatomic brachial plexus reconstruction with nerve autografts. However, it is this author's belief that nerve transfers are often superior means of reconstruction even in situations where traditional methods are tenable options. This belief has scientific support, in particular with regard to restoration of elbow flexion (1).
Nerve injuries are graded on a scale from I to VI (Table 1). Lower grade injuries that do not involve significant scar formation (grades I, II, and III) are expected to recover to a greater extent than any surgical reconstruction can provide and are thus managed observantly. Higher grade injuries that do involve significant scar formation (some grade III and all grade IV, V, and VI) are detected by the combination of flaccid muscle on examination and lack of motor unit action potentials on electromyography at or beyond 3 months following injury. These injuries are expected to benefit from surgical reconstruction.
Nerve Injury Grade
Electrodiagnostic Findings (>3 months after injury)
ischemic; structurally intact
axonal disruption, basal lamina intact
fibrillations and sharp waves present; motor unit potentials (MUPs) normal
axonal disruption, basal lamina (endoneurium) disruption, minimal or no perineurial involvement
fibrillations and sharp waves present; MUPs present, decreased recruitment
axonal, endoneurial, and perineurial disruption, scar formation, "neuroma-in-continuity"
axonal, endoneurial, perineurial, and epineurial disruption; complete transection
combination of different grades of injury within same nerve
Brachial plexus injuries tend to be closed traction injuries. However, the decision-making process to determine the need for nerve transfer applies to all types of injury. Injuries considered to be "sharp", such as knife injuries, are traditionally treated acutely with primary repair. This would be true for brachial plexus injuries that happen to involve the origins of the musculocutaneous nerve. It is arguable, however, for the most proximal of these injuries, such as the level of the upper trunk and certainly the spinal cord roots, that more distal nerve transfers provide equivalent outcomes to primary repair. Closed injuries, or "dull" or traction injuries, are best treated with initial observation and serial exams with, or without, electrodiagnostic studies. Generally, if surgery is required, they are best treated with nerve transfers. The ultimate decision regarding treatment should be a patient-centered, individualized decision addressed by a properly trained peripheral nerve surgeon.
If neurodiagnostic studies have not been performed, they should be obtained for proper surgical planning. It is critical to ensure that proper donor nerves exist before nerve transfer. In this case, for reconstruction of the musculocutaneous nerve function, the ideal donor nerves are the median and ulnar nerves, specifically, components controlling wrist flexion (2,3). Neurodiagnostic studies and physical examination should confirm these neuromuscular units to be fully functional, without impairment. Additional donors include the medial pectoral nerves (4), thoracodorsal nerve, or intercostal nerves. This article will focus on the more common injury involving the upper trunk of the brachial plexus, in which case the median and ulnar nerves are available as ideal donor nerves.
In addition to confirming the need for surgery and the presence of appropriate donor nerves, the peripheral nerve surgeon should include a thorough discussion of the risks to the donor nerves with the patient as part of the informed consent process. These risks include potential injury and loss of function to the wrist flexors, finger flexors, thumb opposers, and ulnar intrinsic musculature, as well as a loss, change, or decrease in sensation in either or both of the median and ulnar nerve distributions. Every effort is made to ensure selection of predominantly wrist flexors, but no donor fascicle at the level of the brachium can be guaranteed to be a "pure" wrist flexor. With proper donor fascicle selection, any effect on other motor or sensory units is expected to be minor and, particularly in the case of sensation, recoverable.
Nerve transfer procedures are generally performed under general anesthesia using microsurgical technique. Individual surgeons may choose to use an operating microscope or surgical loupes. If loupes are used, magnification of 3.5x or higher is recommended. Intraoperative nerve stimulation is essential for proper donor nerve identification. A disposal portable stimulator is perfectly adequate.
The patient is placed in standard supine position with the operative arm abducted at the shoulder onto an arm table. After anesthesia, the surgeon may choose to infiltrate the operative site with an injectable solution of epinephrine to decrease obscuring of the operative field from bleeding. It is critical to ensure that no local anesthetic is used in order to maintain nerve function for proper identification. A tourniquet is not used due to the proximal location of the operative field, and no paralytic agents should be used during the surgical portion of the procedure.
The incision is made along the proximal bicipital groove at the upper inner brachium, represented by the green line in the image below. The incision may be extended as needed for further exposure for other surgical procedures distally or in a zig-zag fashion across the axilla and along the deltopectoral groove proximally, as represented by the red lines. Such a proximal approach would be performed to expose the brachial plexus more proximally to access the medial pectoral nerves.
The dissection is carried through the subcutaneous tissue and the deep fascia. The medial antebrachial cutaneous nerve is identified and protected. The median nerve (MN) should be centered over the brachial artery and most easily identified next, followed by the ulnar nerve (UN) medially. The biceps muscle must usually be elevated laterally in order to identify the musculocutaneous nerve proximally, followed by its branches distally. There should be three branches of the musculocutaneous nerve (MCN) with the biceps branch (BBi) coursing laterally toward the retracted biceps muscle, the brachialis branch (BBr) coursing medially, and the lateral antebrachial cutaneous nerve (LABC) continuing in a straighter path distally as the central of the three branches. The biceps branch generally separates from the musculocutaneous nerve proximal to the separation of the brachialis branch.
The general principle is that the recipient nerve is cut as far proximally as possible and the donor nerve as far distally as possible to allow for a tension-free repair. The biceps and brachialis branches are tested with a nerve stimulator to confirm lack of muscle activity. The nerve branches are dissected as far proximally as possible to allow for more length upon eventual transposition, and the position where the nerve ends will lie, once cut proximally, is estimated. In this way, the surgeon may be able to determine which of the two donor nerves will best match with the respective recipient nerves and at which locations along the donor nerves the dissections need to occur. Often, the brachialis branch is more difficult with which to achieve a tension-free transfer. Thus, it is generally recommended that the donor for this transfer be selected first. It is wise to place the shoulder and elbow through sub-maximal range of motion to confirm an eventual tension-free repair. Once the locations of the donor nerve dissections are determined, the epineurium is opened and grouped fascicles are separated with fine, gentle, microscopic technique. The nerve stimulator is used to select the most pure wrist flexor group fascicles. In general, the motor components of the ulnar nerve are located within the central and lateral portions of the nerve at this level of the brachium, and the motor components of the median nerve are found in its medial portion. Once the ideal donor group fascicles are selected and determined functioning by nerve stimulation, the recipient nerves are cut proximally and transposed toward their respective donors. When adequate length of donor nerve is confirmed, the donor group fascicles are cut distally and transposed toward their respective recipients. Repairs are performed according to surgeon preference, traditionally with interrupted epineurial repair with 9-0 permanent monofilament suture. In the diagram below, the median nerve group fascicle has been transferred to the brachialis branch and the ulnar nerve group fascicle has been transferred to the biceps branch.
Pearls and Pitfalls
Use of a plain epinephrine infiltrate at the incision site prior to prepping and draping will reduce bleeding from the skin and subcutaneous vessels. Use of bipolar electrocautery is encouraged throughout the procedure as necessary to maintain as bloodless a field as possible. These maneuvers will make identification and protection of neural structures easier and therefore safer. Avoidance of systemic paralytics by the anesthesiologist is critical to allow for identification of proper donor nerves. This should be clarified prior to anesthetic induction. Proper localization of the donor nerve dissection site will save both time and potential for donor morbidity by reducing the length and amount of intraneural dissection required. There is some degree of plexus formation within the median and ulnar nerves at this level, and proper selection of predominantly wrist flexor donor group fascicles may require dissection over a few centimeters of nerve length. For this author, avoiding intra-fascicular dissection is preferable to avoiding a longer length of inter-fascicular dissection. Finally, emphasis must be placed on preserving ulnar intrinsic muscle function and thenar muscle function. If a "pure" wrist flexor group fascicle does not exist, and a decision must be made between two group fascicles that are predominantly wrist flexors but also contain other median or ulnar motor components, deference is given to taking the fascicle that may have a combination of wrist flexor innervation and finger flexor innervation and preserving the fascicle that contains a component of intrinsic hand muscle innervation.
The peripheral nerve surgeon may choose to place a surgical drain. The wound is closed in layers and the extremity is wrapped with a gently compressive, bulky wrap, such as gauze and ACE bandage, preferably from the hand or wrist up through the brachium or shoulder. Any evident post-operative hematoma should be drained thoroughly, as blood is quite inflammatory and could create excess scar formation and inhibit nerve regeneration. A shoulder immobilizer is recommended for 1-2 weeks, allowing elbow flexion and extension on a daily basis. Shoulder immobilization is released at 2 weeks and gentle, gradual resumption of normal shoulder range of motion is resumed. Usually, the shoulder is also paralyzed in upper trunk brachial plexus injuries and the range of motion resumed is passive. Physiotherapy is recommended in the recovery phase to promote maximum function. Therapy may be necessary pre-operatively to achieve joint mobility, and this is continued during the early post-operative period. Behavioral physiotherapy for motor re-education is critical to achieve maximum function. At a minimum, this is begun at the first sign of biceps or brachialis muscle activity, anywhere from 3 to 6 months post-operatively, and in the future such therapy may be indicated earlier.
Traditional nerve graft reconstructions of the brachial plexus provided poor functional outcomes. With Oberlin's description of the ulnar-to-biceps nerve transfer in 1994 (2) and Leechavengvongs subsequent report of its use in 1998 (5), elbow flexion power of MRC grade M3 or M4 was achieved. With Mackinnon's addition of reconstruction of the brachialis muscle (3,6), expected outcomes have been even better.
Traditional complications of upper extremity surgery apply to this procedure. The most common complications include surgical site infection and hematoma. Post-operative pain is a risk, including pain syndromes. Particular to peripheral nerve reconstruction surgery is neuroma formation, and as mentioned previously, there is risk of donor nerve morbidity. It is commonly touted that elbow flexion is the most important motor function of the upper extremity from a functional standpoint; thus, even if intrinsic hand function is sacrificed in return for functional elbow flexion, the final functional outcome would be favorable.
1. Garg R, Merrell GA, Hillstrom HJ, Wolfe SW. Comparison of nerve transfers and nerve grafting for traumatic upper plexus palsy: a systematic review and analysis. J Bone Joint Surg Am. 2011 May 4;93(9):819-29. Review
2. Oberlin C, Beal D, Leechavengvongs S, Salon A, Dauge MC, Sarcy JJ. Nerve transfer to biceps muscle using a part of ulnar nerve for C5-C6 avulsion of the brachial plexus: anatomical study and report of four cases. J Hand Surg. 1994 Mar;19A(2):232–7
3. Humphreys DB, Mackinnon SE. Nerve transfers. Op Tech Plast Reconstr Surg 2002;9:89-99
4. Brandt KE, Mackinnon SE. A technique for maximizing biceps recovery in brachial plexus reconstruction. J Hand Surg 1993;18A:726-33
5. Leechavengvongs S, Witoonchart K, Uerpairojkit C, Thuvasethakul P, Ketmalasiri W. Nerve transfer to biceps muscle using a part of the ulnar nerve in brachial plexus injury (upper arm type): a report of 32 cases. J Hand Surg 1998;23A:711-6
6. 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;98:313-318