David J. Slutsky

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A loss of wrist and finger extension can result from myriad causes including brachial plexus injury, radial nerve injury due to trauma, nerve tumors, compression or idiopathic neuritis as well as posterior interosseous nerve (PIN) compression. In the latter case some radial wrist extension may be preserved. Until recently, tendon transfers have been the mainstay of treatment. Nerve transfers have long been used for brachial plexus reconstruction. The principals of nerve to nerve transfer, or neurotization have recently been applied to peripheral nerve injuries with encouraging results. The following discussion will center on neurotization to restore wrist and finger extension.

The radial nerve arises from the posterior cord of the brachial plexus. It receives contributions from C5 - C8 spinal roots. The nerve contains approximately 16,000 myelinated fibers. It runs medial to the axillary artery then at the level of the coracobrachialis it courses posteriorly to lie in the spiral groove of the humerus. In the lower arm it pierces the lateral intermuscular septum to run between the brachialis and brachioradialis. Opposite the head of the radius there are some fibrous bands from the joint’s capsule and immediately distal to this, the nerve is regularly crossed by several prominent veins, the “leash of Henry.” It divides 2 cm distal to the elbow into a superficial radial sensory branch (SRN) and a deep motor branch, the posterior interosseous nerve (PIN). It gives off branches to the extensor carpi radialis longus (ECRL) and brevis (ECRB), brachioradialis (BR) and anconeus before giving off the PIN branch. The PIN continues on between the superficial and deep head of the supinator muscle, to exit on the dorsal forearm. After it emerges from the distal border of the supinator, the PIN sends branches to the extensor digitorum communis (EDC), extensor carpi ulnaris (ECU) , extensor digiti quinti, extensor pollicus longus(EPL) and brevis and the extensor indicis proprius (EIP) in descending order, although there may be considerable variation.

Classification of Nerve Injury
In the Seddon classification, there are 3 types of nerve injury: neuropraxia, axonotmesis and neurotmesis.1 Sunderland subclassified these injuries into 6 types.2 Axonotmesis was divided into axonal injuries with or without an intact basil lamina (2nd and 3rd degree) or with complete scar block (4th degree). Neurotmesis was divided into those with complete transection (5th degree) and a combination of conduction block and transection (6th degree).

For the purposes of nerve transfer, Mackinnon grouped these injuries into three categories: (1) injuries that will recover spontaneously (1st and 2nd degree), (2) Injuries that must be repaired (5th degree or neurotmesis) and (3) Injuries that have partial recovery and will likely need surgery (3rd, 4th and 6th degree).

3 Types of Radial neuropathies
Upper Arm

In the arm region, the radial nerve is often injured in association with some form of unconsciousness. In a Saturday night palsy, an obtunded patient sits with their arm over a chair back or rests his/her head on the lateral surface of their arm. Alternatively the radial nerve can be compressed in the groove between the brachialis and forearm muscles when one person rests their head on the middle third of the arm of another i.e. Honeymooner’s palsy. The patient will typically present with a wrist drop and an inability to extend the fingers, thumb or wrist. In addition, the brachioradialis will be affected along with variable involvement of the triceps. They will also have diminished sensation over the dorsum of the 1st web space.

Electrodiagnostic studies
The NCS typically demonstrates the absence of the superficial radial sensory nerve action potential (SNAP). Motor recordings are more difficult since no muscle is sufficiently isolated from other radially innervated muscles. A surface electrode over the extensor indicus proprius (EIP) results in a volume conducted response from the adjacent radial innervated muscles, which makes side to side amplitude comparisons difficult. Radial nerve recordings using needle electrodes in the EIP are more common as a result, which makes it difficult to approximate the degree of axonal loss by assessing the amplitudes. The EMG however is quite useful, and permits a relatively accurate localization of the lesion. In a spiral groove lesion for example, all 3 heads of the triceps should be normal, with denervation of the brachioradialis and all muscles distal to it.

Posterior interosseous nerve entrapment
In posterior interosseous nerve syndrome the presenting symptoms are weakness and/or paralysis of the extensor muscles, which result in a wrist or finger drop. There may be a history of a fall onto an extended and pronated arm although many cases are spontaneous, especially if due to an underlying lipoma, ganglion or rheumatoid nodule arising from the radiocapitellar joint. The patient will prevent with variable weakness or paralysis of the EPL, EIP, EDC and ECU. Motor function of the ECRB/L should be preserved since they are innervated before the PIN dives between the two heads of the supinator muscle. The patient will hence extend their wrist in radial deviation. As it travels distally through the radial tunnel the PIN may potentially be entrapped by fibrous bands anterior to the radiocapitellar joint, the radial recurrent leash of vessels, the fibrous edge of the ECRB, the proximal border of the supinator i.e. the arcade of Frohse or the distal edge of the supinator muscle.

Electrodiagnostic studies
PIN lesions do not affect the superficial radial SNAP, which should be normal. The compound motor action potential of PIN innervated muscles may show a drop of conduction velocity or amplitude, but this is difficult to assess with surface electrodes. Needle EMG is the best technique for localization, especially with partial lesions. In acute denervation decreased recruitment, increased insertional activity and fibrillation potentials " positive sharp waves are present. In chronic lesions seen after 3-6 months, decreased recruitment may still be seen along with giant motor unit potentials and polyphasia due to peripheral axonal ingrowth.

Indications for Nerve Transfers
The time for reinnervation must take the distance from the injury to the motor endplate into account. As a general rule, motor endplates degrade at about 1% per week. Nerve growth is limited to 1 inch/month or 1 to 1.5 mm/day.4 By this reckoning a nerve will have regenerated 12 inches at 1 year, but 50% of the endplates will be gone. The maximum length that a nerve can grow to restore motor function is hence approximately 13 - 18 inches due to a critical loss of endplates. Proximal radial nerve repairs typically demonstrate superior results as compared to those for the median or ulnar nerves since they are closer to the endplate. Nerve to nerve transfers should therefore be considered in cases of delayed treatment, where the time for nerve ingrowth would exceed this window of time for reinnervation. Proximal radial nerve injuries that are 8-10 months old would be an ideal indication for neurotization for this reason. Brachial plexus lesions and traction injuries that show no EMG evidence of recovery by 3 months may also be appropriate candidates.

Proximal radial nerve lesions that are < 6 months old and PIN lesions < 1 year old are best treated with conventional methods of nerve repair or graft. Concomitant median nerve injuries would preclude use of this transfer.

Nerve Transfers Options
A number of nerve transfers to restore wrist and finger extension have been described. Palazzi and coworkers reported on the use of the motor branch to the brachialis muscle for neurotization of the radial nerve for a C7-C8-T1 avulsion.5 Tubbs et al performed an anatomic study on the anterior interosseous nerve and described a transinterosseous membrane tunnelling technique for transfer to the PIN.6 Ustun et al performed a cadaver study of the widths and lengths of the median motor nerve branches from their points of divergence. They found that the motor branches to the pronator teres, flexor pollicus longus and pronator quadratus were sufficiently long to permit neurotization of the PIN at different levels and in various combinations. The motor branches to the flexor digitorum profundus muscle were too short to use for transfer.7 None of these transfers have enjoyed widespread clinical use however.
Mackinnon et al have demonstrated that redundant median nerve branches to the FDS, FCR or PL may be used for transfer.8 They described two successful cases of transfer of the FDS fascicles to the pronator teres branch to restore pronation.9 They subsequently reported the use of this transfer to the ECRB branch and the PIN in a case of a high radial nerve palsy and a brachial plexopathy with excellent results. 10 In a separate study they noted that the FCU branch of ulnar nerve can also be used, but this required a second incision.11, 12

Relevant Median Nerve Anatomy
Just distal to the cubital fossa, the motor branches of the median nerve consistently collect into 3 fascicular groups.13 There is an anterior group (to the PT and FCR), a middle group (motor to the FDS and hand intrinsics, sensory to the thumb, index and middle fingers) and a posterior group (to the AIN 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 gives off branches to the palmaris longus (PL), flexor carpi radialis (FCR), flexor digitorum superficialis (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 emerges beneath the radial side of the muscle belly of the middle finger superficialis where it is quite superficial and near the palmaris longus tendon.

In a dissection of 31 cadaver arms, Tung and MacKinnon noted that double innervation of the FDS was found in 94% of the specimens.9 The most common branching pattern was a proximal branch that also carried the branch to the PL, and a distal branch a distal branch that arose from the median nerve distal to the origin of the AIN branch. The proximal branch arose 3.1 cm " 1.3 cm distal to the medial epicondyle, and was 2.1 " 0.7 cm long. The distal branch arose 7.4 cm " 2.7 cm distal to the medial epicondyle, and was 2.3 " 0.8 cm long.

Surgical technique (link to FDS transfer to PIN video)
The patient is positioned supine with the arm abducted on an arm board. The procedure is performed under tourniquet control. The tourniquet time is limited to 1 hour and must be released at least 20 minutes before stimulating the nerve. If intraoperative nerve conduction is performed Halothane and muscle relaxants are avoided since they extinguish the nerve response. The nerve should be irrigated with warm saline, since cold nerves do not conduct. A short acting agent such as Fentanyl can be used after infiltration of the wound margins with local anesthetic.

An anteromedial incision is made crossing the antecubital fossa, starting 5 cm above the elbow flexion crease. The medial antebrachial cutaneous nerve is identified next to the basilic vein and protected. Proximal to the elbow, the median nerve can be found medial to the brachial artery. The nerve is followed distally as it passes through the two heads of the PT. At this level the branches arising from the anterior bundle can be seen innervating the FCR and PT. Both the superficial head of the PT and the FDS can be divided and tagged for later repair if needed. The FDS branches along with the FCR and PL fascicles arise from the middle group. They are carefully differentiated from the hand intrinsics using the hand held stimulator. The intrinsic fascicles and sensory fascicles travel distally in the middle group along with the posterior (AIN branch) group which pass underneath the sublimus arch. This arch is divided. The 2nd or distal group of FDS fascicles may arise distal to the sublimus arch. At this point it is helpful to place vessel loops around the branches of interest. A disposable 2 mA nerve stimulator is used to sequentially stimulate the branches. Careful observation of the hand will allow one to discern the individual motor branches by their corresponding muscle twitch (see FDS nerve transfer video). The sensory fascicles can be identified by placed recording ring electrodes on the index and middle fingers, since stimulation will not elicit a muscle twitch, but this is usually unnecessary.

The radial nerve is isolated through the same incision. It can be found between the brachioradialis and brachialis as it divides into the superficial sensory nerve branch and the PIN branch. In PIN palsies with preserved radial wrist extension, the PIN is divided distal to the motor branch of the ECRB (Figure 1 A - G). In proximal radial nerve injuries the ECRB branch is divided and included in the transfer (Figure 2 A - E). These branches are stimulated to confirm that there is total extensor muscle denervation prior to nerve division. An intraoperative EMG can be performed if necessary by placing the needle recording electrode in the ECRB or extensor muscle mass with the reference electrode in the subcutaneous tissue. The presence of a compound motor action potential might alter the prognosis for recovery and require modification of the procedure to a neurolysis. Alternatively, a nerve action potential can be sought by stimulating and recording across a 4 cm segment of the PIN. Kline has estimated that approximately 4,000 myelinated axons are needed to produce a nerve action potential.14

MacKinnon recommends use of the PL and the FDS or FCR branches to the PIN and the ECRB branch. She also states that the strongest donor should be transferred to the ECRB branch to restore strong wrist rather than strong finger extension for maximum function.3 The redundant median nerve fascicles are harvested in close proximity to the PIN to avoid undue tension on the repair site. If need be a short nerve graft can be interposed. Similar to nerve repair, there should be < 8% strain on the repair site.15 For practical purposes, if the repair can withstand the tension during passive elbow motion intraoperatively then a graft is not needed. The anastomosis is performed with 9-0 nylon since the fascicles are typically 1-3 mm in diameter.

Postoperative Rehabilitation
An above elbow splint is applied with the elbow at 90 degrees and the shoulder, wrist, and fingers free. Gentle elbow flexion is started after the first week followed by gradual elbow extension. Motor retraining is akin to tendon transfers. The patient is instructed in active sublimus contractions, which will ultimately produce wrist and finger extension.

Published clinical series on this transfer are still lacking. In Mackinnon’s series a 51 y.o. male with a proxiimal left radial nerve palsy underwent transfer of the nerve branch to the PL and the FDS to the PIN and ECRB branch respectively. He achieved 4/5 power for wrist extension but lacked simultaneous finger extension at 14 months postoperatively. The second patient was a 24 y.o. female following an iatrogenic radial nerve injury at the plexus level. The nerve branch from the PL and FDS were transferred to the PIN. A 2nd FDS branch was transferred to the ECRB. The patient achieved 4/5 power of wrist and finger extension.10

My own experience is limited to two cases. The 43 y.o. male with a high radial nerve palsy featured in the accompanying video underwent transfer of an FDS branch to the PIN proximal to the ECRB branch. By 9 months he had achieved 4/5 power of wrist extension but still lacked independent finger extension. The second case of the 26 y.o. male with a PIN palsy did not achieve any appreciable wrist extension at 6 months followup. Because of the history of a 10 year old nonunion of the medial epicondyle, and the intraoperative findings of complete denervation of the supinator muscle it was uncertain as to whether this represented a much longer standing injury than reported by the patient, and may have contributed to the failure.

It is evident that nerve transfers to restore wrist and finger extension are a viable alternative to tendon transfers when the time for standard nerve repair or grafting has passed. The critical number of axons that need to be transferred are still unknown, but from the extensive work by Dr. MacKinnon, it seems reasonable to perform separate transfers to the PIN and ECRB branch in cases where both finger and wrist extension are needed.

Figure. 1 Posterior Interosseous Nerve Palsy
A. 26 y.o. male with an 8 month old idiopathic PIN palsy. Note the radial wrist extension due to preservation of the ECRB branch.
B. View of the median nerve with exposure of the FCR branch, the PT branch and the middle fascicular group (*) passing under the fibrous sublimus (FDS) arch.
C. Labeling of the median motor nerve branches after nerve stimulation.
D. Close up of the median nerve with the middle fascicular group (*) and the AIN branch, demonstrating its proximity to the PIN.
E. Harvesting of the proximal FDS branch arising from the median nerve.
F. View of the PIN as it runs beneath the brachioradialis with the overlying radial leash of vessels (*).
G. After the completed transfer (*) which has been passed underneath the brachial artery. MN = median nerve Figure 2. High Radial Nerve Palsy
A. 10 month old malunion of the distal humerus with a secondary high radial nerve palsy
B. Dissection of the median nerve with exposure of the AIN and the FDS branch (*)
C. Relative positions of the median nerve (MN) and PIN (*)
D. View of the FDS transfer just prior to anastomosis
E. Completed transfer
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2. Sunderland S. The anatomy and physiology of nerve injury. Muscle Nerve 1990: 13: 771-84.
3. Mackinnon SE WR. Upper Extremity Nerve Transfers. In Slutsky DJ HV, ed. Peripheral Nerve Surgery: Practical Applications in the Upper Extremity. Philadelphia: Elsevier, Inc., 2006: 89-108.
4. Seddon HJ, Medawar PB, Smith H. Rate of regeneration of peripheral nerves in man. J Physiol 1943: 102: 191-215.
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10. Lowe JB, 3rd, Tung TR, Mackinnon SE. New surgical option for radial nerve paralysis. Plast Reconstr Surg 2002: 110: 836-43.
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12. Boutros S, Nath RK, Yuksel E, Weinfeld AB, Mackinnon SE. Transfer of flexor carpi ulnaris branch of the ulnar nerve to the pronator teres nerve: histomorphometric analysis. J Reconstr Microsurg 1999: 15: 119-22.
13. Zhao X, Lao J, Hung LK, et al. Selective neurotization of the median nerve in the arm to treat brachial plexus palsy. An anatomic study and case report. J Bone Joint Surg Am 2004: 86-A: 736-42.
14. Kline DG. Surgical repair of peripheral nerve injury. Muscle Nerve 1990: 13: 843-52.
15. Clark WL, Trumble TE, Swiontkowski MF, Tencer AF. Nerve tension and blood flow in a rat model of immediate and delayed repairs. J Hand Surg [Am] 1992: 17: 677-87.