Sensory Nerve Transfers in the Hand
Following devastating nerve injuries to the upper extremity, patients are often left with insensate areas of the hand. Traditionally, tendon transfers and other reconstructive options would not be considered in the insensate hand. Sensory nerve transfers provide an excellent option for the restoration of critical sensation with a minimal donor-site deficit.1,2 Establishing sensation in these critical areas allows for a more complex reconstruction with significant improvement in hand function. In contrast to primary repair of a proximal nerve injury, which provides only protective sensation at best, distal sensory transfers can provide quality sensation via an end-to-end anastomosis, and protective sensation via an end-to-side anastomosis, which can minimize the sensory downgrading within the donor territory.2 For sensory transfers, a nerve branch that supplies noncritical sensation is sacrificed to restore critical digit sensation, such as the contact surfaces of the thumb and index fingers. In the management of complete brachial plexus injuries, sensation in these fingers can be restored by nerve transfer to the lateral cord contribution to the median nerve. Alternatively, transfer can be performed to the ulnar nerve in the upper arm to restore sensation to the ulnar hand and digits. Then, at a second stage, the nerve branch to the fourth web space can be directly transferred to the nerve that innervates the first web space to supply sensation to the thumb and index finger. As our knowledge of internal nerve topography and anatomy improves, and with the development of reverse end-to-side nerve transfers, the options for sensory nerve reconstruction in the hand have expanded.
Sensory nerve transfers are indicated for restoration of sensation to critical areas of the hand following iatrogenic nerve injury, traumatic nerve injury, and nerve defects following tumor excision.
Although end-to-side nerve transfers have been used in several clinical scenarios, experimental data have shown that, in the absence of some degree of injury to the donor nerve, only sensory axons traverse this repair. The axons transmitted as a result of this technique are not expected to provide much more than protective sensation but do so via a method that prevents noticeable downgrading within the donor territory. As a result, the authors favor this technique for noncritical regions of the hand that would benefit from protective sensation, including the dorsum of the hand and the third and fourth web spaces.
Restoring sensation to the radial side of the index finger and the ulnar side of the thumb are priorities in median nerve palsy.
The median nerve–innervated third web space (when preserved, as in an upper trunk brachial plexus injury), the ulnar nerve–innervated fourth web space, and the radial nerve–innervated dorsum of the hand are less critical sensory distributions and therefore represent the sources from which local axon donors can be recruited.
In a pure median nerve injury, the authors’ preferred strategy for sensory restoration involves transfer of the ulnar-innervated fourth web space common digital nerve to the median-innervated first web space (radial aspect of the index finger and digital nerves to the thumb) via a direct end-to-end repair. Secondarily, restoration of sensation to the donor fourth web space and median-innervated third web space can be achieved using an end-to-side technique as described previously. In this case, the ulnar digital nerve of the small finger is the axon source. This procedure can restore protective sensation to these digits without a noticeable downgrading of the ulnar sensation ( Table 6.1 ).
The sensory loss secondary to ulnar nerve injury involves a significant region of the palm. Nerve transfers for restoration of ulnar sensation involve mobilization of the median sensory contribution to the third web space with an end-to-end transfer to the proximally cut end of the ulnar palmar sensory component. This trades ulnar anesthesia for the less critical third web space numbness.
Protective sensation can be restored to the third web space and dorsal ulnar cutaneous distributions by use of an end-to-side coaptation with the remaining median sensory trunk.
Radial sensory nerve neurotization of the ulnar digital nerve to the thumb can be used to restore partial thumb sensation in cases of a combined proximal median and ulnar nerve injuries.
Sensory nerve transfers are indicated for proximal nerve injuries, where direct repair or nerve grafting will result in protective sensation at best.
Sensory nerve transfers can be performed in patients of all ages; however, results will be superior in younger patients, especially in children.
Sensory nerve transfers are not time dependent, since the sensory end organs can be reinnervated after many years. The results are better, however, if performed within 6 months of the injury.
Sensory nerve transfers are contraindicated in life-threatening or limb-threatening upper extremity injuries.
Sensory nerve transfers are contraindicated in panplexus injuries, where no donor nerves are available.
Sensory nerve transfers do not require any preoperative imaging.
Clinical examination to confirm donor nerve availability and to document insensate areas is critical for preoperative planning.
Two-point discrimination (2PD), the ten test, and patient documentation on a hand diagram are useful adjuncts to document sensory deficits in the upper extremity and to evaluate outcomes following surgery. The ten test consists of applying light touch to the affected digit while simultaneously stroking the same region of the unaffected digit on the opposite hand. The normal digit is assigned a value of 10 for sensibility, and the patient is asked to grade the comparative sensation of the affected digit numerically.3
An extensive knowledge of the topography of both the ulnar and median nerves is required to perform sensory nerve transfers in the hand.
The median nerve ( Fig. 6.1 ) lies between the flexor digitorum superficialis and the flexor digitorum profundus in the proximal forearm.
As it travels distally, it becomes more superficial and is located just deep to the palmaris longus at the wrist crease.
The nerve then enters into the carpal tunnel and travels distally into the hand.
In the forearm, the nerve branch to the pronator teres branches from the anterior aspect of the nerve and courses radially/laterally.
Additional, nerves branches to flexor carpi radialis, palmaris longus, and flexor digitorum superficialis branch from the ulnar/medial aspect of the nerve.
The anterior interosseous nerve (AIN) is the only branch from the radial aspect of the main nerve in the proximal forearm.
After these branches have departed from the main nerve, the majority of the nerve is sensory.
The only exception is an independent fascicular group that is oriented on the posterior radial aspect of the nerve, which will become the recurrent motor branch. This branch exits the main nerve variably, either before the transverse carpal ligament or within its substance.
The palmar cutaneous branch of the median nerve departs from the main nerve ~ 6 cm proximal to the wrist crease on the radial aspect of the nerve.
The internal topography of the median nerve is such that the third web space fascicles are located on the ulnar-most aspect of the nerve. The second web space fascicles are in the middle portion of the nerve. The first web space fascicles are found on the radial aspect of the nerve. The first web space fascicle includes fibers that innervate the radial aspect of the thumb.
The ulnar nerve ( Fig. 6.2 ) travels with the ulnar artery in the distal forearm and is located just deep to the flexor carpi ulnaris.
In the forearm, the topography of the nerve from radial/lateral to ulnar/medial is sensory–motor–sensory. Natural cleavage planes exist between the fascicles dedicated to each modality. Often these cleavage planes can be identified by microvessels running longitudinally along the nerve. Alternatively, they can be identified by tapping a microforceps along the surface of the nerve and noting the area where the forceps will “fall” into the nerve.
The dorsal cutaneous branch of the ulnar nerve departs from the ulnar aspect of the main ulnar nerve at ~ 9 cm proximal to the wrist crease.
Distal to the branching point of the dorsal cutaneous branch, the internal topography of the remaining nerve is such that the radial 60% is sensory and the remaining ulnar 40% is motor.
The ulnar nerve branches further at the level of the leading edge of the hypothenar fascia. The superficial sensory branch travels medially. The deep motor branch dives under the fascia of the hypothenar muscles and wraps around the hook of the hamate to supply the intrinsic muscles of the hand.