Digital Nerve Repair
The aim of a digital nerve repair is to restore functional sensibility to the digit and prevent painful neuroma formation.1 This is best achieved through accurate alignment of the fascicles and a tension-free repair. In order for the nerve to heal with minimal scarring, the tissue bed must also be healthy. In the scenario of significant surrounding traumatized tissue, consideration should be given to delaying repair until a suitable soft-tissue cover can be provided.
Laceration or irrecoverable digital nerve injury is the main indication.
Age is the most critical factor in sensory recovery; however, there are no age restrictions to repair.3 Outcomes are generally better in children, with a sharp decline after the second decade of life. This is due to a combination of superior axonal regeneration and cortical plasticity.
There is no defined time limit for the repair of sensory nerves. If appropriate microsurgical expertise is not readily available, it is better to defer repair for up to 3 weeks rather than perform a poor primary repair. Longer delays can make end-end repair difficult due to retraction and scarring of the nerve ends. In these situations, reconstruction with entubulation repair or graft likely would be required.
A nerve repair under tension is contraindicated.
Segmental loss of nerve or a nerve gap greater than 1 cm after débridement of the traumatized nerve ends is a contraindication.
Nerve avulsion injuries also do poorly with end-end repair and may require grafting.
Nerve injury in an open wound or contaminated wound pending adequate débridement and definitive soft-tissue cover. A stiff, swollen hand would benefit from a period of rehabilitation prior to nerve reconstruction.
A relative contraindication to repair is a mentally unstable patient with a self-inflicted injury pending psychiatric intervention.
A careful neurologic examination, including sensibility testing of the individual digital nerves with two-point discrimination (2PD) and/or Semmes Weinstein monofilaments preoperatively, is critical prior to infiltration of a local anesthetic.
In those with delayed presentation, a digital nerve injury leads to loss of sweating and atrophic changes in the skin.
Young children, in whom sensation examination is difficult, can be evaluated by placing the fingers in water and looking for loss of wrinkling in their skin.
A Tinel sign can help localize the level of axonal regeneration.
The digital artery is often injured in traumatic lacerations; hence it is imperative to assess the digital circulation, including a digital Allen test. Similarly, injury to the flexor tendons must also be excluded.
X-rays should be obtained to rule out fractures or foreign bodies.
The digital nerves arise from the termination of the median and ulnar nerves in the palm.
The common digital nerves are deep to the common digital arteries and bifurcate 1–2 cm proximal to the arteries.
The proper digital nerves become superficial to the arteries at the palmodigital crease. They are particularly superficial at the interphalangeal flexion creases, rendering them more vulnerable to injury.
The common digital nerve at the level of the palm has an average diameter of 3 mm. The proper digital nerve at the level of the finger has an average diameter of 2 mm. The nerve trifurcates distal to the distal interphalangeal (DIP) joint.
Nerve fibers are arranged into fascicles by the perineurium. A proper digital nerve contains 1–4 fascicles ( Fig. 9.1 ).
The patient should be advised regarding a nerve graft or conduit, since it is not always possible to estimate the nerve gap or deficit preoperatively.
Loupe magnification should be used for evaluation of the wound and for exposure and dissection of the nerve, and the operating microscope should be used for alignment and repair.
The patient is placed in the supine position with the hand placed on a stable hand table. A hand table with a supportive leg is strongly recommended any time when using a microscope. A padded tourniquet is placed on the upper arm.
The type of anesthesia depends on associated injuries. General or regional block is appropriate.
We prefer the use of a lead hand to retract the other fingers and thumb.
The arm is exsanguinated with a sterile elastic bandage rather than total exsanguination with an Esmarch bandage for improved visualization of the digital vessels, and the tourniquet is inflated to 250 mm Hg.
An epineurial repair is recommended for digital nerves. This technique is appropriate for small nerves containing only one or two fascicles, such as digital nerves. Since they contain only sensory fibers, matching is not a problem. Fascicular repair has not been shown to have an improved outcome over an epineurial repair. ( Fig. 9.2 )
A 9.0 monofilament nylon suture is sufficiently strong for digital nerve repair since, experimentally, a repair under tension will fail by suture pullout from the epineurium rather than suture breakage. Goldberg et al performed a biomechanical study of 67 cadaver digital nerves. They found that the primary tactile side of the finger tended to have a larger diameter digital nerve. The thumb radial and ulnar, index finger radial, middle finger radial, and small finger ulnar digital nerves have the largest diameters. The average failure load for a normal, intact digital nerve was 6 ± 2 N. Repaired nerves failed at 1 to 2 N. Suture number was the only variable that had a statistically significant effect on repair strength. Four epineural sutures were statistically stronger than 2 sutures. Suture purchase length and gauge did not affect repair strength. Maximal metacarpophalangeal (MCP) joint hyperextension resulted in 4 N of digital nerve tension. When the MCP joint was not hyperextended, proximal interphalangeal (PIP) joint motion did not generate tension. They recommended that an MCP dorsal blocking splint be considered for use after digital nerve repair to prevent MCP joint hyperextension.4
We prefer number 5 jeweler forceps and a curved microsurgical needle holder for the repair. Weck spears are used to keep the field clean of blood.
The incision for exposure largely depends on the initial wound. We prefer a volar Brunner incision when there is injury to associated structures and wide exposure is needed. While the midlateral exposure limits the exposure to the ipsilateral nerve, we feel that it causes less postoperative discomfort, with early finger mobilization.
Thick skin flaps are raised, retracted, and held in place with temporary nylon sutures. Grayson′s ligaments are divided, revealing the neurovascular bundle. The tendon sheath is explored and a tendon repair is performed if necessary. It is essential to leave the nerve to the end to avoid disruption of the repair during manipulation of the tendon for repair.
After the proximal and distal nerve ends are isolated. The microscope is then brought into the field and the epineurium is trimmed under 10× magnification so that the fascicles can be seen pouting ( Fig. 9.3 ). Cleanly cut nerves may not require freshening, but a crush injury will require trimming back to less edematous fascicles. A nerve-holding forceps and nerve-cutting blade are recommended for preparation of the nerve ends.
A resultant gap of 1 cm or less can be accepted proximal to the palmodigital flexion crease because of the elasticity within the nerve. Any gap distal to the palmodigital crease cannot be overcome with nerve mobilization; hence, grafting or use of a conduit is required.
Using forceps, the entire epineurium down to the fascicle is picked up. The suture is passed 3 mm from the edge of the nerve, taking care to avoid traumatizing the underlying fascicle. The first suture is placed on the side of the nerve furthest away from the surgeon. The tail is left long to allow gentle manipulation of the nerve.
The nerve is then flipped 180 degrees and a second suture is placed, leaving a long tail ( Fig. 9.4 ). If there is significant tension with gapping at the repair site, grafting should be considered. One or two additional epineurial sutures are placed on the anterior side ( Fig. 9.5 ); then the nerve is gently rotated over using the two long sutures ( Fig. 9.6 ) and one to two posterior wall sutures are placed ( Figs. 9.7 , 9.8 ).
The finger is then placed through a full range of motion without hyperextension to ensure the repair withstands pullout.
The tourniquet is then released, and hemostasis is achieved before closing the skin.
Postoperative splinting is dictated by any associated tendon injuries. In an isolated injury, the wrist is splinted in neutral position, the MCP joints at 45 degrees of flexion, and the interphalangeal joints left straight.
Early protective active mobilization, as needed in an associated flexor tendon injury, does not hinder sensory recovery. In fact, early motion facilitates nerve gliding, and clinical results are better after protective active mobilization, with less stiffness and less cold intolerance.2 Clare et al examined the outcomes in patients following 40 isolated digital nerve lacerations at a mean of 20 months (range, 12–36 months). Half of the repairs had been splinted beyond the immediate postoperative period and half had not. Splinting did not affect either the measured sensibility or movement of the digit. They concluded that splinting was therefore unnecessary after repair of sharp, un-complicated digital nerve divisions beyond the immediate postoperative period.5
Avoid tension on nerve repair. If a gap is present, proceed with autograft or entubulization.
In acute closed traction rupture, be sure to resect the nerve back to normal-appearing bundle architecture.
Inadequate exposure can lead to missed associated injuries and add difficulty to the nerve repair.
Unrealistic patient expectations are common. Pre-operative counseling on the probability of protective sensation only is essential.
Avoid nerve repair in a bed of lacerated muscle belly, naked tendon, or open synovium. Rotation of undamaged fat can be beneficial.