Paul Binhammer

Case Presentation

A 23-year-old, right-hand-dominant floor installer sustained a utility knife laceration to his left distal palm resulting in numbness in the distribution of the radial digital nerve of the index finger. He was seen in the emergency department and had his laceration repaired and was referred for his nerve injury. He underwent repair of his digital nerve under local anesthesia using 3.5 loupe magnification and 9.0 nylon sutures. The nerve repair was protected with a dorsal block splint for 4 weeks. His healing was uneventful and he was referred to a hand therapist for scar management, desensitization, and nerve reeducation.

He returned 1 year later indicating that his finger did not feel normal. His clinical exam revealed no Tinel sign, normal hydrosis, no dysesthesias, and a two-point discrimination at the level of the distal interphalangeal joint of 5 mm.

Diagnosis

Injury (laceration) of the digital nerve to the radial aspect of the index finger

Anatomy

Digital nerves mainly carry sensory fibers from C6, C7, and C8. The digital nerves arise from either the median or the ulnar nerve at the proximal palm. The ulnar nerve typically gives rise to a digital nerve to the ulnar side of the small finger and the common digital nerve to the ring and small finger. The median nerve divides into common digital nerves to the ring and long fingers, the long and the index fingers, as well as digital nerves to the radial side of the index finger and the thumb. It is not uncommon for there to be crossover branches at the level of the ring finger from the median nerve to supply the ulnar side and from the ulnar nerve to supply the radial side.

The common digital nerves are initially deep to their accompanying arteries but by the level of the metacarpophalangeal joints the nerves have assumed a more superficial level. As well at this level, they have become proper digital nerves.

Within the digits the digital nerves can be found at the level of the interphalangeal joint creases. At approximately the distal interphalangeal joint they terminate. Initially, this may be a bifurcation or trifurcation that is still technically repairable.

The radial digital nerve of the index finger is particularly vulnerable at the metacarpophalangeal joint because it travels over the palmar aspect of the metacarpal head. Similarly the digital nerves of the thumb are devoid of protection as they pass over the palmar surface of the metacarpophalangeal crease. The thumb digital nerves can be subject to external compression injury as in bowler’s thumb.

Characteristic Clinical Presentation

Digital nerve injuries are classified by the mechanism of injury and can be further subdivided as being acute versus chronic.

Acute injuries usually involve an open wound but require that the examiner have a high index of suspicion to rule out a possible nerve injury; in some cases this can only be determined at the time of surgery. Simple lacerations of the digit and palm require careful physical examination to ascertain digital nerve injury. It is useful for the patient to have a point of reference, and therefore examination of a normal digit sets the stage for evaluation of the injured digit. Absence of pinprick sensation at the level of the distal aspect of the middle phalanx on the radial or ulnar aspect of the finger dictates that the nerve is likely injured and mandates exploration of the affected digital nerve. However, some patients report intact pinprick sensation in comparison with other normal digits. In these cases, alteration of two-point discrimination or light touch is helpful in deciding whether exploration is warranted. In borderline cases, it may be appropriate to see the patient 2 or 3 days later in an office setting and repeat the examination in a more controlled environment when the traumatic experience has passed.

Chronic digital nerve injuries present at least 2 weeks after the fact. The patient with a chronic injury demonstrates some degree of anesthesia, paresthesia, or even dysesthesia distal to the site of injury. Physical examination should include more than one sensory modality testing. Additionally the patient may have a Tinel sign at the site of injury, which is sometimes the primary complaint. Further examination of the hand may reveal other missed or existing injuries. Chronic digital nerve injuries are frequently seen in amputation stumps and may have a similar constellation of symptoms or findings.

Finally, examination of a child for a digital nerve injury is difficult. Once again, with an open wound a high level of suspicion is paramount. Sympathetic denervation following injury may be evident in the uncooperative child and aid in the diagnosis. Simple sweat tests such as iodine-starch or an examination for wrinkling after 5 minutes of warm water immersion can be performed.

Management Options

Acute simple digital nerve lacerations are best repaired within 72 hours of injury. They can usually be repaired under local anesthesia using loupes and microsurgical techniques in the operating room. Adequate exposure can be achieved through standard hand incisions, and the repair is achieved using 8–0 or 9–0 nylon. An epineurial repair is required for the digital nerves proper, whereas for the common digital nerves a fascicular repair should be performed to obtain the correct alignment. The technique is simple and minimally invasive to the patient. An operating microscope can also be used, but there is no evidence that this results in an improved outcome. Repair can be performed from the palm to the distal interphalangeal crease and, depending on the branching pattern of the nerve, perhaps a few millimeters beyond.

When repaired without tension, unrestricted motion can be allowed. However, if there are any concerns about tension, a dorsal blocking splint can be used for 4 weeks with the metacarpophalangeal joint held in flexion at 60 degrees and active motion allowed within the splint. Proximal interphalangeal joint (PIPjt) flexion can also be incorporated into the splint; however, one must remember that PIPjt contractures occur easily in the injured digit and may not resolve, despite therapy. Blocking of the PIPjt should be minimal in angulation and short in duration. In addition to splinting postoperatively, a course of hand therapy to include scar management, desensitization, and nerve reeducation is beneficial in reducing hypersensitivity and improving measurable outcomes.

It can be argued that for some injured digital nerves, repair is not required. The sensory and functional loss may be minimal. This includes the ulnar side of the index and long and the radial side of the ring and small fingers. Some surgeons argue that in the elderly, where the recovery of sensation is poor, repair of the digital nerve for these areas is not essential. Although this is true, a major indication for digital nerve repair is the prevention of painful neuromas, which can occur in the elderly and may be prevented by simple repair.

In the acute situation where the laceration is not simple and there is associated nerve loss and a resulting gap, repair may be achieved by flexion and dorsal block splinting, as already described. However, excessive flexion to achieve repair will only delay the associated traction injury, which will occur when splinting is discontinued. In such situations where primary repair is not possible, alternative solutions must be considered. It is also frequently the situation that at the time of injury the wound is contaminated or crushed and nerve reconstruction must be delayed. Similarly, for an initially missed or chronic (practically > 2 weeks since injury) digital nerve injury, as time passes, it becomes progressively more difficult or even impossible to coapt these nerves primarily.

For digital nerves there are several options for the short nerve gap. This includes the possibility of no reconstruction. For digits with at least one intact digital nerve, grafting may not be required. As indicated earlier the deficit is minimal and one must balance the donor site loss and the recipient site gain. Sensibility priority must be given to the thumb and the radial side of the index finger. When reconstruction is required the most conventional reconstruction is a nerve graft. Donor sites include the posterior interosseous nerve at the wrist, as well as the cutaneous sensory nerves of the upper and lower extremities. The posterior interosseous nerve graft has minimal to no donor site loss, provides a good size match for digital nerves, and is probably the graft material of choice.

One of the most common situations in the hand with regard to digital nerves is painful neuroma after amputation. In this situation the goal of neurectomy is to try and both minimize neuroma formation as well as decrease possible contact with the neuroma of overlying sensitive skin. There are a multitude of described techniques, including crushing, sectioning, ligation, epineurial closure, transposition, thermal coagulation, and coapting the two cut ends of the digital nerve. No technique is universally accepted or successful, however. One must balance further proximal dissection of the nerve with the potential problem of painful dysesthesias.

Outcome and Prognosis

Like any other nerve injury, the mechanism of the injury and the age of the patient seem to influence outcomes. Young patients with simple lacerations appear to have the best outcome. Alternatively, older ages, complex lacerations, and chronicity of injury all negatively influence outcome. In general, few patients regain normal sensibility. Although it is difficult to collate results across studies, ˜50% of patients obtain results where a two-point discrimination is less than 10 mm.