Severed axons begin to send out a great number of sprouts within six hours of injury. This occurs proximal to the nerve lesion, at the most distal remaining node of Ranvier. These initial sprouts are usually resorbed; however, permanent sprouts are formed a day later. These grow towards the distal segment and then advance along the endo-neurial tubes (or Schwann cell columns). The regulation of axon growth and orientation is complex and reliant on a variety of biochemical and biomechanical mechanisms. The maximum rate of axonal outgrowth in humans is about 1 mm per day (Fig. 5.3(b)).
Effect on associated tissues Muscle changes
Muscle fibres usually undergo moderate to severe atrophy by three months and moderate to severe fibrosis after about one year. The degree of atrophy and fibrosis varies significantly among individuals and can be affected by infec-tion, muscle stretching, muscle nutrition or the age of the patient. After a three-year period, muscle fibres exhibit progressive fragmentation and disintegration, with the muscle fibres grad-ually being replaced by fibrotic tissue (Bowden and Gutmann, 1944).
Sensory loss
A completely severed nerve will result in loss of sensibility involving the various sensory categories, i.e. light touch, pressure, pain, localization, tem-perature, spatial discrimination (e.g. two-point discrimination) and functional gnosis. Where the nerve lesion is in continuity, the pattern of loss can be variable, e.g. patients with a compression neuropathy may show abnormality when tested with a threshold test such as the Semmes-Weinstein monofilaments, but give a normal response to functional tests such as moving or static two-point discrimination (Callahan, 1995).
Vasomotor changes
Following complete nerve disruption, the dener-vated hand will be warm to the touch for the first 2 to 3 weeks due to vasodilation resulting from paralysis of the vasoconstrictors (Seddon, 1975). After this time, the hand becomes increasingly cool to the touch and readily affected by the surrounding temperature (Sunderland, 1978). Colder weather is troublesome for most patients with nerve injury.
Disruption of sympathetic nerve function affects tissue nutrition, making skin more vulnerable to injury. When injured, denervated skin usually takes longer to heal. Nail changes include ridging and furrowing, slowed growth and hardening. Atrophy of the epidermis results in decreasing prominence of the papillary ridges and there may be reduction or absence of hand sweating. Skin that is smooth and dry is said to have reduced ‘tactile adhesion’
(Moberg, 1962). This facility is important in preventing the slippage of objects when gripping or when performing fine manipulative tasks (Clark, 1999).
Nerve repair
The nerve is repaired as accurately as possible to facilitate the regeneration of axons down the distal connective tissue tubes. The more accurate the matching of sensory to sensory and motor to motor nerve fibres, the better is the potential reinnervation of the end organs (Fig. 5.4).
Where possible, primary repair of the nerve is undertaken. In the presence of wound contamina-tion or associated injuries, secondary procedures are performed when conditions are more favour-able, thereby giving a better result. Where there is a gap in the nerve, grafting with a suitable donor nerve (e.g. sural nerve, medial cutaneous nerve of the forearm) is undertaken to avoid tension at the
Peripheral nerve injuries 61
repair site which will encourage proliferation of scar tissue. It is more difficult to match like axons with a nerve graft; however, because there is complete absence of tension, joint mobilization can commence earlier.
Technique
The proximal and distal nerve stumps are isolated and every attempt is made to preserve the vascular attachments. Epineurial repair is the most common technique and is used for the completely transected nerve. This is the simplest type of repair, requiring minimal magnification and a minimum number of sutures.
Perineurial (or fascicular) repair is the second most commonly used technique of nerve repair.
Higher magnification is required to identify and better align the fascicular groups which should be repaired without tension. This technique allows for greater accuracy in matching fascicles of similar size. Individual fascicular repair is only rarely performed.
Healing of nerve repair
The repaired nerve sheath, whether epineurium or perineurium, takes 3 weeks to gain sufficient tensile strength to withstand stress. The repair is splinted without tension during this time.
Factors affecting nerve regeneration Factors that can affect regeneration of nerve following injury or repair include:
1. The age of the patient (with increasing age there is a reduction in receptor populations, e.g.
Meissner corpuscles).
2. The level of injury (the more proximal the lesion, the less likelihood there is of a favour-able outcome).
3. Associated injuries, i.e. soft tissue loss, frac-tures, tendon injuries.
4. Degree of scar tissue.
5. Accuracy of fascicular alignment.
Digital nerve repair
The digital nerves are the most frequently severed peripheral nerves (Clark, 1999). To avoid tension at the repair site, digital nerve repairs are pro-tected for three weeks with a dorsal hand-based splint that maintains the MCP joints in 50 to 70 degrees of flexion. The finger portion of the splint should allow full IP joint extension. Gentle IP joint exercises can be performed within the splint. The patient should aim for full intrinsic IP joint extension to the limit of the splint to avoid the development of a PIP joint flexion deformity.
Figure 5.4. Nerve suture techniques. (a) Laceration; (b) Epineurial suture; (c) Group fascicular suture; (d) Individual fascicular suture. (Reproduced from Brushart, T. M. Nerve repair and grafting. 1999. In Green’s Operative Hand Surgery (D. P. Green, R. N. Hotchkiss and W. C. Pederson, eds) p. 1387, Churchill Livingstone, with permission.)
In the case of the thumb, a small hand-based thumb post can be fitted which holds the MCP joint in 35 to 40 degrees of flexion while permitting motion at the IP joint. Following the 3-week splinting period, the patient should avoid digital hyperextension for the next 1 to 2 weeks.
Scar massage is begun following suture removal. The patient is instructed in skin care and how to avoid injury to anaesthetic skin. Desensiti-zation exercises are performed at the repair site.
Nerve regeneration is often accompanied by unpleasant paraesthesia or hyperaesthesia. A layer of Opsite Flexifix over the affected area often helps ‘dampen’ these unpleasant sensations (Boscheinen-Morrin and Shannon, 2000). Sensory retraining is begun when moving-touch can be perceived in the fingertip (see p. 68).
Early postoperative management following nerve repair at the wrist
After a median or ulnar nerve repair at wrist level, the hand is rested in a dorsal splint which maintains the wrist in slight flexion to avoid stress on the repair. The splint extends just beyond the tips of the fingers with the thumb remaining free.
The splint is worn for 3 to 4 weeks by which time there is sufficient connective tissue strength to withstand wrist movement (Fig. 5.5).
Nerve injuries at the wrist are frequently asso-ciated with tendon injuries. In the absence of tendon injuries, gentle active finger and thumb movements can be commenced within the splint 1 to 2 days after surgery when the inflammatory response has subsided. Where there has been flexor tendon involvement, the flexor tendon protocol is
used unless the surgeon advises that gentle early active movement is allowed. To minimize stress on the tendon repair, full passive finger flexion range should be established prior to the commencement of active motion.
Scar management and desensitization Sutures are removed 10 to 14 days after surgery.
Scar softening and desensitization at the repair site are commenced. Gentle oil massage should be carried out four to six times a day as part of a home programme. Initially, massage is light otherwise it cannot be tolerated due to hypersensitivity. As tolerance to touch improves, pressure is gradually increased. Extreme hypersensitivity is managed with transcutaneous electrical nerve stimulation (TENS).
Scar tissue that is dense and/or raised is managed with silicone scar gel which is applied to clean, dry, oil-free skin. The gel is also helpful in acting as a ‘shock absorber’ over the repaired nerve.
Patient education
Patient education is an important aspect of man-agement following a peripheral nerve lesion.
Patients should be informed of the following:
1. That muscle wasting increases in the early stages following nerve injury.
2. How to avoid injury and take care of anaes-thetic skin; the patient will need to compensate visually until protective sensation has returned.
3. How to avoid deformity due to muscle imbal-ance by corrective splinting and maintaining mobility of joint and soft tissue structures.
4. That the average rate of nerve regeneration is approximately 1 mm per day.
5. That paraesthesia (tingling or pins and needles) and hyperaesthesia (painful hypersensitivity) are normal manifestations of nerve regeneration and will diminish with time and use of the hand.
Later stage postoperative management (4 to 6 weeks)
Gentle active wrist movements are commenced after 4 weeks. Active wrist extension is initially carried out with the fingers held flexed as there is often considerable tethering of structures, i.e. skin, nerve and tendons, resulting in soft tissue tightness.
Figure 5.5. Following repair of the median or ulnar nerve at the wrist, the hand is rested in a dorsal splint which maintains the wrist in slight flexion.
Peripheral nerve injuries 63
Overcoming soft tissue tightness
A mild flexion deformity of the wrist can be managed with a cock-up wrist splint which holds the wrist in neutral or slight extension. If the flexion deformity involves the wrist and fingers, serial extension casting is commenced between the 4th and 5th weeks. The wrist and fingers are casted in a position of correction that provides only a negligible stretch. The initial cast should hold the wrist and fingers in the position achieved by the patient when asked to actively extend to maximum range (Fig. 5.6).
This position should not cause pain and the fingertips are checked for signs of skin blanching that indicate excessive pressure. The skin, partic-ularly areas of altered or absent sensibility, are checked regularly for signs of pressure areas. The splint is used during the night and intermittently throughout the day. Wearing time may need to be increased slowly from initial periods of 1 to 2 hours. Hand oedema is managed with a lycra pressure glove which will also exert a gentle extension force to the digits.
Tendon adherence
Due to adherence of soft tissue structures at the repair site, active movement of the fingers and thumb occurs as a ‘mass’ action. To promote effective tendon glide, active finger and thumb exercises should be performed individually with stabilization of more proximal joints (Fig. 5.7).
The patient is advised to perform short exercise sessions on an hourly basis with at least 10 to 15 repetitions of movement. Finger movements are
more effectively performed when the wrist is supported in slight extension with a brace or thermoplastic splint.
Protection in cold weather
As the nerve-injured hand is vulnerable to the effects of the cold, the use of a thermal glove for protection in winter is recommended. Prior to exercise, the hand can be soaked in warm water for 10 to 15 minutes to improve comfort and mobility.
Week 6 onwards
Gentle resistance is added to flexion and extension exercises. The patient can attempt to actively extend the fingers and wrist simultaneously if flexor tightness has been overcome. Where soft tissue tightness remains, serial casting is continued until full simultaneous wrist/digital extension has been achieved. This process can sometimes take several months.
The patient is encouraged to use the hand for light daily activity. This may require functional splinting to oppose the thumb in a median nerve lesion or the use of an anti-claw splint in an ulnar nerve lesion. Where gripping is a problem, utensils can be ‘built-up’ with Handitube.
Figure 5.6. Serial plaster casts are used to overcome soft tissue tightness on the volar aspect of the wrist and/or fingers.
Figure 5.7. To promote tendon glide, interphalangeal joints should be exercised with stabilization of the more proximal joints.
Care of denervated skin
Denervated skin becomes smooth, shiny, fragile and prone to injury. Skin should be nourished regularly to maintain suppleness. Apart from the potential danger of heat and sharp implements, patients should be alert to pressure areas that can arise from friction during activity or pressure areas resulting from prolonged contact of the denervated area with splints or simply resting against a hard surface. Frequently used utensils and tools can be padded to avoid these problems.