Hand Nerve Injury Repair

Hippocrates provided one the first known references that stressed the importance of nerves, warning physicians to avoid injuring the nerves within the dislocated shoulders of soldiers during repair. Sushruta, through the observation of different injuries and their results, was able to describe the function of many components of the peripheral nervous system. Galen differentiated nerves from tendons and reported the successful repair of nerves by other physicians, although no record exists that he attempted any repairs (130-200 AD). In the seventh century, Paulus Aegineta was the first to report the use of suture and agglutination to repair nerves. Other physicians who performed early work with nerves and their repair include Rhazes and Avicenna in the ninth century, Ali Abu Ibn Sina in Persia during the 10th century, and Ferrara in Italy in the 17th century.[1, 2, 3, 4, 5]

Despite this early record of physicians attempting to classify and treat nerve injuries, the common belief prior to the 19th century was that nerves did not regenerate. As a result, any kind of major nerve injury was treated nonsurgically or with amputation. During the 19th century, the development of improved microscopic devices, along with enhanced staining techniques, allowed a more detailed examination of nerves and nerve tissue. These new techniques allowed Waller to describe what happens to a nerve once transected, Cruikshank to note the regrowth of nerves, and Cajal (1905) to clarify the events and manner of axon regeneration. These discoveries have since been refined and now form the basis for the current model of axon elongation following injury.[6]

Not until the 20th century and the series of large wars that created many nerve injuries were the clinical and surgical techniques used to repair nerves today successfully refined and implemented. Mitchell, who practiced during the US Civil War, found that nerve injuries were often associated with a burning pain (ie, causalgia) in the distribution of the affected nerve.[7] Tinel characterized a tingling sensation that occurs with regenerating nerves from his work with patients during World War I.

During World War II, Seddon[8] and Woodhall, working in different countries, refined several methods of nerve closure using cable grafts, bridging grafts, and primary and secondary closure. Soon after World War II, Sunderland published his findings on the topographic organization of nerves, which resulted in new techniques regarding the repair of fascicles. Most recently, Millesi[9, 10] pioneered the incorporation of microsurgical instruments and technique, which greatly have improved the results of nerve repair.[11, 4]

Nerve injury can be defined as a defect that results in a disruption of a nerve such that it can no longer transmit an action potential. A wide range of injury types and severities have been classified and should be considered. Two classification schemes have been practiced widely by clinicians to describe nerve injuries (see Table 1). The scheme proposed by Seddon[8] in 1943 uses the terms neurapraxia, axonotmesis, and neurotmesis to describe the severity. The second scheme, proposed by Sunderland, divides the gamut of injury types into 5 degrees that overlap nicely with Seddon's but provide a more specific scale for grading patients.[8, 1, 1, 12, 4, 13, 14]

Table 1. Clinical Progression Based on Degree of Injury (Open Table in a new window)

Neurapraxia (nerve nonfunction), the mildest form of injury, corresponds to a first-degree injury and usually involves demyelination without axon disruption and degeneration. Transient loss of function, or conduction block, results until remyelination occurs. This type of injury has a relatively short recovery time, and full function is expected without intervention by 12 weeks after presentation.[4] Because axons are myelinated to a different extent, function is lost and regained at different times. Motor fibers are the first lost and the last regained, while pain and sympathetic fibers are the opposite.[15] Common examples of this kind of injury include pressure palsies such as those that result from a tourniquet or from sleeping with pressure on a nerve, eg, Saturday night palsy.[4]

Axonotmesis (axon cutting), which occasionally is grouped with neurapraxia, describes the situation when axons, myelin, and associated internal nerve structures are disrupted. This overlaps with second-, third-, and fourth-degree injuries. The characteristic nerve changes that occur with these injuries involve the internal structures, while the external structures remain intact. This means that axons are disrupted and must regenerate, while the epineurium is intact and the nerve looks normal upon macroscopic examination. These injuries often result from situations in which traction overcomes the inelastic internal structures but leaves the elastic epineurium intact.[1, 15, 4, 14]

Second-degree injuries are those in which only the axons of a nerve are disrupted, leaving the endoneurium and the rest of the nerve intact. Because the axon is cut, degeneration and regeneration occur. This is the first stage of injury that shows an advancing Tinel sign. Because the endoneurium is intact, little scar tissue is expected. The nerve should fully regenerate, with complete sensory and motor function regained. Timing varies based on the location of damage relative to the end organs.[4]

Damage to the axons and the endoneurium that leaves the perineurium and epineurium intact is known as third-degree injury. This leaves the blood-nerve barrier intact but provides a disorganized bed through which the axons can travel. This can result in axon misdirection and potentially debilitating consequences. Fortunately, nerve organization is maintained on a large scale with fascicular continuity. Nerve regeneration may be slowed due to the infiltration of scar tissue or a smaller number of axons capable of survival and regeneration. An advancing Tinel sign, although somewhat slowed, should be present. The final result is not likely to be perfect; however, surgery is not normally indicated because targeting from the axon to the end organ is still relatively intact.[1, 12, 4, 14]

The worst form of closed nerve damage, fourth-degree injury, involves damage to all structures except the epineurial covering. Disruption of the perineurium results in a failure of the blood-nerve barrier and electrolyte regulation within the nerve space. This disables the intact axons that are still present so that no conduction down the nerve is possible. Axon regeneration is disorganized, and surgical intervention is required to restore function, although this may not be recognized immediately. At this stage of injury, a Tinel sign is present at the level of injury and does not move distally because the regenerating nerve is kept from advancing by large amounts of scar tissue or debris.

These injuries usually result from very severe stretch or traction injuries. Because the method of injury is similar to second- and third-degree injury and the nerve is continuous, waiting several weeks before surgical intervention is recommended because less serious injuries usually show improvement within the first 3 months.[1, 12, 4, 14]

Neurotmesis (nerve cutting) is the most severe type of injury and corresponds to a fifth-degree injury. This kind of injury results from a disruption in the continuity of the axons and all supporting structures, including the epineurium. This division involves a separation of the nerve ends such that axon regeneration from the proximal end is unlikely to reach the distal end. This class of injury is easy to diagnose because it usually involves an open wound with nerve deficits. Surgical repair is required for any return of function. The time required for return of function after repair depends on the location of the lesion and other variables. One must understand that nerves regenerate slowly, progressing 1-1.5 mm/d or 1 in/mo. Prognosis varies greatly depending on the circumstances but, in general, is guarded.[1, 12, 4, 14]

More recently, Mackinnon[4] described a sixth degree of injury. This new class was characterized to describe lesions with a mixture of the above findings, which occur fairly often and otherwise are known as neuromas-in-continuity. This type of injury has a mixed pattern of regeneration with complete recovery of first- and second-degree injuries and failure of recovery in areas with fourth- and fifth-degree injuries. This makes repair difficult because some fibers within a nerve are intact and should not be disrupted, while others are disrupted and have formed neuromas that must be dissected and repaired. The author feels that the microneurodissection involved with repair of this type of injury is one of the most challenging parts of nerve repair. Once the scar and neuroma(s) have been removed, care should be taken to reattach formerly continuous fascicles.[1, 12, 4, 14]

Frequency

Nerve injuries are involved in a significant number of admissions to hospitals. While most of these injuries involve nerves within the head and spine, 5-10% of patients are found to have a peripheral neuropathy either in isolation or in combination with other injuries.[16] These injuries occur more often in men than in women, with a male-to-female ratio of 2.2:1. They also occur more commonly in the dominant hand, most likely as a result of reflexive protective motions.[17, 13] When researchers looked at the individual nerves of the arm, they found that the digital nerves were damaged most frequently, followed by the median nerve, the ulnar nerve, and, finally, the radial nerve. The most commonly injured digital nerves were in the ulnar and radial distributions.[5] For more information, see the Medscape Drugs & Diseases article Nerve Compression Syndromes of the Hand.

While many possible causes exist for nerve injury, some of the more common involve falls, collisions, motor vehicle accidents, high-velocity missiles, fractures, dislocations, lacerations, or some other form of penetrating trauma. These causes may be grouped into general categories including penetrating injury, which generally involves sharp transection; trauma-type injury, which generally involves some kind of crush component; massive tissue loss; and avulsion or traction injuries, which result in stretching or tearing of the nerve internally due to an excessive amount of tension.

Ischemia and other nonmechanical factors, such as thermal injury, electric shock, radiation, percussion, and even envenomation, also can cause substantial nerve damage. Because of the difficulty in diagnosing a nerve injury in the presence of polytrauma, care must be taken to perform a thorough examination for sensory and motor loss as soon as possible.[18, 13]

All nerve injury results in a predictable response. While all the changes that occur after a nerve injury are usually considered a normal physiologic response, it is a normal response to a very abnormal situation. Depending on the type of injury, the response is different but still somewhat predictable. If the axon is spared, such as in first-degree injury, conduction is interrupted due to demyelination, but it is reinstated whenever the aggravating stimulus is removed and the myelin layers are restored. If the axon, or more, is transected, causing a second- to fifth-degree injury, the response has 2 main phases, degeneration and regeneration, and takes substantially longer.

Table 2. Pathophysiology of Degeneration and Regeneration* (Open Table in a new window)

The first phase of axon injury, degeneration, has been known as wallerian degeneration since Waller's description in 1850. Following injury, changes occur in the axon, cell body, and the Schwann cell covering of the nerve to help prepare for regeneration. Distally, the axon begins to disintegrate and undergoes apoptosis, releasing vesicles of cytosol and organelles. This same process occurs proximally, although the extent varies with the severity of injury and is eventually stopped by the regeneration process. Local Schwann cells and macrophages that migrate across the local vessels clean up the apoptotic debris, creating long, clean endoneurial tubes.

Once the debris has been removed and the macrophages have returned to the peripheral circulation, Schwann cells begin to proliferate and organize themselves into columns that lie within the endoneurial tubes, thus creating what are known as the bands of Bunger.[1, 25, 21, 26, 24, 4, 27, 28, 29, 30, 31, 14]

While the distal portion of the nerve is being cleared and prepared for reinnervation, the proximal portions of the neurons are going through a process of regeneration. Immediately after injury, the cell body swells, the nucleus becomes hypochromatic, and the production of mRNA and proteins is greatly increased.[1, 21, 27] These products are transported down the axon, providing the material and energy for nerve elongation to the distal tip. While the cell body is busy making raw materials, the newly severed axon end begins to sprout, initially sending out transient axonal processes that are retracted and replaced by more permanent filopodia that contain a cytoskeleton and are capped by an expanded region known as the growth cone.

The growth cone has been shown to be the site of axon elongation and is the location of selectivity. This structure sends out many small processes that seek specific markers, which influence the axon in its movements to preferentially select neural tissue and even exhibit a preference for endoneurial tubes that have the same function. For instance, when a motor axon reaches a nerve gap, it moves across small gaps into the severed distal end and finds a motor tube to begin growing down. This helps preserve function but is not specific enough to recreate innervations identical to those that existed prior to injury.[32, 33, 34, 24, 4, 19, 35] Forssman first described this mechanism in 1898, and it has been elucidated further by Cajal, Weir, Weis, and many others.

The axon's response is known to be regulated by chemical signals that exhibit neurotropic and neurotrophic influences. Neurotropic factors are responsible for directing and attracting the regenerating axons. Neurotrophic factors are involved with inducing maturation and elongation of the axon.[36, 37, 38, 29, 39] Macrophages, Schwann cells, and other supporting cells in the area release these chemical factors and attempt to create a microenvironment that promotes axon elongation. Limits exist on directing axon regeneration. If the nerve is too far away, the axons are not strongly attracted to the distal end and eventually stop advancing, resulting in a neuroma. If the nerve is too close, the axons cannot exhibit specificity of function, causing severely aberrant patterns of innervation at the end organ level.[21, 24, 40, 20, 41, 28, 29, 31]

Despite the systems in place for proper reinstatement of nerve function, many different events can impede axon sprouts. Depending on the severity of the injury, the proximal axon stump may have retracted. Usually, more retraction occurs with more severe injuries, and the regenerating axon requires longer to reach the nerve gap. Additionally, the possibility exists that the metabolic demands placed on the cell for regeneration exceed the capabilities of the cell, eventually leading to atrophy.

Large gaps, usually those greater than 15-30 mm, cannot be crossed reliably by axons. This is usually because proliferating Schwann cells or fibroblasts grow between the severed nerve ends and form a physical blockade.[42] Alternatively, if suitable tissue is not found within a set distance, the axon sprouts stop proliferating and take residence in nonneural tissue, forming a neuroma. Neuromas can occur in any situation in which an axon or collateral sprout remains in nonneural tissue.[40, 4, 27, 43, 39]

To accurately diagnose a nerve injury, obtain a thorough history from the patient. The history should include the patient's age, occupation, avocations, hand dominance, and previous injuries and procedures. Additional information that can be helpful includes allergens, medications, past medical history, and the events leading to the injury. In the acute setting, a nerve injury should be expected and sought whenever a patient presents with an open cut or laceration. Also, look for symmetric movement of the limbs because a lack of movement is suggestive of damage.[39]

The physical examination should focus on determining the level of nerve injury and should attempt to differentiate partial from complete lesions. This can be difficult if the patient is not fully coherent. The examination should include a motor evaluation and a sensory evaluation. These tests must be performed in a very detailed manner so that accurate data are found to aid with diagnosis.[6, 44, 45, 39, 46]

The motor evaluation should include tests to determine the range of motion, functionality, and strength in the muscles supplied by the nerve. Each nerve should be tested individually using isolated movements as much as possible. The radial nerve is tested using extension of the elbow, wrist, and fingers. The median nerve innervates both extrinsic and intrinsic muscles of the hand. Extrinsic median motor function can be tested with flexion of the index finger at the distal and proximal interphalangeal joints, thumb flexion on the interphalangeal joints, and radial wrist flexion.

The intrinsic median innervation of the hand can be tested with thumb abduction. The ulnar nerve also has both extrinsic and intrinsic components. The extrinsic ulnar musculature can be tested using proximal interphalangeal flexion of the small finger and ulnar wrist flexion. The intrinsic muscles of the hand, the interossei, are used to test intrinsic innervation. All movements can be compared bilaterally for strength and range of motion. Functionality of muscle innervation can be tested using complex motions such as twisting or manipulating various objects.

The sensory evaluation is used to test for basic protective sensation and 2-point discrimination (2PD) and to map out all areas of paresthesia. The radial nerve supplies the dorsal radial aspect of the hand and the first web space. The median nerve supplies the index finger, thumb, and proximal palm near the thenar eminence by way of the palmar cutaneous branch. The ulnar nerve supplies the ulnar side of the hand and little finger. It also supplies the dorsal ulnar region of hand via the dorsal cutaneous branch of the ulnar nerve. The digital nerves lie adjacent to the distal interphalangeal flexion crease and supply the adjacent areas, but not the fingertips.

Adjuvant findings that can help with localizing innervation deficits are the presence of dry, shiny skin and the presence of skin that does not wrinkle when immersed in water. These findings are the consequence of denervation of the skin. Also, a Tinel test can be used to locate the ends of transected and regenerating axons. If the findings remain ambiguous or unclear after the examination, the patient should be sent for electrodiagnostic procedures so that the extent and grade of the injury can be determined.[19, 46]

Determining the optimal treatment for a patient is a complex task that depends on the results of a complete patient assessment. The questions involved with nerve repair involve 2 main areas, namely, (1) when to do the repair and (2) what type of repair to perform. Primary repair is generally considered within the first few days, delayed primary by the end of the first week, and secondary closure beyond that.[19, 5, 14] Surgical options generally include end-to-end closure, nerve graft placement, vascularized nerve graft placement, and nerve transfer.

Table 3. Selection of Operative Procedure (Open Table in a new window)

Primary closure has been found to be superior to secondary closure.[35, 18] However, to ensure that the repair is successful and undisturbed during the healing process, certain criteria should be met. The condition of the wound itself is important. It should be a limited lesion, preferably a sharp transection, rather than a crush-type injury. If a crush component is present, it should be limited enough that coaptation of nerve ends is still possible after resection of damaged tissue. The wound should be free of debris and contamination that may compromise healing (eg, from infection). Considering whether the wound provides an adequately vascularized bed for the nerve is also advisable because this aids the healing process.

Supporters of delayed repair suggest that waiting provides increased fibrosis and mechanical support to the wound and suture. Some also suggest that waiting to perform the repair allows it to coincide with maximal axoplasmic synthesis, which occurs approximately 3 weeks after injury, although this has not been demonstrated to be beneficial in most patients.[19, 14]

Another consideration is the condition of the patient. Large numbers of concomitant injuries, especially in the same area, present contraindications because of the stress on the patient and the mechanical disruption that can occur during additional repair surgeries. This means that in a complex wound, the nerve repair should be delayed until all other injuries have been treated. The patient should be stable enough to be considered for elective surgery, and the patient should be able to give informed consent.[19, 14]

The final part of the equation concerns the facilities and staff available at the time. In order to maximize the repair effort, microsurgical equipment should be available and the primary surgeon should be familiar and comfortable with this type of surgery.[14, 9]

As long as the above conditions are met and the defect is repaired in a tensionless manner, the result should be favorable. If any of the criteria are not met, waiting and performing a delayed primary or secondary repair of the nerve is better.

Surgically, the most favorable form of repair is the end-to-end approximation. This type of surgery is indicated in sharp transections, in which the ends of the nerve can be found, are relatively intact, and are close enough to be approximated without tension. If the injury is proximal, fascicular repair more fully restores the disrupted pathways. Distally, where the axons are grouped more or less by function and the nerves are much smaller, endoneurial repair can suffice.[19, 14]

In situations in which the wound is more complex, such as with a crushing trauma, burn injury, or failed end-to-end closure that needs resection and repair, alternative methods may be advised. Crush wounds generally create a fairly large nerve gap that cannot be closed without placing the coaptation under significant tension. Some have suggested that in the upper arm, a nerve gap of 4-5 cm[4, 19] requires closure by grafting.

In the small digital nerves, this is reduced to gaps greater than 1 cm, which can usually be closed with finger positioning. Whenever the wound bed is well vascularized and free of contamination, a conventional nerve graft can be performed.[9] In situations that are more rigorous, such as a burn, a vascularized nerve graft can help ensure the survival of the graft and may hasten regeneration.[47, 40, 48] For this reason, vascularized grafts also may be beneficial in proximal injuries when regeneration is likely to take a long time. Despite the increased regeneration speed and graft survival, no significant difference is found in the resulting function or sensation when compared to a normal graft procedure.

Conduits and nerve transfers are the final 2 surgery types, but they are performed only under very specific situations. Conduits are indicated in patients in whom a small nerve gap is present but no autologous nerve is available for transfer (or not enough to completely repair the defect). Instead, conduits can be made out of blood vessels, muscle, and occasionally a synthetic material such as silicon or polyglycolic acid.[42, 49]

Nerve transfers are rarely performed and are reserved for situations of massive injury when other forms of nerve repair are unavailable and tendon transfers are exhausted or impossible. This technique uses the proximal portion of an intact nerve to reinnervate the distal portion of a more important nerve once the proximal portion has been destroyed.[50] This also may be necessary when addressing a delayed presentation of injury or a lesion that is older than 6 months.

Nerve anatomy

Peripheral nerves, such as in the CNS, are arranged to provide an isolated and protected region within which neurons operate. The functional parts of neurons contained within the peripheral nerve are the long processes, axons, and modified dendrites that communicate with distant end organs. These long processes, in the case of axons, carry messages from the anterior horn cells in the spinal cord to effectors and, in the case of dendrites, carry messages to dorsal root ganglia from various sense organs. The axons are surrounded by Schwann cells, which provide nutrition, support, and protection.

Axons can be myelinated, with Schwann cells wrapped around a single axon, or unmyelinated, in which case the Schwann cells cover many different axons without creating the myelin layers. Around each axon and its Schwann cells is a layer of loose connective tissue and fluid that comprises the endoneurium. This combines with the basement membrane of the Schwann cell to create the endoneurial tubes that surround the axons.[4, 51, 52, 39, 31, 14, 53]

The next layer of organization arranges axons into bundles, or fascicles, that travel together. The fascicle number is not constant and can vary extensively through the course of the nerve. Another layer of connective tissue covers each fascicle. This layer is a continuation of the pia-arachnoid layer of the CNS and is one component of the blood-nerve barrier. This layer is made and maintained by concentric lamella of perineurial cells. The perineurial layer is very important to nerve function and must remain intact for the nerve to function.[54, 52, 39, 31, 14, 53]

Above the perineurium lies the epineurium. Inner and outer epineurial layers are present, with loose connective tissue and collagen between them. The inner epineurium is relatively vascular and surrounds individual and bundled fascicles. The thickness of the epineurium varies by location but can comprise 25-75% of nerve cross-section.[55] This allows the epineurium to provide some amount of protective cushioning to the inner structures. Also, the epineurium contains elastin that makes this outer cover slightly elastic. This property facilitates the longitudinal excursion and gliding movements of the nerve.

During rest, the epineurium contracts, creating striations within the nerve termed the bands of Fontana. These disappear with excursion as the epineurium stretches and allows the nerve to extend to full length. Problems arise when the epineurium contracts after nerve transection, causing the ends to retract and the nerve gap to enlarge. Also, the elastic properties of a nerve can be altered greatly by surgery because of adhesions that form around the area of repair. This alters the amount of excursion available and causes the free parts of the nerve to stretch more than normal. The epineurium is primarily a protective layer and, unlike the perineurium, can be removed without detriment to nerve function.[52, 39, 31, 14, 53]

Vasculature and innervation

Neural tissue requires a significant amount of oxygen to function properly, and, as a result, peripheral nerves have an extensive blood supply composed of both external and internal blood vessels. The external vessels run through the perineurium and segmentally supply the intrinsic vessels via the arteriae nervorum. The intrinsic vessels are arranged in a longitudinal plexus made from the vasa nervorum that run through the outer epineurium, inner epineurium, and perineurium. A series of endoneurial vessels that supply the axons themselves is also present. These endoneurial capillaries have epithelial tight junctions that function as the second part of the blood-nerve barrier.

The longitudinal arrangement of the vessels within the nerve may allow the surgeon to perform a fair bit of dissection along the course of the nerve without compromising vascular integrity to the point of ischemia. However, care must be taken to not injure any major nutrient vessels that are associated with the nerve.[54, 39, 31, 46]

Topography

Initially, when Sunderland looked at the fascicular patterns of peripheral nerves in 1947,[56] he described a pattern of twisting and crossing fascicles that branched so often that nerve grafting and intraneural dissection were assumed to be impossible. Fortunately, the pattern described by Sunderland is only true for the proximal part of the nerve. As described by Jabaley in 1980,[51] the fascicular pattern in the distal forearm is much straighter, with less crossing over. Apparently, the crossing over in the proximal portion sorts the nerve fibers into bundles by function. This means that by the time the distal forearm is reached, the fascicles contain nearly pure motor or sensory axons.[4, 14] While the arrangement varies by individual and by nerve, generally, the sensory fascicles are considered to sit more superficially and the motor fibers more dorsal.[57, 9, 58]

Median nerve

The median nerve originates in the brachial plexus as branches from the lateral and medial cords come together.[59] These cords bring fibers from all roots of the brachial plexus, from C5 to T1. The median nerve runs through the anteromedial compartment, through the cubital fossa just medial to the brachial artery, and enters the forearm between the heads of the pronator teres.[4, 52] The median nerve supplies most of the flexor muscles in the forearm and a few muscles in the hand (see Motor targets of the median nerve). Once in the forearm, the median nerve splits into a superficial and deep branch. The superficial branch supplies the pronator teres, flexor carpi radialis, and palmaris longus before the distal portion supplies the flexor digitorum superficialis.

The deep trunk runs medially down the forearm deep to the muscles and supplies motor innervation to the flexor pollicis longus, pronator quadratus, and the portions of flexor digitorum profundus (FDP) that flex the index and long fingers and sends articulate branches to the radial carpal joint via the anterior interosseous. At the wrist, the deep median branch sits between the palmaris longus and flexor pollicis longus, travels through the carpal tunnel with the flexor tendons,[60] and splits into 6 branches once clear of the flexor retinaculum. The recurrent branch of the median nerve innervates the palmaris brevis and the muscles of the thenar eminence, namely the abductor pollicis brevis, opponens pollicis, the first and second lumbricals, and the superficial head of the flexor pollicis brevis.

The remaining branches are sensory and include 3 common digital nerves to the second, third, and fourth digits and 2 proper digital nerves to the thumb. These digital nerves run along the lateral (radial) and medial (ulnar) sides of the fingers with the digital arteries. The median nerve has 2 other sensory branches not yet mentioned that supply the elbow and a palmar cutaneous branch that passes over the top of the flexor retinaculum to innervate the palm.[61, 62]

Motor targets of the median nerve are as follows:

• Pronator teres

• Flexor carpi radialis

• Palmaris longus

• Flexor digitorum superficialis

• Flexor digitorum profundus

• Flexor pollicis longus

• Pronator quadratus

• Palmaris brevis

• Abductor pollicis brevis

• Flexor pollicis brevis - Superficial head

• Opponens pollicis

• First and second lumbricals

Some notable variations in the pattern of innervation exist in the median distribution. In the forearm, the median nerve supplies approximately one half of the FDP muscle, sharing it with the ulnar nerve. However, in 50% of patients, the median and ulnar nerves overlap and the median nerve encroaches on the ulnar innervation to the FDP. However, in the hand, the ulnar nerve tends to extend across the palm and innervate a larger portion of the thenar eminence.[31]

Another clinically important variation is the presence of Martin-Gruber anastomoses in the forearm and Riche-Cannieu anastomoses in the palm. Martin-Gruber anastomoses are believed to be present in approximately 17% of patients, while Riche-Cannieu anastomoses may be present in as many as 70%. Because these anastomoses allow unique patterns of innervation, they can mask lesions by changing the symptoms present.[63, 31]

Ulnar nerve

The ulnar nerve arises from the medial cord and contains fibers from the C7, C8, and T1 roots.[59] It passes through the arm, behind the medial epicondyle, and into the flexor compartment (see Motor targets of the ulnar nerve). In the forearm, the ulnar nerve gives off motor branches to the flexor carpi ulnaris and the medial (ulnar) portion of FDP that supplies the ring and little fingers.[52] Usually, the ulnar nerve gives a small branch to the ulnar artery, known as the nerve of Henle, which is present in approximately 60% of people. The remaining 40% of people have a palmar cutaneous branch.[64] At the level of the wrist, the ulnar nerve passes through the Guyon canal, right next to the hook of the hamate.

In the hand, the ulnar nerve divides into a deep motor and a superficial sensory branch. The deep motor branch supplies all of the intrinsic muscles of the hand: the muscles of the hypothenar eminence, the interossei, the third and fourth lumbricals, adductor pollicis, and the deep head of the flexor pollicis brevis. The superficial sensory nerve supplies sensation to the little finger and the ulnar side of the ring finger. The ulnar nerve also supplies sensation to part of the dorsum of the hand via the dorsal sensory branch that wraps around to the dorsum near the level of the dorsal carpal ligament. This provides sensation in the same distribution as on the volar surface.[61, 31]

Motor targets of the ulnar nerve are as follows:

• Flexor carpi ulnaris

• Flexor digitorum profundus

• Hypothenar muscles

• All interossei[65]

• Third and fourth lumbricals

• Adductor pollicis

• Flexor pollicis brevis - Deep head

Radial nerve

The radial nerve is a branch off the posterior cord and contains fibers from roots C7-T1.[52, 66] This nerve wraps around the humerus in the spiral groove as it passes through the upper arm, supplying motor innervation to all 3 heads of the triceps muscle, the anconeus, the brachioradialis, and a small part of the brachialis muscle before entering the cubital fossa lateral to the biceps tendon. In the forearm, the nerve supplies motor innervation to the extensor carpi radialis longus, extensor carpi radialis brevis, and the supinator before splitting into deep and superficial branches (see Motor targets of the radial nerve).

The deep branch provides motor innervation to the muscles of finger and thumb extension before becoming the posterior interosseous nerve and supplying sensation to the dorsal aspect of the carpal joints. The superficial sensory branch passes through the anatomic snuffbox on its way to supply sensation to the dorsum of the hand, thumb, and first 2.5 fingers. The radial nerve also provides sensation to a portion of the forearm via the posterior cutaneous nerve and to the elbow.[61, 31]

Motor targets of the radial nerve are as follows:

• Triceps (long, medial, lateral)

• Anconeus

• Brachioradialis

• Extensor carpi radialis longus

• Extensor carpi radialis brevis

• Supinator

• Extensor digitorum communis

• Extensor indicis proprius

• Extensor digiti minimi quinti

• Extensor carpi ulnaris

• Abductor pollicis longus

• Extensor pollicis longus

• Extensor pollicis brevis

Nerve repair has been described in virtually all clinical settings. However, experience and common sense dictate the following list of contraindications:

• Medically unstable patient from other injuries and/or illnesses

• Presence of a grossly contaminated wound bed

• Active soft tissue infection in the region of the nerve injury

• Severely compromised nutrition

• Patient unable and/or unwilling to comply with required activity restrictions

• Patient with unrealistic expectations

• Presence of underlying skeletal instability

• Uncertain delineation of zone of injury (requires waiting period)

Serum albumin test

The reason for ordering this test is to determine if the nutritional status of the patient is adequate to allow reconstructive surgery. If the values returned are less than normal, healing may be impaired and surgical repair compromised.

Generally, proceeding is acceptable when albumin levels are greater than or equal to 2.5 g/dL.

Short tau inversion recovery MRI

This is a unique form of magnetic resonance imaging (MRI) that returns pictures that can highlight nerve trauma, although it is less sensitive than other diagnostic tests for determining the degree and location of injury.

The short tau inversion recovery MRI works by placing a phased array of surface coils on the body, thus allowing imaging of a smaller area with a more precise picture.

Both T1- and T2-weighted images can be used to view nerve pathology, which alters the nerve signal from its normally isodense state. T2-weighted images allow the hyperintense signal of nerve injury to be more apparent because of the ability to suppress the fat signal.

This type of test is indicated in patients with nonclassic symptoms, in those with ambiguous findings such as occur in patients with diabetes, for scar and nerve inflammation imaging after the addition of gadolinium, to visualize neuromas, and to determine the length of damaged nerve present.

This technology is not yet consistent enough to play a major role in diagnosis, but it can serve as a useful adjunct to other tests.[67, 13, 53]

Ultrasonography

A study by Endo et al indicated that the reliability of short-axis ultrasonography for assessing digital nerve injuries of the hand is greater than previous research had suggested, while long-axis ultrasonography has reduced efficacy in such evaluations. The report found that of laceration images obtained with long-axis imaging, the injury was demonstrated on 10 images and absent on four. However, all 20 lacerations assessed with short-axis ultrasonography appeared on the image.[68]

Sensory tests[4, 69, 70, 71]

• Tinel test: The patient should feel a sensation when the tip of the severed nerve is tapped.

• Two-point discrimination: Both moving (M2PD) and static (S2PD) are performed with the patient's eyes closed. Results should be consistent enough to exclude the patient guessing. This examination should be very thorough if a partial injury is suggested. S2PD is excellent if less than or equal to 6 mm, and M2PD is considered excellent if less than or equal to 3 mm. A lack of 2PD is poor.

• Semmes-Weinstein test: This uses monofilaments, which allow the application of stimuli using constant force so that administration error is reduced. Findings should be similar to those from S2PD and M2PD.

Motor tests[4, 70, 71]

• Grip and pinch strength: This is usually tested by having the patient squeeze a measuring device that can determine the amount of force the patient produces. Results may be compared to the unaffected hand to grade the deficiency. Testing intrinsic and extrinsic muscles separately can be helpful.

• Muscle bulk: This test looks for atrophy of large muscles such as the first interosseous muscle.

Williams test: Denervated skin responds differently to stimuli. This allows clear demarcation of the affected area in the hand when the injured hand is placed in water. Innervated skin wrinkles when submerged, but denervated skin does not.

Electrodiagnostic techniques have been refined and help a great deal in testing and diagnosing neuromuscular lesions. The 2 most common tests are the nerve conduction study (NCS) or nerve conduction velocity test and electromyography (EMG). EMG is often understood to refer to both tests.[72, 13]

Table 4. Electrodiagnostic Characteristics of Nerve Injury (Open Table in a new window)

 

NCSs use a percutaneous current to stimulate muscle and sensory nerves. This can result in a compound motor action potential or sensory nerve action potential, respectively. An active pickup generates the current that travels along the nerve and is read by another pickup a set distance away in either the efferent or afferent directions. This measures how fast the nerve conducted the stimulus (v = d/t). The current is gradually increased until a maximum response is obtained. This provides the basis for comparison of results over time. The distal ends of transected nerves have an NCS result that gradually falls over 7-9 days postinjury. This test can also help detect complete and partial nerve blocks.[72, 73, 13]

EMG tests record the depolarization potentials of active and spontaneous muscle movement. This is recorded as a muscle unit action potential. Resting muscle is normally electrically silent. Muscle distal to an injury may appear normal for several days after the injury until wallerian degeneration has advanced far enough for the muscle to become denervated and fibrillations to start. This often requires 14-21 days; thus, the best time for EMG diagnosis is 3-4 weeks after injury. Denervated muscle displays positive sharp waves and fibrillations, in that order.[72, 73, 74, 12]

If surgery is not an option, the best choice for patients is supportive therapy to maintain range of motion and muscle tone. This involves 2 main procedures: (1) immobilization to prevent deforming contractures and (2) massage and exercises to maintain blood flow and free movement.[15]

The goal of surgical nerve repair is axonal growth through a repaired or replaced conduit. If no conduit is present, nerve regeneration fails in favor of neuroma creation. Many different techniques have been developed to facilitate repair of the conduits, including approximation, nerve grafting, alternate conduits, and nerve transfer.[75]

End-to-end closure

End-to-end closure involves the approximation of the transected nerve's free ends. This is the simplest repair procedure and usually has the best prognosis. End-to-end closure is indicated in patients in whom the nerve gap is less than 2 cm in length and/or the nerve can be repaired without tension. This defect is closed most often with epineurial or perineurial/fascicular sutures, which is discussed in Closure of defect.[4, 39, 31, 14]

A study by Fakin et al indicated that in epineurial coaptation of digital nerves in adults, the level of success at long-term follow-up with regard to sensory outcome is related only to the surgeon’s level of experience and not to such factors as the patient’s age, whether or not the patient smokes, the time of immobilization, the mechanism of injury, or anastomosis of a digital artery. The study involved 93 coapted digital nerves, with mean follow-up being 3.5 years.[76]

Nerve grafting

This form of repair is ideally suited for bridging the gap between 2 ends that cannot be easily approximated. For many years, the popularity of short nerve grafts and the resulting poor patient outcomes were responsible for the avoidance of this technique. However, a nerve graft, regardless of length, if performed in a tensionless manner, has been shown to generally have better results than an end-to-end approximation performed under tension.[77, 10, 26, 78]

A retrospective study by Felder et al indicated that in patients with severe ulnar neuropathy, sensory recovery can be enhanced via adjunct treatment with cross-palm nerve grafting, either through allograft or autograft. In the study, cross-palm, side-to-side, tension-free grafting was performed, with the grafts extending from the median to the ulnar nerve. The investigators found that protective sensation returned in 21 out 24 patients (87.5%); diminished light touch sensation was restored in 16 patients (66.7%), and normal range sensation returned in six patients (25.0%).[79]

In general, harvesting a nerve graft to repair a defect creates a secondary defect in a different location (see Table 5). For this reason, good donor nerves provide noncritical sensation, and their innervation pattern loss does not leave a body part unprotected. Nerves that meet this requirement include small cutaneous nerves, which lead to a minimal loss in sensation, and nerves that have been severed concomitantly, as in a polytrauma or amputation. Harvest of the graft can be performed in a variety of ways, but removing the graft in a manner that leaves the remaining proximal end buried, thus minimizing possible complications from neuromas at the donor site, is beneficial.[19, 9]

Table 5. Common Donor Sites (Open Table in a new window)

Many different types of grafting have been performed, but very few have worked well. One method used a trunk graft, or a section of a large nerve, that bridged the gap completely without any other grafts. This was similar to the cable grafts developed slightly later, in which smaller nerves were harvested and glued together to create a single large graft. Both of these methods met with little success, largely because the grafts were too thick and their centers took too long to become vascularized, which is a requirement to provide the energy needed for degeneration and reinnervation. Trunk grafting is still used occasionally in situations in which other options have been exhausted, but some have suggested that the graft be cut prior to grafting. This form of delay allows the metabolically active degenerative process to occur in a vascularized bed, before the nerve is free grafted; the idea being that this helps prevent the center and distal ends of the graft from becoming fibrotic.[19]

The use of small cutaneous nerves for grafts minimizes the morbidity of the harvest and has allowed for much better results than using an interfascicular graft. Multiple sections of the donor nerve are used to bridge the gap and are attached between similar fascicles, independent of the other graft pieces. This technique attempts to return some semblance of fascicular continuity. Maintaining the donor nerve vasculature in a free graft form is also possible.

Harvesting the dominant pedicle to the graft and attaching it appropriately in the donor site can allow the graft to survive in suboptimal conditions, such as scar beds, or can accelerate the growth in proximal injuries.[19]

Axons have been demonstrated to preferentially innervate endoneurial tubes of similar function. However, that preference is nonspecific for location. Also, the discovery has been made that over time, regenerating nerves disregard an existing fascicular pattern and progress down extrafascicular paths or create a new fascicle. This means that even in situations in which the fascicles match up, the potential for disastrous disorganization upon reinnervation exists. Thus, take extreme care to align and repair the fascicular patterns because a misalignment can result in a defect of reinnervation. Interestingly, this disorganization can be somewhat overcome through sensory reeducation.

A study by Rinker et al indicated that short-gap digital nerve injuries (ie, 5-15 mm) can be effectively repaired with processed nerve allografts (PNAs). The study, which included 24 patients (37 repairs), found that in 92% of repairs, patients achieved sensory recovery scores of S3-S4 on the Medical Research Council Classification scale.[80]

Conduits

Nonneural tissue and synthetic substances have been used as alternatives for nerve grafts for the last century, although with only marginal success until the 1980s. Since that time, some success has been achieved with arteries, veins, and silicon or bioabsorbable conduits. These have been used with a fair bit of success to bridge gaps less than 3 cm, with good-to-excellent sensory recovery. The use of these tubes is believed to create a microenvironment for the various chemical factors that regulate nerve regeneration, thus enhancing the process.[37, 81, 39, 82, 49, 83]

A prospective, randomized study compared the sensory recovery, cost, and complications of digital nerve repair using autogenous vein and polyglycolic acid conduits. The results concluded that although the cost and sensory recovery after autogenous vein conduit was similar to that of the polyglycolic acid conduit, fewer postoperative complications were found after the autogenous vein conduit technique.[84]

A study by Huber et al comparing end-to-end suture with the implantation of a collagen conduit for the repair of digital nerve injury found that greater functional impairment was reported by the conduit patients at follow-up (6-36 mo). With regard to both procedures, sensory function by the C and A-delta fibers was found to have largely recovered, while mechanical detection thresholds increased due to severe A-beta–fiber function loss.[85]

A study by Isaacs et al indicated that for long-gap acute or subacute digital nerve injuries (15-25 mm), repair with PNAs is more effective than with conduits. For study patients, better average static two-point discrimination was achieved with PNA repairs, at 6.1 mm, than for conduit procedures, at 7.5 mm. Moreover, 40% of the PNA surgeries produced normal sensation (as measured using the American Society for Surgery of the Hand scale), as opposed to 18% of the conduit repairs. In addition, more clinical failures (in which patients lacked protective sensation) were associated with long-gap conduits than with conduits used to repair short gaps (5-14 mm). Results for PNAs and conduits were comparable for short-gap repairs in terms of average static two-point discrimination (7.3 vs 7.5 mm, respectively).[86, 87]

Nerve transfers

This form of nerve repair transfers damaged or minor motor pathways and uses them to reinnervate major pathways, with the hope of restoring some function. This is indicated in severe polyneuropathy and is usually performed only if nerve grafts are unfeasible, the injury is extremely proximal compared to essential end organs, or muscle and tendon transfers have been exhausted or are unavailable. This procedure is not used often in the upper extremities and has primarily been described for cranial nerve injuries.[88]

The results of one study conclude that a reconstruction method of very distal nerve transfer of radial nerve branches to palmar nerves at the level of the proximal phalanx is useful in high median nerve lesions and can restore fingertip sensation. However, very distal nerve transfers might be used in addition to nerve grafting in older patients or in patients with chronic lesions of the median nerve at the wrist.[89]

Preoperative planning is important and includes the diagnostic tests (described in Diagnostic Procedures) that allow localization of the injury. This is important because the grade of injury plays a major role in deciding whether surgery is indicated. Injuries of first to third degree are likely to heal well on their own, without intervention. However, fourth- and fifth-degree injuries need intervention, and the sooner surgery is performed, the better.

Another consideration that becomes apparent based on the patient's history is the mechanism of injury. Sharp transections, because of their minimal tissue damage, can be operated on immediately without worrying whether neural and other tissue close to the wound will die. This certainty does not extend to injuries with a crush component, and these injuries must be handled differently by waiting a few weeks for the degeneration of neural tissue to reach its maximum levels so that healthy nerve tissue can be reached by dissection and repaired by nerve graft.

The issue of how long to wait, ie, whether to perform an immediately primary closure, wait a week, and perform a delayed primary closure or wait more than 2 weeks and perform a secondary closure, has been debated for some time. In general, better results have been achieved following earlier repair, but the conditions that allow for primary repair are fairly stringent. This is discussed in Indications and is one of the most important factors to consider prior to surgery.

Working with patient issues is also extremely important before surgery. During the history and workup, estimate patient compliance. By nature, repairs of peripheral nerves are delicate and can be destroyed within days of the surgery in a noncompliant patient.[90, 91, 92] Because nerve repair is handled as elective surgery, a noncompliant patient may be a candidate for education prior to surgery. If the patient's dominant hand is injured, he or she may need reeducation for the opposite hand to cope during and after surgery and during recovery. Patients also need to be informed, realistically, of the expected outcome. While the results are improving, very few patients recover normal or close-to-normal function. This plays a big role in patient satisfaction later.

Finally, knowing what kinds of activities patients enjoy, their occupation, and their avocations is important because in some patients, this information may be involved in the type of repair and nerve graft site selection. For instance, harvesting the sural nerve is not wise in construction workers who are on their feet in dangerous situations.

Incision

To repair the nerve under optimal conditions, making a large enough incision to facilitate exposure of the proximal and distal ends of the injured nerve is best. Take care to preserve the vascular attachments when exposing the nerves, if possible. Also, because the entry incision inevitably produces a scar that is detrimental to good nerve function, make the incision away from the portion of the wound maintaining the perfusion of the nerve bed.[10, 19, 93]

Repair

The amount of tension on a site of repair or graft is the single largest determinant of success. A repair site under tension has an increased amount of scar tissue and fibrosis and a reduced cross-sectional area, which blocks axonal growth. This is observed in the postoperative period when an advancing Tinel sign stops at or just distal to the repair site. Increased tension on the nerve stretches it out, decreases the cross-sectional area, and increases the pressure within the nerve. With higher pressure, less blood passes through the intrinsic vessels of the nerve. While the peripheral nerves are fairly resistant to ischemia, if the tension is not relieved, ischemic nerve damage and internal fibrosis can occur.[77, 94, 26, 95]

Since the detrimental effect of tension on the site of union was recognized, various strategies have been proposed to lengthen the nerve or shorten the distance between the ends so that the tension is reduced. The most successful and most commonly used method is nerve grafting. A section of nerve can be harvested and placed between 2 severed ends, allowing union without tension. Because excess graft length is typically easy to obtain, current recommendations include harvesting a donor segment at least 15% longer than the nerve gap site.[96, 78]

Other techniques that can be used in extreme or isolated situations include mobilization, bone shortening, and joint repositioning. Mobilization is fairly useful in patients in whom the injured nerve lies outside the center of rotation of a joint, such as the ulnar nerve at the elbow. By transposing the nerve inside the point of rotation, the amount of needed excursion is reduced and the length of nerve is increased. Bone shortening is an extreme method of gaining nerve length and is indicated only in the upper extremity when a humoral osteotomy is also indicated.

Because of the large amount of excursion possible with nerves, joint repositioning can make available significant nerve length. Extensive studies have been performed on cadavers to determine how many millimeters of nerve length flexing and extending various joints can gain. Large amounts of length can be gained with 90° flexion at the elbow and 40° at the wrist. The increase in nerve length is beneficial for a tensionless repair, but the gradual return to full extension once the nerve has started healing inflicts a series of traction injuries. This reduces the number and effect of regenerating axons; thus, the results are inferior to a tensionless nerve graft that allows for maximum extension.[50]

Closure of defect

Surgeons have several options to close the nerve defect. Epineurial closure involves placing several nonabsorbable sutures in the epineurium of the free nerve ends. Use a fairly small suture, 8-0 to 10-0, so that the amount of tissue displaced within the nerve is minimal, as is scar formation from the presence of a foreign body. Place the sutures such that the nerve ends are approximated, but not under enough tension to strangulate the tissue. Excess tension overcomes the capillary perfusion pressure; thus, the nerve is rendered ischemic. The number of sutures required to obtain closure varies depending on the size of the nerve and the size of the gap. Triangulation closure provides the most accurate alignment. This method of closure is recommended for distal injuries in which the nerve fascicles generally have been sorted into discrete sensory and motor functions.[97, 98, 81, 31, 14]

Perineurial closure is involved with fascicular repair. In this method, the outer epineurium is pulled back, exposing the individual fascicles of axons. Care is taken to approximate fascicles of the same function in end-to-end closure or to connect fascicles of similar function in aimed graft repair. The fascicles are then sutured together with a single 10-0 suture placed through the perineurium. If rotation occurs so that the fascicle is no longer lined up, a second suture may be required. Starting at the deepest part of the wound and working toward the surface in a one-way-up anastomotic technique is a good idea.

This type of repair is recommended for more proximal injuries that are not yet well sorted. A variation on this type of repair is the grouped fascicular, in which a number of fascicles are arranged tightly. In this situation, a few sutures are used to approximate and close the whole group.[10, 99, 100, 97, 101, 4, 31, 14]

Closure of wound

Once the nerve is repaired, maximizing the chances for the repair to heal and be functional is important. This can be aided by attempts to reduce the amount of scar tissue and leave the repaired nerve in a well-vascularized bed, away from the incision. Anything that reduces the number of adhesions and lets the repaired nerve regain its ability to glide improves eventual function. In some cases of repair, in which a large amount of trauma and swelling is present, performing adjacent tissue transfers may be necessary to ensure that the repaired nerve is not under pressure by tightly closed soft tissue.[10]

Following surgery, immobilizing the limb is imperative to protect the repair site. Follow basic guidelines for safe splint positioning. This prevents the denervated muscles from remaining in an overstretched position, prevents disfiguring joint contractures, prevents the patient from adapting to the injury, and eventually allows maximum use of the hand. For the first week to 10 days, immobilize the limb completely to prevent disruption of the repaired union. In the second postoperative week, therapy to maintain the range of motion, muscle tone, and blood supply to the area can be started.

Even though the site of repair tolerates movement, the continuing healing process and the sensory deficits warrant a protective splint for several weeks. By the third or fourth week, removing the splint and relying on the patient (after a warning about the dangers of excessive excursion) to slowly increase the amount of movement and function in the affected limb is generally safe.[102, 103, 30, 39]

Following surgery, continuing care for the patient and establishing a therapy routine are important. Continued care and therapy involve 2 main aspects. The continued care aspect is tracking the regenerating axons, using EMG/NCS testing and the Tinel sign, to their successful reinnervation of muscle and sensory end organs. Close follow-up care is essential to detect the arrest of axon regeneration for any reason, especially near the repair, because a second operation may be required to resect scarred or other unfavorable tissue and reconstruct the nerve a second time.

The therapy aspect of follow-up care is essential for functional return. Axon regrowth is only part of the process that must occur for a good functional result because reinnervation of sensory organs has been shown to not correlate with return of function.[104] This finding, when integrated with the improved results in children, suggests that a large central component to nerve repair exists. Several studies have demonstrated that sensory reeducation protocols greatly increase the functional outcome and that the return of function is accompanied by cortical reorganization.[11, 105, 106, 107] In essence, a new part of the brain must figure out what it is like to feel, which is why sensory-rich environments can help speed the process. In addition to the sensory reeducation, hand therapists work with strength and coordination function so that the hand is also functional.[19, 108, 109, 39, 31]

Complications are a concern in every surgery, although being prepared can help minimize the resulting consequences. Additionally, knowing what kinds of situations increase the risk of complications can help decrease the postsurgery complication rate simply by avoiding high-risk procedures.[110, 9]

Potential complications are as follows:

• Hematoma of large or small vessels

• Failure of sutures or grafted structures

• Traction due to adhesions or repair using joint repositioning

• Fibrosis of the nerve or nerve graft if no profusion occurs for long periods (This is especially common if the repair is placed under tension or in an unvascularized bed.)

• Neuroma formation following incomplete repair

  Hand nerve injury repair. Posttraumatic neuroma.

  Hand nerve injury repair. Excision of posttraumatic neuroma.

• Entrapment of the nerve following the repair due to excessive scar tissue formation or pressure from associated wounds

Factors that affect the results of nerve repairs are varied, and only a very few of them rely on physician proficiency. Despite this, awareness of these factors is beneficial to enable the patient and doctor to have realistic expectations of the outcome.[4, 110, 39, 31, 71]

Timing

Better results have been demonstrated when the repair is performed sooner. Good results can be expected if the repair is performed before the 6-month mark. Muscle neuromuscular junctions are usually absent after 15-18 months of inactivity; thus, sooner reinnervation is better.

Patient age

Young patients tend to achieve much better results than older patients, probably due to the elasticity of the CNS and the increased ability to heal.

A literature review by Dunlop et al did not find strong evidence to support the surgical repair of injured digital nerves in adults as opposed to a strategy of no repair. Protective sensation was recovered by most patients in the groups that did or did not undergo repair. Although static two-point discrimination tended to be better in association with nerve repair versus nonrepair, normal levels of sensibility were attained in less than 25% of the operated nerves. Both groups had similar adverse outcomes.[111]

Condition of the wound

A clean, small, noninfected wound fares much better than a wound full of debris or infection or one that is missing a great deal of tissue.

Level of injury

The chances of regeneration are better for injuries that are more distal. Proximal injuries are less likely to regenerate because the distance required for reinnervation is much greater and the percentage of cells lost in the distal portion of the nerve is greater. Also, if the proximal injury is a traction-type injury, it can pull the cell body out of the anterior horn, resulting in neuron death and no regeneration.

Tension of the repair

Tension tends to increase the fibrotic and scar tissue found within the nerve and the repair site, hindering proper axon regrowth.

Gap size

Gap size is different from the amount of tissue loss. The gap size is simply the distance between ends that must be closed. If the nerve is left in the severed condition, it retracts over time, increasing the gap size due to the elastic properties of the epineurium. When the gap size is larger, more of a bridge must be filled and the prognosis is worse.

Mechanism

The manner in which the nerve was injured is also involved in healing, largely because it affects so many of the other categories. From a surgical point of view, a sharp laceration, such as a knife wound, has a good prognosis, can be closed early, and should do fairly well.

Traction injuries, if mild, undergo demyelination. If the stretch is more severe, the axon and endoneurium can rupture, causing a second- to third-degree injury. These usually have a fairly good prognosis.

A wound caused by a large crush component can destroy long stretches of nerve and surrounding tissue. Usually, waiting and observing how much tissue dies is best so that viable nerve ends can be joined, which results in a much better prognosis.

The worst kinds of wounds are those created by projectiles. The rapid changes in momentum tend to damage a great deal of tissue and can leave debris in the wound. These types of injuries usually result in axonotmesis, although spontaneous recovery is possible.

While the factors affecting prognosis seem rather intuitive, communicating a patient's recovery status can be much more difficult. In an effort to create a standard scale for measuring the outcomes of surgery, British physicians created scales for grading sensory and motor function.[31, 71] These have been and remain widely used in clinical practice. Unfortunately, these grades are very subjective, which makes using this system for comparison between institutions difficult to impossible. Other scales have been introduced, such as the Semmes-Weinstein monofilament testing, but they have not been used widely in the literature as a measure of outcome.

Aside from the difficulty in measuring and grading the regenerative outcome of the surgery, patient satisfaction is an extremely important variable that is very difficult to measure. Even if a good-to-excellent functional result is achieved, the patient may be wholly unsatisfied with the result. Patient satisfaction can be improved through upfront communication early in the process about what to expect.

Currently, a large amount of research is investigating the molecular science of nerve growth and the chemical cascades that take place during regeneration. As these mechanisms are elucidated and more is known about the regenerative process, better methods of repair can be determined.