Nerve and Vessel Injury at the Hand
Normal Wrist Anatomy and biomechanics
The wrist is a biomechanically complex joint allowing the wrist to move in extension (up), flexion (down), radial deviation (towards the thumb), ulnar deviation (towards the small finger) and minimal degrees of rotation. Functionally the wrist’s primary role, like the shoulder and elbow, is to position the hand in space. This accurate positioning, allows the hand to perform a vast array of activities that range from skillful manipulation of fine objects to handling heavy objects, including supporting the entire body weight.
To understand the wrist we arbitrarily divide it into 3 layers:
- First layer corresponds to the bones
- Second layer corresponds to the capsule and ligaments
- Third layer corresponds to the remaining soft tissues (tendons, muscles, nerves, vessels)
These three layers are covered by the skin which we will discuss below.
The first layer or bonylayer begins at the metacarpal bases distally (finger side), and extends towards the very end of the forearm bones proximally (elbow side). The bones comprising the wrist include the 5 metacarpal bases, 8 carpal bones (wrist bones) which are organized in 2 rows and the distal ends of the radius and ulna (forearm bones). From distal (finger side) to proximal (elbow side), these bones form the following joints:
The five carpo-metacarpal(CMC) joints, midcarpal joint, radiocarpal joint, ulnocarpal jointand thedistal radio-ulnar joint (DRUJ).
The First CMC joint is at the base of the thumb and articulates with the trapezium, a carpal wrist bone, shaped like a horse’s saddle, that permits a wide range of motion including thumb rotation of 120 degrees, distinguishing humans from other mammals. The First Metacarpal and Trapezium are supported by a strong set of ligaments that surroundthe joint (second layer) allowing stable, thumb opposition against the rest of the fingers. The ability to oppose the thumb is responsible for 40 % of the function generated by the hand. The CMC joints of the 2nd and 3rd digits have minimal motion secondary to short, taut ligaments that surround the joint at the 2nd (index) and 3rd (middle) metacarpals bases and the distal end of the carpal bones including the Trapezium,Trapezoid and Hamate bones. This unit serves as the stable foundation around which the thumb, ring and small finger metacarpals move around. The CMC joints of the ring and small fingers are formed by 4th and 5th metacarpals, capitate and hamate wrist bones. These joints are more mobile and together with the 1st metacarpal create a gutter or cup configuration in the palm of the hand that facilitates the gripping of instruments and small objects.
The carpal bones comprise 8 wrist bones that function as a unit by virtue of having limited motion between each other. Only 7 of the 8 carpal (wrist) bones play a significant functional role in wrist stability and mobility. The 7 important bones are named starting at the distal row (finger side) and from the thumb side: 1- Trapezium, 2- Trapezoid, 3- Capitate, 4 Hamate, and in the proximal row starting on the thumb side the 5- Scaphoid, 6- Lunate and 7- Triquetrum. The Pisiform is the eighth bone but does not play a functional role, however, it can be a source of wrist pain when sustaining a fracture or as a consequence of degenerative arthritis in the pisotriquetral articulation. The carpal bones are almost entirely covered by cartilage and receive their limited blood supply through small vessel branches that enter the bones through little tunnels where the ligaments attach.
Just proximal to the carpal bones (elbow side) are the Radius and Ulna whose articular surfaces form a cup that complements the carpal bones. Together, they form the radiocarpal joint that serves to support the proximal carpal row in a functional ball and socket mechanism. The radius with a much larger articular surface to cradle the carpus, transfers about 80 % of the wrist load. The radius and ulna have a second joint that provides forearm rotation called the Distal Radio Ulnar Joint (DRUJ). The Triangular FibroCartilage Complex is a fibrous structure containing collagenous fibers (ligaments) embedded in a fibrocartilage matrix that complements the distal end of the radius and ulna to complete the cup configuration. The DRUJ provides a cushion between the carpus and the distal ulna and is considered the main stabilizer of the DRUJ. For full normal forearm rotation to occur, the DRUJ is complemented at the elbow by a joint called the proximal radio-ulnar joint (PRUJ).
The Second Layeris composed of the wrist ligaments, a complex intermingling of collagen fibers that join the metacarpals to the carpals and to the radius and ulna, creating stable, balanced wrist motion during our daily activities. These ligaments are taut structures embedded in a thin capsule that function to reinforce key areas of the wrist. Between the carpal bones, a deeper layer of ligaments, called the intra-carpal ligaments, allow a restricted amount of motion and is the reason that the carpal bones function as a unit. The shape and position of the carpal bones is critical for proper function and when injury occurs, it is of vital importance to re-establish the bone anatomic shapes if normal function is to be achieved.
The Third Layer is formed by the tendons, muscles, vessels and nerves.
The tendons about the wrist are divided into flexors (palmar side) and extensors (back hand side). The extensor tendons are in the back of the wrist and travel through 6 independent compartments having 9 of these tendons corresponding to finger motion and 3 tendons corresponding to wrist motion. Compartments 1 and 3 controls the thumb tendons, compartments 2 and 6 the wrist extensors and compartments 4 and 5 the finger extensors.
There are nine flexor tendons that travel through a fibro-osseous tunnel at the wrist called the carpal canal. It is formed by the carpal bones and the transverse carpal ligament. There are three tendons that flex the wrist but only one of these three, the flexor carpi radialis, travels in an independent wrist compartment in the carpal canal.
Radial Artery 
Ulnar Artery
The primary vessels or channels that supply blood to the hand while traveling across the wrist are the Radial and Ulnar Arteries. An arcade of smaller branches that interconnect the radial and ulnar arteriesare safety pathways that allow the continuation of blood flow if the radial and ulnar artery becomes occluded. The blood returns to the heart through the veins which are more prominent in the back of the hand and wrist. The major nerves about the wrist are the Median nerve that travels together with the 9 flexor tendons in the carpal tunnel, the Ulnar nerve that together with the ulnar artery travel through Guyon’s canal and the Radial nerve in the back of the forearm. Terminal smaller branches provide sensation to the superficial and deeper levels within the wrist.
Pathology
Nerves are cables that transmit impulses from the brain and spinal cord that give specific orders to the various body parts. There is a spectrum of nerve injuries depending on the mechanism of injury that have been classified into five grades according to the severity by Dr Sunderland. Grades I and II are minor injuries that will recover function within weeks of the injury without the need for surgery. Grades IV and V represent a more severe injuries that requires surgical intervention for function to be recovered. Grade III has variable results and often requires surgery with a careful evaluation intraoperatively for the various treatment options.
Once a nerve is cut and subsequently repaired, the distal portion of the nerve dies, leaving an empty tube where the regenerating nerve re-grows from the point where the injury occurredto the end of the final target organ. In addition, if the final target organ is a muscle, there is only a certain amount of time available for the nerve to reach the target organ and provide function before it is too late. If this does not happen within an average of six months then the muscles atrophy permanently. Sensory nerves have a more prolonged time frame to reinervate and have the terminal sensory organs regain full sensory function that can be a few years.
Vessels are the pipes where the blood flows through. Their purpose is to supply blood that carries oxygen and other nutrients to the multiple tissues in the body. Tissues survival depends directly on sufficient blood supply being delivered depending on the dynamic oxygen requirements of the various tissues. The vessels that carry the blood from the heart to the periphery are called arteries. The vessels that bring the blood back from the periphery to the heart are called veins. The vessels branch pretty much like a tree from the heart to the periphery. At the level of the wrist there are two main arteries that supply most of the blood for the hand. Two main veins exist that when injured or occluded, will have new veins regenerate as long as the hand has at least one sufficient vein for outflow. When a vein is injured, the blood flow is interrupted and the blood loss can be sufficient enough to risk a person’s life. If only one artery is injured, the other artery if normal has sufficient blood supply to keep the tissues alive in the hand. On occasion, one artery may not be sufficient to provide enough blood supply to the whole hand because of underlying disease and in this instance, ischemia and tissue necrosis with loss of digits will occur. When the two main arteries have been transected about the wrist, it is imperative to re-establish blood flow in a timely fashion to prevent tissue death.
Treatment
Nerve and Vessel (arteries and veins) lacerations about the hand require repair with microsurgical techniques that involve the use of magnification with a microscope, special instruments called jewelers instruments and very fine suture that cannot be handled by the naked eye. The use of a microscope results in accurate repairs and excellent functional outcomes. These techniques require a fellowship in special microsurgery training done in many hand surgery programs. Although a plastic surgeon has microsurgical training, they are not trained to fix or reconstruct complex injuries requiring, bone, tendon, nerve, artery, and vein repairs. This holds true for orthopedic surgeons and general surgeons.
A nerve repair will take approximately 4 weeks for the new axons (nerve fibers) to cross the line of repair and then continues advancing at a pace of approximately 1 mm a day. Therefore, if a nerve laceration that has occurred at 10 cm proximal to the final target organ, will take approximately 100 days plus 30 more days from crossing the repair line for a total of 130 days to reach the final target organ and regain full function. The outcome for nerve repair is influenced by technical factors including; a tension free repair, proper fascicular alignment, appropriate bed, age less than 35 years, health status and smoking habits amongst others.
A vessel repair or reconstruction can be done with several techniques; however, the principles of microvascular surgery should be followed. The average percentage of success in small vessels is 95- 97 %. Patients that have had a vessel repair may have to remain in the hospital for 5 days with blood thinners and other measures to help the vessel repair remain patent.
The nerves and vessels once repaired will require of initial protection in a splint followed by early, progressive and carefully supervised motion by a hand therapist. Therapy geared to nerve repairs will include other different modalities such as nerve glide exercises, desensitization, neuromuscular re-education, bio-feedback, functional electric stimulation, sensory re-education, strengthening and splinting as indicated.