Ring or Middle Finger Neurovascular Island Flaps for Thumb Reconstruction
The neurovascular island flap is an axial-pattern, regional flap that is transferred on a pedicle that contains the artery (common digital along with its digital artery proper extension), digital nerve, and accompanying veins, plus the surrounding fascia. By definition, an island flap does not have a skin component to the pedicle. Island flaps can be designed as composite flaps that contain a variety of tissues, such as bone, cartilage, or tendon. The simplest flaps are elevated as fasciocutaneous flaps. The island flaps provide a unique regional reconstructive option because the arc of rotation of the flap is not tethered by skin contained with the pedicle. The axial blood supply of an island flap allows for reliable transfer of a robust, large surface area compared with random-pattern flaps. The use of island flaps for resurfacing defects of the hands eliminates the need for staged reconstruction to elevate and delay flaps and, finally, divide flap pedicles, as in the case of random flaps. This long delay for flap transfer with random flaps can often result in significant stiffness of the digit or hand.
Island flaps can be classified as follows: by donor or recipient site; by whether arterial flow is anterograde or retrograde; and by composition of the pedicle.1
Classically, neurovascular island flaps are utilized to provide sensation to an insensate thumb by transferring a fasciocutaneous flap from the ulnar border of the middle or ring finger. These flaps offer more durable thick palmar skin coverage, compared with scar tissue, that cannot be achieved by a split-thickness skin graft.
Loss of thumb sensation. The basic function of the thumb is to provide sensate opposition to other digits.2 An insensate thumb can adversely affect hand function and compromise grip and pinch. The patient cannot determine how much pressure to apply and how to manipulate objects without visual feedback. Thumb reconstruction using other insensate flaps may result in recurrent skin breakdown and infections from undue stresses of pinch against skin without protective sensation.
In most cases of sharp laceration causing isolated sensory loss of the thumb, it is possible to repair the digital nerves of the thumb and regain sensory function. Neurovascular island transfer can be beneficial for extensive irreparable digital nerve damage and after revascularization of partial or complete thumb degloving injuries where neurorrhaphy is not possible.
Patients with complex, unreconstructable median nerve lesions can also benefit from the neurovascular island flap. In this case, the skin from the ulnar border of the ring finger can be used, relying on the ulnar-innervated skin to that digit.3
Staged reconstruction of the thumb: In case of a thumb degloving injury, a tubed pedicle groin flap is commonly used for resurfacing, with inadequate sensibility. Although free-toe to thumb transfer can be used to provide additional length and sensibility, some patients prefer to avoid losing a toe.
In some patients, thumb reconstruction takes place without sensory recovery because of avulsion of both digital nerves and the lack of donor nerves or recipient nerves available for the total thumb transfer.
Loss of the thumb pulp from degloving injury: Defects of ~ 1 × 2 cm in size can be treated with a local advancement flap, such as the Moberg advancement flap.4 Larger defects cannot be covered with local flaps. In this scenario, the key to provide sensation to the thumb is to use a neurovascular island flap rather than a cross-finger flap that will not provide any protective sensation.
Chronic scarring and unstable skin: In cases of extensive crush injury to the palmar aspect of the thumb and treatment with a split-thickness or full-thickness skin graft, the patient may have skin coverage, but no soft-tissue padding or sensibility. The neurovascular island flap can provide protective sensation and restore adequate padding for a durable, sensate thumb. This is important for fine manipulation of small objects, precision pinch, pulp pinch, key pinch, check pinch, and object discrimination by tactile gnosis.
Age range: There are no age restrictions. Very young children should be given the benefit of this flap, because of their brain plasticity, to determine how much protective sensation they can have from collateral sprouting before making the decision to perform a neurovascular island transfer. In the elderly with severe atherosclerosis and limited digital joint motion from degenerative joint disease, the surgery would pose a higher risk of further stiffness and for flap failure due to lack of perfusion.
Timing: The timing depends on the initial presentation. In the acute setting, with a severe degloving injury, the flap should be performed before tissue desiccation of exposed bone or tendons. This may be done within 24 to 48 hours; meanwhile, using dressings is preferable to keep the tissues moist until the transfer.
In planned composite osteocutaneous thumb reconstruction with a pedicle groin flap, the neurovascular island flap can be incorporated in the reconstructive part algorithm once the groin flap has been inset and is stable. We prefer to perform the neurovascular island transfer before defatting the flap, so that there is the maximum amount of tissue for adjusting the placement of the flap to minimize tension on the pedicle.
Time limits: Because the neurovascular island flap does not rely on reinnervation of the tissues, there is no time limit on when the procedure can be performed and still provide protective sensation for the patient′s thumb. Ideally, performing the surgery sooner following injury to the thumb allows the patient to have a better chance to relearn and incorporate the new sensory pattern into their functional activities. Children under the age of 5 may have enough brain plasticity to ultimately reorient the sensation as being from the thumb, but most adult patients still perceive the sensation to be coming from the transferred digit. Although this flap does not provide normal sensation to the thumb, there is enough of a protective sensation that when the patient touches a hot or sharp object, there will be enough of a withdrawal response to prevent full-thickness skin damage.
Smoking, from the author′s standpoint, is an absolute contraindication to the flap transfer. It is important that the patient stop smoking for at least 3 to 4 weeks prior to utilizing the neurovascular island flap. Smoking cessation aids, such as nicotine patches, are not helpful because it is the nicotine that is the harmful agent to the flap. Nicotine avoidance treatments are the most successful.
Patients with long-standing, uncontrolled diabetes have increased atherosclerosis and also can be at higher risk for flap ischemia and necrosis.
Connective Tissue Disorders and Vasospastic Diseases
These conditions, particularly severe Raynaud disease, can result in vessel spasticity that may compromise the vascularity transferred with the neurovascular island flap.
Lack of Normal Sensibility in the Donor Digit
Patients with severe median and ulnar nerve injuries have no donor finger for harvesting the neurovascular island flap. It is important for the patient to be cooperative and protect the digit following the transfer to allow for revascularization of the flap. The patient who cannot comply with a rehabilitation program may have stiffness in the donor digits and functional compromise.
Examination and Imaging
Patients with degloving injury of the thumb may have also other digits involved. It is important to ensure that the patient has intact radial and ulnar digital neurovascular bundles in the donor digit.
The patient should have normal sensibility with the ability to detect two-point discrimination (2PD) of 5 mm or less in the donor digit.
It is important to perform a digital Allen test in the donor digit, especially if it was previously injured. The examiner places pressure on both the radial and ulnar sides of the digit with one hand, and then “milks” the digit with another hand until there is blanching of the digit. Alternately one side and then the other can be released to determine whether there is satisfactory flow through both arteries. If this examination is not possible, a noninvasive Doppler examination of the digit can rule out the presence of abnormal flow in one of the digital arteries.
The neurovascular island flap is based on a pedicle containing the proper digital artery and nerve from one side of the donor digit, as well as the surrounding fat, which contains the fine draining veins from the flap. These are part of the venae comitantes that travel roughly parallel with the digital artery.
The arterial supply to the neurovascular island flap originates from the superficial palmar arch, which is formed by contributions from both the ulnar and radial arteries in most individuals. The ulnar artery enters the palm from the Guyon canal and gives off a deep branch that runs with the deep motor branch of the ulnar nerve beneath the pisohamate arcade5 to communicate with the deep palmar arch in the hand. Distal to the pisiform, the ulnar artery continues as the dominant contributor to the superficial palmar arterial arch. The superficial palmar arch lies deep to the palmar aponeurosis and runs superficial or palmar to the tendons of the flexor digitorum superficialis and the lumbrical muscles, as well as the common digital nerves.
The radial artery gives off a superficial branch at the level of the wrist before it continues dorsally along the floor of the first dorsal compartment, containing the extensor pollicis brevis and the abductor pollicis longus ten-dons. This superficial branch passes through the thenar muscles to join the superficial palmar arch in most individuals. In 40% of cases, the superficial palmar arch is supplied only by the ulnar artery without contribution from the radial artery. On the dorsum of the hand, the radial artery dives through the two heads of the first dorsal interosseous muscle and continues as the deep palmar arch. The princeps pollicis and the index radial digital arteries arise from the beginning of this deep arch. The princeps pollicis artery bifurcates deep to the flexor pollicis longus tendon, into the radial and ulnar proper digital arteries of the thumb. The deep palmar arch lies palmar to the palmar interosseous muscles and metacarpal bones. The deep arch is covered by the adductor pollicis muscle, the digital flexor tendons, and the lumbrical muscles.
Three common arch and pass digital arteries branch from the superficial palmar arch and pass toward the second, third, and fourth webspaces to supply the ulnar side of the index finger, the radial and ulnar sides of the middle and ring fingers, and the radial side of the little finger. Each digit has a radial and an ulnar proper digital artery. The common digital arteries bifurcate at the level of the proximal phalanx base to form the proper digital arteries that supply the adjacent digits on either side of the webspace. The ulnar proper digital artery to the little finger arises from the ulnar artery just proximal to the beginning of the superficial palmar arch. The radial digital artery to the index finger arises from the origin of the deep palmar arterial arch. Each digit has a series of transverse palmar arches between the radial and ulnar digital arteries that supply the joints and provide the vascularity to the vincula, which nourish the flexor ten-dons. The proper digital vessels form two or three terminal branches at the level of the distal interphalangeal joints as they arborize to supply the digital pad.
The digits have dorsal and palmar venous systems. The dorsal digital veins are larger than the palmar veins, and a series of interconnective venous arches in the subcutaneous tissue communicate with the large dorsal veins of the hand. Eaton in 1968 described the anatomy of the fine palmar veins that run through the loose areolar tissue surrounding the neurovascular bundle. His cadaver microdissections could not identify true venae comitantes to the digital arteries, but rather demonstrated a series of tortuous branching veins that ran throughout the fatty tissue lateral to the flexor tendon sheath. These fine draining veins should be incorporated into the neurovascular island flap pedicle by including a healthy cuff of fatty tissue surrounding the neurovascular bundle.
Sensory innervation of the neurovascular island flap is through the palmar digital branch of the median or, less often, the ulnar nerve. The median nerve has four branches at the distal edge of the flexor retinaculum: the thenar branch and the first through third common digital branches. The common digital nerves pass deep through the superficial palmar arch and superficial to the flexor tendons in the palm. The common digital nerves divide shortly after passing through the superficial palmar arch, well before the branching point of the common digital arteries. The first common digital nerve divides to become the radial digital nerve to the index finger and the radial and ulnar proper digital nerves to the thumb. The second common digital nerve becomes the ulnar digital nerve to the index finger and the radial digital nerve to the middle finger. The third common digital nerve divides into the proper ulnar digital nerve to the middle finger and the radial digital nerve to the ring finger in most individuals.
The ulnar nerve is located ulnar to the ulnar artery that pass through the Guyon canal. It splits into a deep motor branch and a superficial branch that gives rise to the ulnar digital nerve to the little finger and the common digital nerve to the fourth web space. There may be some communicating branches between the common digital nerves to the third and fourth web spaces spanning a zone along the ring metacarpal, with a potential for crossover between the ulnar and median nerve fascicles. In some individuals, the radial side of the ring finger and the ulnar side of the middle finger may have sensory innervation by the ulnar nerve, but this is quite rare. A dorsal digital nerve branch arises from the proper digital nerves just beyond the metacarpophalangeal joint. While the common digital nerves are deep to the common digital arteries in the three web space canals bordered by the septa of Legueu and Juvara,6 the proper digital nerves run palmar to the proper digital arteries. The digital neurovascular bundles run immediately volar to the Cleland′s ligaments. The digital artery and nerve may course 2 to 3 mm apart in the proximal half of the digit, but they are in close contact in the distal half of the digit.
It is important to have an understanding of the anatomical relationship between the neurovascular structures in the digit and palm during the planning and dissection for the neurovascular island flap.
Instruments and Supplies
Lidocaine without epinephrine, 2–5%, to prevent vessel spasm
Bipolar cautery and hemoclips
General anesthesia is used. This may be combined with an axillary block to provide for immediate postoperative pain relief and vasodilation and to avoid vasospasm.
Donor Site Selection
It is important to determine whether there has been prior median or ulnar nerve injury, as this will affect the donor site from the hand to ensure the use of a sensate flap. The site of the flap must be planned to provide a long enough pedicle to reach the recipient side of the thumb without any tension.
In general, the donor site is not selected from the radial side of the index finger or middle finger because these are key sensate areas that contact the thumb ( Figs. 8.1 and 8.2a,b ). The ulnar side of the little finger also is not used as a donor site because it has a key protective role, as this portion of the hand is frequently placed on a surface when positioning the hand for activities such as writing and typing. Therefore, the most common area for the neurovascular flap is the ulnar side of the ring finger or the ulnar side of the middle finger. The ulnar side of the ring finger is a preferred donor site in patients with poor thumb sensibility due to median nerve injury, because this skin paddle is innervated by the ulnar nerve. Conversely, the ulnar side of the middle finger is recommended if the median nerve is otherwise functioning, as this provides a slightly longer pedicle than the ring finger with a shorter reach to the thumb.
The donor digit must have normal sensory innervation with a robust blood supply from intact neurovascular bundles. To preserve vascularity to the ring finger in case of flap harvest for the ulnar side of the middle finger, both proper digital arteries to the ring finger must be intact. The radial proper digital artery to the ring finger will be divided at its origin during the dissection of the flap pedicle ( Fig. 8.1 ). A digital Allen test can be used to assess the status of the digital arteries on either side of the digit that will be involved in the flap harvest. A hand-held Doppler evaluation may be useful if the digital Allen test is equivocal. Digital arteriography is seldom indicated if there is concern about the patency of both digital arteries to either the donor digit or the adjacent digit based on the history and physical findings. A final assessment of the vessel patency and the adequacy of digital perfusion can be made during the dissection of the pedicle.
Recipient Site Preparation
If the neurovascular island flap is planned for the coverage of an existing thumb tip defect, the wound should be débrided to a clean, healthy base prior to flap transfer. A defect will need to be created in patients who have poor sensibility but who already have skin coverage. It is important to plan the coverage so that any tenuous and adherent scar tissue on the tip of the thumb can be replaced with the sensate flap. If the patient has an insensate thumb with stable skin coverage, the surgeon can then choose the best area for placement of the flap along the pulp of the thumb. The best location for flap placement is at the ulnar pulp of the thumb, with the most distal aspect of the flap positioned 3–4 mm proximal to the thumb tip.7 The area of tissue that is removed from the thumb should be smaller than the area that is planned for flap harvest to minimize tension when the flap is inset ( Fig. 8.2c ). The depth of the excision should include full-thickness skin, with preservation of any underlying septated fat to preserve pulp thickness. If full-thickness skin is removed from the recipient site, this tissue should be saved for use in the resurfacing of the secondary defect following flap transfer. Brunner incisions, which zigzag along the volar aspect of the thumb from the primary defect to the metacarpophalangeal flexion crease, help to avoid scar contracture, which can cause a flexion deformity of the joint ( Fig. 8.2c ). Painful neuromas within the thumb can be dissected out and buried within the thenar muscles, such as the adductor pollicis or abductor pollicis brevis. For accuracy with the flap harvest, a template of the final thumb defect can be created by trimming a sterile paper or a portion of the Esmarch bandage.