Syndactyly is webbing of the digits of the hands or toes. The condition can be complete, incomplete, simple, or complex. Syndactlyl usually is sporadic but may be associated with syndromes. Operative correction is performed in early childhood.
Syndactyly is one of the two most common congenital differences of the upper extremity, the other being polydactyly, occurring in approximately 1 in every 2,000 live births. Syndactyly is most commonly seen in isolation, though it can also be found in association with other anomalies such as Poland sequence, Apert syndrome, and the other acrocephalosyndactyly syndromes. In cases of isolated syndactyly, half of affected patients have bilateral involvement. Syndactyly is twice as common in males and there is a family history in 10 to 40% of cases, most often in an autosomal-dominant inheritance pattern with variable expression and incomplete penetrance. The feet are more frequently involved than the hands, though the majority of cases in the feet are simple, incomplete fusions of the second webspace, which often go unnoticed. In the hand, the third webspace is the most frequently affected (approximately 50%), followed by the fourth (30%), second (15%), and first (5%) webspaces. While syndactyly of the first webspace is uncommon, it tends to be associated with syndromes and part of a highly complex syndactyly of the hand.
Syndactyly is typically classified as either complete, in which the webbing extends to the tip of the digit and involves the nail complex, or incomplete, in which the webspace begins distal to the level of the normal commissure but does not extend to the tip (Fig. 35‑1). Distinction is next made on the basis of the tissues forming the interdigital connection; a simple syndactyly involves only skin and abnormal fibrous tissues, whereas a complex syndactyly includes abnormal osseous or cartilaginous connections. Finally, a syndactyly is termed complicated when the connections are more than simple side-to-side fusions. The Apert hand, central polysyndactyly, and typical cleft hand are examples featuring complicated syndactyly.
The etiology of syndactyly remains unclear. Simple syndactyly can be explained by the failure of programmed cell death within the webspace, which forms the basis of the traditional classification of syndactyly as a failure of differentiation under the original Swanson/IFSSH (International Federation of Societies for Surgery of the Hand) system. However, more complex forms of syndactyly lack a well-accepted cause and there is no current consensus on their appropriate classification. Recently, the IFSSH has proposed adopting the OMT (Oberg–Manske–Tonkin) classification, which categorizes syndactyly in the “abnormal axis formation/differentiation–hand plate” subcategory within the larger category of malformations.
The diagnosis of syndactyly is made by physical examination. The involved digits, the extent of the syndactyly, and the involvement of the nail bed should be noted. The distinction between a simple and complex syndactyly can often be made based on the relative motion of the adjacent digits. Examination should also be performed of the remainder of the upper extremity, the contralateral upper extremity, and the feet. Radiographs of the affected hand should be obtained in most cases, but are generally unnecessary in the case of simple syndactyly. Imaging by ultrasound, magnetic resonance imaging (MRI), or angiography may be helpful in the most complex cases to determine soft tissue and vascular anatomy, or to describe the bony configuration when bone overlap precludes accurate determination by plain radiographs. The authors do not perform routine angiography, MRI, or ultrasound, however. Further workup or referral to other specialists may be indicated in syndromic cases of syndactyly, though these patients are often already involved in a multidisciplinary care team.
35.2.1 Nonoperative Management
There is a limited role for conservative management in syndactyly. Surgical intervention may not be indicated for minor forms of syndactyly in which there is no functional impairment, or it may be at the discretion of the parents. Particularly in the feet, incomplete syndactyly can be relatively inconspicuous, and correction is therefore often unnecessary. Medical tattooing has been proposed to simulate a webspace in the toes, and has even been embraced by some patients in a creative manner.
35.2.2 Operative Treatment
Prior to any surgical correction of syndactyly, it is necessary to understand the normal anatomy of the webspace (Fig. 35‑2). The normal interdigital webspace has an hourglass shape, and is inclined 45 degrees from dorsal to palmar. The second and fourth webspaces are wider than the third webspace, consistent with the greater abduction possible with the index and small fingers. The normal position of the base of the commissure is the midportion of the proximal phalanx (when the fingers are maximally splayed apart), which is approximately halfway between the distal palmar crease and the proximal interphalangeal joint crease. These landmarks place the second and third webspaces at the same level, with the fourth lying slightly proximal. The first webspace “has the configuration of a diamond-shaped tetrahedron,” with triangles of glabrous skin volarly and nonglabrous skin dorsally. The edge of the web extends as a smooth arc from the level of the index metacarpophalangeal joint to a point just distal to the thumb metacarpophalangeal joint.
The abnormalities present in syndactyly can be highly variable. In all cases, there is a deficiency of skin, which can be demonstrated to parents by comparing the measurement of the circumference of two digits held together versus the sum of the circumferences of the individual digits. The deficiency is particularly pronounced in the area of the normal commissure, explaining the need for skin grafts in this area with most releases. The fused digits are connected in the midaxial line along the length of fusion by an abnormal fascia. The digital nerves and arteries are often abnormal with regard to their branching patterns, most frequently bifurcating more distally than normal, especially in more severely affected cases. This distal bifurcation may limit the extent of separation of the digits or may (rarely) necessitate ligation of one of the digital arteries and intraneural separation of the digital nerves. In complex syndactylies, the skeletal union is usually at the level of the distal phalanx, but can be along the entire digit in some cases. Finally, in complicated syndactylies, the anatomy can be highly abnormal, including duplications within the web (polysyndactyly), transversely oriented skeletal elements, joint abnormalities, and abnormalities of the phalanges, to the point where recreation of a normal, useful digit is impossible.
As already noted, surgical treatment is the mainstay for syndactyly. Exceptions include the aforementioned minor cases of syndactyly with no functional impairment, particularly in the feet, as well as highly complex syndactylies in which surgery may in fact worsen function. Severe medical comorbidities can also present a contraindication to surgery.
Timing of Surgery
Historically, release of syndactyly has been performed anytime from the neonatal period to well into childhood. Children establish patterns of prehensile function by 24 months of age, which many feel serves as an upper limit for completing releases. The timing of surgery is often influenced by how much surgery is needed—since most surgeons advocate against releasing neighboring webspaces simultaneously, patients with multiple syndactylies require multiple operations to complete the entire release. Neonatal surgery has been advocated due to the abundance of mobile skin and thus the ability to avoid skin grafts; however, this is not routinely performed by most surgeons. There remains no consensus for ideal timing of surgery, though most surgeons favor performing releases between 9 months and 2 years of age. In most cases, there is no harm in waiting until the child is older, though parents are often eager to have releases performed sooner. Certain cases, however, warrant earlier intervention, specifically release of the first and fourth webspaces, as well as complex cases in which multiple surgeries are planned. In the case of complete syndactylies of the first or fourth webspace (especially in cases where distal bony coalition is present), surgery is best performed early to avoid progressive skeletal deformity and flexion contractures, and to allow for the development of pinch in the case of the first webspace (Fig. 35‑3). In complex cases, the first of multiple surgeries may need to commence earlier than normal to allow for completion of all releases within an appropriate time frame, as well as to allow for possible intervening procedures by other specialists in the case of children with associated anomalies.
A variety of techniques have been described in the history of syndactyly release. Successful techniques will generally adhere to a number of principles, though there are always exceptions. Surgery should only be performed on one side of the digit at a time to avoid vascular compromise to the digit or the skin flaps. The commissure should be resurfaced with some type of skin flap. Zigzag incisions should be used volarly to avoid flexion contractures. When needed, skin grafts should be full thickness. Finally, care must be taken with postoperative dressings and immobilization.