Congenital Hand Differences

Timeline

The limb bud contains a core of proliferating mesenchymal cells covered by ectoderm which thickens at the tip to form the apical ectodermal ridge (AER). The spinal nerves grow into this and an initial capillary network coalesces into a main stem artery, which drains into a marginal vein. The mesoderm organizes into the ZPA and a progress zone (PZ). By day 33, there is a paddle-shape limb with prechondrogenic condensations at sites where cartilaginous skeletal elements will form. Chondrogenesis takes place and myoblasts migrate into the limb and aggregate into ventral and dorsal muscle masses by day 36. At 6 weeks, digital rays form giving the hand a webbed appearance, and at 7 weeks the upper limb rotates by 90 degrees and the elbow flexes bringing the palm anteriorly. Ossification occurs in parallel and apoptosis between rays leads to tissue involution and digital separation. Thus by week 8 a miniature adult upper limb is formed ( Table 45.1 ).

Limb Patterning

Research into the developing chick embryo has provided a great deal of information on the process of limb patterning. In the proximodistal axis, the AER is formed by the Wnt7a gene and secretes FGF 2, 4, and 8 stimulating proliferation and bud outgrowth. AER resection leads to a truncated limb, whilst AER grafting extends the limb. In the anteroposterior (radioulnar) axis, the ZPA is on the ulnar side between the polarizing region (posterior limb bud mesenchyme) and limb bud tip mesenchyme. This secretes Shh, which is the product of the sonic hedgehog gene. If the polarizing region is grafted anteriorly, a mirror set of digits results. In the dorsoventral axis, the somite provides a dorsal signal to the mesenchyme, which dorsalizes ectoderm. Wnt7a from the dorsal ectoderm and Lmx1b from the mesoderm coordinate tendon and ligament formation. Other important genes in limb patterning include HoxD ( HoxD13 mutations are associated with central polydactyly) and Gli3 (mutations lead to polydactylies).

Classification

Syndactyly is classified as follows ( Fig. 45.2 ):

• Incomplete or complete (extending to the distal phalanx)

• Simple (soft tissue only) or complex (with bony synostosis)

• Complicated (a haphazard arrangement of bones with loss of normal ray patterning)

• Acrosyndactyly (fusion of digits distally, from the Greek acro meaning “tip,” with proximal fenestrations commonly associated with constriction ring syndrome).

Clinical Examination

Inspect the syndactylized digits. With complete syndactyly, the nails can be fused with an inward rotation of the digits, which eludes to the presence of a complex distal bony union ( Fig. 45.2C ). The degree of skeletal disruption within a syndactyly tends to parallel the degree of soft tissue abnormality, including that of tendons and digital neurovasculature structures. It is important to examine for the presence of digital creases, which provides valuable information about the passive and active joint range of motions, which may also be abnormal. The status of the joints pre-separation must be established to ensure adequate counseling of patients and their families with regard to the postoperative motion and function. Often a child and family will opt for the correct complement of digital number over function in equivocal cases.

In complicated syndactyly there is more than just a bony synostosis. Often there is a haphazard arrangement including extra skeletal elements concealed within a digit with associated fusions, rudimentary bones, missing bones, abnormal joints, and sometimes cross bones, which can span several rays ( Fig. 45.2D ). It can be uncertain which bones best match with which digits. In acrosyndactyly, there is a complex syndactyly characterized by fusion of the bone distally but not proximally with often fenestrations or sinuses proximally ( Fig. 45.2E ). This is discussed later under Constriction Ring Syndrome (CRS).

Surgical Principles and Management

Web Reconstruction

Multiple techniques have been described using volar, dorsal, or a combination of flaps of varying sizes and designs. The web is generally reconstructed with a proximally-based dorsal flap. Grafts are best avoided in the webspaces as they can lead to problems of poor pliability and delayed healing with associated web creep. Large triangular flaps are practical and versatile ( Fig. 45.3 ). They allow for adjustments to be made around abnormal distal branching of digital vessels and avoid transverse scars that can recontract in the future. They must be designed in such a way as to create a gentle slope from dorsoproximal to volar-distal.

Release of the first web involves the use of Z-plasties (four-flap, Fig. 45.4 ), double, double-opposing, five-flap), transposition flaps (from the dorsum of the hand and index or thumb), pedicled and free flaps depending on the defect size following thumb and index separation. It is important to release the fascia on the first dorsal interosseous and the adductor muscles. Sometimes it is necessary to reinsert the adductor muscle more proximally.

Tips

In a complete syndactyly with separate nails or partly fused nails, on separation of the pulps the skin can generally be advanced to close the resultant defects. In conjoined nails without a ridge, the lateral nail fold requires reconstruction, most commonly with interdigitating flaps from the hyponychium, e.g., Buck–Gramcko “stiletto” flaps ( Fig. 45.5 ). Care must be taken to ensure these paronychial reconstructions are not too bulky and do not leave an excessively narrow tip.

Clinical Assessment

The most common presentation congenitally is in the little finger, with radial deviation of more than 10 degrees, seen at the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints, and a positive family history. An acquired deformity may occur due to growth plate injury or deviation following syndactyly release. Kirner’s deformity may be mistaken for clinodactyly. However, Kirner’s deformity represents a volar and radial curvature at the DIP joint. It most commonly affects females and appears between the ages of 7 and 14 years, often presenting as a painless swelling at the base of the nail.

A common presenting complaint with clinodactyly is cosmesis as milder forms are not associated with any functional impairment. However, with worsening angulation, function can become impaired. Radiographic assessment allows identification of an abnormal or bracketed epiphysis. The status of the growth plate is an important factor in deciding treatment.

Surgical Principles and Management

As the abnormality is primarily skeletal and not corrected with growth, there is little role for splintage or stretches. The main indication for surgery is worsening deviation with associated functional impairment. This commonly occurs with deviations of more than 20–30 degrees. Age and the status of the epiphysis are critical factors. If growing, physiolysis with resection of the lateral physis and interposition of a fat graft can achieve good results with reliance then on the finger growing straight and correcting itself. This is best done before the age of 6 years. Durability of this procedure is influenced by the extent of soft tissue release at the time of surgery. Epiphyseal bar resection with fat grafting is best avoided in small triangular-shaped bones with minimal growth potential and best not repeated to avoid premature epiphyseal closure. With persistent deformity or after completion of growth, correction is best achieved via osteotomy procedures. These include a closing wedge osteotomy, where bony wegde is removed from the longer side in order to correct angulation. If length preservation is more critical an opening wedge, reverse wedge or dome osteotomy ( Fig. 45.7 ) may be more appropriate. However, in this form of correction the soft tissue deficit on the shorter side must be addressed in order to facilitate correction and prevent recurrence in the long term. This can be achieved through techniques such as Z-plasties or local transposition flaps. Our preference whenever possible is for an opening wedge or dome osteotomy (see Fig. 45.7 ) with longitudinal K-wire fixation.

Pathology

Virtually every structure crossing volar to the PIP joint has been implicated in the condition. However, abnormalities can be divided into primary causal anomalies and those that are secondary effects. The primary causes are abnormal lumbrical and flexor digitorum superficialis anatomy. Tightness and aberrant distal insertions lead to a flexor–extensor imbalance. Secondary effects include abnormalities of the retinaculum cutis, volar plate, collateral ligaments lateral bands, central slip, and skeletal changes. Skeletal changes such as joint space narrowing, flexion of the proximal phalangeal head, and flattening of the base of the middle phalanx auger a poorer response to treatment and prognosis ( Fig. 45.9 ).

Clinical Assessment

Functional impairment can occur when the flexion contracture exceeds 60 degrees and multiple digits are involved. Otherwise issues tend to be more cosmetic. Foucher described four types ( Table 45.3 ).

Factors associated with a poor response to treatment include skeletal changes, older patients (teenagers), with stiff joints and those who demonstrate poor compliance with hand therapy.

Surgical Principles and Management

The mainstay of initial management is stretching and splintage. We reserve surgical intervention for patients with a progressive condition despite compliance to hand therapy and flexion contractures of more than 60 degrees. Outcomes from surgery range from 14% to 35% improvement. Outcomes from hand therapy in young children in the Japanese population have been excellent, with an 80%–92% improvement reported though poorer in Caucasian series (mean improvement of 40°).

Radial Polydactyly

Radial, or preaxial, polydactyly occurs in 1 in 3000 live births. It is usually unilateral and sporadic, with no associations. The expanded Wassel classification with contributions from Wood in 1978 and Hung in 1996 is generally the most widely used system ( Table 45.4 ). A significant proportion of cases may not fit completely into this classification and so additional terms such as deviation or triphalangism can be used. Chromosome 7q36, harbouring the regulatory element of the SHH protein, is implicated in the rarer dominantly inherited radial polydactyly. Typically there is some degree of hypoplasia of both duplicates with the radial usually the smaller of the two (see Fig. 45.11A ). The ulnar-innervated intrinsics to the thumb typically insert onto the ulnar-most duplicate and the median-innervated onto the radial duplicate.

Clinical Examination

Other anomalies may be present and so it is always important to examine all four limbs. More specifically, in radial polydactyly the thenar musculature may be hypoplastic, especially with increasing Wassel grade. The extrinsics are usually Y-shaped and asymmetrical with hypoplasia or absence involving the smallest digit. Simultaneous flexion or extension may occur with or without deviation. The joints may be stiff, hypermobile or hypoplastic. Note the presence of creases and hence the chance of active movement following surgery. There may be asymmetry of the pulps and nail beds. The first web may be tight, especially with more proximal duplications.

Surgical Principles and Management

Our preference for timing of surgical correction is between 12 and 18 months of age as long as other more pressing comordities have been addressed. In an unbalanced duplication the thumb to be retained and reconstructed should be first determined. Different parts from each thumb may need to be used in order to provide the best complement of structures for stability and function. Careful dissection of the intrinsic and extrinsic muscles, and tendons and ligamentous structures must be carried out. Pollex abductus – an abnormal deviating radial connection between the flexor and extensor apparatus – must be looked for and divided if present to allow for unrestricted motion and growth. Preservation of an optimal neurovascular supply is important for thumb viability and the preservation of sensibility. One-stage reconstruction, including osteotomies, to ensure correct skeletal alignment is recommended.

Correction of Distal Duplications (Wassel Types I to III)

Correction of Type IV Duplications

Generally, the best approach for these the most common of pre-axial duplications, is the preservation of the ulnar more developed thumb and reconstruction of motion and stability using elements from its radial partner. The principles and steps are similar to those in type II reconstruction with also initial detachment and subsequent reattachment of the intrinsic muscles. Incisions are planned around the radial duplicate to be excised, preserving skin to allow for soft tissue recontouring of the retained thumb. Decisions around which duplicate to retain becomes more complex in a balanced duplication ( Fig. 45.10 ). Skeletal work is often necessary to achieve long-term correction of alignment. The bifid head of the metacarpal must be trimmed to prevent residual exostosis distorting the radial contour of the reconstructed thumb. An additional osteotomy of the metacarpal may be necessary to correct any residual clinodactyly. This involves the removal of a triangular wedge of metacarpal neck to create a perpendicular articular surface. The osteotomy is fixed with a longitudinal K-wire and the radial collateral ligament should be reconstructed if possible. The latter may not be possible due to the extensive dissection required for skeletal realignment, and stability relies upon correct reinsertion of intrinsic musculature in particular the abductor digiti minimi.

Correction of Proximal Duplications (Wassel Types V, VI, and VII)

The basic principles outlined above apply to those of type V, VI, and VII duplications. The ulnar more developed thumb is usually retained and reconstructed ( Fig. 45.11 ). The intrinsic muscles must be repositioned, the lax radial collateral ligament must be tightened and the extrinsic tendons centralized if necessary. Adduction contractures in the first web must also be released to improve function. In type VI duplications where both the metacarpophalangeal (MCP) and carpometacarpal (CPC) joints are abnormal an epiphyseal-sparing chondrodesis at the MCP joint will provide the necessary stability for a functional thumb. In type VII duplications the thumb with the two phalanges is retained if it is the more stable and components of the triphalangeal duplicate used for reconstruction. If the thumbs are comparable in size and functionality then preference should be given to that with the more stable ulnar collateral ligament at the MCP joint. In some instances triphalangeal thumb reconstruction may require an on-top-plasty whereby the distal portion of one thumb is transferred to the proximal part of the other with the better basal joint.