The Temtamy and McKusick10 classification, which is the product of geneticists, classifies duplications by grouping genetically related presentations (Table 2). It provides the most commonly used postaxial classification, with type A being a fully developed digit and type B a rudimentary and pedunculated digit, informally referred to as a nubbin. Type B is more common than type A. Given inheritance patterns, it is assumed that type A is likely multifactorial and type B autosomal dominant.10 Thumb polydactyly inheritance is still unclear. The other types of preaxial polydactyly and high degrees of polydactyly are rare but seem to be passed on in an autosomal dominant fashion on pedigree analysis.10
The Stelling and Turek classification presents the duplication from a tissue perspective: Type I duplication is a rudimentary mass devoid of other tissue elements; type II is a subtotal duplication with some normal structures; and type III is a duplication of the entire “osteoarticular column,” including the metacarpal.1 It is interesting to note that histology of type I duplications shows neuroma-like tissue.26-28 Again, normal is a relative term because, in polydactyly, the duplications are hypoplastic and deviated, with anomalous anatomy.
The Rayan classification describes ulnar polydactyly and was derived from a case study series of 148 patients in Oklahoma (Table 3).29
There are also some complex polydactylies that are not easily classified: ulnar dimelia, cleft hand, pentadactyly, and hyperphalangism. Ulnar dimelia, also known as “mirror hand,” is typically 7 digits with no thumb, but other variations are seen. The radius is often absent, and the elbow is abnormal. There is some debate about whether it is a fusion of 2 hands. Pentadactyly, or the 5-fingered hand, appears as 5 triphalangeal digits with no thumb (Figure 7).
Isolated thumb triphalangism might appear similar to pentadactyly. Miura30,31 pointed out that the radial digit in the pentadactylous hand may be opposable (thumb-like) or nonopposable; in his studies, the patients with the opposable thumb had a metacarpal with a proximal epiphysis (Figure 8). Consequently, the triphalangeal thumb metacarpal with a distal epiphysis is true pentadactyly, whereas that with a proximal epiphysis is hyperphalangism (Figure 9). Treatment of these complex polydactylies involves the same underlying principles as for preaxial and postaxial polydactyly, albeit with additional proximal upper extremity considerations.
When to Operate (Timing)
Ezaki6 recommended surgical intervention at age 6 to 9 months, before fine motor skills have developed with the abnormal anatomy. Cortical learning occurs as the child begins prehensile activities before 6 months, but the risks of anesthesia outweigh functional benefits until the child is older. Waiting until 1 year of age is not uncommon, though surgery at an earlier age may be beneficial if the polydactyly affects hand function.32 It is not uncommon to wait with the more balanced thumb polydactylies to assess thumb function. Hypoplasia might also delay surgical intervention until there is enough tissue inventory for reconstruction. Wassel20 noted that surgical intervention ideally occurs before the supernumerary elements displace the normal elements, as tends to happen with growth. Suture ligation is an option in the neonatal unit for some pedunculated digits.33 Studies have shown satisfactory results in adults treated for polydactyly, if the patient presents later than expected.34
Surgical Considerations
Knavel recommended simple excision, stating that “ablation requires no ingenuity and creates no problems.”5 This belief, though true for some duplications, will not lead to the best outcome for more complex polydactylies. The goal of surgery is a stable and well-aligned thumb for pinch and prehensile activity, as well as a cosmetically pleasing hand. Incisions should not be made linearly along the axis of the digit, as the scar will cause deviation with growth.24
Wassel type I polydactyly might appear incidentally as a broad thumb, in which case it requires no intervention (Figure 10). However, in Wassel types I and II polydactyly with deformity, the Bilhaut-Cloquet procedure is useful for both bifid and duplicated phalanges (Figure 11).5,6,30,32,35 Collateral ligaments may need to be released in type II because of difficulty in opposing the tissue. Cosmetic results with Bilhaut-Cloquet are unpredictable. The original technique required symmetrically sized digits; results today have been improved with microtechniques and preservation of an entire nail.36 Another option is ablation of the more hypoplastic osseous element and soft-tissue augmentation of the residual digit. The theme of ablation and augmentation is seen throughout the literature for the surgical treatment of polydactyly (Figure 12).1
For type III polydactyly, the bifid proximal phalanx is narrowed by resection and realigned with osteotomy of the remaining diaphysis. Type IV polydactyly, the most common thumb duplication, often requires advancement of the abductor pollicis brevis to the base of the proximal phalanx to aid in metacarpophalangeal (MCP) stabilization, abduction, and opposition. The metacarpal head, if broad and with 2 facets, can be shaped to form a single articulating surface. The metacarpal, occasionally with the proximal phalanx, often requires realignment by closing wedge osteotomy. Last, tendons on the resected bony elements should be rebalanced on the remaining digit, and anomalous slips must be released. For instance, given a radial insertion of the long flexor tendon on the distal phalanx, the tendon should be moved centrally. A strong flexor or extensor tendon on the amputated digit should be transferred to the remaining digit.24