Classifications in Brief: The Wassel Classification for Radial Polydactyly
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KeywordsPolydactyly Angular Deformity Duplicate Digit Thumb Reconstruction Congenital Hand
Radial (preaxial) polydactyly is among the most common congenital anomalies of the hand, with an incidence of 0.08 to 1.4 per 1000 live births [1, 11, 17, 21]. Although radial polydactyly is commonly described as “thumb duplication,” it is uncommon that the two thumbs are truly duplicated with equal size and function. More commonly, one “dominant” thumb is more developed anatomically and functionally than the other, leading some surgeons to prefer the term “split thumb” to communicate the concept that neither thumb is fully formed nor complete. Radial polydactyly originally was classified as a “duplication” by the International Federation of Societies for the Surgery of the Hand rather than as a “failure of formation” or “failure of differentiation” . It since has been reclassified as a “malformation,” a failure of axis formation, and/or differentiation of the radioulnar hand plate in the Oberg, Manske, and Tonkin classification, which classifies anomalies based on developmental biology and pathogenesis rather than on morphologic features, and recognizes that processes such as formation and differentiation occur together and not independently . From the genetic perspective, most radial polydactyly occur owing to a sporadic genetic mutation, but triphalangeal thumbs are inherited in an autosomal dominant manner .
Despite ancient descriptions of polydactyly, the first attempts to classify thumb polydactyly were not developed until the 20th century . Egawa  in Japan and Millesi  in Austria organized the various presentations of thumb polydactyly into classification systems in their native languages in 1966 and 1967, respectively. It was not until 1969 when Harry Wassel, as a hand surgery fellow of Adrian Flatt at the University of Iowa, authored “The Results of Surgery for Polydactyly of the Thumb: A Review”,  that a classification system for radial polydactyly was published in the English scientific literature. Wassel’s description of seven types of thumb polydactyly based on the level of skeletal duplication subsequently gained wide acceptance given its simplicity, clarity, and ease of use. Since its original description, many, including the membership of the Pediatric Hand Study Group (an international academic society of pediatric hand surgeons and hand therapists) call this classification system the “Flatt Classification” to acknowledge the intellectual genesis of this classification from Adrian Flatt, MD . The term Flatt Classification has now been adopted in the pediatric hand surgery literature [6, 9, 12, 18, 23, 25, 27].
Wassel’s recommendations for operative technique and surgical timing
Types I, III, V
Bilhaut-Cloquet technique if angular deformity is present
Surgery at 3 years old or older
Leave bifid element intact to prevent angular deformity; bifid element may be removed at skeletal maturity
Earlier surgery has increased risk of physeal damage
Types II, V
Ablation of supernumerary thumb with collateral ligament reconstruction
Early surgical ablation
Ablation of triphalangeal thumb
Timing not discussed
Retain biphalangeal thumb even if rudimentary
Despite the near universal adoption of the Wassel’s classification system, there have been few attempts to validate it and none published until more than 40 years after the Wassel classification was introduced.
Dijkman et al.  assessed the reliability of the Wassel classification system in 2014 by evaluating the intra- and interobserver agreement of seven reviewers (four congenital hand surgeons, a resident, a PhD candidate, and a medical student), who analyzed the radiographs of 40 cases of thumb polydactyly randomly selected from the authors’ series of 520 patients with radial polydactyly. The evaluators classified each patient according to the Wassel classification at two time points 2 weeks apart. The intraobserver reliability of the Wassel classification system showed near-perfect agreement (kappa = 0.87), although the interobserver reliability had only substantial agreement (kappa = 0.65). Type-specific reliability was lowest for Wassel Types II and IV thumbs (intraobserver reliability kappa = 0.40 and 0.50 and interobserver reliability of kappa = 0.33 and 0.34, respectively), indicating only moderate to fair agreement for these subtypes of thumb polydactyly. In addition, the authors evaluated the relationship between reliability of the classification system and years of experience in congenital hand surgery. Notably, there was no relationship between the experience of the evaluator and the reliability of classifying according to the Wassel system.
Cabrera Gonzalez et al.  evaluated the prognostic value of the Wassel classification as modified by Egawa  (which described Type VII thumb polydactyly as a “floating” thumb rather than a triphalangeal thumb) in predicting complications and functional outcome (Tada score ) after thumb reconstruction for radial polydactyly. The authors found a higher complication rate with surgical treatment of Wassel Types III and IV deformities, but not a relationship between functional outcome and Wassel type. The most common complications identified were axial angulation deformity followed by joint instability.
Since the publication of the Wassel classification system, numerous authors have identified deficiencies [2, 4, 5, 8, 14, 22, 28, 29], including concerns that it is not inclusive of all presentations of radial polydactyly, is insufficient to guide surgical intervention, and may not be accurate in skeletally immature patients.
Perhaps the greatest limitation of the Wassel classification is that it does not account for the spectrum of anatomic complexity of radial polydactyly and therefore is limited in its ability to classify all presentations of polydactylous thumbs. The inclusion of the triphalangeal thumb, in particular, has always been controversial. In fact, Flatt’s 1977 modification of the Wassel classification system included only Types I through VI and excluded the triphalangeal thumb, considering this presentation a distinct type of thumb deformity . Similarly, Buck-Gramko and Behrens  modified Wassel’s classification to include bifid trapezium and fully duplicated trapezium as Types VII and VIII, respectively, and considered triphalangism separately. Wood  and Miura  also presented modifications of Wassel’s system to better accommodate triphalangeal thumbs. According to Wood’s modification, Type IV thumbs (duplicated proximal and distal phalanges) are subdivided into three subtypes: Type A (in which both duplicated elements at the proximal phalanx level contain triphalangeal elements) and Type B (only the radial-sided duplication is triphalangeal) ; Miura later added Type C (only the ulnar-sided duplication thumb is triphalangeal) . Wood’s modification also subclassified Wassel’s Type VII triphalangeal thumb into four types: Type A (a duplicated thumb at the level of the metacarpal with a triphalangeal ulnar thumb), Type B (a duplicated thumb at the level of the metacarpal in which both duplicated thumbs are triphalangeal), Type C (a duplicated thumb at the level of the metacarpal with a triphalangeal radial thumb), and Type D (a triplicated thumb in which the central component is triphalangeal).
Dijkman et al.  evaluated the ability of the Wassel system to classify the various presentations of thumb polydactyly and found that 208 of their series of 520 duplicated thumbs (40%) could not be classified using the Wassel system. In addition, they reviewed 1723 cases of radial polydactyly in the medical literature and were unable to classify 96 of 1723 cases (6%) using the Wassel system. The unclassifiable cases were most commonly triphalangeal (63%), deviating (43%), or hypoplastic (39%). Using the Rotterdam classification scheme, they were able to classify all presentations of thumbs in their series of 520 cases.
The Wassel classification of thumb polydactyly, although imperfect, has informed our understanding of the osseous anomalies in radial polydactyly and largely forms the basis of all subsequent classification systems. Its most important limitation is that it does not account for the anatomic complexity of this congenital hand difference, including soft tissue deficiencies and redundancies, axial plane deformities, joint instability, and functionality. Classification systems, such as the Rotterdam classification, that include designations for complex osseous and soft tissue elements may prove more useful in conveying the full extent of the radial-sided hand deformity and for informing surgical technique; nevertheless, an understanding of the Wassel classification system is important as it serves as the foundation for these subsequent classification systems. Finally, to accurately reflect the genesis of the Wassel classification, many pediatric hand surgeons favor referring to this classification system of radial polydactyly as the Flatt classification.
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