Bifid Thumb - observation

Historical Overview

• Thumb polydactyly, or bifid thumb, is a type of radial or preaxial polydactyly that is among the most common congenital anomalies of the hand and the most prevalent duplication in Caucasian and Asian populations. The condition is usually more of a cosmetic than a functional issue, as the supernumerary thumb often contains all of the sensory and motor units that are necessary for function, and often works in concert with the whole hand as a contributing unit.^1,2
• Bifid thumb was recorded in biblical literature as long as 3,000 years ago, but despite these ancient descriptions, the first attempts to classify it 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. Then in 1969, hand surgeon Harry Wassel authored a review that included a classification system for radial polydactyly published in English, and that system is still commonly used today.^2,3

Description

• Bifid thumb is a common hand anomaly involving partial or complete duplication of the thumb that a clinician can identify when conducting a visual examination of the patient.

Pathophysiology

• Thumb duplications result from a failure of the radialulnar axis of the hand plate to form and/or differentiate. The zone of polarizing activity (ZPA) in the posterior part of the developing limb bud is the principal signaling center. The ZPA expresses sonic hedgehog protein, which controls the formation of radial-ulnar features. Abnormal expression of Hox genes, bone morphogenic protein, and the Gli-3 gene all play a role in the evolution of thumb duplications. Most cases of bifid thumb are sporadic and unilateral, but the presence of a triphalangeal thumb is known to exhibit an autosomal dominant inheritance pattern.¹
• The appearance of the duplicated thumb depends on the changes in the skeletal and soft-tissue structures, which vary with the level of the bifurcation and the extent of the duplication.⁴

Instructions¹

1. Obtain an accurate and complete patient history.
2. Observe both of the patient’s thumbs from various angles and compare the two digits.
3. Evaluate the range of motion of the both thumbs, paying particularly close attention to their functional use and status of the thenar musculature and first web space.
4. Take note of the shape of the thumbs (as duplicated thumbs usually retain smaller nails and pulp compared with the unaffected contralateral side).

Associated Conditions and Syndromes

• Blackfan-Diamond anemia
• Cleft palate
• Fanconi’s anemia
• Holt-Oram syndrome
• Imperforate anus
• Tibial defects

Diagnostic Performance Characteristics

• The Wassel classification system is most familiar among pediatric hand surgeons for its simplicity and is the most widely used system for bifid thumb. It describes seven types of thumb polydactyly according to the level of the skeleton at which duplication occurs:
  • Type I: bifid distal phalanx
  • Type II: duplicated distal phalanx
  • Type III: bifid proximal phalanx
  • Type IV: duplicated proximal phalanx
  • Type V: bifid metacarpal
  • Type VI: duplicated metacarpal
  • Type VII: triphalangeal thumb

• Type IV is the most common, followed by types II and VII.¹

• A bifid thumb is present when the clinician observes the presence of a partial or complete duplication of a thumb at any level from the metacarpal to the distal phalanx.

• A negative result is when a single thumb is observed without any form of duplication is seen.

• Although bifid thumb 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.²
• The Wassel classification of thumb polydactyly, although imperfect, has informed clinician’s understanding of the osseous anomalies in bifid thumb 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.²

1. Van Wyhe, RD, Trost, JG, Koshy, JC, et al. The Duplicated Thumb: A Review. Semin Plast Surg 2016;30(4):181-188. PMID: 27895541
2. Manske, MC, Kennedy, CD and Huang, JI. Classifications in Brief: The Wassel Classification for Radial Polydactyly. Clin Orthop Relat Res 2017;475(6):1740-1746. PMID: 27613532
3. Kumar, S. An unusual bifid first metacarpal. Indian J Orthop 2010;44(2):227-9. PMID: 20419015
4. Hung, NN. Bifid thumb type IV in children: transferring an epiphyseal segment of the proximal phalanx with insertion of the abductor pollicis brevis tendon. J Child Orthop 2010;4(6):525-37. PMID: 22132030