Sunday, August 5, 2007

Radial (Thumb) Polydactyly

Radial (thumb) polydactyly occurs in 0.08 per 1000 live births among both black and white populationgroups. It is not as common as ulnar polydactyly. When thumb duplication occurs alone, it is usually unilateral and sporadic. Those involving a triphalangeal thumb may be inherited as an autosomal dominant trait. As with triphalangeal thumb, delta phalanx has often been found to have a strong family history of inheritance. The basic classification system was defined by Wassel in 1969. His classification was based on the level of duplication and the number of bones in the thumb, and has seven groups (types I to VII) according to the level of the bifurcation. The thumb is often hypoplastic (smaller than normal), so the reconstructed thumb is nearly always smaller than the opposite side. Photo credit

Classification--Types I to VII based on level of duplications:
I : bifid distal phalanx (DP)(bone under the finger nail)
II: duplicated DP
III: bifid proximal phalanx (PP) (digit bone nearest the palm)
IV: most common type with duplication of proximal phalanx which rest on broad metacarpal
V: bifid metacarpal (MC) (bone in palm)
VI: duplicated MC
VII: triphalangism


Treatment:
The strategy of surgery is to used the "accessory digit" to augment the thumb that is kept. The duplicated thumb is skeletonized and removed at the MCP joint, retaining all of its soft tissue structures (which help augment size of the remaining thumb). When sharing equal halves, for the reconstruction of a thumb, it is best to preserve a single nail to prevent a nail ridge. Often an “80 percent rule” is followed -- generally keeping a given thumb segment if it is 80 percent of normal thumb size, otherwise a Bilhaut-Cloquet type of thumb-sharing procedure is more likely to be indicated. It is mostly indicated for types I and II deformities.

If there are two equal digits, one should remove the radial one and reconstruct the radial collateral ligament rather than the ulnar collateral ligament (more important for the stability of the thumb). When duplication is at level of joint, types II and IV, the collateral ligament of the digit to be discarded needs to be preserved and sewn to the remaining digit across the retained joint. If the MCP joint remains unstable, fusion of MP joint is considered when the child reaches the age of ten or 12. The tendons of the thumb(s) have to be addressed. EPL (extensor pollicus longus) and FPL (flexor pollicus longus) may be split and insert on both digits, each with its own eccentric insertion. Duplicated EPL and FPL tendons should be transferred over to the remaining digit. The abductor brevis tendon may attach to the radial duplicated thumb and the adductor pollicis usually attaches to the ulnar thumb. The abductor brevis is transferred into an anatomic insertion site on the ulnar thumb (the one being kept). The web space (space between the thumb and index finger) may be contracted. In this case, the dorsal skin of the discarded thumb should be used to assist with making the web space deeper.

Treatment of Type IV should be done very early, because if one waits too long, the supernumerary component displaces the normal component into markedly radial or ulnar deviation. If left, it will continue to grow this way.


References:
Monozygotic Twins Disordant for Thumb Polydactyly; Plastic & Reconstructive Surgery. Vol 113(1):449-451, January 2004; Peterson, Steven L. D.V.M., M.D.; Rayan, Ghazi M. M.D.

Triplicated Thumb; Plastic & Reconstructive Surgery. Vol 100(2):418-421, August 1997; Atabay, Kenan M.D.; Latifoglu, Osman M.D.; Demirkan, Ferit M.D.; Yavuzer, Reha M.D.

Treatment of Congenital Upper Extremity Problems; Plastic & Reconstructive Surgery. Vol 119(5):101e-129e, April 15, 2007; Netscher, David T. M.D.; Baumholtz, Michael A. M.D.

Thumb Polydactyly: Clinical Outcome after Reconstruction; CH Yen, WL Chan, HB Leung, KH Mak; Journal of Orthopaedic Surgery 2006;14(3):295-302

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