Thumb Polydactyly with Symphalangism

PATIENTS AND METHODS

Our Hand Unit encounters about 20 cases of radial polydactyly per year. Over the last 7 years, three cases of thumb polydactyly in which one of the components demonstrated symphalangism were treated by the authors (Table 1). All patients presented as adults and none had a previous history of trauma or surgery. There was no family history of congenital anomalies and there were no other anomalies in any of the patients. Two patients had Wassel Type II and the third patient had Wassel Type III thumb polydactyly (Figs 1–3). In all patients, the radial component was the component showing the symphalangism. In the first two patients, the radial component was excised completely with reconstruction of the radial collateral ligament of the interphalangeal joint. In the third patient, the ulnar component was excised because it was more hypoplastic and deviated. The ulnar collateral ligament of the metacarpophalangeal joint was not disrupted by preserving the bone at the base of the ulnar component. No postoperative complications were noted and the postoperative appearance in the third case was cosmetically acceptable despite the wide bony base (Fig 3d).

DISCUSSION

The anomaly described in the current report does not fit into the classification systems described for thumb polydactyly. Our cases were unique because of the presence of symphalangism in one of the components of polydactyly. Two of our patients had Wassel Type II and the third patient had Wassel Type III thumb polydactyly. All patients presented as adults and hence earlier X-rays (during childhood) were not available. These X-rays would be of interest especially for the diagnosis of Wassel Type II cases with symphalangism. When we reviewed the literature, we found one case of Wassel Type IV thumb polydactyly in which the radial component showed symphalangism (Boutros et al., 1998). This indicates that thumb polydactyly with symphalangism is an entity that can occur with various types of radial polydactyly. It is interesting to note that in all reported cases, the symphalangism was seen in the radial component of the polydactyly.

The combination of thumb polydactyly and symphalangism is also interesting from the embryological point of view. It is well known that upper limb development occurs along three spatial axes (Daluiski et al., 2001). These axes are the proximodistal (which defines the outgrowth of the limb), the dorsoventral (which results in the formation of dorsal structures on the dorsal aspect of the limb and ventral structures on the ventral aspect of the limb), and the anteroposterior (which occurs in the radioulnar direction). The combination of thumb polydactyly and symphalangism is probably rare because it may indicate that two different abnormal molecular events have occurred along two different limb growth axes. Thumb polydactyly is a duplication along the anteroposterior axis and is thought to be due to a deficiency in bone morphogenic protein type 7 (BMP-7). Mice deficient in BMP-7 have various types of radial polydactyly (Daluiski et al., 2001). In the vertebrate limb, BMP-7 is expressed at high level in the interdigital mesenchyme, which normally undergoes programmed cell death. Therefore, loss of BMP-7 allows for survival of these cells which in turn give rise to an extra digit. On the other hand, symphalangism occurs along the proximodistal axis and may be secondary to a deficiency in growth and differentiation factor type 5 (GDF-5) which is normally expressed in areas of future joint formation (Daluiski et al., 2001). It should be noted that the three axes concept in limb development is not as simple as stated (Al-Qattan, 2004). The dorsoventral axis is controlled by three main signaling molecules: Wnt-7a (Wingless-type mouse mammary tumour virus integration site family member 7a), Lmx-1 (a homeoboxgene) and en-1 (engrailed-1 protein). The centre of control of the anteroposterior axis is the zone of polarizing activity (a mesenchyme under the posterior border of the apical ectodermal ridge) which expresses the Sonic hedgehog (Shh) gene. Finally, the outgrowth of the limb along the proximodistal axis is controlled by the apical ectodermal ridge which secretes fibroblast growth factor type 4 (FGF-4). To complicate things further, the three axes of limb growth interact with each other: Wnt-7a of the dorsal ectoderm induces the production of Shh in the zone of polarizing activity, and the latter is involved in a feedback loop with FGF-4 within the apical ectodermal ridge (Yang and Niswander, 1995). Therefore, the combination of radial polydactyly (along the anteroposterior axis) and symphalangism (along the proximodistal axis) cannot be attributed only to the loss of BMP-7 and the effect of GDF-5, respectively. Other factors are probably involved, including a possible disturbance of the interacting mechanisms between the three growth axes mentioned above.