Congenital Anterolateral Tibial Bowing and Polydactyly

Congenital anterolateral tibial bowing and polydactyly (CATBP) is, as its name explicitly states, a syndrome of congenital anterolateral tibial bowing and polydactyly. This rare condition is important because anterolateral tibial bowing usually raises concern for neurofibromatosis type 1; in contrast to NF1, however, there are no neurocutaneous signs in CATBP and the prognosis tends to be better.

CATBP is associated with hallux, first metatarsus, tarsus, and sometimes tibial duplication. The fibula is often without apparent involvement, but may develop proximal overgrowth later. Hand malformations, including syndactyly, pre-axial polydactyly, clinodactyly, and radial deviation of the index finger have also been reported.

Delta Tibia

Based on my admittedly cursory review of the literature, the tibial abnormality in CATBP seems to be what Currarino refers to as "the delta tibia," since both Currarino and Lemire refer to the same literature in citing previous examples of their cases. What follows is based on Currarino's description of the delta tibia.

Frontal radiographs reveal the lateral component of the anterolateral bowing, but the appearance of the curve is nonspecific. The lateral view has more specific features. Lateral radiographs reveal an anterior curve of the tibia with the apex at or slightly below the center of the tibial diaphysis or at the junction between the middle and distal third of the tibia. The tibia also appears divided into two pointed segments with their points at the apex of the curve (pink arrows) with an intervening anterior cortical defect (blue arrow).

Instead of the inward smooth cortical thickening seen in the concavity of the curve in most types of tibial bowing, there is a triangular osseous structure (green arrow) connected to the posterior surface of the two tibial segments.

The fibula is intact, straight, and longer than the tibia, with its proximal end displaced laterally.

References

• Lemire EG. Congenital anterolateral tibial bowing and polydactyly: a case report. J Med Case Reports. 2007 Jul 23;1:54.
• Currarino G, Herring JA, Johnston CE Jr, Birch JG. An unusual form of congenital anterolateral tibial angulation-the delta tibia. Pediatr Radiol. 2003 May;33(5):346-53.