Limb defects are a collection of conditions, each developing through a different prenatal process. Correct classification of these diverse defects is essential in studies to find causes. This study focuses on the preaxial limb defects category, seen in about 1 in 2160 births.

VARIETY OF DEFECTS SEEN

There are 2 lengthwise tissue growth zones in the lower portion of our arms and legs—the preaxial ray gives rise to the larger of the paired bones (radius in the arm, tibia in the leg) as well as the 1st and 2nd digits on the hand and foot.

	The most common preaxial defect is polydactyly or duplication of the thumb (53%). Polydactyly of the big toe is seen in another 7%
	Reduction defects—shortened radius/tibia, missing fingers/ toes—make up about 1/3 of the total.
	About 4% of infants have a finger-like or triphalangeal structure to the thumb or big toe, with 3 bones instead of the usual 2.
	Defects of the radius and hands are 10 times more common than defects of the tibia and feet.
	Multiple preaxial defects are seen in 2.5%.

ASSOCIATED DEFECTS MORE COMMON IN BILATERAL CASES

	Polydactyly of the thumb is typically found only on one hand (over 90%) and is an isolated defect (76%).
	Reduction defects are more likely to have associated birth defects (62%) or chromosome abnormalities (21%). They are also more likely to be bilateral (28% of isolated cases, 49% with associated defects).
	Almost 4/5 of cases with triphalangeal thumbs and associated defects are bilateral, as are only about half of the isolated cases.
	Babies with preaxial defects have much higher than expected rates of esophageal atresia, kidney and heart defects. Many have the VATER association; Holt-Oram syndrome, a genetic disorder, is also common.
	The most common chromosome defects are trisomy 18 (most with reduction defects) and Down syndrome (most with thumb polydactyly).