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With this type of deformity, the parts of the hand, either the bones and/or the tissues, fail to separate in the womb. The most common type of this classification is syndactyly. Syndactyly is the most common congenital hand deformity, in which two or more fingers are fused together. According to some sources, this condition occurs in about five out of every 10,000 live births.
There is a familial tendency to develop this deformity. This deformity usually involves both hands, and males are more often affected than females. If the fingers are completely fused together, it is considered complete. There are two types of syndactyly:
Simple syndactyly: Fusion between only the tissues of the fingers
Complex syndactyly: Fusion between the bones
Another example of failure of the hand to separate is seen in contractures of the hand. Contractures of the hand may also develop as a result of failure of the cells to differentiate in utero. A contracture is an abnormal pulling forward of the digits of the hand, usually caused by problems with the muscles or skin.
One of the common types of this classification includes congenital triggering. Congenital triggering occurs when one of the digits is unable to extend. It is usually seen in the thumb. It may take some time in the child's development before it is noted that the child can not extend the thumb. Some of these cases improve on their own. Surgery is usually not performed until the second year of life, but preferably before the age of 3.