Hypoplasia is a medical term that describes the incomplete development of part of the body. In the case of thumb hypoplasia, a child is born with an underdeveloped thumb, or a thumb that is not in the correct place. This condition is rare. Often, these cases are not easily explained, but some are related to genetic syndromes.
Babies spend nine months growing in the womb. During this time, most develop normally, but some babies can have minor problems in development, and some areas of the body such as the thumb, develop abnormally. In medicine, hypoplasia is a general term that stems from the Greek words for under, which is hypo, and plassein, which means to mold. Therefore a part of the body that is underdeveloped, or undermolded, has a hypoplasia.
There are four major classifications of thumb hypoplasia. A child can have a thumb that has all the functional structures, such as muscles and ligaments, attached to the hand properly, and the development issue only displays itself in the unusually small size of the thumb. Another type of thumb hypoplasia is a child who has a small thumb, and who also has some problems with his or her thumb function.
Cases in which a child also has more serious muscle and bone problems in the thumb can occur, where the child's thumb is not free to move properly. Another possibility is the absence of the structural bones necessary to properly hold the thumb to the rest of the hand. Some children are also born without thumbs, but this developmental issue has its own particular term, which is thumb aplasia.
Some with thumb hypoplasia may benefit from plastic surgery. If the condition is mild, and does not pose serious issues for the child, then he or she may be able to regain much of the natural function through occupational therapy alone. Those with serious cases of the condition, where the thumb does not provide enough function to the hand for everyday life, may receive surgery that moves one of the other fingers on the hand to the thumb position. This surgery is called pollicization.
Thumb hypoplasia can occur on one or both hands. The condition can be genetically linked, and may be present as a symptom of a condition or syndrome such as Holt-Oram Syndrome, or radial club hand. Often, however, the underdeveloped thumb is not genetically-linked.