Metatarsus adductus is a genetic or developmental condition in which the long bones of the foot, called the metatarsals, deviate at an increased angle towards the side of the foot the big toe sits on. Normally these bones are relatively straight, if not usually angled slightly outward. For reasons not quite fully known, during womb development these bones begin to form in a more inward direction. This can be due to genetics, a malformation on one of the bones behind the metatarsals, position of the fetus in the womb, muscle imbalance, or even infant sleeping positions once born. Regardless of the cause, this condition occurs in one out of 1000 live births, and can affect both or just one foot.

Many people have some form of this condition and do not know it, as smaller angle changes are not externally detectible, and can only be seen on x-ray. In these cases, the condition causes minimal to no foot problems, but can have an effect on treatment of other conditions like bunions later on in life. When the angle of the metatarsals compared with the middle of the foot points inward enough (usually greater than 21 degrees as measured on an x-ray), then the condition is considered abnormal and external symptoms can develop. These symptoms include a C-shaped foot, with the outer edge of the foot being convex and the inner border being concave. The arch may appear high as a result (at least initially), and the big toe can separate from the other toes. From this foot structure pain can develop in shoes, tripping can occur, an in-toe walking pattern is seen, and shoes can wear excessively on the entire outer side. Bunions and hammertoes may also develop.

If left untreated, a foot with metatarsus adductus will eventually have to compensate for the inward position to allow the back part of the foot to flatten properly on the ground. Over time, the foot will eventually change to accomplish this, The middle part of the foot translates somewhat to the outside, and the heel rotates outward. This effectively converts the foot into a flat foot, and is sometimes called a skewfoot because of the remaining inward angle of the front of the foot (which is not seen in a traditional flat foot). In certain cases, a skewfoot can result in pain and abnormal foot strain, and also can make shoe fit difficult.

Treatment can consist of a multitude of different forms, depending on the age in which one is diagnosed. Minor cases that have no external deformity and have minimal angle increases may often go untreated, or the foot may be simply stabilized with prescription orthotic inserts. If severe and caught at birth, stretching and manipulation of the foot to reduce the deformity can be done up to three weeks old, and casting to reduce the deformity can be done up through the time a child begins to walk. In toddlers, splints and foot bars can be used to maintain a corrected position. Orthotics inserts are used in older children. Sometimes, simply modifying sitting or sleeping positions can also be of some help. Surgery is used in cases in which correction was not performed early on, or was delayed. There are numerous procedures designed to return the metatarsals to a more natural alignment. They all essentially move the bones back over into a proper position via a series of cuts in the metatarsal bases that slide the bones over. Some procedures also include modification of the ligaments that hold the bones together. The choice of procedure depends on the age of the child and the preference of the surgeon, but they all accomplish the same end: a foot restored to a more natural position.