Underlapping toes usually involve the fourth and fifth toes. (A special form of underlapping toes is called congenital curly toes). The cause of underlapping toes is unknown. It is speculated that they may be caused by an imbalance in muscle strength of the small muscles of the foot.  If deformed toes are flexible, a simple release of the tendon in the bottom of the toe will allow for them to straighten. If the deformity is rigid, surgery may be needed to remove a small portion of the bone in the toe.