An underdeveloped maxilla, abnormal tongue posture and aberrant swallowing patterns are some of the causative factors leading to a crowded narrow mandible. In conjunction with maxillary appliance therapy that directly addresses these factors, appliances are available that will develop the constricted mandible and unravel the lower anterior crowding. The Williams Expander is a fixed appliance that is used during the early mixed dentition stage. It is designed to develop the lower arch and create normal spacing for the alignment of the permanent lower incisors and cuspids.

The framework of the appliance consists of a specially designed midline expansion screw soldered to two long sections of hollow tubing that run along the lingual aspect of the posterior segments. This tubing accepts a preformed .016 or .018 NiTi arch wire that rests against the lingual of the incisors. As the expansion screw is activated and the appliance widens, the NiTi wire expresses itself labially and aids in un-crowding the incisors.