Dental implants are a "root" device, usually made of titanium, used in dentistry to support restorations that resemble a tooth or group of teeth to replace missing teeth. Virtually all dental implants placed today are root-form endosseous implants, i.e., they appear similar to an actual tooth root and are placed within the bone. The bone of the jaw accepts and osseointegrates with the titanium post. Osseointegration refers to the fusion of the implant surface with the surrounding bone. This is what makes the implant resemble the look and feel of a natural tooth. Since dental implants fuse with bone and lack the periodontal ligament, however, they will feel slightly different than natural teeth during chewing. Dental implants can be used to support a number of dental prostheses, including crowns, implant-supported bridges or dentures. They can also be used as anchorage for orthodontic tooth movement. The use of dental implants permits unidirectional tooth movement without reciprocal action.