Dental prostheses usually consist of three components: The first is the implant, which can be thought of as a replacement for the root. The second is the abutment, which fits on the implant and provides a platform for the prosthesis. The third component is the prosthetic tooth itself, which connects to the abutment.
Single tooth replacement and bridges require a tooth to be positioned and placed directly over an implant. If the implant is not correctly positioned, the result can be a misshaped tooth that has to be contoured to work with the bite of the opposing teeth, and must also twist and bend to meet and fit with the implant or abutment. The hybrid prosthesis usually consists of a CAD/CAM (computer aided design/computer aided manufacturing) titanium framework that interfaces with the implants or abutments, and connects the implants together with a bar to create a strong, solid and stable platform upon which to build the teeth.
Because the teeth are built on a bar supported by implants rather than directly on the implants, the positioning of the implants in relation to the positioning of the teeth takes on less importance. In other words, the positioning of the teeth is not reliant solely on where the implants are located. This important feature of the hybrid prosthesis allows for a unique benefit for both the clinicians and patients: the ability to reverse-engineer the prosthesis.
Reverse engineering the prosthesis starts by taking accurate impressions of the patient’s jaws. Then, in the laboratory, upper and lower master models are fabricated that accurately reproduce the position of the implants, the remaining teeth and the gums. A devise called an articulator is used to position the master models in the correct relationship, upper to lower. The articulator also replicates the motions of the jaws.
The teeth can then be positioned on the master model to create the most aesthetic result while achieving optimum function.
This includes selecting the size, shape and shade of the teeth as well as their arrangement. Attaining the best results involves the skilled balance of science and art. While teeth can be positioned in the laboratory on an articulator, the final test of the esthetics and function can only be accomplished in the patient’s mouth, where they are set in pink wax to hold them firmly in place. This “try-in” allows for the repositioning of the teeth before they are permanently set in place.
A highly skilled prosthodontist is necessary to insert, evaluate and adjust the wax try-in of the prosthesis to ensure the correct position of the teeth in every function including biting, chewing, moving side-to-side and up and down, and even at rest. During the wax try-in, aesthetic aspects such as mid-line, smile line, natural bite, shape, size, shade, and position of the prosthesis are also analyzed.
After the wax try-in is finalized, the master model is digitally scanned into a computer. The wax try-in is also scanned and the two images are three-dimensionally superimposed upon each other to create an image that shows the relationship between the position of the teeth and the implants. The technician, with the aid of the computer, can then design the titanium frame that connects the implants together and provides proper support for the teeth.
Once the frame has been digitally designed, all the information is sent to a milling machine that cuts the frame from a solid block of titanium to the exact specifications of the scans and design. The finished frame is then sent back to the laboratory and the teeth are processed on to the frame.
In the traditional mode of dental technology, the frame would have been made to fit the implants and it would have dictated the function and esthetics of the teeth. With reverse engineering, the frame is dictated by the position of the teeth and the implants. This aspect of the hybrid technique enables functional and aesthetic results never before possible: Beautiful permanent teeth that work!