Type and Design of Definitive Prostheses
The type of the prostheses to be fabricated (and the attachments to be used) should be decided prior to the placement of implants. The number and position of implants should be planned based on the design of the intended prosthesis. All the factors discussed in part I section of this course should be taken into consideration while deciding the type of prosthesis.
If an open palate overdenture design is planned for the maxillary arch, a minimum of 4 implants should be planned with a wide anteroposterior (AP) spread (implants configured in canine and first molar bilaterally). The implants should be planned such that they are parallel to each other and emerge through the palatal aspect of the prosthetic teeth. However, if adequate bone is available, planning 6 implants would be advantageous, the reason is even if one or two implants are lost, there would still be a sufficient number of implants left to permit the continuation of the same treatment.
Similarly, if a fixed implant prosthesis is planned for the mandibular arch a minimum of 4 implants should be planned with a wide AP spread (implants configured in canine and first molar bilaterally). However, if sufficient bone is available, 6 implants would be preferable. When an optimal AP spread of implants cannot be achieved due to lack of posterior bone height, the posterior implants may be intentionally angled to improve the AP spread (all-on-4 prostheses). The protocol for the “all-on-4” prosthesis includes the use of four implants in the anterior part of edentulous jaws to support a fixed prosthesis. The two most anterior implants are placed axially, whereas the two posterior implants are placed distally and angled to minimize the cantilever length and to allow the fabrication of prostheses with 10-12 prosthetic teeth. The length of the cantilever should be kept as small as possible when treatment planning an “all-on-4/all-on-5” implant prosthesis. The angulation of the implants can be corrected using multi-unit angle corrections abutments. However, the use of these abutments increases the vertical restorative space requirement by 2-3mm.
An implant overdenture supported by individual attachments may be fabricated by one of the two methods. A conventional complete denture may be fabricated and the retentive elements of the attachments can be picked up chair-side during placement of the denture or the retentive elements can be incorporated in the prosthesis during denture processing. The latter technique is also used for the fabrication of bar-supported overdentures.