Implant Placement
In occlusal rehabilitation using implants as anchors for the reconstruction, the implant unit (the titanium fixture and the abutment) must be looked upon as a tooth root. The degree of bone resorption and other anatomical characteristics also have to be considered. Furthermore, it is important to keep in mind that implant components of approximately the same dimension are intended to serve as supports for teeth of different sizes.
Root and crown width vary greatly among different teeth. According to Wheeler, the mesiodistal and buccolingual average extension of the roots at the cementoenamel junction varies between 3.5 (mandibular central incisor) and 10.0 mm (maxillary molars). The mean width of the tooth crowns in the mesiodistal direction varies between 5.0 (mandibular central incisor) and 10.5 rnm (mandibular molars). When considering only the incisors, canines, and premolars, the regions that are most important from an esthetic point of view, the corresponding figures still differ, but to a lesser degree. The diameter of the threaded portion of the most frequently used fixtures is either 3.75 or 4 mm. At the fixture head, ie, at the collar, the diameter is increased to 4.1 mm. The design of the standard, abutments results in a further increase of the implant width.
To reach a satisfactory esthetic outcome when connecting the superstructure to the implants, the most coronal extension of the crown-abutment junction must be located about 0.5 to 3.0 mm submarginally. This means that in partially edentulous cases in which the neighboring teeth demonstrate intact periodontal-tissue support, the crown-abutment junction (CAJ) has to coincide more or less with the most apical extension of the cementoenamel junction (CEJ) of these teeth. Deeper placement (~ 3 mm) of the crown-abutment junction has to be used when neighboring teeth demonstrate a big difference in position of the cementoenamel junction at buccolingual vs proximal sites. Consequently, in dentitions with reduced periodontal tissue support and exposed root surfaces, the crown-abutment junction most often will be placed more superficially than in a periodontally intact situation, but still at a submarginal position ( 0.5 to 1 mm).
When replacing teeth in the anterior and premolar regions, except in the anterior mandibular region, the idea! distance in the mesiodistal direction between supporting fixtures usually is about 7, 14, or 21 mm. The shortest distance (7 mm) is used when each individual implant serves as an anchor for one crown; the longer distances are used when one or two pontics are placed between two implants. This concept can be applied in totally as well as in partially edentulous situations.
The coordination of the surgical and prosthetic treatment-planning procedures is certainly one of the most important factors in obtaining an ideal esthetic result. In the anterior region, fixtures must be placed so that the screw-access holes are located lingual or palatal to the incisal edge of the crown restoration. Pronounced variances in implant angulation can jeopardize a good final esthetic result. Therefore, an individually designed surgical guide stent will facilitate correct and optimal fixture placement. This in turn will enable prediction of a good esthetic outcome for the implant treatment. When two or more fixtures are placed it is often a necessity and an advantage from an esthetic point of view to place them more or less parallel to each other. Thus, the position and angulation of the first fixture placed will influence the placement of the following ones. These guidelines for implant placement are also applicable in the premolar and molar regions.