Restoration of the Single Implant
. Explain the concept of immediate implant placement and loading, different impression techniques, clinical considerations, and laboratory procedures involved in the fabrication of an implant-supported crown.
Osseointegrated implants have been successfully used to replace single teeth, both anteriorly and posteriorly, and considerable information has emerged regarding design, clinical procedures, survival, failure, and complications. Before discussing the restoration of single implants, however, it is important to consider the clinical data on success and failure.
In 2008 a systematic review determined the 5-year survival of single implants to be 96.8% and that of single crowns to be 94.5%.1 Despite these high survival rates, both biologic and mechanical complications have been reported.1 In fact, another systematic review concluded that crown complications are common.2
A literature review of clinical complications provided the following data on mechanical complications with single crowns attached to single implants: 20% of abutment screws loosened with early screw designs; 7% loosened with newer screw designs and the use of torque devices; and 2% of abutment screws fractured.3 The same literature review reported that 1% of implants fractured, but the data were related to all types of implant prostheses and were not specific to single crowns. However, multiple reports of implant fractures associated with single posterior implants have been published.
A third systematic review presented data from both short-term studies (6 months to 5 years) and long-term studies (5 to 10 years) of single crowns and fixed partial dentures.4 The long-term data on single crowns showed that abutment screw loosening ranged from 1% to 10% in the nine studies that provided data on abutment screw loosening with single crowns. Abutment screw loosening occurred with both external and internal abutment connections.
These data clearly show that mechanical complications occur and that design and material changes have reduced but not eliminated the incidence of complications. Therefore, the use of appropriate design principles is important to minimize the chances of problems developing.
Rangert et al.5 evaluated the forces and moments that occur on Brånemark implants. Based on theoretical considerations and clinical experience with Brånemark implants, these authors presented guidelines for controlling the forces applied to implants. They recommended that the restoration not extend lateral to the implant more than approximately one implant diameter in the molar region and no more than two implant diameters in the incisor region.
In another article, Rangert et al.6 discussed the probable causes of 39 implant fractures. All nine fractures of implants supporting single crowns occurred in the mandibular molar area (eight first molars and one second molar) (Figures 9-1 to 9-5). Several factors that result in adverse loads on implants were discussed, including:
Figure 9-4 Radiographic image of the implant-supported crown made for the implant shown in Figure 9-3 that shows the mesial cantilever.
Implant fractures are not common, and statistical differences in fracture rates based on location in the mouth have not been shown. However, Eckert et al.7 analyzed the incidence of fracture among 4,937 implants. They determined that the only implant fractures associated with single crowns occurred in molar areas. Based on the observation that single implant fractures were associated only with molar crowns, the authors stated, “Despite the lack of statistical significance, this clinical observation makes it appear prudent to consider the single implant–supported molar to be at a higher risk of fracture.”
Rangert et al.8 identified risk factors that increase the load applied to implants. The following factors related to implant single crowns: (1) extension of the occluding surface lateral to the implant; (2) increasing the distance between the occlusal surface and the implant; (3) use of one implant to support the replacement of a multi-rooted tooth; and (4) bruxism or the presence of heavy occlusal forces as evidenced by tooth wear/tooth structure fractures. When a molar is replaced using a single implant, the authors emphasized the importance of controlling occlusion so as to avoid heavy centric occlusal contact. They suggested light centric occlusal contact as a means of avoiding heavy contact.
In light of the guidelines set by Rangert et al.,5 a reasonable conclusion is that anterior implant single crowns can extend laterally a moderate distance beyond the periphery of the implant. This is possible because the maximum occlusal load in the anterior region of the oral cavity is less than the maximum load in the posterior region. Depending on the material used for fabrication of the implant, abutment, and retaining screw and the clamping force achieved when the retaining screw is tightened, an implant-supported crown can resist a specific load before the retaining screw deforms. The practitioner must understand the anticipated load that will be placed on the implant and limit the dimensions of the crown to prevent occlusal overload.
The average dimensions of anterior teeth are 5 to 8.5 mm mesiodistally and 6 to 8.3 mm faciolingually; therefore, adverse leverage is unlikely to be created by the mesiodistal or faciolingual dimensions of the crown. Biomechanical overload of an anterior crown is more likely to occur in an incisocervical dimension, because the distance from the top of the implant to the location of occlusal contact can be substantial, especially if the implant is placed deep below the soft tissue for esthetic reasons (Figure 9-6) or if a significant alveolar defect exists before implant placement.
Figure 9-6 Biomechanical overloading of an anterior crown is more likely to occur in an incisocervical dimension, because the distance from the top of the implant to the location of occlusal contact can be substantial. This problem is especially likely if the implant is placed deep below the soft tissue for esthetic reasons or if a substantial alveolar defect exists before implant placement.
If the previous recommendations about forces and moments5 are applied to posterior single crowns placed on implants, the conclusion is that the crown should not extend lateral to the implant more than one implant diameter. Therefore, for an implant 4 mm in diameter, the maximum lateral cantilever should be about 4 mm; this means that the crown should extend mesially, distally, facially, or lingually only 4 mm lateral to the implant.
The preceding recommendation places limits on the total mesiodistal or faciolingual dimensions of the crown and is designed to provide a conservative, safe guideline that minimizes mechanical overload. The dimensions of a typical premolar do not exceed this guideline, nor do average-sized molars. However, biomechanical overload can occur with molars as a result of excessive occlusal forces or larger than normal mesiodistal or faciolingual dimensions.
Figure 9-7 documents the relative number of fractures found by Rangert et al.6 for implants located in different sites. Biomechanical overload is more likely if a single molar implant is not centered under the crown. In addition, the possibility of overload increases as the distance from the occlusal surface to the implant increases. The combination of a tall crown and an implant that is not centered beneath it compounds the potential for overload.
Figure 9-7 The relative number of fractures that occur with implants located in different sites. (Data from Rangert B, Krogh PHJ, Langer B, Van Roekel N: Bending overload and implant fracture: a retrospective clinical analysis, Int J Oral Maxillofac Implants 10:326-334. 1995.)
Weinberg and Kruger10 mathematically compared four clinical variables that can affect torque production and implant loading: (1) cuspal inclination, (2) implant inclination, (3) horizontal implant offset, and (4) apical implant offset. A knowledge of these factors can help the practitioner design crowns that transfer more favorable forces to the implant system. Cuspal inclination and horizontal implant offset have the greatest effect on torque, and implant inclination and apical implant offset have a lesser impact. The effects are as follows:
The location of the implant in the bone is an important aspect of biomechanical success and crown esthetics with implant single crowns. The implant should be centered mesiodistally in the edentulous space for esthetic and biomechanical reasons. This position equalizes the lever arm developed by the mesial and distal portions of the crown, which project laterally to the implant. When the implant is displaced to the mesial or distal of center and occlusal forces are applied, greater leverage is exerted on the implant and other components than if the implant were centered.
Centering the implant mesiodistally also facilitates the development of a normal emergence profile and permits better morphologic replication of the contralateral tooth than when the implant is displaced to the mesial or distal. If an implant cannot be centered, the practitioner should consider the anatomy of the tooth being replaced to establish the most favorable implant position. For example, in the maxillary incisor region, the mesial surface of an incisor generally has a straighter emergence than the distal surface, which has a more curvilinear emergence from the natural tooth root. When an implant is located off the mesial-distal center of the edentulous space, a design priority should be given to the surface with the straighter or flatter proximal morphology so it matches the contralateral tooth as it emerges from the mucosa.
When locating the implant in bone, the practitioner must take care to avoid approximating adjacent teeth, which can lead to a need for endodontic treatment, damage to the roots, or loss of the implant, or all of these.
The faciolingual positioning of the implant is also important to biomechanical success. Centering an implant beneath a posterior crown helps reduce the potential for biomechanical overload, a factor particularly important for molars or premolars when heavy occlusal forces are present.
In esthetically critical locations, a faciolingually centered position is preferred when existing bone dimensions permit. Lingual positioning of the implant produces a crown with deficient facial cervical contour or one in which porcelain must overlap the facial soft tissue to create the desired cervical crown morphology. The overlapping makes oral hygiene more difficult and will not be esthetically pleasing if the soft tissue recedes apically. Conversely, if the implant is placed too far facially, the facial bone becomes thin and subsequent remodeling may result in soft tissue recession and/or gray discoloration of the overlying soft tissue. Placing an implant too far facially can create such a substantial esthetic challenge that the implant may have to be removed, bone grafting performed, and another implant subsequently placed in a more favorable position (Figure 9-8).
The incisocervical/occlusocervical location of the implant is largely determined by the location of existing bone (Figure 9-9) and the esthetic need to transition from a smaller diameter round form to a larger diameter form with a different geometric perimeter. Typically, implants have been placed apical to the cementoenamel junction of adjacent teeth to permit the required changes in morphology to occur somewhat gradually. While an early textbook11 recommended that implants in the esthetic zone be placed 4 mm or more apical to the cementoenamel junctions of adjacent teeth, it is currently felt that implants should not be positioned this deep but should be located so the implant platform is approximately 3 mm apical to the predetermined midfacial margin of the mucosa.
In some cases, because of the position or dimensions (or both) of the residual bone in the edentulous area, implants cannot be placed in an ideal location. In such cases bone grafting should be used to enhance the location of the implant. If grafting is used and a positional deficit remains, or if grafting is not possible because of the patient’s choice or the added expense, compensatory designs should be used to reduce the overload potential. These design modifications are particularly important with molar implants because of the higher overload potential as a result of heavier occlusal forces and larger crown dimensions. Methods of overload compensation with molar implants include narrowing of the occlusal table, use of wider diameter implants (5 or 6 mm) (Figure 9-9), or use of two implants to support one molar crown (Figure 9-10 A, B).12
The centric occlusal contact between a crown and the opposing dentition should be light when the patient taps the teeth or holds the teeth together without clenching. With this type of contact, shim stock should not be firmly grasped and should just slide from between the crown and opposing tooth or teeth.
When the patient fully activates the masticatory muscles (as in clenching), the shim stock should be grasped with the same intensity that it is grasped between opposing natural teeth. This protocol helps prevent the implant crown from being in heavy occlusal contact when the patient clenches or bruxes the teeth.
Eccentric occlusal contacts should be avoided on posterior single crowns. Whenever possible on anterior teeth, eccentric contact should be shared between the implant crown and adjacent natural teeth during protrusive, lateroprotrusive, and working side movements.
Because occlusal habits such as bruxism increase the forces placed on implant single crowns, an acrylic resin occlusal device should be fabricated that the patient can wear during the times bruxism occurs.