Although nonrigid connectors have been advocated in the literature,18,40 a nonrigid connector in a unilateral prosthesis rarely is indicated for implant-fixed prostheses and may be detrimental. Nonrigid connection does not improve the stress distribution between the different abutments36,38 and has been reported to have caused migration of the natural teeth.41-43 If the nonrigid connector exhibits any clinically observed mobility, it moves more than the implant. As such, the implant-supported part of the restoration is cantilevered to the attachment. In addition, the nonrigid (or mobile) attachment adds cost, creates overcontoured abutments, impairs daily hygiene, and does not decrease the clinical tooth movement.
Reports of intrusion of the natural tooth connected to an implant usually include the use of temporary cement to lute a coping to the natural abutment, leaving the final restoration uncemented on the coping, or the use of a nonrigid connector41 (Figure 17-32). When implants are joined to teeth that act as a terminal abutment, a definitive cement should be used for the natural tooth. The tooth cannot intrude unless it becomes unretained from the abutment (or has a nonrigid connector between the units).
FIGURE 17-32 A, An implant prosthesis fabricated to join implants and teeth in the same prosthesis. Copings were designed for the teeth. B, A panoramic radiograph of the implants and prosthesis in place. C, The teeth intruded from the prosthesis. The cement seal must separate in order for the tooth to intrude (or a nonrigid connection exists between the units).
A possible explanation for tooth intrusion may be that the tooth is pushed vertically 28 microns but wants to rebound only 8 microns. The fixed prosthesis rebounds immediately and pulls on the tooth. The cement seal eventually breaks, causing a space to develop, which is first occupied by air. The prosthesis then acts as an orthodontic appliance and continually pushes the tooth in a vertical direction. Eventually, the space is occupied by saliva, and hydraulics continue the downward force during mastication. The tooth eventually submerges or intrudes from the prosthesis.
Implant Pier (Intermediary) Abutments
A pier abutment is one between two other abutments, sometimes referred to as an intermediate abutment. The intermediate abutment may be an implant or a natural tooth, and each type plays a different role in the overall treatment. When an implant serves as a pier abutment between two natural teeth, the difference in movement between implant and tooth may increase the complication rate compared with one intermediate tooth joined to two terminal implants (Figure 17-33). The pier implant abutment exhibits less movement than the natural teeth terminal abutments and acts as the fulcrum of a class I lever. As a consequence, a compressive force on one end of the prosthesis is converted to a tensile or shear force on the other terminal abutment.18 The cement tensile strength is often 20 or more times less than the compressive strength. Therefore, when the implant acts as a fulcrum, an uncemented abutment (usually the least mobile tooth or least retentive crown) is a common consequence, with decay being the next most common occurrence.
FIGURE 17-33 A pier implant abutment between two natural teeth may cause a cement seal to break on the teeth, especially if one is more mobile than the other.
This problem is magnified by a longer lever arm such as a pontic between the implant and tooth, when the natural tooth or teeth have clinical mobility, the force is lateral on the prosthesis, or the forces are greater than usual. A pier implant abutment may cause complications even when joined to nonmobile teeth as terminal abutments.
Uncemented restorations are a common complication in FPDs even when all aspects of treatment are within acceptable limits. Any condition that may increase this problem, such as the one presently addressed, should be carefully avoided. When bone grafting is not an option and additional implants cannot be inserted, a mobile attachment can be used to help restore the implant pier abutment (Figure 17-34). A nonrigid attachment connects the implant and the least nonmobile tooth to prevent the implant pier abutment from acting as a fulcrum.
FIGURE 17-34 When grafting and additional implants are not an option, a mobile attachment may be used to prevent the pier implant from acting as a fulcrum.
The nonrigid attachment is used between the implant and the more mobile tooth. In conventional fixed prostheses, the “male” portion of a nonrigid attachment usually is located on the mesial aspect of the posterior pontic, and the “female” portion is in the distal aspect of the natural pier abutment tooth. This prevents mesial drift from unseating the attachment.40 However, an implant does not undergo mesial drifting, and the nonrigid connector location is more variable in location.
Natural Tooth Pier Abutments
When a natural tooth rather than an implant serves as a pier abutment between two or more implants, the situation is completely different from the previous scenario. When the two or more implants may support the load of the prosthesis alone, the natural tooth becomes a “living pontic” (Figure 17-35). In other words, the tooth is not needed to support the prosthesis, and the rigid terminal abutments constitute the entire support system for the prosthesis. In absence of the tooth, the dental unit would be a pontic without compromise. Because the tooth has greater mobility than the terminal implants and does little to contribute to the support of the prosthetic load, it is referred to as a pontic with a root, or a “living pontic.” No more than one adjacent site should be a pontic, so a three-“pontic” span does not exist. Therefore, this scenario is best when no additional pontics are between the implants and the tooth. For a natural pier abutment between two implants, a stress breaker is not indicated.
FIGURE 17-35 When a natural tooth serves as a pier abutment between two or more implants, the tooth may act as a “living pontic.” No stress breaker is needed in this situation.
On occasion, multiple implants in a full-arch prosthesis are splinted together to cantilever one or two pontics, yet a healthy, natural tooth is positioned between the implants. The tooth essentially is ignored in the development of the treatment plan, other than the dentist having to fabricate a crown rather than a pontic in the splinted prosthesis (Figure 17-36). One advantage of keeping the natural tooth, even though it does not contribute to the support of the prosthesis, is the proprioceptive aspect of the periodontal complex.34 Implant prostheses have higher bite forces during mastication than natural tooth restorations because of the decrease in occlusal awareness. A living pontic may decrease the interaction of the forces found during function.
FIGURE 17-36 A, The mandibular restoration has five implants and two natural teeth splinted together, so a cantilever may be used to replace the posterior teeth. The five implants support the load of the restoration. The natural teeth pier abutments act as living pontics. B, An intraoral view of the fixed prosthesis. The natural teeth may give some proprioception to the restoration, especially when in the canine position. The support of the restoration is primarily from the implants, which surround the teeth and are splinted together.
Transitional Natural Abutments
On occasion, because of the lengthy aspect of implant treatment, especially when bone regeneration procedures are indicated before implant placement, initially maintaining strategic teeth (even with a poor prognosis) as interim restoration abutments may be desirable. These teeth are often terminal abutments that support a fixed temporary restoration, protect edentulous implant or graft areas from mastication trauma, and avoid the use of a removable soft tissue–borne partial interim prosthesis. These teeth are extracted after initial implant healing, and often these teeth are in an ideal implant site for the final restoration. When this occurs, the implant then is placed in the extraction site as a second surgical phase. This approach is beneficial to provide the patient with a fixed transitional prosthesis and to avoid soft tissue–supported restorations on bone augmentation sites but may extend the overall treatment by 6 months.
The transitional abutment scenario is most common in a full-arch rehabilitation patient who has a full-arch fixed transitional restoration on periodontally involved teeth. The prognosis of these abutments may be poor, mandating their extraction (less than 5-year survival category). However, if all the compromised teeth are extracted, the patient must wear a full immediate denture as a temporary prosthesis while grafting and implant insertion phases are performed. The psychologic and physiologic changes associated with a denture, even if a temporary solution, may have dramatic consequences for the patient. These patients may benefit greatly from a stepped approach in which a few poor, short-term, asymptomatic dental elements are maintained while all others are extracted for the sole purpose of providing the patient with a fixed temporary restoration44 (Figure 17-37).
FIGURE 17-37 A, A panoramic radiograph of hopeless maxillary teeth. B, The teeth were splinted together with a transitional acrylic restoration to replace the missing teeth. C, The prosthesis was removed. D, Implants were inserted in strategic extraction sites, and three teeth remained to support the transitional prosthesis during initial integration of the implants. E, The transitional prosthesis was relined and recemented on the three transitional teeth.
The careful selection of the transitional abutments must not hinder the implant treatment. However, an extended treatment time with additional implant placement surgery may be required. For example, four compromised teeth dispersed throughout the arch may be kept for a fixed transitional restoration. Meanwhile, other sites are extracted, grafted, and implanted. When these implants are healed and ready to be restored, the “temporary” natural abutments may be extracted and additional implants placed. The healed implants now may support the transitional prosthesis. The new implants, on occasion when the bone density and biomechanical factors permit, may be immediately restored with a modified transitional restoration.
The advantage of the transitional abutment procedures is that a fixed prosthesis maintains the patient throughout treatment, protecting the implant surgical site during the submerged healing phase. Disadvantages include additional cost, increased time, risk of implant site contamination if any problem or flare-up of natural abutments occurs, and increased risk for the initial implants because the foundation is not completely sufficient for support until the additional implants are healed. The dentist should weigh the advantages and risks of such a treatment carefully before proposing it to a patient.
A fixed interim prosthesis in an edentulous arch also may be supported by three to six additional implants placed in function immediately at insertion to permit the fabrication of a temporary fixed prosthesis while all other implants are submerged.45 The dentist evaluates these additional implants at the time of final prosthesis fabrication and may or may not include them in the final restoration, depending on their status at that time. Transitional mini-implants also have been developed to that effect.
Caution is needed in using additional implants of normal or minimized dimensions because the volume of bone used for their placement may be of strategic value during treatment and risks being destroyed by fibrous tissue formation or bone resorption when immediately loaded, which may affect a final prognosis. Such treatment options are indicated only on a case-by-case basis.
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