Rationale

Implant-Borne Single Tooth Replacement -- An Illustration and Rationale

Richard K. Rounsavelle, DDS

Copyright 2001 Journal of the California Dental Association.

This article demonstrates a method for the replacement of a single missing tooth with a dental implant system that can simply and easily be incorporated into a general practice. Recent innovations in implant abutment design and impression procedures have resulted in a technique that is very similar to traditional crown and bridge procedures. This article describes a step-by-step protocol for the restoration of a single missing tooth with an implant-borne, cemented crown.

The replacement of diseased natural tooth substance with an optimal material is the essence of restorative dentistry. The restoration should be durable, be reasonably esthetic, and require minimally invasive procedures. Dental schools have taught the art and science of partial coverage cast gold restorations (Figures 1a and b) with the ultimate intent of conserving healthy tooth structure. The implant-borne single tooth replacement, like the partial gold casting, embodies the principle of tooth conservation which all dentists should espouse in serving their patients.

With an implant, the adjacent teeth, when healthy, need no longer be prepared solely for the purpose of providing support for a fixed bridge. On occasion, every dentist has seen a formerly healthy tooth progress through a series of destructive events following restoration for crown retainers. This scenario is now avoidable.

The ITI Solid Abutment System, described herein, heralded a breakthrough in simplifying the restorative process. Dental implants, formerly the province of a few specialists, now may be routinely accomplished by all general dentists.

Treatment Planning

A team approach is best for treating implant patients. The general practitioner should refer his or her patients to a qualified specialist for surgical placement of implant fixtures, along with bone grafting and soft-tissue manipulation where indicated. In consultation with a surgeon, the G.P. will share the responsibility for evaluating such factors as medical history, availability of supporting bone, soft tissue contours, occlusion, and cosmetics. The use of mounted study casts with a diagnostic wax-up of the replacement tooth greatly facilitates the shared evaluation of a given case. In addition to the wax-up, the G.P. may also provide guidance as to implant location by using a surgical stent or template for surgical implant positioning. This appliance may take many forms, but for single tooth replacement, a tooth-borne device is usually most appropriate. This device may transfer from the diagnostic wax-up the facial or lingual anatomy of the proposed final restoration, depending on the surgeon’s preference. This transfer conveys to the surgeon the G.P.’s intent for the final restoration at the time of implant placement (Figure 2).

Standard Abutment-Final Impressions

After the implant has "osseointegrated," or united to the surrounding bone, the patient is ready for restoration. The technique described involves a solid abutment, which is analogous to a tooth preparation (Figure 3); an impression cap and cylinder; and a laboratory replica of the abutment.

First, one places the white impression cap over the implant collar (Figure 4). This impression coping eliminates the need for tissue retraction and, therefore, local anesthesia. To be certain that complete seating has occurred, one should rotate the cap while viewing it from an occlusal direction. If the cap rotates without dislodging and the abutment appears to be centered, it is seated properly. Seating requires firm pressure and, when achieved, will result in a palpable "click" (Figure 5).

Next, the positioning cylinder (Figure 6) is placed into the impression cap. The trick here is to view the abutment again from an occlusal perspective to orient the flat section of the abutment and match it to the corresponding flat area of the positioning cylinder. It, too, will seat with firm pressure and must go completely into place (Figure 7).

One is then ready to take the impression. The best impression material to use is the most rigid: vinyl polysiloxane. Polyether and polysulfide also work well. Nonrigid materials, such as hydrocolloid, are contraindicated. One should be sure to block out any large undercuts created by teeth within the arch. Syringe hydrocolloid works well for blocking.

One should inject and place the tray as for any other crown impression. The cap and cylinder assembly should lift off the abutment and be embedded within the impression (Figure 8).

The actual "prep" or abutment that the lab will use is called an analog or abutment replica (Figure 9). This is a small metal component that one must next mate to the impression component assembly. Again, one carefully matches the flat side of the analog with the flat side of the positioning cylinder and firmly pushes it into place. If the analog does not snap into the impression cap, it is not seated completely (Figures 10 and 11).

The impression is now ready for the lab. The lab will use a prefabricated plastic coping (Figure 12) to act as a base onto which wax is added to create the appropriate contour from which the crown is completed via conventional procedures.

Temporary Crowns

The options for placing a temporary crown depend upon the patient and the situation. A very simple approach is to use temporary cement to place a temporary protective cap available from the manufacturer (Figure 13). If the patient requires a more esthetic temporary crown, one may modify a plastic coping (the same one used in the laboratory phase) and incorporate the coping with acrylic resin into a crown form or clear plastic shell. This latter technique requires more chair time but is greatly appreciated by a patient who needs a tooth and has been without a fixed replacement. The procedure for this is as follows:

First, one shortens, scores, and air abrades the coping in the lab. Then one seats the coping until it snaps into place over the abutment (Figure 15). The crown form or clear matrix filled with acrylic is allowed to set completely on the coping (Figures 16a and b). It is very important to remove all acrylic from any undercuts prior to the final set. The coping "over-crown" is then taken to the lab; and, very carefully, using the salt and pepper technique, one fills in the spaces between the acrylic and the margin of the coping (Figure 17). Once this is achieved, one may finish and polish to create the final crown contour and emergence profile. The final provisional must snap into place on the abutment to prevent soft tissue encroachment over the implant collar. To achieve this fit, one must remove a small amount of acrylic from the occlusal aspect of the provisional internally with a straight fissure lab bur. One then cements with temporary cement and thoroughly removes any gingival excess, as it commonly lodges just below the implant collar.

Delivery of Final Restoration

The beauty of this implant system is its simplicity and similarity to conventional crown and bridge procedures: One tries the crown in place, adjusts contacts and occlusion, and cements. The following are helpful hints on adjusting these crowns:

* One should be very careful in adjusting contacts because there is no periodontal ligament "cushion" on the implant to absorb small discrepancies. If the contact is excessive, the crown will not seat completely.

* One should relieve all eccentric occlusal contacts -- especially those that occupy the extreme periphery of the occlusal table.

* One should make certain that the centric contacts are as close to the long axis of the implant fixture as possible and are light. One should have the patient firmly clench to register contact with a mylar shim stock.

* If temporary cement is to be used, one should be aware that implant-borne crowns tend to dislodge very abruptly and unpredictably when luted with temporary cement (Figure 18).

* It is always a good practice to place an impression cap onto the abutment whenever one is working with the final crown away from the chair. This procedure will prevent the soft tissues from collapsing over the implant collar and from blocking complete seating of the final crown.

If the implant collar extends more than 2 mm subgingivally, one may minimize excess cement by displacing it with an analog prior to final cementing of the crown (Figure 19). When a large volume of cement is removed extraorally, the clean-up of set cement from the subgingival area is greatly facilitated.

Other Abutments

There are several abutments available to deal with other restorative situations and locations. For example, when the missing tooth has sufficient bone and mesiodistal dimensions, as in the molar region, a larger diameter implant with a wider platform is indicated (Figure 20). This increased diameter affords significantly greater implant surface area for osseointegration and a more-ideal emergence profile for the final restoration (Figure 21). A larger bearing area better distributes the load of posterior occlusion. The components and procedures are identical to those used with the standard abutment. Due to the posterior location of most wide-body implants, one would usually temporize with a simple protection cap. Cementation considerations are similar to the standard abutment; however, because these abutments are often shorter than the standard due to reduced interocclusal distance, the additional retention afforded by stronger cement may be needed. Retention may also be enhanced by a lateral set screw.

A narrow implant and abutment can be used for mandibular incisors, maxillary lateral incisors, and some small premolar areas (Figures 22, 23).

Custom Abutments

There will be situations in which the implant collar (the finish line of the restoration) is intentionally located deeply subgingivally to help the restorative dentist develop the optimum esthetics of the final restoration. This usually occurs in the anterior region and the mesial aspect of the maxillary first premolar. If the final crown margin extends to this deep margin location, a significant complication may occur during cementation. Excess cement can be driven over, around, and under the implant collar during crown placement. This subgingival cement can be very difficult, if not impossible, to remove completely; and these retained cement fragments may cause considerable soft tissue inflammation and infection.

The best way to avoid "deep" finish lines affecting implant-borne anterior crowns is to use the custom abutment (Figure 24). The clinician may then elevate the margin to a more coronal location. With the finish line, and thus the excess cement, made more accessible, the complication of excess cement removal is greatly reduced. There are other advantages of using the custom abutment in the anterior teeth. The long axis of the implant may be in a different inclination than that of the final crown. The custom abutment can correct this discrepancy in both labiolingual and mesiodistal directions. The resulting contour enhances tissue health and architecture for an optimum esthetic result (Figure 25).

To maximize the esthetic result, one should sculpt the surrounding tissues with a carefully made provisional crown prior to the final impressions (Figure 26). The technique that uses a plastic coping and stock crown or prefabricated clear shell works well. For the cosmetically demanding patient, one may modify the shade of the provisional crown by removing approximately 1 mm of labial acrylic and directly bonding microfill composite resin to the acrylic surface (Figure 27).

One may fabricate the custom abutment by casting metal to a stock screw-retained abutment. This custom abutment is secured to the implant with the appropriate screw, and the crown is cemented onto it.

Conclusion

This article has described a technique that will greatly facilitate the incorporation of osseointegrated dental implant restorative treatment into the general practitioner’s routine procedures. This technique is just a beginning. The principle and procedures discussed can be expanded to apply to a wide variety of more-involved restorative situations that may be more complicated. As one’s experience and skill increases, the way one thinks about cases with missing teeth will change considerably. The care one provides to patients will progress to a higher level, a level that fully respects the ideals of mentors who encouraged their students to preserve tooth structure with conservative gold restorations.

Author

Richard K. Rounsavelle, DDS, is a general dentist in Torrance, Calif.

To request a printed copy of this article, please contact/Richard K. Rounsavelle, DDS, 23560 Madison St., Suite 111, Torrance, CA 90505.

Legends

Figures 1a and b. Partial gold coverage can be both conservative and esthetic.

Figure 2. Implant positioning guide for single tooth with lingual acrylic of replacement tooth removed.

Figure 3. Standard solid abutment torqued into ITI implant fixture.

Figure 4. Standard abutment impression cap.

Figure 5. Impression cap in place over abutment.

Figure 6. Positioning cylinder for 5.5 mm solid abutment.

Figure 7. Positioning cylinder seated into impression cap.

Figure 8. Impression cap/positioning cylinder imbedded within final impression.

Figure 9. Analog of a 5.5 mm solid abutment.

Figure 10. Analog seated into place in the impression cap/positioning cylinder assembly.

Figure 11. Final laboratory cast for crown fabrication.

Figure 12. Plastic coping for laboratory procedures (seated on analog).

Figure 13. Protective cap for temporization.

Figure 14. Plastic coping shortened and air abraded for inclusion in an acrylic provisional crown.

Figure 15. Modified copings seated on solid abutment.

Figure 16a. Acrylic filled shell seated over modified copings.

Figure 16b. Shell with acrylic and copings incorporated.

Figure 17. Addition of acrylic to establish contour and marginal adaptation.

Figure 18. Final first premolar restoration cemented to place.

Figure 19. Partial displacement of excess cement using laboratory analog prior to cementation.

Figure 20. ITI Wide Neck abutment.

Figure 21. Final restoration showing a more favorable emergence contour achievable with wider implant (Tooth #31 was periodontally compromised).

Figures 22, 23. Very narrow restorations on the ITI Narrow Neck abutments.

Figure 24. Ceramo-metal custom abutment on tooth #8.

Figure 25. Final ceramo-metal crown cemented on the custom abutment shown in Figure 24.

Figure 26. Soft tissue that has been sculpted by a properly contoured provisional crown.

Figure 27. Provisional restoration veneered with microfill composite resin.