New CAD/CAM Designs for Fixed Implant Restorations

Fixed implant-retained partial dentures and single crowns have traditionally been constructed with abutments to which the restorations are cemented, or as block units that are screw retained into the implants.^1-3 PFM frameworks or full-coverage restoration metal castings have been commonly used for the screw-retained format. Zirconia frameworks provide the clinician with additional choices for material selection in tooth-borne as well as implant-supported restorations.4-7 New advances in CAD/CAM technology now make it possible to have screw-retained fixed implant restorations utilizing all-zirconia frameworks to which small portions of add-on porcelain are fired.
The following case represents an innovative approach to zirconia-based, screw-retained implant bridges and crowns.

CASE REPORT
Diagnosis and Treatment Planning
A 75-year-old male patient presented with failing restorations and recurrent caries on several existing teeth. A comprehensive medical and dental exam was conducted. Diagnostic casts were mounted on the Denar 330 articulator (Whip-Mix), intraoral and extraoral photos and a panoramic radiograph (Figure 1) were taken, and a consultation was then performed to discuss treatment options with the patient. After a thorough explanation of advantages and disadvantages of various approaches to treatment, the patient agreed to the following treatment plan:
Extractions would be done on teeth Nos. 2, 15, 18, and 19 and implants would be placed in the Nos. 3, 5, 14, and 19 sites. The patient would be restored with an implant fixed partial denture Nos. 3 x 5 and implant crowns in the Nos. 14 and 19 sites. Very little bone grafting would be needed to house these implants due to an ample quantity and quality of existing bone.

Figure 1. Preoperative panoramic radiograph reveals multiple missing and carious teeth.	Figure 2. Nobel Biocare Replace implant healing abutments in site Nos. 3, 5, and 14.
Figure 3. Replace Select Wide Platform implant (Nobel Biocare) implant healing abutment in site No. 19.	Figure 4. The Denplant Impression Trays (Global Dental Solutions) come in small, medium, and large sizes.
Figure 5. Master impression with impression copings and replicas ready to pour the master cast.	Figure 6. Acrylic verification jig luted together intraorally.
Figure 7. Large contour framework design shown on Procera (Nobel Biocare) software.	Figure 8. Zirconia framework design shown from the occlusal and implant platform views.

Clinical and Dental Laboratory Work
At the next appointment, the extractions and implants were placed. Following a 3-month implant integration period, the 4 above mentioned implants were torque tested and found ready to begin the restorative process (Figures 2 and 3). Healing abutments were removed from all implant sites and impression copings connected. Periapical radiographs were taken to confirm complete seating of the impression copings into the implants. The Strong-Massad Denplant impression tray (Global Dental Impressions) was sized and heat molded to produce an accurately fitted custom tray (Figure 4). The maxillary and mandibular implant level impressions were made using a combination of Aquasil Ultra Putty, Monophase, and Light impression material (DENTSPLY Caulk). The assistant hand mixed the putty and placed it into the trough of the Denplant tray. She then indented the putty with a finger and injected the medium viscosity Monophase material into the arch form indentation. The author injected Light Viscosity impression material around the impression copings intraorally and inserted the impression tray with putty and monophase material. The master impressions were removed from the mouth after a full 5 minutes’ set time according to the manufacturer’s directions (Figure 5). The maxillary master and mandibular impressions, bite registration, and face-bow transfer for the Denar 330 articulator were sent to the dental laboratory team, who poured the impressions in stone and mounted the models.
The laboratory team returned an acrylic verification jig (GC America) to our office. The separate sections of the jig were luted together intraorally with self-curing resin (GC America). The entire jig was then checked against the master cast to confirm that it fit on the cast as it did in the mouth. This verification jig procedure and confirmation assures accuracy of the master cast relative to the implant positions intraorally (Figure 6).
The dental laboratory team then completed a full-contour wax-up of the implant fixed bridge Nos. 3 x 5 and single crowns for Nos. 14 and 19. A small cutback of the wax pattern was then performed and scanned to an electronic file using Procera scanner and software (Nobel Biocare) (Figures 7 and 8). The file was sent to a milling center where all frameworks were robotically fabricated in zirconia. There were now 4 shades of zirconia available for selection to enhance natural looking aesthetics with the completed restorations. The CAD/CAM milled zirconia frameworks were returned to the dental laboratory where add-on porcelain was fired and characterized. The completed restorations thus consisted of all ceramic material, including the interface with the implant platforms.

Figure 9. Completed zirconia frameworks with add-on porcelain, ready for delivery.	Figure 10. Completed zirconia fixed partial denture restoration seated on the Denar 330 articulator (Whip-Mix).
Figure 11. Maxillary screw-retained restorations seated intraorally.	Figure 12. Maxillary screw-retained restorations with composite closure of the screw access holes.

Figure 13. Close-up view of composite seal of screw access holes. The aesthetics of the zirconia/composite interface mimics that of an all-porcelain occlusal surface.

Try-in and Delivery of the Final Restorations
The completed restorations were returned to our office for intraoral try-in and delivery (Figures 9 and 10). Healing abutments were removed from each implant. The restorations were seated and fixed into place with retaining screws (Figure 11). The screws for all restorations were torqued to 35 Ncm using a torque wrench (Nobel Biocare). The occlusion was checked and found to require no adjustments. The patient, a veteran of numerous dental restorations, voiced a complimentary remark on the precision with which his teeth came into contact.
The access holes for the implant screws were sealed intraorally with the following protocol:
1. Inject fast-set, light-body impression material over all implant screws sufficient to cover the screw heads with 2 mm or more depth remaining in the screw access holes. Allow the impression material to set for 3 minutes. Then apply 35% hydrofluoric etch (Porcelain Etch and Silane Kit [Ultradent Products]) to the porcelain at the occlusal region of the screw access hole for a period of one minute after isolating the restorations with a 2 x 2 gauze throat pack, cotton rolls, and mouth prop. Have the dental assistant hold a high-speed evacuation tip adjacent to the porcelain surface to remove irritating vapors which the acid reacts on the porcelain.
2. Apply Etch Arrest (Ultradent Products) into the etchant until its amber color is no longer identifiable. This will neutralize the porcelain etch.
3. Rinse and dry the etch material from the access holes.
4. Apply silane coupling liquid with a microbrush to the etched porcelain. Allow the silane to remain undisturbed for a minimum of one minute. Thin and dry the silane with the chairside air tip.
5. Apply bonding agent (Prime N Bond [DENTSPLY Caulk]) to the etched/silanated porcelain. Using the air syringe, lightly blow off the bonding agent. Light-cure for 10 seconds.
6. Inject composite into the remaining access hole up to the occlusal surface of the restoration. Light-cure this material for 40 seconds.
7. Adjust occlusion with fine and extra fine finishing diamonds. Polish with a felt cone and polishing paste (Luminescence [Premier Dental Products]).
The resulting appearance of the sealed access holes almost makes them disappear as they blend aesthetically with the adjacent porcelain (SR Ivocron [Ivoclar Vivadent]) (Figures 12 and 13). This is in stark contrast to PFM implant screw-retained crowns that we have used in the past. It has been difficult to keep the metal framework substructure from showing through in the occlusal opening, even with opaque sealers and composite placed over from the screw head to the occlusal surface.

DISCUSSION AND CLOSING COMMENTS
The primary advantages of this type of implant restoration include potentially greater resistance to fracture due to the large monolithic framework design, enhanced occlusal aesthetics, and facilitated retrievability of the screw-retained restoration versus cementation methods. If the implant crown or bridge restoration requires reshaping or interproximal adjustment after insertion and use by the patient, it can be easily retrieved and corrected.⁸ A permanently cement retained restoration, however, often has to be destroyed and replaced with a new one in order to remedy the problem. In addition, there is a growing concern regarding excess cement being retained subgingivally with implant crowns and bridges. Peri-implantitis and even loss of the implant can occur if excess cement creates sufficient inflammation and infection to produce bone loss down the length of the implant. The use of screw-retained implant restorations can eliminate this potential problem while new framework designs provide strength and aesthetics that are comparable to cement retained restorations.

Acknowledgement
Grateful appreciation is expressed to Mr. Momo Vasilic (mOmO Laboratories, Anaheim Hills, Calif) for the talented and conscientious efforts exemplified in the dental laboratory procedures needed to complete this case. He can be contacted at momolaboratories.com.

References

1. Lee A, Okayasu K, Wang HL. Screw- versus cement-retained implant restorations: current concepts. Implant Dent. 2010;19:8-15.
2. Sherif S, Susarla SM, Hwang JW, et al. Clinician- and patient-reported long-term evaluation of screw- and cement-retained implant restorations: a 5-year prospective study. Clin Oral Investig. 2010 Sept 1. [Epub ahead of print]
3. Shadid R, Sadaqa N. A comparison between screw vs. cement retained implant prostheses: a literature review. J Oral Implantol. 2010 Nov 23. [Epub ahead of print]
4. Giordano R, Sabrosa CE. Zirconia: material background and clinical application. Compend Contin Educ Dent. 2010;31:710-715.
5. Guess PC, Att W, Strub JR. Zirconia in fixed implant prosthodontics. Clin Implant Dent Relat Res. 2010 Dec 22. [Epub ahead of print]
6. Kapos T, Ashy LM, Gallucci GO, et al. Computer-aided design and computer-assisted manufacturing in prosthetic implant dentistry. Int J Oral Maxillofac Implants. 2009;24 suppl:110-117.
7. Manicone PF, Rossi Iommetti P, Raffaelli L. An overview of zirconia ceramics: basic properties and clinical applications. J Dent. 2007;35:819-826.
8. De Boever AL, Keersmaekers K, Vanmaele G, et al. Prosthetic complications in fixed endosseous implant-borne reconstructions after an observations period of at least 40 months. J Oral Rehabil. 2006;33:833-839.

    

Dr. Strong maintains a full-time private practice in Little Rock, Ark. He is a consultant to various dental companies including Nobel Biocare and DENTSPLY. Dr. Strong is a co-founder of Global Dental Impression Trays. He can be reached at strongdds.com.

Disclosure: Dr. Strong is a consultant to Nobel Biocare and DENTSPLY, and he is vice president of Global Dental Impression Trays.