Juan Carlos Ezquerro Sánchez and Dr Carlos Alomar, Barcelona/Spain
Cast full-arch frameworks have clear disadvantages for both dental labs and dentists as compared to CAD/CAM full-arch frameworks. A few of these include increased remakes, labor-intensive procedures, gold alloy cost and, not less important, inaccuracy, which can have a significant influence on implant survival and undesired bone loss. There is a general consensus in the dental community that CAD/CAM frameworks are not just a valid but a superior alternative to cast full-arch frameworks, but at the time of this writing the author did not find any articles in the literature on this type of restorative solution with Straumann system. The purpose of this article is to evaluate a new solution on the market to allow Straumann users to have the benefit of these advanced restorative techniques.
The CAD/CAM CAM StructSURE system by Biomet 3i was used to scan, design and mill two full-arch bars. Both bars were used to treat maxilla and mandible in the same patient. The maxilla was restored with a primary bar and the mandible with a hybrid bar. Both bars were seated on SynOcta abutments (Straumann). The patient, a 56-year-old male, non- smoker, has worn complete dentures for ten years and needed a fixed solution. His requirements were essentially functional, although the aesthetical aspect couldn't be neglected.
Two hybrid dentures were prescribed, each supported with six Straumann implants. During surgery we observed insufficient bone quality in region 16 and decided not to place an implant there. We were left with five implants in the maxilla and six in the mandible (Fig. 1). After six weeks, good integration had been achieved and restorative treatment could be started. Since we did not place an implant in regio 16, we opted for a solution with mucosal support to avoid excessive lever action on the five integrated implants.
Finally, we decided to use two different frameworks. For the maxilla, we prescribed a telescopic overdenture on a primary bar with galvanized friction elements, and for the mandible a screw-retained hybrid denture. For maximum accuracy and passive fit, we chose the CAD/CAM bars made by Biomet 3i. They are milled from a solid block of TiAlV alloy, are fully biocompatible and stronger than commercially pure titanium frameworks.
As our target was high precision in the frameworks, we followed the working protocols to attain an excellent result. The first step was to take an accurate impression by using a technique known as "rigid impression splints" (FRI), developed by Drs Fernández Arenas and F. Pérez Paniagua and dental technicians José A. Torcuato and Juan M. Fernández Arenas.
The splint was created by connecting cast metal cylinders placed around the impression copings, leaving some clearance around them (Fig. 2).To insert the splint into the mouth, we first fixed an impression coping on the cast with a mixture of fast-setting plaster (20 % Diadent-Siladent and 80% SnapStone) to serve as a guide for the rest of the unit (Fig. 3).
When the guide is inserted, the cylinders on the splint should not touch any other impression coping so as to keep the entire unit passive. Figure 4 shows an impression splint in position, but note that the copings do not touch each other.
To take the impression, the plaster mixture was prepared to a creamy consistency and, with the aid of a syringe, injected into the rings around each analog. Theoretically, the only variation in size would be the minimal expansion occurring when the plaster sets in each ring. Using small quantity, the rigidity of the ring walls and the connections between the rings ensured a very accurate dimensional replica.
After ten minutes setting time, the splints were fitted with individual acrylic perforated impressionTrays.
Precision resin-bonded impression copings are useful for short sections, but with large arches, the flexion effect of the resin makes them less accurate and with angled implants, their elasticity may result in an attachment that does not have the final structure. With the plaster set and the area cleaned, fluid silicone was applied to replicate the soft tissues. At this point, the position of the implants was secured by the splint.
The next step was to insert a resin tray containing some heavy silicone to top up the volume (Fig. 7). In this stage, it is advisable to clean the screw sleeves repeatedly to make it easier to loosen the splint.
It required approximately 45 to 60 minutes for taking impressions of both arches. Although this might seem excessive, it saved us numerous tests and future appointments. We could be practically certain that any framework we fitted to the cast would fit perfectly. The mold was cast using plaster with a very low expansion coefficient to ensure the greatest possible accuracy. Furthermore, the highly rigid metal splint helped to compensate for micromotion while the plaster was setting (Fig. 8). It is important to stress the need of new implant analogs to prevent
future problems with the fit.
We used a Condylator No. 100 set for Gothic arch tracings to determine the centric relation and vertical dimension in edentulous patients (Fig. 9).
The Gerber literature recommends using fast-setting-plaster indexes to fix the position of the arches. This is undoubtedly a very high-precision method, but for practical purposes, hard silicone indexes work equally well and are more comfortable for the clinician and patient. We used the Condylator Vario and Candulor teeth for the mount. Although both prostheses will function as fixed prostheses, we used this configuration to optimize occlusion and function, based on the same type of occlusion as for complete dentures. This type of bilateral balanced occlusion helps to prevent occlusion interferences.
Following an initial test of the diagnostic wax-up, we sent the casts and tooth mounts to Biomet 3i for the bar design. Clear instructions for the dental technicians in the production plant were provided. Before machining the bar, Biomet 3i sent us several images of the framework design for our approval.
These images show our mounts in detail in relation to the casts, final prostheses, implants and soft tissues. We can also see how the CAD design corresponded exactly to the end product and, most importantly, that the accuracy of the fit of the frameworks on the cast and in the mouth was exceptional. The radiographic control confirmed the visual perception on the cast and it is very hard to detect the transition between implants and prosthesis.
As we were not able to place an implant in the posterior part of the first quadrant, we made a primary bar for a telescopic overdenture supported on the distal mucosa with a galvanized-gold retention system.
The primary framework had a cone angle. This type of milling, with a distal cantilever, is extremely complicated and laborious using traditional laboratory methods, especially bearing in mind the perfect fit to be ensured by the galvanized friction elements (which will not tolerate deviations in the milling angle).
We placed three friction elements, rather than a galvanized retainer, over the entire length of the bar because we were concerned that excessive friction would prevent the patient from removing the overdenture from the bar. In the end, the triangle formed by the electroplated elements was more than adequate.
Before inserting the teeth, the friction elements were placed on the upper framework.
We used Attachment Bond by Heraeus Kulzer, which is a dual curing luting resin designed for attachments.
We slightly retouched the upper anterior group a little to improve the aesthetics of the prosthesis, making slight adjustments when mounting the teeth.
We made a mount according to the bilateral pupillary line, occlusal plane and the midline. When trying-in, we noted that the patient's midline was not exactly straight, so we inclined the midline of the incisors slightly to make the final result much more harmonious and satisfactory for the patient.
Conclusion
The CAD/CAM CAM StructSURE bar solution proved to be an optimal alternative for restorations in Straumann cases. Radiographic control of both frameworks showed a level of accuracy very difficult to achieve with traditional methods (Fig. 17). In conclusion, CAD/CAM bars have evolved from being a sporadic alternative to traditional cast methods into a first choice in our daily work.
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