Case Report - A tried and trusted concept in a new formula
Dr Dorit Freitag, ZTM Sebastian Kaufmann, 03 February 2015
Whereas bar-retained overdentures have up to now primarily been made from metal alloys and/or zirconium oxide, for some time now high-performance polymer PEEK has also been available as a workable option. The team of writers describes this prosthetic care option and explains how the material PEEK (JUVORA Dental Disc, JUVORA Dental Ltd.) is able to improve treatment results.
The treatment of toothless jaws using prosthetics is a perennial topic of interest. With an increasing number of people in the 65+ generation, no significant change in this state of affairs can be expected in the future. What is changing, however, are patients’ expectations regarding restoration. Requests for implantsupported restorations are becoming ever more frequent. Modern therapies mean that the treatment team can now offer a multitude of prosthetic implant concepts—from fixed dentures with many implants through to combination dentures anchored onto a lower number of implants. In selecting the right approach to treatment, many factors have to be considered. For example, aspects of general health and anatomy must be taken into account as well as the wishes and financial means of the patient. Moreover, foresight should be exercised and the implications of ageing should not be ignored.
In our implant-based prosthetic portfolio, the bar-retained denture has become a tried and trusted treatment concept for toothless lower jaws. The small number of implants (four implants) that can be inserted into the jaw depending on the available bone, enables a less invasive procedure. The cleanability of the dentures, their stability and ease of insertion and extraction in general lead to high patient satisfaction. In the case study below, bar insertion into a toothless lower jaw is described with details being given of the manufacture of the superstructure and the special material combination.
Material selection for the bar
Up to a few years ago patterns made from a non-precious-metal alloy or titanium were considered to be the means of choice for bar construction. An increasing number of non-metal concepts are now gaining in importance, on the one hand based on the requirement for a prosthesis that is as metal-free as possible as well as for aesthetic reasons. High-precision (CAD/CAM) manufacturing processes enable the use of biocompatible and stable materials. For example, zirconium oxide has become a tried and tested material for the primary bar that is easy to process using CAD/CAM. For the over structure (overdenture) we have up to now worked with an electroplated matrix which did indeed work relatively well, but did not meet demand for a non-metallic solution. Moreover, in many cases, after the dentures had been worn for a lengthy period of time, a high level of material abrasion and wear to the soft electroplated gold on the hard zirconium oxide bar was discernible.
A year and a half ago we discovered a material which, in addition to a high degree of biocompatibility, promised us a long-term successful treatment solution: polyetheretherketone (PEEK). This high-performance polymer is particularly suitable for removable or partially removable dentures as it offers all the advantages of a plastic (light weight, non-metallic, ease of processing etc.), without having the associated restrictive properties. PEEK has been tried and trusted in human medicine for many years being used, for example, to repair defects in the skullcap or as a vertebral spacer. PEEK is bio-inert, tissuefriendly, cytotoxic, non-harmful, nonconductive and thermally insulating.1–3 The semi-crystalline high-performance plastic has impressive mechanical characteristics, low weight, high ultimate strength and outstanding resistance to chemicals. Its modulus of elasticity is similar to that of cancellous bone. PEEK dental bar patterns can be CAD-CAM produced from industriallymanufactured blanks (JUVORA Dental Disc, JUVORA Dental Ltd.) and as described in the case described below. For industrially-produced milled discs the manufacturer uses PEEK-OPTIMA (Invibio) in its pure form which has been in use for decades in human medicine; no colouring additives, adjuvants or processing aids are used, in keeping with our claim to material purity.
Patient case study
Four implants (CAMLOG) were inserted into the toothless lower jaw of the 60-year-old patient. After a six-month healing phase, it was time to manufacture the final overdenture. An open impression was taken using impression posts (CAMLOG) and a custom impression tray (Figure 1). We produced the master with a removable gum mask, which we consider to be essential. The gum mask is a guarantee of optimum basal formation of the restoration and serves as a fit check for the dental bar on the model. The set-up of the prosthesis was defined as early as at the stage of planning the implant positions, and the temporary prosthesis produced accordingly. A duplicate of this prosthesis now served us as a bite template using which the models could be correctly placed in the vertically correct position in the articulator (Figures 2 to 4). The tooth position determined in the planning phase served therefore as the basis for the construction of the primary bar and as a base for the final wax set up.
Fig. 1: Pick-up impression of the healed implants in the toothless lower jaw. – Fig. 2: The duplicate of the temporary dentures acts as a bite template.
Fig. 3: Articulated models. Lower jaw implant model with laboratory abutment. – Fig. 4: Lower jaw model with gum mask in the articulator. – Fig. 5 and 6: The lower jaw model with the applied bar abutment (CAMLOG). The gum mask is removed here. – Fig. 7: The construction of the parallel-walled primary bar.
CAD/CAM-manufactured primary bar (zirconium oxide)
Notwithstanding all due ambitions for dental technology and fine craftsmanship, expense and viability should also come into the equation. Working concepts have to meet economic criteria as well as technical dental requirements. To manufacture a bar manually is timeconsuming and subject to error. We have, for years, been using CAD/CAM technology and prefer to use zirconium oxide as a material for primary bars.
Zirconium oxide is very strong, is tolerated very well by the mucous membrane and has a realistic colour, making it highly suitable for use in implant prosthesis treatment. Apart from material diversity, CAD/CAM-based manufacture meets all requirements in terms of materials, clinical precision and quality. Intraoral cementing is necessary to ensure a perfect fit for the bar—an essential criterion for success. The bar caps (CAMLOG Vario SR) were screwed onto the model (Figures 5 and 6) and the situation digitised on the laboratory scanner (D800 3Shape). After import of the data into the software (3Shape) we constructed a parallel-wall primary bar (Figure 7). The software offers the option of incorporating additional retaining elements into the design. For example, attachments are deposited which resemble the Vario-Soft retentions (bredent). These were adapted to the bar in this case on both distal sides. The CAD data was sent to the milling machine (550i, imes-icore) and milled with a suitable nesting strategy from the zirconium oxide blank (Bio ZW iso, Dental Direkt) (Figure 8). As the profile of the adhesive bases is stored in the software, the recesses for the capsules were precision-milled. The bar, after sintering, fitted the titanium adhesive bases precisely (Figures 9 to 11).
Fig. 8: Zirconium oxide bar in the unsintered state. – Fig. 9: After sintering: exact fit of the zirconium oxide bar on the model.
Fig. 10: Basal view of the zirconium oxide bar with integrated titanium bond bases. – Fig. 11: The bar is transferred to the practice for intraoral bonding. – Fig. 12: The healing caps are unscrewed … – Fig. 13: … and exchanged for titanium bond bases. – Fig. 14: Intraoral adhesion of the bar with the titanium bases (passive fit). – Fig. 15: Try-in of the wax set-up for the lower-jaw prosthesis.
Before we sent the sintered primary bar to the practice for intraoral bonding, a wax set-up was manufactured in the laboratory with prefabricated artificial teeth. The temporary prosthesis acted as a basis. The healing caps were removed from the mouth of the patient, (Figure 12), the bar structures and titanium bonding bases applied to the implant using the securing pins (Figure 13) and the primary bar adhered to the adhesive bases (Multilink Implant, Ivoclar Vivadent) (Figure 14). After the adhesive had hardened, the bar could be removed with the titanium bases and set-up in the patient’s mouth could be finally verified in a try-in (Figure 15). Only slight aesthetic corrections were necessary. The overdenture could now be completed.
CAD/CAM-produced secondary construction (PEEK)
First the excess glue between bar and titanium bases was removed (Figure 16) and the primary bar parallelised at 0° in the milling machine. In this context it should be pointed out that attention should be paid to water cooling during the processing of a zirconium oxide pattern in order to prevent damage to the material structure. It is also important to be aware that only an ultra-smooth zirconium surface will ensure proper function. We work with a laboratory turbine (water cooling) and special diamonds for the milling device (Zrgrinding kit for 0° double crowns, Komet Dental) (Figure 17). The shapecongruent grinders are available in four adapted granularities (processing stages) and in a few steps allow the perfect surface to be created. In the construction of the overdenture, in order to produce an optimum master, the primary bar and the situation of the verified wax set-up are combined in the software. For this first the primary bar was scanned on the model. In a second stage, we scanned the silicone matrix of the setup (with prefabricated teeth) and could now combine the data as desired. Virtual construction took place using a known procedure in the desired design.
Fig. 16: After intraoral bonding: the excess adhesive between bar and titanium bases is now removed. – Fig. 17: The zirconium oxide bar is parallelised and smoothed in the milling device using the laboratory turbine (water cooling). – Fig. 18 and 19: The bar and the silicon wall of the set-up (with prefabricated teeth) are prepared for digitisation in the laboratory
In the construction of the overdenture, in order to produce an optimum master, the primary bar and the situation of the verified wax set-up are combined in the software. For this first the primary bar was scanned on the model. In a second stage, we scanned the silicone matrix of the setup (with prefabricated teeth) and could now combine the data as desired. Virtual construction took place using a known procedure in the desired design. Now the advantage of the digitised silicone matrix was apparent. We were able to look at the position of the teeth using the superimposition function and adapt the pattern under it (Figure 21). Such an accurate adaptation of the secondary structure in the software vastly facilitates subsequent completion of the restoration. The data was sent to the milling machine. As material we chose a high-purity and unfilled PEEK (JUVORA Dental Disc, JUVORA Dental Ltd). The industrially manufactured blanks have CE approval for a definitive partially removable denture (Figure 22). The optimum physical and chemical properties, good fit and low weight impressed us. Light, with a tooth-like colour and biocompatible— according to the manufacturer no material wear is to be expected. After milling (Figure 23) and the separation of the pattern from the blank, a 1:1 copy of the virtual master (Figures 24 and 25) appeared. Without reworking, the secondary structure fitted the model and over the primary bar (Figures 26 and 27). After checking the design using the silicone matrix, manufacture of the prosthesis could commence.
Fig. 20a and b: The design of the secondary structure in the design software resembles a NEM cover. – Fig. 21a and b: Representation in the overlay function enables precise adaptation of the pattern to the required set-up
Fig. 22: The milled-out PEEK structure in the CAD/CAM machine. – Fig. 23: The PEEK blank before separation of the pattern. – Fig. 24: Basal view of the overdenture after separation from the pattern… – Fig. 25: … and from occlusal. Hardly any reworking is necessary. – Fig. 26 and 27: Very good fit of the PEEK secondary pattern over the zirconium bar. – Fig. 28: Checking the design using the silicon wall and pre-fabricated teeth.
As the set-up represented the final tooth-set, it could be relatively quickly transferred to the PEEK pattern (Figure 28). In order to obtain a secure bond between the PEEK pattern and the veneer PMMA, we undertook surface treatment in accordance with the manufacturer’s instructions using the Rocatec procedure (Figures 29 and 30). The conditioned pattern could now be completed with the prefabricated teeth fixed in the silicone wall; we did not consider application of a gumcoloured opaque to be necessary. The polished synthetic teeth were deposited with tooth-coloured plastic and the prosthetic could then be veneered and finished as normal (Figure 31). In this case we used a simple process for a natural result. In the event of high aesthetic requirements, the gum area could be customised with various coloured compounds and the prefabricated teeth also customised. Yet here it was a case of complying with patient’s requirements regarding expectations and options. The prosthesis was neatly finished and polished (Figure 32). The basal sections, among other things, were given a great deal of attention as were the transitions between the PEEK structure and the PMMA synthetic. Before transfer of the denture, the Vario-Soft attachment was applied.
Fig. 29: The PEEK surface is pre-conditioned using the Rocatec process. – Fig. 30: Close-up of the conditioned PEEK surface. – Fig. 31: Completion: Coating with gum-coloured opaque is not necessary in the opinion of the authors. – Fig. 32: The completed replacement teeth on the model.
The finished overdenture was impressive with an aesthetically clean image without any metal structure and a low weight. The prosthesis was immediately accepted by the patient. After removal of the healing caps (Figure 33), the bar abutments were screwed onto the implants and the zirconium oxide primary bar integrated using a corresponding tightening torque (Figures 34a and b). The overdenture was placed without difficulty (Figure 35). Insertion and removal are possible due to the smooth gliding characteristics of the PEEK secondary part on the zirconium oxide bar. The prosthetic sits “firm” in the mouth and integrates harmoniously into the patient’s face. After checking all relevant parameters (aesthetics, function, phonetics), the patient could be discharged with appropriate hygiene instructions.
Fig. 33: Basal view: matrices (Vario-Soft) are installed in the PEEK rider. – Fig. 34a and b: The primary bar is screwed on using appropriate tightening torque. – Fig. 35: The denture is installed. The glide properties between the zirconium oxide bar and the hold of the denture are optimum.
In prosthetic dentistry, PEEK has for some years represented new possi - bilities. In the case described, the secondary structure of a bar-retained over-denture was manufactured from PEEK. The gliding properties on the primary zirconium oxide bar are outstanding. The prosthesis glides more softly than with a metal construction. Simple insertion and removal and hygiene abilities are ensured. A metal cover could be dispensed with, which is of great aesthetic benefit, among other things. Alternatively, the secondary structure could have been manufactured using an electroplated structure, which, in our opinion, would guarantee neither long-term durability nor biocompatibility (nonmetal). The financial aspect too should be taken into consideration for the patient, as gold for the electroplated cover alone would cost several hundred euros. The PEEK material used (JUVORA Dental Disc) has been tried and trusted in use for more than ten years for implants in the medical field (Invibio PEEK-OPTIMA) and the high level of biocompatibility has been proven in several clinical trials. Even in prosthetic dental medicine PEEK has been tried and trusted as a pattern material. The low specific weight, bone-like elasticity, absence of metal, its toughness, combined with almost non-existent material wear, make this material an ideal aid to prosthetic dentistry. The opportunities offered by CAD/CAM-based processing allow for many options. The new formula described in the article (zirconium oxide, PEEK, PMMA) for a tried and trusted concept (bar-retained overdenture) provides for process-optimised material processing (CAD/CAM) and efficient manufacture of non-metal restoration. The perfect fit necessary for long-term success is ensured with the intraoral bonding of the primary bar.