Despite digital progress, the impression remains the most important link between clinical and laboratory dental procedures and is coresponsible for the subsequent accuracy of fit of the final restoration. Mistakes that arise here cannot be corrected during subsequent stages and the precision fit of the prosthetic restoration is influenced by the summation of errors.
The dimensionally accurate three-dimensional reproduction of the oral structures, their surface characteristics and positions are the most important clinical criteria of an impression - especially when taking impressions of implants – because it is not possible to compensate for any inaccuracies of fit with implants, in the same way as it is with natural teeth, due to their osseointegration. Any stress occurring here is inevitably transferred via the implant to the surrounding tissue and is subsequently expressed in a range of distinctive clinical manifestations, such as a loosening of the abutment or peri-implant inflammatory processes and necrosis. Therefore, implant impressions in particular require standardised methods, in order to guarantee stress-free seating of the superstructure. Almost all modern impression materials, such as addition silicones and polyethers, are suitable for achieving a precise transfer impression. However, addition silicones are preferred to polyethers due to their superior physical values. Even the previously highly valued and advantageous properties of polyether (such as early hydrophilicity and snap-set) are now also found in modern addition silicones such as the impression materials from DMG.
Implant impressions can either be taken using the direct (pick-up) or indirect (reposition) process, or in the case of single implants, directly from the polished abutment. In all cases one should either use a singlestage single-phase or two-phase impression method. Depending on the various clinical impression processes, it is necessary to use a range of tray systems. In the case of pick-up impressions it is advisable to utilise ready-for-use or individually manufactured open metal or plastic trays.
With indirect techniques it is possible to use semi-customised ready-for-use metal trays or individual plastic trays. In this case it is essential to use closed-tray systems due to material factors. DMG offers a broad range of materials for use in the generation of impressions for implant-borne prostheses. As such the practice team is able to utilise the entire DMG product range, from the situation impression right through to the precision impression. This allows the team to retain a clear overview, despite the mass of impression products on offer from various suppliers. The alginate alternative, StatusBlue, is well-suited for taking situation and opposing bite impressions. It is flexible and sets rapidly, and can be mixed using the MixStar-eMotion for convenience. The MixStar-eMotion extensively standardises the mixing process, facilitating economical work, delivering bubble-free materials and a predefined time scale through visual and acoustic signals. The precision impression material, Honigum, contains a patented rheologically active matrix, which allows the material to retain its flow characteristics under pressure and to exhibit stability without the application of force. Honigum possesses optimum thixotropic (rheological) properties, this means that the material exhibits flow characteristics when the teeth are immersed in the material, but retains its stiffness at the edge of the tray and offers the requisite degree of resistance when positioned. Honigum-Heavy is particularly well suited for implant impressions because its high Shore-A hardness means that it is possible to guarantee secure fixing of the transfer posts.
Case description
A 76-year old patient arrived at my practice with pain emanating from tooth 23.
Initial findings (fig 1)
The clinical manifestation was an apical swelling on tooth 23 with a degree of mobility 3, caused by a longitudinal root fracture following a root canal filling and prosthetic restoration with a metal-ceramic crown on gold pins. Following an X-ray diagnosis, local anaesthetic was administered and tooth 23 was extracted. The crown was secured between metal-ceramic restorations on teeth 22 and 24 using adhesive bonding as an interim solution. The patient was instructed to return to the practice for a check-up, during which the intraoral diagnosis would take place.
Maxilla: There was an inadequate, purely implant-borne, prosthetically restored partially edentulous arch with a transcoronal screw-retained metal-ceramic bridge restoration. Implants in sites 13, 14, 15, 21, 24 and 25; partial implant in the bone in site 11 following abutment fracture. All implants fully osseointegrated. Physiological occlusion was not assured. However, no function-related temporo-mandibular joint disorder existed.
Mandible: There was an inadequate, fixed and removable, prosthetically restored partially edentulous arch with free-end saddle right and clasped interim denture. After recording the findings, a situation impression was taken of the maxilla and mandible using StatusBlue, in order to generate planning models and for model analysis (figs 2 and 3).
Treatment plan
Replacement of the upper prosthesis, replacement of tooth 23 and re-establishment of optimum occlusal conditions with a luted all-ceramic bridge restoration, Prettau all-zirconium, transparent.
The posterior teeth to be milled fully from zirconium and the anterior teeth additionally covered with ceramic.
The mandible to be restored with implants at tooth 43 and 46 and a fixed prosthetic restoration.
Treatment procedure
First session: First, the prosthetic restoration was carefully removed and the periodontal periimplant condition and implant osseointegration appraised. It was necessary to carefully remove a fractured fixing screw from the implant shoulder at 25 by means of ultrasound. The bridge was subsequently reattached with new fixing screws, with a tightening torque of 15Ncm.
Second session: After removing the bridges, cleaning the implant shoulders, careful gingiva correction with a diode laser, Lina 10-D from Intros, the impression posts for the pick-up impression were screwed in place. The impression was taken with Honigum-Heavy and Honigum-Light using a Miratray implant impression tray from Hager & Werken. However, a mandibular impression tray was used, which was palatally reinforced on the situation model using light-cured tray acrylic. This resulted in maximum stiffness of the tray together with a perfect fit with the dental arch (fig 4). This process enables use of a very precisely defined quantity of impression material around the impression posts (fig 5), which reduces internal stress in the impression material, impression contraction and surplus material (figs 6 and 7). The maxillary bridge was subsequently screwed back in place and checked for stress-free seating.
Third session: During this session three temporary abutment posts were screwed in place at sites
21, 25 and 15 for the facebow registration, as well as an initial provisional centric registration with pink extra hard wax plates and aluminum wax.
Fourth session: Following removal of the maxillary restoration, it was possible to insert an acrylic-based maxillary bite registration plate, produced in the dental lab, with integrated temporary abutment posts for the second centric splinting. The keying was carried out through manual guidance of the mandible in Pattern Resin stops. The impression posts, individually produced in the lab, were subsequently inserted once more. In order to do this, the impression posts were fixed together using a Pattern Resin plate and sawn into three sections:
? block 15–13,
? block 21, and
? block 24–25.
Once the stress-free seating of the posts had been checked and verified, it was possible to cover the thin joints with liquid Pattern Resin (fig 8). A pick-up impression of the individual segments, splinted in this way, was taken with the aid of an individually produced plastic tray, Supertec (fig 9), Honigum-Heavy and Honigum-Light (fig 10).
Fifth session: During the fifth session the fitting of the final abutment took place. Given the intention that the finished work would be produced from transparent partially tinted Prettau all-zirconium, it was necessary in this case, and for aesthetic reasons, to use regular titanium abutments with zirconium caps bonded over the top. This prevented the abutment from being visible through the finished work. A resin mock-up (fig 11) was subsequently checked in the patient’s mouth in order to assess the seating, aesthetics and functionality and a Honigum-Mono pick-up impression was taken over it, for determination of the precise soft tissue situation (fig 12). This resin template can be corrected in this situation as required. The technician subsequently used this item for milling the zirconium and produced a precise copy of the temporary bridge. What you see is what you get (fig 13).
Sixth session: During this session the provisional definitive fixing of the prosthetic work took place (fig 14). This consisted of a Prettau all-zirconium frame (the occlusal sections of the bridge were also produced from highly polished zirconium). Only the anterior teeth, from 13–23, were fitted with labial ceramic covers in the form of veneers (fig 15) for aesthetic reasons.
Conclusion
In implant prosthetics, in particular, it is of decisive significance to precisely determine and fix the three-dimensional position of the individual implants. Using the pick-up technique, high precision impressions are obtained in implant prosthetics. This impression technique must be carried out using customised trays and is best applied with addition silicones because these materials (in particular when taking an impression of multiple implants) deliver more precise impressions and are considerably less sensitive to manipulations in the production process due to their higher Shore-A hardness. The Honigum impression materials, from DMG, offer the experienced practitioner mechanical, rheological and physical material properties that deliver superb results in every clinical situation. Only in this way is it possible to produce dimensionally accurate dental models, which enable the dentist to carry out prosthetic work at the highest functional and aesthetic level with results that deliver a precision fit.