11.2 Impressions

Making an impression, which accurately represents the exact three-dimensional, implant positions and surrounding soft-tissue contours are the starting point of the prosthetic rehabilitation of a fully edentulous patient. The impression procedure is initiated 2–3 weeks after the implant uncovery. The patient would require well-made provisional restorations to sculpt the tissue prior to embarking upon the definitive restoration. In cases of immediately loaded restorations it is possible to initiate the impression procedures by removing the screw-retained provisional restoration. Approximately 3 mm of soft-tissue thickness would be considered optimum for achieving a good emergence for the final restorations. The height of the healing abutment should be such that it rises 1 mm above the height of the soft tissue [1]. In case the tissue level happens to be more coronal to the healing abutment it is prudent to replace the healing abutment with a taller one in order to condition the soft tissue (Fig. 11.1).

Flowchart 2 summarizes the approaches for making impressions for implant-supported restorations.

The conventional metal ceramic restoration can be secured to the implants directly or through the use of intermediary abutments. The abutment-level approach utilizes a multiunit abutment that is torqued on the implant and all the following prosthetic steps are performed over the abutments (Fig. 11.2). The implant-level approach entails using components that directly connect to the implant platform (Fig. 11.3). The abutment-level approach has the advantages of ease of managing all the prosthetic steps such as impressions, jaw relations and trials. Restorative space requirements may dictate which approach the clinician utilizes. When struggling with restorative space the implant-level approach may be more suitable as this will provide the clinician with an extra 2–3 mm of space.

Irrespective of the approach the prosthetic steps remain the same. Only the componentry for both the approaches differs. Impression techniques for implants in edentulous arches are similar to those of single or multiple implants in partially edentulous situations. The closed tray or indirect transfer technique is suited for single-unit cases or multiple implant cases wherein the implants are placed parallel to each other (Fig. 11.4). The open-tray impressions are preferred in cases where implants are placed deeper and have a thick band of soft tissue over them or when implants are not parallel to each other (Fig. 11.5). Full-arch impressions for implant-supported restorations require a high degree of accuracy for which the open-tray or direct pickup technique in a rigid custom tray is preferred [2–5]. The impression copings for closed-tray techniques are engaging type whereas the copings for open-tray techniques for full-arch impressions are non-engaging. This is necessary as engaging closed-tray copings on non-parallel full-arch implant placements will not allow removal of impression material without catastrophic distortion and that can lead to inaccuracies in fit of the prostheses across the arch.

The open-tray technique can be performed by attaching the open-tray impression copings unsplinted with the copings being picked up directly by the set impression material in a custom tray. Alternatively, the copings can be splinted to provide a more rigid fixation of the copings within the impression material (Fig. 11.6). The literature does not show any significant difference between the unsplinted and splinted techniques; however the standard deviation in the splinted techniques is much smaller. The authors support the use of the splinted technique [6]. Several techniques have been used for splinting of the impression copings such as autopolymerizing resin, dual-cured resins, plaster, splinting with prefabricated resin bars and splinting of copings with autopolymerizing resin splints which are sectioned and reconnected after setting [7–9].

The authors prefer to use autopolymerizing resin for splinting of the impression copings. The splinting of the copings should be done on a primary model extraorally. The resin jig should then be sectioned and a custom tray fabricated on top. The resin jigs are then attached in the mouth with a small amount of autopolymerizing resin and a pickup impression is performed using a custom tray (Fig. 11.7).

The two-step approach saves chair-side time and is less messy. However, the splints connecting the copings will need to be sectioned and reconnected intraorally with a quick-setting resin in order to achieve passive fit of all the copings. It is prudent to take radiographs of the copings to ensure proper seating of all the components.

Polyether or polyvinylsiloxane impression materials (Figs. 11.8 and 11.9) are the preferred options for making impressions due to their higher level of accuracy and dimensional stability [10–12]. A custom tray coated with an appropriate tray adhesive is preferred for making the impressions [13, 14]. However, even stock trays designed especially for the open-tray technique could also be used.

11.3 Interocclusal Records

The interocclusal should be made on a rigid base which is screw retained on the implants. A tissue-supported base like the ones made for complete dentures may lead to errors due to the compression of the soft tissues while recording the jaw relation [15, 16]. Often occlusal rims have been made which are directly screw retained to the implants or abutments. Taking jaw relation records using rims like this can be very laborious as they constantly need to be unscrewed to make adjustments. The authors propose a two-piece rim. The primary part is rigid and screw retained to the abutments or implants and has retentive grooves to receive the secondary part. The secondary part snaps directly onto the primary part. It may require some use of denture adhesive. The advantage of this approach is that the clinician can make adjustments to the wax rim without constantly having to unscrew the rim. This makes it more comfortable for the patient and more expedient for the clinician. A facebow record is made to orient the maxillary cast accurately on the articulator (Figs. 11.10–11.12).

11.4 Impression Verification

A verification of the master cast prior to milling will reduce the possibilities of redoing prosthesis framework and thus substantially reduce stress and treatment costs. It is an invaluable step and should not be overlooked by the clinician.

A verification jig connecting all the implants is fabricated on a master cast to verify its accuracy [17]. This jig must be made of a material that is rigid and does not flex [18]. Many clinicians have used acrylic resin jigs. One must be aware that acrylic resin is flexible and may give the clinician a false-positive result. The authors recommend the use of impression plaster, which must be made in appropriate dimensions. This plaster jig must be secured with one screw. Plaster being a brittle material will break in the case of misfit (Figs. 11.13–11.16).

11.5 Framework Trial

Often to transfer soft tissue information to the laboratory technician a pickup impression may be made with the patient’s existing provisional prior to framework try-in (Fig. 11.17). A full contour wax-up with controlled cutback should be performed (Figs. 11.18–11.20).

In complex full-arch cases it is advisable to do a framework try-in to verify passivity of fit has been achieved. The passive fit of the prosthesis is essential to reduce biomechanical complications [19].

The various materials that can be used as framework material options are enlisted in Flowchart 3.

Once the framework is received, it is first inspected for any surface irregularities. The cutback should be evaluated in reference to a putty matrix to ensure a controlled cutback with even thickness. Even if the framework is cast it should utilize machined fit components so that the fit of the restoration to the abutment/implant is not compromised.

The healing abutments or fixed provisionals are removed. The framework is then placed to check for passivity. The distal-most screw is tightened completely first and a radiograph of the contralateral side is taken to ascertain that the framework is fitting flush with the implant/abutment on that side. If the radiograph or screw test detects misfit (Sheffield test ) [19], it must be concluded that the impression was inaccurate. In this instance with a cast framework the framework must be sectioned, fit verified radiographically, joined in the mouth with pattern resin and transferred to the dental laboratory for soldering.

In case of milled frameworks this may not be a possibility. With titanium or zirconia frameworks the entire framework may need to be re-milled after making a new impression.

11.6 Second Interocclusal Record

At the time of the framework trial it is advisable to verify the jaw relation records by doing a second interocclusal record . This is a challenging clinical procedure as the vertical dimension may get inadvertently altered during the new bite record, thereby reducing the space that the lab had left for layering of ceramic. Hence it is best to take this record with help of an anterior jig that is fabricated on the articulator at the desired vertical dimension of occlusion and a rigid posterior recording material. Alternatively the lab may produce the framework with three metal occlusal stops. This bite record can then be taken with a bite registration material. The casts can be remounted at the accurate vertical dimension to fine-tune the occlusal contacts.

11.7 Bisque Trial

All the things that are checked in the framework trial pertaining to fit and accuracy of the prostheses have to be rechecked at the bisque stage. Additionally the aesthetic parameters have to be verified in a detailed manner to confirm the teeth position with the lip dynamics and the smile line of the patient.

The occlusion has to be checked to provide maximum intercuspation at the desired centric relation condylar position. Uniform contacts of equal intensity must be achieved on both sides and the anterior guidance should be effective in providing the desired posterior leeway and disclusion [20–22]. The anterior guidance must be within the envelope of function to respect the chewing pathway, thus avoiding any interference [23].

The tissue surface of the prostheses must be then assessed to verify a positive tissue contact on the edentulous areas between the implants. In the maxillary anterior region lack of such contact can lead to air escaping during speech leading to impediments in specific speech sounds. A modified ridge lap or an ovate pontic is recommended in this region of the prostheses. Similar design must be produced in the posterior regions to avoid food entrapment below the prostheses.

On screwing down the prosthesis over the implants there is usually some degree of soft-tissue blanching. Within limits this blanching is desirable and will disappear in a few minutes. Excessive blanching for a prolonged period may lead to tissue necrosis [24]. In such a situation it may be necessary to reduce the emergence contour of the restoration around the implant.

The undersurface of the prostheses must be highly polished following any adjustment.

Interocclusal contacts are evaluated and communicated to the dental technician. The contacts in the mouth should be similar to the ones on the mounted casts. Minor occlusal corrections in the lab may be necessary by ceramic addition. If the occlusion is significantly different then an occlusal registration will be required.

11.8 Prosthesis Delivery

The soft tissues around the implants should look in pristine health on the day of the prosthesis delivery (Fig. 11.21). Chlorhexidine gel is placed on the soft tissues around the implants [25].

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