Choosing the right type of abutment

Transcription

1 50 Producing custom implant abutments using CAD/CAM Choosing the right type of abutment S. KHALILOVA 1, F. KISTLER 2, S. ADLER 3, S. WEISS 3, S. KISTLER 2 AND J. NEUGEBAUER 2,4 Rapid developments in the field of computer-assisted design and manufacturing CAD/CAM have created many options for choosing the right type of abutment for crown or bridge restorations. This allows the dental technician to determine the necessary design parameters in cooperation with the dentist, as the standard abutments offered by the implant manufacturers often do not result in the best possible rehabilitation for all indications and implant positions. CAD/CAM offers operators a choice between performing chairside modelling and using the design service of the production centres. Introduction Implant systems for crowns and bridge restorations include prefabricated titanium abutments specifically tailored to the relevant implant connector geometry. Depending on the implantological concept, one or more types of angulated abutments may be offered in addition to straight abutments. To allow tension-free insertion of the restoration, some minimal rotation of the abutment relative to the fixture must be permitted so that the position of the antirotational device can be transferred faithfully [1]. But to optimize the transmission of forces to the inside of the implant and to prevent abutment mobility, the connection geometry must allow only a small amount of play of the connector. In cemented restorations, the prefabricated abutments may pose problems if the margin of the implant crown is located far subgingivally, making the removal of excess cement difficult [2]; and if oral hygiene is possible only with great effort, this may lead to peri-implantitis and a risk of implant loss (Figs. 1a and b). However, it was found that when using prefabricated abutments, the insertion of a crown with a natural emergence profile was not possible in all situations, depending on the implant position that could be achieved. Furthermore, if the implant posi- 1a I Peri-implant bone loss following initial mucositis in an unfavorable abutment configuration on implant 26. 1b I Advanced bone loss in non-refractory peri-implantitis. 1 Tashkent State Dental Institute, Tashkent, Uzbekistan 2 Dr Bayer, Dr Kistler and Dr Elbertzhagen, Private Practice, Landsberg am Lech, Germany 3 Implant-Dental-Consult, Landsberg am Lech, Germany 4 Interdisciplinary Policlinic for Oral Surgery and Implantology and Department of Oral and Maxillofacial Plastic Surgery, University of Cologne, Germany

2 tion was relatively low, these abutments exhibited an insufficient vertical dimension and therefore did not provide reliable support for the prosthesis, which resulted in cast-to abutments being introduced to complement certain implant systems [3]. But these precision cast-to abutments are very difficult to use; depending on the quality of the material and workmanship, corrosion problems may materialize, especially in the implant sulcus. As an alternative to the implant manufacturers abutments, third-party modifications of standard abutments became popular some years ago, which were also priced lower than the original system components [4]. However, these abutments often exhibited a lack of fit or in the case of more complicated connection geometries a pattern of transferring loads to the implant that did not follow the design principles of the system vendor [5]. This in turn could lead to screw loosening or fracture of the implant or abutment [6]. Design rules When designing custom abutments, the gingiva region must be shaped as a conical flare so that a physiological crown profile can be achieved. Excessively convex or concave shapes should not be used, as this might result in too much pressure of the structure on the soft tissue or in pronounced niche formation, both of which make conventional oral hygiene measures exceedingly difficult [7]. Furthermore, especially abutments with a concave profile do not offer the option of moving the gingival aspect or the preparation margin further apically following a try-in. Preparation margins should be positioned slightly subgingivally [8]. This implies that the margin will be higher interproximally than vestibularly or orally. The tooth preparation design should ensure that the crown receives balanced support. The preparation shoulders should be between 0.5 and 1.5 mm thick, depending on the crown material used. Particularly in molars, the configuration of the cusps must be taken into account. The height of the abutment should ensure that vertical reduction (as during occlusal adjustments) is possible while still providing sufficient retention for the crown. This can be done automatically by the CAD/CAM design software with appropriate parameter settings. When fabricating custom abutments for single crowns with high interproximal contacts in the presence of ample soft tissue with a discernible vestibular reduction, it is helpful to incorporate additional grooves in the abutment to provide sufficient stability for the crown. By contrast, parallel grooves on abutments supporting a bridge reconstruction Aesculap Ergoperio The New Class of Excellence Ergoperio combines modern design with unique ergonomics and top functionality Flawless performance Easy-to-grip surface Pleasant ergonomics Pioneering aesthetics The Winner of the if DESIGN AWARD 2015 Category Medicine / Healthcare Aesculap a B. Braun company Aesculap AG Am Aesculap-Platz Tuttlingen

3 52 2a I Digital model with registration of the scanbody for the implant system used (Sky unifit titanium base; bredent medical, Senden, Germany). 2b I Definition of the soft-tissue contour for the design of the custom abutments. 2c I Producing the CAD/CAM abutments intended to support a bridge. 2d I Representation of the abutments without a soft-tissue flare but with a conical contour in the emergence profile. 2e I Definition of the parameters of the cementing space for the bridge rehabilitation. 2f I Definition of the cross-section and size of the connectors for the bridge design. are not advisable as they can lead to problems of fit and corresponding tension in these areas in case of insufficient residual rotation at the implant-abutment connection (Figs. 2a to i). Not only the height of the preparation but also the preparation angle will affect the retention of the bridge. Particularly in bridge restorations, a greater wall angle should be provided for higher preparations to ensure that the friction is not too high and that a precise fit can be achieved on cementation [9]. Where a large discrepancy exists between the implant diameter and the profile of the replaced tooth, as is frequently the case when using reduceddiameter implants, the abutment shoulder must be extended to the point where the crown will not

4 53 2g I Final design of the three-unit bridge in the CAD/CAM software (exocad Dental; CADstar, Bischofshofen, Austria). 2h I Radiograph of the restoration. There are no transition lines from the abutments to the bridge. 2i I Clinical check of the ZrO 2 bridge two months after delivery. compress the soft tissue. Otherwise, the compression could result in damage to the soft tissue on insertion. In addition, the precision of fit could be examined only radiologically. Material White abutments have been advocated for aesthetic restorations, especially in the anterior maxilla [10]. When one-piece zirconia/ceramic abutments are used on implants with a precision connector geometry, there is a risk of abrasion due to micro-movements at the antirotational interface. The resulting increased mobility of the ceramic abutments translates into a risk of connector damage, as the connectors are usually designed for metal abutments. The risk of screw mobility is particularly high in allceramic abutments because these have an orthogonal screw hole, which eliminates the traditional advantage of a conical screw with a corresponding clamping effect. The risks associated with such monolithic ceramic abutments (with the potential damage to the connector geometry) can increase as a function of the expertise (or lack thereof) of the respective production centres [11]. It is therefore recommended to use titanium sleeves in the aesthetic zone, to which the customized portion of the abutment can be adhesively connected [12]. Monolithic crowns Particularly in the posterior region, a monolithic crown can be used for a single-tooth restoration instead of an abutment to support a conventional crown. For this purpose, the screw-access hole should be located centrally on the occlusal surface, so that the thickness of the material is sufficient for shaping the cusps and to avoid the presence of a

5 54 3a I Mark-up of the scanbody once the intraoral impression has been taken (Omnicam, Cerec; Sirona, Bensheim, Germany). 3b I Mark-up of the emergence profile for the crown design (Cerec 4.3; Sirona). 3c I Definitive crown design and gingival mask in the construction stage. 3d I The design on the titanium base with the representation of the implant. lingual or vestibular aperture. The transgingival aspect of the abutment can be replaced by a ceramic or composite crown to be connected directly to the titanium sleeve [13]. This results in an axially screwretained crown which does not require cementation. The screw-access hole can be closed permanently with a composite resin in an aesthetically acceptable manner (Figs. 3a to h). Production process Two different concepts have been developed for the fabrication of custom implant abutments. One concept entails custom CAD/CAM abutments that can show an excellent fit and sometimes boast approval by the original implant manufacturer for the relevant milling centre [14]. The other concept entails abutment bases provided by the implant manufacturer to which a custom abutment is added and secured adhesively or using the pressing technique [15]. These CAD/CAM abutments can be produced either centrally or locally, that is, at the laboratory or the dental office. If a production centre is chosen, there are again different ways of arriving at the final design. For example the dental technician can digitize the cast and represent the implant position by a so-called scanbody, then design the abutment itself in the CAD/CAM software model and submit the design data to the production centre for fabrication of the abutment. Here it is important for the position of the scanbody to be accurately detected and assigned to the type of implant used. Alternatively, the cast with the implant analogue and reduced mucosal contours can be sent to the production centre, which then creates a design proposal to be approved by the dental technician or dentist [16]. With local production in the dental practice, the emergence profile of the crown can be determined as soon as the digital impression has been taken. Direct oral impressions eliminate the cast production step, making the abutment or crown available quickly [17,18]. Based on the position of the screw access channel, the implant abutment and the crown are then separated in an additional step,

6 55 3e I Milling preview showing the screw-access channel for the axially screwed abutment crown. 3f I Monolithic crown made of lithium disilicate adhesively connected to the titanium base (SKY unifit titanium base for Cerec; bredent). 3g I Clinical view of the monolithic crown on implant 25. 3h I Control radiograph with partially translucent representation of the implant-supported crown 25 (IPS e.max CAD; Ivoclar Vivadent, Schaan, Liechtenstein). which facilitates an anatomical design. Thus, the transgingival aspect of the anatomical abutment and the crown can be produced in one step [17]. Discussion Custom abutments permit an optimal design of implant-supported restorations in terms of aesthetics and function. Supporting the soft tissue within the emergence profile with the aid of the implant abutment prevents the accumulation of food and bacteria in voids and helps ensure a physiological contour of the peri-implant soft tissue. Depending on the available soft tissue, a convex shape can even extend the soft tissue, allowing the inter-implant structures to grow to their desirable full height [8]. In addition to these aesthetic advantages, custom abutments also have the advantage that cementation is not associated with any complications because the close subgingival preparation margin allows for easy removal of excess cement [2]. This reduces the risk of peri-implantitis triggered by residual cement. When it comes to the adhesive bonding of ceramic abutments to the titanium bases, there have been misgivings regarding the biological stability of the adhesive gap. It is recommended to employ methods where light-activated polymerization maximizes the curing action of the adhesive and produces an adhesive strength that exceeds the load values of the implant/abutment connections [12]. Temporary bonding is contraindicated in this situation because adhesive residue would develop as the bonder dissolves and remain in the immediate vicinity of the implant/bone contact area. A list of references will be supplied by the editorial office on request. Contact address Dr Jörg Neugebauer, PhD Dr Bayer, Dr Kistler and Dr Elbertzhagen Private Practice, Landsberg am Lech Von-Kühlmann-Straße Landsberg am Lech Germany neugebauer@implantate-landsberg.de