Faster, More Predictable Molar Replacement

As with all implant therapy, molar tooth replacement is a restoratively driven modality, and it is now possible to fully synergize the surgical and prosthetic aspects of treatment through digital dentistry. This process is initiated by obtaining both cone-beam computed tomography (CBCT) images and digital impressions. With this digital information, the 3-dimensional position of the final restoration can be fully planned first, and then the surgical anatomy can be properly assessed to accurately and predictably plan the ideal implant position to support the final crown. This digital "top-down" implant planning allows for the delivery of a very streamlined and precise tooth replacement procedure that begins with the end in mind.

Software that allows for the union of CBCT image files (DICOM) and digital impression files (STL) is essential in the overall digital workflow. However, the mere fusion of this information is not sufficient. The software utilized also must allow for the complete, detailed planning of the final restoration as well as the corresponding implant position, in the same program (eg, the authors use NobelClinician^® Software; nobelbiocare.com) (Figure 1). The digital workflow can then be fully realized by precisely planning the 3D position of the final restoration and the technical aspects of the final restorative design to facilitate ideal and proper implant placement. The planned implant placement must allow for the correct biological outcome as well as provide the necessary environment needed for the restorative design to become a reality. The implant placement, which is calculated using a 4-dimensional perspective,¹ can then be transferred from the digital workflow to the oral cavity via guided surgery. This is a key aspect of the digital workflow. The fulfillment of digital implant planning through the use of a 3D-printed surgical guide allows for the implant to be placed in the most predictable fashion with the highest level of precision and accuracy, thereby ensuring the surgical outcome mirrors the planned outcome.²

Selecting the proper diameter implant to be used for a molar site is very important.³ Traditionally, the width of an implant determined the forces it could withstand as well as the beginning point of its emergence profile. However, with the introduction of "platform switching" to many implant systems, the emergence profile of the restoration begins from the width of the implant connection and not the actual width of the implant.⁴ Consequently, a large imbalance between the width of the implant and the width of the connection results in a much narrower initial connection width, requiring the implant to be placed more apically to allow for the space needed to develop the proper emergence profile. Selecting the most appropriate implant connection size to relate to the mesio-distal and bucco-lingual dimensions of the tooth being replaced is imperative to long-term success.⁵ Therefore, placing an implant (eg, the authors use the NobelParallel Conical Connection WP implant; nobelbiocare.com) that is larger than 5 mm in diameter (eg, 5.5 mm in diameter) with a connection size that is also larger than 5 mm in diameter (eg, 5.1 mm in diameter) allows for greater force distribution and tolerances in molar sites. Furthermore, this allows for a much more ideal emergence profile to be developed (Figure 2).

The transfer of the digitally planned implant position to the real position in the mouth via guided surgery is a crucial step in the digital workflow. When the final implant position is planned based on the ideal position, contours, and design of the final restoration, the need for adjunctive procedures such as osseous or soft tissue augmentation can be properly assessed. Benefits of fully guided implant surgery such as shorter procedures, minimal invasiveness, and greatly increased accuracy have been demonstrated throughout the literature.^2,6 The surgical guide is 3D-printed and transfers the precise digital planning from the software to the oral cavity (Figure 3). This step is critical to the restorative success of molar implant therapy because the specific position of the dental implant affects posterior emergence profiles, food impaction between restorations and the natural dentition, as well as esthetics.¹

The placement of a provisional implant restoration delivers restorative design to the implant site. As with any provisional restoration, it can be either screw-retained or cement-retained. However, the main purpose of a provisional restoration is to aid in shaping and contouring the soft-tissue bed to develop the ideal emergence profile.⁷ Therefore, the implant must be placed at an adequate depth, and there must be enough soft tissue present to be molded and sculpted. One technique that has been utilized as an alternative to a provisional restoration is the use of a customized gingival former. This allows for the soft tissues to be formed without having a restoration involved that is in occlusion. This technique works well, but can be time-consuming  chairside. The fabrication of a provisional restoration chairside using classical abutments and techniques can be technique-sensitive and time-consuming  as well. The materials commonly used for these custom solutions are composite resin and polymethyl methacrylate (PMMA) acrylic, both of which are less than kind to the soft tissues.⁸ One recently developed restorative approach involves the use of polyether ether ketone (PEEK) as a material for both gingival formers and provisional abutments that have been pre-milled as stock abutments with generalized emergence profiles built into them (Figure 4 and Figure 5). This provides immense benefit in the ease of fabrication of provisional restorations as well as the use of prefabricated gingival formers. PEEK has been shown to be very kind to the soft tissues, and it is adjustable.⁹ Through the digital workflow, it is now possible to fabricate a screw-retained provisional restoration utilizing PEEK abutments prior to the surgery and deliver it intraorally with some slight modifications on the day of surgery (Figure 6).

If the digital workflow has been conducted properly, then the 3D position of the final restoration and the design of the final abutment have been completed prior to any surgical implant therapy. The final step in the workflow is the design and fabrication of the final restoration. This begins by taking a digital or conventional final impression. In this workflow, a digital impression is preferred to allow for a more streamlined process; however, a conventional impression can also be digitized utilizing a laboratory-based scanner.

Final molar restorations can be designed to be either screw-retained or cement-retained. With evidence mounting that cement retained around implant restorations can result in peri-implant conditions, screw-retained final restorations have become the design of choice.^10,11

Traditionally, a screw-retained crown was fabricated by baking porcelain onto a metal-based custom abutment. Although this provides a favorable solution in that it can be very esthetic, it is also flawed because porcelain may readily chip or break off of these restorations long-term.¹² With new innovations in restorative design, materials such as monolithic zirconia have become a reality and offer a very tissue-friendly, biocompatible option.^13,14

A full-contour zirconia crown provides an esthetic alternative with only minimal porcelain layering using a tooth-colored material. However, until recently, this type of restoration could only be utilized for screw-retained solutions by extraorally cementing the CAD/CAM-created zirconia crowns onto a titanium abutment to create a "screwmentation" type of implant restoration. This can create long-term issues with respect to cement retention and the debonding of restorations.

Historically, the main concern with creating a screw-retained restoration has been the location of the screw access channel, which has limited the use of screw-retained restorations in situations where the trajectory and position of this access channel is an esthetic or functional concern. In addition, when posterior access to the implant site is restricted due to patient factors, such as a limited oral opening, screw-retained restorations can be difficult to deliver.

These difficulties can be overcome by using a screw-retained crown that offers an angulated screw channel (eg, the authors use the NobelProcera Full-Contour Zirconia Implant Crown; nobelbiocare.com) (Figure 7). The technology has allowed clinicians to overcome access issues by re-angulating the screw channel up to 25 degrees, thereby creating screw-retained restorations with favorable esthetics and ease of delivery in the posterior.

Coupled with this technology is the ability to mill the restoration from a highly esthetic, full-contour monolithic zirconia material that fulfills all of the functional and esthetic requirements of a posterior molar site (Figure 8 through Figure 10).

Routine molar tooth replacement therapy using dental implants has recently been elevated to new levels. Using a digital workflow, the process can now be almost flawlessly executed, allowing today's clinician to provide a result that is superior in design, placement, and efficiency. Fully digital implant planning and treatment using highly precise, restoratively driven surgical procedures not only increase the accuracy and predictability of this treatment, but also allow for the delivery of a final result in fewer appointments. Furthermore, the placement of cement-free restorations using new materials and mechanics reduces the incidence of implant-related conditions and is rapidly becoming the standard of care. Today, the fully digital molar implant solution is a reality.

Sundeep Rawal, DMD
Private Practice
Melbourne, Florida

Bobby Birdi, DMD, MSc
Private Practice
Vancouver, British Columbia

Angus Barrie, RDT
Chair, Board of Directors
College of Dental Technicians of British Columbia
Richmond, British Columbia