Recent developments in digital technologies in dentistry have led to significant changes of treatment concepts in prosthodontics. Optical scanning allows digitalization of the initial clinical situation, delivering virtual files of the initial status of the dentition in the standard tessellation language (STL) format. The STL files can be input into a computer-aided design (CAD) software program and analyzed. Digital diagnostics, including digital waxing and trial restorations, can then be performed. Subsequently, the type and design of the planned restorations can be exported as digital files and milled or printed by means of computer-aided manufacturing (CAM) technology to fabricate CAD-CAM trial restorations. After tooth preparation, the planned restorations can be fabricated with the CAD-CAM technology.

Different types of ceramic and composite resin materials have been developed for fabrication, opening up new prosthetic treatments. Less-invasive restorations, such as veneers, onlays, and partial-coverage crowns, can be milled from ceramic or composite resin materials and cemented with adhesive material as an alternative to conventional fixed prostheses.

Studies have shown that conventional preparation of complete coverage ceramic crowns necessitates anterior or posterior tooth reduction ranging between 40% and 70%. In contrast, tooth reduction for ceramic veneers on maxillary central incisors ranges between 17% and 30%. Extensive tooth preparation can cause hypersensitivity and pulpal damage. For this reason, and consistent with the development of materials and adhesive dentistry, current prosthodontic concepts have proposed less-invasive restorations before preparing a crown.

One prerequisite for minimally invasive prosthodontic treatment is thorough pretreatment diagnostics to define the treatment goals. To provide the least invasive approach, clinicians should determine the exact area and amount of tooth preparation needed before clinical treatment. To define the preparation needed, pretreatment diagnostics, including diagnostic waxing of the planned restorations, are essential. The conventional methods of transferring this information to the patient are trial restorations and silicone matrices. Trial restorations reproduce the diagnostic waxing and can be transferred to the existing dentition. They are used to visualize and communicate the treatment outcome to the patient, dentist, and dental laboratory technician. Once approved, silicone matrices of the waxing can be used to define the tooth preparation.

Limitations of this conventional workflow include the time and effort needed to produce a conventional diagnostic waxing and trial restoration and their associated costs. One further limitation is the predictability of the transfer of this plan to the patient. Silicone matrices may help evaluate the area and amount of tooth preparation needed during the procedure, but the predictability of the procedure is not ensured. Silicone matrices deliver 2D information during the preparation and are limited to 1 or 2 buccolingual or mesiodistal slices. To ensure minimal invasiveness and optimal standardization, a 3D preparation guide is desirable.

The 3D information of maxillary and mandibular dentitions obtained by means of optical scans can be used for virtual diagnostics and treatment planning. Furthermore, in clinical situations with reduction of the vertical dimension of occlusion (VDO), a virtual increase of VDO can be performed. Optimal interocclusal clearances can also be determined in the 3D analysis by following the recommendations for different restorative materials. Locations without sufficient clearance are color coded on the 3D image (virtual 3D diagnostics), indicating the amount of tooth substance to be removed based on the 3D virtual diagnostics. To transfer this 3D information predictably to the clinical situation and to ensure least invasive tooth preparation, CAD-CAM preparation guides need to be designed and produced.

In this report, restoration of an extensively worn maxillary dentition without the loss of VDO but with space available is presented—category 2 according to the treatment options in the restoration of an extremely worn dentition. The VDO was increased to obtain space, but some preparation was still needed on the posterior teeth to achieve the minimal clearance for ceramic restorations.

The casts of a previously treated patient were used to evaluate this concept. A 3D diagnosis was performed to determine the needed amount of VDO increase and define the areas with insufficient clearance for the planned reconstructions after a virtual VDO increase. A 3D-printed guide based on the virtual 3D diagnostics was named K-L (Kwon & Lee) preparation guide, and it was used for the minimally invasive preparations.