On the variation of the vertical dimension when mounting analog and digital models in the maximum intercuspation position (MI)

In the allocation of the maxilla and mandible in the maximum intercuspation position, which is used as a standard procedure in reconstructive dentistry, inaccuracies in the process chain both in the analog and the digital workflow produce misalignments of the derived STL files after assembly of the models/orientation of the STL files in the analog and virtual articulator space, so that corrections of the vertical relationship of the mounted analog or digital models are required.
Materials and Methods: The presented in-vivo study includes 53 healthy participants (32 females and 21 males) without functional disturbances of the craniomandibular system, with ages ranging from 18 to 79 years. Conventional precision impressions have been performed of the maxilla and mandible of the subjects. The upper and lower plaster models were mounted conventionally in a dental articulator type Artex CR. After digitalization, they were digitally mounted in the digital environment of the Artex CR digital articulator. In addition, the intraoral full-arch scans of the maxilla and mandible were assembled and consecutively mounted in the digital environment of the Artex CR digital articulator, with intraorally acquired buccal scans while the teeth were seated in the maximum intercuspation position (MI). In the analog workflow, the position of the mandible was determined by the mounting the plaster model of the lower arch with an intraorally achieved silicon wafer in the maximum intercuspation position. The three-dimensional position of the plaster model was transferred to the digital articulator space and defined as the reference value (zero position) of the lower arch. The vertical mounting position of the analog model of the mandible was decreased into the so-called ideal vertical dimension (IVD) by occlusal corrections of the plaster models. The vertical dimension after the occlusal correction was transferred again to the digital articulator; this represents the difference in vertical dimension of the mandibular models after elimination of premature contacts of the plaster models. The extent of the vertical reduction from the reference value to the IVD was defined as difference value 1. In the digital workflow, the full-arch scans (STLs) of the subject’s maxilla and mandible as well as the intraorally taken buccal scans of the right and left side in closed- mouth position were used for the digital mounting of the full-arch scans (STLs) of upper and lower arch of the subject. The derived vertical height of the mounted digital full-arch scans of the mandible was used to form difference value 2.
Results: For the analog clinical gold standard, the mean vertical reduction of the mounting position of the mandibular model was 0.201 mm (SD: ±0.079 mm) is (difference value 1), while the virtual reduction of the digital models indicated by the penetration of the STL datasets when using intraoral full-arch scans and intraoral buccal scans was had a mean value of 0.531 mm (SD: ± 0.136 mm) (difference value 2) . That is the reason why utilizing buccal scans in maximum intercuspation for the digital mounting of intraoral (IO) full-arch scans lead to a significant pronounced vertical decrease of the vertical height of the digitally mounted mandibular STL files in relation to the analog gold standard. This is possible in the digital world, with the digital occlusal overlapping (penetration) of the STL files of the intraoral full-arch scans. Intraorally taken single-tooth scans up to the intraoral quadrant scans of a jaw are a realistic possibility for the purely digital construction of acceptable fixed partial dentures. In comparison, the use of intraoral full-arch scans tends to be problematic without the back-up of additional analog mounting to cope with the vertical mismatch of the lower arch in the mounting of intraorally achieved STL files. Keywords: intraoral scan, maximum intercuspation, mandibular position, vertical relation, instrumental functional analysis, occlusion analysis, digital jaw relation, digital bite registration