Photogrammetric method to measure the discrepancy between clinical and software-designed positions of implants

Statement of problem: The position of dental implants placed with software-guided systems should be highly accurate in order to ensure safety and a passive fit of the immediate prosthesis. Purpose: The purpose of this study was to measure the discrepancy between the clinical and software-planned position of dental implants by applying a photogrammetric method. Material and methods: Two casts were obtained, 1 from the surgical template and 1 from the actual position of the implants on the alveolar ridge of a patient. Photogrammetry was then applied to precisely locate the position of each implant on the casts. Because this mathematical technique required the identification of image points and of the relative spatial coordinates, 4 marks were drilled on the implant screw. The position of the implants was then identified as the geometric center of the 4 marks, while the orientation of the implant axis was represented by a vector normal to the plane fitting the points. A series of 16 convergent images all around the object was made using a high-resolution digital camera. A mathematical method called "rototranslation" was used to superimpose the cast images for the comparison. Results: The tests performed on the casts resulted in an average precision level of 4 μm for the locations and less than 1 degree for the axis of the implants. A series of empirical and numerical tests were performed to assess the performance of the procedure and of the measurement protocol. Conclusions: The photogrammetric method is reproducible and can be used to measure the discrepancy between the software-planned and the real position of dental implants. Considering that the average precision level required for an implant-based prosthesis is approximately 50 μm, the error associated with this method can be considered as negligible.