The Graz hemisphere splint: a new precise, non-invasive method of replacing the dental arch of 3D-models by plaster models.

Three-dimensional (3-D) anatomical models have proven their great value in the field of cranio-maxillofacial surgery. One major disadvantage is the limited representation of the teeth in milled and stereolithographic models. This is mainly caused by the limited resolution of the CT-scan, especially in the plane perpendicular to that of the scan. A new, precise, non-invasive and standardized method of replacing teeth of 3-D models by plaster models is introduced. The accuracy of tooth replacement is analysed. A plastic human skull is scanned with different interscan distances (scan feed), eight 3-D models are fabricated from this data and the positioning precision of the replaced plaster models in the three main axes is examined. Statistical analysis is carried out with a paired samples t-test. A mean positioning deviation of 0.44 and 0.52 mm in all directions is found using a CT feed of 2 and 3 mm. With 4 and 6 mm, the accuracy decreases showing 0.95 mm and 1.08 mm deviation. No significant difference is found between 2 mm and 3 mm scans, but significant differences between 2, 3 mm and 4, 6 mm are found. For the replacement of model teeth, at least three definitive fixed marks are required. With the aid of a hemisphere, used as a marker, the limited resolution in z-direction is overcome. The hemisphere is visible on several scans as semicircles of varying size. In the 3-D model, it allows precise positioning even on the z-axis enabling the exact replacement of teeth for the first time. A scan feed of 3 mm is sufficient for precise tooth replacement.