Lennart Carlsson

Measuring fit at the implant prosthodontic interface

Four centers in the United States and Sweden have been working for 2 years to develop systems and methods for measuring fit at the prosthodontic interface. Two systems are based on stylus contact techniques, one system uses a laser as its reader source, and one system is photogrammetric. All the systems are capable of providing data as three-dimensional x, y, and z axes coordinate values that can be transformed into linear and angular data that characterize the bearing surfaces of abutments or abutment replicas and their mating components in the prosthesis framework. The centroid, a single point computed from the collected data, was the measurement unit, derived for these bearing surfaces, that was used to compare the systems. All four methods can most likely detect misfits that are relevant in the clinical setting; however, only one system can be used intraorally. When any measurement system is assessed, the data should always be examined for repeatability to establish the reliability of the system. This investigation made comparisons among the measurement methods used at the four centers. It was apparent from this study that comparisons of data from measurement systems should be rounded to the nearest 10 microns. The SDs determined in the comparisons were larger than 5 microns and therefore misfits should be calculated in terms smaller than 10 microns. This final point is important to the clinician who relies on research reports about precision of fit when selecting treatment approaches in caring for the implant prosthodontic needs of their patients.

Accuracy of machine milling and spark erosion with a CAD/CAM system

A method for manufacturing crowns and fixed partial dentures based on CAD/CAM has been developed as an alternative to the lost wax technique and the casting of an alloy. In this process two steps are included: milling and spark erosion. The computer-assisted design (CAD) relies heavily on the accuracy of the milling and spark erosion processes to achieve a clinically acceptable restoration. These two processes must be able to produce the crown data generated in the CAD files. This study evaluated the accuracy of the Procera CAD/CAM system in creating specific geometric bodies that were compared with the known dimensions in the CAD files for these bodies. The manufacturing errors of milling (ellipse +/- 6.5 microm, square +/- 3.4 microm, and cylinder +/- 5.8 microm) and spark erosion (ellipse +/- 8.6 microm and square +/- 10.4 microm) were determined. The accuracy of this manufacturing process demonstrated that this system was capable of producing a crown with a clinically accepted range for marginal opening gap dimension of less than 100 microm.