Assessment of the Stress Transmitted to Dental Implants Connected to Screw-Retained Bars Using Different Casting Techniques
Passive fit of the prosthetic superstructure is important to avoid complications; however, evaluation of passive fit is not possible using conventional procedures. Thus, the aim of this study was to check and locate mechanical stress in bar restorations fabricated using two casting techniques. Fifteen patients received four implants in the interforaminal region of the mandible, and a bar was fabricated using either the cast-on abutment or lost-wax casting technique. The fit accuracy was checked according to the Sheffield's test criteria. Measurements were recorded on the master model with a gap-free, passive fit using foil strain gauges both before and after tightening the prosthetic screws. Data acquisition and processing was analyzed with computer software and submitted to statistical analysis (ANOVA). The greatest axial distortion was at position 42 with the cast-on abutment technique, with a mean distortion of 450 μm/m. The lowest axial distortion occurred at position 44 with the lost-wax casting technique, with a mean distortion of 100 μm/m. The minimal differences between the means of axial distortion do not indicate any significant differences between the techniques (P = 0.2076). Analysis of the sensor axial distortion in relation to the implant position produced a significant difference (P < 0.0001). Significantly higher measurements were recorded in the axial distortion analysis of the distal sensors of implants at the 34 and 44 regions than on the mesial positions at the 32 and 42 regions (P = 0.0481). The measuring technique recorded axial distortion in the implant-supported superstructures. Distortions were present at both casting techniques, with no significant difference between the sides.
Implant plane with transfers (a) and impression with open custom trays (b) in the interforaminal region of the mandible (34, 32, 42, 44 regions).
Cast-on abutment technique scheme: transmucosal components at the master model (a), waxing (b), preparation for casting (c), bar after inclusion (d), occlusal (e) and front view (f) of the finished bar.
Sensor supported on implant extension (a), torque of 25 Ncm (b) and placement of sensors at the master model (c).
Six foil strain gauges attached on implant extensions (a), the signals were amplified (b and c) and transferred to a computer (d).
Figure 5. Axial deformation of all implants when the bar is screwed at position 34 for the cast-on abutment (a) and for the lost-wax casting technique (b). Figure 6. Mean axial distortion after tightening the screws of all four implants through the cast-on abutment technique (a) and lost-wax casting technique (b). Figure 7. Axial distortion in relation to the sequence of tightening (34 – 42 – 32 – 44, respectively) represented by numbers 1–4 and loosening (44 – 32 – 42 – 34, respectively), represented by numbers 5–8 of the retention screws for implant position 34 in the cast-on abutment (a) and lost-wax casting techniques (b).
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