PURPOSE. To compare clinical outcomes of machined titanium abutments (machined group) versus cast cobalt-chrome abutments (cast group) and to evaluate in vitro their implant fit.
MATERIALS AND METHODS. This study comprised two parts. In the in vitro part, the implant abutment fit of 5 cast abutments and 5 machined abutments screwed on with a torque of 30 Ncm was qualitatively and quantitatively evaluated using micro-computed tomography (μ-CT) and AgNO3 to reveal connection gaps. In the clinical part, 31 partially edentulous subjects received two single non-adjacent implant-supported crowns at three centres. At impression taking, three and a half months after implant placement, implants were randomized to receive a machined or cast abutment according to a within-
patient study design. Unfortunately, four patients dropped out and one patient lost one implant before randomization, so only 26 patients had their implants randomized.
Outcome measures were: prosthesis and implant failures, any complications, and radiographic peri-implant marginal bone level changes. Patients were followed up to 1 year after loading.
RESULTS. The fit of the implant–abutment connection was assessed in vitro using μ-CT scans. No gaps were revealed at any of the machined or cast abutments tested.
In the clinical part, after randomization, three patients dropped out, no implant failed, but one crown on a cast abutment was replaced. The between-group difference in prosthesis failure was not statistically different (McNemar chi-square test P = 1.0; difference in proportions = 0.039). One complication occurred in each group, the difference not being statistically different (McNemar test P = 1.000; difference in proportions = 0; 95% CI 0.06 to 15.99). Both groups presented statistically significant peri-implant marginal bone loss from implant placement to 1 year after loading, respectively -0.76 ± 1.01 mm for machined and -0.69 ± 0.82 mm for cast abutments, with no statistically significant differences between the two groups (mean difference 0.07 mm; 95% CI -0.54 to 0.67; P = 0.828). Both groups gradually lost marginal peri-implant bone from loading to 1 year after loading but this was not significantly different, respectively -0.06 ± 0.56 mm for machined and -0.10 ± 0.29 mm for cast abutments, with no statistically significant differences between the two groups (P = 0.739; mean difference 0.07 mm; 95% CI -0.12 to 0.16; P = 0.739).
CONCLUSIONS. Our clinical data suggests that implant prognosis up to 1 year after loading is not affected by using machined or cast abutments. In support of these findings, in vitro analysis proved that both types of abutments allow a tight fit with no gaps. Therefore, for the time being dentists should feel free to choose whichever type they prefer. However, these preliminary results need to be confirmed by larger trials with at least 10 years of follow-up.

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