MicroThread™ is the minute thread design on the neck of the Astra Tech implants, introduced as early as 1991. Scientific articles present the ability of MicroThread to ensure positive biomechanical bone stimulation and to maintain marginal bone levels in the long term. Summarized on the following pages, you will find articles about the continuous follow-up of the MicroThread.
Multiple single-tooth implant restorations in the posterior jaws: Maintenance of marginal bone levels with reference to the implant-abutment microgap
Purpose: There has been increasing interest in the role of the implant and abutment microgap and its influence of the establishment of a biologic width across the interface resulting in marginal bone loss of up to 2 mm. Other proposals for crestal bone loss include joint micro-movement, implant geometry and the nature of the implant surface topography.
The present study considered the outcome for maintenance of crestal bone when restoring implants as multiple adjacent single-tooth units in the posterior jaws. In addition all prosthetic complications were recorded.
Material and Methods: 54 patients consecutively enrolled for replacement of 2 or more adjacent teeth posterior to the canines were included in the study. A 4.5 mm diameter tapered implant (Astra Tech ST) was selected as the principle implant for use in the study with either a 4.0 mm or 5.0 mm implant diameter available where circumstances dictated. It was part of the study protocol for treatment to be executed in a transmucosal one-stage manner. All implants were placed at or just below the crest of the ridge according to manufacturer recommendations. Implants benefited from a 3 to 4 month healing phase prior to connection of the definitive single-tooth abutments and fabrication of the individual ceramometal crowns which were all cemented with Temp bond.
Marginal bone levels (MBL) were measured from the implant-abutment junction to the first crestal bone visible at x8 magnification. Where bone levels were coronal to the junction the score given was 0.0 mm in order to avoid positive bias. The geometry of the implant itself was used to ensure accuracy of measurements.
A statistical analysis was undertaken to determine the presence of any correlation for bone loss with gender, jaw, smoking status as well as comparing mesial vs. distal surfaces.
Results: A total of 181 implants were inserted with only one implant removed during the follow up phase of the study, up to 7 years, equating to a 99.4% survival rate. Radiographs of 173 implants were suitable for analysis. The mean MBL for the pooled data from the implant-abutment junction measured -0.65 mm. MBLs by jaw were -0.56 mm for maxilla and -0.7 mm for the mandible, by gender were -0.72 mm for females and -0.53 mm for males, by smoking status were -0.63 mm for non-smokers and -0.77 mm for smokers. The differences were not statistically significant. When comparing mesial and distal surfaces the MBLs measured -0.53 mm and -0.76 mm, (p < 0.001). A total of 28 crowns de-cemented over 7.5 years but recurrent decementation was rare with only 4 further episodes. Porcelain fracture was reported for 13 crowns. Abutment screw loosening was only reported for 4 screws in 4 separate patients.
Discussion and Conclusions: The current study demonstrates the effectiveness of the multiple single-tooth approach when used to replace multiple missing posterior units. Patients reported a high level of satisfaction with the cosmetic and hygienic aspects of the treatment and being individual crowns, retrieval for maintenance was simplified comparative to cumbersome multi-unit prostheses.
The commonest problem was cementation failure which occurred in 17.7% of cases using temporary cement. Given the low risk of screw loosening with the system used (2.2%) it is reasonable to propose the use of stronger cement.
The mean marginal bone loss of -0.65 mm is low when one considers that this was a measure of total bone loss from the microgap including any initial adaptation. This is contrary to what might be expected according to the theory of biologic width. It has been proposed that the influence of joint design, surface topography and medialization of the abutment with respect to the outer edge of the implant may all mediate a positive impact on the maintenance of marginฝ bone. This is reflected in the frequency of no bone loss which measured 23.1% in the maxilla and 16.7% in the mandible where bone could be seen to have an intimate juxtaposition with the implant-abutment junction.
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