Discussion
edentulous maxillae, MIs, RPDs, mandibular MIs, maxillary MIs, implant loss
The primary outcome of this randomized trial was bone level changes of either immediately loaded MI in group A or delayed loaded MI in group B as supplementary abutments under existing RPDs. The assessment of bone heights at MIs on the panoramic radiographs are more sophisticated than expected, and the analyses are still pending. Thus, the present findings are descriptive and explorative in nature and not based on formal statistical hypothesis testing. The outcomes suggest no relevant differences between immediate and delayed loading regarding the survival rate and the stability of strategic MIs contrary to our hypotheses. The median of MI PTVs was higher, and the median of RFA values was lower than the stability values of standard-diameter implants whereby MI PTVs and RFA values strongly correlated. The correlation between insertion torque of MIs and their measured primary stability was rather moderate. The 3-year survival rate of mandibular MIs was markedly higher than of maxillary MIs (99% versus 87%, respectively). The expected relationship between increasing PTVs and the risk of implant loss was confirmed.Some limitations of the trial besides participant`s response merit consideration. First, both upper and lower jaws with various tooth distributions were included. However, it was given that the few remaining teeth provided insufficient and/or imbalanced support for the RPD. Second, the different loading procedures dependent on insertion torque of MIs in group A limit a sound comparison with group B. We would reduce the risk of implant failures in the maxilla as seen in studies in which all maxillary MIs were immediately loaded with the housings [Overall, this is the first estimation of mid-term survival rate and stability data for strategic MIs stabilizing RPDs with unfavorable distribution of natural abutments. Because of the multicenter design (one university hospital and three private practices), a multifaceted patient cohort was treated by experienced dental practitioners and examined by one trained investigator who was not involved in the treatment. The MIs were placed without knowing the loading modus up to the participant’s allocation. For MI stability assessments, two different methods were used whose values strongly negatively correlated.As hypothesized in the design paper [The reference levels for the stability of conventional implants in a successful osseointegration are PTVs < 1 and RFA values > 60 [The possible association between implant diameter and their stability was confirmed by our explorative evaluations of PTVs by MI diameter. The analyses were separated to jaw levels because of the possible differences in the bone quality between the maxilla and the mandible [An expected trend of more MI failures in the immediate loading group A than in the delayed loading group B (8 versus 5 failures or 8% versus 5% 3-year failure rate) was not statistically verifiable. It should be noted that one MI in group A was removed before loading because of its malposition. Immediate loading of conventional implants with mandibular overdentures are poorly documented and result in more implant failures than delayed loading, whereas comparable data for the maxilla are missing [The present MI survival rate of 99% in the mandible corresponds with results from strategic conventional implants (93–100%) [Despite soft relining of RPDs in the case of insertion torque < 35 Ncm in group A and the no-load osseointegration in group B, eleven maxillary MIs were lost, and the survival rate probability was only 87% including the misplaced MI. All were multiple losses occurring in four patients and could be caused by the following reasons. First, two of the participants wore double crown-retained RPDs with multiple abutments in the mandible, and two participants had a natural mandibular dentition. A high occlusal load from the opposing jaw is a suspected cause of MI failures for some researchers [A comparable failure rate of 82.3% in a 2-year prospective cohort study was reported. The study sample size was 31 patients with edentulous maxillae, each received 6 MIs. All overdentures were soft relined for 6 months before the housings were picked-up [Indeed, the predictive value of longitudinal RFA or Periotest measurements for implant failures is feasible, but the evidence is still weak and lacking for MIs [