Posterior jaws rehabilitated with partial prostheses supported by 4x4 mm or by longer implants: a 1- year post-loading randomised controlled trial.

BACKGROUND: The fixed rehabilitation of atrophic edentulous jaws is a challenging treatment because of inadequate bone volumes. In the presence of 4 to 8 mm of bone height, short implants can successfully be used as an alternative to more invasive augmen-tation procedures to allow the placement of long implants. There are a few randomised controlled trials comparing the effectiveness of prostheses supported by short implants with those supported by long implants placed in augmented posterior jaws. The results of these on-going trials having up to 5-year post-loading follow-ups suggest that 5 to 8 mm long implants can be a viable, if not a better alternative to augmentation procedures, especially in posterior mandibles. A more recent clinical question is whether to use short implants also in those situations where long implants could have been used, and how short could an implant be in order to be able to provide a good long-term outcome. AIM/HYPOTHESIS: To evaluate whether 4 9 4 mm dental implants could be an alternative to implants at least 8.5 mm long, which were placed in posterior jaws, in the presence of adequate bone volumes. MATERIAL AND METHODS: 150 patients with posterior (premolar and molar areas) jaws having at least 12.5 mm bone height above the mandibular canal or 11.5 mm below the maxillary sinus, were randomised according to a parallel group design to receive one to three 4 mm-long implants or one to three at least 8.5 mm-long implants, at 3 centres. All implants (TwinKon Universal SA2, Global D, Lyon, France) had a diameter of 4 mm. Implants were loaded after 4 months with definitive screw-retained prostheses. Patients were followed up to 1 year after loading and outcome measures were: prosthesis and implant failures, any complication and peri-implant marginal bone level changes. The sample size was calculated using the Fisher’s exact test: 263 patients per group were needed, however we only had resources to recruit 150 patients. All data analysis was carried out according to a pre-established analysis plan. The patient was the statistical unit of the analyses. Differences in the proportion of patients with prosthesis failures, implant failures and complications between the two groups were compared using the Fisher’s exact test and binomial 95% confidence intervals were computed. Differences of means for radiographic bone levels between groups were compared by Mann-Whitney U test. Comparisons between each time point and the baseline measurements were made by paired Wilcoxon test. All statistical comparisons were conducted at the 0.05 level of significance. RESULTS: 75 patients were randomly allocated to each group; one patient dropped out. Up to 1-year post-loading, 3 patients lost one 4 mm-long implant each, in comparison to 2 patients who lost one long implant each (difference in proportion = 0.013; 95% CI: 0.058 to 0.087; P = 0.506). The failed implants were replaced and the delivery of two prostheses in each group was delayed (difference in proportion = 0.0004; 95% CI: 0.068 to 0.069; P = 0.685). Three short implant patients experienced three complications versus 2 long implant patients (difference in proportion = 0.013; 95% CI: 0.058 to 0.087; P = 0.506). There were no statistically significant differences in prosthesis failures, implant failures and complications. Patients with short implants lost on average 0.53 mm of peri-implant bone and patients with longer implants lost 0.57 mm (mean difference = 0.038 mm; 95% CI: 0.068 to 0.138; P = 0.198). CONCLUSIONS AND CLINICAL IMPLICATIONS: One year after loading 4 mm-long implants achieved similar results as 8.5 mm-long or longer implants in posterior jaws, however 5 to 10 years post-loading data are necessary before reliable recommendations can be made.