Short implants vs. longer implants in vertically augmented posterior mandibles: a randomised controlled trial with 5-year after loading follow-up-

BACKGROUND: While it has been shown that it is possible to vertically augment bone with different procedures, these techniques are associated with significant postoperative morbidity and complications, can be expensive and technique sensitive, and require long treatment periods. Short implants could be a simpler, cheaper and faster alternative to vertical bone augmentation, if they will provide similar success rates. While it is common sense to use longer implants when bone heights allow it, it remains unclear how to proceed in the presence of residual mandibular bone height ranging from between 5 mm to 8 mm. AIM/HYPOTHESIS: To evaluate whether 6.6 mm long implants could be a suitable alternative to longer implants placed in vertically augmented atrophic posterior mandibles. MATERIAL AND METHODS: Sixty partially edentulous patients having 7 to 8 mm of residual crestal height and at least 5.5 mm thickness measured on CT scans above the mandibular canal were randomised according to a parallel group design. They were either to receive 1 to 3 submerged 6.6-mm long implants (Nanotite, External Hex, Biomet 3i, Palm Beach, FL, USA) or 9.6 mm or longer implants of the same type (30 patients per group) placed in vertically augmented bone. Bone was augmented with interpositional anorganic bovine bone blocks covered by resorbable barriers. Grafts were left to heal for 5 months before implant placement. Four months after provisional acrylic prostheses were delivered, they were replaced, after 4 months, by definitive metal-ceramic prostheses. Outcome measures were: prosthesis and implant failures; complications; and radiographic peri-implant marginal bone level changes. All patients were followed up to 5 years after loading. RESULTS: Five years after loading, eight patients dropped out. The augmentation procedure failed in two patients. There were no statistically significant differences for prosthesis and implant failures. Five prostheses failed in four patients of the short implant group vs. five prostheses in five patients in the augmented group (Fishers exact test P = 1.0; difference in proportions = 0.07; 95% CI 0.29 to 0.43). Five short implants failed in three patients vs. three long implants in three patients (Fishers exact test P = 1.00 difference in proportions = 0.00; 95% CI 0.43 to 0.43). There were statistically more complications in augmented patients (25 complications in 21 augmented patients vs. six complications in six patients of the short implant group) (Fishers exact test P < 0.0001; difference in proportions = 0.60; 95% CI 0.38, 0.82). Five years after loading short implants experienced statistically significantly less bone loss (0.82 mm, 95% CI 0.48; 1.16, P < 0.0001) than long implants. CONCLUSIONS AND CLINICAL IMPLICATIONS: When residual bone height over the mandibular canal is between 7 to 8 mm, 6.6 mm short implants could be an interesting alternative to vertical augmentation in posterior atrophic mandibles since the treatment is faster, cheaper and associated with less morbidity. Longer follow-ups may still be needed to confirm these results, however the medium-term prognosis (5 years after loading) of short implants is at least as good as those of longer implants placed vertically in augmented mandibles.