Purpose: Short implants are used in clinical conditions of insufficient vertical bone availability. This study aimed to compare the primary stability of short implants with different macrodesigns placed in different bone densities in vitro.

Materials and methods: One hundred twenty short (6-mm) implants (20/group) were placed at the bone level in commercially available polyurethane blocks representing type I and IV bone quality. The groups were as follows: test A group (4.6-mm diameter with tapered body), test B group (4.8-mm diameter/cylindric microthreaded neck), and test C group (4.8-mm diameter, cylindric body with polished collar, three threads at the intraosseous portion). Implant primary stability was assessed using insertion torque and implant stability quotient (ISQ) values. A blinded calibrated clinician recorded all measurements. Statistical comparisons were completed using a one-way analysis of variance (ANOVA) and Bonferroni posttests.

Results: The insertion torque values (mean ± SD) for groups A, B, and C in type I bone were 52.50 ± 5.25, 49.00 ± 5.98, and 46.25 ± 3.93, and in type IV bone, the values were 14.00 ± 2.05, 15.50 ± 2.76, and 9.75 ± 1.11, respectively. Also, the ISQ values were 67.25 ± 2.760, 69.25 ± 1.67, and 61.80 ± 5.68 (type I bone); and 53.27 ± 1.99, 60.65 ± 2.11, and 51.97 ± 4.51 (type IV bone), respectively. The comparison showed statistical differences in ISQ (Bonferroni adjusted P < .0001) for the A and B groups but also for the A and C groups (type I bone), in soft bone between the A and B groups and between the B and C groups, and also for the insertion torque values for the A and C groups and between the B and C groups in type IV bone.

Conclusion: Short implant macrogeometry defines primary stability. Short implants with 6-mm length and multiple threads can achieve good primary stability in vitro in type I and IV artificial bone.