The purpose of this study was to investigate the effects of the diameter and shape of orthodontic mini-implants (OMIs) on microdamage to the cortical bone during implant placement.
Twenty-eight self-drilling OMIs (Biomaterials Korea, Seoul, Korea; length, 6 mm; diameters, 1.5 and 2 mm; cylindrical and tapered shapes; classified as 1.5C, 2C, 1.5T, and 2T) were placed with a surgical device in the tibias of 7 New Zealand white rabbits (mature males; mean age, 6 months; mean weight, 3.1 kg). Four OMIs of each type per rabbit were placed randomly. Maximum insertion torque (MIT) was measured. Immediately after placement of the OMIs, the block of bone with the OMI was harvested. Cortical bone thickness was measured by using microcomputed tomography, and histomorphometric analyses of the number of cracks (NC), accumulated crack length (ACL), maximum radius of the crack (MRC), and longest crack (LC) were performed. Kruskal-Wallis and Mann-Whitney U tests with the Bonferroni adjustment were done for statistical analyses.
Increased diameter (1.5C<2C and 1.5T <2T) and tapering (1.5C<2T) resulted in increased values of MIT, NC, and LC ( P <0.01, respectively). Similarly, with increased diameters (1.5C and 1.5T <2C and 2T), there were increases of ACL and MRC ( P <0.001, respectively). However, there were no differences in the values of MIT, NC, ACL, MRC, and LC between the cylindrical and tapered OMIs with the same diameters (1.5C and 1.5T, 2C and 2T).
OMIs with larger diameters and tapered shapes caused greater microdamage to the cortical bone; this might affect bone remodeling and the stability of the OMIs.
After reading a number of implant studies in various journals, I came away with the impression that the larger the OMI, the more stable it would be. But is that always the case? Are there situations when differences in size and shape of an OMI cause less stability? According to the authors of this study, “various factors such as age, sex, skeletal pattern, placement location, screw design, type of placement surgery, immediate loading, oral hygiene, and inflammation are associated with their stability.” Also, OMI failures occur most frequently within the first 4 months. Therefore, the purpose of this study was to investigate the effects of the diameter and shape of OMIs with regard to microdamage in the cortical bone during OMI placement.
These researchers placed 4 types of OMIs in 7 rabbits each. The 6-mm self-drilling implants from Seoul, Korea, were classified into 4 subtypes according to diameter (1.5 or 2 mm) and shape (cylindrical or tapered). They used a round-robin method to randomize the placements. When they evaluated the results, they found that excessive placement torque might cause microfracture and ischemia of the surrounding bone, delay the bone healing, and cause failure of the OMIs. In terms of microdamage, tapered OMIs with a 2-mm diameter were identified as not the best option because the tapering and the large diameter resulted in a significant increase of MIT. The OMIs with a larger diameter and a tapered shape resulted in greater microdamage to the cortical bone. This might affect bone remodeling and stability. These results can be used as a guideline for choosing appropriate types of OMIs to prevent or minimize microdamage in daily orthodontic practice.