Primary Stability of Orthodontic Titanium Miniscrews due to Cortical Bone Density and Re-Insertion

Abstract

The increasing demand for orthodontic treatment over recent years has led to a growing need for the retrieval and reuse of titanium-based miniscrews to reduce the cost of treatment, especially in patients with early treatment failure due to insufficient primary stability. This in vitro study aimed to evaluate differences in the primary stability between initially inserted and re-inserted miniscrews within different cortical bone densities. Artificial bone was used to simulate cortical bone of different densities, namely 20, 30, 40, and 50 pound per cubic foot (pcf), where primary stability was evaluated based on maximum insertion torque (MIT), maximum removal torque (MRT), horizontal resistance, and micromotion. Scanning electron microscopy was used to evaluate morphological changes in the retrieved miniscrews. The MIT, MRT, horizontal resistance, and micromotion was better in samples with higher cortical bone density, thereby indicating better primary stability (P < 0.05). Furthermore, a significant reduction of MIT, MRT, and horizontal resistance was observed during re-insertion compared with the initial insertion, especially in the higher density cortical bone groups. However, there was no significant change in micromotion. While higher cortical bone density led to better primary stability, it also caused more abrasion to the miniscrews, thereby decreasing the primary stability during re-insertion.

Effects of Intrabony Length and Cortical Bone Density on the Primary Stability of Orthodontic Miniscrews

“…Hand-held micromotion measurement devices such as PTV and IST are portable and easy to use and have become popular tools for the evaluation of miniscrew micromotion. Previous research has indicated that micromotion is affected by the cortical bone density and porosity, which supports the findings of the current study [ 18 ]. However, the accuracy of these non-invasive easy-to-operate methods for evaluating primary stability is still debated [ 17 , 31 ].…”

“…The porosity of the polyurethane foam blocks was measured via micro-computed tomography (CT) (SkyScan 1076, Kontich, Belgium) ( Figure 2 ). According to previous studies [ 14 , 17 , 18 ], the artificial bone comprised a 2 mm layer of higher density (30, 40, and 50 pcf) polyurethane foam representing cortical bone, and a 20 mm layer of lower density (20 pcf) polyurethane foam representing cancellous bone ( Figure 1 B). Miniscrews were implanted into the artificial bone at different depths (3, 4, 5, and 6 mm) ( Figure 1 C) using an automatic torque device (Admet eXpert8600, ADMET, Norwood, MA, USA) to control the insertion depth [ 19 ].…”

“…Torque testing was conducted according to a previously reported procedure [ 18 , 20 ]. The artificial bone was fixed with a metal clamp, and the miniscrews ( n = 6) were implanted in a clockwise manner using an automatic torque device (Admet eXpert8600, ADMET, Norwood, MA, USA) with a self-drilling system at 5 rpm according to ASTM F-1839-08.…”

“…Horizontal resistance was conducted according to a previously reported procedure [ 14 , 18 ]. A universal testing machine (Instron 5942, Instron, Norwood, MA, USA) was used to measure the horizontal resistance, where the artificial bone was fixed with a metal clamp while a knife-like shear jig applied a tangential load to the miniscrew ( n = 6) at a crosshead speed of 1 mm/min ( Figure 3 ).…”

“…Micromotion analysis was conducted according to a previously reported procedure [ 18 ]. The periotest value (PTV) tester (Medizintechnik Gulden, Modautal, Germany) and implant stability tester (IST) (AnyCheck, Neo Biotech, Seoul, Korea) are two commercial hand-held micromotion measurement devices that obtain micromotion data by repeatedly tapping the head of a miniscrew.…”