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Abstract
Objective
The aim of this study was to evaluate the biomechanical aspects of peri-implant bone upon root contact of orthodontic microimplant.
Methods
Axisymmetric finite element modeling scheme was used to analyze the compressive strength of the orthodontic microimplant (Absoanchor SH1312-7, Dentos Inc., Daegu, Korea) placed into inter-radicular bone covered by 1 mm thick cortical bone, with its apical tip contacting adjacent root surface. A stepwise analysis technique was adopted to simulate the response of peri-implant bone. Areas of the bone that were subject to higher stresses than the maximum compressive strength (in case of cancellous bone) or threshold stress of 54.8MPa, which was assumed to impair the physiological remodeling of cortical bone, were removed from the FE mesh in a stepwise manner. For comparison, a control model was analyzed which simulated normal orthodontic force of 5 N at the head of the microimplant.
Results
Stresses in cancellous bone were high enough to cause mechanical failure across its entire thickness. Stresses in cortical bone were more likely to cause resorptive bone remodeling than mechanical failure. The overloaded zone, initially located at the lower part of cortical plate, proliferated upward in a positive feedback mode, unaffected by stress redistribution, until the whole thickness was engaged.
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Fig 1.
Schematic diagram for the SH1312-7 (Dentos Inc., Daegu, Korea) microimplant with its apical part intruding the PDL space and contacting the root surface. The implant was assumed to be loaded either by an orthodontic force at its head, or apically by the adjacent root in contact with its tip (*a displacement load of 0.1 mm was assumed to be transmitted from the root surface to the microimplant, †5 N was assumed as an orthodontic force).
Fig 2.
Axisymmetric finite element (with non-axisymmetric loading) models simulating. A, Control model which presents a microimplant subject to orthodontic load of 5 N at the head, without root contact; B, a microimplant in contact with root surface, subject to apical excitation of 0.1 mm. Horizontal axis x represents the loading direction. Vertical axis y represents the axis of symmetry.
Fig 3.
Stresses (maximum compressive stress) in control model subject to orthodontic force of 5 N at the head. A, Overall stress distribution; B, magnified view of the cervical area; C, stress band (a cut off stress was specified at 15 MPa for clear observation of stress concentration occurring at the implant/cortical bone junction).
Fig 4.
Stress development in the cancellous bone. Calculation was performed in a stepwise manner by removing the cancellous bone elements subject to stresses of higher than ultimate compressive strength of 10.44 MPa (Table 1). A, Base model; B, Stage 1; C, Stage 2; D, stress band stress band with cut off stress at 10.44 MPa.
Fig 5.
Stresses in cortical bone after the entire peri-implant cancellous bone along the implant has lost structural integrity due to displacement of 1.0 mm at the apex. A, With the cut off stress at 140 MPa (maximum compressive strength of cortical bone, Table 1); B, with the cut off stress at 54.8 MPa (threshold for resorptive remodeling, Table 1).
Fig 6.
Stress development in the cervical cortical bone (due to displacement of 1.0 mm at the apex) calculated in a successive manner by removing the elements from model where resorptive bone remodeling was predicted due to overload. A, Stage 3; B, Stage 4; C, Stage 5; D, Stage 6 models; E, stress band with cut off stress at 54.8 MPa (threshold for resorptive remodeling, Table 1).
Table 1.
Mechanical properties used in this study
| Material | Young’s modulus (GPa) | Poisson ratio | Strength (MPa) | Threshold stress (MPa) |
|---|---|---|---|---|
| Titanium17 | 114.0 | 0.35 | - | - |
| Cortical bone18 | 13.7 | 0.3 | 72-76 (tensile) | 54.8* |
| 140-170 (compressive) | - | |||
| Cancellous bone18,19 | 1.37 | 0.3 | 0.22-10.44 (compressive) | - |
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