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Abstract
Purpose
This study aims at investigating the influence of various insertion torques on thermal changes of bone. A proper insertion torque is derived based on the thermal analysis with two different implant designs.
Materials and methods
For implant materials, bovine scapula bone of 15 - 20 mm thickness was cut into 35 mm by 40 - 50 mm pieces. Of these, the pieces having 2 - 3 mm thickness cortical bone were used as samples. Then, the half of the sample was immersed in a bath of 36.5℃ and the other half was exposed to ambient temperature of 25℃, so that the inner and surface temperatures reached 36.5℃ and 28℃, respectively. Two types of implants (4.5 × 10 mm Bra®nemark type, 4.8 ×10 mm Microthread type) were inserted into bovine scapula bone and the temperature was measured by a thermocouple at 0.2 mm from the measuring point. Finite element method (FEM) was used to analyze the thermal changes at contacting surface assuming that the sample is a cube of 4 cm ×4 cm × 2 cm and a layer up to 2 mm from the top is cortical bone and below is a cancellous bone. Boundary conditions were set on the basis of the shape of cavity after implants. SolidWorks was used as a CAD program with the help of Abaqus 6.9-1.
Results
In the in-vitro experiment, the Microhead type implant gives a higher maximum temperature than that of the Bra®nemark type, which is attributed to high frictional heat that is associated with the implant shape. In both types, an Eriksson threshold was observed at torques of 50 Ncm (Bra®nemark type) and 35 Ncm (Microthread type), respectively. Based on these findings, the Microthread type implant is more affected by insertion torques.
Conclusion
This study demonstrate that a proper choice of insertion torque is important when using a specific type of implant. In particular, for the Microthread type implant, possible bone damage may be expected as a result of frictional heat, which compensates for initial high success rate of fixation. Therefore, the insertion torque should be adjusted for each implant design. Furthermore, the operation skills should be carefully chosen for each implant type and insertion torque. (J Korean Acad Prosthodont 2011;49:168-76)
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Fig. 2.
Schematic presentation of thermocouple position. A: Bra®nemark type (d = 2.8 ± 0.05), B: Microthread type (d = 3.05 ± 0.05).
Fig. 3.
In vitro conditions. A: temperature controller, B: constant temperature bath C: thermocouple and data logger, D: bone drilling procedure.
Fig. 6.
Temperature changes with different insertion torques at the Bra®nemark type. A: 25, B: 35, C: 50, D: 75, E: 100 N.
Fig. 7.
Temperature changes with different insertion torques at the Microthread type. A: 25, B: 35, C: 50, D: 75, E: 100 N.
Table 1.
Heat flux according to time and position
| Heat flux | Boundary condition | |
|---|---|---|
| Upper part condition | Q = μ W×cos θ a×t×cos α | α 60×sin θ Z ≥ 2πr×t×(RPM) |
| Q = 0 | Z < 2πr×t×(RPM) 60×sin θ | |
| Lower part condition | Q = 0 | All area |
W: insertion torque, Q = μ Lv: heat flux, μ : coefficient of kinetic friction, p: pitch of screw thread, r: Implant radius, L: normal force per unit area at screw thread of bone, (RPM): Revolutions per minute of implant, t: time(second), a: width of screw thread, v: linear velocity of a point on implant screw thread.
Table 2.
Maximum temperatures recorded at the 0.2 mm distance with bovin bone (℃)
Table 3.
Maximum temperature throughout simulation (℃)
| Bra®nemark type | Microthread type | |
|---|---|---|
| 25 N | 38.5 | 48 |
| 35 N | 43.9 | 57.3 |
| 50 N | 52 | 71.1 |
| 70 N | 62.8 | 89.6 |
| 100 N | 79 | 117.3 |
Table 4.
Thermal changes of Bra®nemark type implant according to time with sim ulation (at the 0.2 mm distance) (℃
Table 5.
Thermal changes of microthread type implant according to time with simulation (at the 0.2 mm distance) (℃)
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