| Summary: | 碩士 === 國立陽明大學 === 醫學工程研究所 === 87 === A new design interlocking tibial nail was modeled and analyzed by Finite element method in using IDEAS computer aided design package. The designed segmental tibial nail consists of four major parts. A 110 mm long proximal segment has 14 degree flexion angle, 45 mm vertical head length, and two 5mm diameter screws holes in 20mm pitch. A distal segment is 70mm long with two 5mm diameter screw-holes in 25mm pitch. The rest parts are a tighten string and several other segments to accommodate the desired length. The nail has the outer diameter of 10mm and 2 mm wall thickness to fit the normal Chinese tibiae. Two stainless steel cable are attached to the distal segment as the guiding of other inserting segments. Teeth-slots as well as taper lock are used at each connecting parts to each ensure the rigidity of the whole nail.
The strength of designed nail was compared to that of traditional one such as Richard nail. Two types of fracture, complete simple as well as comminuted with 5mm fragment, were simulated in compression, bending, and a combined physiological loading (to simulate the load acting on knee in 20% of a gait cycle).
In both types of nails alone simulation, the weakest point located in the upper distal screw hole for the unified nail, while the weakest of segmental nail was in the connecting junction of distal segments. After nail implanted and in the case of compression load, the highest effective stress occurred in the screw hole of nail (nail-screw interface) for comminuted tibial fracture. While as a complete simple fracture the highest stress presented in the outer screw-cortical bone interface.
In maximum physiological loading, the unified nail had the highest effective stress around the fracture section while the designed segmental nail had its highest nail in the junction of segments closed to the fracture site.
The designed nail might yield when the fracture located around central section of the tibia. Since a effective stress of 665Mpa was calculated which exceeded the yield strength 525Mpa of the cool-rolled stainless steel. When the fracture opened at distal third with over-reaming insertion, the tibia-nail system resisted only 74 to 80 % of the nail only stiffness in bending test. A suitable material for the new design nail should be either titanium or Co-Cr steel
In general, the finite element stress analysis showed that the new designed nail met the strength required in physiological loading.
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