Issue 35

Š. Major et alii, Frattura ed Integrità Strutturale, 35 (2016) 379-388; DOI: 10.3221/IGF-ESIS.35.43 382 I MPLANT TESTING or experiments wee used two types of pedicle-screw, the solid cylindrical and hollow screw. Both types have the same dimensions and geometry of thread. The implants were clamped in an artifical vertebra during test, so that the device simulates the bone-metal contact. The implants were subjected to bending loading. The pedicle-screw is loaded by a pair of forces perpendicular to the screw axis, the two forces are perpendicular to each other. Both forces are offset from the axis so that also create torque. The second bending force reaches its maximum when the first force reaches its second maximum, i. e. the period of the second load is doubled, i. e. T 2 = 2 T 1 . The experiments were carried to the final rapture of specimen. Final rapture is defiden as the complete fracture of the pedicle-screw. Fracture appeared the point where the screw enters into the bone, I. e. at the point highest stresses. Since the bone consists of two basic parts, it is necessary to construct an artificial pedicle so that this fact was simulated. Surface of bone is composed from hard “compact bone” and cover layer of periosteum. The interior of the bone consists of a more flexible and softer tissue so-called “spongy bone”. Artificial pedicle consists of two layers, a hard layer on the surface of 4 mm thick and the rest of the softer. The hollow bolt is fixed tightly by the entire length of the thread, whereas the vicinity of the thread is filled with cement everywhere. Schematic representation of the sample with loading forces is pictured in Fig. 3. During experiments the samples were loaded by this forces L 1 = 200, 300, 400, 500 N and L 2 = 50 N. The experiments were performed the room temperature and loading frequency was f 1 = 24 Hz respectively f 2 = 12 Hz. N UMERICAL MODEL D-model of implant was prepared in SOLIDWORKS software and exported in ANSYS software. The model was meshed and solwed in ANSYS, see Fig.4. The aim of this model was determination of stress and strains in the pedicle-screw. Another finite element model was prepared for calculation of stress-intensity factor along the crack- path. Figure 4 : Meshed finite element model of pedicle screw. First model of pedicle-screw is composed of 556.024 tetrahedral elements (type of element SOLID187) for solid, and 442.052 for hollow screw. In the case of solid screw, as model of the contacts between thread and bone for first 4 mm from point of entry into the bone was used null displacement, for the rest of the contact is allowed to move in a plane perpendicular to the axis of the screw (0.5 mm allowable displacement). Depth of hard part of bone corresponds to almost one loop of the thread. In the case of hollow screw. Conditions of null displacement were applied to all degrees of freedom (due the cement presence) of the nodes along the entire length of the thread in the bone. For second model, model of the crack initiation area, is used refinement of net and the size of the elements is 5 μm, see Fig. 5. The important fact is, that in the region of crack initiation is elastoplastic deformation, while rest of volume is under elastic deformation. Plasticity of material was simulated kinematic hardening. Figure 5 : Meshed finite element model of crack. Figure 6 : Von Misses stress in the thread. F 3