Electro-stimulating implants for animal tests

The animals chosen in the animal tests were female New Zealand white rabbits of 3 — 4 kg weight due to the reason that compared to sheep, this kind of rabbit is easier to handle during the operation and application of electrical stimulation. The designed electro- stimulating implants were planned to be inserted into the rabbit’s distal femurs (Fig. 6.1). For implantation, one hole was drilled on one side of the rabbit tibia and a designed implant was inserted into the hole. The gap between the bone and the designed implant was defined as a bone defect in the animal tests. After animal testing, the new bone growth by electrical stimulation in the gap was checked to determine how much the different stimulation parameters influence the new bone growth in the animal bone. The control unit, connected to all the implants and supplying the stimulation parameters for all implants, was embedded in biocompatible silicon and inserted under the skin of the rabbit’s pelvis.

Fig. 6.1 The rabbit’s right leg in the rabbit skin STL file (left) and electro-stimulating implant position in the rabbit distal femur (right).

The shape and size of the rabbit distal femur indicated that the electro-stimulating implants should be as small as possible to permit the operation procedure. Two kinds of electro- stimulating implants were designed for the animal tests (Fig. 6.2 and Fig. 6.3). Implant

72 design 1 consisted of three parts (two electrodes and one insulator). These three parts were connected by two screws and biocompatible gluten from the top and bottoms sides of the implant. Two biocompatible cables were inserted into the implant to connect the two electrodes. Electric power was supplied to these two electrodes by these two cables. The material of the electrodes and insulator were TiAI6V4 and PEEK, respectively. As in the preliminary test, the liquid showed to pass through the gap into the implant interior and caused a short circuit. For this reason, a new, more robust implant design 2 (Fig. 6.3) was developed to improve the ingrowth of bone cells on the implant surface. The outer shape of the implant corresponds to the shape of the first design. At each plane lateral surface, the implant had a slot for the insulator (NOVO sin, Eschen, Germany), in which a wire electrode (Ti6AI4V, length 7mm, Ø 0.3 mm) was integrated into. The three wire electrodes were connected to the cables in the inside of the electrode from a hole on the implant longitudinal axis upwards. The fourth cable was connected with conductive adhesive to the electrode implant body.

Fig. 6.2 Implant design 1 (left) and its structure (right).

73 Numerical simulation

Besides the complexity of the construction and difficulties of installation, another important criteria for deciding on the final design for the animal tests is electric field distribution. The design that brings a relatively larger activated tissue electric field on the surface of the implant electrodes and in the gaps between implant and bone, can be used for the final animal tests. COMSOL Multiphysics version 4.3b (Comsol AG, Göttingen, Germany) was used to calculate the electric field distribution caused by the two designs of the electro- stimulating implants in the rabbit distal femur models.

Model reconstruction

As shown in figure 6.4, the construction of the CAD implants was accomplished in 3D-CAD- Software Solidworks 2008. The CAD of the rabbit distal femur models were reconstructed from the rabbit CT scans by using the procedure of Kluess et al. [120]. The position of the implants in the rabbit distal femur models were defined according to figure 6.1. To ensure comparable results, the positions were kept constant for both implants in the rabbit distal femur model. The surrounding tissue of the rabbit distal femur was simplified to being blood in order to decrease the complexity of the model reconstruction. To consider being close to a real rabbit distal femur, the surrounding blood cylinder in the simulation has a radius of 15 mm and a length of 30 mm. The rabbit distal femur was located in the middle axis of the cylinder. In the operation, when the implants were inserted into the holes in the rabbit distal femur, blood was filled into the gaps. After a certain time of electrical stimulation, new bone will grow in the gap. Therefore, in the simulation the material in the gap between the bone and the implants was considered in two cases: the gap is blood and the gap is cancellous bone.

74

Stimulation parameters

Frequency and signal wave form, are kept constant compared to the human clinical study, i.e. 20 Hz and sinus wave. To ensure that the most area of the implant surface has the optimum electric field interval (5-70 V/m) [89] which activates tissue in the animal bone, one parametric study was carried out for both designed electro-stimulating implants to define the optimized electric potential on the surface of both implants. It showed that 400 mV and 150 mV of peak electric potentials should be applied to implant design 1 and design 2, respectively.

Material parameters

As the dielectric properties of rabbit distal femur are not available in the literature, the conductivity and relative permittivity of cancellous bone, cortical bone and blood were derived from Gabriel et al. [105,108,109] in both implant models. The material properties of the designed implants in the numerical simulation were used according to the data sheets from the manufacturer (see table 4.1).

In the simulations, the Dirichlet boundary condition and Neuman boundary condition were available (equations 3.16, 3.17). A Dirichlet boundary condition was applied to impose an electric potential, 400 mV and 150 mV, on the surface of the designed implant electrodes, respectively. A Neumann boundary condition was considered for the insulating surfaces of the implants and the exterior boundary of the model.