# Numerical Simulation of Electric Field Distribution around an Instrumented Total Hip Stem
## 4 Discussion  
In the present study, we investigated the general feasibility of an electrostimulative uncemented total hip stem. The bone, being in close proximity to the implant, provides most of the mechanical stability. For this reason, the area close to the implant is of highest interest and thus the electrical stimulation can be limited to the closest bone on both sides of the implant. Therefore, the electric potential values at the cross section of a porcine femur and along a defined raster along the porcine femur defined at depths of 5 mm and 8 mm into the bone were measured in an experiment and compared to results from three different numerical simulation scenarios. Based on two different conductivity values for bone marrow, three case studies were performed in order to discover the most precise model to replicate the physiological behavior of the bone. In the first simulation scenario (case study 1), yellow bone marrow was simulated with low conductivity [22]. The second simulation scenario (case study 2) excluded bone marrow by replacing it with cancellous bone, and the third simulation scenario (case study 3) worked with data from [PERSON] et al. [23], including red bone marrow. In our survey, it was found that case study 3 represented the best approximation to the validation experiment performed. In this case study [24], the predicted potential (see Table 1) was in good agreement with our experimentally measured values at the cross section of the porcine femur, thus supporting our assumption of electric field distribution with the bone tissue. In this context, a closer investigation of the influence of tissue conductivity on the electric field distribution of the electrostimulative total hip revision system has been performed [24]. As the electric properties of biological tissues behave linearly at a fixed frequency, the factor used to decrease the stimulation voltage can also be used to decrease the electric field. During the experiment, the primary goal was to generate a high potential to achieve a value several magnitudes above the error tolerance of the multimeter. This resulted in high gradients of the potential and the electric field as can be observed in Figure 5a. Figure 5: Electric field distribution obtained for case study 3. A voltage of **(a)** 6.0 V and **(b)** 0.2 V was applied at 20 Hz, representing clinical conditions. For clinical use, the electric fields for bone regeneration are required to be between 5 and 70 V/m [6]. In this first approach, a voltage of 0.2 V was selected to achieve the desired electric field strengths. The yellow and red area in Figure 5a can be considered over-stimulated (>70 V/m). This area had to be minimized, which means that the voltage had to be reduced. In contrast, the dark blue area in Figure 5b was not stimulated sufficiently (<5 V/m), and to minimize this area, the stimulation voltage had to be increased. Both these contradictory goals can be accomplished without changes in the voltage by slightly increasing the width of the insulator [25]. A limitation of our experiments was given by the high standard deviations of the measured potential. These were likely due to the drying out and rehydration processes performed during measurements. Another reason for the deviations is the use of bone tissue from adolescent pig, which does not have the same dielectric properties as an adult human bone tissue [26]. Figures 4 and 5 show that apart from readout location L4 at 8 mm depth, the values of the simulation were close to the measured potential in the experiment. The outlier behavior of readout location L4, which had a 99% (depth 8 mm) deviation from simulations 2 and 3, necessitates an investigation of the boundary conditions of the experiment. Due to this reason, the relevant CT scan at the height of L4 was examined in detail. The dark areas within the cancellous bone and marrow are fluids, which increases the conductivity significantly at these locations.