# Numerical Simulation of Electric Field Distribution around an Instrumented Total Hip Stem
## 1 Introduction  
Today, total joint replacement (TJR) is a standard procedure in orthopedic surgery. Adequate osseointegration of the implant components is required for their long-term stability within the bone stock. Surface modifications such as bioactive coatings have been developed extensively to stimulate bone ongrowth, and this increases secondary implant stability [1; 2; 3]. An alternative for the enhancement of bone ongrowth and stable implant fixation may be the application of electrical stimulation of the adjacent bone tissue. So far, in clinical practice, invasive electromagnetically induced osseotherapy has been used for the stimulation of osseoinduction in cases of avascular femoral head necrosis, femoral neck fractures, and pseudarthosis [4; 5]. In this therapeutic approach, based on [PERSON] and [PERSON]'s [6] work, patients are instrumented with a bipolar induction screw implant such as the Asnis III s-series screw, (Stryker Trauma, Kiel, Germany), which consists of two electrodes and a secondary coil [4]. A primary coil is placed around the patient's leg during treatment, generating a low-frequency magnetic field, which transfers energy inductively tothe electrical stimulation system. Sinusoidal magnetic fields in a frequency range between 12 to 20 Hz and with a voltage of 0.7 V are applied [7]. Therefore, an alternating electric field is provided to the surrounding bone tissue. Usually, the stimulating implant is removed after three months of treatment [4].  
A retrospective clinical observation of 53 patients treated with an Asnis III s-series screw showed good postoperative results, i.e., no arthroplasty, no reoperation, and no radiological progress associated with clinical symptoms were observed in 86% of the patients with Steinberg stages I-III after 33 \(\pm\) 20 months postoperatively [7]. It has been proven by several studies that an electric field of 5-70 V/m is beneficial in order to regenerate bone tissue [6; 8; 9; 10]. [PERSON] et al. [11] proved that there was a significant influence of electromagnetic stimulation on the mechanical strength of healing bone at the ovine tibia. In a later animal study, which employed 20 dogs, an increase of 17% new bone formation compared to the control group was seen [12]. The animals were treated with 38 Hz and 0.14 V over 8 weeks with 10 h of stimulation per day. These methods are only used for short-term regeneration of bone tissue over several months and have not been used for long-term electrostimulation of bone cells, bone tissue, or for implant stability, despite the fact that electrical stimulation can also be beneficial for THRs [13; 14; 15]. By supporting ongrowth of bone cells at the implant surface and thereby enhancing fixation strength, revision surgery of the THR may be delayed. Furthermore, for revision implants, the stimulation of bone growth is necessary, especially in large bone defects after the removal of primary implant [16]. Therefore, an electrostimulative acetabular revision system with additional individual patient fixations was developed [17; 18] as there were no previously existing solutions to actively stimulate the ongrowth of bone cells on total hip stems using electromagnetic fields [19].  
In the present study, a commercially available total hip stem was instrumented in order to generate an electric field for enhanced bone regeneration in the surrounding tissue and improved ossseointegration of the implant. For this purpose, finite element simulations of the electric field distribution were conducted and validated by experimental investigations in a porcine femur as proof of concept for an instrumented electrostimulative total hip stem.