## 1 Introduction
### Common Orthopedic Implant Devices, Materials and Biomechanical Characterization
Due to the non-uniform stress distribution involving compressive and shear stress components, implant models have been developed in order to determine mechanical parameters under controlled and standardized conditions. Stainless steel 316L is the most prevalent material for fabrication of implant devices for orthopedic surgery applications. It is highly resistant to the corrosive action of biological fluids due to the high chromium percentage (17-19%). Titanium alloy is lighter than steel for the same strength but it can be easily contaminated when exposed to hydrogen, nitrogen, and oxygen, which make it unsuitable for some medical procedures. The importance of orthopedic implants mechanical behavior becomes even more obvious if considered in the context of healing process types (Section 1.2). A significant number of studies exists on the micromovement of fractured bone components on the healing process. [PERSON] et al. [30] conducted a study to identify the suitable amount of micromovement, location and timing of callus formation. It was found that in case of large micromovement, the amount of newly formed bone within the gap decreased with increasing gap size, suggesting a delay of bone healing. Stimulation of new bone formation by micromovement was mainly effective in the early healing phase (4 weeks postoperatively). Large gaps showed the least new bone formation at the fracture site and the lowest flexural rigidity. [PERSON] et al. [31] presented a study in which 85 tibial fractures of comparable severity were divided into two groups and were fixed with highly rigid fixation (Group 1) and the same type of fixation with axial micromovement applied across the fracture site for 30 min per day (Group 2). It was found that the time to reach stiffness levels equivalent to clinical union was significantly lower in Group 1 than in Group 2 and the mean time to independent weight bearing was lower in Group 2 than in Group 1. Complications and failure of the procedure occurred mostly in Group 1 patients. It was concluded that axial micromovement contributed to improvement of the procedure outcome. Implant devices wear and failure are important issues that must be considered. [PERSON] et al. [32] presented a case study investigating the failure analysis of a 316L stainless femoral orthopedic implant (locking compression plate fixed to the broken femur using locking and compression screws). The macro and micro fractographic analyses revealed that the failure mechanism was high-cycle fatigue and that the implant underwent approximately \(10^{6}\) loading cycles before failure occurred. A Finite Element Analysis (FEA) of the assembly confirmed that the crack initiation sites were located in the region where the highest stress values occurred. Post-failure analysis indicated that the premature fatigue failure (less than 2 years) was most probably caused by walking (high-cycle fatigue). It was concluded that implant geometry and installation procedure could be optimized in order to redistribute the stress concentration. Bone--implant device biomechanical behavior is one of the factors with the highest importance in the outcome of implant devices--based treatment of fractures. [PERSON] et al. [10] reviewed the parameters that quantify the concept of biomechanical behavior for implant devices and the usual testing techniques. It is generally agreed that analytic modeling of the bone--implant device system is extremely difficult and can only be done for simple systems only. Commonly used techniques include FEA, experimental in vitro testing and recently, machine learning algorithms (MLAs). MLAs are extensively used in modeling complex processes with a significant random component. There is a wide range of applications for MLAs in healthcare: [PERSON] et al. [33] proposed an intelligent scalp detection system based on MLAs, [PERSON] et al. [34] developed a Support Vector Machine system to classify dysphagia swallowing sounds, [PERSON] et al.