## 1 Introduction
### Common Orthopedic Implant Devices, Materials and Biomechanical Characterization  
Fracture healing process is closely related to the bone fixation stability and rigidity [17]. Two main types of fixation systems exist (Figure 2):  
* Internal fixation (Figure 2a), which restores bone physiology and enable early mobilization of the limb. The function of the injured bones can be restored and full support for physiological load is ensured by applying internal fixation. The vast majority of internal fixation systems are manufactured from stainless steel or titanium. A review of internal fixation systems and the fracture sites for which they are suitable can be found in [18] and [19]. By means of internal fixation, malunion is a very rare occurrence and mobilization of the patient is very fast. Internal fixation systems have disadvantages, however. The stiffness and Young modulus of the implant material are much higher than in the case of cortical tissue of the human bone. Fixation of the bone fragments with high rigidity materials prevents load transfer to the healing bone, which is unfavorable for fracture callus remodeling [20]. The necessity of surgical removal after the bone healing is another trauma the patient has to undergo. * External fixation (Figure 2b) is considered flexible fixation. External fixator systems consist of elements such as pins, wires (Schanz screws, Steinman pins, Kirschner wires) and belts that are conventionally used as a dynamic fixation of fractured bones. External fixation approach is used for open fractures with massive soft-tissue injuries such as open Type II, Type III fractures (Type II and III are open fractures with a soft tissue laceration larger than 1 cm and 10 cm respectively, minor and severe communication respectively and with simple and complex fracture pattern, respectively) and even in articular fractures, in which the surgical trauma to the limb during fixation is reduced [18]. Several milestones in development of internal fixation systems can be mentioned: [PERSON] et al. [21] developed a plate with a tensioner to provide inter-fragmentary compression during tightening processing, called the dynamic compression plate (DCP); [PERSON] et al. [22] developed another compression plate with limited bone surface contact, with the purpose of preserving the blood supply and protecting the periosteum; [PERSON] proposed a design of combined plate hole (described in [23])--a plate that possessed the advantages of both DCP and locking plate system. The modern locking compression plate design is based on the combined plate hole described in [23]. It is widely accepted that dual plating technique [24] ensures the best results in terms of mechanical stability and clinical outcomes. It was found that PLS plate system [21] provided a similar bio-mechanical resistance to static load in case of Schatzker V tibial plateau fractures. The biomechanical properties of the bone--implant interface are the determinant requirements for implant stability [25]. A good quality of bone healing leads to: (i) direct contact between mineralized  
Figure 2: Internal fixators (**a**) and hybrid external fixator (**b**) used for fixation of tibial plateau fracture. bone tissue and the implant and (ii) an important proportion of the implant surface in intimate contact with bone tissue. Primary stability of the implant is a critical factor in the fracture healing success. Improper primary stability results in excessive interfacial micromotion following surgery [26; 27], which may imply a higher occurrence of migration and implant failure [28]. The majority of implant systems employed in clinical practice have a complex geometry, which leads to three-dimensional complex, non-uniform, multi-axial stress fields [29]. The physiological gait pattern also applies a complex stress field on the implant device.