**5. Conclusion**

*Animal Models in Medicine and Biology*

the necessity to bear weight.

rial have not been discovered yet.

their application in load-bearing regions.

*4.2.3 Bioresorbable metals*

*4.2.1 Polymers*

*4.2.2 Ceramics*

after implantation. However, Ti and stainless steel implants have a higher e-modulus (Ti: e-modulus 100–124 GPa; steel: e-modulus 200 GPa) than bone (e-modulus 6–24 GPa). Moreover, permanent implants can cause "stress-shielding" leading to loss of bone under the plate or between bone and implant, thereby increasing the risk of refractures, designated as peri-prosthetic or peri-implant fractures [29, 31]. Moreover, the FDA has already described possible metallic sensitivity or allergic reactions linked to Ti-based alloys, such as Ti-6Al-4V [66], and studies using vanadium have also shown adverse effects [67]. However, resorbable materials exhibit functional properties by supporting bone formation and in-growth on a molecular level. To date there are only few resorbable materials used in cardiac, dental and neuro-surgery and some not weight-bearing application in orthopedic surgery, but adequate materials for orthopedic and trauma surgeries need good mechanical properties. Therefore, the main focus in biomaterial research is to evolve materials and tools to develop the optimal implant for the respective bone condition under

Poly-L-lactic acid (PLLA) and poly-lactic-co-glycolic acid (PLGA) are the most commonly used polymers considered for their use as osteosynthesis and bone grafts [68]. Disadvantages of polymers include poor mechanical properties (low strength and stiffness, high brittleness) and osteoconductivity. Degradation behavior depends on monomers and can be very slow thereby increasing the risk of adverse

Alternatively, polyhydroxybutyrate (PHB) can serve as a polymeric implant material, which is produced by microorganisms, especially bacteria. However, PHB can induce toxicological effects. However, these effects have been reduced dramatically, raising the potential for its application in clinics. Nevertheless, functional properties (i.e., osteoinductivity or osteoconductivity) of polymeric implant mate-

Ceramics are synthetic bone replacement materials with good biocompatibility and osteoconductivity, thereby showing good osseointegrative and nonimmunogenic effects. Composed of hydroxyapatite (HA), or alpha (α)- and beta (β)-tricalcium phosphates (TCP), ceramics exhibit poor mechanical properties including low yield strength and high brittleness, which make them unattractive for

In comparison to polymers and ceramics, iron (Fe), magnesium (Mg), and zinc (Zn) are more stable, tensile, and load-bearing, respectively. To process Fe-based alloys, the low melting point of Fe constitutes an interesting property. However, Hofstetter et al. demonstrated that limited access to oxygen was associated with slow degradation rates [70]. Metal implants based on Zn display several disadvantages including low rigidity and deformability, as well as corrosion inhibition. Therefore, Zn is likely more suited as an alloying element in combination with other materials. Finally, Mg-based alloys exhibit several advantages including good biocompatibility, resorbability, and favorable biomechanical properties. Moreover, some studies have demonstrated Mg's associated functional properties, especially its ability to support bone fracture healing [33]. For example, recent studies using

effects such as sclerotic areas in bone and fibrous encapsulation [69].

**34**

Here, we summarized fundamental differences in small and large animal models concerning bone quality, composition as well as their individual advantages and disadvantages. Focusing on two major complications in orthopedics and traumatology, we wanted to underline the merits of an animal model by supporting with scientific results obtained from our intensive literature recherché. Implant research is a hot topic in orthopedics and trauma surgery. Based on our expertise, we wanted to give insights into implant technology, materials, and designs. Currently, permanent implants are the state-of-the-art material used to stabilize bone fractures in orthopedics and trauma surgery. However, to develop the ideal implant for a certain bone condition (e.g., osteoporosis and osteoarthritis), the underlying disease and the detrimental outcome on bone (e.g., bone mass, fracture risk, and bone density) have to be taken into account when choosing the implant material (e.g., Ti-, Mg-, Fe-based implants), design (e.g., pin, screw, plate, and scaffold), material properties (e.g., tensile strength, non- or bio-resorbable), and implantation site (e.g., knee, femur, and tibia).

Hence, it is of utmost importance to choose the most appropriate animal model according to the research question and warranted primary outcome measures.
