**4. Orthopedic in vivo implant research**

In vivo studies are essential to investigate novel implant materials and cannot be fully covered by in vitro testing. Preliminary safety tests with new implant materials using in vitro models give some information on acute toxicity and cytocompatibility. Nevertheless, some studies use the term of biocompatibility when testing implant material in vitro. However, biocompatibility tests need living organisms such as animals and humans; therefore, cytocompatibility needs to be correctly used when testing in vitro.

In order to test implant safety, adverse tissue reactions as well as corrosion and wear resistance need to be investigated to guarantee its long-term application in clinics. Hence, in vitro and in vivo tests are essential to evaluate new implant materials regarding cytocompatibility, biocompatibility, and mechanical stability.

The development of bioresorbable metal implants is one of the major goals in orthopedic and trauma surgery. Apparently, the advantages are the unnecessity to remove the implant due to material resorption and the associated avoiding of narcosis, mandatory for the second removal surgery. Since there is an increasing number of patients with metal sensitivity to permanent implants such as titanium (Ti), and long-term complications associated with currently available metal implants cannot be foreseen to date, there is a high demand to develop novel biocompatible and bioresorbable implants with good mechanical properties to stabilize bone fractures.

#### **4.1 Implant design**

To test bioresorbable orthopedic implants in animal models, the implant design and dimension is of utmost importance. Moreover, the implant number and size directly influences the number and species of animals used to test a research hypothesis. The most common implant designs used in small animal models such as mice and rats are cylindrical-shaped pins [61], whereas screws are the commonly

**33**

**Figure 2.**

calculated [61].

performed [65].

**4.2 Implant material**

*Animal Models in Orthopedic Research: The Proper Animal Model to Answer Fundamental…*

used designs in large animal models such as sheep [62] (**Figure 2**). However, there are much more designs that are less commonly used such as plates and discs. More importantly, the size of the implant must be adjusted to the size of the animal which is comparable to common implant sizes in humans. In small animal models, cylindrical rods are used with a simple geometry that makes it easier to analyze implant degradation and behavior. However, these rods have to exactly fit ("press-fit"), otherwise the implants will become unstable and will be lost during investigation. Screws are more reliable when it comes to comparison with humans, since screws are commonly used to stabilize fractures or fix plates in humans [63]. However, the

Dimensions of implants differ according to the sizes of the animals. For example, the most appropriate dimension for cylindrical implants in rabbits is 6 mm in length and 2 mm in diameter, whereas the ideal dimension for large animals including goat, dog, and rabbit is 12 mm in length and 4 mm in diameter, according to ISO guidelines. Proper controls have to be chosen to investigate new implant material. According to ISO standards, it is recommended to use currently certified materials, which are already used in clinics, as a control [64]. In order to properly examine implant material, primary outcomes have to be specified: to test mechanical properties, bone tissue with implants are harvested and undergo pull-out/ push-out tests (cylindrical implants) and torque removal tests (screws) (**Figure 2**). This test usually demonstrates proper integration of the implant in bone [1]. In case of resorbable biomaterials, degradation behavior and bone in-growth are the major primary outcomes besides mechanical properties. Real-time imaging techniques, such as in vivo micro-computed tomography (μCT) in small animals and clinical CT in large animals are used to observe material changes (degradation, bone in-growth, etc.) over the entire study duration. After reconstruction, 3-dimensional (3D) images can be reconstructed, and implant volume loss and bone formation can be

Other studies aim to investigate effects of implant surface modifications on bone formation and bone-implant interaction. To obtain accurate results, surface characteristics including chemical composition and surface topography must be determined. Therefore, visual observation (scanning electron microscopy, μCT) and numerical analysis (energy dispersive X-ray microscopy, profilometry) must be

Conventional alloys currently used in the treatment of fractures include Ti and stainless steel, which are more rigid with desirable advantages including biocompatibility, good resistance to material corrosion, and most importantly, these alloys do not show severe toxic effects on various immune cells and can bear weight soon

*Screws (left image) and cylindrical pins (right image) are often used in orthopedic and trauma research.*

geometry is more complicated which makes the analysis more difficult.

*DOI: http://dx.doi.org/10.5772/intechopen.89137*

## *Animal Models in Orthopedic Research: The Proper Animal Model to Answer Fundamental… DOI: http://dx.doi.org/10.5772/intechopen.89137*

used designs in large animal models such as sheep [62] (**Figure 2**). However, there are much more designs that are less commonly used such as plates and discs. More importantly, the size of the implant must be adjusted to the size of the animal which is comparable to common implant sizes in humans. In small animal models, cylindrical rods are used with a simple geometry that makes it easier to analyze implant degradation and behavior. However, these rods have to exactly fit ("press-fit"), otherwise the implants will become unstable and will be lost during investigation. Screws are more reliable when it comes to comparison with humans, since screws are commonly used to stabilize fractures or fix plates in humans [63]. However, the geometry is more complicated which makes the analysis more difficult.

Dimensions of implants differ according to the sizes of the animals. For example, the most appropriate dimension for cylindrical implants in rabbits is 6 mm in length and 2 mm in diameter, whereas the ideal dimension for large animals including goat, dog, and rabbit is 12 mm in length and 4 mm in diameter, according to ISO guidelines. Proper controls have to be chosen to investigate new implant material. According to ISO standards, it is recommended to use currently certified materials, which are already used in clinics, as a control [64]. In order to properly examine implant material, primary outcomes have to be specified: to test mechanical properties, bone tissue with implants are harvested and undergo pull-out/ push-out tests (cylindrical implants) and torque removal tests (screws) (**Figure 2**). This test usually demonstrates proper integration of the implant in bone [1]. In case of resorbable biomaterials, degradation behavior and bone in-growth are the major primary outcomes besides mechanical properties. Real-time imaging techniques, such as in vivo micro-computed tomography (μCT) in small animals and clinical CT in large animals are used to observe material changes (degradation, bone in-growth, etc.) over the entire study duration. After reconstruction, 3-dimensional (3D) images can be reconstructed, and implant volume loss and bone formation can be calculated [61].

Other studies aim to investigate effects of implant surface modifications on bone formation and bone-implant interaction. To obtain accurate results, surface characteristics including chemical composition and surface topography must be determined. Therefore, visual observation (scanning electron microscopy, μCT) and numerical analysis (energy dispersive X-ray microscopy, profilometry) must be performed [65].
