**2. The usefulness of temporary implants for bone regeneration**

Bone tissue is known for its ability to self-regenerate on its own. However, if the fracture becomes a critical bone defect, the bone loses this ability. From 1934 to the present day, some authors argue that a bone defect becomes critical when it is over two times the diameter of the bone defect [10, 11, 25–27].

These critical-sized defects may result from infection, malformation, and traumatic injuries, which may lead to bone loss in the patient [28–33]. In this case, as bone cannot self-regenerate, it is necessary to use a temporary implant (natural and/or synthetic) to support bone regeneration with cellular incorporation. To achieve this, successfully, it is first necessary to consider the mechanical properties of the native bone tissue.

It is known that the mechanical properties of the bone vary according to age, anatomical location, and bone quality. Within the biomechanical properties of the bone (resistance, stiffness, and fatigue), the elastic modulus is the most attracting variable in research due to its importance to characterize bone pathologies and also in the design orientation of artificial implants. Bone strength and elasticity are anisotropic. The compact bone is stronger under compression and stiffer when loaded longitudinally along the diaphyseal axis than in the transverse radial directions. In trabecular bone, its mechanical properties depend on both the porosity and the architectural desirability of the individual trabeculae [3]. The mechanical properties of human bone are summarized in **Table 1**.

Implants need to be accepted by the human body, where there are guarantees for cell survival in a safe and supportive environment. Moreover, mechanical damage or failure caused by stress shielding must be prevented. The scaffolds need to have an appropriate modulus of elasticity to match bone properties. Scaffolds with a highly porous structure are favorable for cellular activities, including fixation and proliferation, which will contribute to bone neoformation and regeneration and adjust the mechanical properties in terms of Young's strength and modulus [36].

So, an important key factor is concerned with the type of materials to be used in the implant. Biopolymers are biocompatible and biologically active materials as they promote cell adhesion and growth.

To help in the engineering of long bone fracture regeneration, artificial fractures are typically manufactured in models in vivo [23, 37]. Various animal models are studied in vivo before its application in humans.


**Table 1.** *Human long bone properties.*

*Design and Manufacturing*

their structure gives it high tensile strength and its mineral substances impart high

Bone, namely in the diaphysis, is made up of cortical or compact bone that contains its own blood vessels and cells, which aid in its growth and regeneration. It has many types of cells, such as osteoblasts, osteocytes, osteoclasts, and a matrix of non-mineralized collagen (osteoid). Bone tissue comprises several functions, such as: (i) provide structural integrity, and all the necessary support to the soft tissue of the body, constituting the global support of the majority of the muscles, (ii) protect vital organs, and (iii) help to balance the minerals, since the bone tissue stores calcium and phosphate making them more resistant and able to maintain a balance of blood concentration [5–7]. Bone is known to self-regenerate: pos-natal bone maintains an intrinsic capacity for well-ordered growth, remodeling to meet

Major bone defects are the result of injury, trauma, nonunion after a fracture, infection, or abnormality, resulting in long-term deformities, such as limb shortening, leaving patients with reduced bone structure and function [9–11]. It should be

The gold standard treatments for bone defects are still bone grafts. These can be used alone or combined with other materials in order to promote bone healing through osteoinduction, osteoconduction, and osteogenesis [14]. These bone grafts may be from autograft (taken from the patient), allograft (taken from another patient) and xenograft (obtained from an animal) origins or even manipulated with synthetic biomaterials. Additionally, prostheses can also be used, but they usually need a second surgery later on due to some complications that may appear, such as the formation of bone callus or hernias. Autografts are still considered the gold standard treatment due to their osteogenic, osteoinductive, and osteoconductive capacity. However, there is a limitation in tissue extraction from the amount that is required. Allografts taken from other donors or corpses present a high risk of immune rejection, reduced bioactivity and a high risk of pathogen transmission

When there are fractures with a bone defect exceeding a critical size, the bone is not able to self-regenerate and, therefore, requires the use of a temporary implant (natural and/or synthetic) to serve as support and cells to help bone regeneration

The concept of TE was implemented in 1993 by Langer and Vacanti. They specified that "TE is an interdisciplinary field that relates the principles of biology and engineering to the production of tissue functional substitutes" [21]. So, they presented specific characteristics and applications in biodegradable three-dimensional (3D) scaffolds. Ideally, they should be highly porous, having highly interconnected pore networks with a pore size suitable for cells to migrate and differentiate whenever necessary [22]. However, the biggest challenge of scaffolds is related to mass transport of nutrients and secretion of waste in tissue [6]. It is important cells used in 3D cultures of scaffolds be able to mimic the morphology, functionality, and biology of the tissue. These cell cultures are necessary to analyze mechanisms of chronic diseases and the impact of drug treatments or to produce different tissues for major defects in vivo, in this study, the bone. Bioreactors appeared to improve

This chapter intends to perform a critical analysis of the state of the art regarding full bone TE towards the selection of the most appropriate solution of temporary implants. Thus, the optimum conditions (static vs. dynamic), material, cells, and/or the inclusion of growth factors for the repair of large bone defects are discussed. Hence, there are two scientific questions to which this chapter intends to address: (i) which is the most suitable combination of scaffold design and

compressive strength and thus excellent mechanical resistance.

noted that the most transplanted tissue after blood is bone [12, 13].

[19]. In this way, tissue engineering (TE) has emerged [20].

the field of cell culture on 3D support [23, 24].

mechanical needs, and renewal after damage [8].

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[7, 15–18].
