# Biomechanical Study of Proximal Femur for Designing Stems for Total Hip Replacement
## 1 Introduction  
Bone is a dynamic tissue that possesses interesting mechanical properties as a result of its organic and inorganic composition [1]. Such properties lead to relevant features connected to accomplishing the following functions [2]: allowing locomotion, providing rigid support and places for muscle action, providing a protective shield for the internal organs and bone marrow, acting as ion backup for the maintenance of homeostasis minerals, acting as an endocrine organ that regulates the metabolism [3] and maintains an acid-base balance, and providing a hematopoiesis environment inside of medullar spaces [4]. Macroscopic bone anatomy is composed of cortical and trabecular bone; each one has mechanical properties that follow its histological characteristics. On the one hand, cortical bone has a high density and low porosity--these features endow it with great compressive strength, enabling it to contribute to the mechanical role of bone [5]. It is also stiffer with a low toughness, as compared with other biomaterials and human tissues. Its microscopic pores permit vascular and neural supply, and the delivery of nutrients, but its deterioration rapidly compromises mechanical integrity and the loss of strength during ageing [6]. On the other hand, trabecular or spongy bone has high porosity, with pores making up 50-90% of the total volume-- consequently, this reduces its compressive strength but increase its resistance to strain. Likewise, it contributes to providing internal support and increased surface area for red bone marrow, blood vessels and other connective tissues which are in contact and interact with bone [5, 6]. Bones are composed of cells; they participate in process like bone growth, modelling and remodeling. The main cells and functions include: osteoblasts, instructed to create and maintain the skeleton architecture [7; 8; 9]; osteoclasts, able to remove mineral bone matrix and devoted to dissolving calcium phosphate crystals and digesting collagen, linked to diseases such as arthritis, cancer and osteoporosis, among others, when they develop pathogenic roles [10; 11]; and osteocytes, which perform the role of mechanotransduction, the ability to sense mechanical stimuli and translate them into biochemical signals that regulate the action of other cells and communicate with osteoblasts and other associates [12]. These cells, with their self-adaptive abilities, guide bone fate and bone remodeling, the process through which bone undergoes substantial changes in structure, shape, properties and composition according to the mechanical and physiological environment [13]. Bone remodeling is essential in tissue repair, engineering and regeneration approaches, and, being influenced by mechanical cues, it proves urgent to develop and apply reliable biomechanical models in parallel to the development of bone replacements and implants. The bone remodeling process is essentially the same in cortical and trabecular bone, and its functions include: the replacement of primary bone with secondary bone, which is more mechanically competent; the removal of ischemic or microfractured bone; and the promotion of correct calcium homeostasis [2; 14]. This process involves simultaneous resorption and formation, contrary to bone modeling (formation) [15], as result of osteoblasts and osteoclasts action. Both cooperate in the remodeling process in what is called a \"basic multicellular unit\" (BMU). According to the literature, the remodeling process consists of three phases: resorption, reversal and formation [16]. Bone adaptation to mechanical loading, as one of the causes of the bone remodeling process, is referred to as Wolff's law or the \"law of bone formation\". This law suggests that bone density changes in response to variations in the functional forces of the bone. Therefore, bone atrophy occurs when bones are not mechanically stressed, while hypertrophy appears when they are over-stressed [17].