Bone Biomechanics - An Overview

Introduction

Due to its diverse and dynamic nature, bone has a very complicated biomechanical behavior. To comprehend and address fracture issues related to inadequate mechanical behavior that may arise owing to variables like age or specific diseases, bone has long been the focus of study in the discipline of mechanics. Because of the extensive use of specialized terminology in biomechanics, which is drawn from the language of mechanical engineering, it can be pretty challenging for researchers with expertise in bone and mineral metabolism to evaluate data from the literature on the resistance of bone.

What Are the Basic Structure and Functions of Bone?

1. Bones not only serve as the body's physical scaffolding, but also as attachment points for muscles, tendons, and ligaments.

2. Moreover, bones enable movement, safeguard crucial organs, aid in breathing, contribute to homeostasis, and generate a variety of cells in the marrow that are essential for existence.

3. The ability of bone tissue to regenerate and return to a completely functional, pre-injury state makes it special.

4. The effects and linkages between forces applied to a structural or rigid body and the strain created are studied by mechanics and materials science.

5. When investigated, the bone may be viewed as both a tissue and a structure since it serves two fundamental purposes.

6. It supports the body (mechanical process) and safeguards its internal organs by regulating the metabolism of calcium, phosphorus, and magnesium (a physiological function).

7. Bone tissue is more mechanically complicated than other engineering materials because it comprises cortical and trabecular bone, each with unique mechanical properties.

What Forms of Bone Are There in Biomechanics?

The two primary classifications of bones are cancellous bones and cortical bones. Up to 80 % of the skeletal system is made up of cortical bones, while the remaining portion is made up of cancellous bones. Two different kinds of tissue make up bone:

1. Compact (cortical) bone is stronger than trabecular bone and accounts for 80 % of the total bone in the body. It is far denser, considerably more resistant to bending, torsion, and compression, and has a much less part in metabolism. The outer shell of the trabecular bone and the shaft of long bones like the femur and tibia are where it is most frequently found.

2. Cancellous (trabecular or spongy) bone accounts for just 20 % of total bone but has a surface/volume ratio of 10 times that of cortical bone. It is much more dynamic since it reacts to changes in load eight times faster. The vertebral body, pelvis, and other places more susceptible to compression are where it happens.

What Exactly Is Bone Biomechanics?

• Since bone is asymmetrical, its biomechanical characteristics change depending on the direction a force is applied.

• As a result, the strength of the bone will vary depending on whether compression, traction, or shearing forces are used.

• The reference criterion presently used to assess bone strength is known as the mineral mass or bone mineral content (BMC, in grams), which is often represented per unit area as bone mineral density (BMD, in g/cm2).

• However, in recent years, it has been established that bone strength depends not only on mass but also on its geometrical, structural, and material characteristics (mineralization and composition of the matrix) as bone quality.

• For examples of trabecular or cortical bone, as well as for vertebral bodies, compression tests are frequently employed. Tests involving traction, torsion, or bending are often performed on long bones like the femur or tibia. In these, traction forces on the opposing side are combined with compression forces on the side to which the force is applied.

• It is difficult to understand how structural characteristics, material characteristics, and the mechanical behavior of bone relate to one another.

• Understanding this connection is crucial because it reveals how bone responds to continual physiological pressures, identifies the areas most prone to fracture, and enables the diagnosis and treatment of a variety of illnesses related to bone strength.

• Every clinically significant variation of bone quality must modify bone biomechanical performance relative to bone mass since a clinical fracture is ultimately a biomechanical event.

How Is Mechanical Load Detected?

The network of osteocytes, which serve as the body's primary mechanical force sensor, conveys commands to the osteoclasts and osteoblasts that act as effector cells by secreting several cytokines that control the bone microenvironment's concentration. The mere act of defying gravity when standing up straight places stress on bones, but the human body is well suited to this load. A significantly higher load, or more compression and bending, is required to stimulate the body to expend the necessary energy and resources to create stronger bones. Multiples of body weight can be used to calculate the load placed on bones. The ability of activity to promote bone formation increases with the load.

Which Features of Bones Are Crucial for Biomechanics?

• The nature of the exact measurements of bone biomechanical performance and bone mass that are utilized will, of course, affect how relationships between these two variables are evaluated.

• Bone stiffness, strength, toughness, post-yield deformation, fatigue, and creep characteristics are just a few of the assays that may be used to determine bone fragility in terms of biomechanical performance.

• Besides the fact that bone mass is a combination of bone size, shape, and tissue material qualities, it has long been usual practice to normalize bone mass by bone size and report bone density metrics.

• Because of the correlations between bone size, body weight, and common mechanical pressures, evaluation of bone density rather than bone mass eliminates part of the impact of total bone size on bone fragility.

• Moreover, these tests can be applied either cyclically or monotonically, short- or long-term, and at various loading rates.

• These loading conditions include compression, tension, shear, bending, or any combination.

• The most direct correlation between various assays or loading types and fracture incidence is not yet known, even though strength is the most logical choice because it pertains to the bone's ability to withstand a single event's stress.

Conclusion

Currently, it is known that the combination of two factors, bone quantity, and bone quality determines the strength of bones. Optimizing the diagnostic tools, which are primarily based on establishing correlations between the biomechanical variables and the different variables that provide the analysis of the quantity and quality of bone, is crucial to improving the treatments used to treat osteodegenerative diseases like osteoporosis.