Question

1. a) State the Wolff’s law of the transformation of bone. b) What is the fundamental idea in the trajectorial theory of bone structure? c) Discuss in detail the composition and structure of bone explaining why the different bone tissues are organized differently. d) Explain why the bones of the human body are stronger in resisting compression than tension and shear. e) In the human femur, bone tissue is strongest in resisting compressive force, approximately half as strong in resisting tensile force, and only about one-fifth as strong in resisting shear force. If a tensile force of 8000N is sufficient to produce a fracture, how much compressive force will produce a fracture? How much shear force will produce a fracture?

Answer 1

Ans :

1. A) WOLFF'S Law :

  developed by the German anatomist and surgeon Julius Wolff (1836–1902) in the 19th century, states that bone in a healthy person or animal will adapt to the loads under which it is placed. If loading on a particular bone increases, the bone will remodel itself over time to become stronger to resist that sort of loading.The internal architecture of the trabeculae undergoes adaptive changes, followed by secondary changes to the external cortical portion of the bone, perhaps becoming thicker as a result. The inverse is true as well: if the loading on a bone decreases, the bone will become less dense and weaker due to the lack of the stimulus required for continued remodeling.This reduction in bone density (osteopenia) is known as stress shielding and can occur as a result of a hip replacement .The normal stress on a bone is shielded from that bone by being placed on a prosthetic implant.

The remodeling of bone in response to loading is achieved via mechanotransduction, a process through which forces or other mechanical signals are converted to biochemical signals in cellular signaling.Mechanotransduction leading to bone remodeling involve the steps of mechanocoupling, biochemical coupling, signal transmission, and cell response.The specific effects on bone structure depends on the duration, magnitude and rate of loading, and it has been found that only cyclic loading can induce bone formation.When loaded, fluid flows away from areas of high compressive loading in the bone matrix.Osteocytes are the most abundant cells in bone and are also the most sensitive to such fluid flow caused by mechanical loading.Upon sensing a load, osteocytes regulate bone remodeling by signaling to other cells with signaling molecules or direct contact.Additionally, osteoprogenitor cells, which may differentiate into osteoblasts or osteoclasts, are also mechanosensors and may differentiate one way or another depending on the loading condition.

B)  The Trajectorial Theory propounds that these cancellous structures in the femoral neck are due to both tension and compression forces, while modern day concepts of Wolff's Law only acknowledge the action of compression forces: and herein lies the paradox. The Trajectorial Theory and Wolff's Law, when applied to the upper femur, are mutually exclusive. The evidence, anatomical and physiological, indicates that bone forms within the femoral neck solely under the influence of compression forces. This would indicate that the Trajectorial Theory is not appropriate for this region.

C) structure and composition of bone :

  Bone is not uniformly solid, but consists of a flexible matrix (about 30%) and bound minerals (about 70%) which are intricately woven and endlessly remodeled by a group of specialized bone cells. Their unique composition and design allows bones to be relatively hard and strong, while remaining lightweight.

Bone matrix is 90 to 95% composed of elastic collagen fibers, also known as ossein,and the remainder is ground substance.The elasticity of collagen improves fracture resistance .The matrix is hardened by the binding of inorganic mineral salt, calcium phosphate, in a chemical arrangement known as calcium hydroxylapatite. It is the bone mineralization that give bones rigidity.

Structure :  The hard outer layer of bones is composed of cortical bone, which is also called compact bone as it is much denser than cancellous bone. It forms the hard exterior (cortex) of bones. The cortical bone gives bone its smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an adult human skeleton. It facilitates bone's main functions - to support the whole body, to protect organs, to provide levers for movement, and to store and release chemical elements, mainly calcium. It consists of multiple microscopic columns, each called an osteon or Haversian system. Each column is multiple layers of osteoblasts and osteocytes around a central canal called the haversian canal. Volkmann's canals at right angles connect the osteons together. The columns are metabolically active, and as bone is reabsorbed and created the nature and location of the cells within the osteon will change. Cortical bone is covered by a periosteum on its outer surface, and an endosteum on its inner surface. The endosteum is the boundary between the cortical bone and the cancellous bone. The primary anatomical and functional unit of cortical bone is the osteon.

D)  Both cortical bone and trabecular bone are stronger in compression than tension, which reflects the fact that the inorganic phase is stronger in compression than tension. The organic phase contributes to the ductility and toughness of the tissues.

Composite of collagen and hydroxyapatite hydroxyapatite Collagen has a low E, good tensile tensile strength, poor compressive compressive strength Calcium Calcium appatite appatite is a stiff, britle material material with good compressive compressive strength -  anisotropic anisotropic material material that resists resists many forces Bone is strongest strongest in compression, in compression, weakest weakest in shear, intermediate intermediate in tension