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2 Bone Structure and Function
C.M. Riggs1 and A.E. Goodship2
1 The Hong Kong Jockey Club, Sha Tin, Hong Kong
2 Royal Veterinary College, London, UK
Introduction
The skeleton is an extraordinary organ that has evolved to optimize its structure to functional demands. The strength and rigidity of its individual components, bones, maintain the body'’s form, provide a series of interconnected levers upon which forces generated by muscles can act to effect movement and locomotion and afford physical protection to vital internal organs. In addition, it serves as a reservoir for essential minerals, houses haematopoietic tissue, contributes to acid–base balance, serves as a fat repository, sequesters certain toxins (heavy metals) from the circulation and acts as an endocrine organ with released hormones having systemic effects. Furthermore, it is dynamic. Some of its component parts undergo structural adaptation in response to the variation in the loads they experience throughout life, while others, principally those evolved for primary protective functions such as the skull, maintain a similar architecture irrespective of changes in load. The architecture of bone from molecular composition to shape and size of whole bones is maintained by cellular mechanisms that effect modelling, remodelling and repair on an ongoing basis and have the capacity to form large segments of new tissue to fill defects created by injury.
This chapter focuses on the features of a bone that are essential to its mechanical functions.
Bone Architecture
The skeleton is comprised of a set of bones that together form the axial skeleton, including the skull, ossicles, hyoid, vertebrae, ribs and sacrum and the appendicular skeleton, which includes the limb bones.
The cells of individual bones express a genetic blueprint that governs their overall shape at an early stage of embryogenesis. For instance, the developing femur of an embryonic mouse transplanted in utero to the spleen still goes on to form a bone that with minimal mechanical environment is still recognizable as a basic femur (Figure 2.1) (John Chalmers, personal communication). The macroscopic architecture of each bone has evolved to meet functional demands that vary between different species and different anatomical locations in the same species. However, long bones, which make up the majority of the appendicular skeleton, share a fundamentally similar blueprint. The majority of bones comprise a tubular shaft (the diaphysis), optimized to use minimal mass for the greatest strength in resisting bending and twisting. The diaphysis flares at each end, the metaphyses, to form a more bulbous terminus, the epiphysis, with broad, sculptured end surfaces that articulate with adjacent bones. The epiphyses are optimized, to resist compressive loading and reducing pressure and impact loading on articular surfaces. The cortex of the diaphysis and outer shell of the epiphysis is formed of cortical bone that appears solid and has an apparent density (volume fraction [V f] = volume of bone matrix per unit volume of tissue) of approximately 90%. The medulla of the diaphysis is filled with marrow that is comprised predominantly of adipocytes. The cortex steadily thins as it flares towards the epiphysis while the medulla becomes filled with cancellous (trabecular) bone, which becomes progressively more dense (V f) towards the articular surface. The cortical shell may be less than a millimetre thick below articular cartilage and is directly supported by the underlying cancellous bone across the entire joint surface. The trabeculae within the epiphysis are generally arranged in arrays that transmit load from the joint surface to the cortex as it thickens towards the diaphysis. Spaces between the trabeculae are filled with blood and lymphatic vessels, nerve fibres, adipocytes and haematopoietic tissue.
Figure 2.1 Normal femur of an embryonic mouse (a) and one that was transplanted in utero to
