Figure 2.4 Microradiograph of a 100 μm thick, undecalcified section of cortical bone from the proximal diaphysis of the humerus of a two‐year‐old Thoroughbred racehorse that had suffered a catastrophic fracture. Different grey levels reflect mineral density of the matrix. Note lightly mineralized periosteal new bone to the right (dark grey), moderately mineralized ‘young’ secondary osteons (variable densities of grey) and most densely mineralized primary bone (which appears almost white).

Conversely, collagen fibres in lamellar bone are laid down in a highly organized fashion around a plexus of blood vessels. Organic matrix is deposited as thin sheets, or lamellae, each approximately 5 μm thick. The orientation of collagen fibres is largely parallel within each lamella and may be the same or vary between successive lamellae. Osteocytes are present in lower density than woven bone, but are more evenly distributed and are of consistent size and orientation. Lamellar bone may be deposited around part or the entire outer circumference and inner endosteal surface of a bone (circumferential lamellar bone) or may form as concentric lamellae within the small tubular subunits of osteons (Figure 2.5). Osteons are formed during the primary growth of bone around a small central Haversian canal that contains neurovascular components. Lamellar bone has superior mechanical properties to woven bone but is formed more slowly.

A combination of these different matrix organizations is found in a microstructure that is common in long bones of the distal limbs of horses and other animals that have evolved to ambulate soon after birth. Woven bone is deposited at the periosteal surface to form waves or folds that grow out radially several hundred micrometres before branching out circumferentially. Adjacent folds meet to form bridges or domes that enclose a small volume of periosteal soft tissue, which usually includes several blood vessels. Lamellar bone then forms on the interior surface of the woven bone, replacing the soft tissue, to form primary osteons that are elongated around the circumference of the bone and which often contain multiple vessels and associated canals (Figure 2.6). This microstructure is referred to as laminar or plexiform bone and it reflects an evolutionary compromise whereby the rate of bone apposition is accelerated with little detriment to material properties [11].

Figure 2.5 Composite photomicrograph of a transverse section through the lateral margin of the cortex from the mid‐diaphysis of the third metacarpal bone from a two‐year‐old Thoroughbred racehorse. Fluorescent dyes administered systemically to the horse at different times before it died demarcate the mineralization front at the time of administration. Several different bone microstructures are present: (i) circumferential lamellar bone (top left), (ii) Plexiform bone and primary osteons (top right) and (iii) secondary osteons at different stages of formation (bottom half of image). Periosteal surface to the top and palmar to the left. Field width is approximately 10 mm.

Figure 2.6 Diagrammatic illustration of plexiform bone, based on the histological appearance of a transverse section through the cortex of the third metacarpal bone from a neonatal Thoroughbred. Plexiform bone develops around a woven bone template (light shade). Buds of woven bone (dark shade) grow radially outwards for approximately 300 μm from the periosteal surface before expanding circumferentially to join with neighbouring radial struts, thereby forming a three‐dimensional mesh of successive layers of bone linked by radial struts. The spaces that remain within the network fill more slowly with lamellar bone to form primary osteons.

      Source: Riggs and Evans [10]. Reproduced with permission of John Wiley & Sons.

Accretion of new bone at periosteal and/or endosteal surfaces can increase the thickness, overall diameter and mass of the bone. Alternatively, accretion at one surface and simultaneous resorption at another can alter the geometric properties of the bone, increasing its overall diameter for a similar mass of tissue or redistributing a similar mass around the circumference of the shaft (Figure 2.7). Both processes are termed modelling and occur during growth and as a response to changes in the bone's mechanical environment.

Figure 2.7 The geometric properties of bone can be modified through the coordinated action of osteoblasts and osteoclasts, which deposit (red areas in the diagram) and resorb (grey areas) bone at existing surfaces to alter the overall geometry of the bone. This process is called modelling.

Groups of osteoclasts may be recruited to foci on the surface of or within the bone matrix and stimulated to resorb tissue, either as surface layers or as tunnels through the matrix. In the latter case, osteoclasts cut tubes, ‘resorption canals’ approximately 250 μm in diameter, through the bone. The canals are typically orientated parallel to the long axis of the bone, extend over several millimetres and may branch several times (Figure 2.8). Under normal circumstances, osteoclasts and osteoblasts work in synchrony, the latter following the resorptive front, forming fresh matrix on the recently ‘cut’ surface. Osteoid is deposited as sheets, lamellae, in which collagen fibres are aligned in parallel (Figure 2.5). Successive lamellae, between which the alignment of collagen fibres relative to the long axis of the bone may vary, form layers on the surface of bone or around the inner circumference of the resorption canals until only a small central hole that contains blood vessels and lymphatics (the Haversian canal) remains. The resultant structure is called a secondary osteon. This process, whereby osteoclasts are activated and bone is resorbed and subsequently replaced at the same location with fresh tissue, is called remodelling, and the functional unit of cells that performs it is referred to as a basic multicellular unit (BMU). The deepest margin of resorption, where new bone abuts pre‐existing tissue, is called the reversal line and is demarcated by a line of cement, a thin layer of amorphous matrix that joins fresh bone to old. This is an important structural feature in relation to a bone's ability to resist fatigue as it acts to ‘capture’ small cracks that may develop within bone and so prevent their further extension.

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