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Vascular Supply
ОглавлениеBoth cortical and cancellous bone are highly vascularized: it is estimated that around 10% of cardiac output is directed to bone [7]. Arterial supply is through three major sources: (i) a nutrient artery that enters the medulla through a foramen in the diaphysis, (ii) periosteal arterioles that directly penetrate the cortex throughout the diaphysis and (iii) metaphyseal arteries that typically penetrate the bone at or adjacent to the point of insertion of the joint capsule (Figure 2.3).
Dense cell populations within cortical bone require substantial blood supply to sustain high demands for oxygen and nutrients and to remove waste products associated with normal metabolism and homeostatic processes. Cortical bone is perfused by a combination of arterial blood supplied from the main nutrient artery in addition to smaller arteries in the periosteum. The nutrient artery ramifies within the medulla and anastomoses with metaphyseal vessels. Under normal conditions, the medullary circulation provides vessels that perfuse the inner 80% of the cortex. Arterioles that originate from periosteal vessels supply the outer shell of the cortex although they have the capacity to supply a much greater proportion of the bone following injury. Blood flow is predominantly centrifugal. Capillaries pass through cortical bone in Volkmann's canals, which are generally orientated perpendicular to the long axis of the bone. These branch at right angles to give rise to smaller vessels that are contained with Haversian canals that lie in the centre of osteons and are usually parallel to the long axis of the bone. Osteons, and hence vessels within them, branch regularly, thereby providing an intricate network of vessels perfusing cortical bone: osteocytes in healthy bone reside within 300 μm of a capillary. The anastomosing network between medullary and periosteal blood supplies gives cortical bone a dual blood supply. This is important following injury or surgery, when one or other of the supplies may be disrupted.
Figure 2.3 Diagrammatic illustration of blood supply to a long bone of the appendicular skeleton. Arterial supply has three sources: (i) nutrient artery, which passes through the cortex into the medulla in the mid‐diaphyseal region via a nutrient foramen, (ii) periosteal arteries, which supply the outer circumference of the cortex, and (iii) metaphyseal vessels, which supply the epiphyseal and metaphyseal regions. All three networks share anastomoses.
Disruption to blood supply and the subsequent effects on oxygen tension have a profound effect on bone cell activity. Hypoxia has been shown in vitro to increase the number, size and bone‐resorbing activity of osteoclasts and inhibit the bone‐forming activity of osteoblasts [7]. Conversely, when oxygen tension is above normal, osteoclast function is suppressed and osteoblast activity increased.