Orthopaedic Surgery/Blood Supply of Long Bones

Blood Supply of Long Bones

Vascular supply of long bones depends on several points of inflow, which feed complex sinusoidal networks within the bone. These in turn drain to various channels Through all surfaces of the bone except that covered by articular cartilage.

=General Anatomy= One or two main diaphyseal nutrient arteries enter the shaft obliquely through one or two nutrient foramina leading to nutrient canals. Their sites of entry and angulation are almost constant and characteristically directed away from the growing epiphysis. Except for a few with double or no foramina, 90% of long bones have a single nutrient foramen in the middle third of the shaft. The nutrient arteries divide into ascending and descending branches in the medullary cavity. These approach the epiphysis dividing into smaller rami. Near the epiphysis they anastamose with the metaphyseal and epiphyseal arteries.

Metaphyseal arteries are the branches of neighbouring systemic vessels. They enter the long bones in the region of the metaphysis.

Epiphyseal arteries form the periarticular vascular arcades formed on non-articular surfaces of the long bones. Numerous vascular foramina are present at the end of long bones, some which are occupied by arteries but most by veins. The nutrient arteries of epiphyses form many intraosseous anastamoses as they pass towards the articular surfaces within the trabeculae. Near the articular cartilage they form serial anastamotic arcades from which arise end-arterial loops which pierce the thin hypochondral compact bone to enter and traverse the calcified zone of articular cartilage before returning to the epiphyseal sinusoids.

Epiphyseal and metaphyseal arteries quantitatively exceed the diaphyseal supply which they can complement when the latter is destroyed. Nutrient arteries are responsible for two-thirds of blood supply of the diaphysis and one-third of blood supply of each of the metaphyses.

Periosteal plexuses are formed by arteries from the neighbouring muscles. Muscular arteries contribute vascular arcades with longitudinal links to the fibrous periosteum forming an external plexus. From this external plexus a capillary network permeates the deeper osteogenic periosteum. However at muscular attachments periosteal and muscular arteries are confluent.

Inside the medullary cavity the ascending and the descending branches of the shaft give
 * Centripetal branches to a hexagonal mesh of medullary sinusoids draining into a wide thin-walled central venous sinus.
 * Cortical branches passing through endosteal canals feed fenestrated capillaries in the Haversian systems. These cortical capillaries conform in pattern to the Haversian canals, which are longitudinal with oblique interosteotomic connections.

The cortical capillaries also make capillary and venous connections with the periosteal plexuses mentioned above.

The central venous sinus drains veins, which trace the paths of nutrient arteries, sometimes piercing the shaft as independent emissary veins.

Modern research has emphasized
 * A centrifugal type of flow of blood through the cortical bone in the shafts of the long bones
 * A centripetal type of flow through the medulla.

This is in contrast to the earlier concept of substantial centripetal arterial flow of blood into the cortex form the periosteal vessels. Though the concept of exclusively centrifugal supply to the cortex has received increasing support, some have described an appreciable centripetal arterial flow especially to the outer cortical zones from the periosteal capillaries.

Blood supply of the immature bones is similar but the epiphysis is a discrete vascular zone separated from the metaphysic by the growth plate. Epiphyseal and metaphyseal arteries enter on both sides of the growth cartilage, with anastomoses between them being few or absent. Growth cartilage receives its blood supply form both the sources and also form an anastamotic collar in the adjoining perichondrium. Young periosteum is more vascular with its vessels communicating more freely with those of the shaft than in the adults and has more metaphyseal branches.

Physiologically blood flow in the bone occurs at the rate of 5 – 20 ml / min in 100 gm of wet bone tissue representing approximately 4 – 10 % of the resting cardiac output. The rate of blood flow is highest in the metaphysic. Control and variation of blood supply is due to stimulating factors such as sympathetic nerves, acid metabolites and increased or decreased carbon – di – oxide tension.

=Applied Anatomy= Response of the bone to circulatory disturbances: Like any other tissue in the body necrosis of the bone can occur due to interruption of its blood supply resulting from physical injury (trauma) or form non-traumatic occlusion of blood vessels (Caisson’s Disease). In such aseptic necrosis of bone, there could be two situations
 * 1) Bone with some residual supply:Involved bone is invaded by vascular fibrous stroma and the necrotic bone is gradually resorbed and replaced by new bone. Part of bone with blood supply undergoes atrophy with decalcification.
 * 2) Bone with completely no blood supply:Involved bone shows increased density because there is no blood supply to remove the mineral salts. Sequestrum is an example of the effect of loss of blood supply to bone.

Thereby increased density of bone may represent
 * Zones of hypertrophic living bone
 * Necrotic bone with reactive bone about it
 * Simply necrotic bone

In situations where there is increased circulation, bone responds by hypertrophy and increase in length. Increase in length of lower limb with an arterio-venous aneurysm is an example.

Circulatory disturbances in specific regions of the long bones:
 * 1) Legg–Calve–Perthes Disease: Circulatory disturbance to the capital femoral epiphysis plays an important role in the pathogenesis of the disease.
 * 2) Physeal Trauma: Epiphyseal vessels are responsible for nourishing the reproductive cells of the physis. Interruption of these vessels results in irreparable damage to the growth plate. Metaphyseal vessels supply calcium and Vitamin D via serum and phosphates via erythrocytes, aiding in the calcification of the matrix, removal of degenerate cells, and deposition of lamellar bone. The metaphyseal vessels are of no nutritional significance to proliferating chondrocytes of the physis.

Metaphysis:
 * 1) Haematogenous Osteomyelitis: In children infective foci are known to localize in the metaphysis within two hours following intravenous inoculation. This is explained by the nature of blood supply to this region. The last ramifications of the intramedullary nutrient artery to the metaphysic turn down in sharp loops and empty into a system of large sinusoidal veins where the rate of blood flow is decreased. This creates an ideal medium for the bacteria to settle down and proliferate. In the adult there is a free anastomosis between the metaphyseal and epiphyseal arteries and thereby osteomyelitis, when it does occur may appear anywhere. In infants a few vessels do cross the physis. At eight months the physeal cartilage becomes a barrier that becomes definitively established by 18 months. Therefore in infants osteomyelitis can lead to septic arthritis.
 * 2) Metastasis: Metastatic deposits are more common in the axial skeleton than the appendicular skeleton. However when they occur they are known to deposit in the region of the metaphysis. This is attributed to the fact that the metaphyseal circulation is richer compared to epiphyseal or diaphyseal circulation.

Diaphysis
 * 1) Intramedullary Reaming: Cortical reaming and nail insertion injures the medullary vascular system resulting in avascularity of significant portions of the cortex. Medullary reaming disturbs the intracortical transport mechanism by altering the high intramedullary pressure required for venous drainage into the periosteal veins and the pulsating intramedullary pressure necessary for nourishment of osteocytes. Research has shown that there is more rapid revascularization with nails placed without preparatory reaming compared with nails placed after reaming. Hence there is a shift of interest to interlocking nailing systems that do not require reaming.
 * 2) Fracture Healing: Periosteal circulation plays a very important role in healing of fractures. Soft tissue stripping thereby, while performing an internal fixation of a fractured bone must be kept to a minimum to encourage the participation of periosteum and its circulation in fracture healing.