Bone Anatomy Introduction
Bones are living tissue. They can be divided by type into long bones, short bones, and flat bones. Avascular Necrosis AVN usually affects the ends of long bones especially the femur, humerus, and tibia.
Long bones grow from a cartilage model. The shaft of the bone or diaphysis has a different growth centre from the ends or of the bones. There may be more than one growth centre in each epiphysis, the hip has three for instance. The division between the shaft and the end of the bone is where it grows in length, this is the growth or epiphyseal plate. In an adult, the growth plate disappears as the bone stops growing and the ends of the bone unite with the shaft.
The bone is covered by a fibrous sleeve, the periosteum, which contains blood vessels and nerves. Muscles, tendons, and ligaments attach to the periosteum rather than directly onto the bone. The joint surfaces of the bone are covered by hyaline cartilage which is very smooth and provides the almost frictionless movement with the adjacent bone. The joint is surrounded by a capsule of ligaments and is filled with synovial fluid which is both a lubricant and nutrient for the cartilage.
Like all tissue bone needs a constant blood supply and so there are arteries, capillaries, and veins throughout the bone. Even the dense cortical bone has capillaries in it. There are holes or foramen in the bone cortex through which arteries, veins, and nerves enter the medulla. Because of the role of bone marrow in the production of blood cells the medulla is quite well supplied with blood vessels.
During growth when the ends of the bone are effectively separate from the shaft they have to have their own separate circulation. Separate arteries and veins enter the ends of the bones as they don’t cross the growth plate. Even after the bone has stopped growing the epiphysis has independent blood supply from the diaphysis.
The cortex of the bone is mainly supplied with blood from the periosteum sleeve around it and so it’s circulation is largely independent of the arteries in the spongy bone.
Avascular Necrosis AVN
AVN starts with a disturbance of circulation to part of the bone which then dies. One of the other names for AVN is a ‘bone infarct’. As the circulation to the diaphysis, epiphysis, and cortex of the bone are largely separate we can understand how the part with the weakest blood supply is affected by avascular necrosis AVN more frequently. That is why the commonest sites for avascular necrosis AVN are in the epiphysis of the long bones but affecting the spongy bone of the medulla (bone marrow) rather than the outer cortex of the bone.
The cortex generally survives the initial necrosis but loses a lot of its strength because of the loss of the spongy bone interior. Often the cortex then gets damaged and collapses changing shape and leading to arthritis in the joint. The necrotic part of the bone can also regenerate if the blood supply returns to normal. This can happen without there being any collapse of the cortex of the bone in which case the bone heals completely, without any long term consequences.