Bursa are small fluid-filled flattened sacs. They are a bit like two thin sheets of plastic wrap with a drop of water between them. When rubbed, the opposing sides of the sac gently slide back and forth over each other. For this reason, they appear throughout the body between structures that are predisposed to friction and subsequent wear. Bursa are therefore commonly seen between tendon and tendon, tendon and bone, and skin and bone.
There are two sorts of bursa. The first type is an anatomical bursa. These are bursa that you are born with and placed throughout the body where mother nature has pre-empted problems with friction. The second type, adventitious bursa, are formed by the body later in life in response to unanticipated shearing forces. Bursa seem to get a bad rap amongst most people, but they are there to help. The problem is, that when the forces that the bursa are designed to protect us from become so great, they too are overwhelmed. In this instance, the bursa itself becomes inflamed and very painful. This is known as bursitis.
Regarding anatomical bursa of the foot and ankle, retrocalcaneal bursitis is the most infamous. The retrocalcaneal bursa lies between the underside of the Achilles tendon just as it begins to insert into the back of the heel bone. Abnormal foot function and tight calf muscles combine to irritate this bursa giving rise to pain behind the ankle. Close by, lying between the skin and the back of the heel bone is the subcutaneous calcaneal bursa. This bursa succumbs to the same forces acting on the retrocalcaneal bursa, only it also has to deal with irritation from footwear. It appears as a reddened, tender swelling right on the back of the heel bone.
Adventitious bursitis is common in feet, because unlike other parts of the body, they are squashed into footwear. Toe deformities and crooked feet create all sorts of lumps and bumps that get pressed on by shoes, thereby causing the intervening skin to shear against underlaying bone. Adventitious bursitis present as reddened, sore and swollen areas of skin where footwear is rubbing it. The classic adventitious bursitis is the bunion. The bunion is the name given to bursitis overlying the inside of the protruding 1st knuckle. This occurs in association with a joint deformity in which the 1st toe becomes deviated towards the 2nd toe.
The above describe just a few of the possible bursitis conditions of the foot and ankle. However, whatever their location, a podiatrist can help. Podiatrists are trained to identify irregularities in foot shape and function that are contributing to the bursitis. Treatment might include the application of protective devices to reduce shear, provision of foot orthoses (customised arch supports) and exercises to improve foot function, and expert footwear advice.
By Craig Cheney - Physiotherapist & Podiatrist
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