Object lesson No 55

30th March 2001 at 01:00
You know the song. It's the African-American spiritual about leg bones connecting to knee bones, knee bones connecting to thigh bones and so on up to the "head bone" via the "elbone".

The lyrics tell of Ezekiel's vision of the Valley of Dry Bones. Whatever their religious significance, they reveal a superficial knowledge of anatomy. For example, the Old Testament prophet leaves out the knee's complex system of elastic bands, cushions and lubricating fluids. Without that "dem bones" are just not "gonna walk around". The man of God clearly had his head bone on loftier matters.

When he knelt in prayer, Ezekiel should have given more thought to the joint that was supporting him. The knee is the hinge between the thigh bone (femur) and the shin bone (tibia) and, as the largest joint in the body, has to withstand great stresses. The smooth chunky ends - or condyles - of the two bones are cushioned from each other by elastic cartilage and shock-absorbing pads of tissue called menisci.

Balanced on top of the femur is the bit that would be judged in a knobbly knees contest. Known as the kneecap or patella this protects the knee and stops the tibia moving too far forward.

Cushioning the kneecapfrom the femur and tibia are fluid-filled sacs called bursas. Should you have a liking for polishing floors or praying (watch out, Ezekiel) then beware. Your bursas can become inflamed, resulting in housemaid's or clergyman's knee.

Four ligaments bind the femur and tibia together and the whole lot is encased in a fibrous capsule lined with a soft tissue called synovium. This secretes a fluid that allows the tibia to move easily and the kneecap to swing up and down on the top of the femur.

Two main muscles operate the knee - the quadricep, which runs down the front of the thigh and straightens the leg, and the hamstring, which runs down the back of the thigh and bends it. Tendons, vulnerable to overuse, connect the muscles to the bones. Jumper's knee can afflict basketball players who have worked their kneecap tendon so much it resembles a worn-out elastic band.

Knees are great at bearing the body's weight, but not so good when it comes to sideways movement, as many sportsmen and women know to their cost.

Dislocated bones and torn cartilages are all-too common reasons to be stretchered off the pitch muttering, "Damn bones, damn bones, damn dry bones".

Stephanie Northen

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