A joint is the place where two bones meet. In a mobile joint such as the knee, hip, shoulder or elbow, the ends of the bones are covered with a layer of smooth cartilage, a few millimetres thick, and enclosed in a sealed sac, the capsule, whose lining secretes the synovial fluid that lubricates the surfaces. Bands of tough tissue, the ligaments, run from bone to bone across the joint and limit its movements to the intended ones. In the knee two crescents of cartilage, the menisci, sit between the bones and spread the load.
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Example 9.3 (Sprain, tear, dislocation)
Twist an ankle on a kerb: the ligaments on the outside are stretched beyond their limit — a sprain; the joint swells as the capsule bleeds and fluid accumulates. Pull hard on a cold hamstring: some of its fibres rupture — a muscle tear. Fall on an outstretched arm: the head of the humerus may leave its socket — a dislocation, the ligaments and capsule stretched or torn as it goes. None is a broken bone; all can take longer to heal than one.
Example 9.7 (Holding a bag)
A 5kg bag (weight 49N) hangs from the hand, 35cm from the elbow; the biceps tendon attaches 5cm from it. The biceps pulls with 49×35/5=343N — seven times the load, the weight of 35kg — and the elbow joint is pressed together with about 300N. Muscles, tendons and cartilage are built for these forces; what damages them is a sudden force in the wrong direction, or a load repeated beyond what the tissue can repair.
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