Tendons are structures that link muscles to bone. Like ligaments, tendons have a relatively simple structure. They comprise more-or-less parallel bundles of fibres (made of collagen, which is a fascinating helical protein) surrounded by a gel (the matrix) which is largely made of water held in place by branching molecules called proteoglycans (the water is held between the branches of the proteoglycans like the decorations on a Christmas tree).
As we age the proteoglycans tend to wear out, and less water can be stored in the matrix. This means that the matrix gets stiffer and so less elastic - it cannot absorb so much of the energy of constant stretching and relaxation. As this happens so the fibres come under greater unprotected strain, and they start to break up.
We see this on ultrasound scan as a loss of the normal architecture of the tendon (the fibre bundles start to become disorganised and clump together). This exposes the fibres to further strain, splits start to form in the matrix, and the fibres start to degenerate further. The body tries to heal the splits, but often the healing tissue is weaker than before, so the shoulder function starts to become noticeably poorer. All the while the blood supply in the tendon (fragile at the best of times) is trying to cope with the problem but with the splits and broken fibres it is difficult for the blood supply to get to the damaged parts. These parts then have to function without a good oxygen supply, and the tissue gets ‘ischaemic’. This is often felt at night when we stretch the tendon (either by lying on the shoulder, or conversely, when the arm is lying across the body) and it reacts with the uncomfortable ache, felt down the arm, which wakes the patient up.
Tears of the rotator cuff (RC) tendon are almost inevitable: it is a fortunate person who reaches the age of 60 years without some form of RC tearing, although - equally fortunately - most RC tears are ‘silent’, that is, after a short period of an ache, perhaps after unusual gardening activity for instance, the shoulder settles down and there is no pain or noticeable change in the function - until the next unguarded moment. It is thought that about 60-70% of people over the age of 60 years have a RC tear of some form: about half have a partial tear, and the other half a full or complete tear (see Rotator cuff disease).
Rotator cuff disease