Degenerative Disease of the Rotator Cuff
The shoulder is a highly mobile and relatively unstable joint with the largest range of motion of all the body's joints. The shoulder is designed to allow us to put our hands in front of our vision so we can watch as we perform complex manual manoeuvres, a defining feature of primates and humans. The shoulder has a group of muscles which stabilise and move the joint, a group known as the rotator cuff, which forms a tendinous cuff around the head of the humerus so that it can exert the necessary forces.
As the muscles approach their insertions on the humeral head they become more and more fibrous until they become wholly tendinous. Many bodily tendons are cylindrical and long but the shoulder tendons are flatter structures which coalesce over the top of the humeral head. The rotator cuff has a relatively poor blood supply and little or no ability to heal and with time and physical stresses tears appear which are often painful but not always so. Rotator cuff tears are a major part of a shoulder surgeon's work and rotator cuff surgery is common, complex and demands detailed physiotherapy follow up for successful outcomes.
How rotator cuff tears develop is not completely clear but there are two main ideas, the extrinsic and the intrinsic hypotheses. In the extrinsic idea the cuff tears are caused by repeated impingement against other structures in the shoulder joint. Neer, an eminent shoulder surgeon, popularised the name of impingement syndrome and described the typical impingement beneath the arch made up of the anterior acromion, the coracoacromial ligament and the acromioclavicular joint. The supraspinatus tendon is vulnerable as it impinges in flexion with internal shoulder rotation and much degeneration occurs in this tendon with extensions at times into the infraspinatus tendon.
The lateral part of the scapula, the acromion, has a characteristic anatomical shape and radiological studies have indicated that the hooked shape is connected with cuff degeneration but not necessarily causally linked. Osteophytes, bony outgrowths, develop underneath the acromioclavicular joint and these are compressed against the tendons of the cuff on repeated movement. If a younger worker does a lot of overhead work bleeding and swelling can develop in the tendons and with a series of injuries with time this can develop into tendon scarring and inflammation. In older patients, for example over forty years old, the process can progress to bone spurs and partial or complete cuff tears.
If the arm is taken back into a throwing action, into the cocking part of the process, the edge of the shoulder socket can impinge again and again against the underneath of the supraspinatus tendon. Each time this occurs a small amount of trauma happens in the tendon which can lead to minor tears, particularly occurring in throwing athletes. The lesser tuberosity, the supraspinatus and biceps tendons can impinge against the coracoid process. Overall most cuff tears may be partly explained by these three impingement types.
The intrinsic view holds that the external factors may be contributory but that the fundamental underlying process is age-related degeneration inside the tendons themselves. This helps explain why young people rarely suffer cuff tears and that tears increase strongly with age, for example after fifty years old. Under the supraspinatus tendon near to its insertion onto the greater tuberosity is an area which has been called the critical zone and postulated to have a poor vascular supply. This could increase the risk of injury and poor healing in this area but further studies have not confirmed this idea so degenerative changes in the tendons may still be important.
It is likely that the cause of rotator cuff degeneration is a combination of both extrinsic and intrinsic causes, with the tendon tending to fail first in the areas of greatest load, leading to rupture of small tendon fibres. This causes increased forces being loaded onto close fibres, detachment of some fibres from bone, reduction in cuff power and compromised blood supply due to kinked fibres. Wound healing could be reduced and this would impair the possibility of repair.