What the rotator cuff is
The rotator cuff is a group of four tendons — supraspinatus, infraspinatus, subscapularis and teres minor — that wrap around the head of the humerus like a closed fist around a ball. Their job is not so much to lift the arm as to keep the head of the humerus centred while the deltoid does the lifting. Without that centring, the shoulder loses efficiency and pain and a sense of giving way appear.
When one of those tendons tears, the balance is lost. Hence the very recognisable pattern: it hurts to lift the arm, strength is missing, and it is worse at night, because lying down removes the effect of gravity that during the day kept the joint decompressed.
Why it tears
There are two different stories behind a cuff tear, and they are not treated the same way:
The degenerative tear is the more common one from the age of 50. The tendon gradually wears, loses its blood supply and eventually gives way, often without any injury to account for it. It comes on gradually, sometimes after a trivial effort that was only the last straw.
The traumatic tear happens after a fall onto the arm, a sudden wrench or a dislocation. It is more common in younger people, the pain and loss of strength are immediate, and here timing does matter: a tendon repaired early is of better quality and less retracted than one that has been torn for months.
The decision to operate or not
Not every rotator cuff tear needs surgery. There are tears visible on MRI in people with no symptoms at all, and there are shoulders that work perfectly well with a torn tendon because the rest of the musculature has learned to compensate.
What is weighed in the consultation is not just the image but the combination of four things: how much strength you have lost, how much it limits your actual life, your age and functional demand, and whether the tear is acute or has been there for years. The same MRI can lead to opposite decisions in two different people.