What happens inside the shoulder
Between the head of the humerus and the acromion — the bony ridge forming the roof of the shoulder — there is a gap of only a few millimetres. The rotator cuff tendons pass through it, along with a gliding sac called the bursa. Every time you lift your arm, that assembly has to slip through that gap.
Subacromial impingement appears when that space narrows, or when the humeral head is not held properly centred during movement. The tendon rubs and becomes inflamed, and once inflamed it takes up more room, so it rubs still more. That loop is why the pain tends to worsen if nothing is done.
Why the space narrows
It is almost never a single cause. Most often it is a combination of:
A muscular control problem. The rotators and scapular stabilisers are what keep the humeral head centred and low during elevation. When they weaken or lose coordination — through inactivity, overload or after an injury — the head rides up slightly and the space closes. This is the commonest cause and the good news, because it is the most treatable.
The shape of the acromion. Acromions can be flat, curved or hooked. The last of these leaves less space to begin with, and that shows on an X-ray.
Repeated movement. Swimmers, painters, electricians, padel players: anyone spending hours with the arm above shoulder height subjects that space to thousands of repetitions.
Why it is almost never operated on
Impingement is a mechanical problem, and a mechanical problem can be corrected by changing the mechanics. When scapular control and external rotator strength are restored, the humeral head returns to where it belongs and the space recovers without anything being touched.
Surgery is considered when pain persists after several months of properly conducted conservative treatment — and ‘properly conducted’ means directed, consistent physiotherapy, not a few scattered sessions — or when there is a clear structural cause no amount of rehabilitation is going to change.
