Impingement, tendinopathy and sub-acromial pain
Pain in your shoulder on lifting your arm to shoulder height? Pain lying on your side at night? Pain after periods of increased activity such as gardening? You may well be experiencing shoulder pain as a result of what lots of people refer to as ‘impingement’.
I use the inverted commas as the term impingement is a controversial one and one which many health care professionals are trying to move away from using. Why? First you need to understand a little about the anatomy and mechanics of the shoulder.
The diagram above shows the sub-acromial space of the right shoulder as viewed from the side. The floor of the space is the humeral head (the ball at the top of the arm bone). The roof at the back is formed by a bony outcrop from the shoulder blade called the acromion, the roof at the front is formed by the coraco-acromial ligament. The tendons of the rotator cuff sit within the sub-acromial space, as does the sub-acromial bursa.
The sub-acromial bursa is the largest bursa in the body. Like all bursae it consists of two membranous surfaces separated by a small amount of lubricating fluid. Its job is to protect and lubricate movement of the rotator cuff tendons on the bony roof of the space. It can commonly become thickened and painful.
As we lift our arm, the sub-acromial space begins to narrow. This narrowing is most marked when the arm is at shoulder height. Above this, the space starts to increase again. This can often give rise to the classic ‘painful arc’ sign – where pain is worst when the arm is lifted at shoulder height and decreases the further the arm is raised. This is often described as shoulder impingement.
Shoulder impingement was first described in 1852, but it was Charles Neer in 1972 who presented the theory of impingement that is still used today, but it is a theory that some want to change. Neer suggested that ‘impingement’ was due to the roof of the sub acromial space ‘coming down’, either due to poor control of the shoulder blade or bony spurs on the roof. He suggested that this squashes, traps and irritates the tendons of the rotator cuff and bursa causing pain and inflammation, eventually leading to structural damage in the form of rotator cuff tears. His proposed answer to this problem was acromioplasty – removal of the front of the acromion to stop it from irritating the structures in the sub-acromial space. This operation gained huge popularity.
So why is the term impingement controversial? Well first of, and this may be just semantics, but the thought of things getting repeatedly trapped until they tear and fall apart doesn’t conjure up a good image in your mind does it? Especially not when it’s keeping you awake at 2am. Things getting trapped is never good thought.
Secondly there is also evidence to suggest that in people with sub-acromial pain, the sub-acromial space is not narrowed by the roof (acromion) coming down but rather the floor and contents coming up i.e. the tendons of the rotator cuff becoming thickened and enlarged (tendinopathic), and that removing the acromion with an acromioplasty won’t change that. Therefore some suggest that ‘tendinopathic shoulder pain’ or ‘sub-acromial pain syndrome’ to describe pain over the side of the shoulder has more diagnostic value.
Tendons attach muscle to bone, transmitting their force to produce movement. It is easy to think of them as being inert (i.e. not much happens inside them). But they are far from it; our tendons are constantly responding to the load that we expose them to, be that a session in the gym or two hours of typing on the keyboard. When we load a tendon it responds by re-inforcing itself. This process can be thought of as a healing process. Sometimes this healing process goes wrong and we call this tendinopathy, this gives rise to pain and altered tendon structure. The main change in structure is that the tendon becomes much thicker. In the sub-acromial space this can be a problem, particularly when the tendon is sensitised and painful.
Tendons need to be exposed to load to keep them healthy. Jeremy Lewis, an eminent shoulder physiotherapist, describes tendons as being like sharks – they need to keep moving or they die, they want to be loaded but not overloaded. Lots of factors affect our tendon health and can increase risk of tendinopathy; raised BMI (weight), hormonal fluctuation (peri-menopausal/menopausal), raised cholesterol, age and repetitive work patterns to name but a few.
The problem is that ‘load’ and ‘overload’ are relative terms. Someone who is very sedentary and has some of the above risk factors may overload their rotator cuff tendons with relatively innocuous activity. This causes the tendons to become enlarged within the sub-acromial space. Tendinopathic tendons hate being squashed, so they don’t like it when the space narrows and they also get painful when lying on the affected side. They can’t deal with load very well so they don’t like lifting the arm very much. They also dislike being stretched so sometimes this can cause pain when lying on the non-affected side and the painful arm rests across the body putting the tendons on a little stretch.
Tendinopathy doesn’t just affect the sedentary though. It’s very common in the athletic population due to persistent overload and not allowing time for appropriate recovery. Once established, tendinopathic pain can be difficult to shift – just ask someone with tennis elbow! The problem is that pain makes you use the arm less, thus underloading the tendon (remember they want to keep moving – like sharks!). Then any load that is applied can be painful. This can quickly become a vicious cycle.
We know that tendinopathic tendons can return back to normal with exercise, particularly eccentric loading. A very high percentage of people with tendinopathic shoulder pain should get better with physiotherapy and not need surgery. Treatment consists of graded loading exercises. Sometimes, depending on their symptoms, people will benefit from injection of steroid and local anaesthetic in to the sub-acromial space. Contrary to popular belief there is little difference in outcomes between guided injections and blind (non-guided) injections. Sometimes we like to use guided injections for the added diagnostic value though – if we inject local anaesthetic into the sub-acromial space and all of the pain goes we know that the pain must be coming from there.
People with persistent sub-acromial pain, who have not responded to a twelve week course physiotherapy +/- injection may benefit from a surgical procedure called arthroscopic sub-acromial decompression (ASD). Particularly when something structural like a bony spur or calcification is narrowing the sub-acromial space. This surgery removes the bursa and shaves some bone from the sub-acromial space, creating space and allowing the tendon to recover.
In summary sub-acromial pain is generally thought to be tendinopathic pain, and it does not generally arise due to the tendon getting trapped or pinched as ‘impingement’ suggests. More it is the tendon becoming thickened and enlarged in the space, and this is a process that has many contributory factors. The good news is that the vast majority get better with appropriate physiotherapy. There are lots of other potential causes of shoulder pain so seeing a shoulder specialist who deals with this day in day out is key. So, if you think that you may have tendinopathic shoulder pain, get in-touch.
