Shoulder Impingement Syndrome: Is it a bone problem that needs a surgical solution?

You head to the doctor because you keep having this nagging shoulder pain every time you move your arm. It hurts to scratch your back and grab plates out of the cabinet. It’s getting harder to wash your hair and put your shirt on every morning.

Your doctor does some tests and diagnoses you with shoulder impingement syndrome - or SIS for short.

Now you want to learn what this means for you because you want to get back to living your life without shoulder pain. You want to reach to the top shelf, play tennis, lift weights, and play with your kids without shoulder pain.

You may have tried physical therapy for a while and gotten no relief, and now you may be considering surgery to cut down bones that your doctor says are shaped wrong...


A quick overview of shoulder impingement

The conventional understanding of impingement is that it occurs with a “compression, entrapment, or mechanical irritation of the rotator cuff structures and/or long head of the biceps tendon either beneath the coracoacromial arch or between the undersurface of the rotator cuff and the glenoid of glenoid labrum” (Ludewig 2009). When you lift your arm up, the space between the bone, tendons, and bursa becomes smaller, causing something to jam into something else.

This impingement is coupled with pain and difficulty with movement, and generally seems to lead to more loss of movement, flexibility, and strength.


What are some of the proposed causes of impingement?

If you’re like many others, you may have been told that the problem is that your bones are just the wrong shape. You may have been told that your clavicle is simply too long, and that is causing premature impingement at the acromioclavicular junction. A surgeon may have suggested that shaving down the bone is your best course of action. In the past, this perspective was widespread and considered almost indisputable fact.

This perspective, however, is not a complete picture of why shoulders may “impinge.”


Given the complexity of the shoulder joint, many factors are now believed to contribute to impingement syndrome.


Given the complexity of the shoulder joint, there are many factors that can contribute to impingement syndrome.  The actual structure of the joint, surrounding supports, and also the function (or lack thereof) of these structures contribute greatly to a healthy shoulder (Escamilla 2004). Because the joint has so many things working together in a very small space, any structure that takes up some of that space can result in reduced range of motion (Fongemie 1998).

This chart provides a clear view of several possible issues that are believed to contribute to impingement based on two studies (Fongemie 1998Escamilla 2004).



Rotator cuff - Weak, Inflamed, Muscle imbalance, Poor stabilization dynamically

Capsule - Hypomobile, Hypermobile, Contracture (adhesive capsulitis)

Scapula - Postural changes over time, Position, Decreased or restricted motion, Poor neuromuscular control, paralysis


Bursae - Inflamed, Thickened

Rotator cuff - Tendinitis, Thickened, Partial-thickness tear

Acromion and Acromioclavicular (AC) joint - Abnormal shape of bone or joint, Os acromiale, Type 2 and type 3 acromions, Spurs, Poorly healing or unhealed fracture

Humeral head - Abnormalities from birth, Poor healing of fracture


Notice many of these issues are well within your control. Everything in the “functional” category are things that you can affect through proper muscular retraining.

One area to draw your attention to is the “scapula” section of the functional column. Scapula position is dramatically altered by muscle function. The mention of “postural changes over time” is also a reflection of muscular activity. Your posture is a reflection of how muscles throughout your body hold your bones. If you’ve ever used a computer for 30 minutes, you know just how easy it is to train your muscles to hold your body in a bad position!

The “structural” issues are where doctors start recommending surgery, and the way those issues contribute is actually not as clear as once thought. Many of them can be improved without surgery, suggesting strongly that they should not even be considered "structural issues" at all!


How is the shoulder supposed to work?

In short, all the muscles that directly surround the shoulder joint and the muscles that are attached to related bones  harmoniously. If they are not balanced in terms of flexibility, strength, and mobility, smooth functioning of the shoulder is impossible.

Many people have heard of the rotator cuff. The rotator cuff (muscles: infraspinatus, supraspinatus, teres minor and subscapularis) stabilizes the humerus to the glenoid cavity (part of the scapula). It gets some additional help to stabilize with the capsule, labrum, and ligaments.

The rotator cuff is not the only set of muscles that stabilize though. The rotator cuff, along with some other muscles like the deltoids, helps with all the dynamic movements of the shoulder - rotation, flexion, abduction, and more.

In normal movement of the arm, the rotator cuff helps stabilize the glenohumeral joint in place, but it also allows the humerus to move within the joint.

What else happens when your arm flexes? The scapula should protract, upwardly rotated and tilt posteriorly. Movements occur at the acromioclavicular and sternoclavicular joints. The clavicle elevates and begins to retract the higher your arm goes.

All of these motions are simultaneously occurring to keep your shoulder stable but allow for an incredible range of movement (Ludewig 2009). Look back at the chart of structural and functional issues, and you can imagine how each of these things could have considerable impact on the stability and movement of your shoulder joint.



What does research say about functional issues and impingement?

In short, your muscles can definitely lead to shoulder impingement.

As you saw above, there are a lot of things that can affect movement at the shoulder joint. Many different studies offer various perspectives on muscular/soft tissue (like tendons and ligaments) issues that can lead to shoulder impingement.

Keramat in 2015 proposed that  “Posterior Capsule Tightness may cause anterior superior translation of the humeral head and anterior acromion tipping through scapular dyskinesis”. He also mentions the importance of the rotator cuff in stabilizing the position of the humerus in the glenoid cavity.

Either of these can lead to “early fatigue, dysfunction of these muscles, and eventually impingement.”

Another study found that different muscles showed differing levels of activity depending on symptoms (Diederichsen 2009).

And researchers have also looked at imbalance of strength between the internal and external rotators of the shoulder. An assessment of shoulder torque in those with normal shoulders was compared with those with impingement syndrome (both before and after surgery).

Before and after surgery, those with chronic impingement syndrome had weaker rotator strength and a lower internal rotator to external rotator ratio (Leroux 1994). (Studies on the rotator cuff like this one are part of the reason many shoulder rehab and post rehab programs obsess over establishing rotator cuff strength).

Not only is adequate muscle recruitment and balance important, but so is the timing of the recruitment. With a systematic review, Struyf 2014, found that patients with SIS and glenohumeral instability had increased upper trapezius activity and decreased activity of the lower trapezius and the serratus anterior muscles.

The pec minor can also be a major issue. Because the shortening of the pectoralis minor can change the resting position of the scapula, it can drastically affect shoulder function (Rosa 2016).


To summarize: the muscles all around the shoulder joint and around the scapula have a big impact on how comfortable your movements are.


What about acromion shape and shoulder impingement?

In short, recent research on acromion shapes and shoulder impingement show that the acromion shape has very little effect on shoulder impingement.

Of course, the acromion shape gets mentioned a lot in discussions about surgery for shoulder impingement.

Surgeons hold a strong belief that the bone shape is the deciding factor (and thus needs to be cut/shaved). This is a belief at the core of surgical intervention.

Unfortunately, this is not a belief that is based on rigorous evidence.

Many times, you'll find claims that type II and type III acromion processes are a big part of shoulder impingement.

But then you see plenty of research that shows acromion shape is just not related to impingement.

Take for example Mayerhoefe in 2009.  These authors felt that the shape of the acromion did not relate well to the narrowing of the subacromial space.

Debate also exists about which part of the acromion to even assess. 

Balke et al in 2013, using x-ray, looked at the following characteristics of the acromion: type, slope, tilt, lateral angle, and index in 150 patients. A third of the patients had full-thickness rotator cuff tears, another third had subacromial impingement, and the final third were controls. One of their findings was:

Only 2% of the controls had a type-III acromion according to Bigliani et al. (1986), as compared to 20% in the impingement and cuff-tear patients. Acromion of type III was "common" in both impingement and rotator cuff-tear groups.

There are a couple ways to read this.


Some shoulder surgeons might say:

"That means the type-III acromion contributes to impingement AND rotator cuff tears!”

This is a weak argument, as the prevalence of 20% does not come close to establishing a causal relationship.

Assuming it is true, however, there must clearly be other factors involved in the development of the problem if only 20% of the people with tears and impingement have this acromion type.


A closer look at acromion shape and shoulder impingement 

To even begin to understand whether the acromion type is actually related to the impingement or tears, you need to look at how prevalent the acromion types are in the general population without symptoms.

So how prevalent are Type III acromion in the general population without symptoms? Let's look at Vahakari 2010.

These researchers looked at 305 people between the ages of 20 and 80, all asymptomatic. 

They found that in this asymptomatic population 14.1% had the type III acromion. 81.3% had the type II acromion.

Look at those numbers again. 

In the asymptomatic population, type II and type III acromions are QUITE prevalent.  


Which means that acromion shape probably has nothing to do with shoulder impingement.

If people can be perfectly asymptomatic with type II and type III acromion processes (these are the two types that often get labeled as problematic), what is the likelihood that the acromion itself is the major cause of the problem? best!


Acromion shape in the context of functional contributions to shoulder impingement

The study we looked at earlier, Mayerhoefe in 2009, also mentions occupation as being a factor. People who are involved in heavy lifting seem to be greatly affected by impingement.

Following up with 37,402 workers, those who had shoulder complaints with their shoulders or their necks and shoulders had high loads of shoulder activity in their job.

They were most likely to end up getting surgery for shoulder impingement.

The authors recommended people like this look for ways to reduce their heavy lifting in order to avoid the need for surgery (Svendsen SW 2013).


Think about this: if shoulder impingement is made worse by heavy lifting, does it make sense to blame bone shapes?

There are few jobs where heavy lifting involves holding heavy objects overhead or out to the side (where impingement may be felt).

Instead, heavy lifting often means having your arms at your sides and the object in front of you.

If less heavy lifting has the possibility of reducing the problem, it strongly suggests that something about the muscle activity is affecting comfort levels -especially since heavy lifting does nothing to cause direct bone impingement.


In what way might that affect the joint? Heavy lifting has obvious effects on the muscles around the joint - and only a miniscule effect on the joint itself. The humerus is actually being pulled down and away from the acromion during heavy lifting and does not cause impingement! 


The simplest answer is this. Heavy lifting trains the shoulder muscles to get really good at one specific position. The muscles get bad at the overhead position and create improper shoulder joint mechanics. 

So the key to fixing shoulder impingement would be to restore muscle function in the overhead position (slowly and gradually).



For many years, shoulder impingement was considered to be a problem of bone shapes. These days, lack of coordination and strength around the shoulder joint have become areas of more focus.

Muscles are the more obvious and more likely contributors to shoulder impingement. 

Conventional medical approaches to dealing with the muscles are not perfect, though.

Because shoulder impingement was thought to be purely a bone problem for so long, physical therapy has been limited in scope and often ends up quite frustrating.

Passive treatments involving heat, ice, ultrasound, and stretching may occasionally help some people, but to really deal with the problem, you need to retrain muscles of the shoulder and the rest of the body.


Stop cursing your shoulder and start helping it!

If you’ve been fighting a losing battle with shoulder pain, I know how you feel.

For years, I was unable to lift my arm out to the side or out in front of me without pain, snapping, and popping in the joint. It took me years to realize that I could fix the problem myself. As I was solving that problem (and many others with my body!), I started helping others with their problems.

The Shoulder Fix is a program designed to help you with your shoulder problems. Impingement is just a way to say that your muscles need help, and with the Shoulder Fix, you can learn step-by-step how to help them!

Train your body to be more flexible, strong, and resilient! 

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