The term subacromial impingement can be traced back to orthopedic surgeon, Charles Neer at 1972, who argued that the primary cause for shoulder pain was the attrition of the suprapinatus tendon and subacromial bursa due to abrasion under the overlying acromion.
This led to a massive increase in the rate of subacromial decompression surgery (not surprisingly, also invented by Charles Neer). 
To this day, despite the rise of the biopsychosocial model and a modern understanding of pain, the majority of shoulder complains are still primarily attributed to the impingement of the rotator cuff by the overlying acromion.[1]

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Here are 5 reasons why it may be time to finally abandon the term “impingement” for good!

1. Acromiohumeral distance is not associated with shoulder pain and disability

Acromiohumeral distance is the distance between the inferior surface of the acromion and the humeral head. It has been suggested that people with subacromial impingement have and superiorly migrating humerus which caused pain and dysfunction. Which doesn’t appear to be the case with many inconsistent findings. 
As evident by 2020 sysematic review which found no relationship between the acromiohumeral distance and pain in adults with subacromial pain syndrome. [2]

2. No association between pectoralis minor length and shoulder pain

It is hypothesized that a shortened  pectoralis minor leads to a decrease in the acromiohumeral distance by causing anterior tipping of the scapula. However, this doesn’t appear to be the case as pectoralis minor length is poorly associated with acromiohumeral distance, as well as with shoulder pain and mobility, in people with chronic shoulder pain. [3] The same goes for levator scapulae muscle. 
Yet, these muscles can still be tender and senstitive to touch, but there is no evidence that these muscles caused any shoulder pain. [4]

3. The humeral head doesn’t stay centered in healthy people during arm elevation

The humeral head translates quite a bit in healthy people without shoulder pain. The translations is average 6 mm A-P translation and 2.5 mm S-I translation during shoulder abducation, it doesn’t stay centered as it might be assumed. [5]

4. Those who impinge have an up-sloping acromion

This 14 years old paper actually found that those with subacomial impingement have an upsloping acromion versus a hooked or down sloping acromion. 
Which doesn’t support the thinking that bony impingement by any part of the acromion leads to subacromial impingement syndrome. [6]

5. There is no difference between bursectomy and acromioplasty surgery

A bursectomy surgery is the removal of the bursa and acromioplasty surgery involoves a shaving of the acromion and a bursectomy procedure. 
While many studies have shown that acromioplasty is considered a successful procedure to reduce mechanical compression, they compared acromioplasty surgery with conservative treatment. These studies didn’t take into account the effect of placebo or bursecotmy surgery on its own which sometimes is considered a sham procedure. 
This study by Kolk et al at 2017 peformed an acromioplasty surgery to one group and a bursectomy surgery to the other. It found  that there were no difference in rotator cuff tear rates and functional outcomes at 9 to 14 years follow up.This shows that not only doesn’t acromiplasty no protect against full-thickness rotator cuff tears, it didn’t result in a clinically relevant improvement in shoulder function or relief of pain in patients. [1]
The results of these studies show that even conservative interventions should focus on more than just the potential decrease in subacromial space and highlights the importance of other other biopsychosocial factors. [2]

If you're interested in learning more about this topic, we highly recommend you this great lecture by Jared Powell:

Click here

References:
1. Kolk A, Thomassen BJ, Hund H, de Witte PB, Henkus HE, Wassenaar WG, van Arkel ER, Nelissen RG. Does acromioplasty result in favorable clinical and radiologic outcomes in the management of chronic subacromial pain syndrome? A double-blinded randomized clinical trial with 9 to 14 years' follow-up. Journal of shoulder and elbow surgery. 2017 Aug 1;26(8):1407-15.

2. Park SW, Chen YT, Thompson L, Kjoenoe A, Juul-Kristensen B, Cavalheri V, McKenna L. No relationship between the acromiohumeral distance and pain in adults with subacromial pain syndrome: a systematic review and meta-analysis. Scientific Reports. 2020 Nov 26;10(1):1-4.

3. Navarro-Ledesma S, Fernandez-Sanchez M, Luque-Suarez A. Does the pectoralis minor length influence acromiohumeral distance, shoulder pain-function, and range of movement?. Physical Therapy in Sport. 2018 Nov 1;34:43-8.

4. Navarro-Ledesma S, Fernandez-Sanchez M, Struyf F, Martinez-Calderon J, Morales-Asencio JM, Luque-Suarez A. Differences in scapular upward rotation, pectoralis minor and levator scapulae muscle length between the symptomatic, the contralateral asymptomatic shoulder and control subjects: A cross-sectional study in a Spanish primary care setting. BMJ open. 2019 Jun 1;9(6):e023020.

5. Massimini DF, Boyer PJ, Papannagari R, Gill TJ, Warner JP, Li G. In-vivo glenohumeral translation and ligament elongation during abduction and abduction with internal and external rotation. Journal of orthopaedic surgery and research. 2012 Dec;7(1):1-9.