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AC Joint Impingement: What Is It and How Is It Treated?

| | Shoulders
AC joint impingement

Shoulder pain can be disabling.  Simple chores like grabbing something from the overhead cabinet or putting on a sweater can be almost impossible at times.   A common cause of shoulder pain found on MRI or diagnosed by your doctor is AC Joint impingement. What is the AC joint?  And what does it mean for it to be “impinged”?  Let’s dig in.

What Is the AC Joint?

The acromioclavicular (AC) joint is where the shoulder bone called the acromion meets the collar bone (clavicle) and forms a joint.  It is the bony point on the top of the shoulder.  If you put your fingers on your collar bone and follow it back towards your shoulder you will feel a bony bump which is the AC joint. The AC joint provides important stability to the shoulder joint.

The space immediately underneath the AC joint is called the subacromial space and is depicted by the red area in the illustration below.  The subacromial space contains the rotator cuff tendons and bursae (1).

What Is AC Joint Impingement?

AC joint Impingement is a painful condition that occurs when the space beneath the acromion bone is narrowed.   This narrowing can result in irritation of the rotator cuff tendons and bursa.  A bursa is a fluid-filled sac that reduces the friction on tendons and muscles as they cross bony surfaces.  A tendon is thick collagen tissue that connects muscles to bones.  The rotator cuff tendons provide important support and enable movement in the shoulder.   If severe,  impingement can cause tears in the rotator cuff tendons.  Patients with AC joint impingement typically have pain with elevation of the arm and or while lying on the shoulder (2).

There are three grades of impingement (3).

Grade 1:  Inflammation of the bursa and tendons

Grade 2:  Progressive thickening and scarring of the bursae

Grade 3:  Rotator cuff tendon degeneration and tears

What Causes AC Joint Impingement

Three major structures in the shoulder can cause AC joint impingement (4).


Inflammation and or thickening of the bursae often time referred to as bursitis can cause impingement

Rotator Cuff Tendons

Inflammation, thickening or tears in the rotator cuff tendons can cause impingement.

AC Joint

Injury and strain of the AC joint can lead to instability which in turn can cause ligament thickening and AC joint arthritis.  Both of these cause impingement.

What Can Be Done for AC Joint Impingement?

Activity Modification

Overuse or repetitive motions can cause impingement and therefore rest is important.

Physical Therapy

Physical therapy programs focusing on improving range of motion, rotator cuff, and scapula strengthening have been associated with good outcomes (5).


Safe anti-inflammatory agents such as fish oil and curcumin are appropriate and can reduce swelling.  Nonsteroid anti-inflammatory agents such as Motrin and Advil have significant side effects which include acceleration of arthritis, GI bleeding and increased risk of a heart attack.

Steroid Injections

While often recommended as a treatment option to those who do not respond to physical therapy, steroids are toxic to orthopedic tissue and can lead to rotator cuff tendon shrinkage and reduced ability to heal (6).


Shoulder decompression surgery is a common surgery for AC joint impingement.  Also known as an Acromioplasty, it involves removing part of the bone and ligaments from the AC joint.  The rationale is that the AC joint and ligaments are pressing on and injuring the rotator cuff tendons.  A high-level study has demonstrated that shoulder surgery with the removal of a portion of the AC joint is no better than physical therapy (7).

To understand what an Acromioplasty is and why it can lead to shoulder instability click on the video below.

Nonsurgical Options

Bone marrow-concentration injections are effective in the treatment of shoulder and AC joint arthritis.  In 2015 we treated 102 patients with shoulder arthritis and or rotator cuff tears.  On average patients reported a 48.8% improvement after bone marrow concentrated injections (9).  Patients avoided the risks associated with surgery and extensive rehabilitation.  Bone marrow concentrate is rich in mesenchymal stem cells which can differentiate into cartilage, bone, and tendon as well as facilitate repair (10).

In Conclusion

The AC joint is an important joint in the shoulder that provides support.  The AC joint is compromised of the acromion bone from the shoulder and the collar bone (clavicle).  Underneath the AC joint is the subacromial space that contains the rotator cuff tendons and bursa.  AC joint impingement occurs when there is the narrowing of the subacromial space and puts the rotator cuff and bursa at risk for injury.  Injury, instability, and arthritis of the AC joint can cause AC joint impingement.  Treatment options include activity modification, physical therapy, and medications.  Steroids are toxic and should be avoided.  Decompression surgery involves the removal of a portion of the acromion and ligaments which destabilizes the shoulder and puts additional stress on the rotator cuff tendons.  Surgery has been shown to be no more effective than PT.  Bone marrow concentration injections are an effective, non-surgical option in the treatment of AC joint impingement.

1. Ludewig PM, Braman JP. Shoulder impingement: biomechanical considerations in rehabilitation. Man Ther. 2011;16(1):33-9. doi: 10.1016/j.math.2010.08.004

2.Garving C, Jakob S, Bauer I, Nadjar R, Brunner UH. Impingement Syndrome of the Shoulder. Dtsch Arztebl Int. 2017;114(45):765-76. doi: 10.3238/arztebl.2017.0765.

3.Khan Y, Nagy MT, Malal J, Waseem M. The painful shoulder: shoulder impingement syndrome. Open Orthop J. 2013;7:347-51. doi: 10.2174/1874325001307010347

4. Escamilla RF, Hooks TR, Wilk KE. Optimal management of shoulder impingement syndrome. Open Access J Sports Med. 2014;5:13-24. doi: 10.2147/OAJSM.S36646.

5.Dickens VA, Williams JL, Bhamra MS. Role of physiotherapy in the treatment of subacromial impingement syndrome: a prospective study. Physiotherapy. 2005;91(3):159–164.

6.Kennedy JC, Willis RB. The effects of local steroid injections on tendons: a biomechanical and microscopic correlative study. Am J Sports Med. 1976;4(1):11-21.DOI: 10.1177/036354657600400103.

7. Kukkonen J, Joukainen A, Lehtinen J, et al. Treatment of non-traumatic rotator cuff tears: A randomised controlled trial with one-year clinical results. Bone Joint J. 2014;96-B(1):75-81.DOI: 10.1302/0301-620X.96B1.32168

8. Ketola S, et al. Long term acromioplasty results no better than structured exercise for anterolateral shoulder pain. Presented at: European Society of Sports Traumatology, Knee Surgery and Arthroscopy Congress; May 4-7, 2016; Barcelona.

9. Centeno CJ, Al-Sayegh H, Bashir J, Goodyear S, Freeman MD. A prospective multi-site registry study of a specific protocol of autologous bone marrow concentrate for the treatment of shoulder rotator cuff tears and osteoarthritis. J Pain Res. 2015;8:269-76. DOI: 10.2147/JPR.S80872.

10. Pittenger MF, Mackay AM, Beck SC, et al. Multilineage potential of adult human mesenchymal stem cells. Science. 1999;284(5411):143-7.DOI: 10.1126/science.284.5411.143.

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