Left Shoulder AC Separation | Why It’s More Common, How it Happens, and More

Written by on October 29, 2021 — Medically reviewed by John Doe

working out to prevent shoulder AC Seperation

About 90% of the world is right-handed. This preference generally also means that most of us use our right arms more than our left. That’s all well and good. But, this natural bias also makes left shoulder AC separation more common. (1)

This, then, opens up a bunch of questions, like:

  • Why do people fall on their non-dominant shoulders?
  • Why do we even have dominant arms?
  • What treatment options are there?

I’m addressing all these today. But, to give you some more context, let’s start with the injury’s basics.

What is an AC joint separation?

It’s when there’s a dislocation between the tip of your shoulder blade and the end of your collarbone. Respectively, these bones are your acromion and clavicle.

Thus, why the injury is called acromioclavicular joint separation. Or, simply, shoulder separation.

The ligaments and muscles that surround your AC joint could also sustain damage. For some, there might even be a visible deformity on top of your shoulder blade.

Shoulder separation injury accounts for about 12% of all shoulder injuries. Half of that comes from athletes who take part in contact sports. (2)

What causes an AC joint separation? 

Shoulder separation happens because of a direct blow to your AC joint.

Common examples of these situations include:

  • Falling and landing on the outer part of your shoulder
  • Violent trauma right on the AC joint
  • Falling on an extended arm.

These traumas disrupt muscles and ligaments that surround your AC joint, causing injury.

Does hand dominance affect shoulder separation?

Actually, yes. It’s a protective mechanism that helps make sure your dominant side doesn’t get hurt.

Based on a recent study, most people tend to fall on their non-dominant side by age 4 and up. (4)

And, since most people are right-handed, we’re generally also more likely to fall on our left sides.

Diagnosing a shoulder separation

During your physical examination, your physician will check for asymmetries between your shoulders. Any bumps, swelling, or deformity on your left shoulder may be a sign of an AC joint problem.

Your doctor will also check for your shoulder range of motion and strength. Clicking/grating noise, muscle weakness, and stiffness may be a sign of rotator cuff and AC joint injuries.

Finally, your doctor might also order an X-ray. This is to check the integrity of your bones. A fractured collar bone and/or a wider gap between your bones are tell-tale signs of a separated shoulder.

What are the symptoms of shoulder separation? 

The signs and symptoms of a shoulder separation will be the same for either side. These include (5):

  • Bruising or any skin discoloration on the shoulder and collar bone
  • Difficulty sleeping on the affected shoulder
  • Intense shoulder pain right after a fall or direct blow
  • Limited shoulder range of motion
  • Pain relief when crossing your affected shoulder towards your body
  • Radiating pain to the neck or arm during arm movement
  • Swelling on the tip of the shoulder blades
  • Tenderness on the collarbone or tip of the shoulder blade

Note that while the side of the injury won’t change how it presents itself, its severity will.

That being said, there are 6 types of shoulder joint separations according to the Rockwood Classification.

6 types of shoulder separation (Rockwood Classification)

These types are ranked according to the severity of the damage done to the following structures:

  • AC ligament
  • AC joint
  • CC ligament, and
  • Deltoids and trapezius muscles

Your doctor will do a physical exam and order a few imaging tests to accurately diagnose your injury. But, here’s an overview of what he/she will be looking for:

What are the treatment options?

There are two types of treatment for shoulder separation:

  • Conservative treatment
  • Surgical treatment

Treatment will depend on what type of AC joint separation you have and the demands you place on your left shoulder.


Types I and II are mild shoulder separation. They’re manageable via non-surgical means.

Some with Type III usually go through a period of rehabilitation before considering surgery.

Treatment includes (5):

  • Immobilization – A protective sling is usually used for a couple of days. This is to support the weight of your arm and to limit the stress on your rotator cuff.
  • Pain Control – Analgesic medications decrease pain and inflammation. Examples are Ibuprofen and Paracetamol tablets, and corticosteroid injection.
  • Physical Rehabilitation – This will depend on your symptoms. Your physical rehab can last for a couple of weeks to months.  It focuses on different aspects of your recovery –from immobilization, range of motion, strengthening, and eventually to task-specific training. (7)


Types IV, V, and VI require surgery because of the clear separation in your joint.

Also, there needs to be a proper fixation between your displaced bones so your ligaments, muscles, and joint capsule heal properly.

Here are a few commonly used procedures in shoulder surgery:

  • Stabilization with metallic devices – This fixates the dislocated joint and fractured collarbone using K-wire, Bosworth screws, hook plates, and others.
  • Using a suspensory device – This is a less invasive technique that doesn’t use screws or other metals to correct the dislocation.
  • Ligament and tendon reconstruction – Orthopaedic surgeons repair soft tissue with a graft taken from another part of your body or from a donor.

How long does it take for a separated shoulder to heal?

Recovery time differs depending on the surgical approach and rehab protocol. But, you can expect to be physically ready to go back to work after 3 months, all the way up to 12 months after surgery. (8)

Unfortunately, it may take even more time for your left AC separation to get better. This is true despite not having any significant strength difference between the scapular muscles of your dominant versus non-dominant arms. (9)

The simple reason behind this is that you’re less likely to use your left arm. This means strengthening the muscles on your left side takes more time.

6 Easy tips to prevent left AC joint separation

  1. Warm-up before a game or even intensive household chores
  2. Make sure to wear the appropriate equipment before any game
  3. Have a proper strength and conditioning program especially for the upper body
  4. Learn to fall correctly to distribute forces
  5. Give your body the recovery time it needs in between strenuous activities
  6. Don’t get involved with physical activities if you’re not fully healed from a previous injury


Why is there a bump on top of my left shoulder?

You most likely have a torn ligament that caused your collarbone to pop right on top of your shoulder.

This bump, also known as “step deformity”, is a sign of separation between your collarbone and shoulder blade. The larger the distance, the more severe the injury.

Is it normal for your shoulders to be uneven?

 Yes, it is normal but only to a certain extent. Our dominant shoulder is much higher because of muscle imbalance. Since you tend to favor one side, it has more developed muscles than the non-dominant arm. As long as you don’t have any symptoms such as pain, it is normal.

 But a shoulder that is noticeably higher and with associated symptoms may indicate:

  • Upper arm bone length asymmetry
  • Underdeveloped shoulder blade
  • Scoliosis

If you have doubts, it’s best to get it checked by your physician to get a proper diagnosis.

Why is one arm more dominant than the other?

There are two theories behind why we have dominant arms:

First, we humans developed our speech and language control in the left hemisphere of our brain. This same hemisphere controls the movements of the right side of the body. This enables us to use our right arm more when we need to express ourselves, such as through writing or hand gestures.

The other theory is a bit more complicated as it involves our genes. Specifically, a gene associated with handedness.

One is a D gene, the other is a C gene. Both of them are passed down through our genetic heritage.

  • The D gene is more frequent in the world and promotes right-hand preference.
  • The C gene is less common, but when present, hand preference is determined randomly. So there is a 50-50 chance of being right or left-handed. (11)


A separated shoulder can be tough to deal with. It’s even tougher if it happens to your off-hand. But with your newfound knowledge, you should be better equipped to deal with a separated AC joint.


  1. de Kovel, Carolien G F et al. “A large-scale population study of early life factors influencing left-handedness.” Scientific reports vol. 9,1 584. 24 Jan. 2019, doi: 10.1038/s41598-018-37423-8
  2. Warth, Ryan J et al. “Acromioclavicular joint separations.” Current reviews in musculoskeletal medicine vol. 6,1 (2013): 71-8. doi: 10.1007/s12178-012-9144-9
  3. 3.Martetschläger, Frank et al. “The Diagnosis and Treatment of Acute Dislocation of the Acromioclavicular Joint.” Deutsches Arzteblatt international vol. 116,6 (2019): 89-95. doi: 10.3238/arztebl.2019.0089
  4. Herdea, Alexandru et al. “The Relationship between the Dominant Hand and the Occurrence of the Supracondylar Humerus Fracture in Pediatric Orthopedics.” Children (Basel, Switzerland) vol. 8,1 51. 15 Jan. 2021, DOI: 10.3390/children8010051
  5. Kiel J, Kaiser K. Acromioclavicular Joint Injury. [Updated 2021 Jul 25]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from:  https://www.ncbi.nlm.nih.gov/books/NBK493188/
  6. Sirin, Evrim et al. “Acromioclavicular joint injuries: diagnosis, classification and ligamentoplasty procedures.” EFORT open reviews vol. 3,7 426-433. 17 Jul. 2018,  DOI: 10.1302/2058-5241.3.170027
  7. LeVasseur, Matthew R et al. “Acromioclavicular Joint Injuries: Effective Rehabilitation.” Open access journal of sports medicine vol. 12 73-85. 28 May. 2021, DOI: 10.2147/OAJSM.S244283
  8. Cheema, Sana G et al. “Publicly Accessible Rehabilitation Protocols for Acromioclavicular Joint Reconstruction Are Widely Variable.” Arthroscopy, sports medicine, and rehabilitation vol. 3,2 e427-e433. 18 Mar. 2021, doi: 10.1016/j.asmr.2020.10.007
  9. Day, Joseph M. et al. ‘Arm Dominance Does Not Influence Measures of Scapular Muscle Strength and Endurance in Healthy Individuals’. 1 Jan. 2015 : 87 – 95. http://dx.doi.org/10.3233/PPR-150056 
  10. Maleitzke, Tazio et al. “Can an acute high-grade acromioclavicular joint separation be reduced and stabilized without surgery? A surgeon’s experience.” Archives of orthopaedic and trauma surgery vol. 140,12 (2020): 2021-2027. doi: 10.1007/s00402-020-03630-0
  11. Scientific American “What causes some people to be left-handed, and why are fewer people left-handed than right-handed?” SCIENTIFIC AMERICAN, a Division of Springer Nature America, Inc. Jun 21, 2004. Accessed Sept. 24, 2021. https://www.scientificamerican.com/article/what-causes-some-people-t/