Shoulder Impingement Part 2: Primary vs Secondary Impingement and why it matters

Look at any anatomical photo of the shoulder and you’ll notice there’s a small space between the acromion and the humerus. However, this tiny hole, known as the subacromial arch, isn’t a void space like photos make it out to be. In fact, in real life there are numerous structures which sit here including bursa and other rotator cuff muscles, and as a result you actually impinge the structures every time you move your arm.

I know what you’re thinking now: “Ok Andrew, so what IS impingement then? You just spent Part 1 telling me that’s what’s wrong with my shoulder and now you’re saying impingement is normal? What gives?”

The truth is impingement is actually normal, it’s when it becomes excessive or abnormal that issues arise. Let’s take a closer look:

What causes abnormal impingement? 

The next thing we need to consider in a diagnosis is WHAT KIND of shoulder impingement you are experiencing: primary or secondary? Here are the main differences between the two:

• Primary Shoulder Impingement: essentially means that the impingement is the main problem. This is usually due to anatomical reasons such as osteophyte (bony projections that form along joint margins) build up on the acromion.
• Secondary Shoulder Impingement: this is when the humeral head isn’t centred within the shoulder joint causing impingement when the arm is moved. Common causes include specific upper body exercises, poor posture, or muscle imbalances such as rotator cuff weakness.

The rotator cuff along with other muscles, such as the deltoid, work to move your arm in space. Being a ball and socket joint most people assume that the shoulder is relatively stable, but it’s more similar to a golf ball sitting on a tee (turned on it’s side) then it is a snug fit of a lock and key.

The rotator cuff muscles work to keep the golf ball and tee connected, but often times these small muscles are weak or overpowered by the bigger deltoid muscles. When this happens the humerus can shift upwards and impinge the rotator cuff between the head of the humerus and the acromion leading to pain or tendinopathy.  Other reasons for impingement could be restrictions of the shoulder, shoulder blade (scapula) or the thoracic spine.

How do we differentiate?

Essentially there are a range of “special tests” that physiotherapists use to diagnose impingement. Two of the most popular tests are the Hawkins test and the Neer test. While in University it’s essential to effectively demonstrate these tests, I don’t use them as a diagnostic tool clinically. This is because the Neer test will essentially push the humeral head into the acromion (ouch) and the Hawkins test will grind the cuff under the subacromial arch (double ouch!).

These tests are not 100% effective in determining a diagnosis so it’s important to note functional activities that may cause you discomfort. These may include:

• Brushing your hair
• Taking off your shirt
• Undoing your bra strap
• Driving a car

These functional activities will give your physiotherapist the best clue in diagnosing your specific type of impingement before prescribing an individualized treatment and exercise plan for rehabilitation, which we will look at in Part 3 next week.

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