Indianapolis Colts Andrew Luck Injury Discussion Part 2 – Anatomy of the Shoulder Part 1

I’m continuing my “Injury Discussion” with Andrew Luck’s shoulder injury as he continues to make his strides in preparation for the 2018-2019 NFL Football Season. As many of you know he missed all of the 2017-2018 NFL Season due to a shoulder injury. He didn’t miss any games in the 2016-2017 NFL Season, but did miss two games during the 2015-2016 NFL Season.

As many of you recall from reading Part 1, he was injured during Week 3 of the 2015 NFL Season. In today’s post, we’re going to take a look at shoulder and what actually comprises of the shoulder.

The Shoulder “Complex” actually is built by several joints:

A. Scapulothoracic Joint – scapula and the thoracic spine and rib cage

B. Glenohumeral Joint – glenoid fossa of the scapula and the head of the humerus

Shoulder Joint Diagram Shoulder Joint – Serior
C. Acromioclavicular Joint – acromion process of the scapula and the clavicle


D. Sternoclavicular Joint – sternum and the clavicle



All four of these joints play a role in the functionally of the shoulder when healthy and each joint influences the other. I’m going to briefly talk about the Sternoclavicular (SC) Joint and the Acromioclavicular (AC) Joint. For the main reason is that the focus on Andrew’s Luck shoulder is not about the SC or AC Joint.

The SC Joint is a stable joint held in place by small ligaments that are very strong. The main purpose of the SC Joint allows for movement of the humerus about the scapula. As you try to perform different movements of the scapula and humerus the clavicular portion of the SC Joint has to rotate or elevate and depress to allow for the movements to occur.

The AC Joint, similar to the SC Joint, allows the scapula and humerus to move. It has to do small micromovements so that you can move your arm through large ranges of motion. The AC and SC joints are influenced by one another which in turn influence range of motion of the scapulothoracic and glenohumeral joint.

The Scapulothoracic (ST) Joint is not a true synovial joint, like the other three. It is more of an articulation the the scapula about the rib cage and thoracic spine. The ST Joint is influenced by the availability of movement from the AC and SC Joints but it heavily influences the movement of the Glenohumeral joint. Literature and research usually state that for every 2 degrees of Glenohumeral movement, the ST Joint moves about 1 degree, so a 2:1 work ratio.

It’s been well documented and shown through instagram, twitter, and other blogs how important proper “rhythm” between the scapulothoracic and glenohumeral joint is. Much shoulder movement dysfunction can be traced back to improper mechanics and movement at the ST junction. What allows for proper movement of the ST junction are the variety of muscles that do attach to the scapula.

Lastly, the Glenohumeral (GH) Joint is what is most commonly known to the general public as the “shoulder joint.” It’s a ball and socket synovial joint that has the potential for a lot of range of motion. Since the GH joint is a ball and socket it naturally sacrifices stability for the ability to have a great range of motion.

To protect the joint from all the range of motion it has, you have a shoulder capsule, ligaments, and muscles that encapsulate the bones to protect it. Some of the structures we’ll highlight in this discussion: the shoulder labrum, the rotator cuff muscles, and the biceps brachii.

The Biceps Brachii, is recognized primarily as an elbow flexor. Every one does biceps curls at the gym. That’s what you usually think. But the biceps plays a role in the shoulder because it attaches on the scapula. Actually it has two attachment sites on the scapula. It helps provide stability to the shoulder, especially on the front of the shoulder. The Biceps can play a role in shoulder pain, especially if you’ve exhausted working other muscles of the shoulder.


The Rotator Cuff Muscles, commonly known as the SITS Muscles. It’s composed of the supraspinatus, infraspinatus, teres minor, and subscapularis. The rotator cuff muscles all directly cross the GH Joint. They are smaller muscles in comparison to other muscles of the body but play a vital role in shoulder health.

  • Subscapularis – the only muscle located on the anterior side of the scapula, is also the only internal rotator of the humerus of the four rotator cuff muscles.
  • Infraspinatus – located below the spine of the scapula on the posterior side of the scapula, it assists with external rotation of the humerus.
  • Supraspinatus – located above the spine of scapula on the posterior side of the scapula, it assists with initiation of the shoulder abduction and shoulder flexion.
  • Teres Minor – located on the lower border of the scapula, it assists with external rotation of the humerus.

The Glenoid Labrum is fibrocartilaginous rim around the glenoid cavity. The labrum makes the glenoid cavity deeper to provide more stability to the GH Joint.

The labrum will highlight the next post because it is what Andrew Luck tore. He tore the posterior aspect of his labrum, which is less common compared to the anterior aspect of the labrum.

With basic shoulder anatomy out of the way, next post will look in-depth at the practical application to Andrew Luck’s injury.


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