What Type Of Joint Is The Shoulder?

By Jen Neuenfeldt, OTR/L

What Type Of Joint Is The Shoulder?

 

The shoulder is a complex joint made up of muscles and tendons.

Image of the shoulder.

Fun Fact: The shoulder joint is the most mobile joint in the human body.

So you want to know what type of joint is the shoulder? Because the shoulder joint is a ball and socket joint, it has 3 degrees of freedom or motion than a hinge joint like the elbow or knee, which only has 2( flexion/extension). Also, because the shoulder is the most mobile joint in the body, it’s the most unstable joint in the body due to the amount of motion it allows. The shoulder’s main stability is provided by 4 muscles, which are the muscles that make up the rotator cuff. If those muscles become weak or deficient, the shoulder mechanics can become disrupted, leading to problems such as tendonitis, arthritis, or instability.

Bones of the Shoulder

The shoulder joint is made up of 3 bones; the clavicle, the humerus, and the scapula. These three bones make up 4 joints; however, the most important joint of the shoulder is the glenohumeral joint. The AC joint is where the acromion of the scapula meets the clavicle. The sternoclavicular joint is where the clavicle meets the sternum. The scapula thoracic joint is where the scapula meets the ribcage. These 3 previously mentioned joints work together to secure the scapula to the body, and the glenohumeral joint allows the arm’s functional motion.

The shoulder joint is complex and susceptible to injuries.

Image of shoulder anatomy.

Scapular Motion

The motion of the shoulder is made up of two main motions. The first part of shoulder motion comes from the scapula or big triangular-shaped bone you feel on your back, which is the bone in which the “socket” portion of the shoulder projects. The scapula can move up and down or side to side based on muscles and how they pull on it. With a reduced motion of the scapula due to stiffness or weakness, you can have decreased motion when attempting to raise or move the shoulder.

Glenohumeral Joint

The second portion of motion when moving the shoulder and the motion that most think of when moving the shoulder is the humerus’s motion (the large long bone that makes up the upper arm) and how it rotates on the glenoid (the cup that sits on the scapula). The motion allowed at this joint is flexion, extension, circumduction, scaption, abduction, and adduction. Motion at this joint allows you to reach overhead, reach behind your back, reach across your body, and reach out to the side.

Scapula/Humeral Rhythm

The shoulder only moves and works properly when the shoulder’s two main motions work together in harmony.  The humerus and the scapula’s relationship is important because approximately 60 degrees of motion comes from the scapula’s movement and 120 degrees of motion comes from the humerus moving on the glenoid. Together, you can have 180 degrees of motion, which allow movement of your arm from the side of your body to overhead for functional use and reaching.

 

Because of the shoulder joint’s complexity and the number of muscles and structures that go into making the shoulder work, it is no wonder the number of individuals plagued by shoulder pain or injury yearly.

By: Jen Neuenfeldt, OTR/L

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