What Type Of Joint Is The Shoulder?
By Jen Neuenfeldt, OTR/L
What Type Of Joint Is The Shoulder?
So you want to know what type of joint the shoulder is? Because the shoulder joint is a ball and socket joint, it has 3 degrees of freedom or motion. The ball part of the joint is the head of the humerus. The socket of the shoulder is protected by the labrum.
Is the Shoulder a Hinge Joint?
A hinge joint like the elbow or knee, only has 2 motions or flexion and extension. The shoulder does not fall into this category mainly due to it being the most mobile joint in the body. Also, It is 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 tendonitis, arthritis, or instability.
Fun Fact: The shoulder joint is the most mobile joint in the human body.
Rotator Cuff Tendons
Bones of the Shoulder
The shoulder joint has 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.
Please read this article on Proximal Humerus Fractures.
Bones of the shoulder
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, 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 them. 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 allows 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.
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