Rotator cuff
Shoulder anatomy
How the shoulder works: the ball and socket, the labrum and capsule, the four rotator cuff tendons, and the bones and deltoid around the joint.
Read the transcript
Introduction
Dr Kalman Piper is an orthopaedic surgeon specialising in shoulder surgery, based in Sydney, Australia. This video explains how the shoulder works, the same way Dr Piper explains it to patients in consultation.
Ball and socket
If you look at the shoulder from the front, it's a ball and socket joint. The ball or head is part of the humerus, the bone of the upper arm. The socket is part of the scapula, or shoulder blade. It's called the glenoid.
The socket
The humeral head is larger than the glenoid and the glenoid is shallow. The shoulder joint is often described as a golf ball sitting on a tee. That lets the shoulder move in almost any direction, but it also means it's easier for the ball to slip out of the socket or dislocate.
Inside the joint
To help keep the ball in the socket, a rim of tough tissue called the labrum runs around the edge of the socket. It makes the socket deeper and helps hold the ball in place. When a shoulder dislocates, the labrum can tear away from the edge of the socket. The long head of the biceps starts inside the joint, at the top of the socket. Its tendon runs over the top of the ball, out of the joint and down the front of the arm.
The capsule
The ball and socket are sealed inside a joint capsule. It's a thin, flexible lining that holds the joint fluid in. Normally it's loose and baggy, so the arm can move freely. In a frozen shoulder, the capsule becomes inflamed, then thickens and shrinks, and the shoulder becomes painful and stiff.
Four tendons
Attached to the ball are four large tendons. There's one at the front, one on top, and two more at the back.
Muscle to bone
A tendon connects muscle to bone. Most of the muscles that move the ball in the socket are attached to the shoulder blade. When a muscle contracts, it pulls on its tendon. The tendon pulls on the bone, rotates the ball in the socket, and therefore moves the arm.
What each one does
The tendon at the front internally rotates the arm, the two at the back externally rotate the arm and the one on top elevates the arm. Together, these four tendons are called the rotator cuff.
Around the shoulder joint
A part of the shoulder blade called the acromion projects over the top of the shoulder like a roof, and it forms the point of the shoulder. The clavicle, or collarbone, is connected to the acromion and they form a small joint called the AC joint. At the front of the shoulder is the coracoid, a small hook of bone. The short head of biceps and a muscle called coracobrachialis hang off its tip by a shared tendon, called the conjoint tendon.
The deltoid
Finally, the deltoid muscle wraps around the whole shoulder. It's the big muscle that gives the shoulder its rounded shape and adds power when elevating the arm or heavy lifting. It originates from the collarbone, the acromion and the shoulder blade, and attaches about halfway down the arm.
Close
That covers the basic anatomy of the shoulder. The other videos in this series build on it, to explain common shoulder problems and how Dr Piper treats them.