Frozen Shoulder - What is Happening to the Shoulder?

Frozen Shoulder

Frozen Shoulder or Impingement?

Frozen Shoulder Syndrome - What's Going On Inside the Shoulder?

The causes of frozen shoulder syndrome are still poorly understood. About 50% seem to stem from an injury to the shoulder (such as a fall on an outstretched arm) and these are called secondary frozen shoulders.

The other half the time they appear for no apparent reason, and these are called primary frozen shoulders.

Although we don’t know why they happen we do know a lot about what goes on inside the frozen shoulder.

Shoulder Anatomy

The first thing to understand is that the shoulder is a modified ball and socket joint. The ball is at the top of the arm bone (humerus) and the socket is a shallow cup on the end of the scapula (shoulder blade).

This is a good design to give mobility to the shoulder joint but it makes it inherently unstable. To improve the stability of the shoulder, a cuff of four muscles (called the rotator cuff) braces the joint, as well as a complex plethora of tough internal ligaments.

Surrounding the gleno-humeral joint (shoulder joint) is a bag called the capsule. When the arm is raised above the head, this capsule is fully stretched, and when the arm is lowered to the side, the capsule hangs down in a small pouch-like sack (plica).

The synovial capsule contains up to 60ml of synovial fluid. This fluid helps to lubricate the joint and gives the joint surfaces nutrients for repair. Cells lining the joint membrane produce the synovial fluid.

Internal cameras have shown that during the "frozen phase" of a frozen shoulder the capsule may shrink to less than half its normal size!

Sticky Capsule

In frozen shoulder syndrome (adhesive capsulitis), this small sack starts to stick to itself, hence the name of the condition.

As it becomes sticky, the synovial fluid drains away and can often reduce to about 5ml. This makes the joint dry and crackly.

The stickiness is brought on through massive localized inflammation. This inflammation spreads into other shoulder soft-tissues and can cause swelling in other shoulder sacks (bursae).

It has been my experience that this situation may occur the other way around as well. Often a frozen shoulder results from a non-treated biceps tendonitis, or triceps tendonitis.

Both the biceps and triceps tendon run into the ball and socket joint. The tendons, like the muscles, are covered by a clingfilm-like sheath, which gets inflamed and becomes swollen, so the tendon can no longer slide smoothly as the arm is moved.

This quickly leads to a vicious cycle. The tendons become even more swollen and night pain commences.

There is very little free space inside the shoulder joint and all of the tendon sheaths eventually blend together (they are continuous).

This means that thousands of microscopic cells of inflammation can easily make their way from sheath to sheath and eventually the whole shoulder becomes engulfed in a rapid and massive inflammatory cycle.

Inflammation