4 Phases of Humeral Head Arthritis
Glenohumeral joint osteoarthritis, A.K.A shoulder arthritis, typically stems from progressive wear and tear of the cartilage over the years. The cartilage that covers the surface of the humeral head and glenoid is akin to Teflon on a ball bearing. The cartilage is typically 2 millimeters to 3 millimeters thick - not very thick at all.1 This dampens the contact between the bones.
Phases of arthritis on the humeral head.
Osteoarthritis in the shoulder typically progresses through phases.1
(A) At first the cartilage gets soft.
(B) The second phase is when cracks start to develop on the surface.
(C) In the third phase we see that the cartilage starts to “fibrillate”, otherwise known as deterioration and flaking.
(D) Finally, the cartilage has been worn away and the surface of the bone is exposed.
By this point the joint has lost its ability to act as a smooth, gliding surface.
Now this doesn’t mean that the cartilage has worn away over the entire surface of the bone in a joint. It occurs at different rates for different parts of the joint.
Getting back to the ball bearing example, this would be like if the Teflon would be irregular with pits.
Wear progression of cartilage on the humeral head.
The cartilage can progressively wear away once the surface becomes irregular. Once it has thinned out the bones will start to rub on each other, which is where the term bone-on-bone comes from. This is stage 4 for the cartilage loss.
The tricky part is that we don’t know how fast the arthritis will progress. Each shoulder has different amounts of arthritis and different rates of arthritis progression.