Thursday, December 27, 2012

The glenoid - the key to shoulder arthroplasty

The restoration of the humeral (ball) side of the arthritic shoulder is relatively straightforwardThe real challenge in surgical reconstruction exists on the socket side of the joint.

The normal shoulder socket is shallow so that the range of motion of the joint is optimized. The shape of the socket has two important functions.

First it distributes forces from the arm to the shoulder blade.

A comfortable force transmission requires that the load be distributed evenly across the joint surface.
When the joint surface is not congruent with the ball, we have load concentration (recall the princess and the pea) and a painful joint.
The second function of the socket is to provide a geometry that centers the ball within it.
 The primary mechanism for this centering is concavity compression - the same mechanism by which a golf ball is centered on the tee. The tee is concave and gravity provides the compressive force. Without these factors, the ball rolls out of the tee. In the shoulder, the stability of the joint is provided by the concave shape of the socket and the muscles pressing the ball into this concavity.
This is nicely demonstrated by this video prepared by my colleague John Sides. In subsequent posts, we'll discuss some of the ways that load distribution and stability can be provided when the arthritic shoulder is surgically reconstructed.


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