Friday, January 13, 2017

Stress shielding after humeral component insertion

These are the x-rays of  of a patient with a stiff painful right shoulder after a  hemiarthroplasty performed five years earlier presenting for revision arthroplasty.

Note that the distal two thirds of the stem is tightly impacted in the humeral diaphysis so that forces applied to the prosthetic head bypass the proximal one third of the bone. As a result of this stress shielding bone has been lost from the humerus above the diaphyseal metaphyseal junction (red arrows).

Interestingly, this prosthesis has a bone ingrowth surface in its metaphyseal portion. Should it require removal, there may be a risk of tuberosity fracture.

We avoid ingrowth surfaces and tight diaphyseal fit of the distal stem in our approach to humeral fixation. Instead we attempt to prevent stress shielding using impaction grafting to distribute the load evenly from the prosthesis to the humerus. For more about impaction grafting see this link.

Shown below is a diagram of an impaction grafted stem and an x-ray of an impaction grafted stem 6 years after total shoulder arthroplasty with no evidence of stress shielding.