What is stress shielding?

Stress shielding of the humerus in press-fit anatomic shoulder arthroplasty: review and recommendations for evaluation

Stress shielding has been defined as "Osteopenia occurring in bone as the result of removal of normal stress from the bone by an implant"

These authors point out that a wide variety of humeral implants and implant fixation methods are now available, each of which has the potential for changing the distribution of loads applied to the humerus. Over time, changes in load distribution can be expected to produce changes in the bone - some areas will developed increased density while others will loose bone density. These changes are commonly referred to as 'stress shielding'. The clinical effects of stress shielding (symptoms, risk of fracture, etc) are not well documented.

In this report the authors review the current literature on press-fit fixation of the humeral component during total shoulder arthroplasty and propose minimum requirements for radiographic descriptions of stress shielding of the humeral component during total shoulder arthroplasty and propose minimum requirements for radiographic descriptions of stress shielding.

Signs of stress shielding are thought to include cortical thinning, osteopenia, spot welds, and condensation lines. The challenge is that each of these is not an 'all or none' characteristic; each has at least 50 shades of grey. The authors offer these two examples of medial calcar osteolysis (blue arrow) 2 years after a total shoulder arthroplasty and osteopenia and proximal lateral cortical thinning (green arrow) 2 years postoperatively.

 

 

The challenges in diagnosing 'stress shielding' are (1) the radiographic appearance can be altered by the technique of the X-ray

 

(2) it is difficult to determine the degree to which the findings were present prior to the arthroplasty and 

(3) characteristics like cortical thinning and osteopenia are difficult to quantitate on plain films.

We conclude that diagnosing the presence and degree of  'stress shielding' can be a challenge.

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