Stemless humeral component - is stress shielding an issue?

Every type and design of humeral component changes the way in which load is transferred from the articular surface to the bone. Thus, adaptive changes (read: "stress shielding) are inevitable.

The authors of Stress shielding following stemless anatomic total shoulder arthroplasty set out to assess the radiographic proximal humeral bone adaptations seen following stemless anatomic total shoulder arthroplasty. They prospectively followed 152 stemless total shoulder arthroplasties utilizing a single implant design performed by 21 shoulder arthroplasty surgeons.

At 2 years postoperatively, "stress shielding" was noted in 61 (41%) shoulders. A total of 11 (7%) shoulders demonstrated major stress shielding with 6 occurring along the medial calcar. At the final follow-up, no humeral implants were radiographically loose or migrated. There was no statistically significant difference in clinical and functional outcomes between shoulders with and without stress shielding. Thus the clinical importance of these adaptive changes was not demonstrated.

In reviewing some of the figures from this article, "stress shielding" may not be the principal cause of bone resorption (yellow arrow). Consider this comparison of the 3 month (left) and the 2 year (right) anterior posterior views of the same shoulder. The 2 year film was interpreted as "demonstrating severe stress shielding along the medial calcar".  Note, however that the cement originally beneath the glenoid component (green arrow) is no longer seen on the 2 year film and that the two year film appears to show shift of the glenoid component's position and lucencies around the pegs (red arrow). These findings bring up the possibility of particulate debris as a contributing factor to the humeral bone resorption at the yellow arrow.

Another figure (below) shows sequential axillary radiographs (L to R: 6 weeks, 6 months, 12 months, and 24 months postoperatively) and was interpreted as showing moderate stress shielding along the medial calcar and greater tuberosity regions. However, it appears that that the proximal humeral bone may have been cystic (yellow arrow) leading to what may be humeral component loosening (red arrows).

Thus it is difficult to know whether the changes seen in the humeral bone are a result of "stress shielding" or are a result of problems with humeral and glenoid component fixation, such as those shown below in other figures from this paper.

While the authors described that "Glenoid radiolucent lines were noted in 28 (18%) patients. These were Lazarus grade 1 in 6, grade 2 in 5, grade 3 in 13, and grade 4 in 2", the relationship of these findings to humeral adaptive changes was not reported.

These authors concluded "the occurrence and severity of bony adaptations had no effect on short-term patient clinical outcomes."

For more detail on this topic see Stemless humeral components: stress shielding

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