Abstract
Introduction
Total shoulder arthroplasty (TSA) is an effective treatment to restore shoulder function and alleviate pain in the case of glenohumeral arthritis [1]. Stress shielding, which occurs when bone stress is reduced due to the replacement of bone with a stiffer metallic implant, causes bone resorption of up to 9% of the humeral cortical thickness following TSA [2]. Shorter length stems and smaller overall geometries may reduce stress shielding [3], however the effect of humeral head backside contact with the resection plane has not yet been fully investigated on bone stress. Therefore, the purpose of this study was to quantify the effect of humeral head contact conditions on bone stresses following TSA.
Methods
3D models of eight male left cadaveric humeri (68±6 years) were generated from CT data using MIMICS. These were then virtually prepared for reconstruction by an orthopaedic surgeon to accept a short-stem humeral implant (Exactech Equinoxe® Preserve) that was optimally sized and placed centrally in the humeral canal. The humeral head was positioned in the inferior-medial position such that contact was achieved on the medial cortex, and no contact existed on the lateral cortex. Three different humeral head backside contact conditions were investigated (Figure 1); full backside contact (FULL), contact with only the inferior-medial half of the resection (INF), and contact with only the superior-lateral half of the resection (SUP). Cortical bone was assigned an elastic modulus of 20 GPa and a Poisson's ratio of 0.3. Trabecular bone was assigned varying stiffness based on CT attenuation [4]. A joint reaction force was then applied representing 45˚ and 75˚ of abduction [5]. Changes in bone stress, as well as the expected bone response based on change in strain energy density [6] was then compared between the intact and reconstructed states.
Results
For cortical bone, the full backside contact altered bone stress by 28.9±5.5% compared to intact, which was significantly less than the superior (37.0±3.9%, P=0.022) and inferior (53.4±3.9%, P<0.001) backside contact conditions. Similar trends were observed for changes in trabecular bone stress relative to the intact state, where the full backside contact altered bone stress by 86.3±27.9% compared to intact, compared to the superior and inferior contact conditions, which altered bone stress by 115.2±45.0% (P=0.309) and 197.4±80.2% (P=0.024), respectively. In terms of expected bone response, both the superior and inferior contact resulted in an increase in bone volume with resorbing potential compared to the full contact (Figure 2).
Discussion and Conclusions
The results of this study show that full humeral head backside contact with the humeral resection plane is preferable for short stem humeral TSA implants with the head in the inferior-medial position. As expected, the superior contact typically increased resorption potential in the medial quadrant due to the lack of load transfer, however interestingly the inferior contact increased resorption potential in both the lateral and medial quadrants. Analysis of implant micromotion showed that medial liftoff of the implant occurred, which resulted in a lack of load transfer in the most medial aspect of the resection plane.
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