CORS_ForceProfile2022
Evaluating the Effect of Varying Force Profiles on the Stress Analyses of Shoulder Humeral Implants
Presenting Author: David E. Cunningham
Contributing Authors: Gregory Spangenberg, Dr. Dan Langohr, Dr. George Athwal, Dr. Jim Johnson
Purpose
Experimental and finite element (FE) analyses have commonly been implemented in the evaluation of shoulder implants. These approaches can provide insight into the effects of implant design variables; however, the results of these studies are limited to the boundary conditions imposed on the implant-bone construct. To date, these studies have employed loading protocols isolated to simulating forces in the coronal plane with special interest in arm abduction. The objective of this study was to evaluate the effects of loading magnitude and load direction that represent a wide range of other activities of daily living and to determine if these protocols are also warranted during the evaluation of shoulder implants. We employed a FE model of the humeral implant in the proximal humerus and assessed the stresses in the medial calcar region, as this location is critical from the perspective of stress shielding.
Methods
One patient-specific (sex: male, age: 79, mass: 85 kg, height: 170 cm) three-dimensional model of the proximal humerus was created from CT data using Mimics (Materialise, Leuven, Belgium). A stemless reversed humeral implant model (Perform Stemless™ Reversed Shoulder System, Wright Medical Group) was positioned into the humeral model. We evaluated a number of separate loading conditions based on the joint reaction forces experienced during normal daily physiological activities (Fig. 1) (Orthoload, Berlin, Germany). We examined abduction as well as six wide-ranging activities of daily living (which are not typically modelled in studies to date). Forces were directed through the centre of rotation of the humeral implant and loads were applied linearly using an iterative solver. The maximum principal stresses were averaged over a bone volume of 45 mm3 located at the medial humeral calcar region.
3D Loading Plot of (Left) Humerus during Clinically-Relevant Motions
Figure 1: Varied Anatomical Loading Profiles on a 3D Humeral Model. This interactive plot of the left humerus shows the variation of loading vectors that are applied to the glenohumeral articualtion during everyday activities. Please note that if this plot is being viewed on a phone you may have to rotate the screen to a landscape orientation for the best viewing experience.
Results
The range of variation in stresses developed in the medial calcar region was [0.025 MPa – 0.137 MPa] (Fig. 2) over all loading cases, indicating a large variability of possible stresses developed at the glenohumeral articulation during loading of the shoulder throughout daily physiological activities. The maximum relative increase in average calcar principal stress when compared to the commonly assessed “abduction motion” was observed in the “two-handed steering motion” which reflected a 140% increase in average stress. It was notable that the forward flexion motion resulted in a 62% increase in average calcar principal stress when compared to the abduction motion. These results suggest that out-of-coronal-plane activities warrant inclusion in both computational and biomechanical experimental studies of this joint.
Figure 2: Average Maximum Principal Stresses in the Medial Calcar Region During Varying Loading Profiles .
Conclusion
The precedent methods of evaluating the efficacy of humeral prosthetics may not fully assess the stress state developed in the humerus when applying a limited number for force profiles. The results of this investigation show differences in the resultant stresses developed in the proximal humerus during physiologically relevant activities. It is therefore suggested that future work utilize a larger loading profile than simple abduction, particularly for the assessment of these implants.