EFFECTS OF DELTOID LOADING ON ACROMIAL STRESS: A FINITE ELEMENT ANALYSIS STUDY

The effect of deltoid loading on acromial stress is an important part of understanding the mechanisms by which humeral position can play a role in exposing regions of the acromion to fracture risk, which can be severely debilitating (1). Understanding the effect that the position of the humerus has on acromial stress could be valuable for shoulder injury prevention and rehabilitation. Previous research has investigated the effect of deltoid loading and implant location on acromion stress in reverse total shoulder arthroscopy (2), however, no studies have investigated the effect of deltoid loading direction representing a wide range of shoulder positions. Three-dimensional (3D) computational models of the scapula were generated from CT scans of five upper extremity cadavers (age: 63±26 years). The computational model was simplified to impart the force from the three main deltoid heads at their scapular insertion points. A total deltoid load of 1000N was apportioned between the three heads according to published load ratios during abduction and flexion (3), and blended linearly at 15° intervals in between. Simulations were run between 0° plane of elevation (shoulder abduction) and 90° plane of elevation (shoulder flexion), as well as between 0° elevation angle and 90° elevation angle, both at 15° intervals. ABAQUS CAE (Dassult Syste?mes, France) finite element analysis software was then used to compute the resulting stress on the acromion, with the inferior portion of the scapula rigidly constrained. The outcome variable was the maximum principal stress in the cortical bone in each of the three levy regions of the acromion. Heatmapping was performed to illustrate the effects of deltoid load direction on acromial stresses for all elevations and planes of elevations investigated (Figure 1). It can be seen that in Levy regions 1 and 2, stresses are larger at higher planes of elevation when the humerus is approaching anatomic flexion. Analysis of the stress distribution on the acromion and spine (Figure 2) showed that due to the nature of load transfer, which is likely primarily in bending and torsion, the location of maximum stresses changes as the neutral axis effectively varies as the loading angle is altered from vertically downwards (0° elevation angle) to horizontal (90° elevation angle). The results of this study demonstrate that deltoid load direction has a marked influence in the distribution of bone stresses. Levy regions 1 and 2 appear susceptible to being subjected to higher stresses for a wide region of humeral positions. Moreover, Levy region 3 is consistently subjected to lower stresses compared to regions 1 and 2. These findings have important implications with regard to applying loading in both experimental and computational biomechanical studies when determining worst-case stress scenarios. This information could be important when planning rehabilitation programs and preventing acromion fractures in at risk patient groups. Future work should evaluate the effect cortical bone density has on acromion stress. For any figures or tables, please contact the authors directly.