SECEC Grammont Award 2025: the resting arm position influences impingement-free range of motion and the distalization and lateralization shoulder angle in reverse shoulder arthroplasty using Imascap preoperative planning software

Preoperative planning software enables virtual implantation of reverse total shoulder arthroplasty (rTSA) and assessment of impingement-free range of motion (ROM). Recent studies have shown that scapular resting position varies significantly among individuals and impacts impingement-free ROM. The resting position of the arm also varies between individuals, yet its impact on simulated outcomes and whether this parameter should be integrated into planning programs remains unclear. The objective of this study was to assess the effect of the resting arm position in the coronal plane on simulated impingement-free ROM and on implant positioning metrics, specifically the lateralization shoulder angle (LSA) and distalization shoulder angle (DSA) in rTSA planning. A prospective computational modeling study was conducted using a commercially available three-dimensional planning software (Imascap, Plouzan&#xe9;, France). Thirty computed tomography scans of patients with primary osteoarthritis or cuff tear arthropathy were planned independently by 9 experienced surgeons. The resting abduction angle (RAA) was implemented as proxy for the resting arm position in the coronal plane and determined by the humeral diaphyseal axis and vertical scapular axis. Each plan was simulated with 10 distinct resting arm positions (between -5&#xb0; and 40&#xb0; of abduction), resulting in 2,700 virtual cases. Impingement-free ROM, LSA, and DSA were evaluated for each resting arm position. One-way analysis of variance and Pearson correlation analyses were conducted to determine the relationship between RAA and key planning outcomes. RAA significantly influenced simulated ROM parameters across all tested arm positions (P < .0001). Increasing RAA, resulted in greater adduction (R= 0.72, P < .0001), internal (R= 0.23, P < .0001), and external rotation (R= 0.23, P < .0001), while inversely affecting abduction (R= 0.44, P < .0001), forward flexion (R= 0.26, P < .0001), and extension (R= 0.31, P < .0001). Significant differences were found in LSA and DSA values across the different simulated resting arm positions (P < .0001). LSA showed a weak positive correlation with RAA (R= 0.34, P < .0001), whereas DSA exhibited a strong inverse relationship (R= 0.63, P < .0001), indicating that a more abducted resting arm position led to higher LSA and lower DSA values without altering implant position. The resting arm position significantly affects virtual impingement-free ROM in Imascap planning software, highlighting the importance of incorporating humeral position into rTSA preoperative planning. Additionally, because LSA and DSA measurements are substantially influenced by the arm position despite an unchanged implant configuration, consideration of humeral orientation may be necessary when interpreting these metrics.