Understanding Functional Internal Rotation Variability After Reverse Total Shoulder Arthroplasty Through Muscle Activation Analysis: A Pilot Investigation

The use of reverse total shoulder arthroplasty (RSA) has increased in recent years, due to its application versatility. However, long-term outcomes for functional internal rotation (fIR), also known as hand-to-back movement, vary significantly, and thereby impact patients' independence. Although previous studies examined various factors associated with fIR outcomes, the reason for fIR variability remains unclear. This study investigated muscle activations and their correlations with fIR levels, which was not previously explored. A prospective study (s62153) included 16 pain-free RSA patients at minimum 1 year postoperatively, where their Tampa-17 kinesiophobia score was obtained. During a motion analysis study, patients performed 3 hand-to-back movements, while their kinematical patterns were captured using reflective markers. Anatomical landmarks, identified during a static trial, were used to scale a generic upper-body musculoskeletal model in OpenSim4.0 towards patients' anthropometry. Afterwards, dynamic vertical hand position relative to ground was determined. Movement start and end were defined as the timepoint before hand velocity increase and the timepoint at maximal hand position. Hand position was normalized relative to C7 and the highest patient's level, such that fIR level ranged between 0% and 100%. Electromyography signals, captured by surface and intramuscular electrodes, were processed and normalized to 9 maximal isometric voluntary contraction (MIVC) trials. Signals with a signal-to-noise ratio below 3dB were excluded. For visualization, patients were divided into 3 groups by rank of their mean fIR level, aiming for equally-sized groups. Spearman's Rank correlations(R) were computed for root-mean-squared muscle activations, MIVC internal rotation strength, and Tampa score with fIR level. Asterisk* indicated significance (p<0.05). Posterior deltoid, upper and lower trapezius, and teres major had highest activation among all muscles in high-level patients. Lower-level patients showed increased activation of pectoralis major*, middle and lower trapezius, serratus anterior, teres minor*, and latissimus dorsi*. Internal rotation strength (R=0.28) and Tampa scoring (R=−0.32*) correlated with fIR level. Posterior deltoid, upper and lower trapezius, and teres major appeared crucial for reaching high fIR levels. The overall difference between low and high fIR levels was muscle overactivation without functional outcome, likely driven by fear of movement. This highlights the importance of muscle control training in RSA rehabilitation. Additionally, correlation between muscle strength and fIR outcome emphasizes the value of strength training. ** Shared last authorship of Lennart Scheys and Filip Verhaegen.