Purpose <p>The effects of different posterosuperior rotator cuff tear patterns, ranging from isolated supraspinatus tears to partial or complete infraspinatus involvement, on glenohumeral joint contact mechanics in reverse total shoulder arthroplasty (RTSA) remain unclear. This study aimed to investigate how varying levels of posterosuperior cuff tear (PCT) severity affect three-dimensional center-of-pressure (COP) trajectories, joint contact forces (JCFs), and muscle-tendon forces using a musculoskeletal shoulder model of RTSA.</p> Methods <p>Three-dimensional marker trajectory data from eleven male participants were collected during 120° of shoulder abduction in the coronal plane and used as consistent inputs for the RTSA model. Model simulations were performed for various stages of PCT severity. For comparison with each stage of the model, an intact cuff model of RTSA was also simulated.</p> Results <p>As PCTs progressed, the COP trajectories increasingly shifted in the superior (<i>P</i> &lt; 0.001), anterior (<i>P</i> &lt; 0.001), and medial (<i>P</i> &lt; 0.001) directions. Additionally, the peak inferior and posterior JCFs decreased by 5.6% (<i>P</i> = 0.02) and 20.0% (<i>P</i> = 0.002), respectively (i.e., the contact area moved in a more anterosuperior direction). Conversely, preserving more posterosuperior cuffs increased muscle-tendon forces in the subscapularis and pectoralis major.</p> Conclusion <p>Preserving the middle-inferior infraspinatus can restore joint contact mechanics and mitigate increased muscle-tendon forces in RTSA. These findings can help orthopedic surgeons in deciding whether to repair the rotator cuff during RTSA to mitigate increased muscle-tendon forces and restore the transverse force couple between the anterior and posterior cuffs, thereby improving glenohumeral joint stability.</p>