Hypothesis: We hypothesized that the malpositioning of the humeral component can preclude the long-term success of anatomical total shoulder arthroplasty. The goal of this study was to evaluate the mechanical consequences of superior and inferior malpositioning of the humeral head. Materials and methods: A numerical musculoskeletal model of the shoulder joint allowing natural humeral head translation was used to simulate a loaded abduction movement controlled by muscular activation. An inferior and superior malpositioning of 5 mm were compared to an optimal positioning. Impingements, articular contact pattern, and cement stress were evaluated. Results: Inferior malpositioning of the humeral head induced impingement and limited the abduction level, while superior malpositioning increased the subluxation risk. Both inferior and superior malpositioning increased the stress level within the cement mantle. Discussion: This numerical study highlights the importance of an anatomical reconstruction of the glenohumeral surfaces for the success rate of anatomical total shoulder arthroplasty.