Purpose The acromioclavicular ligament complex (ACLC) is the primary stabilizer against horizontal translation with the superior ACLC providing the main contribution. The purpose of this study was to evaluate the specific regional contributions in the superior half of ACLC, where the surgeon can easily access and repair or reconstruct, for posterior translational and rotational stability. Methods The superior half of ACLC was divided into three regions; Region A (0°–60°): an anterior 1/3 region of the superior half of ACLC, Region B (60°–120°): a superior 1/3 region of the superior half of ACLC, and Region C (120°–180°): a posterior 1/3 region of the superior half of ACLC. Fifteen fresh-frozen cadaveric shoulders were used. Biomechanical testing was performed to evaluate the resistance force against passive posterior translation (10 mm) and the resistance torque against passive posterior rotation (20°) during the following the four conditions. (1) Stability was tested on all specimens in their intact condition ( n  = 15). (2) The ACLC was dissected and stability was tested ( n  = 15). (3) Specimens were randomly divided into three groups by regions of suturing. Stability was tested after suturing Region A, Region B, or Region C ( n  = 5 per group). (4) Stability was tested after suturing additional regions: Region A + B (0°–120°), Region B + C (60°–180°), or Region A + C (0°–60°, 120°–180°, n  = 5 per group). Results The translational force increased after suturing Region A when compared with dissected ACLC ( P  = 0.025). The force after suturing Region A + B was significantly higher compared to the dissected ACLC ( P  < 0.001). The rotational torque increased after suturing Region A or Region B compared with dissected ACLC ( P  = 0.020, P  = 0.045, respectively). The torque after suturing the Region A + C was significantly higher compared to the dissected ACLC ( P  < 0.001). Conclusion The combined Region A + B contributed more to posterior translational stability than Region B + C or Region A + C. In contrast, combined Region A + C contributed more to posterior rotational stability than Region A + B or Region B + C. Based on these findings, surgical techniques restoring the entire superior ACLC are recommended to address both posterior translational and rotational stability of the AC joint.