The effects of the anterior and posterior sternoclavicular joint (SCJ) soft tissue structures on joint dislocation strength by sequential sectioning the ligaments and capsule of twenty-eight SCJs were evaluated. The medial clavicle of each specimen was initially loaded in the anterior and posterior directions to provide control values for joint laxity. The anterior or posterior ligaments and capsular structures of the SCJs were then selectively cut and the specimens retested for laxity and then loaded to failure simulating either anterior or posterior dislocation. Testing of intact specimens showed that the posterior ligaments were stiffer than other structures in that it was significantly more difficult to posteriorly displace the SCJ than in any other direction and that the capsule was the important anterior structure affecting joint laxity. Load-to-failure testing showed that it required 50% more force to create a failure by posterior dislocation than by anterior dislocation. The results of this study explain the clinical rarity of posterior sternoclavicular joint dislocations.