Numerous biomechanical studies have shown that the primary restraint to anteroposterior translation of the clavicle is the ligamentous thickenings of the acromioclavicular joint capsule. Debski et al showed in one such study that the strongest of these ligaments is the superior one, verifying the findings of several other authors. They reported that the superior ligament supplies around 50% of the strength against anteroposterior translation, and it is thickest in its posterior aspect. Additionally, the posterior AC ligament adds an additional 25% of the overall strength. For this reason, these ligaments should be preserved when performing a distal clavicle resection. The length of distal clavicle that can be taken and still preserve stability of the joint is highly debated in the literature. The conoid and trapezoid ligaments are the primary restraints to vertical translation at the AC joint. Renfree and Wright review the published anatomic findings around the AC and SC joints, and come to similar conclusions as above.
Renfree KJ, Wright TW. Anatomy and biomechanics of the acromioclavicular and sternoclavicular joints. Clin Sports Med. 2003 Apr;22(2):219-37.
PMID:12825527 (Link to Abstract)
Debski RE, Parsons IM 4th, Woo SL, Fu FH. Effect of capsular injury on acromioclavicular joint mechanics. J Bone Joint Surg Am. 2001 Sep;83-A(9):1344-51.
PMID:11568197 (Link to Abstract)