Ligament mechanics during three degree-of-freedom motion at the acromioclavicular joint

The development of effective treatment and reconstruction procedures for in juries to the soft tissues around the acromioclavicular (AC) joint relies o n a comprehensive understanding of overall joint function. The objective of this study was to determine the magnitude and direction of the in situ for ces in the AC capsular and coracoclavicular ligaments as well as the result ing joint kinematics during application of three external loading condition s while allowing three degree-of-freedom joint motion. A robotic/universal force-moment sensor testing system was utilized to determine the in situ fo rces in the soft tissue structures and the resulting joint kinematics. The clavicle translated 5.1+/-2.0, 5.6+/-2.2, and 4.2 +/-1.9mm during applicati on of a 70 N load in the anterior, posterior, and superior directions, resp ectively, representing almost a 50% increase over previous studies using si milar load magnitudes. In response to an anterior load, the magnitude of in situ force in the superior AC ligament (35+/-18N) was found to be greater (p<0.05) than the force in the trapezoid and conoid ligaments. In contrast, the magnitude of in situ force in the conoid (49+/-22N) was significantly greater (p<0.05) than all other ligaments in response to a superior load. A dditionally, the directions of the force vector representing the conoid and trapezoid were different, being located in opposing quadrants of the poste rior axis of the scapula with this loading condition. Our data suggest that the kinematic constraints placed on the AC joint during loading affect the resulting joint motion and that the magnitude and direction of force in ea ch ligament are affected by the coupled motions that occur. Based on the di fferences in magnitude and direction of the in situ force in the coracoclav icular ligaments with each loading condition, surgical procedures should re construct these ligaments in a more anatomical manner or treat them separat ely to prevent joint degeneration. (C) 2000 Biomedical Engineering Society. [S0090-6964(00)00106-5].