Functional anatomy and vertical biomechanics of the acromioclavicular joint

Objective To determine the role of acromioclavicular ligament in maintaining the stability of acromioclavicular joint. Methods In 12 cadaveric specimens of normal shoulder joint which had been routinely treated by formalin, the coracoclavicular ligaments (trapezium and conical ligaments) were dissected and exposed after soft tissue was removed from the surface. The distribution of the insertion and starting points, appearance and attachment area of the trapezium and conical ligaments were observed. The lengths of the 2 ligaments, the coronal and sagittal lengths of the clavicular attachment area, the distances from the most lateral point to the distal end of the clavicle, and the angles at the coronal and sagittal positions of the 2 ligaments were measured. Subsequently, the 12 cadaveric specimens were randomly divided into 4 groups (n=3). Group A retained the intact acromioclavicular ligament, group B the intact coracoclavicular ligament, group C the intact trapezium ligament and group D the intact conical ligament. In an electronic machine for versatile mechanical tests, a 100 mm/min load speed was applied for destructive static stretching of the ligament specimens in the vertical direction. The load-displacement curves were recorded and drawn by a computer in connection with the biomechanical testing machine. The rupture strengths of the 4 ligaments were recorded. Results The average lengths of the conical and trapezium ligaments were 10.6 mm and 12.5 mm, respectively. The coronal and sagittal lengths of the clavicular attachment area of the conical ligament averaged 13.4 mm and 5.8 mm, respectively. The coronal and sagittal lengths of the clavicular attachment area of the trapezium ligament averaged 14.2 mm and 8.7 mm, respectively. The distances from the most lateral points of the conical and trapezium ligaments to the distal clavicle averaged 35.5 mm and 23.6 mm, respectively. The average angles at the coronal and sagittal positions were 6.2° and 11.3° for the conical ligament and 38.7°and 6.9° for the trapezium ligament, respectively. The average tensile force was 201.3±1.9 N for the acromioclavicular ligament rupture, 374.6±1.4 N for the coracoclavicular ligament rupture, 192.3±4.3 N for the trapezium ligament rupture, and 345.7±1.1 N for the conical ligament rupture. Conclusions The roles and contributions of the conical, trapezium and acromioclavicular ligaments are different in maintaining the stability of the acromioclavicular joint. In anatomical reconstruction of the acromioclavicular joint, it is more important to reconstruct the conical ligament and to repair the acromioclavicular ligament simultaneously as much as possible. Key words: Acromioclavicular joint; Anatomy; Biomechanics; Coracoid ligament; Acromioclavicular ligament