[Biomechanical properties of the acromioclavicular joints]

The purpose of this study was to investigate the biomechanical properties of the cartilagenous element of the acromioclavicular joint. Experimental specimens of the distal end of the clavicle were selected from operative excisions from patients with acromioclavicular joint dislocation. Dynamic stiffness, tan delta, cartilagenous thickness, elastic modulus and creep modulus were measured with a Biomechanical Spectrometer. The results were as follows: There was a negative correlation between age and values of dynamic stiffness as well as between age and the values of elastic modulus. The values of dynamic stiffness, tan delta, cartilagenous thickness, elastic modulus and creep modulus correlated significantly with the number of days after injury. The values for the anterior parts, excluding dynamic stiffness, were markedly higher than those for the other parts. Aging of the articular cartilage may be a factor in the lowering of dynamic stiffness and elastic modulus. Degenerative change is evidenced by a decrease in cartilaginous thickness and a softening of the articular cartilage. The anterior part of the articular surface showed good load-relaxation and viscoelastic properties. The anterior parts may act as the center of the rotational axis of the clavicle and the fulcrum of the horizontal adduction of the scapula when the arm is raised.