Etiology of ossificantion of the superspinous ligament : from the view point of bio-mechanical factors

The etiology of ossification of the spinal ligaments remains unclear. We selected the ossification of the supraspious ligament (OSSL) as a method to clarify the etiology of ossification. We exmained 728 cases (345 males, 383 females) of outpatients to reveal the frequency of OSSL. OSSL can be divided into three types according to the shape and size. 70 cases with OSSL and 30 cases enployed as non-OSSL group were analyzed roentgenographically. We proceeded to calculate the stress on the supraspinous ligament on the plane model and to discuss causative factors of ossification of the spinal ligaments from the view point of biomechanical factors. The following result were ptained : 1) Frequency of OSSL increased with aging and its hendency was maked especially in male. 2) In type II and type III of OSSL, the apwx of thoracic lyphosis is chiefly seen at the Th_7 nad Th_8, but the highest rate of ossigication is almost always found at Th_<11> and Th_<12>. 3) In type III, thoracolumbar kyphosis (T_<10>-L_2) increased and lumber lordosis (L_1-L_5) decreased. 4) Stress on the supraspinous ligament depends mainly on the body weight and the distance from the center of gravity to the center of intercertebral disc. 5) The stress on the supraspinous ligament increase with increaseing thoracic kyphosis and reaches a maximum at the point which is two vertebral bodies caudad from the apex of thoracic kyphosis.