Frequency and size of ossifications in the caudal attachments of the ligamentum flavum of the thoracic spine. Role of rotatory strains in their development. An anatomic study of 121 spines.

Ossifications in the caudal attachments of the ligamentum flavum were studied on 121 dried thoracic spines. Most of them were aged. Frequency increased in a caudal direction. At T12, frequency depended on the orientation of the zygapophyseal joints of T11-12 which determined the range of rotation. When they were of thoracic orientation, allowing large range of rotation, ossifications were present in 83% of cases. If orientation was lumbar, allowing poor range of rotation, ossifications became much less frequent:33%. The size of ossifications was also studied. The smaller were in T1 and T2. In T11 and T12, size also depended on the orientation of the zygapophyseal joints of T11-12. If it allowed important range of rotation, ossifications were large sized. In the opposite case, they were small sized. Ossifications of caudal attachments of ligamentum flavum appear to be a normal feature of the aged lower thoracic spine. Their frequency in a given motion segment seems to be influenced by rotational strains.