[The antero-posterior diameter of the cervical spinal canal in the ossification of the posterior longitudinal ligament (author's transl)].

1. The antero-posterior diameter (APD-Fig. 1A) of the cervical spinal canals in 38 cases of symptomatic ossification of the posterior longitudinal ligament (OPLL) and 29 cases of asymptomatic OPLL was measured for each vertebra, and the thecoperiosteal diameter (TPD-Fig. 1B) of the cervical spinal canals in the same cases was measured. 2. The APD in the cases of symptomatic OPLL was found to be significantly smaller than those of asymptomatic OPLL. 3. The TPD in symptomatic OPLL was also found to be more significantly smaller than those of asymptomatic OPLL. Measurements of 9 mm or less were considered liable to be associated with cord compression. 4. The growing rate of the ossification was calculated in 13 cases of symptomatic OPLL which we have followed up radiologically for more than 2 years. The growing rate varied considerably from case to case. The average growing rate per one year was 4.07 mm in axial length and 0.67 mm in thickness on antero- posterior direction. 5. The incidence of OPLL was examined.