The K-line in the cervical ossification of the posterior longitudinal ligament is different on plain radiographs and CT images

Background: The K-line, which is a virtual line between the midpoints of the antero-posterior canal diameter at C2 and C7, can be useful for determination of surgical procedures for cervical ossification of the posterior longitudinal ligament (OPLL). Although K-line is originally measured with plain radiogram, computed tomography multiplanar reconstruction (CT-MPR) is applied for K-line measurement by several surgeons. The purpose of the present study was to analyze whether there is a difference in K-lines obtained from radiographs of standing patients and those obtained from CT-MPR images of supine patients. Methods: The present study included 65 patients with cervical OPLL underwent surgical treatment. We investigated the K-line (+ or −) before surgery, measured from lateral cervical spine radiographs taken in standing patients in a neutral position (X-P-based K-line) and CT-MPR mid-sagittal images obtained in supine patients (CT-based K-line). The X-P-based and CT-based K-lines were compared and differences between them were assessed. Results: The-X-P-based K-line was found to be (+) in 35 patients and (−) in 30 patients. Four of 35 patients with an X-P-based K-line (11%) showed a change from K-line (+) to (−) in CT-based K-line measurements. One of 30 patients with an X-P-based K-line (3%) showed a change from (−) to (+) in CT-based measurements. Conclusions: The K-line should be measured with plain radiogram of standing patients because X-P-based K-line and CT-based K-line can be different.