Usefulness of 3-dimensional Measurement of Ossification of the Posterior Longitudinal Ligament (OPLL) in Patients with OPLL-induced Myelopathy

Ossification of the posterior longitudinal ligament (OPLL) is a 3-dimensional (3D) disease. 3D method of measurement is superior to the conventional method in terms of evaluating the clinical state of symptomatic patients with OPLL. Higher 3D OPLL ratio has an adverse effect on the spinal cord and is associated with deteriorating Japanese Orthopedic Association score. Study Design. Retrospective study. Objective. To evaluate the usefulness of 3-dimensional (3D) measurement of ossification of the posterior longitudinal ligament (OPLL), and identify the reliability of a novel 3D method of measurement. Summary of Background data. OPLL is not a 2-dimensional (2D) disease, but rather a 3D disease. Therefore, conventional measurement of parameters using radiography may not be suitable for evaluating OPLL. However, there is no study that investigated the correlation between 3D parameters of OPLL and clinical outcomes. Methods. 50 patients (40 males and 10 females; mean age 57.2 yr) with symptomatic OPLL were enrolled. Neurological and clinical outcome scales were measured using the Japanese Orthopedic Association (JOA) score, visual analogue scale, short-form health survey (SF-36) and neck disability index. A 3D model was reconstructed with digital imaging and communications in medicine files from axial computed tomographic images using software (MIMICS; Materialise, Leuven, Belgium) to obtain the following parameters: 3D volume of OPLL, 3D volume of the spinal canal (confined to the vertebral level involving OPLL), spinal canal diameter, thickness of OPLL, and length of OPLL. All patients were divided into 2 different groups, the mild myelopathy group (JOA score ≥18, n = 11) and the severe myelopathy group (JOA score ≤17, n = 23). Results. The 3D OPLL volume did not correlate with clinical outcomes (r =-0.275, P = 0.116). 3D OPLL ratio and OPLL thickness had a significant negative relationship with JOA score (r =-0.502, P < 0.005 and r =-0.437, P < 0.05, respectively). In particular, 3D OPLL ratio was an independent risk factor for increased severity of myelopathy (B =-29.71, P < 0.05). The mild myelopathy group showed lower 3D OPLL ratio than the severe myelopathy group (0.092 vs. 0.148, P < 0.05). Conclusion. 3D method of measurement is superior to the conventional method in terms of evaluating the clinical state of symptomatic patients with OPLL. Higher 3D OPLL ratio has an adverse effect on the spinal cord.