Flexional distance index: A new prognostic indicator of neurological outcomes at 4 years after cervical laminoplasty for K-line (+) ossification of the posterior longitudinal ligament.

OBJECTIVE We aimed to investigate whether K-line in the neck flexion (FK-line), flexion angle (FA), or flexion distance index (FDI) could predict the recovery rate of the Japanese Orthopedic Association score (RR-JOA) at 4 years after cervical laminoplasty (LP) for ossification of the posterior longitudinal ligament (OPLL). METHODS A new index, i.e., the FDI, which is based on the degree of neck flexion and the OPLL size on a lateral radiograph. "Flexional distance" is the distance from C2 to C7 in neck flexion, and "distance to OPLL" is the maximal distance from the line of the flexional distance to OPLL. FDI was defined as follows: FDI = flexional distance/distance to OPLL. Twenty-three patients with K-line (+) OPLL were evaluated at 4 years after LP (follow-up rate, 92%). We investigated the relationships between preoperative radiological factors, including FK-line, FA, and FDI, and RR-JOA at 4 years postoperatively. RESULTS Preoperative FK-line and FA were significantly related with the RR-JOA at 1 year postoperatively, but not at 4 years postoperatively. Preoperative FDI was significantly positively correlated with the RR-JOA at 1 year and 4 years postoperatively (P = 0.0132, r = 0.504 and P = 0.0183, r = 0.484, respectively). Preoperative FDI < 2.5 was associated with worsening of the RR-JOA at 4 years postoperatively, with a probability of 80% DISCUSSIONS: FDI could predict the RR-JOA at 4 years after LP for OPLL. Decompression with fusion may be recommended for patients with preoperative FDI < 2.5. LEVEL OF EVIDENCE 4.