PURPOSE: There is no definitive evidence that additional instrumented fusion following laminoplasty suppresses the progression of ossification of the posterior longitudinal ligament (OPLL). Recently, we reported a novel method involving the creation of three-dimensional (3D) model from computed tomography images to measure the volume of OPLL accurately. The study aim was to evaluate whether laminoplasty with instrumented fusion suppresses the progression of OPLL in comparison with stand-alone laminoplasty by our novel 3D analysis. METHODS: The present study comprised of a group of 19 patients (14 men, five women) with OPLL treated with posterior decompression and fusion (PDF group), and a group of 22 patients (14 men, eight women) treated with laminoplasty alone (LP group). The volume of OPLL was evaluated three times during the follow-up period, and the volume change of OPLL was compared between the two groups. RESULTS: The PDF group (2.0 ± 1.7 %/year; range, -3.0 to 5.3) demonstrated lower annual rate of lesion increase compared to the LP group (7.5 ± 5.6 %/year; range, 1.0-19.2) (p < 0.001). In a notable thing, the annual rate of increase from the 2nd to the 3rd measurement significantly decreased compared with that from the 1st to the 2nd measurement in the PDF group (p < 0.05). CONCLUSION: This is the first study to prove a possible suppressant effect of additional posterior instrumented fusion on OPLL progression using novel 3D analysis.