PTH and ossification of the posterior longitudinal ligament of the spine: A basic experiment comparing the phenotype of ligament cells from normal subject or patients with OPLL revealed that the cells from OPLL showed the osteoblast-like phenotype in response to PTH, but cells from ligament without ossification did not show any osteoblastic properties, [23] suggesting that ligament cells of OPLL are responsive to PTH associating with bone formation.