INTRODUCTION: Hypertrophic change of ligamentum flavum (LF) is one of the major causes of canal stenosis in the lumbar spine. However, the mechanism of ligament hypertrophy has not been well elucidated. The purpose of this study was to clarify the mechanism of hypertrophic change of LF by histopathological examination and their relationship with clinical feature such as instability.
METHODS: One hundred and thirty two LF specimens surgically harvested from patients with degenerative lumbar canal stenosis (DLCS) and 14 LF specimens from lumbar disc herniation (LDH) used as control were histologically analyzed. Immunohistochemical staining of bone morphogenetic proteins (BMPs) and their receptors were also performed using avidin‐ Biotin complex (ABC) method or fluorescent method for double staining with cell markers. Clinical data of each patient was reviewed from patients' clinical records. Thickness of LF was measured on axial view of MRI with computer software, and mechanical instability was evaluated on dynamic X‐ray.
RESULT: Histological examination of degenerated LF showed ruptured elastic fibers with irregular arrangement, reduced elastic component and increased collagen tissue. The cellularity was well correlated with thickness in MRI, and the number of cells in DLCS with instability was significantly higher than that in DLCS without instability. Immunohistochemistry revealed that the BMPs and their receptors were expressed in many kinds of cells even in center of flavum. The average percentage of BMP receptors positive cells was significantly higher in the DLCS with mechanical instability than that in without instability and those in LDH. Sox 9 and Msx2 co‐expressed some BMPRs positive cells.
DISCUSSION: Histological examination revealed that not only the degeneration of elastic fibers but increase of cell number, especially of chondrocytes, were important changes in hypertrophy of LF. The result of the present study suggested that BMP signaling and mechanical stress play an important role in these changes.