Repetitive tensile stress to rat caudal vertebrae inducing cartilage formation in the spinal ligaments: a possible role of mechanical stress in the development of ossification of the spinal ligaments

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Object

Mechanical stress has been considered one of the important factors in ossification of the spinal ligaments. According to previous clinical and in vitro studies, the accumulation of tensile stress to these ligaments may be responsible for ligament ossification. To elucidate the relationship between such mechanical stress and the development of ossification of the spinal ligaments, the authors established an animal experimental model in which the in vivo response of the spinal ligaments to direct repetitive tensile loading could be observed.

Methods

The caudal vertebrae of adult Wistar rats were studied. After creating a novel stimulating apparatus, cyclic tensile force was loaded to rat caudal spinal ligaments at 10 N in 600 to 1800 cycles per day for up to 2 weeks. The morphological responses were then evaluated histologically and immunohistochemically.

After the loadings, ectopic cartilaginous formations surrounded by proliferating round cells were observed near the insertion of the spinal ligaments. Several areas of the cartilaginous tissue were accompanied by woven bone. Bone morphogenetic protein–2 expression was clearly observed in the cytoplasm of the proliferating round cells. The histological features of the rat spinal ligaments induced by the tensile loadings resembled those of spinal ligament ossification observed in humans.

Conclusions

The findings obtained in the present study strongly suggest that repetitive tensile stress to the spinal ligaments is one of the important causes of ligament ossification in the spine.

Abbreviations used in this paper: ALL = anterior longitudinal ligament; BMP = bone morphogenetic protein; OSL = ossification of the spinal ligament; PLL = posterior longitudinal ligament.

Article Information

Address reprint requests to: Takeshi Maeda, M.D., Department of Orthopaedic Surgery, Graduate School of Medical Sciences, Kyushu University, Maidashi 3–1–1, Higashiku, Fukuoka, 812–8582, Japan. email: maeken@ortho.med.kyushu-u.ac.jp.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    The stimulating apparatus. A: A schematic drawing of the apparatus. The moving parts, painted in light gray, consisted of a linear actuator, moving bed, and proximal immobilizer. The fixed parts, painted in dark gray, consisted of a distal immobilizer and a supporting plate with strain gauges. B: Photograph of the stimulating apparatus.

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    Anteroposterior radiograph of a rat after pin insertion into the eighth caudal vertebra (CV). The ellipse indicates the target caudal vertebrae from the second to the fifth caudal vertebra.

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    A radiograph of a control rat in a lateral view. The ellipse indicates the target caudal vertebrae from the second to the fifth caudal vertebra.

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    Representative photomicrographs showing the spinal ligaments of Group IV (A, B, and F), Group I (C and D), and Group III (E) rats. A: Ectopic cartilaginous tissue formation in the PLL (open triangles) at lower magnification. B: The PLL of the specimen in A at higher magnification. The proliferation of chondrocyte-like cells (arrows) and round cells (black arrowheads) were observed within the PLL. C: The PLL (open triangles) of a control rat at lower magnification. D: The PLL of the specimen in C at a higher magnification. E: The PLL with cell proliferation only in round cells (black arrowheads). F: The development of woven bone (arrows) in the ALL. A schematic caudal spine with arrowheads at the bottom of each photomicrograph indicates the number of the caudal vertebrae and the locations for the observation in the photomicrographs. H & E, original magnifications × 20 (A and C) and × 100 (B and D–F).

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    A schematic drawing of the target sites for a histological evaluation between the second and fifth caudal vertebrae. The frames indicate the six target sites for PLL and the six target sites for ALL, each of which contained a ligament insertion area.

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    Representative photomicrograph of the ectopic cartilaginous formation within the PLL of a Group IV rat. A schematic caudal spine with an arrowhead at the bottom of the photomicrograph indicates the number of the caudal vertebrae and the location for the observation in the photomicrograph. Safranin O, original magnification × 200.

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    Representative photomicrographs of the expression of S100 protein in the mature chondrocyte-like cells in the PLL of a Group III rat (A) and in the PLL of a control rat (B). Original magnification × 400.

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    Representative photomicrographs of the expression of Sox9 in the mature chondrocyte-like cells (arrows) in the PLL of a Group III rat (A) and in the PLL of a control rat (B). Original magnification × 400.

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    Representative photomicrographs of the expression of BMP-2 in the proliferated cells (arrows) adjacent to cartilaginous tissue formation (arrowheads) in the PLL of a Group III rat (A) and in the PLL of a control rat (B). Original magnification × 400.

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