| 摘要 |
The center of the present invention comprises a body having a lower end (3) and an upper end (5), comprising between both lower and upper ends a wall with apertures (9). The configuration of the wall and the apertures is such as to allow a radial contraction of the wall when the center is axially compressed. There are areas liable to deformation in at least the neighbouring portion of the lower end (3) that facilitate the axial contraction of the wall (7), which prevents the lower end (3) from being radially outwardly deformed.
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