A retention device includes a one-piece longitudinal anchor member having opposed first and second ends, a nut retaining part formed between the first and second ends, a through hole extending through the nut retaining part, and an engagement surface around the through hole. A screw rod extends through the through hole and engages threadedly a nut, and is turnable between folded and unfolded positions, in which the screw rod is moved toward and away from the first end, respectively. The nut is movable within the through hole between engaging and non-engaging positions. The nut engages and disengages the engagement surface in the engaging and non-engagement positions when the screw rod is in the unfolded and folded positions, respectively.