Fig. 1

A graphic of the trajectories of the initial and second punctures. The solid line represents the initial trajectory of the puncture needle and the dotted line the trajectory of the second puncture. The intravertebral area in gray is the cement-filled cleft. a Planned puncture: The initial puncture trajectory lies along the border between the cleft and surrounding cancellous bone, and the needle tip lies in the cleft (near the surrounding bone) when the initial puncture is completed. When the initially injected cement is restricted to the cleft, a second puncture is made via the same pedicle entry point. b Central-clefted puncture: The pedicle entry point and trajectory are chosen at random. When the initial puncture is completed, the needle tip lies in the center of the cleft. Frequently, the second puncture fails because the path to the region lacking cement is blocked by the (hardened) initially injected cement (white arrow) or the lower border of the pedicle (black arrow)