Figure 4.

(A) Needle is starting to impinge but not perforate vessel. (B) Needle has transfixed the vessel on to bone but has not perforated the vessel. Aspiration here will render a negative result. The hand movement required during aspiration may reposition the needle into the vessel, or the increased resistance of the bone with subsequent injection may allow vessel perforation. (C) Vessel is perforated. The operator, unaware and reassured by the negative aspiration, may proceed to injection of bolus. (D) Even if the vessel remains only partially transfixed to the periosteum, the vertical height of the bevel of the needle and the pressure differential between the resistance of the bone to forward injection pressure and the pierced vessel may see the bolus delivered largely in the vessel as the route of least resistance.