Magnetic resonance imaging of the distribution of local anesthetic during the three-in-one block

The three-in-one technique of simultaneously blocking the femoral, the late ral femoral cutaneous (LFC), and the obturator nerves by a single injection of a local anesthetic was first described in 1973, and it was suggested th at the underlying mechanism was one of cephalad spread resulting in a block ade of the lumbar plexus. Today, the technique is widely used in surgery an d pain management of the lower limb. Many investigators have, however, repo rted suboptimal analgesia levels, particularly in the obturator nerve. The purpose of this prospective study was to trace the distribution of a local anesthetic during a three-in-one block by means of magnetic resonance imagi ng (MRI). Seven patients scheduled for surgery of the lower limb were analy zed with the aid of a primary MRI and then received three-in-one blocks usi ng 30 mL of bupivacaine 0.5% under the guidance of a nerve stimulator. A se condary MRI was performed to determine the distribution pattern of the loca l anesthetic. It emerged that the local anesthetic blocks the femoral nerve directly, the LFC nerve through lateral spread, and the anterior branch of the obturator nerve by slightly spreading in a medial direction. No involv ement of the proximal and posterior portions of the obturator nerve was obs erved, nor was there any cephalad spread that could have resulted in a lumb ar plexus blockade. We therefore conclude that the basis of the three-in-on e block is confined to lateral, medial, and caudal spread of the local anes thetic, which effectively blocks the femoral and LFC nerves, as well as the distal anterior branch of the obturator nerve. Implications: We demonstrat e by using magnetic resonance imaging that the mechanism of a three-in-one block is one of lateral, caudal, and slight medial spread of a local anesth etic with subsequent blockade of the femoral, the lateral femoral cutaneous , and the anterior branch of the obturator nerves. It does not involve ceph alad spread of the local anesthetic with blockade of the lumbar plexus.