Distal Conduction and Metacarpal or Transthecal Block

Distal conduction blocks at the shoulder, elbow, or wrist are only complementary blocks in pediatrics, and their indications result from the failure of a proximal brachial plexus block procedure, especially through the axillary route. These blocks are not usual in children and require fairly extensive experience. They have been fully described elsewhere.^[201] An easy technique is the metacarpal block, also called a transthecal block. Local anesthetic is injected within the flexor tendon sheath that surrounds the synovial sheath of the flexor tendon of each digit and within which run the four digital nerves supplying the relevant finger. The technique is suitable for any traumatic or elective surgery on the relevant digit.^{[228] [229] [230]} The block is performed with the hand supinated after identification by palpation of the head of the relevant metacarpal bone. An intradermic needle is inserted perpendicular to the skin, right in the center of the skin projection of this metacarpal head ( Fig. 45-10 ), until bone contact is made. To avoid subperiosteal injection, the needle is then slightly withdrawn before injecting 1 to 3 mL of local anesthetic (until resistance is felt). Within 2 minutes, all four digital nerves (two ventral and two dorsal) are fully anesthetized in more than 95% of patients.

This technique has many advantages over the classic digital ring block. It is safer because no terminal artery is close enough to cause complications; it is simpler because only a single injection is required; and it is less painful in conscious patients, even though the injection is not pain free. Caution should be taken in the case of infection to avoid bacterial contamination of the synovial sheath; in this case, the technique is contraindicated. The only real drawback of the technique is the possibility of puncturing the flexor tendon, which is unpleasant, even painful.