Title: Nerve Stimulator Assisted Posterior Tibial Nerve Block Improves Latency and Success Of Complete Sensory Block.
Benjamin Newell, M.D., Robert Doty, Jr, M.D., Mark Kendall, M.D., Radha Sukhani, M.D., Robert J. McCarthy, Pharm. D.
Northwestern University Feinberg School of Medicine, Chicago, Il

Introduction: The posterior tibial nerve (PTN) is a mixed sensory/motor nerve that innervates the skeletal structures of the mid and forefoot as well as supplying cutaneous innervations to the majority of the plantar area of the foot.1 Traditionally, PTN block is performed for mid and fore foot surgery in the para-medial malleolar area without nerve stimulator assistance.2 The PTN can also be blocked proximally (7cm above the medial malleolus) in the sub-fascial plane between the flexor hallucis longus and flexor digitorum
longus tendons.1 The purpose of this prospective randomized study was to compare the frequency of successful PTN at the distal traditional site with and without peripheral nerve stimulator (PNS) assistance and to compare the onset of surgical anesthesia at the distal and proximal block sites.
Methods: After IRB approval and written informed consent, ASA I-III patients having PTN block for foot surgery participated. Subjects were randomized (sealed envelops) to one of three groups; proximal site with PNS (n=31), distal site with PNS (n=32), distal site without PNS (n=30). The distal block site was midpoint between the tip of the medial malleolus and the medial border of the Achilles tendon (or posterior to the posterior tibial artery if palpable) and 2 cm proximal to the tip of the medial malleolus. The proximal site was 7 cm above the tip of the medial malleolus, slightly anterior to the medial border of the Achilles
tendon. For PNS assisted blocks, a 50mm 22g insulated stimulating needle was connected to a nerve stimulator set at 1.0 mA, 0.1 msec, and 2 Hz. The evoked motor response end point for local anesthetic injection was planter flexion of the 2nd – 5th toes, and/or, adduction or abduction of the great toe obtained at  0.5 mA. For the non-PNS approach, the needle was advanced in the same direction until bone contact and then withdrawn 1-2 mm. Levobupivacaine 0.625%, 0.15 ml/kg (maximum 15 ml) was used for all blocks. An independent investigator graded the evolution of sensory block every 2 min for 10 min, and then every 5 min for 30 min post-injection in the distribution of the medial plantar, lateral plantar, and medial calcaneal nerves as: 0 = normal sensation, 1 = analgesia (dull pinprick felt) and 2 = anesthesia (pinprick not felt). PTN block was considered successful if the patient had surgical anesthesia (absence of sensation) in the distribution of the PTN. The frequency of successful PTN block was compared among groups using a 2 statistic. Kaplan Meier curves and the log-rank test were used to compare time to complete block.
P<0.05 was required to reject the null hypothesis.
Results: Patient characteristics (weight, body mass index, gender) were similar among groups. The frequency of successful PTN block was greater with nerve stimulator guidance (distal 100%, proximal 93.5%), compared to the traditional distal method (73.3%) (P=0.02). Median latency to complete block (fig) was less for the distal site with PNS (8 min 95% CI - 7 to 9 min) compared to the distal site without PNS (25 min, 95% CI - 16 to 33 min) (P<0.01). Median latency of complete block at the proximal site (10 min, 95% CI – 5 to 14 min) was greater than the distal site with PNS (P=0.01), but less that the distal site without PNS (P=0.02).
Conclusion: The important finding of this study is that nerve stimulator guided distal PTN block improves the success and decreases the latency to onset of complete sensory block. In addition our data suggests that the proximal approach to PTN may be a useful alternative to the traditional distal approach. This method may be particularly useful in patient with restricted access to the distal site.
References:
1. Sarrafian. Anatomy of the Foot and Ankle: Descriptive, Topographic, Functional (2nd edition), Williams & Wilkins 1993.
2. Brown, DL. Regional Anesthesia and Analgesia, WB Saunders, Philadelphia, 1996

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