INTRODUCTION: Flexor digitorum profundus (FDP) muscle is a muscle in the forearm that flexes distal phalanges of 2nd to 5th digits. Since FDP is innervated by anterior interosseous nerve and ulnar nerve, needle electromyography of this muscle is useful for evaluation of proximal median neuropathy, anterior interosseous neuropathy or ulnar neuropathy. Ulnar approach is used for the needle electromygraphic examination of the FDP3 and 4, but the test method for FDP 1 and 2 is not exactly known. Since median nerve and ulnar artery are located between FDP and flexor digitorum superficialis (FDS), needle insertion for needle electromyography of the FDP muscle requires more caution. The aim of this study is to provide an accurate and safe approach to needle insertion site for flexor digitorum profundus 1 and 2, through identifying anatomical relationship between palmaris longus tendon, FDP muscle and the neurovascular bundle using ultrasonography.
Methods: Sixty upper limbs of 30 healthy subjects were evaluated with ultrasonography. Ultrasonographic examination of FDP 1 & 2 was transversely performed on the anterior surface of the forearm at the junction of the middle and distal third between the medial epicondyle and ulnar styloid process. The distances and angles from the palmaris longus tendon (PL) to the FDP 1 & 2 and median nerve (MN), were measured (Figure 1): Angle_MN, angle between lateral volar surface of forearm and median nerve; Angle_FDP1M, angle between horizontal volar surface of forearm and medial margin of FDP1; PL_FDP1S, distance between PL and upper margin of middle FDP1; PL_FDP1D: distance between PL and lower margin of middle FDP1; PL_FDP2S, vertical distance from PL to upper margin of FDP2; PL_FDP2D, vertical distance from PL to lower margin of FDP2. The middle angle between FDP1M and MN (Angle_mid_FDP1) was calculated using Angle_MN and Angle_FDP1M. The middle angle between FDP1M and FDP2M (Angle_mid_FDP2) was calculated using Angle_FDP1M and Angle_FDP2M.
Results: Median Angle_MN, Angle_FDP1M, Angle_FDP2M, Angle_UA, Angle_mid_FDP1, and Angle_mid_FDP2 was 45.4 (30.3-61.2), 77.0 (49.9-104.2), 124.6 (90-151.6), 132.3 (11.4-158.6), 62.0 (43.4-81.1), and 100.6 (70.0-125.4), respectively. PL_FDP1S and PL_FDP1D was 13. 3 (6.9-22.7) and 20.9 (13.7-35.5), respectively. PL_FDP2S and PL_FDP2D was 10.4 (4.6-19.0) and 18.4 (10.1-29.6), respectively. Considering maximal Angle_MN was 61.2 degrees, the risk rate to the median nerve during needle electromyographic examination of FDP1 was zero, and the accuracy of FDP1 was 91.7%. The accuracy of FDP2 was 100%, and the risk rate to the ulnar artery was zero.
Conclusion: The medial approach to palmaris longus tendon for the needle electromyographic examination of FDP 1 and 2 is useful and safe. The safe range for needle insertion of FDP 1 using PL was from 62 to 77.1 degrees. The safe range for FDP 2 was from 90 to 100.3 degrees.
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