SCIENTIFIC ARTICLE Desensitizing the Posterior Interosseous Nerve Alters Wrist Proprioceptive Reflexes Elisabet Hagert, MD, PhD, Jonas K. E. Persson, MD, PhD Purpose The presence of wrist proprioceptive reflexes after stimulation of the dorsal scapholunate interosseous ligament has previously been described. Because this ligament is primarily innervated by the posterior interosseous nerve (PIN) we hypothesized altered ligamento-muscular reflex patterns following desensitization of the PIN. Methods Eight volunteers (3 women, 5 men; mean age, 26 y; range 21–28 y) participated in the study. In the first study on wrist proprioceptive reflexes (study 1), the scapholunate interosseous ligament was stimulated through a fine-wire electrode with 4 1-ms bipolar pulses at 200 Hz, 30 times consecutively, while EMG activity was recorded from the extensor carpi radialis brevis, extensor carpi ulnaris, flexor carpi radialis, and flexor carpi ulnaris, with the wrist in extension, flexion, radial deviation, and ulnar deviation. After completion of study 1, the PIN was anesthetized in the radial aspect of the fourth extensor compartment using 2-mL lidocaine (10 mg/mL) infiltration anesthesia. Ten minutes after desensitization, the experiment was repeated as in study 1. The average EMG results from the 30 consecutive stimulations were rectified and analyzed using Student’s t-test. Statisti- cally significant changes in EMG amplitude were plotted along time lines so that the results of study 1 and 2 could be compared. Results Dramatic alterations in reflex patterns were observed in wrist flexion, radial deviation, and ulnar deviation following desensitization of the PIN, with an average of 72% reduction in excitatory reactions. In ulnar deviation, the inhibitory reactions of the extensor carpi ulnaris were entirely eliminated. In wrist extension, no differences in the reflex patterns were observed. Conclusions Wrist proprioception through the scapholunate ligament in flexion, radial deviation, and ulnar deviation depends on an intact PIN function. The unchanged reflex patterns in wrist extension suggest an alternate proprioceptive pathway for this position. Routine excision of the PIN during wrist surgical procedures should be avoided, as it alters the proprioceptive function of the wrist. Type of study/level of evidence Therapeutic IV. (J Hand Surg 2010;35A:1059 –1066. Copyright © 2010 by the American Society for Surgery of the Hand. All rights reserved.) Key words Electromyography, ligaments, posterior interosseous nerve, proprioception, wrist. T he complexity and intricacy of wrist stability is well recognized among hand surgeons. To en- hance our understanding of the elaborate kine- matics and kinetics of the carpus, a multitude of scien- tific studies have been conducted, and more than 1500 publications are presently available on PubMed related to the biomechanics of the wrist joint. Recently, another facet of wrist stability has begun to be explored—that of From the Karolinska Institutet, Department of Molecular Medicine and Surgery, Section of Orthopaedics; and Karolinska Institutet, Department of Clinical Neuroscience, Section of Neurophysiology, Stockholm, Sweden. The authors wish to thank the Department of Hand Surgery, Stockholm Soder Hospital, Stockholm, Sweden, for providing the opportunity to conduct the experiment in their facilities. The authors also wish to thank Mr. Michael Gehlen for invaluable help with extraction of data from the KeyPoint and Hans Pettersson, PhD, for assistance with concerns regarding statistics. Received for publication November 30, 2009; accepted in revised form March 15, 2010. Support for this study was provided by the Karolinska Institutet, Stockholm, Sweden. No benefits in any form have been received or will be received related directly or indirectly to the subject of this article. Corresponding author: Elisabet Hagert, MD, PhD, Hand & Foot Reconstructive Surgery Center, Storängsv. 10, 11542 Stockholm, Sweden; e-mail: elisabet.hagert@ki.se. 0363-5023/10/35A07-0002$36.00/0 doi:10.1016/j.jhsa.2010.03.031 ©  ASSH  Published by Elsevier, Inc. All rights reserved.  1059