Compressive Damage to the Deep Branch of the Lateral Plantar Nerve Associated with Lameness Caused by Proximal Suspensory Desmitis FERENC TO ´ TH, DVM, JIM SCHUMACHER, DVM, Diplomate ACVS, MICHAEL SCHRAMME, DVM, PhD, Diplomate ECVS, TROY HOLDER, DVM, H. STEVE ADAIR, DVM, MS, Diplomate ACVS, and ROBERT L. DONNELL, DVM, PhD, Diplomate ACVP Objective—To describe pathologic changes in the deep branch of the lateral plantar nerve (DBLPN) of horses determined to be lame because of proximal suspensory desmitis (PSD), and to report the outcome after treatment by excision of a segment of the horses’ DBLPN. Study Design—Retrospective case series. Animals—Adult horses (n ¼ 16). Methods—Horses determined to be lame on one or both pelvic limbs because of PSD were treated by excision of a segment of the DBLPN, and 30 nerves were examined histologically. Owners were contacted to obtain information about the horses  6 months after surgery. Results—Histologic changes suggestive of chronic nerve compression were identified in both nerves of 11 bilaterally lame horses and in the lame limb of 5 unilaterally lame horses. The nerve of the sound limb of 2 of 3 unilaterally lame horses that had bilateral nerve resection also had histologic changes compatible with nerve compression. Ten of 16 horses (62.5%) with follow-up information returned to soundness after excision of the DBLPN. Conclusions—Pathologic changes of the DBLPN associated with compression may complicate PSD of the pelvic limbs. Excision of the nerve may resolve lameness caused by PSD. Clinical Relevance—Horses lame because of PSD of the pelvic limb may remain lame after desmitis has resolved because of compression of the DBLPN. Excising a portion of this nerve may resolve lameness. r Copyright 2008 by The American College of Veterinary Surgeons INTRODUCTION I NFLAMMATION OF the proximal aspect of the interosseous medius muscle, or suspensory ligament (i.e. proximal suspensory desmitis, PSD) of the pelvic limb, is a common cause of equine lameness, 1 and is most commonly diagnosed in competition horses 4–10-year-old. 1,2 Lameness associated with PSD of the pelvic limbs may be either acute or insidious in onset. Horses that have excessively straight hocks or hyperextension of the metatarsophalangeal joints appear to be predisposed to developing PSD of the pelvic limbs. 3 The suspensory ligament of the pelvic limb is innervated by the lateral and medial plantar metatarsal nerves, which originate from the deep branch of the lateral plantar nerve (DBLPN), a branch of the lateral plantar nerve that ramifies from the tibial nerve. 4 The proximal aspect of the suspensory ligament, along with the distal- most segment of the DBLPN, is rigidly confined by the plantar aspect of the 3rd metatarsal bone, the axial borders of the 2nd and 4th metatarsal bones, and the plantar fascia covering the suspensory ligament. Diagnosis of PSD of the pelvic limbs is based on the results of clinical examination, perineural analgesia, ultrasonographic examination of structures on the Work was performed at the University of Tennessee and North Carolina State University. Address reprint requests to Ferenc To´ th, DVM, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, 2407 River Dr., Knoxville, TN 37996. E-mail: ftoth@utk.edu. Submitted September 2007; Accepted February 2008 From the Departments of Large Animal Clinical Sciences and Pathobiology, College of Veterinary Medicine, University of Tennessee, Knoxville, TN; and the Department of Clinical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC. r Copyright 2008 by The American College of Veterinary Surgeons 0161-3499/08 doi:10.1111/j.1532-950X.2008.00385.x 328 Veterinary Surgery 37:328–335, 2008