33rd Annual Scientific Meeting proceedings

Palmar digital neurectomy is one of the oldest described equine surgical techniques, described in the Grand Marèchal, Expert et Francais published in Lyon in 1701. The operation became much less widely used towards the end of the last century, as the complications came to be viewed as unjustified. Ironically, the advent of the latest technology, MRI, resulted in a resurgence of interest in the procedure. It became apparent the prognosis with some conditions, notably deep digital flexor tendonitis, was poor and therefore there were limited other therapeutic options.

Technique
Many techniques have been described for palmar digital neurectomy. Cryotherapy received some attention in the 80’s. It was shown that percutaneous cryotherapy was relatively ineffective [1]. It has been described that embedding the transected nerves in bone, such as in a drill hole, can reduce the incidence of neuroma. A low rate of neuroma formation and post operative lameness has been reported in two case series [2]. Use of a laser to transect the nerve did not appear beneficial in one study [3]. Neurotoxins, to reduce neuroma formation, were unsuccessful [4]. The guillotine technique has remained most popular. A modification of this, the pull through, involves dissection of the nerve in two small incisions proximal and distal in the pastern region. The nerve is then transected distally, pulled through from the distal incision to the proximal, and transected proximally [5]. One series reported very good success rates, while another study reported no statistical difference between guillotine and pull through technique results. However, this study was not highly powered (25 and 15 in each group), and results were generally better for the guillotine technique, other than an initially higher success with pull through [6]. It can be concluded that no studies so far have been sufficiently powered to provide conclusive evidence that any technique is superior.

Prognosis
The published results of neurectomy are surprisingly consistent over the years. In 1888 22/35 (63%) horses were described as successful and a cost benefit analysis is included [7]. In 1993, 40 of 57 (70%) horses were considered successful, with four painful neuroma and 14 recurrence of lameness [8]. In 2014 40/50 (80%) of horses returned to athletic use at the same level or which 32 (64%) were still sound and performing well after one year [9]. This study also provides excellent “survival” data and provides clear evidence that linear and core lesions of the deep digital flexor tendon are poor prognostic indicators. One study using the pull through technique showed outstanding results, 37 of 41 (88%) horses sound at one year follow up with an estimated mean time to lameness of 4.14 +/- 0.33 years [5]

Complications
Regrowth of the palmar digital nerve is widely described as a complication. There has not been a published dissection of a regenerated nerve, and the reports do not appear to consider proximal progression of the disease as another explanation of recurrent lameness. The procedure changed from a neurotomy to neurectomy – removing a section of nerve - at the end of the nineteenth century.

Painful neuroma are commonly cited as the most common complication and cause of recurrent lameness [8]. A scoring system for histological evaluation has been developed [1]. As above, the precise cause of post operative lameness is seldom established.

Rupture of the deep digital flexor tendon is the most serious complication [10]. This condition can be associated with multiple, progressively more proximal neurectomy procedures [8]. It has also been described following repeated corticosteroid medication of the navicular bursa [11].

Much the most common complication is persistent or recurrent lameness, which may or may not be associated with cutaneous sensation around the coronary band. The duration of soundness following the procedure is typically in the region of two years [9,10]

Regulation
Due to the potential danger to a rider on a horse with “numb” feet, many organisations ban use of horses following neurectomy. The International Federation for Equestrian Sports (FEI) is unequivocal: horses are not eligible to compete when … part of a limb is hyposensitive... Hyposensitive limbs include … a neurectomy. In the UK there has been controversy recently as the FEI rules have been extended to domestic organisations such as British Dressage. In France, a knowledge certificate makes recording mandatory.  In Denmark there is national legislation: “Any type of … treatment … which aims to hide disease symptoms so that the horse can train and participate in competitions, is not permitted”.

Conclusion
The literature we have does establish a success rate, of approximately 80%, and a duration of action of approximately two years. There is one clear conclusion that linear and core lesions of the deep digital flexor tendon have a poor prognosis. The best results reported are with the pull through procedure.

For such a commonly performed procedure, the gaps in the literature are considerable. There is minimal discussion of the appropriate proximo-distal level, nor of the importance or otherwise of the dorsal digital nerves. Similarly there is no investigation of the significance of skin sensation after surgery, nor of change in skin sensation over time. Finally, there is no report or investigation of any rider or horse fall following neurectomy.

References

1. Schneider, R.K., Mayhew, I.G. and Clarke, G.L. (1985) Effects of cryotherapy on the palmar and plantar digital nerves in the horse. Am J Vet Res 46, 7–12.
2. Walmsley, J.P. (2006) Palmar digital neurectomy with intra-osseous nerve transposition: technique and results.  Proceedings of the European Association of Veterinary Orthopaedics and Traumatology 13
3. Dabareiner, R.M., White, N.A. and Sullins, K.E. (1997) Comparison of Current Techniques for Palmar Digital Neurectomy in Horses. Proceedings of the American Association of Equine Practitioners 43
4. Fubini, S.L., Cummings, J.F. and Todhunter, R.J. (1988) The Use of Intraneural Doxorubicin in Association with Palmar Digital Neurectomy in 28 Horses. Vet Surgery 17, 346–349.
5. Maher, O., Davis, D.M., Drake, C., Myhre, G.D., Labbe, K.M., Han, J.H. and Lejeune, S.S. (2008) Pull-Through Technique for Palmar Digital Neurectomy: Forty-One Horses (1998-2004): PALMAR DIGITAL NEURECTOMY IN HORSES. Veterinary Surgery 37, 87–93.
6. Oosterlinck, M., Pille, F., Lubbers, C., Haspeslagh, M. and Martens, A. (2020) Guillotine versus pull‐through technique for palmar digital neurectomy: A retrospective study on 40 horses. Equine Vet Educ 32, 37–41.
7. Reekie, W. (1888) Plantar Neurotomy. Journal of Comparative Pathology and Therapeutics 1, 329–332.
8. Jackman, B.R., Baxter, G.M., Doran, R.E., Allen, D. and Parks, A.H. (1993) Palmar Digital Neurectomy in Horses 57 Cases (1984–1990). Vet Surgery 22, 285–288.
9. Gutierrez-Nibeyro, S.D., Werpy, N.M., White, N.A., Mitchell, M.A., Edwards, R.B., Mitchell, R.D., Gold, S.J. and Allen, A.K. (2015) Outcome of palmar/plantar digital neurectomy in horses with foot pain evaluated with magnetic resonance imaging: 50 cases (2005-2011): Palmar/plantar digital neurectomy outcome. Equine Vet J 47, 160–164.
10. Wylie, C.E., Payne, R.J., Bathe, A.P., Greet, T.R.C., Head, M.J., Boys-Smith, S.J. and Powell, S.E. (2015) An Epidemiological Investigation of the Aid of Magnetic Resonance Imaging in Determining Long-Term Prognosis for Soundness Following Palmar/Plantar Digital Neurectomy for Chronic Foot Pain: Abstracts. Equine Vet J 47, 15–16.
11. Dabareiner, R.M., Carter, G.K. and Honnas, C.M. (2003) Injection of corticosteroids, hyaluronate, and amikacin into the navicular bursa in horses with signs of navicular area pain unresponsive to other treatments: 25 cases (1999-2002). Journal of the American Veterinary Medical Association 223, 1469–1474.