The operation known by the above terms is undertaken with the object of depriving some part of an animal of sensation when affected with an incurable disease. It is resorted to in cases of navicular disease, of side-bone, ring-bone, and other forms of lameness of a chronic and painful character, more especially in the region of the feet.
Unnerving, although always effective in depriving the part of sensation, is attended with a varying measure of success, largely dependent upon the judgment of the surgeon. If undertaken upon a subject of navicular disease, for instance, it is important that the foot shall have sufficient strength of horn to receive the nails, without risk of binding or pricking in the course of shoeing. When deprived of sensation, injuries of this kind are not recognized by the horse, and he continues to use the diseased and unfeeling foot without that care which he would otherwise take of it, and without showing any signs of lameness. The presence of corns, which may fester in a foot deprived of sensation, is another element of danger, since in the absence of pain and lameness it is liable to pass unnoticed, until irreparable mischief is wrought. It is, however, an error to suppose that reparative power is lost when the purely sensory nerves are divided. If a pricked foot or one with a festered corn is detected before serious inflammatory changes have taken place, recovery may be looked for, if judiciously treated.
Neurectomy, if performed on suitable subjects, is undoubtedly a valuable operation, and may add two or three years or more to the usefulness of an animal.
The operation is distinguished topographically as the high operation and the low operation. These terms are somewhat confusing to the beginner, as there is another neurectomy frequently performed, but adopted much later. It is called median, and is the highest on the limb, the median nerve being situated on the inner aspect of the forearm as shown at Fig. 502. For the present the old nomenclature must be retained. The high operation consists in division and removal of a portion of the trunk of the plantar nerve immediately above the fetlock. Here it runs a little to the front of the border of the back tendons (fig. 502), and, being immediately beneath the skin, affords the surgeon facilities for division without the risk of injuring other structures. It is rather more superficially placed on the outside, and as its bifurcation takes place somewhat lower down than on the inner side, the incision in the skin may be made a little farther down the limb in the former than the latter. It is usual to divide both nerves, though not always necessary, as, for instance, when side-bone exists on one aspect of the pastern only. Moreover, by leaving the nerve of one side intact, the foot will continue to enjoy a certain amount of feeling, which will not only give security to the animal's movements, but ensure some degree of caution in the use of the limb, which will be to the advantage of the neurotomized half of the foot.
It is necessary to cast the animal prior to operation, and if total anaesthesia is to be produced, a period of some fifteen hours' fasting is recommended; indeed, lasting by way of preparation for throwing is by many veterinary surgeons considered a necessary precaution against risk of rupturing some of the abdominal organs, especially the stomach and bowels.
Fig. 502. - Tenotomy and Neurectomy. Localities of the various operations.
1, Median neurectomy. 2, Neurectomy of the ulnar nerve. 3, Tenotomy of deep flexor tendon. 4, High plantar neurectomy. 5, Low plantar neurectomy. 6, Cunean tenotomy.
If, immediately the horse is cast, the precaution is taken to apply a powerful india-rubber band (with hook-and-eye) to act as a tourniquet, bleeding from the wound is prevented, and the operator has no difficulty in dissecting the nerve from its surroundings.
The site of the operation is closely clipped, and washed with soap and water, and afterwards dressed with an antiseptic, the same precautions being taken in respect to instruments and appliances to be used. A longitudinal incision about three-cmarters of an inch in length is then made with a sharp scalpel, and the underlying connective tissue divided until a clear view of the nerve is obtained. A blunt-pointed needle (fig. 503, and Plate LIII, fig. 2) with an eye in it is then passed under it. While so placed, the end of a piece of carbolized silk or gut is passed through the eye of the needle and drawn back under the nerve, which may now be raised from its bed and divided by a pair of blunt-pointed scissors. The chief difficulty attaching to this rather delicate operation is to recognize the nerve when exposed, and to distinguish between it and the artery which runs alongside of it. In old horses, the victims of many blisters, these two structures are somewhat firmly attached together, and require careful dissection.
Three - quarters of an inch of the nerve trunk must now be removed from that portion connected with the foot. Then the wound must be irrigated with an antiseptic, and covered with a pad of cotton-wool supported by a clean linen bandage, and subsequently treated by the ordinary antiseptic method.
Many modern practitioners who are adept at this operation make a transverse incision, and cut down upon the nerve with very little disturbance of the adjacent tissues. The nerve is divided by means of a combined needle and knife (fig. 504). In order to remove a sufficient length of it, through an aperture no more than half an inch long (fig. 505), the insensitive end that has just been divided is picked up by strong pliers, and pulled out until sufficient is exposed for removal, as above advised.
Fig. 503. - Neurotomy Needle.
Fig. 504. - Neurotomy Needle and Knife combined.
Fig. 505. - High Plantar Neurectomy by Transverse Incision.
Next in frequency of performance is the low operation, in which the posterior branch of the plantar nerve (fig. 502) is divided as it passes along beside the perforans tendon, midway between the fetlock and the coronet. The mode of procedure is the same as that described for the higher neurectomy. It is sometimes resorted to in navicular and other diseases of the posterior part of the foot, but it is not so uniformly successful as when the main trunk is divided above the fetlock. It has the advantage, however, of leaving a certain degree of sensibility in the front part of the foot, which greatly adds to the safety of the animal's movements. Dealers in unsound horses are much in favour of the low operation, as the scar resulting from it is not readily seen, and the unwary are in this way imposed upon.
Fig. 506. - Low Plantar Neurectomy. Raising the digital nerve by an aneurism needle threaded with carbolized silk or gut.
Fig. 507. - Low Plantar Neurectomy. Severing the digital nerve held out by carbolized silk or gut.
Median neurectomy (fig. 508) is sometimes performed for the relief of lameness affecting some part of the limb from the fetlock upwards, such as that arising from "knee splints" and other ossific depositions which have not yielded to ordinary measures of treatment. Division of the ulnar branch (fig. 509) is also practised for removing sensibility of parts at the back of the knee.
Plate LII. NEURECTOMY: THE HIGH PLANTAR OPERATION.
1. Making the Incision. 2. Needle passed below the lateral digital nerve and threaded with carbolized silk or gut. 3. Severing the Nerve. 4. Removing portion of the Nerve. 5. The Operation completed.
Horses which have been unnerved, although free from lameness, usually give some indication of the fact. When the hand is passed over the site of operation the foot is sharply raised as though the animal had been asked to hold it up, or the horse flinches on the application of slight pressure over the nerve end, which always remains sore for some time after division. In many cases a nodule of more or less hard material forms on the end of the upper division of the nerve, which can be felt on either side of the leg where the incision was made. When these exist they form serious ground of suspicion as to neurectomy having been performed, and all that is needed to settle the point is a few pin-pricks over the pastern. If the nerves have been divided there will be no snatching up of the foot, as occurs when sensibility of the skin is intact.