Luxation, or displacement without fracture of the bones forming a joint, is comparatively rare among cattle. It most frequently occurs in the stifle joint, where dislocation of the kneepan (patella) takes place. A glance at the skeleton ([Pl. XXV]) will show the relations better than they can be described. It will be observed that the small, irregularly shaped bone (patella) plays on the anterior rounded part of the lower edge of the thigh bone (femur) and between it and the upper end of the shank bone (tibia). The outer ridge on the lower end of the thigh bone is less prominent than the inner one, so that displacement, when it does take place, is by slipping outward. Such an accident may occur from direct injury or external force, as a blow, or from slipping. When it does occur the symptoms produced are somewhat alarming. The animal is unable to draw the leg forward, and either stands with it thrown back with the toe pointing downward, or, if it should succeed in getting its weight upon it, holds it firmly on the ground, fearing to move it. Examination of the outside of the joint will disclose the situation of the patella outside its proper place. If the operator is not familiar with the normal appearance of the joint, it is well to make a comparison between the injured and the sound one. If compelled to move, the animal does so with great difficulty, jerking the leg which it is unable to bring forward, hopping with the other, and partially dragging the injured one.
Treatment.—The treatment is simple. A rope 20 feet long should be applied around the fetlock of the affected leg, passed forward between the front legs and up over the opposite side of the neck, back over the withers, and wrapped once behind the elbow around that portion of the rope which passes between the front legs. The leg is then drawn away from the body and forcibly pushed forward by an assistant, while another person tightens up the slack in the rope until the affected leg is off the ground in front of the supporting leg. The rope is then drawn taut and the assistant grasps the tail and pulls the cow toward the affected side. The animal makes a lurch to keep from falling, contracts the muscles, and the patella slips into place with a sharp click, and the animal walks off as if nothing had happened. If the animal resists this method of handling, it may suffice to manipulate the dislocated kneepan by shoving it inward and forward with the heel of the hand while the affected leg is drawn well forward. Unless some precaution is taken the accident is liable to recur, as the ligaments have been stretched by the dislocation till they no longer hold the bone with that firmness necessary to retain it. The animal should be tied and the foot fastened forward, so that the patient can just stand on it comfortably, by means of a rope or strap around the fetlock carried forward between the front legs, around the neck, and tied on the breast.
Should this accident occur more than once it is a good practice to apply a blister around the joint, as in the formula recommended for sprain of shoulder, and observe the precautions as to restraint and subsequent treatment there recommended. With this one exception dislocations in the ox occurring independently of other complications are rare.
Dislocation with fracture may occur in any of the joints, and if one is suspected or discovered, examination should always be made for the other before treatment is applied. When a fracture occurs near a joint the force sufficient to rend the bone is liable to be partly exerted on the immediate tissues, and when the bone gives way the structures of the joints may be seriously injured. It occasionally happens that the injury to the joint becomes the most important complication in the treatment of a fracture. In order clearly to understand the reason for this a few words are necessary in relation to the structure of joints.
The different pieces constituting the skeleton of the animal body are united in such manner as to admit of more or less motion one upon another. In some of the more simple joints the bones fitting one into another are held together by the dense structures around them, admitting of very little or no movement at all, as the bones of the head. In other joints the bones are bound together by dense, cartilaginous structures, admitting of only limited motion, such as the union of the small bones at the back part of the knee and hock (metacarpal and metatarsal). In the more nearly perfect form of joint the power of motion becomes complete and the structures are more complex. The substance of the bone on its articular surface is not covered with periosteum, but is sheathed in a dense, thin layer of cartilage, shaped to fit the other surfaces with which it comes in contact (articular). This layer is thickest toward its center when covering bony eminences, and is elastic, of a pearly whiteness, and resisting, though soft enough to be easily cut. The bones forming an articulation are bound together by numerous ligaments attached to bony prominences. The whole joint is sealed in by a band or ribbonlike ligament (capsular ligament) extending around the joint and attached at the outer edge of the articular surface, uniting the bones and hermetically sealing the cavities of the articulation. This structure and the articular surface of the bone is covered by a thin, delicate membrane, known as the "synovial membrane," which secretes the joint oil (synovia). This fluid is viscid and colorless, or slightly yellow, and although it does not possess a large quantity of fat, its character somewhat resembles oil, and it serves the same purpose in lubricating the joints that oil does to the friction surface of an engine. Although the tissues of the joint when used in a natural way are able to withstand the effect of great exertion, when unnaturally used, as they are very delicate and complex, they are liable to inflammatory and other changes of a very serious nature. The synovial membrane, and in fact the whole structure of the joint, is susceptible to injury and serious inflammatory derangement, and the capsular ligament is liable to be distended from excessive secretion of synovia. The latter process may be almost noninflammatory, and attended with little inconvenience or importance other than a blemish to the animal, which in cattle is not serious. It may occur on the back part of the leg above the fetlock or on the inner and fore part of the hock, corresponding in its location to windgalls and bog spavin of the horse. Continuous support by bandages will generally force reabsorption, and as the limb is not subjected to violent action, as in the case of the horse, the affection is not so liable to recur.