CHAPTER 106 Vertebral Facet Joint Syndrome
In horses, as in humans, a complex of inflammatory processes in and around vertebral facet joints can lead to a painful condition known as facet joint syndrome. The initiating incident in this syndrome is damage to the facet joint, primarily to the joint capsule and adjacent connective tissue and sometimes the cartilage or subchondral bone. In horses this primary incident often involves traumatic injury, when the horse slips, falls, flips over, gets cast, or injures itself in some other way.
Because of the rich innervation of the facet joint tissues, the inflammatory reaction caused by the primary trauma starts a cascade of reactions, which is more obvious than in the joints of the limbs. One of the first reactions in a horse is stabilization of the vertebral column by muscular contraction and spasm of the epaxial and subaxial muscles. This response can be limited to the segment that is related to the injured vertebral facet joint, but a larger portion of the vertebral column can be involved. This contraction can be short, hours to days, or it can last weeks or even months. It is very likely that the prolonged contraction or spasm is painful and partially immobilizes the vertebral column.
As a consequence of this immobilization, the facet joint is more or less fixed in one position, which makes regaining later mobility a painful and possible joint-damaging experience. Because of the muscle spasm, no normal sequence of contraction-relaxation occurs in the muscle, and its normal supportive function is less effective. Consequently, when excessive force is applied to the vertebral column again, repetitive injury to that facet joint can result.
Because of the contribution of the vertebral column to the normal gait pattern in the horse, as described in Chapter 104, Evaluation of Back Pain by Clinical Examination, horses with vertebral facet joint syndrome are often presented for lameness. In acute cases the pain in and around the facet joints can be so severe that the horse does not want to move at all and just stands with the hind feet parked out behind the hindquarters and the back lowered, a position that closes the facet joints in the thoracic and lumbar portions of the vertebral column, causing less tension in the joint capsules. The muscles of the back and hindquarters are contracted. This posture can be difficult to differentiate from tying-up syndrome; however, with vertebral facet joint syndrome serum activities of creatine kinase (CK), lactic dehydrogenase (LDH), and aspartate aminotransferase (AST) are within normal limits or only slightly elevated.
Horses with severe pain in the thoracic or lumbar vertebral column segments may also have signs that resemble colic, with restlessness, pawing, looking back, or standing with the hind limbs parked behind the pelvis, similar to urination posture. The heart rate can be as high as 60 to 80 beats per minute in those horses, but an examination for colic will be unremarkable.
In more chronic cases, locomotion can be altered and the horse may have a stiff back. At the trot, the propulsion of the hind limbs may be reduced unilaterally or even bilaterally. Because of the attachment of the forelimbs with ligaments, tendons, and muscles to the thoracic vertebrae, motion of the forelimbs also affects the position of the thoracic facet joints. At the trot, because of the diagonal gait, the thorax moves between the scapulae, with a maximum extent of motion in the dorsal processes of the withers of 1 to 3 cm to left and right. Pain in the facet joints of the thorax thus can lead to less mobility in the thoracic segment of the vertebral column and an altered gait pattern at the trot, best described as stiff or shortened.
During the canter, the thorax moves less, except during downward transitions, when there is more loading of the facet joints. This also occurs after a jump or when the horse goes downhill. Therefore, when thoracic facet joints are involved, signs mentioned by the owner or rider can include being stiff, reluctance to go downhill or jump, and refusal or difficulty in doing the extended trot. The trainer or rider may note a preference of the horse to go left or right after a jump, which is quite often thought to be related to an injury to the distal limb but also can be caused by facet joint problems.
Because lateral bending is a prominent mobility feature of the thoracic vertebrae, lateral bending of the horse may be reduced when thoracic vertebrae are involved with facet joint syndrome. This can be observed during circles and serpentines and during lateral dressage gaits such as shoulder-in, travers, and half-pass, as well as in turns made in barrel racing, polo, and eventing, or between fences when jumping. Tightening the girth or putting the saddle on also can reveal an adverse reaction from the horse because these actions load thoracic facet joints.
When lumbar facet joins are involved in facet joint syndrome, the most affected gait is the canter because in canter the dorsoventral flexion of the lumbar part of the vertebral column is most pronounced. Also lateral gaits such as shoulder-in, travers, and half-pass can show alterations when lumbar facet joints are involved because rotation in the lumbosacral region of the column makes an important contribution to these gaits. In racing, dorsoventral flexion of the lumbar segment of the column is a prominent contribution to the propulsion phase of the hind limbs, and loss of performance may be caused by lumbar facet joint syndrome.