Swayback, lordosis, appears in three forms in horses. It may be present in the newborn foal, develop in young animals during the first year of life, or appear in old horses at any time after fifteen or so years of age. The old age form is more common in brood mares and, particularly, multiparous broodmares. The new born and juvenile forms occur in both colts and fillies.
The newborn form is a component of neonatal contracture, the so-called contracted foal syndrome. This form is more commonly a scoliosis or a combination of scoliosis with a small degree of lordosis. Scoliosis is a lateral or sideways bending of the spine; that is, it is at about a right angle to lordosis. This condition is discussed at length in the two references on this site and will not be considered further here.
At postmortem examination there are no specific lesions to be seen in the horse with old age lordosis. Weight bearing over the years and multiple foals gradually stretch the ligamentous tissues of the vertebral column, allowing the column to sag into the swaybacked position.
Very few cases of juvenile lordosis have been examined at postmortem, and, as is so often the case, understanding of the disease process is hampered by lack of pathological data. In those cases which have been examined, however, there is a clear-cut and distinct anatomical reason for the lordosis. I emphasize anatomical because even though the lesion which causes the lordosis is evident, as well see next, the cause of that lesion is not known.
The lesion is a complete or partial failure of development of the articular processes of two or more vertebrae. Fig. 1 is a schematic drawing of a head-on (or hind-on) view of a typical vertebra. (2)
Fig. 1: Schematic end-on view of a typical thoracic vertebrae.
There are three articulations between each pair of vertebrae: two intervertebral articulations - formed by the articular processes - and the intervertebral disk. The failure of development of the intervertebral articulations is aplasia when complete and hypoplasia when incomplete and usually occurs near or at the 12th and 13th thoracic vertebrae.
Fig. 2: Schematic of vertebra with hypoplasia of the articular facet to the left and aplasia to the right.
With two of the three articular linkages missing or too small, the vertebral column is unstable, and the body weight acting at the center of mass, Fig. 3, pulls the body into the lordotic position.
Fig.3: Schematic of lordosis, in this case in an ASB with hindlegs stretched to the rear.
American Saddlebred Lordosis
I know that this is a subject of some tenderness and I shall try to be judicious in my presentation. My goal as always has been to do well by the horse. That usually is in the best interests of horsemen as well though I know that what I say and do is not always (hardly ever!) appreciated in the short run.
Low back or sway back has been recognized in the ASB (American Saddlebred) for a number of years. Although numbers are not available, anecdote indicates that the condition is more common in that breed than in any other. Unfortunately, I have not had the opportunity to examine one of these animals at postmortem, despite a long-standing plea for such an opportunity. Lordosis in the ASB is of the juvenile type, but we simply do not know if the lesion described above is present. I have seen young ASB foals so markedly lordotic that it was obvious that they could never perform and undoubtedly were destroyed and buried. As with some other conditions in other breeds the choice has apparently been to ignore the problem when possible and bury it when not possible.
In order to make any progress, then, it is necessary to try to construct the pathogenesis with the information, albeit incomplete, available. The show requirements for the ASB are unique. In the standing position the neck is elevated, the forelegs straightened, the thoracolumbar spine (the back) is lowered, and the hindlegs are stretched back, Fig. 3. This is a unique variant of the normal collected stance for a horse, Fig. 4.
Fig. 4: Schematic of the usual collected stance of a horse.
As discussed in The Lame Horse (reference on the web site) collection is the normal response of a horse to an unusual or frightening event. The neck elevates, the head tips nose down and the back is rounded up as the hind feet move forward beneath the body. Collection can, of course, be induced by a rider using the reins and leg aids.
The ASB is unique in that the animal is trained to collect the head, neck, and forelegs but not to collect the hindlegs. Indeed, the hindlegs move in the opposite direction.
The elevation of the neck induces a reflex lowering of the thoracolumbar spine. This is countered in usual collection by the forward placement of the hind feet but is not countered in the ASB because the hind feet move backward. Indeed this backward movement of the hindlegs mechanically tends to lower the thoracolumbar spine. The stretching out of the hindlegs is enhanced by the practice of elevating the tail, often with the training aid of a tail set. Elevating the tail induces an extensor reflex in the hindlegs, so that they straighten and move backward. (This extensor reflex induced by lifting the tail is routinely used with cattle to inhibit kicking since the cow cannot kick unless she can first flex her leg. That is true of the horse as well, but the horse can override the reflex easily while the cow is less able to do so.)
So far, then, we have an induced but reversible hollowing or lowering of the back into a lordotic or nearly lordotic position. The more successful horses will be those which assume this stance most prettily (I cant think of a better word). The horse more likely to be selected for breeding is that individual successful in best achieving this stance together with, of course, high marks for movement at the required gaits. [I am simplifying by omitting consideration of the gaits and their evaluation for the moment.]
Given that selection for stance is a determinant, those horses will tend to be selected for breeding which either best achieve the stance or are predisposed to the stance. That is, the horse which is born with a low back- some degree of lordosis - has an advantage to start with over the horse which must be trained to this stance. Once one begins to select in this way it is almost inevitable that the low back will be perpetuated in the offspring of that first horse with a low back, and the end stage of low back is lordosis. By selection, therefore, the back becomes fixed in the lordotic position. What used to be achieved by training has now become part of the basic, inherent conformation of the horse.
The static or stance position is reflected in the movement or action of the ASB. The neck is elevated, the forehand action high and bouncy. The hindlegs, too, have a bouncy action. The overall picture is reminiscent (to me, at least) of children bouncing on a trampoline and is somewhat more obvious in some three-gaited horses. This action appears to be the result of the unusual, incomplete collection characteristic of the ASB. Both the forelegs and the hindlegs tend to reflex extension which leads to the stiffly bouncing up and down movement. The low back is of no advantage; it is simply the by-product of the positions of the neck, forelegs, and hindlegs.
Once again, I recognize that this is a delicate subject and that I seem to be saying that the American Saddlebred is all wrong, the stance is all wrong, the gait is all wrong, and so on. That is certainly not what I intend to convey though I recognize that is what will probably be conveyed. In order to achieve some understanding of lordosis in this breed I have had to proceed in the above manner. One may well ask why lordosis needs to be understood at all since it is not a lethal or debilitating disease. I need to understand it because that is what I do, and, in my opinion horsemen need to understand it because it is an abnormality, and breed integrity should entail the elimination of abnormalities. It can be done.
(1) Lordosis is from the Greek word meaning : bent backward.
(2) You can find photographs of swayback and the affected vertebrae in Chap. 8 of The Lame Horse.