CHAPTER 107 Sacroiliac Disease
The skeletal structures involved in sacroiliac disease include the pelvic bones, vertebral column, and sacroiliac joints where the ventral aspect of the ilium, the ilial wing, comes into close contact with the sacral bone (Figure 107-1). At this junction there are two synovial joints, with a thin layer of fibrocartilage at the ilial facets and hyaline cartilage at the sacral facets.
At the ventral aspect of the sacroiliac joint, several ligamentous structures provide support, including the capsule of the joint itself and a package of small fibrous ligaments known as the ventral sacral ligaments. Also some ligaments originating from the sacral bone attach muscles to the great trochanter of the femur. On the top of the pelvic bones, the dorsal sacroiliac ligaments originate from the tuber sacrale. These include a long part, which adheres to the fibrous structures of the pelvis (the sciatic ligament), and a smaller, short part that adheres to the sacral bone and the coccygeal vertebrae and ligaments (Figure 107-2). Also, from the ventral aspect of the ilial wing, the interosseus sacroiliac ligament supports the sacroiliac joint.
The muscles of the caudal portion of the vertebral column and pelvis are major contributors to the stability of the sacroiliac region. Muscles on the dorsal aspect are the semimembranosus and semitendinosus, but also the psoas muscles on the ventral part of the pelvis are important contributors to stability. Under normal conditions, the muscles in this region maintain the mobility of the sacroiliac joints within their physical limits. However, when the muscles are not powerful enough to compete with the external forces resulting from falling, slipping, tipping over, and other similar accidents, primary ligament damage and primary or secondary joint injury are possible. Weakness in the same muscles can also be associated with sacroiliac injury. Muscle weakness can have many causes, including fatigue from improper training techniques, and overuse and repetitive stress injuries can arise in young horses that are not yet capable of the desired level of performance.
All the above structures, as well as the adjacent structures of the lumbar segment of the vertebral column, can contribute to sacroiliac injury. Although the range of motion in the sacroiliac junctions is small, any pain that results from the change in range of motion is magnified by the lever distance between the point of rotation (sacroiliac joints) and the distal portion of the limb, thus causing remarkable changes in gait when lesions are present.
Horses with sacroiliac pain may be presented with a great diversity of owner or rider complaints, including reduced stride length in one or both hind limbs, asymmetry of the hind end with one hip lowered, or atrophy of the croup muscles. Owners report changes in both the rhythm and quality of the walk, with a more lateral element to the walk seen especially during serpentines or circles. Reduced stride length may be seen in one or both hind limbs. There are no clear signs at the trot, just a little stiffness with slightly reduced propulsion and engagement. Downward transitions from canter to trot or from trot to walk can be of lower quality, with a disturbance in the rhythm in the new gait in the first strides after the transition. Going downhill, even when the incline is small, can be difficult, and refusal to jump in hunting, cross-country, or eventing is a common rider complaint.
Signs of sacroiliac problems are usually most evident during cantering because the pattern of this gait involves unilateral loading of the hind limbs, with a phase when the inside hind limb carries the entire body load after the air phase of the stride. This introduces unilateral loading of the sacroiliac region. When sacroiliac lesions are present, the horse will often alter its gait to relieve the unilateral loading of the sacroiliac region by cantering in a bunny-hop fashion, which results in less separation of footfalls. Horses with sacroiliac pain also change leads or cross canter frequently. As a result of this augmentation of signs in the canter, disciplines that are judged on the quality of the canter, for example, dressage and western pleasure, or that need a good quality of canter for optimum performance, such as in hunting, jumping, and racing, are represented more frequently in the population of horses that are evaluated for sacroiliac pathology at our practice.
Behavioral issues as kicking, rearing, striking, and bucking as well as not wanting to go forward are often associated with sacroiliac pathology. Riders frequently report that these behaviors occur when the horse is asked to canter, and they wonder whether this is a training or behavioral issue or whether the problem can be attributed to pain.
Some horses do not show evident signs of sacroiliac disease until ridden under saddle. However, severe sacroiliac injury, such as acute dislocation of one of the sacroiliac joints or rupture of the sacral ligaments, can cause severe, even nonweightbearing, hind limb lameness.