Sprain of the ventral sacroiliac ligaments can be caused by repeated trauma, such as constant hyperflexion or hyperextension or by an acute traumatic event such as a fall or flip.
The SI joint is designed to provide of the pelvis versus mobility, so when the SI joint is forced beyond its normal range of motion the ligaments are weakened & stretched beyond their elastic range causing tearing of some or all of the ligament fibres. The tearing of fibres causes inflammation and swelling into the ligament which causes pressure that further separates the fibres and causes pain and weakness in the ligament. Without sufficient ligamentous support the joint becomes and will not follow the normal path of articulation. Abnormal articulation often causes irritation and pinching of the surrounding soft tissue structures such as the joint capsule, the articular cartilage and tendons resulting in pain. Over time this will lead to degenerative joint disease. Horses with long backs or those with rump high conformation are predisposed to sacroiliac strain and subsequent sacroiliac subluxation. Horses performing high intensity exercise over fences or at especially with quick turns or abrupt stops are also at a higher risk of developing sacroiliac subluxation as these activities involve forceful flexion and extension of the sacroiliac joint. Standardbred are at higher risk of lateral subluxation of the sacroiliac joint as the pace requires the horse to move its limbs in lateral pairs. Thoroughbred racehorses are predisposed as they constantly gallop. It is reported that mares experience sacroiliac subluxation more than stallions, which may be due to the sexual dimorphism seen in the equine pelvis. Clinically subluxation of the sacroiliac joint is seen as chronic, low grade pain, hind limb lameness such as a shortened stride, or a refusal to jump or extend a gait, “lump-hollow- lump” or “hunter’s bump” in the lumbosacral region, prominence of one or both of the tubers, a flattening of the pelvic angle, decreased flexion of the sacroiliac joint and dorsal subluxation of the lumbar vertebrae. Some individuals may exhibit heat & pain on palpation of the lumbar and sacral regions of the back, hyperlordosis, roach back, or cold back. Some horses may experience stress fractures of the pelvis. When the sacroiliac joint experiences abnormal articulation the muscles that cross that joint may be strained or may develop . Gait abnormalities may also develop as the horse attempts to avoid the pain associated with movement of the sacroiliac joint. Hypertrophy of the , the muscles of the as well as hypotrophy of the is commonly seen in horses affected by sacroiliac subluxation. Chronically DJD has been shown to occur. Diagnosis of sacroiliac subluxation is generally made through clinical signs and symptoms, palpation, + flexion tests and thermography. Diagnosis is often difficult as some horses are stoic and do not show much pain or discomfort. Radiography and CT scan are of little use in the diagnosis of sacroiliac subluxation due to the high muscle mass and density in the lumbosacral region. may provide the best visual of the SI joint and definitive diagnosis.
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