The Sacrum
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By Wendell Sammet
The sacrum is of great importance concerning the hind limb assembly. It is attached to the pelvis by the sacral articulating cartilage at the hip joint. The two fused sacral cartilages, including the three sacrum, form the roof of the pelvic cradle. The number of sacral vertebrae that enter into the forming of the sacrum differs in various breeds. It is three in dogs; the first is the largest, the second and third join together with the largest. (Figure 1)
The sacroiliac joint on each side of the pelvis is formed by the articulating surface of the wing of the ilium and sacrum. This is a result of the fusion of the transverse processes of the sacrum vertebrae. The sacroiliac joint is a stablilizing joint in which the articular surfaces are coated with fibrocartilage and supported by dorsal and ventral sacroiliac ligaments. This union allows a direct transfer of force from the hind limb to the vertebral column.
The anterior of the sacrum articulates with the last broad, strong lumbar vertebrae of the vertebral column attaching it to the dog’s body. The function of the lumbar (loin) is the transmission of force from the hind limb assembly to the rest of the body. The posterior of the sacrum is joined with twenty coccygeal vertebrae (tail) which gradually decrease in size to the end of the tail.
The sacral vertebrae and the fourth coccygeal vertebrae form the croup. The angulation of the croup determines the “tail set.” The seven vertebrae form a slightly curved area in the dog’s back. It is important that you know the difference between the pelvis angle and the angle of the croup; which is determined by the slope of the croup, or the sacrum and the first four coccygeal vertebrae. The pelvis angle is determined by the angle of the pelvis which is attached to the sacrum at the sacroiliac articulating cartilage. The angle of the pelvis is measured from the iliac spine to the tip of the toe. These angles are independent of each other. Although this is a rigid joint, the angle at which the pelvis is attached to the sacrum can and does vary from dog to dog, leading into a variety of pelvic angles.
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