Principles of Manual Medicine
Pelvic Anatomy
The pelvic girdle consists of three bones (the sacrum and the two hip bones) and three joints (the symphysis pubis and the two sacroiliac joints). The sacrum is formed by the fused elements of the sacral vertebrae and articulates superiorly with the last lumbar vertebra (L5) and caudally with the coccyx. In many instances, the sacrum functions as an atypical lumbar vertebra between the two innominate bones, with the sacroiliac joints as atypical zygapophysial joints.
The three joints of the pelvis consist of the symphysis pubis and the two sacroiliac joints. The symphysis pubis is a fibrocartilaginous joint with strong superior and inferior ligaments and a thinner posterior ligament. The sacrum is wedge shaped and fits vertically between the two iliac bones. Being self-locking, analogous to the keystone of an arch, the sacrum becomes increasingly stable with increasing loads. Much of the integrity of the sacroiliac joint depends on ligamentous structures with the posterior ligaments making the greatest contribution to pelvic stability.
The integrity of the sacrum is maintained by a group of strong anterior ligaments, posterior, inferior, and superior to this cartilaginous joint, with posterior ligaments contributing most significantly to the structural integrity of the sacroiliac joint. The structural configuration of the pubic ligaments, in conjunction with the sacro-iliac ligaments, provides the sacrum with a high degree of anterior/posterior stability. However, dislocation of the pubic symphysis can result in separation of the two pubic bones. This can result in displacement of the iliac bones, a decrease in the frictional coefficient of the articular cartilage of the sacroiliac joints, and a decrease in anterior/posterior sacral stability.
The sacroiliac joints are true arthrodial joints with a joint space, articular capsule, and articular cartilage. They are unique in that the cartilage on the sacral side is hyaline cartilage and on the ilial side is fibrocartilage. They are L- or C-shaped in contour, with a shorter upper arm and a longer lower arm and vary significantly from individual to individual. The joint contour usually has a depression on the sacral side at approximately S2 and a corresponding prominence on the ilial side. The shape of the sacroiliac joint varies markedly from individual to individual and from side to side in the same individual. During the aging process, there is an increase in the grooves on the opposing surfaces of the sacrum and ilium that appears to reduce available motion. It is of interest to note that the age with highest incidence of disabling back pain (25-45 years) is the same age where the greatest amount of motion is available in the sacroiliac joints. Occasionally, the opposing joint surfaces are quite flat and do not have the interlocking joint bevel change at S2 or the ilial prominence within the sacral depression. This type of sacroiliac joint is much less stable, and the possibility of superior and inferior translatory movement, or shearing, exists.
Six groups of hip and thigh muscles attach to the pelvic girdle and lower extremities. These hip muscles strongly influence the movement of the two innominates within the pelvic girdle. Anterior to the sacroiliac joints are two highly significant muscles, the psoas and piriformis. The psoas muscles cross over the anterior aspect of the sacroiliac joints in its travel from the lumbar region to insert into the lesser trochanter of the femur. The right and left piriformis muscles originate from the anterior surface of the sacrum, travel through the sciatic notch, and insert into the greater trochanter of the femur. Muscle imbalance in any of these groups affects pelvic girdle function. Imbalance in piriformis length and strength strongly influences movement of the sacrum between the innominates. Imbalance of the pelvic diaphragm is significant in patients with rectal, gynecological, and urological problems.
Click here to join Dr. Philip E. Greenman, D.O., F.A.A.O., as he presents an overview of the pelvis.
Click here to join Dr. Philip E. Greenman, D.O., F.A.A.O., as he presents an overview of the pelvis.
