Biomechanics of the Lumbar Spine During Pregnancy
The vertebral bodies in the caudal lumbar spine are larger than those in the cervical and thoracic spine, which allows them to accommodate the added weight and stress when a person is in an upright posture. The functional units of the vertebral bodies can be divided into anterior and posterior portions. The anterior portion, comprising the vertebral bodies and the intervertebral discs between them, provides support and weight-bearing strength as well as shock absorption. The posterior portion of the functional unit, which is the non-weight-bearing portion, includes the vertebral column that houses and protects the spinal cord. It also contains the facet joints, which are diarthrodial joints. Like all other diarthrodial joints in the body, they have a joint cavity between the articulating bones and are freely moveable. Their role is to direct movements of the functional unit as a whole in flexion, extension, and lateral bending. The load and shearing forces acting on the lumbar spine are greater than those on the cervical and thoracic spine because of the larger amount of body mass the lumbar spine must support as well as the lordotic curve in the lumbar spine. These factors make the lumbar spine more vulnerable to degenerative changes and disc herniations.
During pregnancy, a woman develops postural changes that are necessary for her to maintain balance in the upright position. The average weight gain with a single fetus is 10 to 12kg. The increasing weight is distributed primarily in the woman's abdominal girth. After 12 weeks of pregnancy, the uterus can no longer be contained within the pelvis and the mass moves superiorly and anteriorly. As the abdominal muscles are stretched and tone is diminished, they lose their ability to contribute effectively to the maintenance of neutral posture. With these biomechanical changes, it was thought that lumbar lordosis increased; however, studies have shown that the lordosis remains the same or increases only slightly. Instead, what seems to happen is that the entire spine shifts to a more posterior position, and the center of gravity as a whole tends to move in a posterior and caudal direction.
As pregnancy continues, production of the hormone relaxin increases ten-fold, reaching its peak between weeks 38 and 42.[8,9] Relaxin creates joint laxity, which is necessary to allow the pelvis to accommodate the enlarging uterus. Joint laxity is more pronounced in multiparous women than it is in nulliparous women. In the lumbar spine, joint laxity is most notable in the anterior and posterior longitudinal ligaments. This weakens the ability of static supports in the lumbar spine to withstand the shearing forces. As a result, there may be an increase in discogenic symptoms and/or pain coming from the facet joints. In the pelvis, joint laxity is most prominent in the symphysis pubis and the sacroiliac joints. The symphysis pubis continues to widen throughout pregnancy from its normal width of 0.5mm to a maximum of approximately 12mm. With this widening, there is the risk of vertical displacement of the pubis and the possibility of rotatory stress on the sacroiliac joints. The sacroiliac joints themselves tend to be extremely stable joints; they have anterior and posterior tight ligamentous structures as well as a curved and sigmoid articular surface that limits movement. Movement in the sacroiliac joint can be dramatically increased throughout pregnancy, however. This movement can stretch pain-sensitive structures, causing sacroiliac pain.
Cite this: Managing Back Pain During Pregnancy - Medscape - Jan 08, 1997.