Spinal stability

Dr. Usama Said . biomechanics magazine

The spine is a multi-segmented organ consisting of approximately 24 mobile segments. The compressive strength of each vertebrae increases from the cervical to the 5th lumbar level. The strength of the cancellous bone making up the vertebrae is the same at each level; therefore, the increasing strength as one progresses caudally is due to the increasing size of the vertebrae from C3 to L5. Cortical bone carries more of the load as the individual ages. Cancellous bone resorption also occurs. Studies have shown that the posterior portion of the vertebrae, the facets, carry 18 percent of the compressive loads of the spine. The anterior and posterior longitudinal ligaments add stability to the spine but predominately are active in traction and offer very little stability in shear or compression. The ligamentum flavum between the lamina provides a compressive force across the disc and acts as a holding force in flexion. The capsule of the facets, the supraspinatus ligaments, and the inter-transverse ligaments add further stability to the spine. The iliolumbar ligaments are major stabilizers of the L4, L5 lumbar vertebrae on the sacrum.

The intervertebral disc is the most important structure between vertebrae. From L1 to S1 the discs account for 33 percent of the length of the column of the spine. The annulus fibrosus is a multifibrous structure which contains the nucleus palposus, a remnant of the notochord. Weight is distributed across the end plate of each disc complex. With compression, the disc bulges centripetally. With flexion, however, traction and compression rotation occur, and the disc may be displaced in multiple directions. Within the intact annulus, the elastic limits of the disc cannot be exceeded without vertebral fracture. The end plate is the most susceptible; the vertebral body is second.

Muscles acting on the lumbar spine are the extensors--predominately the sacrospinalis group. The flexors, the quadratus lumborum, psoas, iliacus, and abdominal muscles act on the spine when the pelvis is fixed. Lateral bend is achieved by unilateral contracture of the quadratus lumborum muscle

Important tips to help prevent back injuries.

• Don't lift by bending over. Instead, bend your hips and knees and then squat to pick up the object. Keep your back straight, and hold the object close to your body.

• Don't twist your body while lifting.

• Push, rather than pull, when you must move heavy objects.

• If you must sit for long periods, take frequent breaks and stretch.

• Wear flat shoes or shoes with low heels.

• Exercise regularly. An inactive lifestyle contributes to lower-back pain.