The lower segment of the spine is also known as the lumbar spine. The lumbar portion of the spine consists of 5 separate bones or vertebrae. The vertebrae are each separated by a cushion known as the intervertebral disc. The disc consists of a tough, outer shell known as the annulus fibrosis, and a soft inner core called the nucleus pulposus. Besides cushioning the vertebrae, the discs allow for free movement of the spine. Posteriorly, the vertebrae are joined at each consecutive level by a pair of synovial joints, known as the facet joints. The facet joints stabilize the spine and also allow for normal motion. Finally, an important element of the lumbar spine is the muscle surrounding the vertebral column, or the paraspinal musculature. The paraspinal muscles play an important role in stabilizing the spine, preventing excessive stress on the intervertebral discs and the facet joints.
Causes of Lower Back Pain
Low back pain can be acute or chronic. Pain usually is the result of injury to or degeneration of the intervertebral discs, facet joints, or paraspinal muscles. The most common causes of low back pain include:
- Muscle strain or muscle spasm
- Disc herniation or disc degeneration
- Arthritis of the facet joints
Muscle Strain or Spasm
The paraspinal muscles consist of a thick column of muscle that stabilizes the spine. The muscles can be easily injured, particularly in sedentary individuals with weakened or atrophied musculature. An improper technique used when lifting heavy objects can lead to muscle strain, as can over-exertion from sports or other physical activity.
Disc Herniation or Disc Degeneration
The term disc herniation refers to the protrusion of a portion of the soft central disc material (the nucleus pulposus) through a tear in the outer lining of the disc (the annulus fibrosis). As an analogy, think of pounding a jelly-filled doughnut with a hammer, as the inner filling “herniates” through the outer portion of the doughnut. The herniated disc material can press on adjacent spinal nerves, causing low back pain. Additionally, pressure on the lumbar nerves can cause radiating pain in the legs, known as sciatica.
Arthritis of the Facet Joints
Over time, stress on the facet joints of the spine can lead to degenerative changes, also known as osteoarthritis. As in any joint, osteoarthritis is a condition associated with pain on joint motion. Additionally, degeneration of the facet joints can lead to instability of the spine, and possibly misalignment of the spine, resulting in more severe, chronic pain.
Diagnosis of Low Back Pain
Initial evaluation of low back pain includes a medical history and physical exam. The physical examination is useful to evaluate the range of motion of the lumbar spine and to help establish evidence of nerve problems in patients with associated sciatica. The diagnostic next step includes routine x-rays of the lumbar spine. X-rays can show evidence of disc degeneration (narrowing or calcification of the disc) and of facet joint degeneration. X-rays can also detect less common, but more severe conditions such as fractures of the spine. Finally, patients with evidence of nerve impingement or with long standing pain may undergo MRI examination. MRI is excellent at demonstrating disc and facet joint disease, spine misalignment, and paraspinal soft tissue abnormalities.