Facet Joints and Low Back Pain

Low back pain is the second most common reason for a visit to a doctor's office. Many medical specialties treat this highly prevalent condition. Low back pain is the leading cause of disability in people younger than 45 years of age. Billions of dollars are spent directly and indirectly for the treatment of low back pain. Often, low back pain has been attributed to lumbar disc herniation or to a dysfunctional intervertebral disc. However, there are several other entities that may cause low back pain, including the zygapophysial joints, the sacroiliac joints, spinal nerve roots, muscles and ligaments, viscerosomatic pain, and other non-spinal causes.

Anatomy
The zygapophysial joints are often incorrectly referred to as facet joints. The facet represents the articular cartilage surface in numerous small joints in the body including the phalanges, costotransverse and costovertebral joints. We will refer to these joints as "L-z-joints" lumbar zygapophysial joints.

The L-z-joints are paired, true synovial joints endowed with cartilage, capsule, meniscoid, and synovial membrane. The articular facets exhibit variation in both the shape of the articular surfaces and general direction in which they face. The superior articular process is larger, posteriorly and medially facing and has a concave appearance. The inferior articular process is anteriorly and laterally facing. The L-z-joint space has a potential capacity of 1-2 cc=s. Each L-z-joint is innervated by a medial branch of the primary dorsal rami. The capsule serves to limit bending forces and to resist a backwards sliding motion during extension. This capsule is highly innervated with nociceptive and autonomic nerve fibers. The synovium is believed to contain nociceptors that may only serve to regulate blood flow.

Biomechanics
The only movement permitted by the L-z-joint is a sliding movement in a vertical direction, which is executed during flexion and extension of the vertebral column. The maximum pressure applied to the L-z-joints occurs during extension. The capsular ligaments protect the posterior annulus of the disc from excess flexion and torsion forces. The L-z-joints play an important role in limiting rotation about to the intervertebral disc.

The precise cause of most L-z-joint pain remains unclear. However, several mechanisms have been postulated. It has been well established clinically, with fluoroscopic controlled injections, that L-z-joints cause pain. The pain is variable. Often the pain is localized only to the back. However, radicular pain has also been observed. During hyperextension, the capsule stretches. This may cause pain. Arthritic changes of the L-z-joint can also cause low back pain, but this is inconsistent as various studies support both painful and pain free low backs. No noninvasive pathognomonic finding or a constellation of findings exclusively identify L-z-joint mediated pain from other sources of low back pain.

Signs and Symptoms
A variety of entities present with low back pain. There exist considerable overlap with patients who have L-z-joint pain. Most patients often only have axial pain. However, some patients complain of radiating pain, which occasionally travels into the buttocks and into the lower extremities.

Differential Diagnosis
The spine physician must exclude spinal versus nonspinal causes of low back pain. Some of the spinal causes include discogenic pain, nerve root compression, ligamentous or muscular strain/sprain, and sacroiliac joint pain. Neoplasms, infection, or fractures could be the primary or secondary causes of low back pain. Nonspinal causes include genitourinary, gynecologic, gastrointestinal, or systemic disease. Another cause of low back pain is psychogenic pain. This should be evaluated and addressed appropriately.