The human spine is a complex structure of individual bones, called vertebrae, that provides both support and flexibility. It allows for a wide range of movements while protecting the delicate spinal cord. Within this system, small connections play a significant role in maintaining spinal health and function.
What Are Facet Joints?
Facet joints, also known as zygapophyseal or Z-joints, are a pair of small, specialized connections located at each level of the spine. These are synovial joints, similar to the knee or hip, allowing for smooth movement between bones. Each spinal segment, from the neck (cervical region) through the mid-back (thoracic region) to the lower back (lumbar region), features two facet joints, positioned on either side of the vertebrae.
These joints form where the bony projections, called articular processes, of two adjacent vertebrae meet. The superior articular process of one vertebra connects with the inferior articular process of the vertebra directly above it. Together with the intervertebral disc, these bilateral facet joints form a “three-joint complex” at each spinal level, facilitating movement and maintaining the vertebral column’s structural integrity.
Detailed Anatomy of Facet Joints
The ends of the bones within each facet joint are covered by a layer of articular cartilage, a slick, spongy material that allows the bony surfaces to glide against each other. This cartilage helps to absorb forces and reduce friction during spinal movements.
Each facet joint is encased by a fibrous joint capsule, a watertight sac of connective tissue that surrounds the entire joint. This capsule, along with supporting ligaments, helps to hold the two sides of the joint together and maintains its integrity. Within this capsule, synovial fluid is present, which lubricates the joint and provides nourishment to the cartilage, minimizing friction.
Facet joints also possess a specific nerve supply that transmits sensation, including pain. Small nerves known as medial branch nerves provide this innervation. Each facet joint receives nerve signals from two medial branches: one from its own vertebral level and one from the level above it.
The Crucial Role of Facet Joints in Spinal Movement and Stability
Facet joints are instrumental in guiding and limiting spinal movements, contributing to both flexibility and stability. Their unique orientation varies across different regions of the spine, which dictates the type and range of motion possible at each level. This design prevents excessive movement that could lead to injury to the spinal cord or other delicate structures.
In the cervical spine (neck), facet joints are generally angled at about 45 degrees, allowing for a wide range of movements including flexion, extension, lateral bending, and rotation. This orientation enables the neck’s notable mobility. In contrast, the facet joints in the thoracic spine (mid-back) are angled more vertically, around 60 degrees, which restricts flexion and extension but facilitates rotation and lateral bending.
The lumbar spine (lower back) facet joints are oriented closer to the sagittal plane, at 90 degrees, favoring flexion and extension while limiting rotation. This alignment helps the lumbar spine bear significant loads and protects against excessive twisting and anterior shear forces. The combined action of facet joints, along with intervertebral discs and ligaments, ensures the spine can move effectively while remaining stable.