The spine contains a large number of joints. Individually, these joints tend to have very limited ranges of motion. However, collectively, these joints allow for an impressive range of total spine motion through a variety of planes while maintaining relative stability. The versatility of spine motion is made possible by the contributions of multiple types of joints that are specialized to their tasks.
Discs and Related Joints
The most visibly evident joints of the spine are the disc joints. These are formed by special pads (discs) that rest between and are firmly attached to most of the vertebrae. These discs have jelly-like centers that are contained by interwoven, fibrous tissue. Healthy discs contribute strength and stability to the spine while allowing motion between the segments. The discs also serve as primary shock absorbers of the spine.
In the cervical (neck) region, immediately adjacent to the disc joints are additional small joints known as "uncovertebral joints." There are five pairs of uncovertebral joints in the cervical region. These joints provide some added stability to the neck in side-bending positions while permitting a large range of motion in flexion and extension, as described by Kapandji in his text, "The Physiology of the Joints."
Facet Joints
The movements between most of the vertebrae are governed by what has been referred to as a "three-joint complex" that includes the disc joints and two smaller joints located on the back side of the spine. These smaller joints, known as "facet joints," are anatomically similar to joints found in the fingers. They have smooth, cartilage surfaces. They are surrounded by a flexible, ligament capsule, and the space between the joint surfaces is filled with a slippery lubricant called synovial fluid.
In their text, "Clinical Biomechanics of the Spine," White and Panjabi describe how these facet joints assume varied orientations at different levels of the spine. The fit and orientation of the facet joints largely determines the torsional stiffness in each region of the spine.
Other Spine Joints
The topmost vertebra of the spine is named for the Greek God Atlas. Atlas is often illustrated supporting the globe of the earth on his muscular shoulders. In place of the earthly globe, the atlas vertebra supports the skull on a sturdy pair of shoulders, known as "articular processes." These form joints with matched bumps at the base of the skull, known as "articular condyles."
There is no disc separating the atlas and the second vertebra. The segment is connected by joints that are somewhat similar to those connecting the skull and atlas. An additional joint is formed by a peg that projects up from the second vertebra into a space at the inside front of the ring-shaped atlas. This unique joint allows the skull and the atlas to pivot around the peg. This arrangement is also the reason that the second vertebra is named the "axis." A study by J. Dumas et al, published in the journal, "Surgical and Radiological Anatomy," reported that greater than 50 percent of total neck rotation happens between the atlas and the axis.
References
- "The Physiology of the Joints, Volume 3"; I. A. Kapandji; 2008
- "Clinical Biomechanics of the Spine, 2nd Editions"; A. White and M. Panjabi; 1990
- "Surgical and Radiologic Anatomy"; "Rotation of the Cervical Spinal Column: A Computed Tomography In Vivo Study"; J. Dumas, M. Sainte Rose, P. Dreyfus, et al; 1993
