Normal Vertebral Motion

In this chapter, we focus on the anatomy, kinematics, and biomechanics of the vertebral column. The goal is to give the reader an understanding and appreciation of the role of cardinal plane movement and coupled movement in normal vertebral motion. Comprehension of normal vertebral motion is critical for one to achieve a clinical perspective as to the pathogenesis of vertebral dysfunction.

VERTEBRAL MOTION

Certain conventions are used in describing all vertebral motion. The vertebral motion segment consists of the superior and inferior adjacent vertebrae and the intervening disk and ligamentous structures. By convention, motion of the superior vertebra is described in relation to the inferior. Motion is further defined as the movement of the superior or anterior surface of the vertebral body. In describing rotation, the anterior surface is used rather than the elements of the posterior arch. For example, in rotation of T3 to the right in relation to T4, the anterior surface of T3 turns to the right and the spinous process deviates to the left. Therefore, remember that descriptions relate to the anterior or superior surfaces of the vertebral body. In addition to describing characteristics of a vertebral motion segment, we also speak of movement of groups of vertebrae (three or more).

Vertebral motion is also described in relation to the anatomically oriented cardinal planes of the body using the right-handed orthogonal coordinate system. Most of the clinical literature relates to the anatomically described cardinal planes and axes (Fig. 5.1), while the biomechanical research literature uses the coordinate system extensively. Motion can be described as rotation around an axis and translation along an axis with the body moving within one of the cardinal planes. By convention, the horizontal axis is the x-axis, the vertical axis is the y-axis, and the anteroposterior axis is the z-axis. The coronal plane is the xy plane, the sagittal plane is the yz plane, and the horizontal plane is the xz plane. The ability to rotate around an axis and to translate along an axis results in 6 degrees of freedom for each vertebra. Vertebral motion can then be described as having an overturning movement (rotation around an axis) and/or a translatory movement (translation along an axis).

TERMINOLOGY

At the present time, convention in clinical practice describes vertebral motion in the following terms: forward bending, backward bending, side bending right and left, and rotation right and left. These motions are oriented to the cardinal planes of the body. It is imperative that one understands that the context of the following descriptions is kinematical. Kinematics is defined as that phase of mechanics concerned with the study of movement of rigid bodies, with no consideration of what has caused the motion.

Forward Bending

In forward bending, the superior vertebra rotates anteriorly around the x-axis and translates somewhat forward along the z-axis. In forward bending (Fig. 5.2), the anterior longitudinal ligament becomes somewhat more lax, anterior pressure is placed upon the intervertebral disk displacing the nucleus posteriorly, and the posterior longitudinal ligament becomes more tense as do the ligamentum flavum and the interspinous and supraspinous ligaments. The transverse processes of the superior segment move more anteriorly. The inferior zygapophysial facet of the superior vertebra moves superiorly in relation to the superior zygapophysial facet of the inferior vertebra. This has been described as “opening” or “flexing” of the facet.

Backward Bending

In backward bending, the vertebra rotates backward around the x-axis and moves posteriorly along the z-axis (Fig. 5.3). The anterior longitudinal ligament becomes more tense. There is less tension on the posterior longitudinal ligament, the ligamentum flavum, and the interspinous and supraspinous ligaments. The transverse processes of the superior segment move more posteriorly. The inferior zygapophysial facet of the superior segment slides inferiorly in relation to the superior zygapophysial facet of the inferior vertebra. The facets are spoken of as having “closed” or “extended.” Forward bending and backward bending result in an accordion-type movement of the opening and closing of the zygapophysial joints. If something interferes with the capacity of a facet joint to open or close, restriction of motion of either forward bending or backward bending will result.

Side Bending

In side bending, there is rotation around the anteroposterior z-axis, translation along the horizontal x-axis, and rotation around the vertical y-axis. The z-axis and x-axis directions are dependent on the direction of side bending; however, the y-axis direction (rotation) can vary, as it is dependent on the vertebral segment involved. In side bending to the right, the right zygapophysial joint “closes” and the left zygapophysial joint “opens.” Interference with a facet’s capacity to open or close can interfere with its segmental side-bending and rotatory movement.

Figure 5.1 Anatomically described cardinal planes and axes.

Rotation

Rotation of a vertebra is described as rotation around the y-axis with the translatory movement being dependent on the vertebral segment involved. Rotation is always a component of side bending with the exception of the atlantoaxial joint.

Figure 5.2 Vertebral forward bending.

COUPLED MOVEMENTS

Coupled motion by definition is the rotation or translation of a vertebral body about or along one axis that is consistently associated with rotation or translation about a second axis.¹ Coupling of spinal motion is a phenomenon that is derived from the kinematics of the individual vertebra, the anterior-posterior curvature, and the connecting ligaments of the spine. Robert W. Lovett, MD, in his quest to understand the pathogenesis of scoliosis, dispelled the theory that there are four movements of the spine since neither rotation nor side-bending movements were “pure.”² Panjabi further discriminated it stating, “When we flex the spine in the sagittal plane, the flexion rotation is the main motion and the accompanying anterior and inferior/superior translatory motions are called the coupled motions.”³ The phenomenon of coupling has been well documented experimentally and clinically in all areas of the vertebral column⁴, ⁵, ⁶, ⁷; however, controversy remains regarding the conclusions that Drs. Lovett⁸, ⁹ and Harrison Fryette¹⁰ made as to the direction of coupled rotation in the various areas of a side-bent spine.

Dr. Lovett contributed to our understanding of spinal motion with his observation that two dominant factors controlled spinal motion, one being the articulating facets and the other the bodies of the vertebra. By separating the spine through the pedicles into two columns, he studied the coupling behavior of the anterior part, which consisted of the vertebral bodies and intervertebral disks, and the posterior part, which consisted of the laminae and neural arch. When the column of vertebral bodies was side bent under load, they collapsed toward the convexity. When the column of facets was similarly side bent, it behaved like a flexible ruler or blade of grass; rotation into the concavity was necessary before it could be side bent. These experiments suggest that in the intact spine, to the degree that the facets are in control, they direct and govern rotation.¹⁰ If the facets are not controlling motion, side bending can occur with
coupled rotation to the opposite or convex side; if the facets are controlling motion, side bending can occur after the spine rotates into the concavity or into the direction of the side bending. As discussed below, the amount of “control” each vertebral segment facet is provided is dependent on the anterior-posterior curvature of the spine and the orientation of the facets to the horizontal plane. An understanding of vertebral anatomy and spine kinematics is crucial in understanding coupling mechanics and vertebral dysfunction.

Figure 5.3 Vertebral backward bending.

Neutral Mechanics

Neutral mechanics, or its synonym type I mechanics, results in coupled movement of side bending and rotation to opposite sides. Neutral mechanics occur in the thoracic and lumbar spine; in the absence of dysfunction (or anatomical deviation) when the patient is in the erect position with normal anteroposterior curves, the facets are not controlling motion. For example, in the lumbar spine, with a normal lumbar lordosis present, side bending of the trunk to the left results in rotation of lumbar vertebrae to the right in three-dimensional space (Fig. 5.4). This motion behavior is derived from bending forces on the vertebral bodies and associated ligaments; the facets do not control motion.

If one looks at the behavior of each of the lumbar vertebra in relation to the segment below, they do not all side bend and rotate to opposite sides. In fact, in this example, the middle segment maximally side bends and rotates to the opposite side. The segments below the apex also side bend and rotate to opposite sides in a gradual fashion. The segments above the apex side bend and rotate to the same side, and the curve gradually reduces. However, for descriptive purposes, the neutral mechanical behavior is described as being side bent and rotated to opposite sides in three-dimensional space.

Figure 5.4 Neutral (type I) vertebral motion.

CLINICAL PEARL

Look at a model of the vertebral column. Now imagine the motion that would occur if the posterior aspect (neural arch) of the vertebra were removed, leaving only the bodies of the vertebra. Because the facets of the thoracic and lumbar spine are oriented at a 60- to 90-degree angle from horizontal, there is a fairly large anteroposterior range in which they have no effect on motion. This is the neutral range. Outside this range, the bending forces are placed on the facets allowing them to control motion.

Nonneutral Mechanics

Nonneutral mechanical coupling, or its synonym type II mechanics, results when side bending and rotation of vertebrae occur to the same side. This takes place when there is alteration in the anteroposterior curve into forward or backward bending, which places bending forces onto and allows the facets to control motion.

CLINICAL PEARL

To demonstrate, stand and forward bend at the waist and place both fingers overlying the posterior aspect of the transverse processes of the lumbar spine. Introduce side bending to the right. You will feel fullness occur under the fingers of your right hand, interpreted as resulting from posterior orientation of the right transverse processes during a right rotational response to the right-side bending coupled movement (Fig. 5.5). Return to the midline before returning to the erect posture.

Nonneutral (type II) mechanics include the coupling of all three arcs of vertebral motion and all 6 degrees of freedom.¹¹ Nonneutral coupling results in significant reduction in freedom of motion. It is for this reason that the vertebral column appears to be at risk for dysfunction when nonneutral mechanics are operative.

Type III Mechanics

Type III refers to the observation that when motion is introduced in the vertebral column in one direction, motion in all other directions is reduced. To demonstrate the phenomenon, have your patient sit erect on an examining couch and passively introduce rotation of the trunk to the right and left. Ascertain the range and quality of movement. Now have the patient slump on the table with a posterior thoracolumbar convexity and again introduce trunk rotation to the right and left. Note the reduction in range and the restricted quality of movement during the range with the patient in this slumped position. The phenomenon of type III vertebral motion is therapeutically applied during localization to dysfunctional segments. Introduction of motion above and below a dysfunctional vertebral segment can be accurately localized to a single vertebral motion segment that will then be treated by introduction of some activating force.

Figure 5.5 Nonneutral (type II) vertebral motion.

VERTEBRAL ANATOMY

The vertebral column consists of 33 segments. There are usually 7 cervical segments, 12 thoracic segments, 5 lumbar segments, 5 fused sacral segments, and 4 coccygeal segments. Anomalous development occurs in the spine and is most common in the lumbar region where four or six segments are occasionally found. The lumbar region is also the site of the greatest number of anomalous developmental changes, particularly in the shape of the transverse processes and zygapophysial joints. The vertebral motion segment consists of two adjacent vertebrae and the intervening ligamentous structures. The typical vertebra consists of two parts, the body and the posterior neural arch. The vertebral body articulates with the intervertebral disk above and below at the vertebral end plate. The posterior arch consists of the two pedicles, two superior and two inferior zygapophysial joints, two laminae, two transverse processes, and a single spinous process. Two adjacent vertebrae are connected, front to back, respectively, by the anterior longitudinal ligament, the intervertebral disk with its central nucleus and surrounding annulus, the posterior longitudinal ligament, the articular capsules of the zygapophysial joints, the ligamentum flavum, the interspinous ligament, and the supraspinous ligament.

The anterior-posterior curves of the vertebral column develop over time. The primary curve at birth is convex posteriorly. The first secondary curve to develop is in the cervical region, which becomes convex anteriorly when the infant begins to raise its head. The second curve develops in the lumbar region
on assuming the biped stance. This curve is convex anteriorly. Appropriate alignment of the three curves of the vertebral axis is an essential component of good posture (Fig. 5.6).

Figure 5.6 Normal vertebral curves.

Cervical Region

Atlas

The atlas and axis are structurally and functionally different from the vertebrae in the lower cervical region. The atlas (Fig. 5.7) is considered atypical as it does not have a vertebral body or intervertebral disk and because it consists primarily of a bony ring with two lateral masses. On the posterior aspect of the anterior arch is a small joint structure for articulation with the anterior aspect of the odontoid process of the axis. Each lateral mass consists primarily of the articular processes. The shape of the superior articular process is concave front to back and side to side. The long axis of each superior articulation diverges approximately 30 degrees from anteromedial to posterolateral. This results in an anterior wedging of the long axis of these joints. The superior articular processes articulate with the concave shaped, similarly oriented condyles of the occiput. The inferior articular processes of the atlas are quite flat, but when the articular cartilage is attached, they become convex front to back and side to side. These inferior articular processes articulate with the superior articular process of the axis. The transverse processes are quite long and are palpable in the space between the tip of the mastoid process of the temporal bone and the angle of the mandible.

Figure 5.7 Atlas (C1).

The primary movement of the occipitoatlantal articulation is forward and backward bending. There is a small amount of coupled side bending and rotation to opposite sides. This motion is consistently controlled by the uniquely shaped articulation and its ligamentous attachments. Left rotation of the occiput on the atlas is associated with anterior displacement of the right occipital condyle on the concave and anteriorly convergent right articular process of the atlas, and posterior displacement of the left occipital condyle on the concave and posteriorly divergent left articular process of the atlas. As the occiput turns to the left, its articular capsule tightens, displacing the occipital condyles to the left, resulting in side bending to the right¹² (Fig. 5.8).

Figure 5.8 Left rotation of the occiput over the center of the odontoid O, the base of the occiput is displaced to the left by 2 to 3 mm along the direction indicated by the vector V (right-side bending).