Measuring Spinal Motion

Standardized terminology and conventionalized measuring systems are necessary to define spinal kinematics and biomechanics accurately. A three-dimensional Cartesian coordinate system1 has been accepted as a frame of reference to define spinal motion ( Fig. 3.1 ). Spinal movements are characterized by two distinct types of motion: rotations (angular motions) and translations (linear motions). Each type of motion is described relative to each of the three axes of motion (x, y, and z). Clinically, rotation solely about the x-axis is referred to as flexion/extension, y-axis rotation is referred to as axial rotation, and z-axis rotation is referred to as lateral bending. Clinically, translation without rotation is referred to as subluxation. Both types of movement (translations and rotations) are important for understanding normal and pathological spinal behaviors.

Although usually reported in terms of motion in individual planes, the different spinal motions are linked, or coupled, together. Coupling refers to the simultaneous motions (rotations and/or translations) that occur secondary to the intended main motion (rotation and/or translation).1–3 For example, axial torque produces primarily axial rotation of C1 on C2, but also produces significant coupled y-axis translation, with C1 positioned farther caudally at neutral than when rotated left or right.3 Similarly, coronal plane torque at the CVJ produces primarily lateral bending at the occipitoatlantal segment (C0-C1) and at the atlantoaxial segment (C1-C2), but it also produces substantial coupled axial rotation.4 The secondary coupled motions are usually smaller than the primary motion but are sometimes the same order of magnitude.1,4 Posture, injury, fixation, applied load vector, angle of articulations, and other factors can influence the amount of coupling that occurs.

Biomechanical Flexibility Testing

The majority of information from the laboratory relating to the biomechanics of the CVJ has been derived from an experimental method known as flexibility testing. Flexibility testing is performed in vitro, using cadaveric spine segments of two or more vertebrae that have been stripped of muscle tissue, leaving the ligaments and bone structures intact. Although animal models are sometimes used experimentally in other regions of the spine where porcine, equine, or bovine anatomy is similar to human anatomy, the unique anatomy of the CVJ precludes the use of most animal models except some primates for studying CVJ biomechanics in vitro.5

For a flexibility test, load (torque, linear forces, or combination) is applied to the spinal segments, and the resulting spinal motions are measured.6 One or several levels of the spine can be examined simultaneously with a flexibility test. Results are usually reported in terms of the motion of a single vertebra with respect to its adjacent inferior vertebra: a motion segment, or the sum of motion across the key motion segments. Load-deformation responses can be analyzed for parameters such as stiffness, flexibility, range of motion (ROM), neutral zone (NZ) or lax zone (LZ), elastic or stiff zones (SZ), and axes of rotation.6,7 All of these biomechanical parameters are distinct and unique for each spinal level, and several parameters are sensitive indices of spinal instability. The information generated from in vitro flexibility testing represents the contributions of the bony articulations and ligamentous attachments between each motion segment without the stabilizing contributions of the muscles. Flexibility testing is only an in vitro technique; technically, it is not possible to perform such testing in living organisms because of ethical and practical limitations.

Load-Deformation Responses of the Craniovertebral Junction

Flexibility testing produces load-versus-deformation curves that characterize the unique behavior of the CVJ. These curves show applied load versus angular or linear displacement. Qualitatively, these plots represent a unique biomechanical “fingerprint” of the joint and the specimen being tested. Several parameters may be extracted from these curves for quantitative and qualitative analysis ( Fig. 3.2 ).

ROM, LZ, SZ, NZ, flexibility, and stiffness are important parameters that can be measured from the load-deformation curve. ROM is defined as the displacement between the neutral or resting position of the motion segment and the limit of its physiological motion.7,8 The neutral position is defined as the posture where minimal joint stresses occur and where minimal muscular effort is required to maintain the spatial orientation.8 This neutral or resting position is best approximated by the halfway point of the bilateral LZ. The LZ is the portion of the ROM where the ligaments are lax and small forces produce large vertebral displacements, represented on the load-deformation curve by the range where load is near zero ( Fig. 3.2 ).7 The NZ is a subset of the LZ, representing where only frictional joint resistance occurs.7,8 The SZ is the steep portion of the load-deformation curve near the edge of the ROM, where the ligaments become stretched and stiffness increases, causing resistance to any further movement. Flexibility is the amount of deformation in response to a unit load. This parameter is a measurement of the inherent “elasticity” or “stretchability” of the specimen.6 Flexibility is not constant over the entire ROM of the specimen. Flexibility should be measured from the steeper linear portion (SZ) of the load-deformation curve ( Fig. 3.2 ).5–7 Stiffness is the inverse of flexibility. It is the amount of resistance to a unit increment of displacement in the specimen. Bone-on-bone contact is more stiff/less flexible, whereas a stretching ligament is less stiff/more flexible.

It may seem trivial to extract lax and stiff components from load-deformation curves instead of just evaluating the ROM; however, these separate indices are more sensitive measurements of injury and instability than ROM.8,9 Pathological conditions may affect the lax and the stiff regions differentially, leaving the total ROM constant. These terms are important for understanding how the spine becomes unstable and how fixation devices behave, even though the terms cannot be measured in vivo.

The normal values for ROM, LZ, and SZ at C0-C1 and C1-C2 are summarized in Fig. 3.3 and Table 3.1 . The motion characteristics of the different levels of the CVJ are due to the geometry of the vertebrae and skull base, the shapes of the joints, and the arrangements of the ligaments. Neither the C0-C1 nor C1-C2 joints have an intervertebral disk. The ball-and-socket shaped C0-C1 joints allow slightly more flexion and extension than the other levels of the cervical spine, although they are quite rigid in axial rotation and lateral bending. The biconvex articular surfaces of the C1-C2 joints allow gliding and wide rotation of C1 around the dens. The atlantoaxial motion segment is the most flexible motion segment in the entire spine with respect to axial rotation, allowing a bilateral ROM of 80 degrees or more. More than half of all cervical axial rotation occurs at the atlantoaxial motion segment, a point surgeons must consider when deciding to fuse the atlantoaxial joints. Both C0-C1 and C1-C2 allow less lateral bending than the subaxial cervical motion segments, which average ~8 degrees unilaterally.3

Coupled Motion

As mentioned, coupling refers to the secondary motions that occur simultaneously with a primary motion. The coupling patterns can be measured at each motion segment. The coupling patterns can be used to differentiate the normal from the unstable spine. Coupling can be displayed graphically by using individual load-deformation curves that represent the simultaneous motion that occurs in each of the three primary reference planes.

Normally, lateral bending and axial rotation are strongly coupled at the C1-C2 joints.3,4,10 This pattern is due to the sloping conformation of the C1-C2 articular surfaces, which make these motions interdependent.3–5 The direction of the coupled motions is often opposite that of the main motion (e.g., left axial rotation is coupled with right lateral bending at C1-C2).

Maintaining different postures affects the direction of the angular coupling patterns at the CVJ.4,10 For example, when the neck is in a neutral or extended position, coupled right axial rotation occurs during left lateral bending at C1-C2. However, when the neck is in a flexed position, coupled left axial rotation occurs during left lateral bending.

Axes of Rotation: Instantaneous Center of Rotation and Helical Axis of Motion

The instantaneous center of rotation,3 an important parameter that reflects the kinematic behavior of the spine, can be used to differentiate the normal spine from the injured, unstable spine. The instantaneous center of rotation can be measured during flexibility testing. The term center of rotation refers only to rotation in a single plane. It is the point within the plane of motion about which the vertebra rotates. The approximate locations of instantaneous centers of rotations for C0-C1, C1-C2, and C2-C3 are depicted in Fig. 3.4 .

The three-dimensional analog of the instantaneous center of rotation, called the instantaneous helical axis of motion, measures the axis or line in space (rather than the point on a plane) about which the vertebra rotates at a given instant in time.3 If the vertebra is also allowed to slide along this axis, the helical axis of motion forms a complete (6 degrees of freedom) description of motion. The motion of a vertebra from one location in space to another can be uniquely described by specifying the orientation of the helical axis of motion, the angle of rotation about it, and the distance of translation along it. The instantaneous center of rotation represents the intersection of the instantaneous helical axis of motion with the plane of interest ( Fig. 3.5 ).

A collection of instantaneous centers of rotation or instantaneous helical axes of motion accumulated at intervals during a rotational movement of the spine (the centrode) can help assess instability.11,12 If the joint movement is pure rotation without translation, all instantaneous axes in the centrode will coincide. However, if some sliding (translation) occurs at the joint, the distribution of the points or lines will widen. In addition, if the orientation of the lines of the helical axis of motion is parallel, the rotation is true. If the angles of the lines of the instantaneous helical axis of motion vary widely, the joint is wobbling.12