A Radiographic Study of Spondylolisthesis with Special Reference to Stability Determination

Abstract

In a paper published elsewhere (1), an improved method for the detection of spondylolisthesis has been described, as well as a method of determining the degree of stability or instability of the slipped vertebral body. These methods are briefly reviewed here in the light of more extensive experience gained with additional cases subsequently seen. Anatomical Considerations The term “pars interarticularis” is a descriptive one, but has caused some confusion because it does not appear in most classical texts. It refers to the isthmus of bone lying between the superior and inferior articular processes. Actually, this segment is a portion of the lamina and separates the vertebral body, pedicles, superior articular processes, and transverse processes on the one hand, from the inferior articular processes and spinous process on the other. This is illustrated in Figure 1, in which the pars interarticularis is indicated by wire on both the photograph of a lumbar vertebral body and on roentgenograms in the anteroposterior, lateral, and oblique positions. A superior-inferior view is also shown, but this projection, of course, is not applicable clinically. The oblique projection has been found to be the most accurate, and will reveal a defect even when other views fail to do so. Oblique views of the lumbosacral spine are employed routinely in all examinations of this region. A labeled diagram of a representative oblique view of the lumbosacral spine is shown in Figure 2 with an accompanying roentgenogram. The defects can also occasionally be seen on the anteroposterior projection; when bilateral, they can usually be detected in straight lateral views. Normally, the vertebral body is prevented from anteroposterior displacement by paraspinous ligaments, muscles, and the superior and inferior apophyseal joints. The superior joints prevent posterior displacement; the inferior prevent anterior displacement. If the function of the inferior intervertebral (apophyseal) joints is faulty, the other supporting structures may not be adequate to prevent anterior displacement of the vertebral body. Thus, in case of a bilateral defect in the pars interarticularis of a vertebra, the vertebral body becomes susceptible to anterior displacement. The mechanism of spondylolisthesis becomes readily apparent when one considers that, in cases of bilateral defects of the pars interarticularis, the vertebral body is held securely superiorly but not inferiorly. With sufficient stress, the vertebral body will swing forward about the superior fixed points in the arc of a circle (Fig. 3), and the lower margin of the vertebral body will move through a greater space than the upper margin. With sufficient strain upon the capsule of the superior apophyseal joint, it will stretch and allow a small amount of anteroposterior motion and, in these rarer instances, the superior and inferior surfaces of the slipped vertebral body will move through equivalent spaces.