Viewed from behind (see Fig. 3.1), the articular facets of the lumbar zygapophysial joints appear as straight surfaces, suggesting that the joints are planar. However, viewed from above (Fig. 3.2), the articular facets vary both in the shape of their articular surfaces and in the general direction they face. Both of these features have significant ramifications in the biomechanics of these joints and, consequently, of the lumbar spine, and should be understood and appreciated.

Figure 3.2 A top view of an L3–4 zygapophysial joint showing how the joint space and articular facets are curved in the transverse plane. I, inferior articular process L3; S, superior articular process L4.

In the transverse plane, the articular facets may be flat or planar, or may be curved to varying extents (Fig. 3.3).² The curvature may be little different from a flat plane (Fig. 3.3D) or may be more pronounced, with the superior articular facets depicting a C shape (Fig. 3.3E) or a J shape (Fig. 3.3F). The relative incidence of flat and curved facets at various vertebral levels is shown in Table 3.1.

Figure 3.3 The varieties of orientation and curvature of the lumbar zygapophysial joints. (A) Flat joints orientated close to 90° to the sagittal plane. (B) Flat joints orientated at 60° to the sagittal plane. (C) Flat joints orientated parallel (0°) to the sagittal plane. (D) Slightly curved joints with an average orientation close to 90° to the sagittal plane. (E) C-shaped joints orientated at 45° to the sagittal plane. (F) J-shaped joints orientated at 30° to the sagittal plane.

Table 3.1 The incidence of flat and curved lumbar zygapophysial joints at different segmental levels. (Based on Horwitz and Smith 1940.³)

The orientation of a lumbar zygapophysial joint is, by convention, defined by the angle made by the average plane of the joint with respect to the sagittal plane (see Fig. 3.3). In the case of joints with flat articular facets, the plane of the joint is readily depicted as a line parallel to the facets. The average plane of joints with curved facets is usually depicted as a line passing through the anteromedial and posterolateral ends of the joint cavity (see Fig. 3.3). The incidence of various orientations at different levels is shown in Figure 3.4.

Figure 3.4 The orientation of lumbar zygapophysial joints with respect to the sagittal plane: incidence by level. (Based on Horwitz and Smith 1940.³) x-axis, orientation (degrees from sagittal plane); y-axis, proportion of specimens showing particular orientation.

The variations in the shape and orientation of the lumbar zygapophysial joints govern the role of these joints in preventing forward displacement and rotatory dislocation of the intervertebral joint. The extent to which a given joint can resist forward displacement depends on the extent to which its superior articular facets face backwards. Conversely, the extent to which the joint can resist rotation is related to the extent to which its superior articular facets face medially.

In the case of planar zygapophysial joints, the analysis is straightforward. In a joint with an oblique orientation, the superior articular facets face backwards and medially (Fig. 3.5A). Because of their backward orientation, these facets can resist forward displacement. If the upper vertebra in a joint attempts to move forwards, its inferior articular processes will impact against the superior articular facets of the lower vertebra, and this impaction will prevent further forward movement (see Fig. 3.5A).