Chapter 3 The zygapophysial joints – detailed structure
The lumbar zygapophysial joints are formed by the articulation of the inferior articular processes of one lumbar vertebra with the superior articular processes of the next vertebra. The joints exhibit the features typical of synovial joints. The articular facets are covered by articular cartilage, and a synovial membrane bridges the margins of the articular cartilages of the two facets in each joint. Surrounding the synovial membrane is a joint capsule which attaches to the articular processes a short distance beyond the margin of the articular cartilage (Fig. 3.1).
Figure 3.1 A posterior view of the L3–4 zygapophysial joints. On the left, the capsule of the joint (C) is intact. On the right, the posterior capsule has been resected to reveal the joint cavity, the articular cartilages (AC) and the line of attachment of the joint capsule (- – -). The upper joint capsule (C) attaches further from the articular margin than the posterior capsule.
The articular facets of the lumbar vertebrae are ovoid in shape, measuring some 16 mm in height and 14 mm in width, and having a surface area of about 160 mm2. The facets of upper vertebrae are slightly smaller than these values indicate; those of the lower vertebrae are slightly smaller.1
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.
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).
Figure 3.5 The mechanics of flat lumbar zygapophysial joints. A flat joint at 60° to the sagittal plane affords resistance to both forward displacement (A) and rotation (B). A flat joint at 90° to the sagittal plane strongly resists forward displacement (C) but during rotation (D) the inferior articular facet can glance off the superior articular facet. A flat joint parallel to the sagittal plane offers no resistance to forward displacement (E) but strongly resists rotation (F).
Similarly, the medial orientation of the superior articular facets allows them to resist rotation. As the upper vertebra attempts to rotate, say, anticlockwise as viewed from above, its right inferior articular facet will impact against the right superior articular facet of the vertebra below, and further rotation will be arrested (Fig. 3.5B).
Maximum resistance to forward displacement will be exerted by the superior articular facets that are orientated at 90° to the sagittal plane, for then the facets face fully backwards and the entire articular surface directly opposes the movement (Fig. 3.5C). Such facets, however, are less capable of resisting rotation, for during rotation the inferior articular facet impacts the superior articular facet at an angle and is able to glance off the superior articular facet (Fig. 3.5D).
Joints orientated parallel to the sagittal plane afford no resistance to forward displacement. The inferior articular facets are able simply to slide past the superior articular facets (Fig. 3.5E). However, such joints provide substantial resistance to rotation (Fig. 3.5F).
In essence, therefore, the closer a joint is orientated towards the sagittal plane, the less it is able to resist forward displacement. Resistance is greater the closer a joint is orientated to 90° to the sagittal plane.
In the case of joints with curved articular surfaces, the situation is modified to the extent that particular portions of the articular surface are involved in resisting different movements. In curved joints, the anteromedial end of the superior articular facet faces backwards, and it is this portion of the facet that will resist forward displacement. As the upper vertebra attempts to move forwards, its inferior articular facets will impact against the anteromedial portion of the superior articular facets of the vertebra below (Fig. 3.6A). The degree of resistance will be proportional to the surface area of the backward-facing, anteromedial portion of the superior articular facet. Thus, C-shaped facets (Fig 3.6A) have a larger surface area facing backwards and afford greater resistance than J-shaped facets (Fig. 3.6B), which have only a small portion of their articular surface facing backwards.