Chapter 1 The lumbar vertebrae

The intervertebral joints

The lumbar vertebral column consists of five separate vertebrae, which are named according to their location in the intact column. From above downwards they are named as the first, second, third, fourth and fifth lumbar vertebrae (Fig. 1.1). Although there are certain features that typify each lumbar vertebra, and enable each to be individually identified and numbered, at an early stage of study it is not necessary for students to be able to do so. Indeed, to learn to do so would be impractical, burdensome and educationally unsound. Many of the distinguishing features are better appreciated and more easily understood once the whole structure of the lumbar vertebral column and its mechanics have been studied. To this end, a description of the features of individual lumbar vertebrae is provided in the Appendix and it is recommended that this be studied after Chapter 7.

Figure 1.1 The lumbar vertebrae and how they appear in the entire vertebral column.

What is appropriate at this stage is to consider those features common to all lumbar vertebrae and to appreciate how typical lumbar vertebrae are designed to subserve their functional roles. Accordingly, the following description is divided into parts. In the first part, the features of a typical lumbar vertebra are described. This section serves either as an introduction for students commencing their study of the lumbar vertebral column or as a revision for students already familiar with the essentials of vertebral anatomy. The second section deals with particular details relevant to the appreciation of the function of the lumbar vertebrae, and provides a foundation for later chapters.

It is strongly recommended that these sections be read with specimens of the lumbar vertebrae at the reader’s disposal, for not only will visual inspection reinforce the written information but tactile examination of a specimen will enhance the three-dimensional perception of structure.

A typical lumbar vertebra

The lumbar vertebrae are irregular bones consisting of various named parts (Fig. 1.2). The anterior part of each vertebra is a large block of bone called the vertebral body. The vertebral body is more or less box shaped, with essentially flat top and bottom surfaces, and slightly concave anterior and lateral surfaces. Viewed from above or below the vertebral body has a curved perimeter that is more or less kidney shaped. The posterior surface of the body is essentially flat but is obscured from thorough inspection by the posterior elements of the vertebra.

Figure 1.2 The parts of a typical lumbar vertebra: AP, accessory process; iaf, inferior articular facet; IAP, inferior articular process; L, lamina; MP, mamillary process; NA, neural arch; P, pedicle; RA, ring apophysis; saf, superior articular facet; SAP, superior articular process; SP, spinous process; TP, transverse process; VB, vertebral body; vf, vertebral foramen.

The greater part of the top and bottom surfaces of each vertebral body is smooth and perforated by tiny holes. However, the perimeter of each surface is marked by a narrow rim of smoother, less perforated bone, which is slightly raised from the surface. This rim represents the fused ring apophysis, which is a secondary ossification centre of the vertebral body (see Ch. 12).

The posterior surface of the vertebral body is marked by one or more large holes known as the nutrient foramina. These foramina transmit the nutrient arteries of the vertebral body and the basivertebral veins (see Ch. 11). The anterolateral surfaces of the vertebral body are marked by similar but smaller foramina which transmit additional intra-osseous arteries.

Projecting from the back of the vertebral body are two stout pillars of bone. Each of these is called a pedicle. The pedicles attach to the upper part of the back of the vertebral body; this is one feature that allows the superior and inferior aspects of the vertebral body to be identified. To orientate a vertebra correctly, view it from the side. That end of the posterior surface of the body to which the pedicles are more closely attached is the superior end (Fig. 1.2A, B).

The word ‘pedicle’ is derived from the Latin pediculus meaning little foot; the reason for this nomenclature is apparent when the vertebra is viewed from above (Fig. 1.2E). It can be seen that attached to the back of the vertebral body is an arch of bone, the neural arch, so called because it surrounds the neural elements that pass through the vertebral column. The neural arch has several parts and several projections but the pedicles are those parts that look like short legs with which it appears to ‘stand’ on the back of the vertebral body (see Fig. 1.2E), hence the derivation from the Latin.

Projecting from each pedicle towards the midline is a sheet of bone called the lamina. The name is derived from the Latin lamina meaning leaf or plate. The two laminae meet and fuse with one another in the midline so that in a top view, the laminae look like the roof of a tent, and indeed form the so-called ‘roof’ of the neural arch. (Strictly speaking, there are two laminae in each vertebra, one on the left and one on the right, and the two meet posteriorly in the midline, but in some circles the term ‘lamina’ is used incorrectly to refer to both laminae collectively. When this is the usage, the term ‘hemilamina’ is used to refer to what has been described above as a true lamina.)

The full extent of the laminae is seen in a posterior view of the vertebra (Fig. 1.2D). Each lamina has slightly irregular and perhaps sharp superior edges but its lateral edge is rounded and smooth. There is no medial edge of each lamina because the two laminae blend in the midline. Similarly, there is no superior lateral corner of the lamina because in this direction the lamina blends with the pedicle on that side. The inferolateral corner and inferior border of each lamina are extended and enlarged into a specialised mass of bone called the inferior articular process. A similar mass of bone extends upwards from the junction of the lamina with the pedicle, to form the superior articular process.

Each vertebra thus presents four articular processes: a right and left inferior articular process; and a right and left superior articular process. On the medial surface of each superior articular process and on the lateral surface of each inferior articular process there is a smooth area of bone which in the intact spine is covered by articular cartilage. This area is known as the articular facet of each articular process.

Projecting posteriorly from the junction of the two laminae is a narrow blade of bone (readily gripped between the thumb and index finger), which in a side view resembles the blade of an axe. This is the spinous process, so named because in other regions of the vertebral column these processes form projections under the skin that are reminiscent of the dorsal spines of fish and other animals. The base of the spinous process blends imperceptibly with the two laminae but otherwise the spinous process presents free superior and inferior edges and a broader posterior edge.

Extending laterally from the junction of the pedicle and the lamina, on each side, is a flat, rectangular bar of bone called the transverse process, so named because of its transverse orientation. Near its attachment to the pedicle, each transverse process bears on its posterior surface a small, irregular bony prominence called the accessory process. Accessory processes vary in form and size from a simple bump on the back of the transverse process to a more pronounced mass of bone, or a definitive pointed projection of variable length.^1,² Regardless of its actual form, the accessory process is identifiable as the only bony projection from the back of the proximal end of the transverse process. It is most evident if the vertebra is viewed from behind and from below (Fig. 1.2D, F).

Close inspection of the posterior edge of each of the superior articular processes reveals another small bump, distinguishable from its surroundings by its smoothness. Apparently, because this structure reminded early anatomists of the shape of breasts, it was called the mamillary process, derived from the Latin mamilla meaning little breast. It lies just above and slightly medial to the accessory process, and the two processes are separated by a notch, of variable depth, that may be referred to as the mamillo-accessory notch.

Reviewing the structure of the neural arch, it can be seen that each arch consists of two laminae, meeting in the midline and anchored to the back of the vertebral body by the two pedicles. Projecting posteriorly from the junction of the laminae is the spinous process, and projecting from the junction of the lamina and pedicle, on each side, are the transverse processes. The superior and inferior articular processes project from the corners of the laminae.

The other named features of the lumbar vertebrae are not bony parts but spaces and notches. Viewing a vertebra from above, it can be seen that the neural arch and the back of the vertebral body surround a space that is just about large enough to admit an examining finger. This space is the vertebral foramen, which amongst other things transmits the nervous structures enclosed by the vertebral column.

In a side view, two notches can be recognised above and below each pedicle. The superior notch is small and is bounded inferiorly by the top of the pedicle, posteriorly by the superior articular process, and anteriorly by the uppermost posterior edge of the vertebral body. The inferior notch is deeper and more pronounced. It lies behind the lower part of the vertebral body, below the lower edge of the pedicle and in front of the lamina and the inferior articular process. The difference in size of these notches can be used to correctly identify the upper and lower ends of a lumbar vertebra. The deeper, more obvious notch will always be the inferior.

Apart from providing this aid in orientating a lumbar vertebra, these notches have no intrinsic significance and have not been given a formal name. However, when consecutive lumbar vertebrae are articulated (see Fig. 1.7), the superior and inferior notches face one another and form most of what is known as the intervertebral foramen, whose anatomy is described in further detail in Chapter 5

Only gold members can continue reading. Log In or Register to continue