Anuli fibrosi

As described in Chapter 2, each anulus fibrosus consists of collagen fibres running from one vertebral body to the next and arranged in concentric lamellae. Furthermore, the deeper lamellae of collagen are continuous with the collagen fibres in the fibrocartilaginous vertebral endplates (see Ch. 2). By surrounding the nucleus pulposus, these inner layers of the anulus fibrosus constitute a capsule or envelope around the nucleus, whereupon it could be inferred that their principal function is to retain the nucleus pulposus (Fig. 4.1).

Figure 4.1 The anulus fibrosus as a ligament. The inner fibres of the anulus which attach to the vertebral endplate form an internal capsule that envelopes the nucleus pulposus. The outer fibres of the anulus which attach to the ring apophysis constitute the ‘ligamentous’ portion of the anulus fibrosus.

In contrast, the outer fibres of the anulus fibrosus are attached to the ring apophysis (see Ch. 2). For various reasons it is these fibres that could be inferred to be the principal ‘ligamentous’ portion of the anulus fibrosus. Foremost, like other ligaments they are attached to separate bones, and like other ligaments they consist largely of type I collagen, which is designed to resist tension (see Ch. 2). Such tension arises during rocking or twisting movements of the vertebral bodies. During these movements the peripheral edges of the vertebral bodies undergo more separation than their more central parts, and the tensile stresses applied to the peripheral anulus are greater than those applied to the inner anulus. In resisting these movements the peripheral fibres of the anulus fibrosus are subject to the same demands as conventional ligaments, and function accordingly.

As outlined in Chapter 2 and considered further in Chapter 8, the anulus fibrosus functions as a ligament in resisting distraction, bending, sliding and twisting movements of the intervertebral joint. Thus, the anulus fibrosus is called upon to function as a ligament whenever the lumbar spine moves. It is only during weight-bearing that it functions in concert with the nucleus pulposus.

Anterior longitudinal ligament

Conventional descriptions maintain that the anterior longitudinal ligament is a long band which covers the anterior aspects of the lumbar vertebral bodies and intervertebral discs (Fig. 4.2).¹ Although well developed in the lumbar region, this ligament is not restricted to that region. Inferiorly it extends into the sacrum, and superiorly it continues into the thoracic and cervical regions to cover the anterior surface of the entire vertebral column.

Figure 4.2 Classic descriptions of the anterior longitudinal ligament (ALL) and the intertransverse ligaments (ITL). The arrows indicate the span of various fibres in the anterior longitudinal ligament stemming from the L5 vertebra.

Structurally, the anterior longitudinal ligament is said to consist of several sets of collagen fibres.¹ There are short fibres that span each interbody joint, covering the intervertebral disc and attaching to the margins of the vertebral bodies (Figs. 4.2, 4.3). These fibres are inserted into the bone of the anterior surface of the vertebral bodies or into the overlying periosteum.^2,3 Some early authors interpreted these fibres as being part of the anulus fibrosus,⁴ and there is a tendency in some contemporary circles to interpret these fibres as constituting a ‘disc capsule’. However, embryologically, their attachments are always associated with cortical bone, as are ligaments in general, whereas the anulus fibrosus proper is attached to the vertebral endplate.² Even those fibres of the adult anulus that attach to bone do so by being secondarily incorporated into the ring apophysis (Ch. 2), which is not cortical bone. Because of these developmental differences, the deep, short fibres of the anterior longitudinal ligament should not be considered to be part of the anulus fibrosus.

Figure 4.3 A median sagittal section of the lumbar spine to show its various ligaments. ALL, anterior longitudinal ligament; ISL, interspinous ligament: v, ventral part; m, middle part; d, dorsal part; PLL, posterior longitudinal ligament; SSL, supraspinous ligament. LF, ligamentum flavum, viewed from within the vertebral canal and in sagittal section at the midline.

Covering the deep, unisegmental fibres of the anterior longitudinal ligament are several layers of increasingly longer fibres. There are fibres that span two, three and even four or five interbody joints. The attachments of these fibres, like those of the deep fibres, are into the upper and lower ends of the vertebral bodies.

Although the ligament is primarily attached to the anterior margins of the lumbar vertebral bodies, it is also secondarily attached to their concave anterior surfaces. The main body of the ligament bridges this concavity but some collagen fibres from its deep surface blend with the periosteum covering the concavity. Otherwise, the space between the ligament and bone is filled with loose areolar tissue, blood vessels and nerves. Over the intervertebral discs, the anterior longitudinal ligament is only loosely attached to the front of the anuli fibrosi by loose areolar tissue.

Because of its strictly longitudinal disposition, the anterior longitudinal ligament serves principally to resist vertical separation of the anterior ends of the vertebral bodies. In doing so, it functions during extension movements of the intervertebral joints and resists anterior bowing of the lumbar spine (see Ch. 5).

Posterior longitudinal ligament

Like the anterior longitudinal ligament, the posterior longitudinal ligament is represented throughout the vertebral column. In the lumbar region, it forms a narrow band over the backs of the vertebral bodies but expands laterally over the backs of the intervertebral discs to give it a serrated, or saw-toothed, appearance (Fig. 4.4). Its fibres mesh with those of the anuli fibrosi but penetrate through the anuli to attach to the posterior margins of the vertebral bodies.³ The deepest and shortest fibres of the posterior longitudinal ligament span two intervertebral discs. Starting at the superior margin of one vertebra, they attach to the inferior margin of the vertebra two levels above, describing a curve concave laterally as they do so. Longer, more superficial fibres span three, four and even five vertebrae (see Figs 4.3 and 4.4).

Figure 4.4 The posterior longitudinal ligament. The dotted lines indicate the span of some of the constituent fibres of the ligament arising from the L5 vertebra.

The posterior longitudinal ligament serves to resist separation of the posterior ends of the vertebral bodies but because of its polysegmental disposition, its action is exerted over several interbody joints, not just one.

Ligamentum flavum

The ligamentum flavum is a short but thick ligament that joins the laminae of consecutive vertebrae. At each intersegmental level, the ligamentum flavum is a paired structure, being represented symmetrically on both left and right sides. On each side, the upper attachment of the ligament is to the lower half of the anterior surface of the lamina and the inferior aspect of the pedicle (Figs. 4.3, 4.5). Its smooth surface blends perfectly with the smooth surface of the upper half of the lamina. Traced inferiorly, on each side the ligament divides into a medial and lateral portion.^5–7 The medial portion passes to the back of the next lower lamina and attaches to the rough area located on the upper quarter or so of the dorsal surface of that lamina (see Fig. 4.5). The lateral portion passes in front of the zygapophysial joint formed by the two vertebrae that the ligament connects. It attaches to the anterior aspects of the inferior and superior articular processes of that joint, and forms its anterior capsule. The most lateral fibres extend along the root of the superior articular process as far as the next lower pedicle to which they are attached.⁷

Figure 4.5 The ligamentum flavum at the L2–3 level. (A) Posterior view. (B) Anterior view (from within the vertebral canal). The medial (M) and lateral (L) divisions of the ligament are labelled. The shaded areas depict the sites of attachment of the ligamentum flavum at the levels above and below L2–3. In (B), the silhouettes of the laminae and inferior articular processes behind the ligament are indicated by the dotted lines.

Histologically, the ligamentum flavum consists of 80% elastin and 20% collagen.^7,8 Elastic fibres proper are found throughout the ligament but at its terminal ends the ligament contains modified fibres consisting of elastin and microtubules, and known as elaunin.⁸

As an elastic ligament, the ligamentum flavum differs from all the other ligaments of the lumbar spine. This difference has prompted speculation as to its implied unique function. Its elastic nature has been said to aid in restoring the flexed lumbar spine to its extended position, while its lateral division is said to serve to prevent the anterior capsule of the zygapophysial joint being nipped within the joint cavity during movement. While all of these suggestions are consistent with the elastic nature of the ligament, the importance of these functions for the mechanics of the lumbar spine is unknown. It is questionable whether the ligamentum flavum contributes significantly to producing extension,⁹ and no disabilities have been reported in patients in whom the ligamentum flavum has been excised, at single or even multiple levels. Biomechanical studies have revealed that the ligamentum flavum serves to pre-stress the intervertebral disc, exerting a disc pressure of about 0.70 kg cm⁻²,¹⁰ but the biological significance of this effect remains obscure.

A plausible explanation for the unique nature of the ligamentum flavum relates more to its location than to its possible biomechanical functions. The ligamentum flavum lies immediately behind the vertebral canal, and therefore immediately adjacent to the nervous structures within the canal. As a ligament, it serves to resist excess separation of the vertebral laminae. A collagenous ligament in the same location would not function as well. A collagenous ligament could resist separation of the laminae, but when the laminae were approximated, a collagenous ligament would buckle. Were the ligament to buckle into the vertebral canal it would encroach upon the spinal cord or spinal nerve roots and possibly damage them. On the other hand, by replacing such a collagenous ligament with an elastic one, this buckling would be prevented. From a resting position, an elastic ligament stretches and thins. When relaxed again, the ligament simply assumes its original thickness. Buckling does not occur or is minimal. Therefore, by endowing the ligamentum flavum with elastic tissue, the risk of nerve root compromise is reduced.

Interspinous ligaments

The interspinous ligaments connect adjacent spinous processes. The collagen fibres of these ligaments are arranged in a particular manner, with three parts being identified (see Fig. 4.3).¹¹ The ventral part consists of fibres passing posterocranially from the dorsal aspect of the ligamentum flavum to the anterior half of the lower border of the spinous process above. The middle part forms the main component of the ligament, and consists of fibres that run from the anterior half of the upper border of one spinous process to the posterior half of the lower border of the spinous process above. The dorsal part consists of fibres from the posterior half of the upper border of the lower spinous process which pass behind the posterior border of the upper spinous process, to form the supraspinous ligament. Anteriorly, the interspinous ligament is a paired structure, the ligaments on each side being separated by a slit-like midline cavity filled with fat. This cavity is not present more posteriorly.

Histologically, the ligament consists essentially of collagen fibres, but elastic fibres occur with increasing density in the ventral part of the ligament, towards its junction with the ligamentum flavum.^8,12

The fibres of the interspinous ligament are poorly disposed to resist separation of the spinous processes; they run almost perpendicularly to the direction of separation of the spinous processes. Indeed, X-ray diffraction studies have indicated a greater dispersal of fibre orientation than that indicated by dissection, with many fibres running roughly parallel to the spinous processes¹³ instead of between them. Accordingly, contrary to traditional wisdom in this regard, the interspinous ligaments can offer little resistance to forward bending movements of the lumbar spine.¹³

Supraspinous ligament

The supraspinous ligament lies in the midline. It runs posterior to the posterior edges of the lumbar spinous processes, to which it is attached, and bridges the interspinous spaces (see Fig. 4.3). The ligament is well developed only in the upper lumbar region; its lower limit varies. It terminates at the L3 spinous process in about 22% of individuals, and at L4 in 73%; it bridges the L4–5 interspace in only 5% of individuals, and is regularly lacking at L5–S1.^11,14

Upon close inspection, the nature of the supraspinous ligament as a ligament can be questioned. It consists of three parts: a superficial; a middle; and a deep layer.¹⁴ The superficial layer is subcutaneous and consists of longitudinally running collagen fibres that span three to four successive spinous processes. It varies considerably in size from a few extremely thin fibrous bundles to a robust band, 5–6 mm wide and 3–4 mm thick, with most individuals exhibiting intermediate forms.¹⁴

The middle layer is about 1 mm thick and consists of intertwining tendinous fibres of the dorsal layer of thoracolumbar fascia and the aponeurosis of longissimus thoracis (see Ch. 9).

The deep layer consists of very strong, tendinous fibres derived from the aponeurosis of longissimus thoracis. As these tendons pass to their insertions on the lumbar spinous processes, they are aggregated in a parallel fashion, creating a semblance of a supraspinous ligament, but they are clearly identifiable as tendons. The deepest of these tendons arch ventrally and caudally to reach the upper border of a spinous process, thereby constituting the dorsal part of the interspinous ligament at that level. The deep layer of the supraspinous ligament is reinforced by tendinous fibres of the multifidus muscle (see Ch. 9).

It is therefore evident that the supraspinous ligament consists largely of tendinous fibres derived from the back muscles and so is not truly a ligament. Only the superficial layer lacks any continuity with muscle, and this layer is not present at lower lumbar levels. Lying in the subcutaneous plane, dorsal to the other two layers and therefore displaced from the spinous processes, the superficial layer may be rejected as a true ligament and is more readily interpreted as a very variable condensation of the deep or membranous layer of superficial fascia that anchors the midline skin to the thoracolumbar fascia. It affords little resistance to separation of the spinous processes.¹³

At the L4 and L5 levels, where the superficial layer is lacking, there is no semblance of a longitudinally orientated midline supraspinous ligament, and the true nature of the ‘ligament’ is revealed. Here, the obliquely orientated tendinous fibres of the thoracolumbar fascia decussate dorsal to the spinous processes and are fused deeply with the fibres of the aponeurosis of longissimus thoracis that attach to the spinous processes.

Intertransverse ligaments

The so-called intertransverse ligaments (see Fig. 4.2) have a complicated structure that can be interpreted in various ways. They consist of sheets of connective tissue extending from the upper border of one transverse process to the lower border of the transverse process above. Unlike other ligaments, they lack a distinct border medially or laterally, and their collagen fibres are not as densely packed, nor are they as regularly orientated as the fibres of true ligaments. Rather, their appearance is more like that of a membrane.³ The medial and lateral continuations of these membranes suggest that rather than being true ligaments, these structures form part of a complex fascial system that serves to separate or demarcate certain paravertebral compartments. Indeed, the only ‘true’ ligament recognised in this area is the ligament of Bourgery which connects the base of a transverse process to the mamillary process below.³

In the intertransverse spaces, the intertransverse ligaments form a septum that divides the anterior musculature of the lumbar spine from the posterior musculature, and embryologically the ligaments arise from the tissue that separates the epaxial and hypaxial musculature (see Ch. 12). Laterally, the intertransverse ligaments can be interpreted as dividing into two layers: an anterior layer, otherwise known as the anterior layer of thoracolumbar fascia, which covers the front of the quadratus lumborum muscle and a posterior layer which blends with the aponeurosis of the transversus abdominis to form the middle layer of thoracolumbar fascia (see Ch. 9).

Towards the medial end of each intertransverse space, the intertransverse ligament splits into two leaves (Fig. 4.7).²³ The dorsal leaf continues medially to attach to the lateral margin of the lamina of the vertebra that lies opposite the intertransverse space. Inferiorly, it blends with the capsule of the adjacent zygapophysial joint. The ventral leaf curves forwards and extends forward over the lateral surface of the vertebral bodies until it eventually blends with the lateral margins of the anterior longitudinal ligament. In covering the lateral aspect of the vertebral column, it forms a membranous sheet that closes the outer end of the intervertebral foramen. This part of the leaf is marked by two perforations which transmit structures into and out of the intervertebral foramen. The superior opening transmits the nerve branches to the psoas muscle. The inferior opening transmits the ventral ramus of the spinal nerve and the spinal branches of the lumbar arteries and veins.

Figure 4.7 The ventral and dorsal leaves of the intertransverse ligament. (Based on Lewin et al. 1962,²³ with permission.) D, dorsal leaf; MB, medial branch of dorsal ramus; V, ventral leaf; VR, ventral ramus of spinal nerve.

Enclosed between the ventral and dorsal leaves of the intertransverse ligament is a wedge-shaped space, called the superior articular recess. This recess serves to accommodate movements of the subadjacent zygapophysial joint. It is filled with fat that is continuous with the intra-articular fat in the joint below, through the foramen in its superior capsule. The superior articular process of this joint projects into the bottom end of the recess, and during extension movements of the joint, its inferior articular process moves inferiorly, pulling the superior articular recess, like a sleeve, over the medial end of the superior articular process. During this process the fat in the recess acts as a displaceable space-filler. At rest, it maintains the space in the recess but is easily moved out to accommodate the superior articular process. A reciprocal mechanism operates at the inferior pole of the joint, where a pad of fat over the vertebral lamina maintains a space between the lamina and the multifidus muscle into which the inferior articular process can move.

Transforaminal ligaments

The transforaminal ligaments are narrow bands of collagen fibres that traverse the outer end of the intervertebral foramen. Five types of such bands have been described, according to their specific attachments (Fig. 4.8):²⁴

Figure 4.8 The transforaminal ligaments. (Based on Golub and Silverman 1969.²⁴) (A) Superior and inferior corporotransverse ligaments. (B) Superior transforaminal ligament. (C) Middle transforaminal ligament. (D) Inferior transforaminal ligament.

Transforaminal ligaments are not always present. The overall incidence of all types is around 47%, with the superior corporotransverse being the most common type (27%).²⁴ For two reasons, they are not strictly ligaments. Firstly, their structure resembles bands of fascia more than ligaments proper. Secondly, except for the inferior corporotransverse ligament, they do not connect two separate bones, and the midtransforaminal variety is not connected to any bones. Accordingly, they are more correctly interpreted as bands of fascia, and in view of their location it is most likely that they represent thickenings in the ventral leaf of the intertransverse ligament.

Mamillo-accessory ligament

A tight bundle of collagen fibres of variable thickness bridges the tips of the ipsilateral mamillary and accessory processes of each lumbar vertebra (Fig. 4.9). This structure has been called the mamillo-accessory ligament²⁵ but it is not a true ligament because it connects two points on the same bone. Moreover, its cord-like structure resembles a tendon more than a ligament, and indeed it has been interpreted as representing a tendon of the semispinalis musculature in the lumbar region.²⁵ The ligament may be ossified, converting the mamillo-accessory notch into a bony foramen. The prevalence of this change was found in one study to be 10% at the L5 level,²⁵ while in another study it was 28% at L5, 10% at L4 and 3% at L3.²⁶

Figure 4.9 The mamillo-accessory ligaments (MAL). AP, accessory process; MP, mamillary process. Note the foramina under the ligaments, through which pass the medial branches of the lumbar dorsal rami.

The ligament has no biomechanical significance, but its significance lies in the fact that it covers the medial branch of the dorsal ramus of the spinal nerve as it runs through the mamillo-accessory notch. Furthermore, when the ligament is ossified, the foramen it forms can be an apparent anomaly evident on CT scans.²⁷ Ossification of the ligament, however, is a normal phenomenon without any pathological significance. It has been suggested that the ligament may be a site of entrapment of the nerve beneath it,²⁸ but this has not been verified clinically.