Anat Clin (1983) 5 : 17-28 9 Springer-Verlag 1983

Original Papers Innervation of the Zygapophyseal Joints of the Lumbar Spine Pierre Auteroche Service de Chirurgie Osseuse et Vert6brale du Professeur R Louis, H6pital H6tel-Dieu, 6, place Daviel, F-13224 Marseille Cedex, France

Summary. This study based on the examination of 200 joints points out the great interindividual variability concerning the innervation of the posterior or zygapophyseal articulations of the lumbar spine. Furthermore, their innervation varies in a given individual according to the level and the side of the spine under consideration. The nerves supplying these joints are of multiple origins. The zygapophyseal articulations receive their innervation by the anterior, lateral or posterior aspect.. Rather than following a strictly metameric pattern, such nerve supply involves at least three spinal levels. The first part of this study deals with the descriptive anatomy of the best known neural structure innervating these articulations, i.e. the dorsal ramus of the spinal nerve, as well as the other nerves supplying the posterior joints. These anatomical data are then followed by a tentative synthesis of the different modes of innervation. This part of the paper treats both the origins of the nerves and the features of their distribution to the joints. Finally, the results of this study are discussed with respect to the metameric arrangement of nerve supply.

Innervation des articulations zygapophysaires du rachis lombaire R~sum~. Ce travail portant sur 200 articulations met l'accent sur la grande variabilit6 de l'innervation des articulations zygapophysaires (articulations post6rieures) du rachis lombaire, d'un individu/t l'autre, mais aussi pour un mame individu /t des 6tages diff6rents et d'un c6t6/t l'autre. Nous insistions sur la pluralit6 d'origine des nerfs, leur Offprint requests: P Auteroche, 4, rue Henri Cheneaux, F-13008 Marseille, France

abord articulaire aussi bien antdrieur, latOral, que postOrieur, leur distribution qui loin de suivre une r6partition m6tambrique stricte prend en compte un minimum de trois 6tages rachidiens. I1 s'agit dans un premier temps d'une 6tude d'anatomie descriptive de l'616ment le plus connu fi savoir la branche post6rieure du nerf spinal, mais aussi de tous les autres +lbments nerveux qui jouent un r61e dans cette innervation; c'est ensuite une tentative de synth6se des diff6rentes modalitbs d'innervation que ce soit dans l'origine des nerfs ou dans la modalitb de l'apport; c'est enfin une confrontation entre notre ~tude et la disposition m6tam6rique. Key words: Lumbar spine - Zygapophyseal joints - Innervation

Many studies have been devoted to radicular or medullar compression caused by the vertebral bodies and intervertebral discs, and to the anatomy of the spinal nerve roots and their anterior rami. Conversely, little attention has been given in the literature to the dorsal rami of the spinal nerves. Due to the relatively high frequency of lumbalgia of uncertain etiology and to the need for a better understanding of the pathology of the posterior spinal (zygapophyseal) joints this study was undertaken to elucidate the basic innervation of these articulations in the region of the lumbar spine. Embryology and Comparative Anatomy The dorsal rami of the spinal nerves develop with a slight delay compared to the ventrolateral rami, i.e. the lateral ramus precedes the medial one. Era-

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P Auteroche: Innervation of the Zygapophyseal Joints of the Lumbar Spine

bryological development regularly progresses from the deep to the superficial regions, from the lateral to medial sectors and in the craniocaudal direction. These findings are based on embryological studies of the dorsal rami of the spinal nerves and especially on the sequential aspect of such development. According to Lazorthes and Juskiewenski [13] the analysis of data from studies of comparative anatomy shows that the spinal cord undergoes progressive functional organization. Indeed, from its initially segmental organization, the spinal cord differentiates to form two functional units with respect to the development of the limbs, i. e. the cervical and lumbar swellings. These swellings of the spinal cord reflect a phenomenon of desegmentation whereas the thoracic cord can be likened practically to a simple segment of passage inserted between the swellings. As the differentiation of the limbs becomes more pronounced from the functional standpoint the functional unity of the spinal cord swellings becomes more obvious. This functional unity is thus best developed in the lumbar: swelling of homosapiens and correlates with the evolutionary adaptation to the erect position. The differential growth of the spinal cord and vertebral column, causing an apparent upward shift of the cord, does not affect the different spinal segments to the same extent, i.e. the growth differential is greatest in the region of the swellings of the spinal cord, especially the lumbar one. These embryological, chronological, medullar and comparative findings lead to several comments. The segmental arrangement and metameric organization of the spine apparently represent a necessary state in the development of the nervous system which is submitted to great variations according to the developmental stage and level of the spine under consideration. Although segmentation seems to be a model of organization in the very small embryo and in the thoracic region of the spine, it is not such a model in the adult human or the cervical and lumbar regions of the spine. Indeed, upright posture in humans has allowed to ~); the corollary to this is that man, like the mast of a ship, remains erect in equilibrium with respect to the different forces against which his powerful muscles and tendons act as braces. Much discussion has been given to the metameric organization of the nervous system. The anterior rami of a given spinal nerve seem to be organized in this way in the thoracic region. However, the question arises as to whether a metameric pattern can be applied at the level of the plexuses in the brachial, lumbar and lumbosacral regions. This question can also be applied to the dorsal

rami of the spinal nerves. Rather than to try to describe a simple system relating one nerve to one zygapophyseal articulation, this study is an attempt to identify the multiple origins of the innervation of the posterior joints of the spine. Classical Data

Many classical authors propose that the zygapophyseal joints are innervated by the dorsal rami of the lumbar spinal nerves. These rami have motor, sensory and neurovegetative fibers and innervate the muscles of the vertebral gutters and a large cutaneous surface. The posterior rami, of smaller caliber than their ventral homologs, branch of the spinal nerves at practically right angles. From their point of origin the dorsal rami run outwards, backwards and slightly downwards in contact with the superior articular process of the subjacent vertebra. Each ramus is accompanied by a branch of the dorsospinal artery. In the region just posterior to the lower part of the intercostiform muscle the dorsal ramus divides to give off one lateral and one medial branch. The lateral branch, carrying sensory and motor information, leaves the plane of the articular processes and runs over the m. intertransversarius to enter the longissimus dorsi and iliocostalis. The lateral branch finally emerges in the subcutaneous tissue at a point approximately three vertebrae below its point of origin. The short slender medial branch is almost exclusively a motor nerve. This branch describes a laterally convex curve as it skirts around the superior articular process of the subjacent vertebra. It then runs obliquely downwards and backwards to pass between the mamillary process and the interstyloid bundle to come to lie in the notch between the mamitlary and accessory processes. The medial branch continues its course between the intermamillary and mamillostyloid bundles of the subjacent intertransversarius medialis giving off small fibers to this muscle. The nerve then turns inwards and downwards, approaching the dorsal surface of the multifidus spinae where it gives off terminal flaments to this muscle and the interspinales (Figs. 1 and 2). According to Lazorthes [l 2, 13] the nerves supplying the lumbar zygapophyseal joints arise from the dorsal ramus of the spinal nerve just after the former pass through the lateral fascia to enter the posterior region. The dorsal ramus is not in intimate contact with the spinal bone (except in cases where fibrous tissue sometimes presses the nerve against the bone of the neck of the inferior

P Auteroche: Innervation of the Zygapophyseal Joints of the Lumbar Spine

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merous but the most voluminous and longest. Some articular rami, displaying a vertical course, run towards the articular interface, while others, running obliquely, approach the lower part of the articular capsule by passing over its lateral surface. The innervation of the lumbosacral junction presents n o special features except that the articular rami seem to be less numerous and of smaller caliber. Finally, neither the truncus sympathicus nor the sinuvertebral nerve (ramus meningeus nervi spinalis) give off rami to the zygapophyseal articulations. Luschka described branches arising from the sympathetic root which run towards the arches and different processes of the vertebrae (no further precision was given by the author). Fig. 1 Distribution of the dorsal ramus of the spinal nerve in the lumbar spine (according to Lazorthes and Winckler): 1 articular nerves, 2 interspinal muscle, 3 posterior branch of the spinal nerve, 4 intermamillary bundle, 5 mamillostyloid bundle, 6 interstyloid bundle, 7 lateral intertransversary bundle Distribution de la branche post~rieure du nerf rachidien de l'ttage lombaire (d'aprts Lazorthes et Winkler). 1 nerfs articulaires, 2 muscle intertpineux, 3 branche posttrieure du nerf rachidien, 4 faisceau intermamillaire, 5 faisceau mamillo-styloidien, 6 faisceau interstylo'idien, 7 faisceau intertransversaire externe

Fig. 2 Drawing to show the innervation of the posterior articulations in the horizontal plane (according to Lazorthes) Innervation des articulations post~rieures dans un plan horizontal (d'aprts Lazorthes)

articular process). On the average six articular rami, arising from the dorsal ramus run towards the superior and inferior articular processes. Of these articular rami, those presenting an ascending course (about four or five rami) are the least nu-

Personal Studies

Study Technique Dissection was carried out on adult spines (often from elderly subjects) preserved in formalin in the anatomy laboratory. The twenty cadavers (12 male, 8 female) were each dissected symmetrically at five different spinal levels, i.e. a total of 200 posterior articulations. The specimens examined comprised the spine from T 11 to its caudal termination and all the tissue lying medial to a vertical line passing through the posterior third of the right and left iliac crests. The twenty specimens studied thus contained essentially the spine and the muscles including the skin in back and the great vessels in front (aorta, inferior vena cava and their bifurcation). Each specimen thus measured roughly 50 cm in length and 25 cm in width. The whole specimens were first studied after which they were cut transversely at different pre-established spinal levels. In every case the spinal level and side of transection and sex were noted. Dissection, carried out once the dorsal ramus of the spinal nerve had been identified, was accomplished using fine instruments and magnifying eyeglasses.

Approach Dissection was always begun by the superior approach. For each spinal level the vertebral pedicle was rasped to expose the spinal nerve. The nerve was then identified out to its anterior ramus. At this point in the procedure, we began to look for the neural structures running posteriorly. After this step the anterior approach was used, involving hemicorporectomy, foraminotomy and partial (supe-

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P Auteroche: Innervation of the ZygapophysealJoints of the Lumbar Spine

rior) transversectomy often associated with rasping beginning at the upper part of the cut through the articular processes. Dissection in this manner allowed to visualize the bifurcation of the spinal nerve into its ventral and dorsal rami, the smaller nerves branching directly off the ventral ramus and the division of the dorsal ramus into its medial and lateral branches. Dissection was pursued by the lateral approach along the medial branch or branches running in contact with the superior articular process. Dissection then continued by the posterolateral and finally the posterior route. In a few cases dissection was initiated by the posterior approach. However this technique often requires the cutting of small nerve filaments supplying the muscles and posterior articulation since the posterior route lacks successive structures to guide dissection and the neural rami studied are essentially of very small caliber.

Descriptive Anatomy It is necessary to examine two complementary aspects to the innervation of the zygapophyseal joints. The first of these aspects is related to the neural structure considered to be the main source of innervation, i.e. the dorsal ramus of the spinal nerve. The second aspect involves the identification of all the other possible sources of innervation. These notions account for the different routes of dissection used in this study.

The Dorsal Ramus of the Lumbar Spinal Nerve Origin of the Dorsal Ramus. In its typical form, i.e. in the region from L2 to L5, the dorsal ramus arises 1.5 cm distal to the origin of the spinal nerve (1 cm lateral to the emergence of the spinal nerve from the intervertebral foramen). The angle formed by the division of the spinal nerve into its ventral and dorsal rami is always less than 90 ~ (most often 50 to 60~ Near its origin on the posterior surface of the spinal nerve the dorsal ramus is surrounded by abundant fatty tissue. In this region it is not in close contact with the spinal bone. Veins and arteries (in no special pattern) accompany the dorsal ramus. Its caliber is much less than that of the ventral ramus (smaller by a factor of one quarter to one fifth). In addition to the above-described typical aspect, the origin of the dorsal ramus is considerably variable. In the upper region of the lumbar spine the dorsal ramus originates more proximally on the spinal nerve. For

example, at the level of the thoracolumbar junction it arises 6 to 7 mm distal to the point of emergence of the spinal nerve from the intervertebral foramen, whereas in the lower lumbar spine this value is close to 2 cm. The caliber of the dorsal rami also progressively increases down to the level of L4 after which it then decreases in the region of the lumbosacral junction. As examination progresses from the upper to the lower part of the lumbar spine the angle between the dorsal and ventral rami also progressively declines (about 30 to 45 ~ near the lumbosacral junction) (Fig. 3).

Course of the Dorsal Ramus. The dorsal rami usually run over a short distance of about 3 to 5 mm in a direction obliquely downwards, backwards and outwards. Each ramus lies in the angle formed between the superior articular process and costiform process. In this zone the dorsal ramus is ensheathed in fatty tissue and thus is not in direct contact with the bone. As the ramus runs along this course it comes to lie in the posterior lodge of the spine. Once again, these rami display a considerable degree of anatomical variability. Their course progressively lengthens from the upper to the lower parts (lumbosacral junction) of the lumbar spine. Indeed, the course of the dorsal ramus is often extremely short at the level of the thoracolumbar junction (and in 30% of cases at the lumbar level) where it almost immediately gives off two branches. The obliquity of its course also becomes more pronounced from the upper to lower parts of the lumbar spine. Along its course the dorsal ramus turns round the superior articular process and passes posterior to the costiform process. As a general rule it then runs behind the proximal part of the upper margin of the transverse process, displaying more or less close contact with the bone. Indeed, in close to 60% of cases cleavage between the nerve and osteoperiosteal structures in this zone is relatively easy to achieve, as opposed to the veritable union often afforded by the fibrous tracts connecting the nerve to the superior articular process. As shown further on, the dorsal ramus may even be connected to the articular process by fine nerve fibers. Conversely, in about 40% of cases the dorsal ramus adheres tightly to the transverse process. In such cases partial transversectomy (from front to rear) is required to avoid damage to the nerve as it runs along the posterior surface of the upper part of the transverse process. As examination progresses from the upper to lower parts of the lumbar spine, the trunks of the dorsal rami increasingly give off fine branches in bouquet-like fashion (each

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Fig. 3 The dorsal ramus: origin, course and division to form lateral and medial branches Origine, trajet, et division de la branche post~rieure en deux branches lat6rale et m6diale Fig. 4 A defile (source of neural injury) lying between the base of the costiform process and superior articular process Un d6fi16 (source de conflit) situ6 entre la base de l'apophyse costiforme et la base de l'apophyse articulaire sup6rieure

bouquet comprising three or four branches). These branches run in the anteroposterior direction to meet and adhere to the anterior and lateral surface of the superior zygapophyseal joint (Fig. 4).

Termination of the Dorsal Ramus. In its typical pattern of termination the distal trunk of the dorsal ramus corresponds to the lateral branch which has approximately the same direction and diameter as the dorsal ramus. Conversely, the medial branch can be likened to a sort of posteroinferior collateral of the dorsal ramus rather than a true terminal branch. - The lateral branch, carrying sensory information from and motor information to the muscles, divides about one centimeter distal to its origin into two or three filaments which

run towards the muscles. The course o f the lateral branch of the dorsal ramus is posterior to the junction between the medial and middle thirds of the costiform process. It is rather easy to dissect the lateral branch from the posterior surface of the ancostiform process since its distal filaments are of relatively large caliber. The lateral branch appears as a flattened neural structure embedded in the muscle tissue. Clear exposure of the nerve requires partial superomedial transversectomy and lateral displacement of the neighboring muscles. Dissection in this way allows to expose the cobweb-like pattern of the lateral branch and its collateral filaments. In many cases examined in this study the lateral branch of the dorsal ramus terminated in arborescent pattern to innervate the

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P Auteroche: Innervation of the Zygapophyseal Joints of the Lumbar Spine

muscles and skin, rather than in distinct terminal fibers. The muscles innervated by the lateral branch are those previously described in the section on classical data. Conversely, the relatively high frequency (close to 25% of cases) of terminal filaments to the zygapophyseal joint should be underlined. In most cases the articulation is innervated by a branch arising in recurrent fashion from the superoposterior aspect of the lateral branch of the dorsal ramus. This nerve supplying the joint runs laterally upwards and backwards on the superior articular process to join the articulation laterally, near its upper end. It may originate either directly from the lateral branch of the dorsal ramus or in some cases from the bouquet-like termination of the lateral branch. Investigation of each spinal level demonstrated that the nerve supplying the zygapophyseal joint is most often present in the lower segments of the lumbar spine but not necessarily in symmetrical fashion. Dissection by the posterior approach demonstrated in a few cases the existence of very fine filaments (one or two) arising from the terminal fibers of the lateral branch of the dorsal ramus (especially from the most medial branches in the muscle). These very fine filaments run along the lateral margin of the inferior articular pillar to terminate on the lateral surface of the articular capsule, sometimes anastomosing with the articular nerve filaments arising from the medial branch of the dorsal ramus. The medial branch of the dorsal ramus. This neural structure was initially the main object of this study since it seems to be an integral part of the neuroarticular system. The medial branch arises most often from the medial surface of the dorsal ramus of the spinal nerve. Dissection relatively frequently showed that its origin may be on the posterior surface of the dorsal ramus. In such cases it was necessary to divide the main trunk or smaller terminal rami of the lateral branch of the dorsal ramus to expose to medial branch. This study also showed that a posterior or posteromedial origin of the medial branch apparently occurs more often in the caudal part of the lumbar spine. The medial branch generally arises in close proximity to the origin of the dorsal ramus, i.e. less than one cm and sometimes just a few mm distally. In a few cases the dorsal ramus gives rise directly to a veritable arborescence of fibers corresponding to the medial branch. The angle between the dorsal ramus and its medial branch is relatively acute, about 30 to 40 ~. The angle is even smaller in cases where the medial banch aarises from the posterior aspect of the dorsal ramus. In some of these cases the medial branch cannot even be identified be-

cause its initial part is directly adherent to its lateral homolog. The caliber of the medial branch is only one third to one quarter that of the lateral branch. The apparent diameter of the medial branch also decreases in the craniocaudal direction (the caliber of the smallest medial branch still in the normal range was one fifth that of the first lumbar medial branch). In almost every case there is only one medial branch per side at each level of the lumbar spine. Nevertheless, three cases of twin medial branches were found in this study, i.e. symmetrical twin branches at L3 in one subject and a right-sided twin branch at L2 in another. The medial branch runs over the superior articular process for a distance of 7 to 12 mm. Its proximal part along the lateral surface of the articular process (at the junction of the middle and lower third of the process) is relatively easy to dissect. The medial branch then runs into the dihedral angle situated at the junction formed from behind by the root of the articular process and from in front by the base of the costiform process. Certain authors [2, 3, 19] refer to this angle under the terme 'osteoligamentous defile'. Dissection in this region was always very difficult, i.e. in the area where the course of the medial branch changes from an oblique, downward, backward and medial slant to a strictly posterior direction. Indeed, in many cases it was necessary to open the joint and remove the articular bone in order to preserve the articular capsule and the nerve running along its inferior and lateral surface. This difficulty led us to repeat the bony dissection and to study the anatomy on dry bone specimens. In the post mortem material from elderly subjects a veritable fissure was often found in the proximal part of the roots of the upper articular and costiform processes. In a few cases it was even necessary to free the nerve on each side of the bony fissure in order not to damage the intraosseous part of the nerve. Accordingly, it is easy to understand the possible neural injury caused by the bone in this region, and the even greater risk of neural insult in the presence of osteophytic outgrowth. Subsequent to its passage of one or two mm through this zone the medial branch emerges at the posterior surface of the joint with the subjacent vertebra. The emergence of t h e medial branch is on the posterolateral surface of the zygapophyseal joint, i.e. well behind the costiform process, also affording protection to the ventral branch of the spinal nerve, and should thus not be damaged when surgical maneuvers are carried out on the nerves of the posterior joints of the lumbar spine. Additionally, the process can be used as a topographical landmark and point

P Auteroche: Innervation of the ZygapophysealJoints of the Lumbar Spine of support during surgical procedures. In its course the medial branch, generally unaccompanied by other structures, crosses behind the slender articular blood vessels (usually one artery and one or two veins) and gives off filaments to the articulation. There are usually three to five ascending filaments near the lateral part of the joint, each entering the articular capsule after a course of 2 to 4 ram. These neural filaments give rise to very slender fibers on the lateral and then the posterior surface of the joint. In cases of a twin medial branch of the spinal nerve the homologous neural trunks run parallel to each other, without interconnecting fibers, until they reach the posterior surface of the joint. At this point in their course anastomotic fibers are found between them as they penetrate the bone to form ascending and descending filaments. The medial branch of the spinal nerve terminates on the posterior surface of the zygapophyseal joint. After its emergence at the inferolateral pole of the articulation the medial branch, describing a superiorly concave curve, gives off three types of filaments. The ascending filaments (usually 3 or 4) distribute to the posterior surface of the joint. Although apparently not fewer in number than the descending filaments they are much thinner, i.e. they comprise many slender ramiffed fibers rather than a few large fibers grouped together as in the case of the descending filaments. Indeed the overall form of the medial branch of the spinal nerve resembles an 'S'. The first curve of the 'S' runs along the inferolateral margin of the upper articulation while the second much longer curve lies along the upper pole of the subjacent articulation. It can be noticed that the upper concave part of the medial branch lies closer to the supra- and not the subjacent articulation. Thus, the ascending neural filaments fan out over a short distance to reach the upper joint. Conversely, the descending filaments remain grouped together over a distance of several millimeters before they reach (sometimes forming fine ramified fibers) the subjacent joint. Posterior nerve supply to the upper joint is thus by the lateral route whereas that of the lower joint is predominently medial. The medial branch of the dorsal ramus thus forms a loop with superior concavity on the posterior surface of the zygapophyseal joint and gives off fine nerve filaments which run upwards to the suprajacent articulation. The terminal part of the medial branch runs inwards to give rise to the following: a) one or two nerve filaments which course upwards along the posteromedial surface of the joint. These filaments are present in about only one fifth of cases; b)superficial filaments arising

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very medially with respect to the spine as a whole. These filaments innervate the muscle and fascia; c) one or two much larger nerve filaments which descent to reach the upper part of the subjacent joint. In their downward course, these filaments often pass behind small blood vessels. The one or two descending filaments progressively decrease in size in the craniocaudal direction and thus are sometimes very difficult to identify at the level o f the lumbosacral junction. The techniques used in this study did not allow to follow the terminal nerve filaments where they run through the articular capsule. However, points of anastomosis were seen relatively often. In the region of the inferomedial part of the suprajacent joint the filaments arising from the terminal part of the medial branch merge with the filaments descending from above or with the terminal filaments of a collateral arising from the suprajacent lateral branch. Indeed, in about one third of the joints examined, the final articular terminals of a given medial branch did not innervate the immediately subjacent joint but rather another more caudal joint. These terminal nerve filaments also fan out to innervate the muscle and skin. A more detailed study of them would thus have required greater optical amplification than that used in this study. To sum up the study of the dorsal rami of the spinal nerves with respect to their innervation of the posterior joints of the spine, the following points should be underlined: a) the importance of the nerve filaments arising anteriorly; b ) t h e marked variability of the nerve distribution to the joints; c) the predominance and intimate nature of the osteoarticular innervation by the medial branch of the dorsal ramus; d) the twin, ascending and descending, innervation of the supra- and subjacent joints by a given medial branch; e ) t h e highly ramified and anastomotic aspect of the terminal nerve filaments, which rather than forming a segmental mode of innervation, corresponds to the concept of the lumbar spine as a single unit submitted to numerous constraints. The second part of this study deals with the other neural structures which contribute to the innervation of the posterior joints of the spine.

Other Neural Structures Innervating the Zygapophyseal Joints The results of this study and review of the literature lead to consider the following three types of neural structures: nerves arising from the spinal nerve itself, proximal to its division into ventral and

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P Auteroche: Innervation of the Zygapophyseal Joints of the Lumbar Spine

Fig. 5 Example of the anterior nerve filaments branching off the trunk of the dorsal ramus Exemple des branches ant6rieures issues du tronc de la branche post~rieure

Fig. 6 Nerve filaments arising from the trunk of the spinal nerve to innervate the zygapophyseal joint Des rameaux du tronc du neff rachidien ~i destin6e articulaire

dorsal rami; nerves arising from the ventral ramus of the spinal nerve; the sinuvertebral nerve.

Nerves Arising from the Spinal Nerve Proximal to the Origin of the Ventral and Dorsal Rami. Fine rami (two to five on average) of the spinal nerve were fairly often seen to inervate the posterior articulations in this study. Exposure of these rami required dissection by the superior approach to the intervertebral foramen to isolate the spinal nerve. After freeing the spinal nerve from the surrounding fibrous tissue, it can be displaced anteriorly to expose fine rami which run in the anteroposterior direction to reach the facing posterior joint. These rami approach the joint by its anterior aspect. Smaller nerve filaments arising from them

run upwards along the lateral surface of the joint. The rami of the spinal nerve innervating the posterior joints are more numerous and constant in the caudal part of the lumbar spine. In about one fourth of cases, these rami arise from a common nerve trunk originating a few millimeters proximal to the division of the spinal nerve into its ventral and dorsal rami (Figs. 5 and 6).

Nerves Arising J?om the Ventral Ramus of the Spinal Nerve. In most of the cases studied the ventral ramus gave off a single branch from its posterior or lateral surface at a point about 1 to 1.5 cm distal to the bifurcation of the spinal nerve. This branch of the ventral ramus presents a recurrent ascending course describing a curve with inferior and posteri-

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or concavity. In its course it divides to give off filaments to the muscle and one or two more medial filaments which run to the lateral surface of the zygapophyseal joint. The articular innervation from the ventral rami also seems to increase in density in the craniocaudal direction. In should be noted that the nerves arising from the ventral rami are not necessarily symmetrical.

A The Sinuvertebral Nerve. A survey of the literature led to examine in this study the degree to which the sinuvertebral nerve contributes to the innervation of the posterior joints of the lumbar spine. The sinuvertebral nerve has two roots of origin, i.e. a cerebrospinal and a sympathetic root. The cerebrospinal root arises from either the spinal nerve [9] or the spinal ganglion (Luchka). Lying below this root is the slender sympathetic root which usually arises from the most posterior ramus communicans of the subjacent spinal ganglion. In some cases, the sympathetic root arises from the ganglion or is of mixed origin (ramus communicans and ganglion). This purely metameric pattern of the sinuvertebral nerve is truly typical of only the thoracic segment of the spine. However, studies by Laux et al. [10] and by Lazorthes demonstrated that although a truly metameric arrangement of the sinuvertebral nerves may be lacking, there consistently exists 'conjugate innervation' which is equivalent to metameric distribution. In the lumbar spine in particular the arrangement of the sinuvertebral nerves is very similar to that in the thoracic segment. The present study confirms the previously mentionned reports, especially with respect to the notion that both the truncus sympathicus and sinuvertebral nerves do not give off nerve fibers to the zygapophyseal joints in the thoracic and lumbar spine. This finding is in contrast to the innervation of these joints by nerve fibers arising directly from the spinal nerve. Histological study of these nerve fibers may help to elucidate their rote. Indeed, it is conceivable that the major component of vegetative innervation of the articular capsules passes through these neural structures.

Synthetic View of the Zygapophyseal Innervation and Systematization Modes of Innervation Origin of the Nerves. The dorsal rami of the spinal nerves are the major source of innervation. These rami successively give off the following types of

i* *

aG

Fig. 7 Origins of the different nerves distributing to the lumbar zygapophyseal joints: 1 dorsal ramus of spinal nerve, 2 medial branch of dorsal ramus, 3 lateral branch of dorsal ramus, 4 ventral ramus of spinal nerve, 5 trunk of spinal nerve prior to its bifurcation into ventral and dorsal rami Sch~matisation des diffrrentes origines des nerfs des articulations zygapophysaires du rachis lombaire: 1 branche post~ricure du nerf rachidien, 2 branche mrdiale de la branche postrricure, 3 branche latrrale de la branche postrrieure, 4 branche antrrieure du nerf rachidien, 5 tronc du nerf rachidien avant sa division en branche antrrieure et branche post~rieure

nerves to the articulations: a) nerve filaments arising from the main trunk of the dorsal ramus. As a general rule they innervate the anterior and lateral parts of the articulation; b)filaments arising from the medial branch o f the dorsal ramus. Some of these filaments innervate the anterior and lateral parts of the suprajacent joint while others run to the posterior part of the supra- and subjacent joints (ascending and descending filaments); c) nerve filaments arising from the lateral branch of the dorsal ramus. These filaments generally run in recurrent fashion to the suprajacent joint. However, there are some descending filaments which run along the inferolateral border of the posterior surface of the joint to merge with the filaments innervating the superolateral part of the subjacent joint. The terminal parts of the nerve filaments originating from the medial and lateral branches of the dorsal ramus are anastomosed to one another. Finally, a significant part of the anterior innervation of each joint is derived from the nerves arising from the ventral ramus and the spinal nerve itself (proximal to its bifurcation) (Fig. 7).

Modes of Distribution. The innervation of the zygapophyseal joints is derived from two sets of nerves,

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P Auteroche: Innervation of the Zygapophyseal Joints of the Lumbar Spine

i.e. an anterior and lateral set and a posterior set. The anterior and lateral nerves comprise essentially the filaments originating from the main trunk of the spinal nerve, the dorsal ramus and the proximal part of its medial branch. Nerve filaments arising from the proximal segment of the lateral branch of the dorsal ramus and from the ventral ramus sometimes constitute a complement to these nerves. The number and size of the anterior and lateral nerves progressively increase in the craniocaudal direction. They innervate mainly the suprajacent articularion (this is especially true of the most anterior nerves). Proper understanding of the posterior set of nerves requires that the lumbar vertebra be examined with respect to its articulations with the vertebrae lying immediately above and below it. When studied in this way, each lumbar vertebra presents three poles in respect to the innervation of its zygapophyseal joints: a) The superior pole of the suprajacent articulation. Innervation of this pole is derived from the ascending terminal filaments of the anterior and lateral nerves (mentionned above). These filaments first give off fine anterior and lateral articular fibers and then ramify on the superior pole of the posterior surface of the joint. Such innervation is relatively sparce (very fine terminal filaments); b) The inferior pole of the suprajacent articulation. Innervation in this zone is by fine short filaments arising from the upper concave part of the medial branch as it runs over the posterior surface of the articulation. These filaments can be divided into a lateral, a median and a minor medial group; c) The superior pole of the subjacent articulation. The nerves supplying this region are much larger and longer than those of the previously mentionned pole. They reach the superior pole of the articulation to ramify on its midline, lateral and medial (less consistently) aspects. The anastomoses of the different nerves (between mainly the filaments arising from the lateral and medial branches of a given dorsal ramus, or between the medial branches of two contiguous dorsal rami) usually lie along the lateral margin of the posterior surface of the joint. In the joints studied in this paper, the medial nerve filaments were consistently scarce, i.e. usually corresponding to very fine fibers innervating the muscle and skin (Figs. 8 and 9).

Systematization The spinal nerves consist of three types of nerve fibers: efferent somatic motor fibers whose axons lie in the anterior grey column of the spinal cord;

Innervation of the lumbar zygapophyseal joints: left posterior oblique view. L1-L2 Division of the spinal nerve to form ventral (A) and dorsal (P) rami. L2-L3 Division of the dorsal ramus into medial (M) and lateral (L) branches. L3-L4 Nerve filaments arising from the trunk of the spinal nerve and the dorsal ramus. L4-L5 Recurrent nerve arising from the ventral ramus of the spinal nerve. L5-$1 Nerve filaments arising from the trunk of the spinal nerve. (Note that the drawing has been greatly simplified so that the different modes of innervation can be associated) Sch6matisation de l'innervation des apophyses zygapophysaires du rachis lombaire: sch6ma trois quart post6rieur gauche. L1L 2 Division du nerf rachidien en deux branches ant~rieure (A) et post6rieure (P). L2-L3 Division de la branche post6rieure en deux branches m6diale (M) et lat6rale (L). Ls-L4 Rameaux issus du tronc du nerf rachidien et du tronc de la branche post~rieure. L4-L~ Branche r6currente issue de la branche ant6rieure du neff rachidien. Ls-SI Branches issues du tronc du nerf rachidien. Simplification sch6matique volontaire (permettant en r+alit6 l'association de ces diffbrentes modalitbs)

afferent somatic sensitive fibers whose cell bodies lie in the spinal ganglia; sympathetic motor, vasomotor, secretory and sensitive fibers. The nerve fibers which make up the peripheral nerves carry efferent, motor information and afferent, sensory

P Auteroche: Innervation of the Zygapophyseal Joints of the Lumbar Spine

Innervation of the lumbar zygapophyseal joints : posterior view, Division of the dorsal ramus to form medial and lateral branches. Ascending nerve filaments to the suprajacent joint (these filaments are short and few in number). Descending nerve filaments to the subjacent joint (these filaments are long and numerous. Anastomoses among these different nerves. (Note that the drawing has been greatly simplified so that the different modes of innervation can be associated) Sch6matisation de l'innervation des articulations zygapophysaires du rachis lombaire: vue post6rieure. Division en branche m6diale et lat+rale. Branches ascendantes (articulation sus-jacente) (peu nombreuses et courtes). Branches descendantes: nombreuses, longues (articulation sous-jacente). Anastomoses entre ces diff6rentes branches. Simplification sch6matique volontaire (permettant en r6alit6 l'association de ces diff6rentes modalit6s)

information relating to the different types of sensibility. The spinal cord presents a veritable segmental metameric pattern allowing the cord to be divided i n t o myelomeres. Much work has been devoted to identify the motor and cutaneous sensitive territories of each myelomere corresponding to each pair of spinal nerve roots. In this way, certain authors have proposed the existence of a radicular or more precisely a radiculomedullar

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metameric pattern. This concept of the spinal cord with its paired spinal nerve roots leads to envisage a sort of neural segment corresponding to each myelomere. Each of these segments or neuromeres would comprise a system of peripheral innervation and connections with the laterovertebral sympathetic trunks (the most typical example of such a neuromere would be the thoracic spinal nerve). This metameric concept has greatly aided our understanding o f the neural components of the spine and seems to be appropriate when applied to the spinal cord. Conversely, the practical application of this concept becomes problematic with respect to the ventral rami of the spinal nerves, e.g. the formation of plexuses with very rich anastomotic networks, and presents serious limits when one' attempts to apply it to the dorsal rami, and thus to the innervation of the lumbar zygapophyseal joints. Indeed, the present study shows that the 'functional unit' of such articular innervation spans at least three vertebral levels. Furthermore, neural anastomoses are frequently found between the filaments arising from the same branch of the dorsal ramus, from different branches at the same level and from branches lying at different spinal levels. It was also found that a given articular nerve was intimately connected (by means of collaterals, terminal filaments or anastomoses) to the musculofascial and cutaneous structures adjacent to and at some distance from it or its corresponding articulation. Accordingly, four osteoarticular structures of the lumbar spine can be easily identified along the course of a given nerve from its origin near the intervertebral foramen to its points of innervation of the posterior joints and finally out to its termination in the muscle and skin. Finally, this study underlines the highly variable modes of innervation of the zygapophyseal joints of the lumbar spine, i.e. inter- and intraindividual variability in the craniocaudal direction sometimes associated with a lack of symmetry. Thus, radicular topography seems to markedly contrast with the metameric concept when applied to the innervation of the zygapophyseal articulations of the lumbar spine.

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P Auteroche: Innervation of the Zygapophyseal Joints of the Lumbar Spine

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12. Lazorthes G, Gaubert J (1956) Innervation des articulations interapophysaires vertrbrales. C R Assoc Anat, Lisbonne 1956 13. Lazorthes G, Juskiewenski S (1964) Etude comparative des branches postrrieures des nerfs dorsaux et lombaires et de leurs rapports avec les articulations interapophysaires vert6brales. Bull Assoc Anat XLIX ~ Rrunion, Madrid, 10 septembere 1964, pp 1025--1033 14. Lazorthes Y, Verdie JC, Lagarrique J (1976) Thermocoagulation percutanre des nerfs rachidiens/t visre analg~sique: neurochirurgie 22: 445-453 15. Maigne R (1974) Origine dorso-lombaire de certaines lombalgies basses. Rfle des articulations inter-apophysaires et des branches postrrieures des nerfs rachidiens. Rev Rhum Mal Osteoartic 41:781-789 16. Mooney V, Robertson (1975) The facet syndrom. Clin Orthop Rel Res 115:i46-149 16a. Ogsbury JS, Simon RKH, Lehman (1977) Facet "denervation" in the treatment of low back. Syndrom Pain 3: 257-263 17. Shealy CN (1976) Facet denervation in the magement of back and sciatic pain. Clin Orthop 115:157-164 18. Shealy CN (1974) Technique for percutaneous spinal facet rhizotomy. Procedure technique series. Radiotonics, Inc., Burlington, Massachussets 19. Yinchuani (1978) Anatomical observations on lumbar nerve posterior rami. Chin Med J 4:492