A spatial understanding of the skeletal anatomy is crucial for the surgeon. Bony landmarks are used for guidance to neural compressive pathology. In the degenerative and/or pre-operated spine, normal anatomic features are distorted, imbedded in cicatrix, or absent. Safe root decompression in such circumstances can be challenging and the surgeon often must do so by dissecting along the “lateral corridor” which is the inter-connecting pathway of the facet joints, the pars interarticularis, and the inferior articular processes. The dynamic anatomy of the dorsal vertebral connections is complex. In normal anatomy, each facet joint has a stabilization contribution from the contralateral side. There are five ways by which the surgical removal of bone can destabilize a facet. The particular architecture of the joint also has relevance to this stability.
The classical depiction of muscle anatomy (and their differentiating planes) at the lower lumbar region is erroneous. A better understanding of this anatomy will allow the surgeon to use the lateral intramuscular plane for dissection to the lateral vertebral/pedicle region. Functionally, the lumbar musculature can be divided into three main groups: multifidus, pars thoracis, and pars lumborum. These groups are arranged for spinal stability during elevational movement of the spine much like cables supporting the boom of a construction crane.
For the surgeon, important knowledge of the neural anatomy is primary in reference to the nerve roots. The dermatomal distributions of their ventral rami, especially, allow for more specific delineation of the pain generating compressive process. Anatomical variation of root anatomy must also be understood especially as it pertains to conjoined nerve roots which can be damaged if their presence is unappreciate after review of the MRI, or at surgery. The afferent nociceptive neural pathways from the skeletal and discogenic spine are not clearly understood and there is evidence that sympathetic innervation/pathways may be involved. The gate control theory of somatic pain transmission accounts for clinical features of pain and also for the paradoxical pain of root compression.
Keywords: lumbar musculature, lumbar skeletal, facet joint, facet destabilization, transitional vertebrae, lumbo-sacral plexus, dermatomes, gate control theory, sympathetic nerves, sinovertebral
Get the fundamentals down and the level of everything you do will rise.
Much of the following anatomic presentation is drawn from Clinical Anatomy of the Lumbar Spine and Sacrum, 4th ed., by Bogduk and Endres. 1 The reader is directed to this invaluable text as a companion reference for clarification and review. Additional illustrations here are provided when they have particular clinical/operative relevance.
2.1 Space Definitions
Subarticular space: below medial edges of facets through which root travels prior to entering foramen (▶ Fig. 2.1).
Lateral recess: same as “subarticular space.”
Foramen: space extending laterally under pars and limited superiorly (cephalad) and inferiorly (caudally) by pedicles.
Foraminal or intraforaminal: within space of foramen.
Intracanalicular: all space in canal medial to foramen.
Extraforaminal: space lateral to foramen (edge of pars).
Safe sublaminar space: The epidural space below the superior lateral half of each lamina is between the attachments of the ligament flava to that lamina. It is dorsal to the lateral aspect of the dural sac and approaches the medial aspect of the exiting root. Thus, it provides the surgeon with a relatively safe space for exposure/passage without jeopardy to the dura and nerve root (▶ Fig. 2.2).
Fig. 2.1 (a) Foraminal roof: dorsal margin of foramen. (b) Lateral margin of foramen.
Fig. 2.2 Relatively safe access to epidural space under lamina between attachments of ligamentum flavum.
2.2 Muscular Anatomy
The classic depiction of the lumbar musculature, as divided by vertical sagittal planes, 2 is not wholly accurate at the lowest lumbar levels. 3 For the surgeon, it can be simplified into three functional components at the lower lumbar levels.
The multifidus is the largest component. It consists of overlapping bundles of paraspinous fibers that remain medial to the lateral edge of the facets (encompassed by a shiny fascia) extending caudally to the L4–L5 joint. Importantly, at this level, the fibers fan out to the ilium/sacrum with the most lateral part extending nearly horizontally into the ilium. Thus, these fanning fibers roof the more caudal lumbosacral articulation.
The multifidus, primarily oriented along the spinous processes, provides functional dynamic stability of the vertebrae in relation to each other. At its caudal end, it is anchored at the ilium/sacrum, and the ilial attachment is robust and nearly transversely oriented. The medial fibers of the multifidus are vertically oriented, attaching at right angles to the posterior margins of the spinous processes. This orientation establishes the multifidus’ vector of action as primarily that of posterior sagittal rotation (and resistance to anterior rotation). This vector allows for relatively minimal resistance to anterior translation.
The pars thoracis of the erector spinae musculature (longissimus thoracis and iliocostalis lumborum) is represented by the aponeurosis of erector spinae (AES) extending from the level of L3 caudally, merging with the fascia of the multifidus medially, and constituting the deep fascia at these lower levels. There may be some pars thoracis muscle fibers medially (i.e., on the underside of the AES adjacent to the lateral fascia of the multifidus).
The primary function of the pars thoracis is related to its connection from the pelvis to the thorax, mainly spanning the lumbar region, and is involved in stabilization of the upper body in response to gravitational forces. Thus, it allows for a controlled gravitational descent in bending, and for tension stabilization and lifting in the action of elevating the upper body in relation to the pelvis. The action of pars thoracis in the upright position increases lumbar lordosis (posterior rotation) and its unilateral action produces lateral flexion.
Pars lumborum of the erector spinae has also been classically separated into components of the longissimus thoracis and the iliocostalis lumborum. However, the function of these two components is essentially similar. Together, they attach at each level from the ancillary and transverse processes, inserting on the lateral ilium and an aponeurotic cephalad extension from this (lumbar intermuscular aponeurosis). Their main vector of action is horizontal (posterior), adding antitranslational resistance. Their vertical vector can produce lateral coronal and posterior rotational force.
Construction crane analogy: The human, as an upright biped, must have the capacity to interact physically with the ground and so must have the capacity to re-erect to an ambulatory status from that level. Evolutionary forces have created an efficient muscular/mechanical system to this effect. Certain types of construction cranes have been engineered with a stabilization system analogous to the human anatomic one (▶ Fig. 2.3).
Lateral intramuscular plane: a distinct fatty plane of separation exists between the pars lumborum and the multifidus below L3. This plane is directed to the juncture of the transverse process and superior articular process. In contradiction to the usual anatomical depiction, this plain is not vertically sagittal; rather, it is oblique. At its upper portion (L3–L4), this obliquity is longitudinal with the axis of the spine. At L5–S1, this obliquity turns laterally toward the ilium, thus following the lateral edge of the multifidus in a J-shaped curve. This plane can be found just medial to the ilium and provides a clear avenue of access for stabilization procedures at these levels (▶ Fig. 2.4).
Fig. 2.3 Construction crane demonstrating correlation of its lifting and stability mechanics with that of the human anatomy.
Fig. 2.4 Lateral intramuscular plane as oblique and “J” shaped.
2.3 Skeletal Anatomy
A thorough and three-dimensional understanding of the anatomy of the lumbar vertebral bodies and sacrum/ilium, and their interconnections, is essential. The bony anatomy here represents the signposts directing to the neural elements needing decompression. Certain landmarks are crucial for appropriate instrumentation application. These anatomic features will be represented in the technical presentations of this book.
2.3.1 Operative Landmarks to the Lateral Corridor
In certain instances, the normal anatomy has been distorted either through degenerative changes or from previous surgery(s). In the latter situation, technical difficulties arise from previous bony removal and/or from obscurity rendered by cicatrix. Thus, the surgeon must often identify a bony landmark and use it as a road map to advance to the pathology needing decompression via the lateral corridor. There are two such main landmarks and their directive relationship to the roots must be understood:
The spinous process/lamina: If this is present, it can be followed down/laterally to safely access the appropriate root:
Inferiorly (caudally) along the medial inferior articular process (IAP) for access to the root that is enumerated one greater than the designated number of the associated spinous process.
Superiorly (cranially) along the pars to access the root of same enumeration as the spinous process (▶ Fig. 2.5).
Fig. 2.5 Access to lateral corridor by following intact spinous process to lamina.
Both of these approaches lead to the facet joint, which is the other major landmark and which can be used to advance decompression sequentially further caudally and/or cephalad.
The facet joint: Often, the spinous processes have previously been removed and a facet joint can be found safely by dissection laterally. The medial aspect of the joint will direct the surgeon to the underlying root either to its proximal portion by exploring along the IAP or more distally at the foramen by exploration at the pars. Importantly, once the joint is exposed, the surgeon can advance caudally along this lateral corridor from pars–lamina–IAP to next facet, etc. (or cranially: IAP–lamina–pars to facet, etc.), for multiple root decompressions with less danger of dural disruption than in attempts to establish root anatomy from a more medial position (see Chapter 7) (▶ Fig. 2.6).
Fig. 2.6 Access to lateral corridor by direct exposure of intact facet.
2.3.2 Zygapophyseal (Facet) Joints
Functional integrity: The surgeon must know the functional stabilization/destabilization anatomy of the facet joints, with the understanding that the stability of a facet joint has contribution from the contralateral side. This knowledge is important in the consideration of various techniques for root decompression. Five ways in which facet destabilization can occur are:
The loss of functional integrity of the superior articular process/facet (rare).
The loss of functional integrity of the IAP/facet (▶ Fig. 2.7).
The loss of functional integrity of both struts of the IAP—the ipsilateral pars and the ipsilateral lamina (▶ Fig. 2.8).
The loss of functional integrity of the ipsilateral pars and the contralateral lamina (▶ Fig. 2.9).
The loss of functional integrity of both pars (bilateral facet joint destabilization) (▶ Fig. 2.10).
Fig. 2.7 Facet destabilization from loss of structural integrity of inferior articular process.
Fig. 2.8 Facet destabilization by loss of structural integrity of the two struts of the IAP: ipsilateral lamina and pars.
Fig. 2.9 Facet destabilization from loss of structural integrity of ipsilateral pars and contralateral lamina representing bilateral contribution to stability.
Fig. 2.10 Bilateral facet destabilization from loss of structural integrity of both pars.