1 Applied Anatomy and Percutaneous Approaches to the Lumbar Spine After careful review of the anatomy of the spine in humans and in various primates, Putz and Müller-Gerbl concluded that the lumbar portion of the vertebral column has the ideal structure to simultaneously optimize the functions of mobility and stability.1 However, low back pain is patients’ predominant reason for seeking medical advice in the modern era, and it is necessary for a spine surgeon to have a thorough knowledge of clinical and surgical anatomy of this region. Because the present text focuses on percutaneous endoscopic disk surgeries, the discussion is restricted to the anatomy relevant to endoscopic spine surgery. • Although identifying the appropriate vertebral level for a percutaneous procedure can be easily accomplished by fluoroscopy, knowledge of the surface anatomy is necessary for a better topographic orientation for surgery. • The most prominent and possibly the only palpable landmarks of the lower back are the lumbar spinous processes. In comparison to thoracic spinous processes, they present a flatter surface at the posterior tip. • The L4 and L5 spinous processes are shorter than other lumbar segments and are sometimes difficult to palpate, especially the L5 spinous process. The L4 spinous process is the last spinous process that shows movement on palpation during flexion–extension range of motion (ROM). • Generally, the L4 spinous process is in a horizontal plane with the superior boundary of the iliac crest; in 20% of the population, however, the iliac crest is in line with the L5 spinous process.2 • The tips of the transverse processes are located ~ 5 cm from the midline and are not palpable. • The level of the superior border of the iliac crest in relation to the corresponding disk space is important in the surgeon’s decision-making process. • When viewed from above, the vertebral bodies of the lumbar spine are large and kidney-shaped. • They are larger in males than in females. • The pedicles of the lumbar vertebrae are short and stout. • The superior vertebral notch is less distinct than in the cervical region, but the deep inferior vertebral notch forms the roof of the neural foramen. • The transverse processes project posterolaterally after originating from the junction of the lamina and pedicle on the same side. • They lie anterior to the articular process but posterior to the intervertebral foramen (IVF). • The lumbar transverse processes are quite long, with those of L3 being the longest. • The intertransverse distance at L4–L5 is much smaller than at L3–L4 and is even smaller at L5–S1. • The accessory process projects from the posteroinferior part of the transverse process with the corresponding lamina. • Two superior articular processes are found for each lumbar vertebra, with a hyaline cartilage–covered facet at the end of each process. • The facets are oriented in a vertical plane. Each lumbar articular facet faces posteromedially. • The orientation of the superior articular facet varies with different vertebral levels; for example, the L4 superior facets (L3–L4 joint) are more sagittally oriented than the L5 facets (L4–L5 joint). Also the L5–S1 joint is more coronally oriented than the L5 facet (L4–L5 joint). • There are two inferior articular processes, each with a facet that exactly conforms to the superior facet of the vertebral body below. • The facets of the superior and inferior articular processes of a vertebra form a zygapophyseal joint. • The facet joint is a synovial type of joint, with a surrounding joint capsule (Fig. 1.1). The IVF is an area of great importance for percutaneous endoscopic procedures, both because it harbors the exiting nerve root and other vascular structures and because it is the area of entry for endoscopic spinal access (Fig. 1.2). Fig. 1.2 Anatomy of the intervertebral foramen and its boundary. (From Laser Anatomy videodisc series Clinical and Imaging Anatomy of the Lumbar Spine and Sacrum by Wolfgang Rauschning, MD, PhD.) The boundaries of the foramen contain two mobile joints—intervertebral disk (IVD) and zygapophyseal joints. Because of the mobility of these joints, the dimensions of the foramen change dynamically. The boundaries are: • Roof: inferior vertebral notch of the pedicle of the superior vertebra, and ligamentum flavum at its outer free edge • Floor: superior vertebral notch of the pedicle of the inferior vertebra, and posterosuperior margin of the inferior vertebral body • Anterior wall: the posterior aspect of the adjacent vertebral bodies, the IVD, lateral expansion of the posterior longitudinal ligament, and the anterior longitudinal venous sinus • Posterior wall: posteriorly bounded by the superior and inferior articular process of the facet joint at the same level as the foramen, and the lateral prolongation of the ligamentum flavum • Medial wall: dural sleeve • Lateral wall: a fascial sheet and overlying psoas muscle • Spinal nerves (combined ventral and dorsal root in the root sheath) • Dural root sleeve, which becomes continuous with the epineurium of the spinal nerve at the distal end of the foramen • Lymphatic channels • The spinal branch of a segmental artery, which, after entering the foramen, divides into three branches to supply the posterior arch, neural and intracanal structures, and posterior part of the vertebral bodies • Communicating veins between internal and external vertebral venous plexuses • Two to four recurrent meningeal (sinuvertebral) nerves • Adipose tissue surrounding all the structures • The greatest superoinferior dimension of the IVF is at L2–L3, and the dimension decreases from superior to inferior, which means it is smallest at L5–S1. • The anteroposterior dimension remains more or less constant at all levels in the lumbar spine and is less than the superoinferior dimensions (at L5–S1, however, the anteroposterior dimension is greater than the superoinferior dimension). • From L1 to L4, the IVF has a shape similar to an inverted pear, whereas at L5 the foramen is more oval. • IVF dimensions in the male are slightly larger than in the female. • With aging and degeneration, the foraminal dimensions change. • More recent knowledge of the ligaments suggests that ligaments are found mainly at three locations in the foramen: the internal, intraforaminal, and external zones. • Internal ligaments are found in the inferior aspect of the medial portion of the foramen, attaching the posterolateral aspect of the disk to the anterior surface of the superior articular process, bridging the superior vertebral notch of the inferior vertebra, and converting the notch into a compartment through which veins commonly course. • The intraforaminal ligaments have three basic types: • External ligaments have a common attachment to the base of the transverse process. From this they fan out in three different directions: superior, inferior, and transverse, attaching to the vertebral body of the same or a lower vertebra. They also form many small compartments through which neurovascular elements pass to or from the spinal canal. • During percutaneous endoscopic lumbar diskectomy, the ligaments are not distinguished separately, because they have a minimal effect on the success of the operation. • The plexus of veins surrounding the external aspect of the vertebral column is called the external venous plexus. • The veins can be divided into anterior and posterior, depending on their location in relation to the vertebral body. • They communicate with segmental veins and also with the internal venous plexus through the IVF and transosseous channels. • The internal venous plexus is composed of veins located beneath the bony elements of the vertebral foramina (e.g., laminae, spinous processes, pedicles, and bodies) and embedded in a layer of loose adipose tissue. • The veins contain many interconnected longitudinal channels coursing anteriorly and posteriorly to the canal, forming the Batson plexus. • The peculiarity of this plexus is that it is a valveless plexus. • Most surgical bleeding is from the venous plexus around the foraminal area. This can be a potential hazard in the foraminal adipose tissue, requiring careful attention during dissection in the area. For successful surgery, it is essential to avoid or to control bleeding. • External arterial supply to the lumbar spine is from the lumbar segmental arteries. • The lumbar segmental arteries send spinal branches to the vertebral canal through the IVF. • On entering the IVF, each spinal branch divides into three further branches. • One branch courses posteriorly to supply the laminae, ligamentum flavum, spinous processes, articular processes, posterior epidural tissue, and dura. • The anterior branch supplies the posterior aspect of the vertebral body. • A third branch (neural branch) courses to the spinal nerve and supplies the ventral and dorsal roots. • It is important to review the foraminal sagittal MRI for any abnormal arteries in the lower part of the foramen at the entry site for the procedure. An abnormal artery can be a surgical contraindication. The neural structures important to percutaneous procedures are described here. • The spinal cord generally ends at or around the lower level of the L1 lumbar vertebra, due to differential growth of the spinal cord and vertebral somites. • Dorsal and ventral roots, originating around the level of the thoracolumbar junction, travel through the lumbar cistern as the cauda equina before entering the dural sheath. • The lumbar nerve root has to travel a considerable distance into the nerve root canal (the region in which the nerve splits from the dura to the lateral boundary of the relevant IVF) with a more oblique course before reaching the destined IVF. • The enlargement at the end of the dorsal root just proximal to the point at which it joins the ventral root is called the dorsal root ganglion (DRG). • The DRG increases in diameter from L1 to L5. • Because the S1 root canal is short, the DRG for S1 mostly lies intraspinally. • Hasegawa et al (1996) categorized the ganglia topographically as intraspinal (within the canal), foraminal (within the IVF), and extraforaminal.3 • The DRG of the L1–L5 root lies mostly inside the IVF, with the upper ganglia lying more laterally and lower ganglia more medially in the IVF. • As mentioned, the S1 root ganglion is mostly intraspinal. • Care must be taken to prevent manipulation or heat injury to the DRG, because the most frequent complication is a postoperative dysesthesia. Fig. 1.3 Triangular safe zone (Kambin triangle): a zone for safe access to the pathology (disk herniation) via endoscopic instruments.
1.1 Introduction
1.2 Surface Anatomy
1.3 Osseous Anatomy
1.3.1 Vertebral Bodies
1.3.2 Pedicles
1.3.3 Transverse Processes
1.3.4 Articular Processes
Superior Articular Processes
Inferior Articular Processes
Facet Joints
1.4 Anatomy of the Intervertebral Foramen
1.4.1 Boundaries of the Foramen
1.4.2 Structures in the Intervertebral Foramen
1.4.3 Characteristics of the Intervertebral Foramen
1.4.4 Accessory Ligaments of the Intervertebral Foramen
The first type runs from the base of the pedicle to the inferior border of the same vertebral body. The recurrent meningeal nerve and a branch of the spinal artery are commonly found in the compartment formed by this ligament.
The second type of ligament attaches to the angle formed by the posterior end of the pedicle with the base of the transverse process and extends to the posterolateral part of the same vertebral body. A large branch of segmental artery travels through this anterosuperior compartment.
The third type of ligament originates from the upper anterior portion of the superior articular facet, extending to the posterolateral body of the vertebra above. The exiting root lies directly over this ligament.
1.5 Vascular Anatomy of the Foraminal Region
1.5.1 Venous Supply
External Venous Plexus
Internal Venous Plexus
1.5.2 Arterial Supply
External
Internal
1.6 Nerves of the Lumbar Spine in Relation to the Intervertebral Foramen
1.6.1 Dorsal and Ventral Roots and Spinal Nerves
1.6.2 Dorsal Root Ganglia
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Percutaneous Endoscopic Lumbar Diskectomy: Transforaminal Approach
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Percutaneous Endoscopic Interlaminar Lumbar Diskectomy: Structural Preservation Technique for L5-S1 Herniated Nucleus Pulposus
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