Lumbar Spine Make-Up

The typical vertebral column is composed of 33 vertebrae. The lumbar spine usually has five mobile lumbar vertebrae, denoted as L1–L5. As a group, the lumbar vertebrae create a lordotic curve. The vertebral bodies increase in size as the spinal column descends, because of the increasing demands of load bearing. The lumbar vertebrae have distinct features that make them discernible from the cervical and thoracic vertebrae. Most notable are the large vertebral bodies, which consist of cancellous bone surrounded by cortical bone. These bodies are wider transversely than they are deep anteroposteriorly. They also develop into a wedge shape as they descend the column, with the L5 vertebra having the greatest difference between anterior and posterior height. This difference creates the lumbosacral angle. The pedicles are short and thick, arising from the upper third of the body. The transverse processes are thin, long, and flat in the anteroposterior (AP) plane. The articular processes are vertical and large with a rounded enlargement on the posterior border, known as the mammillary process. The superior facets face posteromedially with a somewhat concave surface. The inferior facets project downward and face largely laterally and anteriorly in concordance with the superior facets, with a slightly convex articulating surface. The inferior facet of the L5 vertebrae differs by having a flat articulating surface that faces largely anteriorly. The spinous processes are short and broad and project perpendicularly from the body.

Transitional Segments

The typical lumbar spine has five lumbar vertebrae, but up to 10% to 15% of the population is recognized with an anatomical variant known as a lumbosacral transitional vertebra. The optimal method of classifying transitional segments was outlined by Castellvi in 1984. Castellvi described a classification system using radiographic imaging, identifying four groups of lumbosacral transitional vertebrae based on their morphological characteristics ( Fig. 1.1 ). Type I includes a dysplastic transverse process, either unilateral (Ia) or bilateral (Ib), presenting as triangular in shape, and measuring at least 19 mm in width. Type II exhibits incomplete lumbarization/sacralization, either unilateral (IIa) ( Fig. 1.2 ) or bilateral (IIb), with a large transverse process that follows the contour of the sacral ala. These are recognized as incomplete by the appearance of a diarthrodial joint between the transverse process and the sacrum. Type II is most consistently associated with lower back and buttock pain. Type III describes complete lumbarization/sacralization, either unilateral (IIIa) or bilateral (IIIb), in which the transverse process has made complete fusion to the sacrum. Type IV is mixed, with these patients exhibiting characteristics of type II on one side and type III on the other. This system is useful in classifying the morphology of the transitional segments, but it does not provide enough accurate information for numbering the involved segments.

Castellvi classification system: Ia, Ib, IIa, IIb, IIIa, IIIb, IV.

Left Castellvi type IIa transitional vertebrae.

Anteriorly Based Fusions and Approaches

The three ways to approach the spine anteriorly are direct anterior, oblique, and lateral. They all provide excellent access to the disc space and end-plate preparation, and, if necessary, allow placement of implants with large footprints that minimize the risk of subsidence. More importantly, we know from biomechanical studies that the anterior and middle columns carry about 80% of spinal loads. When taking this into consideration along with Wolff’s law of bone remodeling in response to applied stress, a fusion mass has better potential for healing if placed anteriorly, where it is under direct compression. Not only is an anterior fusion under direct compression, but the anterior and middle column provide 90% of osseous contact between vertebrae, as well as a more vascular bed. Distraction with a large interbody implant provides better neuroforaminal decompression. Additionally, it can be very powerful in deformity correction in the coronal and sagittal planes. These approaches are not suitable for patients that have central canal stenosis, bony lateral recess stenosis, or high-grade spondylosis.

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  Direct anterior: In the direct anterior approach, the patient is positioned supine and this is followed by a median, paramedian, or mini-Pfannenstiel incision. This provides a retroperitoneal corridor with direct access to the disc space. Presurgical advanced imaging is necessary, as it will determine the limitations imposed by visceral structures. Most often, the direct anterior approach is used to access the L4–L5 and L5–S1 disc spaces, with higher levels limited by the degree of retraction on the vascular and renal structures. There are drawbacks associated with this approach such need for an access surgeon, risk of vascular injury, and the possibility of retrograde ejaculation secondary to injury to the hypogastric plexus. Previous abdominal surgeries are not an absolute contraindication to this approach, but should be taken into consideration as they might make the exposure more difficult. Placement of a ureteral stent can aid in the approach, and with a skilled approach, revision anterior surgery can be done.

  
  
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  Oblique: For the oblique approach to the anterior spine, the patient is placed in a lateral decubitus position. Along the flank, the surgical corridor is between the retroperitoneum and the psoas muscle. This approach does not require retraction of vascular structures or violation of the psoas muscle. As such, the levels accessible to this approach are from L1 to S1. Although vascular structures are not manipulated directly, they are still at risk given their proximity with the surgical field. The risk of retrograde ejaculation is still present. Because the approach involves dissection through the abdominal musculature, patients can develop hernias or pseudohernias secondary to denervation of the flank musculature. Furthermore, the approach requires experience to adequately assess its obliquity without inadvertent entry into the canal or violation of the lateral annulus. Clinical experience suggests that this is more technically challenging than the standard straight anterior approach and to date no current data is available on revision oblique approaches.

  
  
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  Lateral: The patient is placed in the lateral decubitus position. For this approach, the surgical corridor is retroperitoneal but transpsoas. The disc spaces accessible through this approach are T12–L1 to L4–L5. Preoperative images need to be obtained to determine if the planned region of the lumbar spine can be accessed through this corridor, with particular attention to the relationship between the top of the iliac crest and the level to be accessed. Care must be taken at the L4–L5 level to avoid injuring the femoral nerve. This approach allows bilateral access to the lumbar spine, which is advantageous for coronal plane deformity correction, as it is better managed from the convex side. The lumbar plexus is at risk during a transpsoas approach, its position drifting more anteriorly with more-caudal levels. About 5% of patients complain of sensorimotor disturbances post surgically. Because the approach is to the lateral abdominal musculature, the possibility of hernia and pseudohernia are also present, but are minimized with careful dissection to prevent denervation. This approach allows resection of the disc transversely all the way across, but the risks of this are contralateral vessel or visceral injury.

Posteriorly Based Fusions and Approaches

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  Posterior: For this approach, the patient is positioned prone, which is followed by a midline approach. The fusion procedure involves decortication of the lamina, with or without the spinous processes, and typically involves the facet joints. This fusion mass is in the least advantageous position biomechanically, as it must support all the loading via a cantilever loading technique, thus experiencing minimal compression compared with its anterior counterpart. Even with a solid posterior fusion, there have been instances of persistent anterior column pain demonstrated by discography and later confirmed by clinical improvement after anterior interbody fusion. Although this fusion has long been the workhorse of the available surgical techniques, it is not without its problems. Revision surgery through a posterior approach is common. The main increased risks from this are bleeding of the scar bed, distorted landmarks depending on the prior intervention, and possible incidental durotomy in patients with significant laminectomy defects.

  
  
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  Posterolateral: The posterolateral fusion is the more commonly used posterior approach and involves not only the lamina and facet, but also the transverse processes. This puts the axis of loading closer to the fusion mass and in a more biomechanically advantageous position. A drawback of this technique is that it requires more stripping of the muscles to gain access to the grafting area. A variation of this is the paraspinal or Wiltse-type approach. This variation provides similar access with less muscle disruption and for many years was a mainstay for fusion in young adults with spondylolysis. It has also been used in a minimally invasive fashion as a way of accessing the paraspinal or Wiltse plane. However, more typically, the minimally invasive approaches have involved a transforaminal lumbar interbody fusion.

Interbody Fusion From a Posterior Approach

A number of techniques over the years have used this method of obtaining interbody fusion. It began with the posterior lumbar interbody fusion, which involves significant lamina resection and side-to-side dural mobilization to access the disc space and place bone graft into this area. The next advancement was the so-called transforaminal lumbar interbody fusion, which was originally described as a single-sided approach that allowed cleaning out of approximately two-thirds of the disc space and obtained an interbody fusion with bone graft and/or structural interbody support. These techniques use a working window in which the thecal sac and traversing nerve root form the medial border and the exiting nerve root of the proximal vertebra forms the lateral border. This technique has subsequently been a workhorse approach when done bilaterally and combined with Smith-Peterson osteotomy for both extensive disc clean-out and bone grafting structural interbody support, which allow significant sagittal plane realignment.

Osteoclasts

Osteoclasts are specialized cells derived from the monocyte-macrophage lineage; these cells degrade bone to allow for normal and pathological bone remodeling. Differentiation into an osteoclast requires receptor activator of nuclear factor kappa-B ligand (RANK Ligand) although its function is regulated by many other cytokines. Upon recruitment to areas of bone targeted for resorption, precursor cells will fuse with each other to form multinucleated cells. These cells have powerful acid-producing and enzyme-secreting machinery to resorb calcified bone and degrade the extracellular matrix. Bone mass and quality have been shown to be directly related to osteoclast activity, with all acquired forms of osteoporosis resulting from increased activity of these cells relative to osteoblasts. Bisphosphonates have been developed to mitigate osteoclast-mediated bone loss. Another therapy aimed at slowing bone resorption by osteoclasts is calcitonin. Normally, this peptide is closely linked to bone metabolism and has been shown to reduce vertebral osteoporotic fractures in postmenopausal women when administered on a daily basis. Besides their role in bone health, osteoclasts have been shown to be involved in osteoblast differentiation, mobilization of hematopoietic cells from the bone marrow into the bloodstream, and immune responses.

Osteoblasts

Osteoblasts are mesenchymal cells involved in depositing and maintaining bone architecture. They do so by producing organic components, such as bone and collagen, and the inorganic components of the calcium/phosphate matrix. Additionally, they have an important role in maintaining bone health by regulating osteoclast function through the production of a decoy receptor for RANKL, the osteoprotegerin molecule. Parathyroid hormone (PTH) is closely linked to calcium metabolism and has been shown to have anabolic effects on bone physiology. When administered at low and intermittent doses, activation of the PTH pathway, as shown with teriparatide, can act through osteoblasts to improve bone mass and architecture, providing physicians with another tool to combat the deleterious effects of osteoporosis. As bone matures, osteoblasts can become trapped in the matrix they deposit, turning into residing osteocytes. These osteocytes act as mechanoreceptors to orchestrate the appropriate balance between osteoblasts and osteoclasts as they function to maintain adequate bone homeostasis.

Wolff’s Law

Wolff’s law states that bone will remodel in respond to the stresses applied to it. In this way, bone that is exposed to higher loads will respond by increasing its mass to better resist external pressures. The opposite is also true, where bone that experiences decreased loads will adapt by reducing its mass, such as in long-term bedridden patients. This concept has important implications in spine fusion, where increased loading can be helpful to promote bone formation and improve the likelihood of arthrodesis. From previous studies we know that 70% to 80% of axial loads through the spine pass anteriorly through the vertebral bodies. Here interbody fusion devices seem to have their best utility; the compressive load that is applied across the device provides optimal conditions for bone formation.