Introduction

The development of a cell-based, biological replacement to restore, maintain, and improve the function of damaged tissues and organs has become the en vogue frontier to developing potential novel approaches to patient cures. The intervertebral disc (IVD) undergoes very extensive degenerative changes (Figure 63-1) with the various macro- and microtraumas that come with age and daily life activities. Individual differences have been demonstrated in young individuals exhibiting the disc of an elderly person and vice versa. It is generally accepted that an extremely prevalent rate and degree of asymptomatic disc degeneration exists in the general population. Therefore, differentiating normal aging from symptomatic pathological degeneration is very difficult and cannot be assessed by simply identifying the most abnormal disc on imaging (Figure 63-2). At this time, controlled provocative lumbar discography (Figure 63-3) remains our best clinical test to identify a physiologically painful structurally compromised IVD. The term ‘‘discogenic low back pain” is the term often used to indicate degenerative disc disease associated with concordant pain.

FIGURE 63-1 Macroscopic pathoanatomy evident in disc degeneration compared side by side to healthy intervertebral discs.

FIGURE 63-2 T2-weighted sagittal MRI demonstrating segmental degenerative disc desiccation and bulging at the L4-L5 level.

FIGURE 63-3 Provocative lumbar diskography demonstrating a structurally compromised fissured L5-S1 IVD with a physiologic concordant pain elicitation under controlled pressure as well as adjacent nonphysiologic and nonpainful discs at L4-L5 and L3-L4 with intact morphologies.

Relating recent findings regarding the molecular mechanisms in initiating or propagating degenerative alterations of the IVD will be crucial to the ultimate success in the developments of biologic strategies to cure discogenic low back pain.

Functional Anatomy of the Intervertebral Disc

Intervertebral discs transmit loads from body weight and muscle activity as well as provide flexibility to the spine. The discs consist of three highly specialized structures: the endplates, the annulus fibrosus, and the nucleus pulposus The two cartilaginous endplates form the inferior and superior interface between the disc and the adjacent vertebrae, thereby enclosing the disc axially. The annulus fibrosus is made up of several lamellae consisting of parallel collagen fibers interspersed by elastin fibers. Surrounded by the annulus fibrosus is the nucleus pulposus, the gelatinous core, which consists of randomly organized collagen fibers, radially arranged elastin fibers, and a highly hydrated aggrecan-containing gel. The highly hydrated proteoglycans in the nucleus pulposus are essential to maintain the osmotic pressure and therefore have a major effect on the load-bearing properties of the disc.

It is also important to note that the intervertebral disc is a largely avascular structure. With increasing age, as growth and skeletal maturation proceed, degenerative processes begin to change the morphology and therefore the function of the disc. The most widely accepted conceptual model of spinal segmental degeneration was proposed by Kirkaldy-Willis.¹ In this model, the nucleus pulposus of degenerated discs is characterized by a decreased water and proteoglycan content leading to the loss of its gel-like appearance and hydrostatic properties. Degenerative changes of the annulus fibrosus are less obvious, but result in irregular lamellae with the collagen and elastin networks becoming more disorganized. Replacing the gel-like structure of the nucleus pulposus with fibrocartilaginous tissue results in decreased flexibility and therefore often in cleft formation with fissures. Up to 50% of the cells show signs of necrosis and some of them reveal signs of apoptosis, potentially resulting in cell loss from the disc.² Although there is broad consensus about these hallmarks of degeneration, the question of whether revascularization and/or reinnervation of the inner parts of the disc may occur during degeneration is still a topic of debate.³ Although studies have described revascularization, possibly accompanied by reinnervation, of the inner parts of the IVD, it is not completely clear at which stage of degeneration these occur.³ Clarification of this question is of special importance since the interplay between neovascularization and neoinnervation might be of crucial importance for the pain sensation caused by degenerated discs. Answers to these questions may ultimately determine the rate-limiting step to the potential of biologic cures of symptomatic degenerative disc disease.

Causes of Degenerative Disc Disease

Degenerative disc disease is a complex process with a multifactorial etiology. Nutritional effects, mechanical load, and genetics all likely have contributory pathologic effects on the IVD. Of these, nutrition and removal of waste products likely play a special role in realizing the potential that intradiscal biologics may ultimately hold. Insufficient nutritional supply of the cells is thought to be the major obstacle contributing to degenerative disc disease. Cells of the IVD face the precarious situation of having to maintain a huge extracellular matrix with a ‘’fragile’’ supply of nutrients that is easily disturbed because the IVD is avascular and nutrition is dependent on diffusion. Because of the size of the intervertebral disc, the nutrients need to diffuse from a capillary network in the vertebral bodies, through the endplates and the disc matrix to the cells in the nucleus of the disc. The supply of nutrition becomes more restricted as the originally cartilaginous endplates become calcified as the degenerative process progresses. As glucose and oxygen is restricted because of diffusion distances, the removal of metabolic waste such as lactic acid becomes critically impaired. Measurements have demonstrated that as oxygen concentrations were very low in the nucleus and increased toward the disc surface, the lactic acid concentration showed the reverse profile. The buildup of lactic acid results in an intradiscal environment with a lowered pH. Low oxygen concentrations and acidic pH adversely affect the synthetic activity and proteoglycan synthesis rates of disc cells. This toxic environment may lead to a fall in proteoglycan content and therefore to degenerative disc disease. This suboptimal environment may lead to increased cell death and therefore reduced cell numbers in the disc.⁴ Ultimately, the result of poor nutritional supply of the IVD is that very few remaining cells are confronted with the task of maintaining an extensive matrix. Unfortunately, it likely holds true that the progression of matrix degeneration becomes irreversible once the cell density falls below a minimal threshold.

Therapeutic Biologic Strategies

Biologic treatments for the degenerated IVD have scarcely been utilized to regenerate or cure the painful, deteriorated disc and restore biological function. Thus far, intradiscal biologics are classified into four approaches:

FIGURE 63-4 Intradiscal injection of a “naked” biologically active factor—done to facilitate the sustained release of an agent into the cellular matrix.

FIGURE 63-5 Modification of the gene expression of resident disc cells in vivo (direct gene therapy) utilizing a viral vector resulting in transformation and sustained expression of the active protein Z.

FIGURE 63-6 Supplementation with autologous implantation of in vitro cultivated and modified cells. Cultivated cells can be genetically modified in vivo before implantation (indirect gene therapy), seeded into a scaffold, or simply implanted directly.

FIGURE 63-7 Stem-cell based gene therapy. Mesenchymal stem cells can be cultivated as progenitor cells and either injected directly into the disc matrix or be differentiated in vitro into a disc cell and then injected.

These applications aim for sustained delivery of biologically active substances to the disc that should drive regeneration or conserve the status quo. The nature of the respective active factor is hereby defined by our knowledge of the molecular mechanisms active in the disc during the various stages of degeneration. The applicability of the various approaches is largely dependent on our current knowledge of disc cell biology, the state of degeneration of the intervertebral disc, and potential safety issues.