Macro- and Microanatomy of Degenerative Disc Disease

 

Wolfgang Rauschning, MD, PhD, Department of Orthopaedic Surgery,

Academic University Hospital, S-751 85 UPPSALA, Sweden

 

The cascade of lumbar motion segment degeneration includes internal disc disruption, disc dysfunction due to the delamination of the annulus fibrosus, and also slackening and incompetence of the outermost annulus, longitudinal ligaments, inter-and supraspinous ligaments and instability/subluxation of the facet joints, all reflecting the dysfunction of the spinal segment. In the early stages of DDD these stabilizing structures are anatomically intact, although relaxed and therefore not functioning properly due to altered mechanics and insertion sites.

 

Intradiscal therapy and genetic engineering with the aim of decelerating, halting or even reversing this degenerative cascade, such as disc cell culture injection may become an alternative to fusion surgery. The biological acceleration of fusions would appear to be an alternative option. The problem with such biological options, however, is the deleterious impairment of segmental spinal mechanics that exert enormous forces on the stabilizing anatomical elements.

 

In degenerative disc disease the impairment of nutrition pathways into the disc and the inability of the disc to dissipate toxic metabolic products, create an extremely hostile intradiscal environment with low pH, the formation of protease, cytokinines, prostaglandines, hypoxidity, dehydration, loss of proteoglycans and thereby turgor (swelling pressure). This toxicity leads to irritation of the fine nociceptive nerve endings which over the age of 50 penetrate the miniscule crevices of the endplate which thereby becomes painful. The toxic environment also causes necroptosis of disc cells. Disc cell cultures injected into degenerated discs have a rather limited number of life cycles. It therefore has been stated that biochemical and biological treatment should be complemented with mechanical measures that restore some of the normal kinematics and biomechanics of the motion segment.

 

In early stages the internal disruption of the disc and early endplate changes reflect the disturbance of fluid transport through the endplate, and also a disequilibrium between the intradiscal and the intravertebral=intraosseous pressure. In later stages the cross-linkages between the annular collagen lamellae are progressively broken by a combination of malnutrition and mechanical attrition. Later stages of the disease encompass gross delamination of annular lamellae, sometimes with vacuole formation and the separation of the inflamed outermost annulus fibrosus from the remainder of the disc.

 

When tears of fissures sever the outermost annulus fibrosus, blood vessels are sprouting into the disc, frequently accompanied by nociceptive pain fibers (neovascularization). Larger and long-standing annular tears are typically sealed by a callus-type cellular granulation tissue which is richly vascularized and innervated. This granulation tissue is the pathoanatomical substrate of the High Intensity Zone (HIZ) that is frequently observed in the posterior central portion of degenerated discs on MR scans of patients complaining of non-dermatomal (mechanical) low back pain, "discogenic pain", but also in subjects without any such symptoms at all. Endoscopic and other minimally invasive treatment options for the various stages of DDD are discussed along with the pathoanatomical changes.

In the lumbar and lumbosacral spine the cascade of degenerative disc disease (DDD) is demonstrated  in view of the currently available surgical treatment options. The pathoanatomy of "low-back-pain" and "radiculopathy" is mirrored against current treatment options, ranging from chemonucleolysis, percutaneous disc ablation, a variety of laser disc ablation options, coblation, and IDET, to hydrogel nucleus prosthesis, PDN, a wide array of fusion techniques such as cages for PLIF and ALIF applications, femoral ring and precision crafted allograft fusions and artificial disc prostheses. As an intriguing alternative, the concept of neutral dynamic distractive stabilisation of the lumbar spine in painful mechanical dysstabilities and spinal stenosis in younger patients is briefly outlined.

 

We also conducted a cadaveric-experimental study pertaining to posterior percutaneous or endoscopic surgical approaches to the intervertebral discs. The study clearly showed that any uni or biportal approach to the lower lumbar spinal discs carries potential risk for injury or violating blood vessels or neural structures, in particular the delicate dorsal root ganglia.

 

In all postsurgical specimens of patients who had had posterior lumbar surgery, extensive scar transformation of the back muscles was consistently observed. Not only were the erector trunci muscles affected, but also the deep short oligosegmental muscles which account for the proprioception and fine-tuning of segmental mobility. In short as well as in long instrumentation, the scarring extended one or two levels above and below the intended instrumentation. All back muscles are contained in a non-expansile osseoaponeurotic compartment. When contracted, they constitute a powerful "dorsal soft tissue column" which stabilises the lumbar spine. Surgery must minimise violation of these muscles to avoid failed back surgery sequelae.