Research Review by Dr. Shawn Thistle©

Date:

Aug. 2008

Study Title:

Current understanding of lumbar intervertebral disc degeneration: A review with emphasis upon etiology, pathophysiology, and lumbar Magnetic Resonance Imaging

Authors:

Beattie P

Author’s Affiliation: Physical Therapy, Department of Exercise Science, University of South Carolina, USA.

Publication Information:

Journal of Orthopaedic & Sports Physical Therapy 2008; 38(6): 329-340.

Summary:

Lumbar intervertebral discs (IVDs) are thought to be critically important for support and stability of the spine, and yet remain extremely controversial when discussing back pain etiology and treatment.

IVDs have a remarkable capacity to maintain stability while permitting intersegmental motion, even under extreme loading and dynamic movement demands. Unfortunately, they are also susceptible to some of the most severe degenerative changes of any tissue in the body, which can contribute, directly or indirectly, to lower back pain clinical syndromes. IVDs still conjure much controversy in the medical and manual therapy fields, stemming from lingering uncertainty about their contribution to pain syndromes, how to identify them clinically as pain generators, and how to best treat them.

In order to facilitate evidence-based discussion with patients and colleagues, this review will provide an executive summary of this paper, which presented a review of the current knowledge on IVD pathology regarding degenerative disc disease (DDD).

Etiology of Disc Degeneration
  • the strongest predictors of DDD are genetic factors, not environmental factors (smoking, manual labour, vibration exposure etc.) as was once thought
  • these genetic factors influence the size and shape of spinal structures, as well as synthesis and breakdown of IVD structural components
  • studies show a high prevalence of family history of DDD in patients who are diagnosed with a herniated disc before age 21
  • exposure to smoking and a history of heavy manual loading have not been supported as meaningful etiological factors for DDD
Normal Lumbar IVDs
  • a healthy IVD is composed of concentrically arranged layers of fibrocartilage that surround an amorphous, proteoglycan gel (nucleus)
  • this creates a hydraulic system that can transmit compressive, shear, and tensile forces
  • healthy discs create a “spacer effect” – maintaining ideal vertical distance between vertebrae, allowing for proper spinal ligament tension and facet joint function
Four Tissue Layers of the Disc and Other Anatomical Considerations
  1. Outer Annulus - consists of dense, type I collagen arranged in a lamellar configuration; reinforced by the anterior and posterior longitudinal ligaments; contains almost all of the IVD’s neurovascular supply.
  2. Inner Annulus - also contains thick, type I collagen fibers but they lack parallel organization, differentiating into a transitional zone closer to the inside.
  3. Transition Zone - composed of thin, fibrous tissue.
  4. Nucleus Pulposus - composed mainly of water held in a glycosaminoglycan suspension (containing chondroitin and keratin sulphate); loosely bound together by irregularly arranged elastin and type II collagen.
  • the vertebral endplate is a flat plate of cartilage 0.1-1.6mm in thickness acting as a semipermeable barrier between the subchondral bone of the vertebral body and the IVD
  • the portion of the endplate most adjacent to the vertebral body is primarily composed of hyaline cartilage that has a weak attachment to the subchondral bone – this is a “weak spot” when the IVD is exposed to any sort of trauma
  • IVDs are mainly avascular, and hence must rely on diffusion for transport of small molecules and bulk fluid flow for transport of larger molecules
  • in healthy young adults, IVDs can lose ~ 1 mm of height throughout the course of a day – note that degenerative discs generally do not display this diurnal height variation
  • disc height loss of even a few millimeters can overload facet joints and reduce foraminal cross-sectional area
Pathophysiology of IVD Degeneration
  • ”Disc Degeneration” encompasses a wide array of morphological and biochemical changes that can manifest in many ways, most commonly as a bulge or herniation, and involve specific regions of the IVD, or the entire structure
  • common features of degeneration (DDD – degenerative disc disease) include changes to the nucleus, annulus, and adjacent osseous structures, which can occur over the course of a lifetime
  • signs of degeneration most commonly appear in the third decade of life, and are considered universal by the 7th-8th decade (that is, it could be considered a normal part of aging)
  • a primary age-related change that is thought to contribute to DDD is reduced nutritional capacity within the disc
  • as thinner type II collagen is replaced by thicker type I collagen in the nucleus, tissue-fluid exchange is blocked
  • loss of hydrostatic pressure in the disc results from microtrauma in one, or many regions of this disc, which eventually reduce disc stiffness and alter fluid pressure
  • the endplate is a typical failure point following axial trauma, and are increasingly being shown to have a strong association with DDD
  • injuries to the endplate can also immediate decompress the nucleus, directly affecting hydrostatic properties
  • repeated motion can cause persistent stresses on the longitudinal ligaments and outer annulus, leading to osteophyte formation
  • Schmorl’s Nodes result from endplate injuries and are associated with bone marrow edema, but are generally not thought to result in clinical pain syndromes
Annular Tears - three types occur independent of age and one another:
1) Peripheral Rim Lesions
in the outer annulus; likely occur after trauma; can cause pain due to high concentration of pain receptors in this region of the disc
2) Circumferential Tears are splits between the layers of the annulus, thought to occur after repetitive compressive stress
3) Radial Fissures are annular disruptions that spread outwards – these are the primary lesions associated with DDD as they provide pathways for nuclear material to migrate outwards
  • annular tears have limited capacity to heal, particularly as they occur closer to the center of the disc
  • in outer disc regions, healing occurs through the inflammatory process but results in poorly remodeled scars; inner region tears tend to accumulate granulation tissue that does not restore tensile properties of the tissue
  • both of these processes will also impair oxygen diffusion within the disc, leading to further degeneration
  • further, it is ironic that the body’s attempt to heal annular tears may actually proliferate pain generation, as the healing area reduces its pain stimulation threshold
Disc Bulges - when enough nuclear material migrates through a radial tear but remains contained, a bulge can occur (also called a contained herniation)

Disc Herniation - when nuclear material breaks free of the outer annulus and migrates to the neural foramen, resulting in an inflammatory cascade, dorsal root ganglia and nerve root irritation, radiating leg pain, etc.

Imaging Considerations
  • it is now widely accepted that imaging findings of DDD do not correlate well with clinical presentation or treatment outcome
  • on MRI – endplate injuries can be visualized as having high signal intensity on a T2 weighted image – this is known as a “modic sign”
Clinical Relevance & Rehabilitation
  • although it is highly likely that pain sensitive fibers in the outer annulus or other structures associated with the IVD could generate LBP, the literature to date cannot provide a valid system for identifying them and appropriately directing treatment
  • it is interesting to note that weight-lifting, when performed properly, does not result in adverse loads on the IVDs (this is supported by data indicating that weight-lifters do not have an elevated prevalence of DDD)
  • in patients with more advanced DDD, prolonged end range loading should be avoided (especially in the morning), and it should be kept in mind that injured tissues will not regain tensile strength
  • patients with DDD should also be advised to avoid prolonged periods of sitting, especially in a flexed position
  • ideal treatment for DDD remains controversial

Conclusions & Practical Application:

Clinicians should be mindful of the powerful, yet often overlooked psychological effects of the diagnosis “degenerative disc disease”. Many patients view it is a condition certain to progress, leading to a lifetime of pain and disability. The psychological impact of MRI/imaging findings can further enhance this process. Therefore, prudent clinicians should become skilled in communicating to patients that DDD is a normal process of aging. Further, while it can be associated with pain, only in rare cases does it represent serious disease, and it should not prevent the person from participating in normal activities.

From a treatment perspective, conservative care is the first line option, and interventions such as spinal manipulation, McKenzie approach, and lumbar stabilization exercise have all shown short-term effectiveness for relieving pain, despite not showing any direct effect on the DDD process. It should also be noted that lumbar traction has not been consistently shown to be effective for treating disc pathology, despite the prevalence of marketing materials from companies and practitioners who use expensive traction devices.