As with other parts of the human body, intervertebral discs are subject to the adverse effects of aging, normal daily activity, and injury. As adults age, the nucleus propulsus (NP) gradually contains less water and proteoglycans, and the outer annulus fibrosus (AF) acquires small localized fissures and tears. Such degenerative processes in intervertebral discs progress over time and are irreversible, and compromise the structural integrity and mechanical function of the spine. Patients with disc degeneration or injury may present with decreased spinal flexibility, stability, and load bearing capacity, as well as pain that waxes and wanes or is chronic. The current gold standard for assessment of disc degeneration is a subjective ranking system, based on gross morphological features. Edward Vresilovic, M.D., Ph.D., and his colleagues at Penn State Hershey Bone and Joint Institute, however, are working to change this by developing a quantitative, biomechanical approach toward disc degeneration investigation, diagnosis, and treatment. Vresilovic explains, “The function of intervertebral discs is largely mechanical, and it should be possible to use objective measures to quantify changes in the internal stress-strain environment of discs during physiologic loading.” Until now the inner workings of the intact disc under load have not been adequately characterized due to difficulties in measurement without invasively altering the disc.
Vresilovic and his group of research colleagues have developed a non-invasive technique that allows measurement of internal displacement and strain within both the AF and NP. In this technique, a high-resolution magnetic resonance (MR) sequence images intact human cadaveric lumbar discs under axial compression (1.2 times body weight, moderate physiological stress) in combination with neutral, flexion, or extension positions. The high-resolution MRI s are evaluated with digital texture analysis to assess two-dimensional internal strain within the disc tissue. Findings based on this model, described in a recent published report (O’Connell GD, et al. J. Orthop Res. 2011;29:545- 55.), showed that in degenerated discs (versus normal discs), more of the applied load was placed directly on the AF, likely due to the depressurized NP seen with degeneration. Moreover, in all of the discs studied, regardless of disc degeneration and with all positions studied, the posterior AF exhibited higher compressive axial and higher tensile radial strains than the other AF regions (see figure),
and this may predispose it to failure and tears. Structurally, the posterior AF has thinner, less organized lamellae than the anterior AF, and this paired with the higher strain may contribute to NP herniation. Vresilovic is optimistic about the usefulness of this model for advancing our understanding of internal disc mechanics. He notes, “This model allows us to quantify stress, strain, and shear without the use of invasive technologies like microbeads or wires which alter the disc itself. Because of our success with these initial studies, moving forward we plan to employ three-dimensional imaging and this will allow for more complete three-dimensional strain analysis.”
Understanding changes in load transduction at the cellular level is central to developing new treatments focused on reversing the mechanical breakdown of the disc components. As normative data continue to be gathered using this quantitative approach, the potential opens for its use in evaluating surgical treatment, such as discectomy, nucleotomy, implants, or other clinical interventions.
Edward Vresilovic, M.D., Ph.D.
- Orthopaedic surgeon, spine surgery, professor, orthopaedics and rehabilitation
- Fellowship: Spine surgery, University of California at San Diego, San Diego, California
- Fellowship: Joint replacement, Thomas Jefferson University Hospital, Philadelphia, Pennsylvania
- Residency: Orthopaedic surgery, Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania
- Medical School: University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania