Abstract
Introduction: In vivo measurements of intradiscal stresses are difficult. McNally measured stress profiles in human discs. It is unclear why some exhibit stress peaks in posterior annulus while others do not. Therefore finite element (FE) models are useful to improve the knowledge of stress distribution in the disc. We compared experimental and numerical stress in discs under axial loading, in non degenerated and degenerated disc.
Methods: The FE disc model resembles one fourth of a full disc. The annulus contains both matrix and fibers, while the nucleus only consists of matrix. Similar load profiles were applied and model predictions of matrix stress were compared to experiments (stress profilometry).
Results: Both experimental data and numerical simulations exhibit a peak of axial stress in posterior annulus and lower peaks in anterior annulus. Simulating a “normal” disc results in a uniform matrix stress profile from posterior to anterior. By reducing the fixed charged density (FCD) to 50% in both nucleus and annulus, stress profiles become non-uniform. Stresses in the nucleus decrease. Axial annulus stresses exhibit peaks on anterior and posterior side. Stress peaks increase when FCD decrease under the same loading.
Discussion: The size of the peaks computationally depends on the FCD in discs. Decreasing the FCD shows development of stress peaks in the annulus. A uniform stiffness is seen in nucleus region, but not in annulus. The hydrostatic pressure, due to the FCD, is not high enough to evenly distribute the load over the whole disc. The posterior stress peaks may explain why hernia develops particularly in the posterior annulus.
Correspondence should be addressed to Ms Alison McGregor, c/o BOA, SBPR at the Royal College of Surgeons, 35–43 Lincoln’s Inn Fields, London WC2A 3PE.
