Stepwise reduction of functional spinal structures increase disc bulge and surface strains

Previous studies postulated that an axial compression of lumbar intervertebral discs causes a complex strain pattern on the annulus. This pattern is not fully understood, since most studies measured only the uniaxial ultimate tensile strain of the annulus. The aim of this study was to investigate surface strains and their relation to disc bulging. This work was extended to study some defects that are relevant for the intermediate process of finite element modeling. Six specimens (L2–3) with a median age of 51 years were utilized for this in vitro study. Specimens were loaded with pure moments (2.5–7.5 N m) in the principal directions. The anatomy was subsequently reduced in three steps: (1) ligamentous and bony posterior structures, (2) anterior and posterior ligaments and (3) nucleus. Measured were ranges of motion, three-dimensional disc bulging and surface strains of the outer annulus. Lateral bending showed the largest axial strains (9.7%) for intact specimens, which increased to 15.1% after the removal of posterior structures. Disc bulging was largest in flexion with 1.56 mm, which increased to 2.06 mm after step (1). Defect (2) caused that flexion yielded the largest axial strains with 22.6% and 2.17 mm of bulging. We could also determine a constriction effect of these ligaments. Nucleotomy did not essentially increase anterior disc bulging in flexion, but inward disc bulging increased by 0.55 mm, in extension. Due to the increase in the complexity of finite element models, it is difficult to obtain data from the literature for validation purposes. This study presents new data, which assist in the development of such models.