Date Published: April 6, 2017
Publisher: Public Library of Science
Author(s): Cornelis P. L. Paul, Magda de Graaf, Arno Bisschop, Roderick M. Holewijn, Peter M. van de Ven, Barend J. van Royen, Margriet G. Mullender, Theodoor H. Smit, Marco N. Helder, Chun Kee Chung.
Lumbar hernias occur mostly in the posterolateral region of IVDs and mechanical loading is an important risk factor. Studies show that dynamic and static overloading affect the nucleus and annulus of the IVD differently. We hypothesize there is also variance in the effect of overloading on the IVD’s anterior, lateral and posterior annulus, which could explain the predilection of herniations in the posterolateral region. We assessed the regional mechanical and cellular responses of lumbar caprine discs to dynamic and static overloading.
IVDs (n = 125) were cultured in a bioreactor and subjected to simulated-physiological loading (SPL), high dynamic (HD), or high static (HS) overloading. The effect of loading was determined in five disc regions: nucleus, inner-annulus and anterior, lateral and posterior outer-annulus. IVD height loss and external pressure transfer during loading were measured, cell viability was mapped and quantified, and matrix integrity was assessed.
During culture, overloaded IVDs lost a significant amount of height, yet the distribution of axial pressure remained unchanged. HD loading caused cell death and disruption of matrix in all IVD regions, whereas HS loading particularly affected cell viability and matrix integrity in the posterior region of the outer annulus.
Axial overloading is detrimental to the lumbar IVD. Static overloading affects the posterior annulus more strongly, while the nucleus is relatively spared. Hence, static overloading predisposes the disc for posterior herniation. These findings could have implications for working conditions, in particular of sedentary occupations, and the design of interventions aimed at prevention and treatment of early intervertebral disc degeneration.
Lumbar disc herniation (LDH) and disc protrusion are phenomena occurring with degenerative disc disease (DDD), often causing acute exacerbation of pain symptoms [1–3]. Disc degeneration usually begins in the third decade of life and peak incidence of LDH is in the fourth and fifth decade . The caudal segments are affected more commonly (i.e. L5-S1 more than L4-5) and the posterolateral corner of the lumbar intervertebral disc (IVD) is the most common site to herniate . Besides genetic factors and extrinsic factors like smoking and obesity, spinal loading conditions have been identified as risk factors for developing both disc degeneration and a lumbar hernia [6–9]. In fact, a recent observational study in a general adult population endorsed the finding of Wilder et al.  that physical loading and sitting hours (static axial load on the spine) are the most important risk factors for developing a lumbar hernia .
In the current study we show that with prolonged axial overloading of caprine lumbar IVD; 1) significant height loss occurs without changes in the exterior pressure distribution over the disc, 2) general cell death and matrix disruption occurs in all disc regions with high dynamic overloading and 3) static overloading results in a posterior annulus region specific breakdown, with significant cell death and matrix disintegration and relative sparing of the nucleus region. Therefore, we conclude that the manner (dynamic or static) of axial loading influences the effect on the various regions and structures of the IVD differently and prolonged static axial overloading primes the lumbar caprine IVD for posterolateral herniation.