Article

Intradiscal pressure, shear strain, and fiber strain in the intervertebral disc under combined loading.

Institute of Orthopaedic Research and Biomechanics, University of Ulm, Ulm, Germany.
Spine (Impact Factor: 2.16). 05/2007; 32(7):748-55. DOI: 10.1097/01.brs.0000259059.90430.c2
Source: PubMed

ABSTRACT Finite element study.
To investigate intradiscal pressure, shear strain between anulus and adjacent endplates, and fiber strain in the anulus under pure and combined moments.
Concerning anulus failures such as fissures and disc prolapses, the mechanical response of the intervertebral disc during combined load situations is still not well understood.
A 3-dimensional, nonlinear finite element model of a lumbar spinal segment L4-L5 was used. Pure unconstraint moments of 7.5 Nm in all anatomic planes with and without an axial preload of 500 N were applied to the upper vertebral body. The load direction was incrementally changed with an angle of 15 degrees between the 3 anatomic planes to realize not only moments in the principle motion planes but also moment combinations.
Intradiscal pressure was highest in flexion and lowest in lateral bending. Load combinations did not increase the pressure. A combination of lateral bending plus flexion or lateral bending plus extension strongly increased the maximum shear strains. Lateral bending plus axial rotation yielded the highest increase in fiber strains, followed by axial rotation plus flexion or axial rotation plus extension. The highest shear and fiber strains were both located posterolaterally. An additional axial preload tended to increase the pressure, the shear, and fiber strains essentially for all load scenarios.
Combined moments seem to lead to higher stresses in the disc, especially posterolaterally. This region might be more susceptible to disc failure and prolapses. These results may help clinicians better understand the mechanical causes of disc prolapses and may also be valuable in developing preventive clinical strategies and postoperative treatments.

0 Bookmarks
  • [Show abstract] [Hide abstract]
    ABSTRACT: The effects of intervertebral disc (IVD) degeneration on biomechanics of the lumbar spine were analyzed. Finite element models of the lumbar spine with various degrees of IVD degeneration at the L4-L5 functional spinal unit (FSU) were developed and validated. With progression of degeneration, intersegmental rotation at the degenerated FSU decreased in flexion-extension and left-right lateral bending, intradiscal pressure at the adjacent FSUs increased in flexion and lateral bending, and facet joint forces at the degenerated FSU increased in lateral bending and axial rotation. These results could provide fundamental information for understanding the mechanism of injuries caused by IVD degeneration.
    Computers in biology and medicine 09/2013; 43(9):1234-40. · 1.27 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Intervertebral disc degeneration is implicated as a major cause of low back pain. There is a pressing need for new regenerative therapies for disc degeneration that restore native tissue structure and mechanical function. To that end we investigated the therapeutic potential of an injectable, triple interpenetrating network hydrogel comprised of dextran, chitosan and teleostean, for functional regeneration of the nucleus pulposus of the intervertebral disc in a series of biomechanical, cytotoxicity and tissue engineering studies. Biomechanical properties were evaluated as a function of gelation time, with the hydrogel reaching ~90% of steady-state aggregate modulus within 10 hours. Hydrogel mechanical properties evaluated in confined and unconfined compression were comparable to native human NP properties. To confirm containment within the disc under physiological loading, toluidine blue labeled hydrogel was injected into human cadaveric spine segments after creation of a nucleotomy defect, and the segments were subjected to 10,000 cycles of loading. Gross analysis demonstrated no implant extrusion, and further, that the hydrogel interdigitated well with native NP. Constructs were next surface-seeded with NP cells and cultured for 14 days, confirming lack of hydrogel cytotoxicity, with the hydrogel maintaining NP cell viability and promoting proliferation. Next, to evaluate the potential of the hydrogel to support cell-mediated matrix production, constructs were seeded with mesenchymal stem cells (MSCs) and cultured under pro-chondrogenic conditions for up to 42 days. Importantly, the hydrogel maintained MSC viability and promoted proliferation, as evidenced by increasing DNA content with culture duration. MSCs differentiated along a chondrogenic lineage, evidenced by up-regulation of aggrecan and collagen II mRNA, and increased GAG and collagen content, and mechanical properties with increasing culture duration. Collectively, these results establish the therapeutic potential of this novel hydrogel for functional regeneration of the NP. Future work will confirm the ability of this hydrogel to normalize the mechanical stability of cadaveric human motion segments, and advance the material towards human translation using preclinical large animal models.
    Tissue Engineering Part A 01/2014; · 4.64 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: It is difficult to study the breakdown of lumbar disc tissue over several years of exposure to bending and lifting by experimental methods. In our earlier published study we have shown how a finite element model of a healthy lumbar motion segment was used to predict the damage accumulation location and number of cyclic to failure under different loading conditions. The aim of the current study was to extend the continuum damage mechanics formulation to the degenerated discs and investigate the initiation and progression of mechanical damage. Healthy disc model was modified to represent degenerative discs (Thompson grade III and IV) by incorporating both geometrical and biochemical changes due to degeneration. Analyses predicted decrease in the number of cycles to failure with increasing severity of disc degeneration. The study showed that the damage initiated at the posterior inner annulus adjacent to the endplates and propagated outwards towards its periphery in healthy and grade III degenerated discs. The damage accumulated preferentially in the posterior region of the annulus. However in grade IV degenerated disc damage initiated at the posterior outer periphery of the annulus and propagated circumferentially. The finite element model predictions were consistent with the infrequent occurrence of rim lesions at early age but a much higher incidence in severely degenerated discs.
    Journal of biomechanics 10/2013; · 2.66 Impact Factor