Dependency of disc degeneration on shear and tensile strains between annular fiber layers for complex loads.

BACKGROUND One of the first signs of disc degeneration is the formation of circumferential tears within the annulus fibrosus. It is assumed that high shear and tensile strains between the lamellae mainly cause the initiation of these failures. However, it is not known which load application and which degree of disc degeneration could lead to the highest strains and therefore, might induce the formation of tears. Therefore, the aim of this finite element (FE) study was, to find load combinations that would yield highest shear and tensile strains in differently degenerated discs. MATERIALS AND METHODS A three-dimensional FE-model of a motion segment L4-5 was utilized in different degrees of disc degeneration (healthy, mild, moderate, and severe). The degenerated models consider the reduction of disc height, endplate curvatures, the osteophyte formation, the increase of nucleus compressibility, and the decrease of fiber and ligament stiffness. An axial compression load of 500 N together with moments of 7.5 Nm in single and combined load directions were simulated. RESULTS High strains for the healthy and degenerated discs were predicted for load combinations, particularly for the combination of lateral bending plus flexion or extension. The maximum strains were located in the postero-lateral region of the disc. In comparison to the healthy disc, the maximum strains increased slightly for the mildly and moderately degenerated disc. Strains decreased strongly for the severely degenerated disc. With progressive degeneration, the size of the region of maximum strains diminished and the location transferred from the inner annulus to the adjacent bony endplates. CONCLUSIONS The results could be a possible explanation for the initiation of circumferential tears. The mildly degenerated disc model, which represents early stages of life, suggests that circumferential tears could primarily occur at these stages, especially for the load combinations of lateral bending plus axial rotation and lateral bending plus flexion.

[1]  Stephen J Ferguson,et al.  Simulated influence of osteoporosis and disc degeneration on the load transfer in a lumbar functional spinal unit. , 2004, Journal of biomechanics.

[2]  Lutz Claes,et al.  Validity and interobserver agreement of a new radiographic grading system for intervertebral disc degeneration: Part II. Cervical spine , 2005, European Spine Journal.

[3]  V K Goel,et al.  Effect of Disc Degeneration at One Level on the Adjacent Level in Axial Mode , 1991, Spine.

[4]  Lutz Claes,et al.  Application of a calibration method provides more realistic results for a finite element model of a lumbar spinal segment. , 2007, Clinical biomechanics.

[5]  M M Panjabi,et al.  Disc Degeneration Affects the Multidirectional Flexibility of the Lumbar Spine , 1994, Spine.

[6]  Antonius Rohlmann,et al.  Analysis of the influence of disc degeneration on the mechanical behaviour of a lumbar motion segment using the finite element method. , 2006, Journal of biomechanics.

[7]  Adams Ma,et al.  Prolapsed Intervertebral Disc: A Hyperflexion Injury , 1982 .

[8]  B. Vernon‐roberts,et al.  Annular tears and disc degeneration in the lumbar spine. A post-mortem study of 135 discs. , 1992, The Journal of bone and joint surgery. British volume.

[9]  L. Claes,et al.  Validity and interobserver agreement of a new radiographic grading system for intervertebral disc degeneration: Part I. Lumbar spine , 2005, European Spine Journal.

[10]  N. Boos,et al.  The Course of Macroscopic Degeneration in the Human Lumbar Intervertebral Disc , 2006, Spine.

[11]  Lutz Claes,et al.  The risk of disc prolapses with complex loading in different degrees of disc degeneration - a finite element analysis. , 2007, Clinical biomechanics.

[12]  Lutz Claes,et al.  Validity and interobserver agreement of a new radiographic grading system for intervertebral disc degeneration: Part I. Lumbar spine , 2006, European Spine Journal.

[13]  A. Patwardhan,et al.  A follower load increases the load-carrying capacity of the lumbar spine in compression. , 1999, Spine.

[14]  I W Nelson,et al.  Mechanical initiation of intervertebral disc degeneration. , 2000, Spine.

[15]  B. Vernon‐roberts,et al.  Intervertebral disc degeneration , 1993, European Spine Journal.

[16]  L. Claes,et al.  The relation between the instantaneous center of rotation and facet joint forces - A finite element analysis. , 2008, Clinical biomechanics.

[17]  Hendrik Schmidt,et al.  The relation between intervertebral disc bulging and annular fiber associated strains for simple and complex loading. , 2008, Journal of biomechanics.

[18]  E. Spangfort,et al.  The lumbar disc herniation. A computer-aided analysis of 2,504 operations. , 1972, Acta orthopaedica Scandinavica. Supplementum.

[19]  B. Vernon‐roberts,et al.  Degenerative changes in the intervertebral discs of the lumbar spine and their sequelae. , 1977, Rheumatology and rehabilitation.

[20]  B. Vernon‐roberts,et al.  Pathogenesis of Tears of the Anulus Investigated by Multiple‐Level Transaxial Analysis of the T12‐L1 Disc , 1997, Spine.

[21]  U. Ebeling,et al.  Are there typical localisations of lumbar disc herniations? A prospective study , 2005, Acta Neurochirurgica.

[22]  Lutz Claes,et al.  Application of a new calibration method for a three-dimensional finite element model of a human lumbar annulus fibrosus. , 2006, Clinical biomechanics.

[23]  A. Osborn,et al.  Computed tomography and differential diagnosis of the extruded lumbar disc. , 1983, Journal of computer assisted tomography.

[24]  Mark D. Brown,et al.  Measurement of Cadaver Lumbar Spine Motion Segment Stiffness , 2002, Spine.

[25]  Kevin F. Spratt,et al.  Classification of Age-Related Changes in Lumbar Intervertebral Discs , 2002 .

[26]  Lutz Claes,et al.  Intradiscal pressure, shear strain and fiber strain in the intervertebral disc under combined loading , 2006 .

[27]  Thomas R Oxland,et al.  Accuracy and repeatability of a new method for measuring facet loads in the lumbar spine. , 2006, Journal of biomechanics.

[28]  P. Brinckmann,et al.  Interlaminar Shear Stresses and Laminae Separation in a Disc: Finite Element Analysis of the L3‐L4 Motion Segment Subjected to Axial Compressive Loads , 1995, Spine.

[29]  M. Adams,et al.  THE BIOMECHANICS OF BACK PAIN , 2003 .

[30]  F Cantraine,et al.  A Cadaveric Study Comparing Discography, Magnetic Resonance Imaging, Histology, and Mechanical Behavior of the Human Lumbar Disc , 1992, Spine.

[31]  M. Adams,et al.  Gradual Disc Prolapse , 1985, Spine.