Mark Weidenbaum

Degeneration affects the anisotropic and nonlinear behaviors of human anulus fibrosus in compression

Axial and radial specimens of non-degenerate and degenerate human anulus fibrosus (AF) were tested in confined compression to test the hypothesis that degeneration significantly affects the compressive properties of AF. Due to the highly oriented structure of AF, a secondary objective was to investigate anisotropic behaviors of AF in compression. Uniaxial swelling and stress relaxation experiments were performed on site-matched samples of anulus from the anterior outer region of L2-3 intervertebral discs. The experimental stress-relaxation behavior was modeled using the finite deformation biphasic theory and a finite-difference approximation scheme. Significant effects of degeneration but not orientation were detected for the reference stress offset, sigma(offset), and parameters describing the compressive stiffness (i.e. reference aggregate modulus, H(A0), and nonlinear stiffening coefficient, beta). Average values were 0.13+/-0.06 and 0.05+/-0.05 MPa for sigma(offset), 0.56+/-0.21 and 1.10+/-0.53 MPa for H(A0) and 2.13+/-1.48 and 0.44+/-0.61 for beta for all normal and degenerate specimens, respectively. No significant effect of degeneration or orientation were detected for either of the parameters describing the strain-dependent permeability (i.e. reference permeability, k0 and strain-dependent permeability coefficient, M) with average values for all specimens of 0.20+/-0.10 x 10(-15) m4/N-s and 1.18+/-1.30 for k0 and M, respectively. The loss of sigma(offset) was compensated with an elastic stiffening and change in the shape of the equilibrium stress-strain curve with H(A0) for degenerate tissues almost twice that of normal tissues and beta less than one sixth. The increase in reference elastic modulus with degeneration is likely related to an increase in tissue density resulting from the loss of water content. The significant effects of degeneration reported in this study suggested a shift in load carriage from fluid pressurization and swelling pressure to deformation of the solid matrix of the AF. The results also suggest that the highly organized and layered network of the anulus fibrosus, which gives rise to significant anisotropic effects in tension, does not play a major role in contributing to the magnitude of compressive stiffness or the mechanisms of fluid flow of the anulus in the confined compression configuration.

Degeneration and aging affect the tensile behavior of human lumbar anulus fibrosus.

Studyb Design Samples of human lumbar (L3-L4) anulus fibrosus from four different anatomic sites (anterior outer, posterolateral outer, anterior inner, posterolateral inner), ranging from normal to severely degenerate, were studied in uniaxial tension and measured for water content. Objectives To evaluate the effects of aging and degeneration on the tensile properties and hydration of the anulus fibrosus in a site-specific manner, The relationship between hydration and parameters of the tenaile behavior were investigated. Summary of Background Data Degeneration and aging have been shown to be related to dramatic changes in the composition and structure of the anulus fibrosus. The associated changes in the tensile, compressive, and shear properties of the anulus fibrosus have not been documented. Numerical studies using finite element models have attempted to simulate the degenerative process by incorporating estimated mechanical properties meant to represent the degenerate anulus fibrosus. Their results present findings that suggest that altered material properties of the anulus fibrosus affect the mechanics of the entire intervertebral disc. Methods Samples of human lumbar anulus fibrosus were classified by grade of degeneration based on a morphologic grading scheme. Multiple layer anulus specimens from four sites in the disc were tested in uniaxial tension under quasistatic conditions in a physiologic saline bath. The tensile modulus, prission;s ratio failure stress and strain, the strain energy density to failure, and the corresponding hydration were determined for each test sample. Result The possons ratio, Failure stress, and strain energy density of the anulus fibrosus were found to be affected significantly by degeneration, with some evldence of a sensitivity of the tensile modulus to gradeof degeneration. All nmaterial properties were found to exhibit a significant and greater dependence on site within the disc than on degenerative grade. Weak corelations between aging and the Poissons ratio and strain energy density were observed. Water content of anulus fibrosus tissue was not affected by degeneration or again, although correlations with tensile properties fwere observed. Conclusions The dramatic changes in morphology, composition, and structure that occur in anulus fibrosus with agin and degeneration are accompanied by specific variations in the tensile properties, Which were generally small in magnitude. Position of the anulus fibrosus within the intertebral disc, particularly in the radial direction, appeared to be the most important variable affecting anulus fibrosus tensile properties. This dependence on position did not change with either aging or degeneration. Results from the present study may be useful in future finite element models to assess how altered material properties of the anulus fibrosus during degeneration and aging may affect the mechanics of the entire intervertebral disc.

Regional variation in tensile properties and biochemical composition of the human lumbar anulus fibrosus.

Study Design The structure-function relationship of anulus fibrosus of nondegenerate lumba intervertebral discs was investigated. Objectives The tensile properties and biochemical composition of single lamella specimens from human anulus librosus and their variations with anatomic region were determined. Summery of Background Data Regional differences in composition and ultrastructure suggest differences in tensile properties. Methods Single lamella specimens were isolated from the anulus, equilibrated in 0.15 ± NaCl and tested in uniaxial tension using a slow strain-rate protocol. Adjacent specimens were used to determine biochemical composition (Including nycration, collagen, proteonlycan, and hydroxypyridinium crosslink density). Tensile properties, biochemical composition, and anatomic location were compared. Results Significant radial and circumferential variations in tensile properties of anulus were detected, with the anterior being, stiffer than the posterolateral regions, and the outer being stiffer than the inner regions. Conclusions The regional differences in tensile properties may result predominantly from structural rather than compositional variations and may contribute to the clinical frequency of anulus failure in the postarolateral region.

Tensile properties of nondegenerate human lumbar anulus fibrosus

Study Design The in vitro tensile behavior of multiple‐layer samples of anulus fibrosus were investigated from nondegenerate intervertebral discs. Objectives To quantify the intrinsic tensile behavior of nondegenerate anulus fibrosus and the variations with position and age in the intervertebral disc. Summary of Background Data Tension is an important loading mode in the anulus fibrosus. The tensile behavior of single‐ and multiple‐layer samples of anulus fibrosus has been shown to vary with specimen orientation, position in the disc, and environmental conditions. Little is known of the changes in these site‐specific tensile properties of the anulus with aging or degeneration of the intervertebral disc. Methods Multiple‐layer specimens of anulus fibrosus were harvested with an orientation parallel to the circumference of the disc. Constant strain rate and uniaxial tensile tests were performed in 0.15 mol/l NaCl at slow strain rates to measure the intrinsic properties of the collagen‐proteoglycan matrix of the anulus fibrosus. The tensile modulus, failure stress, failure strain, and strain energy density were determined. Statistical analyses were done to evaluate regional and age‐related differences in these properties. Results Significant radial and circumferential variations in the intrinsic tensile properties of anular samples were detected. The anterior anulus fibrosus had larger values for tensile moduli and failure stresses than the posterolateral anulus. Also, the outer regions of the anulus had greater moduli and failure stresses and lower failure strains than the inner regions. Strain energy density did not vary significantly with region. Significant, but very weak, correlations were detected between tensile properties and age of the intervertebral disc. Conclusions The observed variations in tensile behavior of multiple‐layer anulus samples indicate that larger variations in tensile modulus and failure properties occur with radial position in the disc than from anterior to posterolateral regions. This pattern is likely related to site‐specific variations in the tensile properties of the single‐layer samples of anulus fibrosus lamellae and the organization of successive lamellae and their interactions. The results of the present study suggest that factors other than age, such as compositional and structural variations in the disc, are the most important determinants of tensile behavior of the anulus fibrosus.

Compressive mechanical properties of the human anulus fibrosus and their relationship to biochemical composition.

To enhance understanding of the biomechanical role of the intervertebral disc, the compressive properties and biochemical composition of nondegenerate samples of anulus fibrosus were determined as a function of radial position, region, and level. Because of the large swelling propensity of this tissue, a method was developed to test excised specimens while maintaining their in situ geometry and hydration. Using an analysis based on linear biphasic theory, the compressive modulus, hydraulic permeability, and isometric swelling pressure of the anulus fibrosus were determined and correlated with the tissue composition. The findings indicate that the anulus fibrosus is inhomogeneous, with regional and radial variations in both material properties and biochemical composition. The results of this study suggest that both structural and compositional factors may determine the mechanical behavior.

Coronal and sagittal plane correction in adolescent idiopathic scoliosis : A comparison between all pedicle screw versus hybrid thoracic hook lumbar screw constructs

Study Design. This was a retrospective cohort study using a previously matched convenience sample of 34 patients. Objective. This study sought to determine the relative corrective benefits of these 2 types of constructs in the correction of coronal and sagittal curves in patients with adolescent idiopathic scoliosis (AIS). In addition, the 2 constructs were compared for coronal and sagittal balance. Summary of Background Information. Recent clinical research suggests that thoracic pedicle screw constructs (all-screw constructs) are more effective than hybrid lumbar screw thoracic hook constructs (hybrid constructs) in correcting spine deformity. Methods. The sample consisted of patients with AIS who underwent isolated posterior spinal fusion and instrumentation. Seventeen patients underwent fusion using all-screw constructs, and 17 underwent fusion with hybrid constructs; preoperative and postoperative radiographs and measurements were compared. Results. There was no significant difference observed when comparing the 2 groups, although there was a trend toward better correction of the main thoracic curve in the all-screw construct group (P = 0.089). In the all-screw group, mean thoracic kyphosis decreased from 29.6° to 19.4° (P = 0.012). Sagittal balance changed in the hybrid group from −21.2 mm to 8.2 mm, and in the all-screw group changed from −28.8 mm to 1.5 mm. The major curve in the hybrid group improved from 54.06° to 20.25° and improved from 54.88° to 15.06° in the all-screw group. Conclusions. There was no statistically significant difference comparing the 2 groups, although a trend was observed toward better correction of the main thoracic curve in the all-screw construct group. The all-screw group demonstrated a significant decrease in kyphosis, which was not seen in the hybrid group. Hybrid constructs were comparable to all-screw constructs in the correction of coronal plane deformity and sagittal balance.

The anisotropic hydraulic permeability of human lumbar anulus fibrosus. Influence of age, degeneration, direction, and water content

STUDY DESIGN Experimental investigation to determine the effect of intervertebral disc degeneration on the kinetic behavior of fluid in human anulus fibrosus. OBJECTIVES To measure the hydraulic permeability coefficient of anulus fibrosus specimens in the axial, circumferential, and radial directions to determine the anisotropic permeability behavior of nondegenerate and degenerate human intervertebral discs over a range of ages. SUMMARY OF BACKGROUND DATA Fluid, a major component of normal intervertebral discs, plays a significant role in their load-supporting mechanisms. Transport of fluid through the intervertebral disc is important for cell nutrition and disc viscoelastic and swelling behaviors. The hydraulic permeability coefficient is the most important material property governing the rate of fluid transport. However, little is known about the anisotropic behavior of this kinetic property and how it is influenced by disc degeneration. METHODS Using a permeation testing apparatus developed recently, testing was performed on 306 axial, circumferential, and radial anulus fibrosus specimens from the posterolateral region of 30 human lumbar (L2-L3) discs. A new method, flow-controlled testing protocol, was developed to measure the hydraulic permeability coefficient. RESULTS The hydraulic permeability coefficient of anulus fibrosus depended significantly on the disc degenerative grade (P = 0.0001) and flow direction (P = 0.0001). For the nondegenerate group (Grade I), the hydraulic permeability was significantly anisotropic (P < 0.05), with the greatest value in the radial direction (1.924 x 10(-15) m4/Ns) and the lowest value in the circumferential direction (1.147 x 10(-15) m4/Ns). This anisotropic kinetic (flow) behavior of anulus fibrosus varied with disc degeneration. For the Grade III specimen group, there was no significant difference in hydraulic permeability coefficient among the three major directions (P = 0.37). With disc degeneration, the hydraulic permeability coefficient was decreased in the radial direction and increased in the axial and circumferential directions. The variations of hydraulic permeability coefficient from nondegenerate discs (Grade I) to mildly degenerate discs (Grade II) in each direction were significant (P < 0.05). However, the changes in permeability from Grade II to Grade III groups were not significant (P > 0.05) except in the circumferential direction (3.8% increase; P < 0.05). CONCLUSIONS The hydraulic permeability of human nondegenerate anulus fibrosus is direction-dependent (i.e., anisotropic), with the greatest permeability in the radial direction. With disc degeneration, the radial permeability of anulus fibrosus decreases, mainly because of decreased water content, and the axial and circumferential permeability coefficients increase, mainly because of structural change, leading to more isotropic permeability behavior for Grade III discs.

Sagittal alignment of the spine and pelvis in the presence of L5-s1 isthmic lysis and low-grade spondylolisthesis.

Study Design. A radiographic study of 82 patients with L5–S1 spondylolysis or spondylolisthesis of less than 50% displacement of L5 on S1. Objective. To measure and describe the sagittal alignment of the spine and pelvis in patients with spondylolysis before the development of a large secondary deformity associated with progression of the spondylolisthesis. Summary of Background Data. Several publications have addressed the alignment of the spine and pelvis as an important factor in the occurrence, symptomatology, progression, and treatment of spondylolysis and spondylolisthesis. To our knowledge, this is the first report to systematically document the native sagittal alignment of affected patients and compare them to a large control population. Materials and Methods. The sagittal alignment in this cohort of 82 patients was compared with a control population of 160 patients without symptoms of back pain or radiographic abnormalities of the spine and pelvis that was the subject of a previous study. Results. Patients with spondylolysis and low-grade spondylolisthesis demonstrate increased pelvic incidence, increased lumbar lordosis, but less segmental extension between L5 and S1 than in a normal population. Conclusions. These data suggest that differences in the sagittal alignment of the spine and pelvis may influence the biomechanical environment that results in the development of spondylolysis and progressive spondylolisthesis.

Streaming potential of human lumbar anulus fibrosus is anisotropic and affected by disc degeneration

The streaming potential responses of non-degenerate and degenerate human anulus fibrosus were measured in a one-dimensional permeation configuration under static and dynamic loading conditions. The goal of this study was to investigate the influence of the changes in tissue structure and composition on the electrokinetic behavior of intervertebral disc tissues. It was found that the static streaming potential of the anulus fibrosus depended on the degenerative grade of the discs (p = 0.0001) and on the specimen orientation in which the fluid flows (p = 0.0001). For a statically applied pressure of 0.07 MPa, the ratio of streaming potential to applied pressure ranged from 5.3 to 6.9 mV/MPa and was largest for Grade I tissue with axial orientation and lowest for Grade III tissue with circumferential orientation. The dynamic streaming potential responses of anulus fibrosus were sensitive to the degeneration of the disc: the total harmonic distortion factor increased by 108%, from 3.92 +/- 0.66% (mean +/- SD) for Grade I specimens to 8.15 +/- 3.05% for Grades II and III specimens. The alteration of streaming potential reflects the changes in tissue composition and structure with degeneration. To our knowledge, this is the first reported data for the streaming potential of human intervertebral disc tissues. Knowledge of the streaming potential response of the intervertebral disc provides an understanding of potentially important signal transduction mechanisms in the disc and of the etiology of intervertebral disc degeneration.

A comparative biomechanical study of spinal fixation using Cotrel-Dubousset instrumentation

A biomechanical study was performed comparing the stiffness and stability of Cotrel-Dubousset (CD) spinal instrumentation with that of segmentally wired Harrington distraction rods and segmentally wired Luque rods under conditions of single-level instability. The axial and torsional stiffness coefficients of each system were determined on a customized geometric spine simulator fashioned from stainless steel. The relative stability of each instrumentation system was then compared by mounting the fixation systems on bovine thoracic spines from 12-week-old calves, destabilized by anterior vertebrectomy to create simulated two column instability. Thirteen spines were tested. Each specimen was tested under axial and torsional loading conditions while monitoring with a personal computer-based data acquisition system was performed. The stability of first-and second-level CD instrumentation was tested on the bovine specimens. First-level CD instrumentation involved double-hook fixation one level above and below the level of instability. Second-level CD instrumentation involved fixation two levels above and below the level of instability without fixation at the intermediate level. In axial loading, double-level wired Harrington distraction rods, double-level wired Luque rods, and first-level CD rods were 26.5%, 18.4%, and 21.5%, respectively, as stable as second-level CD instrumentation. In torsion, double-level Harrington, double-level Luque, and second-level CD rods were 13%, 64%, and 34%, respectively, as stable as first level CD instrumentation. Locking hooks, double-hook configurations, and stabilizing transverse traction devices of the CD contributed to its greater stability. First-level CD instrumentation is recommended for rotational instability while second-level CD instrumentation is preferred for axial instability.