Biomedical subjects

Joseph J Sarver

Publications and source records attributed to Joseph J Sarver.

6 recordsLinked to original sources

PubMed2005

Mechanical differences between lumbar and tail discs in the mouse.

The mouse lumbar and tail discs are both used as models to study disc degeneration; however, the mechanical behavior of these two levels has not been compared. The objective of this study was to compare the elastic and viscoelastic mechanical properties of lumbar and tail discs of the mouse under axial compression-tension loading. We hypothesized that tail discs would have a larger transition zone (e.g., neutral zone) and would be less stiff in compression. To test these hypotheses, lumbar and tail bone-disc-bone motion segments were loaded in axial compression and tension. The nonlinear elastic mechanical behavior was examined using a tri-linear curvefit. Elastic behavior of lumbar and tail discs was most different in the low-stiffness transition region (neutral zone), where lumbar discs were nearly twice as stiff over half the axial displacement. In addition, viscoelastic behavior, which was examined using a stretch-exponential curvefit, also showed large lumbar and tail differences, where lumbar discs compressed by 60% of their original height and tail discs by 98% after static creep compression. These results demonstrate that tail discs undergo far more axial displacement than lumbar discs under the same load. These findings are relevant to rodent tail models where chronic loads are applied in vivo to study mechanical pathways of degeneration. Furthermore, the tri-linear model, used here to curvefit the nonlinear compression-tension data, quantified stiffness in the transition zone for the first time, which may prove useful in future disc mechanical studies.

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PubMed2004

Altered disc mechanics in mice genetically engineered for reduced type I collagen.

STUDY DESIGN: Mechanically test lumbar discs of transgenic mice in compression-tension and torsion. OBJECTIVES: Determine if a reduction in type I collagen results in decreased disc mechanics. SUMMARY OF BACKGROUND DATA: Quantitative relationships between disc structure and function would improve the understanding of disc generation and are essential relationships for functional tissue engineering. The reduced type I collagen transgenic mouse has been used in structure-function studies of bone and tendon, but not intervertebral discs. Methods for testing mouse discs have recently been developed, making disc structure-function studies possible. METHODS: Microradiographed and mechanically tested lumbar discs from control and collagen-reduced mice in both compression-tension and torsion were used. Disc area and polar moment of inertia were determined from radiographic data, stiffness from mechanical data, and apparent modulus from geometric and mechanical data. RESULTS: Collagen-reduced discs had a larger area and polar moment of inertia compared to controls. The linear and torsional stiffness of collagen-reduced and control discs were not significantly different. Finally, the apparent modulus of collagen-reduced discs was significantly less than controls in compression (73% of control) and torsion (50%). CONCLUSIONS: Compared to controls, collagen-reduced discs had reduced apparent modulus in both loading directions, suggesting that the transgenic disc tissue was mechanically inferior to controls. These results are consistent with the widely accepted functional role of type I collagen in disc mechanics, and therefore supports the use of transgenic mice to study structure-function relationships of the disc. Future work will focus on quantifying structure-function relationships related to degeneration, as well as those relevant to the design of tissue-engineered disc replacements.

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PubMed2004

Young investigator award winner: validation of the mouse and rat disc as mechanical models of the human lumbar disc.

STUDY DESIGN: Measure the mechanical properties of the mouse and rat disc in compression and torsion. OBJECTIVES: Validate mouse and rat disc as a biomechanical model of the human disc by comparing the normalized properties in compression and torsion loading. SUMMARY OF BACKGROUND DATA: Rodents have been widely used as models to study disc degeneration; however, mechanical assessments of the rodent disc have been limited. Mouse and rat disc mechanical properties have not been determined. METHODS: Mechanically test mouse and rat motion segments from both the lumbar and the caudal levels in axial compression and torsion. Normalize the stiffness using disc geometry and compare with human motion segment stiffness taken from the literature. Compare lumbar and caudal levels with each other within each species, and test for correlation between mechanics and body weight. RESULTS: The average compression stiffness, normalized by geometry, was 2-4 MPa and compared well with human motion segment stiffness in compression (3-9 MPa). The average torsion stiffness, normalized by disc geometry, was 5-11 MPa and compared well with human motion segment stiffness in torsion (2-9 MPa). Differences between the lumbar and caudal levels were observed. For the caudal tail, no correlation between body weight and any compression property was observed, but for the lumbar spine, some correlations were observed. CONCLUSIONS.: This study provides validation for the mouse and rat disc as a mechanical model of the human disc. Correlations between lumbar spine properties and animal body weight provide support for the use of quadruped animal lumbar spines as mechanical models of the bipedal human spine. The differences between lumbar and tail mechanics need further exploration. These findings are important in light of the extensive use of the rodent in disc studies and the expected future utility of genetically engineered mice.

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PubMed2004

The effect of overshooting the target strain on estimating viscoelastic properties from stress relaxation experiments.

BACKGROUND: Tendon's mechanical behaviors have frequently been quantified using the quasi-linear viscoelastic (QLV) model. The QLV parameters are typically estimated by fitting the model to a single-step stress relaxation experiment. Unfortunately, overshoot of the target strain occurs to some degree in most experiments. This has never been formally investigated even though failing to measure, minimize, or compensate for overshoot may cause large errors in the estimation of parameters. Therefore, the objective of this study was to investigate the effect of overshoot on the estimation of QLV parameters. METHOD OF APPROACH: A simulated experiment was first performed to quantify the effect of different amounts of overshoot on the estimated QLV parameters. Experimental data from tendon was then used to determine if the errors associated with overshoot could be reduced when a direct fit is used (i.e., the actual strain history was used in the curve fit). RESULTS: We found that both the elastic and viscous QLV parameters were incorrectly estimated if overshoot was not properly accounted for in the fit. Furthermore, the errors associated with overshoot were partially reduced when overshoot was accounted for using a direct fit. CONCLUSIONS: A slow ramp rate is recommended to limit the amount of overshoot and a direct fit is recommended to limit the errors associated with overshoot, although other approaches such as adjusting the control system to limit overshoot could also be utilized.

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PubMed2003

Effect of altered matrix proteins on quasilinear viscoelastic properties in transgenic mouse tail tendons.

Tendons have complex mechanical behaviors that are viscoelastic, nonlinear, and anisotropic. It is widely held that these behaviors are provided for by the tissue's composition and structure. However, little data are available to quantify such structure-function relationships. This study quantified tendon mechanical behaviors, including viscoelasticity and nonlinearity, for groups of mice that were genetically engineered for altered extracellular matrix proteins. Uniaxial tensile stress-relaxation experiments were performed on tail tendon fascicles from the following groups: eight week old decorin knockout, eight week old reduced type I collagen, three week old control, and eight week old control. Data were fit using Fung's quasilinear viscoelastic model, where the model parameters represent the linear viscoelastic and nonlinear elastic response. The viscoelastic properties demonstrated a larger and faster stress relaxation for the decorin knockout and a smaller and slower stress relaxation for the three week control. The elastic parameter, A, in the eight week control group was significantly greater than in the collagen reduction and three week control groups. This study provides quantitative evidence for structure-function relationships in tendon, including the role of proteoglycan in viscoelasticity. Future studies should directly correlate composition and structure with tendon mechanics for the design and evaluation of tissue-engineered constructs or tendon repairs.

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PubMed2003

Methods for quasi-linear viscoelastic modeling of soft tissue: application to incremental stress-relaxation experiments.

The quasi-linear viscoelastic (QLV) model was applied to incremental stress-relaxation tests and an expression for the stress was derived for each step. This expression was used to compare two methods for normalizing stress data prior to estimating QLV parameters. The first and commonly used normalization method was shown to be strain-dependent. Thus, a second normalization method was proposed and shown to be strain-independent and more sensitive to QLV time constants. These analytical results agreed with representative tendon data. Therefore, this method for normalizing stress data was proposed for future studies of incremental stress-relaxation, or whenever comparing stress-relaxation at different strains.

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