Rat disc torsional mechanics: effect of lumbar and caudal levels and axial compression load

Abstract

Background context: Rat models with altered loading are used to study disc degeneration and mechano-transduction. Given the prominent role of mechanics in disc function and degeneration, it is critical to measure mechanical behavior to evaluate changes after model interventions. Axial compression mechanics of the rat disc are representative of the human disc when normalized by geometry, and differences between the lumbar and caudal disc have been quantified in axial compression. No study has quantified rat disc torsional mechanics.

Purpose: Compare the torsional mechanical behavior of rat lumbar and caudal discs, determine the contribution of combined axial load on torsional mechanics, and compare the torsional properties of rat discs to human lumbar discs.

Study design: Cadaveric biomechanical study.

Methods: Cyclic torsion without compressive load followed by cyclic torsion with a fixed compressive load was applied to rat lumbar and caudal disc levels.

Results: The apparent torsional modulus was higher in the lumbar region than in the caudal region: 0.081+/-0.026 (MPa/degrees, mean+/-SD) for lumbar axially loaded; 0.066+/-0.028 for caudal axially loaded; 0.091+/-0.033 for lumbar in pure torsion; and 0.056+/-0.035 for caudal in pure torsion. These values were similar to human disc properties reported in the literature ranging from 0.024 to 0.21 MPa/degrees.

Conclusions: Use of the caudal disc as a model may be appropriate if the mechanical focus is within the linear region of the loading regime. These results provide support for use of this animal model in basic science studies with respect to torsional mechanics.

Figure 1

Characteristic nonlinear torque-angular displacement: a) lumbar b) caudal. The area within the curve represents the hysteresis area, a measure of viscoelastic dissipation. Note that both curves are nonlinear and that the lumbar region has a smaller and stiffer neutral zone. The parameters quantified in this study are shown on (a): torque range, and (b): stiffness values.

Figure 2

Mean (standard deviation) of torsional properties in the lumbar and caudal regions with and without axial load. a) Neutral zone apparent shear modulus (MPa), b) Apparent shear modulus in linear region (MPa), c) Maximum shear stress (MPa), and d) hysteresis area (MJ/m³). One asterisk represents significant difference vs. no axial load case, and two asterisks denote significant difference vs. lumbar (p<0.05).

Figure 3

Comparison of apparent shear modulus (G) for rat (present study, a) lumbar, axially loaded, b) caudal, axially loaded, c) lumbar, pure torsion, d) caudal, pure torsion) and human. Cases e) to g) are from axially loaded human motion segments: e) Kleinstueck et al. [20], f) Abumi et al. [17], and g) Beckstein et al. [19]. Cases h) to l) correspond to human motion segments tested in pure torsion: h) Farfan et al. [22], i) McGlashen et al. [18]. Data from j) healthy discs and k) discs with annular tears are from Haughton et al. [21]. In cases that do not show error bars the source reports only an average value. Human apparent modulus was calculated as either tangent or secant modulus as described in Elliott and Sarver [2]