The goal of non-fusion stabilization is to reduce the mobility of the spine segment to less than that of the intact spine specimen, while retaining some residual motion. Several in vitro studies have been conducted on a dynamic system currently available for clinical use (Dynesys). Under pure moment loading, a dependency of the biomechanical performance on spacer length has been demonstrated; this variability in implant properties is removed with a modular concept incorporating a discrete flexible element. An in vitro study was performed to compare the kinematic and stabilizing properties of a modular dynamic lumbar stabilization system with those of Dynesys, under the influence of an axial preload. Six human cadaver spine specimens (L1-S1) were tested in a spine loading apparatus. Flexibility measurements were performed by applying pure bending moments of 8 Nm, about each of the three principal anatomical axes, with a simultaneously applied axial preload of 400 N. Specimens were tested intact, and following creation of a defect at L3-L4, with the Dynesys implant, with the modular implant and, after removal of the hardware, the injury state. Segmental range of motion (ROM) was reduced for flexion-extension and lateral bending with both implants. Motion in flexion was reduced to less than 20% of the intact level, in extension to approximately 40% and in lateral bending a motion reduction to less than 40% was measured. In torsion, the total ROM was not significantly different from that of the intact level. The expectations for a flexible posterior stabilizing implant are not fulfilled. The assumption that a device which is particularly compliant in bending allows substantial intersegmental motion cannot be fully supported when one considers that such devices are placed at a location far removed from the natural rotation center of the intervertebral joint.

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Ligaments are regularly subjected to repetitive loading in vivo. Typically, mechanical studies focus on repetitive loading protocols of short duration, while those characterizing damage accumulation over a longer duration (i.e., fatigue studies) are lacking. The aims of this study were as follows: (a) to demonstrate that damage does accumulate in ligament tissue subjected to repetitive loading and (b) to evaluate existing and new methods for characterizing fatigue damage accumulation. It was hypothesized that ligaments would accumulate damage with repetitive loading as evidenced by failure at stresses well below ultimate tensile strength, creep curve discontinuities, and by reductions in stiffness during loading. Eight normal medial collateral ligaments from female New Zealand white rabbits were cycled in tension, between 0 MPa and 28 MPa, to failure or until 259,200 cycles, whichever came first. Medial collateral ligaments that did not fail were subsequently loaded to failure. Displacement rates (dl(max)/dt) as well as primary, secondary, and tertiary creeps were monitored as indices of damage accumulation and impending mechanical failure. Additionally, the relative utilities of tangent, secant, and chord stiffness parameters were critically evaluated. Finally, new uses for the second derivative of force-displacement data were explored. Three out of eight ligaments failed during testing, demonstrating that ligaments can fail in fatigue under moderate tensile stress in vitro. The evaluation of displacement rates (dl(max)/dt), as well as primary through tertiary creep patterns, were not well suited to predicting failure in normal ligaments until rupture was all but imminent. Tangent stiffness, which was calculated from a mathematically defined start of the "linear region," was surprisingly constant throughout testing. Secant stiffness dropped in a predictable fashion, providing a global indicator of tissue stiffness, but did not provide any insight into fiber mechanics. Chord stiffness, on the other hand, appeared to be sensitive to fiber recruitment patterns. The second derivative of force-displacement data proved to be a useful means of (a) objectively defining the start of the linear region and (b) inferring changes in fiber recruitment patterns within ligament tissue. Tangent, secant, and chord stiffnesses highlight different attributes of ligament responses to loading; hence these parameters cannot be used interchangeably. Additionally, the second derivative of the force-displacement curve was introduced as a useful descriptive and analytical tool.