Intervertebral Disc Response to Cyclic Loading—An Animal Model

Abstract

The viscoelastic response of a lumbar motion segment loaded in cyclic compression was studied in an in vivo porcine model (N = 7). Using surgical techniques, a miniaturized servohydraulic exciter was attached to the L2-L3 motion segment via pedicle fixation. A dynamic loading scheme was implemented, which consisted of one hour of sinusoidal vibration at 5 Hz, 50 N peak load, followed by one hour of restitution at zero load and one hour of sinusoidal vibration at 5 Hz, 100 N peak load. The force and displacement responses of the motion segment were sampled at 25 Hz. The experimental data were used for evaluating the parameters of two viscoelastic models: a standard linear solid model (three-parameter) and a linear Burger's fluid model (four-parameter). In this study, the creep behaviour under sinusoidal vibration at 5 Hz closely resembled the creep behaviour under static loading observed in previous studies. Expanding the three-parameter solid model into a four-parameter fluid model made it possible to separate out a progressive linear displacement term. This deformation was not fully recovered during restitution and is therefore an indication of a specific effect caused by the cyclic loading. High variability was observed in the parameters determined from the 50 N experimental data, particularly for the elastic modulus E1. However, at the 100 N load level, significant differences between the models were found. Both models accurately predicted the creep response under the first 800 s of 100 N loading, as displayed by mean absolute errors for the calculated deformation data from the experimental data of 1.26 and 0.97 percent for the solid and fluid models respectively. The linear Burger's fluid model, however, yielded superior predictions particularly for the initial elastic response.

Role of endplates in contributing to compression behaviors of motion segments and intervertebral discs

“…Disc stiffness and relaxation levels were previously reported to return to pre-loaded levels after 18 h of recovery yet disc height before and after loading was not measured (Johannessen et al, 2004). Nonrecoverable deformations were previously reported in a porcine model under cyclic loading (Ekstrom et al, 1996), and non-recoverable deformations in cartilage were also reported under strains as low as 2% (Langelier and Buschmann, 2003) consistent with our findings. Nonrecoverable deformations in motion segments are consistent with the hypothesis that minor damage to vertebral bodies can lead to progressive disruption of the adjacent discs (Adams et al, 2000).…”

Viscoelastic Relaxation and Recovery of Tendon

“…Not only did the spines fail to recover during an unloading period equal in length to the loading period, 8,21 most still had not completely recovered in unloaded periods double or more the length of time of loading. Gedalia et al observed that the lumbar structures failed to recover after more than double the amount of time allotted for creep testing.…”

“…21 Likewise, Eckstrom et al induced creep in the porcine spine over 1 h, and observed its recovery for an hour following the removal of load. 8 Gedalia et al performed similar testing in lumbar structures, loading them for 50 min and monitoring recovery after 50 min. 10 Finally, Solomonow et al subjected cat spines to cyclic loading for 50 min, and observed the recovery for 7 h. 32 Interestingly, each of these studies found that the recovery of creep in the spine requires more time than the duration over which the load was applied.…”

Dynamic stiffness and damping of human intervertebral disc using axial oscillatory displacement under a free mass system

“…The experiment set-up can explain different values in comparison to other dynamic studies [11,23], as well as the preparation of the specimens and the choice of the viscoelastic model [6].…”

“…To achieve these models, it is necessary to know the mechanical properties of the different structures of the human body, and we have focused first on the intervertebral disc. Most studies of the dynamic behaviour of the intervertebral disc have focused on the physiological loading of daily life and have considered creep [13], cyclic loading [2, 6,14,15,16,17], fatigue with a frequency around 1 Hz [1,19] or impact tests [22]. The goal of this study is to characterise the dynamic stiffness and damping of human intervertebral discs subjected to vibration from 5 to 30 Hz in a sitting position in a car.…”