AbstractWith a reported lifetime prevalence as high as 80% lower back pain (LBP) is responsible for a significant socioeconomic burden. The condition is often associated with disc degeneration which, in turn, leads to alterations of the load transfer mechanism between spinal structures. The extent of the mechanical changes arising from disc degeneration is poorly understood.
This study aimed at developing an in vitro testing modality for the six degrees of freedom (DOF) characterisation of the load-displacement behaviour in healthy and degenerated isolated spinal disc specimens (ISDs). A new six-axis control system was developed to allow operation of the University of Bath spine simulator under load control conditions alongside the existing position control. Preliminary validation tests confirmed analytical inverse and symmetric relationships between stiffness and flexibility matrices obtained from a linear elastic specimen. The six DOF response of six porcine ISD specimens was investigated under load and position control. The resulting stiffness and flexibility matrices were constructed using linear regression to estimate the magnitude of the 36 elements of each matrix. Inversion of flexibility matrices lead to 15 out of 18 non-zero elements being comparable to the equivalent in the stiffness matrix. For three out of six principal elements, differences were greater than 38%.
The effect of disc degeneration was investigated in a further set of ISD specimens. Severe posterolateral herniation was simulated by total nucleotomy; acute nucleus calcification was reproduced by nucleotomy and subsequent injection of bone cement into the created void. Posterolateral herniation mainly affected bending stiffness, with an increase of 47% compared to healthy; nucleus calcification mainly affected flexion-extension stiffness, with a 44% increase compared to healthy. Both methods of degeneration resulted in substantial increases in load range and energy dissipation, indicating that under such conditions, higher annulus loads were induced for equivalent ranges of motion, potentially affecting surrounding highly innervated structures.
The limitations associated with using linear regression to fully characterise the highly non-linear load-displacement behaviour of ISD were highlighted and an alternative approach, involving the use of spring and damper models, was proposed. Tuning of models to experimental data demonstrated an average root mean squared error reduction of 56% compared to linear regression. Furthermore, energy dissipation behaviour of the ISDs, measured by the area enclosed by the hysteresis loop, was replicated by this modelling approach to within 18% of experimental.
This study has provided a blueprint for investigating the load-transfer behaviour of healthy and degenerated ISD specimens. The equivalence assumption between position and load control testing protocols was demonstrated to not always hold, with sizeable differences noticed in half of the principal elements in stiffness and inverted flexibility matrices. The pitfalls associated with the use of linear regression to describe the highly non-linear load-displacement responses of the ISD were highlighted and an alternative technique was proposed. This exhibited the added advantage of capturing the energy dissipation characteristics of the specimens with a reasonable margin of error.
|Date of Award||29 Mar 2023|
|Sponsors||The Enid Linder Foundation|
|Supervisor||Patrick Keogh (Supervisor), Tony Miles (Supervisor) & Sabina Gheduzzi (Supervisor)|
- stiffness matrix
- spine testing