Abstract
The aims of this study were to assess the damage and failure strengths of lamellar fibrous tissues, such as the anterior annulus fibrosus (AF), and to develop a mathematical model of damage propagation of the lamellae and inter-lamellar connections. This level of modelling is needed to accurately predict the effect of damage and failure induced by trauma or clinical interventions. 26 ovine anterior AF cuboid specimens from 11 lumbar intervertebral discs were tested in radial tension and mechanical parameters defining damage and failure were extracted from the in-vitro data. Equivalent 1D analytical models were developed to represent the specimen strength and the damage propagation, accounting for the specimen dimensions and number of lamellae. Model parameters were calibrated on the in-vitro data. Similar to stiffness values reported for other orientations, the outer annulus was found stronger than the inner annulus in the radial direction, with failure at higher stress values. The inner annulus failed more progressively, showing macroscopic failure at a higher strain value. The 1D analytical model of damage showed that lamellar damage is predominant in the failure mechanism of the AF. The analytical model of the connections between lamellae allowed us to represent separately damage processes in the lamellae and the inter-lamellar connections, which cannot be experimentally tested individually.
Highlights
A number of surgical techniques in the intervertebral disc, such as needle puncture during nucleus replacement, discography, or simple annulus suturing, can cause direct or progressive damage leading to degeneration (Carragee et al, 2009; Michalek et al, 2010; Nassr et al, 2009)
The experimental data presented here suggests that, in radial loading, the annulus has an almost linear behaviour up to about 8 to 20% strain depending on location, followed by an apparent stiffness decrease until the first instance of local failure
The stress level at the occurrence of damage and failure depends on the radial location of the samples
Summary
A number of surgical techniques in the intervertebral disc, such as needle puncture during nucleus replacement, discography, or simple annulus suturing, can cause direct or progressive damage leading to degeneration (Carragee et al, 2009; Michalek et al, 2010; Nassr et al, 2009). Modelling the failure mechanisms of the inter-vertebral disc can provide a better understanding of those effects beyond direct observation. While disc degeneration is caused by both mechanical and biochemical factors (Adams and Roughley, 2006; Alini et al, 2008), damage, failure, and fracture during or directly following an intervention can have a purely mechanical origin.
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