Viscoelastic properties of a spinal posterior dynamic stabilisation device

Abstract

The purpose of this study was to quantify the frequency dependent viscoelastic properties of two types of spinal posterior dynamic stabilisation devices. In air at 37°C, the viscoelastic properties of six BDyn 1 level, six BDyn 2 level posterior dynamic stabilisation devices (S14 Implants, Pessac, France) and its elastomeric components (polycarbonate urethane and silicone) were measured using Dynamic Mechanical Analysis. The viscoelastic properties were measured over the frequency range 0.01-30Hz. The BDyn devices and its components were viscoelastic throughout the frequency range tested. The mean storage stiffness and mean loss stiffness of the BDyn 1 level device, BDyn 2 level device, silicone component and polycarbonate urethane component all presented a logarithmic relationship with respect to frequency. The storage stiffness of the BDyn 1 level device ranged from 95.56N/mm to 119.29N/mm, while the BDyn 2 level storage stiffness ranged from 39.41N/mm to 42.82N/mm. BDyn 1 level device and BDyn 2 level device loss stiffness ranged from 10.72N/mm to 23.42N/mm and 4.26N/mm to 9.57N/mm, respectively. No resonant frequencies were recorded for the devices or its components. The elastic property of BDyn 1 level device is influenced by the PCU and silicone components, in the physiological frequency range. The viscoelastic properties calculated in this study may be compared to spinal devices and spinal structures.

In vitro oxidative degradation of a spinal posterior dynamic stabilization device

“…The degraded components and BDyn device, with the degraded components, were viscoelastic throughout the frequency range tested. The degraded BDyn 1 level device storage stiffness and loss stiffness were less than the storage stiffness (95.56–119.29 N/mm) and loss stiffness (10.72–23.42 N/mm) for the normal BDyn 1 level device. However, the reductions in viscoelastic properties of the PCU and silicone components, due to the in vitro degradation process, are significantly different for specific frequencies.…”

“… Storage stiffness ( k' ) against ln( f ) for (A) normal and degraded BDyn device (BDyn), (B) normal and degraded PCU component (PCU), and (C) normal and degraded silicone (Sil) component (mean ± 95% confidence intervals). Normal data are from a previous study …”

“…Similar to the previous study, custom‐designed grips were used to clamp the titanium alloy rods and/or titanium alloy elastomer housing of the BDyn device. The devices were secured by twelve horizontal screws.…”

“…Six silicone and six PCU components ( Figure 2) were obtained from S14 Implants (Pessac, France) and were used for a previous study. 9 These components, which were sterilized with ethylene oxide (EtO) (Steriservices, Bernay, France) for the previous study, were degraded using a 20% hydrogen peroxide (H 2 O 2 ) and 0.1 M cobalt (II) chloride hexahydrate (CoCl 2 .6H 2 O) oxidative solution. The in vitro accelerated ageing of the components was performed at 378C in a Grant JBN18 water bath (Grant Instruments, Royston, UK).…”

“…Since the human lumbar spine has been reported to be resonant between 4–5 Hz in the seated position, the frequency‐dependent viscoelastic properties of the BDyn device, and its elastomeric components, were quantified by Dynamic Mechanical Analysis (DMA) . By applying an oscillating force to a multi‐component structure and analyzing the out‐of‐phase displacement response, the storage ( k' ) and loss ( k'' ) stiffness were calculated to characterize the viscoelastic properties .…”