Biomechanical investigation of posterior dynamic stabilization systems of the lumbar spine

  • Biomechanische Untersuchung von posterioren dynamischen Stabilisierungssystemen der lumbalen Wirbelsäule

Beckmann, Agnes; Markert, Bernd (Thesis advisor); Morlock, Michael (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Book, Dissertation / PhD Thesis

In: Report. IAM, Institute of General Mechanics 10
Page(s)/Article-Nr.: 1 Online-Ressource : Illustrationen, Diagramme

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

In vitro tests are a state-of-the-art method to evaluate the biomechanical behavior of instrumented cadaveric spine models. However, some biological materials and some materials for medical devices – e.g. polycarbonate urethane (PCU) – are influenced by environmental parameters such as temperature, moisture and loading rates. Therefore, first scope of this thesis work is the development of a spine test rig including an environmental chamber. Posterior dynamic stabilization systems (PDSS) should have a considerable flexibility but nevertheless sustain dynamical high-cycle loadings in the body. Clinical cases of fatigue failure of devices containing polyether ether ketone(PEEK) material are discussed in the literature recently. Therefore, second scope of this thesis work is the development of a finite element (FE) model to predict the cycles to failure of PDSS containing PEEK material. As a favorable sagittal balance improves the outcome of fusion surgeries, the surgeon can manually correct the lordotic angle by applying a dorsal compression. Therefore, third scope is the investigation of the effect of dorsal compression procedures on the lumbar spine instrumented with a PCU-based PDSS using the FE method. For this thesis work, a novel spine test rig is developed including an environmental chamber that allows pure moment tests in flexion-extension (FlexEx), lateral bending (LatB) and axial rotation (AxRot) direction of multisegmental cadaveric spine specimens. In total, three in vitro studies of lumbar spines instrumented with thePEEK-based BalanC PDSS, PCU-based Transition PDSS and PCU-based MOVE-P PDSS are performed and evaluated. Additionally, two studies are undertaken to calibrate FE material models of PEEK and PCU material. Based on the experimental results, a native and two instrumented FE lumbar spine models are developed andvalidated. Using these instrumented FE spine models, the cycles to failure of the BalanC PDSS and effect of dorsal compression procedures on the spine instrumented with the Transition PDSS are evaluated. The experimental results of the BalanC study show that the BalanC PDSS reduces the range of motion (RoM) of the instrumented segments for all trials, except of AxRot at L3-L4. Also, the Transition and MOVE-P PDSS stabilize the segments to a comparable extent, although the FlexEx-RoM is higher reduced with the Transition PDSS while AxRot RoM is higher reduced with the MOVE-P PDSS. The predicted cycles to failure of the BalanC PDSS are 16.7·10^6 cycles which corresponds to a mission time of about 2 years. The simulation of the lordotic angle corrections suggest, that dorsal compression of 1 mm results in a Transition PDSS without initial pretension for standing postures of patients.

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