In vitro mechanical characterization of the human natural and treated vertebrae

Abstract

The present thesis illustrates the research carried out during PhD studies in Bioengineering. The research objective was to investigate the in vitro biomechanical properties of human thoracolumbar natural and treated vertebral body, underwent to prophylactic vertebroplasty, to provide a comprehensive analysis on the stress/strain distribution and the failure mechanics of natural and
augmented vertebrae and to obtain indications on the performance of the treatment itself. The research was carried out mainly at the Biomechanics Laboratory of the Department of Industrial Engineering at the University of Bologna and partially at the Laboratory of Medical Technology (LTM) Rizzoli Orthopaedic Institute (Bologna). Vertebra is the main topic of the studies reported in this thesis. The results are related to vertebra as organ level. Spine and vertebrae have been widely investigated in the past, however, given the complexity of such structures a deep understanding of biomechanical properties is necessary to improve treatments and reduce the negative outcome of spine pathologies. Therefore,
investigation of the spine and vertebrae is still an open challenge within the scientific community. Vertebral compression fractures are the most common fracture type related to osteoporosis, with an estimated 1.4 million new fractures occurred worldwide every year. These fractures are associated with pain, decreased quality of life and large health care costs. The most promising approach to reduce the consequences of osteoporosis, is to diagnose the bone loss early and begin treatment strategies before fractures occur. In the last years prophylactic augmentation has been proposed as an alternative to pharmacological treatments in order to reduce the fracture risk of osteoporotic vertebrae or to prevent adjacent fractures after augmentation. This treatment is meant to increase the strength and the structural support of weak vertebrae, by injection of an augmentation material
into the vertebral body. The associated risks (cement leakage and subsequent neural damage; tissue necrosis due to residual monomer and to the exothermal reaction; increased risk of fracture in the adjacent vertebrae) have raised questions about the efficacy and safety of the vertebroplasty in general. Furthermore, it is still debated whether prophylactic augmentation actually strengthens the
treated vertebra. Therefore, there is a need for a clearer understanding on the cost-benefit trade-off. In the light of this debate, in-depth knowledge of the mechanical behaviour and failure of prophylactic-augmented vertebra is of fundamental importance to understand vertebral biomechanics and improve diagnosis and prophylactic treatments. The main objective of this work was to investigate the biomechanical behavior of the natural
and augmented human vertebrae in terms of mechanical properties and strain distribution in the elastic regime up to failure to obtain indications about the efficacy of the treatment. To overcome some limitations of the current in vitro test, some methodological studies were developed to improve and make more accurate in vitro biomechanical test on vertebrae. To obtain a greater reproducibility and repeatability of test, an in vitro anatomical reference frame for human vertebrae was defined and validated for the first time. An investigation was developed to examine the effect of different experimental boundary conditions (with and without discs) in the human vertebra and to elucidate if testing a singlevertebra
specimen (which provides a number of practical advantages) is an acceptable alternative to a three-adjacent-vertebrae-segment (which can be assumed closer to physiological), when measuring the principal strains (magnitude and direction) on the surface of the vertebral body, in the elastic regime. The experimental methods developed were implemented during in vitro destructive e nondestructive
test to investigate the biomechanical behaviour of human natural and augmented
vertebrae. Studies about the vertebral strain distribution were based on an integrated approach, which combined different measurement methods (strain gauges and digital volume correlation) for a more comprehensive investigation. Through the strain gauge technique, very precise and punctual strain information was collected but only at the cortical bone level, while the technique of DVC allowed to capture the internal full-field strain distribution and quantify internal microdamage initiation/evolution under loading. In conclusion this thesis is a comprehensive investigation of the biomechanical properties of natural and treated human in terms of strain distribution (both in the cortex and trabecular bone),
and failure mechanics to obtain indication of the efficacy of prophylactic vertebroplasty.

Date of Award	12 May 2016
Original language	English
Awarding Institution	University of Bologna
Sponsors	University of Bologna
Supervisor	Luca Cristofolini (Supervisor) & Gianluca Tozzi (Supervisor)