ABSTRACT of the Doctoral Thesis

Development of a Patient-Specific Finite Element Model for Augmented Vertebral Bodies

by Michael Kinzl (2012)

Vertebral compression fractures due to osteoporosis affects approximately 700,000 patients in the US and 1.4 million in Europe. The percutaneous injection of bone cement into the vertebral body, known as vertebroplasty, is able to restore the stiffness and strength of fractured vertebral bodies and leads to immediate and lasting pain relief in the majority of cases. Of concern, however are the increased risk of adjacent level fractures and the alteration of stress transfer associated with vertebroplasty. Therefore, the objective of this thesis was to develop and validate a patient-specific, homogenized finite element (hsFE) model for vertebroplasty that can be used to improve clinical outcomes and to reduce the mentioned complications.

In the first study, forty-nine human vertebral body sections were prepared by removing the cortical endplates and were augmented with either standard or low-modulus poly-methyl-methacrylate (PMMA) bone cement. All specimens were scanned with a high-resolution CT system and were tested in compression before and after augmentation. Apparent stiffness and strength as well as contact pressure distributions between the specimens and both loading plates were obtained. This data was used to compare the effects of standard and low-modulus cement augmentation, to generate the hsFE models and to validate them. The second work package aimed at characterizing and understanding the mechanical behaviour of the PMMA/bone composite in the augmented region. Biopsies were extracted from the augmented region, scanned with a micro-CT system and tested in compression to develop a non-linear material model for PMMA/bone composites. Non-linear micro-FE models of the biopsies were used to investigate various parameters affecting the mechanical behaviour of PMMA/bone composites. In the final work step, the previously obtained data and results were used to develop and validate the hsFE approach. The hsFE models featured non-Linear, morphology based material behaviour of bone, patient-specific geometry with explicit modeling of the cortical shell as well as real cement distributions and the non-linear PMMA/bone material model. The developed methods allowed fully automatic generation of the hsFE models from CT images.

The studies of this thesis showed (1) that low-modulus cement has the potential to reduce complications while providing sufficient strengthening of the vertebral body, (2) that the material properties of the augmented region are mainly determined by the porosity, (3) but are also influenced by polymerization shrinkage and PMMA/bone interface properties and (4) that the developed hsFE models provided good qualitative and quantitative predictions of the apparent as well as of the local mechanical behaviour.

In conclusion, the developments and investigations of this thesis contributed to a better understanding of the biomechanics of vertebroplasty, explained the material behaviour of the augmented region and provided a novel, accurate simulation tool for future use in pre-clinical studies.

revised 120906 (hjb)