T. Van Cleynenbreugel

Individualised, micro CT-based finite element modelling as a tool for biomechanical analysis related to tissue engineering of bone

Load-bearing tissues, like bone, can be replaced by engineered tissues or tissue constructs. For the success of this treatment, a profound understanding is needed of the mechanical properties of both the native bone tissue and the construct. Also, the interaction between mechanical loading and bone regeneration and adaptation should be well understood. This paper demonstrates that microfocus computer tomography (microCT) based finite element modelling (FEM) can have an important contribution to the field of functional bone engineering as a biomechanical analysis tool to quantify the stress and strain state in native bone tissue and in tissue constructs. Its value is illustrated by two cases: (1) in vivo microCT-based FEM for the analysis of peri-implant bone adaptation and (2) design of biomechanically optimised bone scaffolds. The first case involves a combined animal experimental and numerical study, in which the peri-implant bone adaptive response is monitored by means of in vivo microCT scanning. In the second case microCT-based finite element models were created of native trabecular bone and bone scaffolds and a mechanical analysis of both structures was performed. Procedures to optimise the mechanical properties of bone scaffolds, in relation to those of native trabecular bone are discussed.

Validation of x-ray microfocus computed tomography as an imaging tool for porous structures

X-ray microfocus computed tomography (micro-CT) is recently put forward to qualitatively and quantitatively characterize the internal structure of porous materials. However, it is known that artifacts such as the partial volume effect are inherently present in micro-CT images, thus resulting in a visualization error with respect to reality. This study proposes a validation protocol that in the future can be used to quantify this error for porous structures in general by matching micro-CT tomograms to microscopic sections. One of the innovations of the protocol is the opportunity to reconstruct an interpolated micro-CT image under the same angle as the physical cutting angle of the microscopic sections. Also, a novel thresholding method is developed based on matching micro-CT and microscopic images. In this study, titanium porous structures are assessed as proof of principle. It is concluded for these structures that micro-CT visualizes 89% of the total amount of voxels (solid and pore) correctly. However, 8% represents an overestimation of the real structure and 3% are real structural features not visualized by micro-CT. When exclusively focusing on the solid fraction in both the micro-CT and microscopic images, only an overestimation of about 5% is found.

Trabecular bone scaffolding using a biomimetic approach

The current treatment of large bone defects has several disadvantages. An alternative for using grafts or bone cement for the filling of bone cavities is the use of a bone scaffold that provides a temporary load-bearing function. This paper describes a biomechanical design procedure for a personalized implant with a geometry that has a good fit inside the defect and an internal architecture that provides a scaffold with optimized mechanical properties. These properties are optimized for a load-bearing application, for avoiding stress shielding in the bone surrounding the implant and for activation of osteoblasts seeded inside the scaffold. The design is based on medical images both of the defect and of healthy bone tissue that is representative for the tissue being replaced by the scaffold. Evaluation of the scaffolds mechanical properties is done with high-resolution finite element analyzes of the scaffold and healthy bone. This allows matching of the scaffold and bone mechanical properties, thus giving the scaffold its biomimetic properties.

Structural and radiological parameters for the nondestructive characterization of trabecular bone.

AbstractTrabecular bone is characterized by compositional and organizational factors. The former include porosity at microlevel and mineralization. The latter refer to the trabecular architecture. Both determine the mechanical properties of the trabecular bone. The aim of this study is to investigate the relationship between the mechanical properties and the local HU value, the bone mineral density, the in vitro histomorphometric properties assessed by means of microcomputed tomography, and the Youngs modulus determined by ultrasound measurement. Also the correlation between local HU values based on CT data of the full bone and HU values based on CT data of excised trabecular bone cylinders is investigated. Therefore density and strength related parameters of 22 trabecular bone cylinders retrieved from a fresh cadaver femur were measured by using different techniques. The mean HU value of the excised bone samples is very highly correlated with the pQCT density (R2=0.95) and the μCT-based morphometric parameter BV/TV (R2=0.95). The mean HU values, determined from the CT images of the planned and excised bone samples, are less highly correlated (R2=0.75). The Youngs modulus EUS determined from the ultrasound measurement is highly correlated with the maximal stress σ max (R2=0.88) but not with the mechanically determined Youngs modulus Emech (R2=0.67). The maximal stress σ max correlates well with the density parameters (R2 varies between 0.76 and 0.86). On the contrary the mechanically determined Youngs modulus Emech does not correlate well with the density parameters (R2 varies between 0.52 and 0.56). The absorbed energy Eabs during the deformation is only highly correlated with the maximal stress σ max (R2=0.83). The inclusion of structural parameters besides a density related parameter did improve the prediction of the Youngs modulus and the maximal stress. In conclusion, it seems that the HU value from clinical CT scanning is a good predictor of the local bone density and volume fraction. A combination of local density and a measure of the structural anisotropy is clearly needed to achieve good predictions of bone mechanics.

Structural and radiological parameters for the characterization of jawbone

The success of dental implants has been highly correlated with the jaw bone quantity and quality. In contrast to a precise determination of bone volume, bone quality is usually assessed by a crude grading method (Lekholm and Zarb 1985). A new pre-operative classification of bone quality is mandatory. As CT is becoming a standard procedure in the pre-operative planning of dental implants, the Hounsfield units of the selected implant sites are available and a classification like the one proposed by Norton and Gamble proposed in 2001 seems most suitable. The question remains how these Hounsfield values relate to the structural and mechanical properties of the jawbone. The aim of this study was to determine the Hounsfield values of selected bone sites on a CT scan of the jaw and to investigate the relationship between this radiological parameter and structural parameters.