Michel Dalstra

Fabric and elastic principal directions of cancellous bone are closely related

Cancellous bone architecture and mechanics are intimately related. The trabecular architecture of cancellous bone is considered determined by its mechanical environment (Wolffs law), and the mechanical properties of cancellous bone are inversely determined by the trabecular architecture and material properties. Much effort has been spent in expressing these relations, but the techniques and variables necessary for this have not been fully identified. It is obvious, however, that some measure of architectural anisotropy (fabric) is needed. Within the last few years, volume-based measures of fabric have been introduced as alternatives to the mean intercept length method, which has some theoretical problems. This paper seeks to answer which of four different fabric measures best predicts finite element calculated mechanical anisotropy directions. Twenty-nine cancellous bone specimens were three-dimensionally reconstructed using the automated serial sectioning technique. A series of large-scale finite-element analyses were performed on each of the three-dimensional reconstructions to calculate the compliance matrix for each specimen, from which the mechanical principal directions were derived. The architectural anisotropy was determined in three-dimensional space for each specimen using mean intercept length (MIL), volume orientation (VO), star volume distribution (SVD) and star length distribution (SLD). Each of the architectural anisotropy results were expressed by a fabric tensor. Architectural main directions were determined from the fabric tensors and compared with the FE-calculated mechanical anisotropy directions. All architectural measures predicted the mechanical main directions rather well, which supports the assumption that mechanical anisotropy directions are aligned with fabric directions. MIL showed a significant, though very small (1.4 degrees), deviation from the primary mechanical direction. VO had difficulty in determining secondary and tertiary mechanical directions; its mean deviation was 8.9 degrees. SVD and SLD provided marginally better predictors of mechanical anisotropy directions than MIL and VO.

Adaptive bone remodeling and biomechanical design considerations for noncemented total hip arthroplasty

Clinical problems with noncemented total hip arthroplasty (THA) stems, directly or indirectly related to load transfer, include mid-thigh pain due to relative (micro) motions or excessive endosteal interface stresses, subsidence and loosening due to inadequate primary stability and fit, and proximal femoral bone atrophy due to stress shielding. In this article, the load-transfer mechanisms associated with noncemented THA stems and their resulting stress patterns are discussed in relation to design features, bonding characteristics, and materials choice. Nonlinear finite-element models and computer simulation programs for strain-adaptive bone remodeling have been used for this study. Canal-filling, fully bonded metal stems have been found likely to cause proximal bone atrophy, possibly leading to long-term failure of the implant/bone composite. The use of flexible (isoelastic) materials and/or press-fit fixation reduces stress shielding, but also reduces the potential for interface stability. The stem material, the stem shape, and the coating geometry interact in relation to the load-transfer mechanism, and it is suggested that optimal combinations of these characteristics can be determined through the computer simulation methods presented.

The role of an effective isotropic tissue modulus in the elastic properties of cancellous bone.

Conceptually, the elastic characteristics of cancellous bone could be predicted directly from the trabecular morphology--or architecture--and by the elastic properties of the tissue itself. Although hardly any experimental evidence exists, it is often implicitly assumed that tissue anisotropy has a negligible effect on the apparent elastic properties of cancellous bone. The question addressed in this paper is whether this is actually true. If it is, then micromechanical finite element analysis (micro-FEA) models, representing trabecular architecture, using an effective isotropic tissue modulus should be able to predict apparent elastic properties of cancellous bone. To test this, accurate multi-axial compressive mechanical tests of 29 whale bone specimens were simulated with specimen-specific micro-FEA computer models built from true three-dimensional reconstructions. By scaling the micro-FEA predictions by a constant tissue modulus, 92% of the variation of Youngs moduli determined experimentally could be explained. The correlation even increased to 95% when the micro-FEA moduli were scaled to the isotropic tissue moduli of individual specimens. Excellent agreement was also found in the elastic symmetry axes and anisotropy ratios. The prediction of Poissons ratios was somewhat less precise at 85% correlation. The results support the hypothesis; for practical purposes, the concept of an effective isotropic tissue modulus concept is a viable one. They also suggest that the value of such a modulus for individual cases might be inferred from the average tissue density, hence the degree of mineralization. Future studies must clarify how specific the tissue modulus should be for different types of bone if adequate predictions of elastic behavior are to be made in this way.

AGE VARIATIONS IN THE PROPERTIES OF HUMAN TIBIAL TRABECULAR BONE

We tested in compression specimens of human proximal tibial trabecular bone from 31 normal donors aged from 16 to 83 years and determined the mechanical properties, density and mineral and collagen content. Youngs modulus and ultimate stress were highest between 40 and 50 years, whereas ultimate strain and failure energy showed maxima at younger ages. These age-related variations (except for failure energy) were non-linear. Tissue density and mineral concentration were constant throughout life, whereas apparent density (the amount of bone) varied with ultimate stress. Collagen density (the amount of collagen) varied with failure energy. Collagen concentration was maximal at younger ages but varied little with age. Our results suggest that the decrease in mechanical properties of trabecular bone such as Youngs modulus and ultimate stress is mainly a consequence of the loss of trabecular bone substance, rather than a decrease in the quality of the substance itself. Linear regression analysis showed that collagen density was consistently the single best predictor of failure energy, and collagen concentration was the only predictor of ultimate strain.

A three-dimensional finite element model from computed tomography data: A semi-automated method

Abstract Three-dimensional finite element analysis is one of the best ways to assess stress and strain distributions in complex bone structures. However, accuracy in the results may be achieved only when accurate input information is given. A semi-automated method to generate a finite element (FE) model using data retrieved from computed tomography (CT) was developed. Due to its complex and irregular shape, the glenoid part of a left embalmed scapula bone was chosen as working material. CT data were retrieved using a standard clinical CT scanner (Siemens Somatom Plus 2, Siemens AG, Germany). This was done to produce a method that could later be utilized to generate a patient-specific FE model. Different methods of converting Hounsfield unit (HU) values to apparent densities and subsequently to Youngs moduli were tested. All the models obtained were loaded using three-dimensional loading conditions taken from literature, corresponding to an arm abduction of 90°. Additional models with different amounts of elements were generated to verify convergence. Direct comparison between the models showed that the best method to convert HU values directly to apparent densities was to use different equations for cancellous and cortical bone. In this study, a reliable method of determining both geometrical data and bone properties from patient CT scans for the semi-automated generation of an FE model is presented.

Bone strength and material properties of the glenoid

The quality of the glenoid bone is important to a successful total shoulder replacement. Finite element models have been used to model the response of the glenoid bone to an implanted prosthesis. Because very little is known about the bone strength and the material properties at the glenoid, these models were all based on assumptions that the material properties of the glenoid were similar to those of the tibial plateau. The osteopenetrometer was used to assess the topographic strength distribution at the glenoid. Strength at the proximal subchondral level of the glenoid averaged 66.9 MPa. Higher peak values were measured posteriorly, superiorly, and anteriorly to the area of maximum concavity of the glenoid joint surface known as the bare area. One millimeter underneath the subchondral plate, average strength decreased by 25%, and at the 2 mm level strength decreased by 70%. The contribution of the cortical bone to the total glenoid strength was assessed by compression tests of pristine and cancellous-free glenoid specimens. Strength decreased by an average of 31% after the cancellous bone was removed. The material properties of the glenoid cancellous bone were determined by axial compression tests of bone specimens harvested from the central part of the glenoid subchondral area. The elastic modulus varied from approximately 100 MPa at the glenoid bare area to 400 MPa at the superior part of the glenoid. With the elastic constants used a predictor of the mechanical anisotropy, the average anisotropy ratio was 5.2, indicating strong anisotropy. The apparent density was an average 0.35 gr. cm-3, and the Poisson ratio averaged 0.263. According to our findings the anisotropy of the glenoid cancellous bone, details concerning the strength distribution, and the load-bearing function of the cortical shell should be considered in future finite element models of the glenoid.

Glenoid bone architecture

This article describes regional variations in trabecular bone architecture in terms of density and orientation within six glenoid specimens. The mean donor age was 56 years and ranged from 31 to 72 years. An automated imaging technique based on 3-dimensional serial sectioning was used for the direct examination of the glenoid cancellous bone structures. Subchondral plate thickness was on average 1.9 mm and ranged from 1.2 mm to 2.9 mm. The volume fraction of trabecular bone varied from 11% to 45% with peak values at the posterior glenoid vault. On graphic 3-dimensional reconstructions, the glenoid appeared as platelike trabeculae, radially oriented perpendicular to the subchondral plate and interconnected by thin rods. These views also displayed regional variations throughout the glenoid, reflecting differences in the macroscopic appearance. Quantitative structural analysis revealed different degrees of anisotropy at the glenoid cancellous region, predominantly transverse isotropy. Resemblance to direct weight-bearing cancellous bone such as the proximal tibia was evident.

Mechanical properties of the normal human tibial cartilage-bone complex in relation to age

OBJECTIVE: This study investigates the age-related variations in the mechanical properties of the normal human tibial cartilage-bone complex and the relationships between cartilage and bone. DESIGN: A novel technique was applied to assess the mechanical properties of the cartilage and bone by means of testing the cartilage-bone complex. BACKGROUND: Up to now, mechanical testing of cartilage and bone has been reported separately, and little is known about the mechanical behaviour of both tissues when examined as a unit. METHODS: Cylindrical human proximal tibial cartilage-bone complex specimens from 31 normal donors aged 16-83 years were tested in compression. The deformation was measured simultaneously in bone and cartilage to obtain the mechanical properties of both tissues. RESULTS: The stiffnesses and elastic energies of both cartilage and bone showed an initial increase, with maxima at 40 years, followed by a steady decline. The viscoelastic energy was maximal at younger ages (16-29 years), followed by a steady decline. The energy absorption capacity did not vary with age. Stiffnesses and elastic energies were correlated significantly between cartilage and bone. CONCLUSIONS: The present study demonstrates that similar age-related trends were seen in cartilage and bone, as if they behaved as a single mechanical unit. RELEVANCE: The basic information presented here on the mechanical properties of cartilage and bone and the correlations between them reveals the unit function of both tissues that are of importance for the understanding of the etiology and pathogenesis of degenerative joint diseases, such as arthrosis.

Changes in the stiffness of the human tibial cartilage-bone complex in early-stage osteoarthrosis

Cylindrical human tibial cartilage-bone unit specimens were removed from 9 early-stage medial osteoarthrotic (OA) tibiae (mean age 74 years) and 10 normal age-matched tibiae (mean age 73 years). These specimens were divided into 4 groups: OA, lateral comparison, medial age-matched, and lateral age-matched and were tested to 0.5% bone strain with a novel technique to obtain the stiffnesses of both cartilage and bone simultaneously. We found a pronounced reduction in the stiffnesses of OA cartilage and subchondral bone when compared with the medial age-matched group. OA cartilage was significantly thinner than that of the lateral comparison and the medial age-matched control groups. However, this reduction in thickness was not correlated with the reduction in stiffness for OA cartilage. The stiffnesses did not correlate between OA cartilage and bone, whereas the stiffness relationships between cartilage and bone remained significant in the three control groups. Our findings suggest that both cartilage and bone in early-stage OA are mechanically inferior to normal, and that OA cartilage and bone have lost their unit function to mechanical loading.

Alendronate treatment improves bone-pedicle screw interface fixation in posterior lateral spine fusion: an experimental study in a porcine model.

The bone–screw interface has been indicated as the weak link in pedicle screw spine fixation. Bisphosphonate treatment may have the effect of improving bone–screw interface fixation in spine fusion by inhibiting bone resorption. An experimental study was conducted using a porcine model to evaluate the influence of alendronate treatment on bone–pedicle screw interface fixation. Eleven pigs in the treatment group received alendronate 10xa0mg/day orally for threexa0months postoperatively. The other 11 pigs served as a control group. Posterior lateral fusion with the CD Horizon pedicle screw system was performed with autograft on the lumbar spine on all animals. Biomechanical torsion test and histomorphometric parameters of screw fixation were evaluated threexa0months after the operation. The maximum torque and initial angular stiffness of the treatment group was higher than that of the control group, but there was no statistical significance. The bone–screw contact surface was 23.3u2009±u200910% for the treatment group and 9.8u2009±u20095.9% for the control group (Pu2009<u20090.01). This study indicated that alendronate treatment increased bone purchase of stainless steel screw surfaces.