Micro-finite element (μFE) analysis has recently been introduced for the detailed quantification of the mechanical interaction between bone and implant. The technique has been validated at an apparent level. The aim of this study was to address the accuracy of μFE analysis at the trabecular level. Experimental displacement fields were obtained by deformable image registration, also known as strain mapping (SM), of dynamic hip screws implanted in three human femoral heads. In addition, displacement fields were calculated using μFE analysis. On a voxel-by-voxel basis, the coefficients of determination (R(2)) between experimental and μFE-calculated displacements ranged from 0.67 to 0.92. Linear regression of the mean displacements over nine volumes of interest yielded R(2) between 0.81 and 0.84. The lowest R(2) values were found in regions of very small displacements. In conclusion, we found that peri-implant bone displacements calculated with μFE analysis correlated well with displacements obtained from experimental SM.

With the introduction of high-resolution peripheral quantitative computed tomography (HRpQCT) scanners, detailed three-dimensional in vivo imaging of human radii has become possible. Current recommendations for clinical measurements of the forearm are to scan a relatively small part of the radius (9-mm axial field of view). Micro-finite element (μFE) models based on the measured volume have been shown to correlate better to measured failure load than density-based parameters 1 . Current μFE modeling simulates compression testing between platens in the 9-mm slab. We hypothesized that strength prediction can be improved further when more realistic bone loading is simulated.

Low bone mass is highly prevalent among patients receiving endosseous implants. In turn, the implantation prognosis in low-density skeletal sites is poor. However, little is known about the mechanostructural determinants of implant anchorage. Using metabolic manipulations that lead to low bone density and to its rescue, we show here that anchorage is critically dependent on the pen-implant bone (PIB). Titanium implants were inserted horizontally into the proximal tibial metaphysis of adult rats 6 weeks after orchiectomy (ORX) or sham ORX. Systemic intermittent administration of human parathyroid hormone (1-34) [iahPTH(1-34)] or vehicle commenced immediately thereafter for 6 weeks. The bone-implant apparatus was then subjected to image-guided failure assessment, which assesses biomechanical properties and microstructural deformation concomitantly. Anchorage failure occurred mainly in PIB trabeculae, 0.5 to 1.0 mm away from the implant. Mechanically, the anchorage performed poorly in ORX-induced low-density bone, attributable mainly to decreased trabecular number. iahPTH(1-34) rescued the PIB density and implant mechanical function by augmenting trabecular thickness (Tb.Th). However, implant biomechanical properties in low-density bone were relatively insensitive to implant surface treatment that affected only the osseointegration (%bone-implant contact). These results support a model wherein anchorage failure involves buckling of the weakest trabecular struts followed by sequential failure of the stronger trabeculae. Treatment with iahPTH(1-34) induced thicker struts, which were able to delay and even prevent failure of individual elements, thus implicating trabecular thickness as a prime target for enhancing implant anchorage by systemic bone anabolic therapy. (C) 2010 American Society for Bone and Mineral Research.

Fractures in the distal radius are amongst the most common in humans. Their incidence is increasing due to an aging population leading to a higher percentage of osteoporotic patients with an increased risk of forearm fractures. Hence, an accurate prediction of bone strength in the human radius is of major interest. Bone strength depends on bone geometry and internal architecture. With the recent introduction of a new generation of highresolution 3D peripheral quantitative computed tomography (HR-pQCT) systems, direct quantification of structural bone parameters has become feasible. Furthermore, it was recently demonstrated that bone mechanical competence can be derived from HR-pQCT based micro-finite element modelling (μFE) [Pistoia et al, 2002]. The goal in this study was to gain more insight into bone mechanical competence at the human distal forearm in an elderly population. Specifically, our aim was to find the optimal region for assessing fracture prediction based on μFE bone mechanics.

The Effect of Hydroxyapatite (Endobon) Particles on Implant Osseointegration in Osteoporotic Trabecular Bone Melanie M. Pucher1, Andrea E. Tami1, Thomas L. Mueller2, Gerrit H. van Lenthe2, Ralph Mueller2, Pierre M. Montavon3, Keita Ito1 1AO Research Institute, Davos, Switzerland; 2Institute for Biomechanics, Swiss Federal Institute of Technology, Zurich, Switzerland; 3Clinic for Small Animal Surgery, University of Zurich, Zurich, Switzerland

Pelvic and femoral neck bone surface strains were recorded in five full-body human cadaver vehicle–pedestrian impacts. Impacts were performed at 40km/h using automotive front ends constructed to represent those used in previously reported finite element simulations. While experimental kinematics and bone strains closely matched model predictions, observed pelvic fractures did not consistently agree with the model, and could not be solely explained by vehicle geometry. In an attempt to reconcile injury outcome with factors apart from vehicle design, a proxy measure of subject skeletal health was assessed by high-resolution quantitative computed tomography (HRqCT) of the femoral neck. The incidence of hip/pelvis fracture was found to be consistent with low volumetric bone mineral density and low trabecular bone density. This finding lends quantitative support to the notion that healthy trabecular architecture is crucial in withstanding non-physiological impact loads. Furthermore, it is recommended that injury criteria used to assess vehicle safety with regard to pedestrians consider the increased susceptibility of elderly victims to pelvic fracture.