Papers by Harry van Lenthe
Femoroplasty is a procedure where bone cement is injected percutaneously into a weakened proximal... more Femoroplasty is a procedure where bone cement is injected percutaneously into a weakened proximal femur. Uncertainty exists whether femoroplasty provides sufficient mechanical strengthening to prevent fractures in patients with femoral bone metastases. Finite element models are promising tools to evaluate the mechanical effectiveness of femoroplasty, but a thorough validation is required. This study validated a voxel-based finite element model against experimental data from eight pairs of human cadaver femurs with artificial metastatic lesions. One femur from each pair was left untreated, while the contralateral femur was augmented with bone cement. Finite element models accurately predicted the femoral strength in the defect (R² = 0.96) and augmented (R² = 0.93) femurs. The modelled surface strain distributions showed a good qualitative match with results from digital image correlation; yet, quantitatively, only moderate correlation coefficients were found for the defect (mean R² =...
Archives of oral biology, Jan 16, 2017
Aging is accompanied by a series of changes in mature tissues that influence their properties and... more Aging is accompanied by a series of changes in mature tissues that influence their properties and functions. Collagen, as one of the main extracellular components of cartilage, becomes highly crosslinked during aging. In this study, the aim was to examine whether a correlation exists between collagen crosslinking induced by artificial aging and mechanical properties of the temporomandibular joint (TMJ) condyle. To evaluate this hypothesis, collagen crosslinks were induced using ribose incubation. Porcine TMJ condyles were incubated for 7 days with different concentrations of ribose. The compressive modulus and stiffness ratio (incubated versus control) was determined after loading. Glycosaminoglycan and collagen content, and the number of crosslinks were analyzed. Tissue structure was visualized by microscopy using different staining methods. Concomitant with an increasing concentration of ribose, an increase of collagen crosslinks was found. The number of crosslinks increased almos...
I In nt tr ro od du uc ct ti io on n: : In osteoporotic bone, remodelling is altered, whereby bon... more I In nt tr ro od du uc ct ti io on n: : In osteoporotic bone, remodelling is altered, whereby bone resorption exceeds bone formation and consequently bone volume and bone contact decreases in the peri-implant region1. As a result, implant loosening and/or failure may occur. Current approaches to reduce the effect of remodelling on secondary implant stability are based on pharmaceutical methods (e.g. bisphosphonates, parathyroid hormones). The objective of this study was to determine if a simple alternative could also preserve the bone mantle around a screw implant. Endobon® (Biomet) is a non-resorbable osteoconductive hydroxyapatite (HA) ceramic which is being used clinically as filling material for the management of bone defects. We hypothesized that implanting such particles in the peri-implant region in an osteoporotic rat model could maintain a more dense and functional peri-implant bone structure, by shift remodelling events. M Ma at te er ri ia al ls s a an nd d M Me et th ho od ds s: : Sixty-five 12 week old female Wistar rats were examined. The rats were ovariectomized (OVX) 4 weeks prior to implantation for osteoporosis induction. The osteoporotic state of each animal was verified with in vivo bone mineral density (BMD) measurements of the distal femoral metaphyses at the time of ovariectomy and implantation using a novel high-resolution peripheral quantitative computed tomography (pQCT) system (XtremeCT, Scanco Medical, Bassersdorf, Switzerland). The animals were randomly separated into five groups representing the different time-points of euthanasia after implantation. Each animal received two self-tapping titanium screws (Ø 1.7 x 5 mm), one in each proximal tibia. For reproducible screw positioning, a custom made surgical aiming device was used. Prior to screw implantation in the right tibia, the drill-hole was filled with 6 mg HA ceramic particles with an average size of 63 to 100 μm. The left tibia was used as a control, whereby the screw was implanted without any HA particles. Animals were euthanized 1 hour, 2, 4, 6, and 8 weeks post implantation. The proximal tibiae were explanted and scanned with a micro-computed tomography system (μCT40, Scanco Medical, Bassersdorf, Switzerland) at 16 μm isotropic resolution. Thereafter, 9/13 tibiae of each group were preserved for histomorphometric analysis (primary outcome) while the remaining 4/13 animals were used for biomechanical pull-out tests (secondary outcome). A constrained Gaussian filter with sigma of 1.2 and a support of 1 was used to suppress the noise in the μCT40 data. Subsequently, segmentation was used to digitally separate bone, HA, and implant in the images. For this purpose special image processing was applied to the direct vicinity of the implant to remove imaging artifacts from the Ti implant2. For quantitative analysis, a volume of interest (VOI) was defined in a 200 μm thick peri-implant region. Bone and HA volumes were analyzed in a region including 130 slices (approx. 2 mm) in axial screw direction, starting at the fully threaded region of the screw tip. In the HA-implanted side, the peri-implant bone mantle was analyzed as the (BV+HAV)/TV %, whereas only BV/TV % was taken into account for the control side. For mechanical testing each bone was partially embedded in PMMA, i.e. not including the implant head, to hold it firmly in a custom made jig of the testing machine (Instron, Darmstadt, Germany). The implant was then pulled under displacement control (1 mm/min) until failure and ultimate force was determined. Finally, a one-way ANOVA with Tukey HSD post-hoc testing was used to determine if the difference between HA-implanted and control side changed over time, with a significant threshold of p < 0.05.
A sequence {a n } n≥0 is said to be asymptotically r-log-convex if it is r-log-convex for n suffi... more A sequence {a n } n≥0 is said to be asymptotically r-log-convex if it is r-log-convex for n sufficiently large. We present a criterion on the asymptotical r-log-convexity based on the asymptotic behavior of a n a n+2 /a 2 n+1. As an application, we show that most P-recursive sequences are asymptotic rlog-convexity for any integer r once they are log-convex. Moreover, for a concrete integer r, we present a systematic method to find the explicit integer N such that a P-recursive sequence {a n } n≥N is r-log-convex. This enable us to prove the r-log-convexity of some combinatorial sequences.
INTRODUCTION. A clinically relevant way to investigate the biomechanical behavior of the trapezio... more INTRODUCTION. A clinically relevant way to investigate the biomechanical behavior of the trapeziometacarpal (TMC) joint is to evaluate its contact patterns during isometric tasks. This will help us to obtain a better understanding of the onset of osteoarthritis (OA) and will eventually improve prevention and treatment strategies of this highly disabling disease. METHODS. CT scans of the hand region of 20 female volunteers (mean age: 60.8 years) were taken in three different configurations: relaxed neutral, lateral pinch and power grasp, using a radiolucent jig with embedded load cell (Brown University, USA). Four subjects showing radiological signs of OA were excluded from the study. Scans were segmented using Mimics (Materialise, Belgium) and 3D models of the first metacarpal (MC1) and the trapezium were created. The articular area of each bone was quantified based on manual measurements performed on the 3D bone models. A custom-written Matlab code based on the finite deformation b...
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Papers by Harry van Lenthe