Peter M de Bakker

University of British Columbia - Vancouver, Vancouver, British Columbia, Canada

Are you Peter M de Bakker?

Claim your profile

Publications (4)9.17 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Understanding proximal femur fracture may yield new targets for fracture prevention screening and treatment. The goal of this study was to characterize force–displacement and failure behaviours in the proximal femur between displacement control and impact loading fall simulations. Twenty-one human proximal femurs were tested in two ways, first to a sub-failure load at a constant displacement rate, then to fracture in an impact fall simulator. Comparisons of sub-failure energy and stiffness were made between the tests at the same compressive force. Additionally, the impact failure tests were compared with previous, constant displacement rate failure tests (at 2 and 100 mm/s) in terms of energy, yield force, and stiffness. Loading and displacement rates were characterized and related to specimen stiffness in the impact tests. No differences were observed between the sub-failure constant displacement and impact tests in the aforementioned metrics. Comparisons between failure tests showed that the impact group had the lowest absorbed energy, 24% lower maximum force and 160% higher stiffness than the 100 mm/s group ( for all), but suffered from low statistical power to differentiate the donor age and specimen BMD. Loading and displacement rates for the specimens tested using impact varied during each test and between specimens and did not show appreciable viscoelasticity. These results indicate that constant displacement rate testing may help understand sub-failure mechanical behaviour, but may not elucidate failure behaviours. The differences between the impact and constant displacement rate fall simulations have important ramifications for interpreting the results of previous experiments.
    Journal of Biomechanics. 11/2014; 47.
  • [Show abstract] [Hide abstract]
    ABSTRACT: To gain insight into a new technology, a novel facet arthroplasty device (TFAS) was compared to a rigid posterior fixation system (UCR). The axial and bending loads through the implants and at the bone-implant interfaces were evaluated using an ex vivo biomechanical study and matched finite element analysis. Kinematic behaviour has been reported for TFAS, but implant loads have not. Implant loads are important indicators of an implant's performance and safety. The rigid posterior fixation system is used for comparison due to the extensive information available about these systems. Unconstrained pure moments were applied to 13 L3-S1 cadaveric spine segments. Specimens were tested intact, following decompression, UCR fixation and TFAS implantation at L4-L5. UCR fixation was via standard pedicle screws and TFAS implantation was via PMMA-cemented transpedicular stems. Three-dimensional 10 Nm moments and a 600 N follower load were applied; L4-L5 disc pressures and implant loads were measured using a pressure sensor and strain gauges, respectively. A finite element model was used to calculate TFAS bone-implant interface loads. UCR experienced greater implant loads in extension (p < 0.004) and lateral bending (p < 0.02). Under flexion, TFAS was subject to greater implant moments (p < 0.04). At the bone-implant interface, flexion resulted in the smallest TFAS (average = 0.20 Nm) but greatest UCR (1.18 Nm) moment and axial rotation resulted in the greatest TFAS (3.10 Nm) and smallest UCR (0.40 Nm) moments. Disc pressures were similar to intact for TFAS but not for UCR (p < 0.04). These results are most applicable to the immediate post-operative period prior to remodelling of the bone-implant interface since the UCR and TFAS implants are intended for different service lives (UCR--until fusion, TFAS--indefinitely). TFAS reproduced intact-like anterior column load-sharing--as measured by disc pressure. The highest bone-implant moment of 3.1 Nm was measured in TFAS and for the same loading condition the UCR interface moment was considerably lower (0.4 Nm). For other loading conditions, the differences between TFAS and UCR were smaller, with the UCR sometimes having larger values and for others the TFAS was larger. The long-term physiological meaning of these findings is unknown and demonstrates the need for a better understanding of the relationship between spinal arthroplasty devices and the host tissue as development of next generation motion-preserving posterior devices that hope to more accurately replicate the natural functions of the native tissue continues.
    European Spine Journal 03/2012; 21(8):1660-73. · 2.47 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Results of recent imaging studies and theoretical models suggest that the superior femoral neck is a location of local weakness due to an age-related thinning of the cortex, and thus the site of hip fracture initiation. The purpose of this study was to experimentally determine the spatial and temporal characteristics of the macroscopic failure process during a simulated hip fracture that would occur as a result of a sideways fall. Twelve fresh frozen human cadaveric femora were used in this study. The femora were fractured in an apparatus designed to simulate a fall on the greater trochanter. Image sequences of the surface events related to the fractures were captured using two high-speed video cameras at 9111 Hz. The videos were analyzed with respect to time and load to determine the location and sequence of these events occurring in the proximal femur. The mean failure load was 4032 N (SD 370 N). The first surface events were identified in the superior femoral neck in eleven of the twelve specimens. Nine of these specimens fractured in a clear two-step process that initiated with a failure in the superior femoral neck, followed by a failure in the inferior femoral neck. This cadaveric model of hip fracture empirically confirms hypotheses that suggested that hip fractures initiate with a failure in the superior femoral neck where stresses are primarily compressive during a sideways fall impact, followed by a failure in the inferior neck where stresses are primarily tensile. Our results confirm the superolateral neck of the femur as an important region of interest for future hip fracture screening, prevention and treatment research.
    Journal of biomechanics 07/2009; 42(12):1917-25. · 2.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Magnetic resonance imaging (MRI) is a promising medical imaging technique that we used to assess femoral neck cortical geometry. Our primary objective was to assess whether cortical bone in the femoral neck assessed by MRI was associated with failure load in a simulated sideways fall, with and without adjustment for total bone size. Our secondary objective was to assess the reliability of the MRI measurements. We imaged 34 human cadaveric proximal femora using MRI and dual-energy X-ray absorptiometry (DXA). MRI measurements of cross-sectional geometry at the femoral neck were the cortical cross-sectional area (CoCSA(MRI)), second area moment of inertia (x axis; Ix(MRI)), and section modulus (x axis; Zx(MRI)). DXA images were analyzed with the standard Hologic protocol. From DXA, we report the areal bone mineral density (aBMD(DXA)) in the femoral neck and trochanteric subregions of interest. The femora were loaded to failure at 100 mm/s in a sideways fall configuration (15 degrees internal rotation, 10 degrees adduction). RESULTS AND OBSERVATIONS: Failure load (N) was the primary outcome. We observed that the femoral neck CoCSA(MRI) and Ix(MRI) were strongly associated with failure load (r (2)=0.46 and 0.48, respectively). These associations were similar to those between femoral neck aBMD and failure load (r (2)=0.40), but lower than the associations between trochanteric aBMD and failure load (r (2)=0.70). We report that MRI holds considerable promise for measuring cortical bone geometry in the femoral neck and for predicting strength at the proximal femur.
    Osteoporosis International 11/2006; 17(10):1539-45. · 4.04 Impact Factor