AN APPROACH TO EXAMINE THE EFFECT OF TAPER ANGLE AND THREADING ON PERIPROSTHETIC BONE REMODELING FROM BONE-ANCHORED AMPUTATION PROSTHESES

INTRODUCTION The most common problems experienced by transfemoral amputees using socket prostheses are soft tissue pain and a limited range of motion around the hip joint [1].  Recently, intraosseous transcutaneous amputation prostheses (ITAP) have been developed as an alternative to the standard socket prostheses for amputees.  A current shortcoming of ITAP is the change in the local mechanical loading at the bone-implant interface leading to bone resorption.  The clinical consequences of this bone loss are increased risks of bone fracture and implant loosening [2].  The purpose of this study was to develop a finite element modeling approach to examine the effect of ITAP fixture threading and taper angle on femoral bone remodeling. METHODS An intact femoral geometry was generated using Mimics software (Materialise, Leuven, Belgium) from CT scans obtained from the VAHKUM database [3]. Twelve ITAP (six threaded and six unthreaded) implants of varying taper angles were designed using SolidWorks (Waltham, MA). Implants were registered and aligned within the femoral diaphysis, and the implant-femur assembly was meshed with quadratic tetrahedral elements; elements at the bone-implant interface shared identical nodes to represent full osseointegration.  Bone elements were assigned inhomogeneous linear-elastic material properties based on CT Hounsfield units. Implant material was modeled as titanium alloy Ti6Al4V (E=114 GPa, ν=0.3), which is commonly used for prostheses due to its superior strength and biocompatibility. Boundary conditions and loads applied to the finite element models were taken from Tomaszewski et al. [4], which were linearly scaled to correspond to an individual with a mass of 70.1 kg and a height of 170 cm.  All models were solved using ABAQUS Standard v6.1 (Providence, RI). Strain energy density was calculated for each implanted femur and compared to those of an intact femur. RESULTS Considerable energy was transferred to the ITAP (Figure 1). Consequently, the periprosthetic cortical bone in the implanted femur had a significantly lower strain energy density than that of the intact femur (Figure 1). DISCUSSION AND CONCLUSIONS It is critical that implant geometry is optimized to decrease periprosthetic bone resorption and reduce the incidence of bone fracture and implant loosening.  Changes in strain energy density following prosthetic implantation is a driving stimulus for bone remodeling, and our future work will incorporate adaptive bone remodeling algorithms into our simulations

THE INFLUENCE OF INDIVIDUAL MUSCLE GROUPS ON FEMORAL STRAIN DISTRIBUTION DURING WALKING

INTRODUCTION Mechanical strain resulting from muscle forces in locomotion plays an important role in the maintenance of bone health [1]. Depending on their magnitude and line of action, these muscle forces may lead to an overall increase or decrease in mechanical strain [2]. Thus, the exclusion of specific muscle groups in finite element models may have a significant impact of simulation results. Our purpose was to quantify the influence of individual muscle groups on the femoral strain distribution during walking using the finite element method. METHODS Kinematic and kinetic data were collected from ten males (age 24.9 ± 4.7 yrs; height 1.7 ± 0.1 m; mass 70.1 ± 8.9 kg) walking overground at 1.25 and 1.75 m/s. Joint reaction forces and moments were calculated using standard inverse dynamics procedures. Muscle and hip contact forces were quantified using musculoskeletal modeling [3] with a static optimization routine (cost function = sum of squared muscle stresses) [4]. For both walking speeds, two instances in stance were examined in the finite element models, coinciding with the first (Peak 1) and second peak (Peak 2) of the axial hip contact force. A finite element model of a young, healthy femur was generated from the VAHKUM database (http://www.ulb.ac.be/project/vakhum/) and scaled to average subject size. Bone was assigned inhomogeneous linear-elastic material properties based on apparent density [5]. The femur was physiologically constrained at the lateral epicondyle, center of the patellar groove, and femoral contact point [6]. Muscle and hip contact forces were applied as point loads. Seven different simulations were run in ABAQUS Standard v6.1 (Providence, RI) for each of the two instances in stance at both 1.25 and 1.75 m/s. For baseline analyses, all the muscle loads were included. For subsequent analyses, muscles forces from specific muscle groups (Hip Adductors, Hip Abductors, Hip Flexors, Hip Extensors, Hip Internal Rotators, and Hip External Rotators) were orderly removed.  Principal strains were quantified along the anterior, lateral, medial, and posterior aspects of the femoral periosteal surface. RESULTS & DISCUSSIONS For all simulations, principal compressive and tensile strains were greatest on the medial and lateral aspect of the femur, respectively (Figure 1). Strain magnitudes for baseline analyses were consistent with in vivo measurements [7], ranging from 1,500-2,000 με and increasing with walking speed. Strains were higher during Peak 1, compared to Peak 2, and removal of specific muscle groups had a greater influence on the strain distribution at this instance in stance. Removal of the hip extensors, abductors, and internal rotators resulted in an overall increase in the femoral strain distribution (Figure 1), suggesting these muscles have a prophylactic action to reduce femoral bending. On the other hand, removal of the hip flexors, adductors, and external rotators had a negligible effect of the femoral strain distribution (Figure 1), but these muscles were minimally activated during walking. CONCLUSIONS Specific muscle groups make important contributions to the femoral strain distribution during walking, and failure to include these muscles in finite element models will lead to erroneous conclusions regarding femoral strain magnitudes

THE INFLUENCE OF EFFECTIVE MASS ON IMPACT FORCE AND ACCELERATION

Accelerometry is often used as a means to quantify the osteogenic or injury potential of impacts. This paper uses a series of four experiments to demonstrate theoretically, mechanically, and experimentally that increasing the effective mass of an impact can lead to an increase in impact force with a corresponding decrease in acceleration. The four experiments included: 1) mass spring models, 2) shoe impact testing, 3) cadaver impact simulation, and 4) an in vivo study manipulating knee angle during running. Results were consistent with the aim, illustrating a limitation for the use of accelerometers for impact assessment. In order to appropriately interpret the results from accelerometry it is necessary to quantify the effective mass of the impact. Failure to account for the influence of effective mass can lead to erroneous conclusions about impact severity

Vertebral arteries do not experience tensile force during manual cervical spine manipulation applied to human cadavers

Background: The vertebral artery (VA) may be stretched and subsequently damaged during manual cervical spine manipulation. The objective of this study was to measure VA length changes that occur during cervical spine manipulation and to compare these to the VA failure length. Methods: Piezoelectric ultrasound crystals were implanted along the length of the VA (C1 to C7) and were used to measure length changes during cervical spine manipulation of seven un-embalmed, post-rigor human cadavers. Arteries were then excised, and elongation from arbitrary in-situ head/neck positions to first force (0.1 N) was measured. Following this, VA were stretched (8.33 mm/s) to mechanical failure. Failure was defined as the instance when VA elongation resulted in a decrease in force. Results: From arbitrary in-situ head/neck positions, the greatest average VA length change during spinal manipulation was [mean (range)] 5.1% (1.1 to 15.1%). From arbitrary in-situ head/neck positions, arteries were elongated on average 33.5% (4.6 to 84.6%) prior to first force occurrence and 51.3% (16.3 to 105.1%) to failure. Average failure forces were 3.4 N (1.4 to 9.7 N). Conclusions: Measured in arbitrary in-situ head/neck positions, VA were slack. It appears that this slack must be taken up prior to VA experiencing tensile force. During cervical spine manipulations (using cervical spine extension and rotation), arterial length changes remained below that slack length, suggesting that VA elongated but were not stretched during the manipulation. However, in order to answer the question if cervical spine manipulation is safe from a mechanical perspective, the testing performed here needs to be repeated using a defined in-situ head/neck position and take into consideration other structures (e.g. carotid arteries). Keywords: Spinal biomechanics; cerebrovascular accidents; spinal manipulation; stroke; vertebral artery dissection

Characterizing the Hydrology of Shallow Floodplain Lakes in the Slave River Delta, NWT, Canada, Using Water Isotope Tracers

The relative importance of major hydrological processes on thaw season 2003 lakewater balances in the Slave River Delta, NWT, Canada, is characterized using water isotope tracers and total suspended sediment (TSS) analyses. A suite of 41 lakes from three previously recognized biogeographical zones—outer delta, mid-delta, and apex—were sampled immediately following the spring melt, during summer, and in the fall of 2003. Oxygen and hydrogen isotope compositions were evaluated in the context of an isotopic framework calculated from 2003 hydroclimatic data. Our analysis reveals that flooding from the Slave River and Great Slave Lake dominated early spring lakewater balances in outer and most mid-delta lakes, as also indicated by elevated TSS concentrations (\u3e0.01 g L-1). In contrast, the input of snowmelt was strongest on all apex and some mid-delta lakes. After the spring melt, all delta lakes underwent heavy-isotope enrichment due to evaporation, although lakes flooded by the Slave River and Great Slave Lake during the spring freshet continued to be more depleted isotopically than those dominated by snowmelt input. The isotopic signatures of lakes with direct connections to the Slave River or Great Slave Lake varied throughout the season in response to the nature of the connection. Our findings provide the basis for identifying three groups of lakes based on the major factors that control their water balances: (1) flood-dominated (n=10), (2) evaporation-dominated (n=25), and (3) exchange-dominated (n=6) lakes. Differentiation of the hydrological processes that influence Slave River Delta lakewater balances is essential for ongoing hydroecological and paleohydrological studies, and ultimately, for teasing apart the relative influences of variations in local climate and Slave River hydrology

Climate-driven Shifts in Quantity and Seasonality of River Discharge over the past 1000 Years from the Hydrographic Apex of North America

Runoff generated from high elevations is the primary source of freshwater for western North America, yet this critical resource is managed on the basis of short instrumental records that capture an insufficient range of climatic conditions. Here we probe the effects of climate change over the past ~1000 years on river discharge in the upper Mackenzie River system based on paleoenvironmental information from the Peace-Athabasca Delta. The delta landscape responds to hydroclimatic changes with marked variability, while Lake Athabasca level appears to directly monitor overall water availability. The latter fluctuated systematically over the past millennium, with the highest levels occurring in concert with maximum glacier extent during the Little Ice Age, and the lowest during the 11th century, prior to medieval glacier expansion. Recent climate-driven hydrological change appears to be on a trajectory to even lower levels as high-elevation snow and glacier meltwater contributions both continue to decline

Tibial Strains are Sensitive to Speed, but not Grade, Perturbations During Running

Tibial stress fractures are thought to result from a fatigue-failure process where bone failure is highly dependent on peak strain magnitude. Little is known regarding the mechanical loading environment of the tibia during graded running despite the prevalence of this terrain. To probe the sensitivity of the mechanical loading environment of the tibia to running grade, tibial strains were quantified using a combined musculoskeletal-finite element modeling routine during graded and level running. Seventeen participants ran on a treadmill at $\pm$10{\deg}, $\pm$5{\deg}, and 0{\deg} while force and motion data were captured. At each grade, participants ran at 3.33 m/s and a grade-adjusted speed, that was 2.20 m/s and 4.17 m/s for uphill and downhill conditions, respectively. Muscle and joint contact forces were estimated using inverse-dynamics-based static optimization. These forces were applied to a participant-informed finite element model of the tibia. 50th percentile pressure-modified von Mises strain was lower ($\leq$-130 $\mu\varepsilon$) during downhill running compared to level and uphill running at 3.33 m/s. However, neither 95th percentile strain (peak strain) nor the volume of bone experiencing strains $\geq$4000 $\mu\varepsilon$ (strained volume) were different between grades (F(4)$\leq$3.28, p$\geq$0.01). In contrast, peak strain and strained volume were highly sensitive to running speed (F(1)$\geq$10.61, p$\leq$0.001), where a 1 m/s increase in speed resulting in a 9 % and 155 % increase in peak strain and strained volume, respectively. Overall, these findings suggest that faster running speeds, but not changes in running grade, may increase the risk of developing a tibial stress fracture

An efficient method for detecting connectivity in neural ensembles

Modern technology is allowing researchers to collect data from neural ensembles with a large number of units, and the analysis of interaction between these units can be very time consuming. Estimation of pairwise connectivity is the most common method of determining the neural `network' but usually necessitates the production of numerous histograms for each pair considered. We present a method which will indicate which pairs in a network represent potential connections and thereby simplify the postexperimental analysis. The technique uses cross-interval information to create an n x n matrix which represents all possible connections in an n neuron ensemble and can be calculated recursively on-line. The performance of this technique is analyzed with respect to data size and strength of the connections. It is compared to 2 similar techniques that are also presented here, one in which perfect knowledge of the timing of the excitation is known, and one in which the timing can be bounded.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29794/1/0000136.pd

On the statistics of binned neural point processes: the Bernoulli approximation and AR representation of the PST histogram

Neural point processes are often approximated by partitioning time into bins, each with a Bernoulli distribution of firing, in order to simplify the mathematical description of their properties. Some of the basic statistics of a neural process are compared using the Bernoulli approximation and the actual Poisson representation. It is seen that in general the Bernoulli approximation is an accurate model only for small λΔ where λ is the intensity and Δ is the width of the time bin. This discrete representation leads to a model of the PST histogram as an AR system, where the parameters depend upon the driving signal s(t) , the refractory effect r(t) and the binwidth Δ . This AR representation is used to predict the PST histogram given s(t) , r(t) and Δ . Estimates of s(t) and r(t) are derived within this parameterization and results discussed for several types of recovery functions given a constant s(t) . AR techniques are used to estimate the AR parameters from the PST histogram of a simulated point process, from which both s(t) and r(t) are estimated.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47434/1/422_2004_Article_BF02331344.pd

Effects of Stride Length and Running Mileage on a Probabilistic Stress Fracture Model

The fatigue life of bone is inversely related to strain magnitude. Decreasing stride length is a potential mechanism of strain reduction during running. If stride length is decreased, the number of loading cycles will increase for a given mileage. It is unclear if increased loading cycles are detrimental to skeletal health despite reductions in strain. Purpose: To determine the effects of stride length and running mileage on the probability of tibial stress fracture. Methods: Ten male subjects ran overground at their preferred running velocity during two conditions: preferred stride length and 10% reduction in preferred stride length. Force platform and kinematic data were collected concurrently. A combination of experimental and musculoskeletal modeling techniques was used to determine joint contact forces acting on the distal tibia. Peak instantaneous joint contact forces served as inputs to a finite element model to estimate tibial strains during stance. Stress fracture probability for stride length conditions and three running mileages (3, 5, and 7 miles·d−1) were determined using a probabilistic model of bone damage, repair, and adaptation. Differences in stress fracture probability were compared between conditions using a 2 × 3 repeated-measures ANOVA. Results: The main effects of stride length (P = 0.017) and running mileage (P = 0.001) were significant. Reducing stride length decreased the probability of stress fracture by 3% to 6%. Increasing running mileage increased the probability of stress fracture by 4% to 10%. Conclusions: Results suggest that strain magnitude plays a more important role in stress fracture development than the total number of loading cycles. Runners wishing to decrease their probability for tibial stress fracture may benefit from a 10% reduction in stride length