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This paper presents real world acceleration data for an ambulance driving up and over a curb. A full scale reenactment was performed for a litigated case in which a patient on a gurney in an ambulance claimed a variety of bodily injuries after the ambulance struck a curb. A height and weight matched surrogate rode on the gurney during the tests. Results demonstrated that peak vehicle and occupant accelerations never exceeded 1.1 g’s. To address the claimed injuries, the accelerations likely sustained by the patient were compared to those experienced during daily life. Since ambulances are wide vehicles that travel fast on potentially narrow arterial, collector or local roadways, curb or median impacts may occur during the normal course of driving. Thus, these results may be useful for forensic experts in dealing with similar cases involving claimed injuries following curb impacts.

Job design that is protective of the lumbar spine may inadvertently increase stresses on the thoracic spine, potentially leading to thoracic injury. In this study, we determined the ratio of thoracic to lumbar loading during various tasks, including stoop and squat lifts. Loading on the thoracic spine was calculated based on previously reported thoracic intradiscal pressures and cross-sectional areas, and compared to loading on the lumbar spine calculated using the same methodology. Results demonstrated that the ratio of thoracic and lumbar loading is not uniform and varies with the posture used during manual material handling tasks. Specifically, the loading of the thoracic spine increased during squat lifts, as compared to stoop lifts, whereas the loading on the lumbar spine decreased during squat lifts. This study adds to the body of knowledge that there are trade-offs between squat and stoops lifts and neither are without risks

Prostaglandin E2 (PGE2) has been shown to stimulate both bone resorption and formation in experimental animals, leading to augmentation of trabecular and cortical bone. The amino bisphosphonate alendronate (ALN) is a potent inhibitor of bone resorption. The objectives of this study were to examine if PGE2 stimulation of bone formation was dependent on bone resorption and if the bone accrued as a result of PGE2 treatment contributed to bone strength. The 48 female Sprague-Dawley rats were assigned to six groups as follows: five groups (8/group) were ovariectomized at the age of 6 months. One group was sacrificed 2 months later to establish baseline conditions, and four groups were treated for 25 days with (1) vehicle, (2) PGE2 at 3 mg/kg/day, (3) ALN sc at 0.8 μg/kg/day, and (4) PGE2 + ALN at the respective doses. The sixth group served as nonovariectomized untreated controls. Histomorphometric analysis of 6–10 μm thick tibial sections after in vivo fluorochrome double labeling showed that treatment with PGE2 alone increased endo-cortical mineral apposition rate and bone formation rate, stimulated production of bone trabeculae in the marrow cavity, and increased cortical porosity. Combined ALN + PGE2 treatment prevented the resorption induced by PGE2 but not the stimulation of bone formation on endocortical and periosteal surfaces and resulted in a significant increase in cortical thickness. Consistent with these observations, the femoral midshaft tested to failure in three-point bending showed a significant increase in strength in the PGE2 + ALN group (181 ± 15 N) compared to time 0 controls (145 ± 23 N) or to the ovariectomized vehicle-treated group (141 ± 28 N). Similarly, there was a statistically significant increase in femoral midshaft stiffness. There was also a statistically significant increase in the ash content of bones from animals treated with PGE2 + ALN relative to vehicle-treated animals. No statistically significant differences among groups were observed in femoral neck or vertebral strength or stiffness. These findings show that PGE2 stimulation of bone formation is not blocked by inhibition of bone resorption and that combined treatment with PGE2 and the bone resorption inhibitor ALN increased the mechanical strength of ovariectomized rat femoral shaft compared to nonovariectomized animals.

Hip fractures can be separated into femoral neck (cervical or intracapsular) or trochanteric (extracapsular). Trochanteric fractures have been associated with up to twice the short-term mortality of cervical fractures in the elderly. Fracture type may be influenced by the fall direction and local differences in proximal femur strength properties. We previously demonstrated that fall characteristics and body habitus, in addition to femoral bone mineral density, play a dominant role in the prediction of hip fracture in elderly fallers. To examine the association of these determinants with hip fracture type, we assessed fall characteristics, body habitus, and site-specific bone mineral density measurements in 112 elderly hip fracture patients (85 women and 27 men, mean age 85 years) 1 week after an acute hip fracture. Trochanteric BMD was 13% lower in women and 11% lower in men for patients with trochanteric fracture than in those with femoral neck fracture (p < 0.01). A stepwise multiple logistic regression indicated that trochanteric BMD (decrease of 1.0 SD, adjusted OR 4.6, 95% Cl 2.0-9.5, p < 0.0001) and femoral neck BMD (increase of 1.0 SD, adjusted OR 3.0, 95% Cl 1.6-5.9, p = 0.0003) were independently associated with trochanteric fracture. Fall characteristics, body habitus, gender, and age were not associated with hip fracture type. We conclude that a relatively low trochanteric BMD or a high femoral neck BMD was associated with a trochanteric hip fracture and that site-specific trochanteric BMD determinations should be measured when assessing risk of trochanteric hip fractures in the elderly.

As with any structure, the structural capacity of the proximal femur depends on the applied loads and these can vary as a function of impact direction during a fall. However, despite its potential importance in hip fracture risk assessment, the relative importance of impact direction is unknown. To investigate the role of impact direction in hip fracture, we developed a detailed finite element model of the proximal femur. We analyzed four loading configurations that represent a range of possible falls on the greater trochanter. Our results indicate that a change in the angle between the line of action of the applied force and the axis of the femoral neck from 0 degrees (representing a direct lateral impact) to 45 degrees (representing a posterolateral impact) reduced structural capacity by 26%. This weakening of the femur with changes in impact direction is comparable to the weakening associated with 2-3 decades of age-related bone loss. Our result elucidates the independent contribution of fall mechanics to hip fracture risk by identifying an aspect of the fall (the direction of impact) that is an important determinant of fall severity. The results can also be incorporated into a refined clinical method for assessment of hip fracture risk that accounts for the complex interactions between fall severity and bone fragility.

Despite radiographic and histologic evidence of trabecular bone density changes within and adjacent to osseous metastases, there currently exist no data to demonstrate whether these changes are important in predicting the risk of fracture. To determine if these density changes result in significant reductions in mechanical properties, trabecular bone specimens were prepared from lower thoracic and lumbar vertebrae from two cadavers with radiographic, gross, and histologic evidence of lytic and/or blastic osseous metastases. Each specimen was classified as normal, lytic, or blastic based on appearance in fine-grain radiographs of 8-9 mm thick coronal plane sections. Specimens were tested to failure in uniaxial compression, and tissue and apparent densities were measured. Mean tissue densities were within normal ranges. The mean apparent density for all specimens combined was within the normal range for human vertebrae, and the mean apparent density for radiographically normal (0.131 g/ml) and lytic (0.111 g/ml) specimens was less than the mean apparent density of blastic (0.182 g/ml) specimens (p0.02). The moduli of lytic and blastic specimens were less than for normal specimens (p0.025). The strength of lytic specimens was less than normal (p = 0.057), but the strength of blastic specimens was not (p0.1). Apparent density explained significant fractions of the variations in both modulus (p0.001) and strength (p0.001). The data suggest that blastic changes associated with osseous metastases to trabecular bone disrupt the normal dependence of trabecular mechanical properties on apparent density, but lytic changes do not.(ABSTRACT TRUNCATED AT 250 WORDS)

The implantation of fixation devices increases the primary stability after surgery significantly. Little is known about the comparison of flexibility of instrumented and posterolateral fusions or the lumbosacral distractions spondylodesis (LSDS). The purpose of the study was to compare the flexibility of instrumented and posterolateral fusions or LSDS after simulation with polymethylmethacrylate (PMMA). Dynamic testing from 0-16 Nm in flexion/compression and 0-900 N in compression was performed on 14 lumbosacral spines. The spines were tested intact, after laminectomy, instrumentation with an internal fixateur L5-S1, and simulation of posterolateral fusion L4-S1 or LSDS and thereafter a pseudoarthrosis was simulated. The laminectomy resulted in a significant increase of flexibility and posterior distraction in flexion and compression. The stabilization with an internal fixateur or PMMA simulation for posterolateral fusion or LSDS resulted in a significant decrease of the flexibility. Creation of a unilateral pseudoarthrosis did not effect the flexibility. The instrumented fusion as well as the PMMA simulated bony posterolateral fusion and LSDS resulted in this model in a significant stabilization of the laminectomy. After unilateral fusion the flexibility was not significantly increased.

Because fall experiments with volunteers can be both challenging and risky, especially with older volunteers, we wished to develop computer simulations of falls to provide a theoretical framework for understanding and extending experimental results. To perform a preliminary validation of the articulated total body (ATB) model for passive falls, we compared the model predictions of fall direction, impact location, and impact velocity as a function of disturbance type (faint, slip, step down, trip) and gait speed (fast, normal, slow) to experimental results with young adult volunteers. The three-dimensional ATB model had 17 segments and 16 joints. Its physical characteristics, environment definitions, contact functions, and initial conditions were representative of our experiment. For each combination of disturbance and gait speed, the ATB model was left to fall passively under gravity once disturbed, i.e., no joint torques were applied, until impact with the floor occurred. Finally, we also determined the sensitivity of the model predictions to changes in the model’s parameters. Our model predictions of fall angles and impact angles were qualitatively in agreement with those observed experimentally for ten and seven of the 12 original simulations, respectively. Quantitatively, the model predictions of fall angles, impact angles, and impact velocities were within one experimental standard deviation for seven, three, and nine of the 12 original simulations, respectively, and within two experimental standard deviations for ten, nine, and 11 of the 12 original simulations, respectively. Finally, the fall angle and impact angle region did not change for 92% and 95% of the 74 input variation simulations, respectively, and the impact velocities were within the experimental standard deviations for 78% of the 74 input variation simulations. Based on our simulations and a sensitivity analysis, we conclude that our preliminary validation of the ATB model for passive falls was successful. In fact, these ATB model simulations represent a significant step forward in fall simulations. We believe that with additional work, the ATB model could be used to accurately simulate a variety of human falls and may be useful in further understanding the etiology and mechanisms of fall injuries such as hip fractures.

This study assessed changes in balance self-efficacy (BSE) over 1 year in community-dwelling elderly, compared changes in BSE between fallers and nonfallers, and assessed the relationship between specific balance and mobility risk factors for side falls and BSE scores. Elderly fallers (n = 67; 80.2 ± 5.9 years) and nonfallers (n = 75; 79.4 ± 4.9), categorized based on self-reported falls over 1 year, were tested at baseline on postural sway, hip-abduction strength, lateral-stepping velocity, tandem walk, and get-up-and-go and given a BSE questionnaire. Fallers had lower BSE scores than nonfallers did (141.6 ± 33.5 and 154.9 ± 25.4; p = .008). BSE did not change over 1 year. In stepwise regression, BSE scores were predictive of time on the get-up-and-go, mediolateral sway, and tandem walk independent of age, height, and strength (p < .001). The BSE scale might be useful for screening individuals at risk for injurious falls because it is inexpensive and noninvasive.

The Quick Step measures reaction time and lateral stepping velocity. Upon a visual cue, participants step to the side as quickly as possible. Instrumentation includes floor pads with pressure-sensitive switches and two timers. In all, 109 older adults who had experienced a recent fall, 46 older adult nonfallers, and 24 young adults volunteered for testing. Reliability for reaction time and stepping velocity was good to excellent (intraclass correlation = 0.69–0.85). Multivariate analysis of variance revealed a significant difference between groups, p < 0.01, but not between stepping directions, p = 0.62–0.72, for both reaction time and stepping velocity. Reaction times were different among the three groups, p < 0.01, with the young adults having the fastest times and the older adult fallers having the slowest times. Lateral stepping velocity was faster among the young adults than for the two older groups, p < 0.01, but did not differ between the older adults, p = 0.29. It is concluded that the Quick Step is a simple and reliable tool for determining reaction time and lateral stepping velocity, and that this test can be used to detect a significant difference in reaction time between older adult fallers and nonfallers.

When vehicle-specific stiffness coefficients cannot be acquired, stiffness coefficient values that are representative of the desired vehicle type, class, wheelbase or weight are routinely used for accident reconstructions. Since the original compilation of representative vehicle stiffness data almost 20 years ago, changes in crash testing standards and other safety and technological improvements in vehicular design have affected vehicle stiffness. While generic frontal stiffness data have been recently updated to reflect these vehicular changes, rear and side stiffness data have not. Structural, geometric and inertial data for over 300 passenger cars and light trucks were collected. Among the vehicles targeted were the top-selling cars, SUVs, vans and pickups for model years 1990 to 2012. Results indicated that all vehicle types demonstrated increases in mean stiffness over the time period considered. SUVs were, on average, the stiffest vehicle type in the front, rear and side. There was a correlation between vehicle wheelbase and stiffness, with longer vehicles having greater stiffness than shorter vehicles. Vehicle class also affected stiffness. In the front and rear, mid-size passenger cars had the greatest mean “A” and “B” stiffness coefficients of all passenger cars. By contrast to the front and rear, mean side stiffness of all passenger cars classes was similar. In conclusion, the updated structural stiffness and geometric data presented here for the front, rear and side, provide an accurate representation of today's market for use in crash reconstructions.

The recovery of young adults from trips of increasing severity was studied. Our null hypothesis was that lower extremity strength, and reaction time, step time, step distance and step velocity measured in a volitional stepping task would not explain a significant portion of the variance in the magnitude of the threshold trip duration for which recovery is no longer possible. Ten males and 11 females (average age 26.8 and 28.4 years old, respectively) were subjected to trips of increasing duration until recovery was no longer possible with a single step. The average threshold trip duration for which subjects were no longer able to recover with a single step was 681±169 m s. The threshold trip duration significantly increased as lower extremity strength increased and volitional reaction time decreased (multiple stepwise linear regression: R2=0.52, p=0.001). The other volitional step parameters and the subject characteristics were not significantly associated with the magnitude of the threshold trip duration. These results suggest that some trip-related falls may be due to slower reaction times and/or reduced lower extremity strengths.

Since falling to the side and impacting on or near the hip increase hip fracture risk, we examined the fall direction and pelvis impact location resulting from four disturbances (faint, slip, step down, trip) at three gait speeds (fast, normal, slow) in 14 young adults instructed not to attempt recovery. We hypothesized that certain disturbances such as faints and slips and slow walking speed were more likely to result in an impact on the hip. For each trial, the fall direction, impact location and pelvis impact velocity were measured. The results showed that both disturbance type and gait speed significantly affected fall direction and impact location (analysis of covariance with repeated measures, p< or =0.0001) with a significant interaction (p

Pathologic fractures from a reduction in bone mass and strength are a debilitating complication affecting the quality of life of individuals with metastatic lesions. There are a number of existing animal models for studying the effects of bone metastases experimentally, but these models are unsuitable for measuring structural changes in metastatic bone. Our goal was to present an in vivo model for directly investigating the densitometric and structural consequences of tumor-induced osteolysis in long bones. One femur from female Sprague Dawley rats was implanted with Walker Carcinosarcoma 256 malignant breast cancer cells or with a Sham implant. After 28 days, the animals were killed, and both femora of each animal evaluated using histomorphometry, densitometry, and mechanical testing. Compared to Sham-operated controls, we found an 11% decrease in bone mineral content, a 9% decrease in bone mineral density using dual energy X-ray absorptiometry, and a 16% decrease in bone density using peripheral quantitative computed tomography in the group with tumor cell implants. In addition, failure torque was decreased by 35% compared to the contralateral controls and by 41% compared to the Sham-operated controls. Torsional stiffness in the tumor cell-implanted femora was decreased by 35% compared to contralateral controls and by 39% compared to Sham-operated controls. Bone density was only weakly to moderately associated with bone strength in our model. By creating reproducible localized tumor-induced osteolytic lesions in a long bone, this model provides the most direct evaluation of the structural consequences of bone metastases. In the future, this model may provide a method for determining the effects of new therapeutic approaches on the preservation of bone mass and bone strength in the presence of metastatic bone disease.

Given that 90% of hip fractures result from a fall, individuals who fall frequently are more likely to be at greater risk for fracture than one-time fallers. Our aim was to determine whether performance variables associated with injurious falls could be used to distinguish frequent fallers from both one-time fallers and nonfallers.A total of 157 men and women (77.4-5.4 years) were recruited and categorized into one of the following three groups based on falls status over the previous 12 months: nonfallers (n = 48), one-time fallers (n = 56), and frequent fallers (more than one fall) (n = 53). All subjects were evaluated on functional mobility and lower extremity strength and power.Using multivariate analysis of covariance with height as a covariate, nonfallers were significantly faster than both one-time and frequent fallers during the Get Up and Go (a test involving lower extremity strength and power, and mobility) and faster than one-time fallers on the Tandem Gait (p.01). There were no significant differences between groups for other mobility variables or for laboratory measures of strength and power. Because one-time and frequent fallers were similar on all measures. they were grouped as "fallers" in discriminant analysis. The Get Up and Go discriminated between the fallers and nonfallers with a final Wilks's Lambda of .900 (p.001) and correctly classified 72.4% of fallers and nonfallers before crossvalidation and 71.2% of the cases after validation.Given that the Get Up and Go discriminates between fallers and nonfallers and is associated with lower extremity strength and power, fall prevention strategies should focus on improving both functional mobility and lower extremity strength and power.

Objective: Pathological processes in bone can lead to fatal health consequences. Therefore, it is important to study factors that possibly influence the activity of bone cells. The mast cell is a normal component of bone, storing and producing many potent bioactive substances. One of the most important factors to influence mast cell number, function, and phenotype is the c-kit ligand. A defect of the c-kit receptor leads to mast cell deficiency. In literature oversight, evidence for the importance of mast cells in skeletal homeostasis is compiled. Methods: To investigate the influence of c-kit receptor deficiency on bone mass, geometry, microstructure, and strength, 30 femora of profoundly c-kit receptor deficient mouse mutants and 30 control group animals aged 8--20 weeks were phenotypically characterized using peripheral quantitative computed tomography (pQCT), micro-computed tomography (\muCT) and 3-point-bending. Results: The femora of the c-kit receptor and therefore mast cell deficient animals were significantly altered in bone mass and geometry but not in bone density and microstructure. The mutants had a lighter femur with a thinner shape. The lower load bearing capacity of the femora of mast cell deficient mouse mutants is more likely explained by the smaller amount of bone material than due to a change in intrinsic material properties. Technical considerations: With the little dimensions of mouse bones, it is of prime importance to have precise methods to phenotypically characterize the bone. The pQCT allows the separate assessment and analysis of trabecular and cortical bone density, as well as a statement about bone geometry. Beyond it, the \muCT-technique delivers a 3-D analysis of bone microstructure, which so far was only achieved with 2-D histomorphometry. \muCT is an efficient alternative to destructive histological preparations allowing further biomechanical testing of the same specimens to also deliver measures for bone strength.

Background: Our hypothesis that multiple, equally tensioned strands of hamstring graft used for reconstruction of the anterior cruciate ligament are stronger and stiffer than ten-millimeter patellar ligament grafts was tested biomechanically with use of tendons from cadavera. Methods: In the first part of the study, we measured the strength and stiffness of one, two, and four-strand hamstring grafts, from fresh-frozen cadaveric knees, that had been tensioned equally when clamped. In the second part of the study, we compared four-strand grafts to which tension had been applied by hand and then clamped with similar grafts to which tension had been applied with weights and then clamped. The grafts for the two experiments were obtained from thirty-four paired and ten unpaired knees. We also studied the effects of cooling on the biomechanical properties of grafts by comparing patellar ligament grafts tested at 13 degrees Celsius with those tested at room temperature. Results: Two equally tensioned gracilis strands had 185 percent of the strength and 210 percent of the stiffness (1550 ± 428 newtons and 336 ± 141 newtons per millimeter, respectively) of one gracilis strand (837 ± 138 newtons and 160 ± 44 newtons per millimeter, respectively). Two equally tensioned semitendinosus strands had 220 percent of the strength and 220 percent of the stiffness (2330 ± 452 newtons and 469 ± 185 newtons per millimeter, respectively) of one semitendinosus strand (1060 ± 227 newtons and 213 ± 44 newtons per millimeter, respectively). Four combined strands (two gracilis strands and two semitendinosus strands) that were equally tensioned with weights and clamped had the additive tensile properties of the individual strands. With the numbers available, four combined strands that were manually tensioned and clamped were not found to be significantly stronger or stiffer than two semitendinosus strands that were equally tensioned with weights (p > 0.07). Conclusions: Four combined strands that were equally tensioned with weights and clamped were stronger and stiffer than all ten-millimeter patellar ligament grafts that have been described in previous reports. All strands of a hamstring graft must be equally tensioned for the composite to have its optimum biomechanical properties. Clinical Relevance: Because of the well recognized donor-site morbidity associated with the use of patellar ligament grafts for reconstruction of the anterior cruciate ligament, multiple-strand hamstring-tendon grafts have become an increasingly popular choice. Our data demonstrate that equally tensioned four-strand hamstring-tendon grafts have initial tensile properties that are higher than those reported for ten-millimeter patellar-ligament grafts; thus, from a biomechanical point of view, they seem to be a reasonable alternative.

Many bones within the axial and appendicular skeleton are subjected to repetitive, cyclic loading during the course of ordinary daily activities. If this repetitive loading is of sufficient magnitude or duration, fatigue failure of the bone tissue may result. In clinical orthopedics, trabecular fatigue fractures are observed as compressive stress fractures in the proximal femur, vertebrae, calcaneus and tibia, and are often preceded by buckling and bending of microstructural elements. However, the relative importance of bone density and architecture in the etiology of these fractures is poorly understood. The aim of the study was to investigate failure mechanisms of 3D trabecular bone using micro-computed tomography (\muCT). Because of its nondestructive nature, \muCT represents an ideal approach for performing not only static measurements of bone architecture but also dynamic measurements of failure initiation and propagation as well as damage accumulation. For the purpose of the study, a novel micro-compression device was devised to measure loaded trabecular bone specimens directly in a micro-tomographic system. The measurement window in the device was made of a radiolucent, highly stiff plastic to enable X-rays to penetrate the material. The micro-compressor has an outer diameter of 19 mm and a total length of 65 mm. The internal load chamber fits wet or dry bone specimens with maximal diameters of 9 mm and maximal lengths of 22 mm. For the actual measurement, first, the unloaded bone is measured in the \muCT. Second, a load-displacement curve is recorded where the load is measured with an integrated mini-button load cell and the displacement is computed directly from the \muCT scout-view. For each load case, a 3D snap-shot of the structure under load is taken providing 34 \mum nominal resolution. Initial measurements included specimens from bovine tibiae and whale spine to investigate the influence of the structure type on the failure mechanism. In a rod-like type of architecture as seen in the whale spine, structural failure was described by an initial buckling and bending of structural elements followed by a collapse of the overloaded trabeculae. In the more plate-like bovine tibial architecture, buckling and bending could not be observed. Failure rather seemed to occur instantaneously. In conclusion, micro-compression in combination with 3D \muCT allows visualization of failure initiation and propagation and monitoring of damage accumulation in a nondestructive way. We expect these findings to improve our understanding of the relative importance of density, architecture and load in the etiology of spontaneous fractures of the hip and the spine. Eventually, this improved understanding may lead to more successful approaches to the prevention of age-related fractures.

Bone marrow contains a population of rare progenitor cells capable of differentiating into bone, cartilage, tendon, and other connective tissues. These cells, referred to as mesenchymal stem cells, can be purified and culture-expanded from animals and humans and have been shown to regenerate functional tissue when delivered to the site of musculoskeletal defects in experimental animals. To test the ability of purified human mesenchymal stem cells to heal a clinically significant bone defect, mesenchymal stem cells isolated from normal human bone marrow were culture-expanded, loaded onto a ceramic carrier, and implanted into critical-sized segmental defects in the femurs of adult athymic rats. For comparison, cell-free ceramics were implanted in the contralateral limb. The animals were euthanized at 4, 8, or 12 weeks, and healing bone defects were compared by high-resolution radiography, immunohistochemistry, quantitative histomorphometry, and biomechanical testing. In mesenchymal stem cell-loaded samples, radiographic and histologic evidence of new bone was apparent by 8 weeks and histomorphometry demonstrated increasing bone formation through 12 weeks. Biomechanical evaluation confirmed that femurs implanted with mesenchymal stem cell-loaded ceramics were significantly stronger than those that received cell-free ceramics. These studies demonstrate that human mesenchymal stem cells can regenerate bone in a clinically significant osseous defect and may therefore provide an alternative to autogenous bone grafts.

We measured the step response of a surrogate human pelvis/impact pendulum system at force levels between 50 and 350 N. We then fit measured response curves with four different single-degree-of-freedom models, each possessing a single mass, and supports of the following types: standard linear solid, Voigt, Maxwell, and spring. We then compared model predictions of impact force during high-energy collisions (pendulum impact velocity ranging from 1.16 to 2.58 m/s) to force traces from actual impacts to the surrogate pelvis. We found that measured peak impact forces, which ranged from 1700 to 5600 N, were best predicted by the mass-spring, Maxwell, and standard linear solid models, each of which had average errors less than 3 percent. Reduced accuracy was observed for the commonly used Voigt model, which exhibited an average error of 10 percent. Considering that the surrogate pelvis system used in this study exhibited nonlinear stiffness and damping similar to that observed in simulated fall impact experiments with human volunteers, our results suggest that these simple models allow impact forces in potentially traumatic falls to be predicted to within reasonable accuracy from the measured response of the body in safe, simulated collisions.

Background. Resistance-training intervention studies have demonstrated meaningful health benefits in older adults; however, most have used exercises performed at specific intensities on expensive equipment, which limit their widespread applicability. We tested whether two self-paced, less expensive exercise protocols could be effective and safe for modifying neuromotor performance and functional capacity in community-dwelling adults 65-95 years of age. Methods. One hundred and thirty-one subjects were randomized to a novel resistance training, walking, or control group. Subjects determined their level of resistance or walking intensity (self-paced) on a session-by-session basis. Muscle strength, balance, reaction time, stair climbing speed, and a timed pen pickup task were measured before and after the intervention period. Exercisers met three times per week for 10 months. Results. Significant improvements in tandem stance and single-legged stance with eyes open times and stair climbing speed were seen in both exercise groups. In addition, resistance trainers improved their muscle strength and ability to pick up an object from the floor and reduced the number of missteps taken during tandem walking, and walkers reduced tandem walking time. Controls showed no significant improvement in any variable. Conclusions. The two self-paced exercise protocols were effective at improving neuromotor performance and functional capacity in the study sample and show promise as a safe, effective, cost-efficient, acceptable exercise model for primary and secondary prevention in the general population of community-dwelling older adults.

The aim of this study was to determine the efficacy and safety of 16 weeks of self-paced resistance training or walking protocols on neuromotor and functional parameters in active, community-dwelling older adults. Twenty-two sequentially recruited older adults were randomly assigned to one of two exercise groups: self-paced resistance training and self-paced walking. Static and dynamic balance, upper and lower extremity reaction times, muscle strength, and stairclimbing speed were measured before and immediately after 16 weeks of exercise. Preliminary data showed that 16 weeks of self-paced. progressive, lower body resistance training improved balance (one-legged stance with eyes open, 68%). reaction time (10%), muscle strength (160%), and stair climbing speed (28%), while a self-paced walking program improved balance (one-legged stance with eyes open, 51%), stair climbing speed (16%), and in certain circumstances muscle strength (25%), in active, community-dwelling older adults.

The purpose of this study was to evaluate the effects that operative mobilization and transposition of the ulnar nerve have on both neural excursion and mechanical properties. Twelve dogs underwent ulnar nerve transposition and postoperative casting. Four animals were killed at 3 weeks and four animals were killed at 6 weeks. Four animals had their casts removed at 3 weeks, were allowed to ambulate, and were killed at 6 weeks. Operated and contralateral control nerves were compared. Neural excursion was measured near the elbow and 12 cm proximally. The nerves were harvested and their mechanical properties determined. Repeated measures analysis of variance revealed significant differences in longitudinal excursion between control and experimental groups at both sites. Ultimate strain, ultimate strength, and modulus were significantly reduced in the experimental groups. No differences were seen in cross-sectional area or stiffness between control and experimental groups. Analysis revealed no independent effect of the rehabilitation method. Results of this study indicate that significant changes in neural excursion, ultimate strain, ultimate strength, and modulus occur following ulnar nerve mobilization and transposition and that these changes persist throughout the early postoperative period.

A partially saturated linear polyester based on poly(propylene fumarate) (PPF) was synthesized for potential application in filling skeletal defects. The synthesis was carried out according to a two-step reaction scheme. Propylene glycol and fumaryl chloride were first combined to form an intermediate fumaric diester. The intermediate was then subjected to a transesterification to form the PPF-based polymer. This method allowed for production of a polymer with a number average molecular weight up to 1500 and a polydispersity index of 2.8 and below. The polymeric backbone structure was investigated through the use of FTIR and NMR. Kinetic studies of the transesterification allowed mapping of the molecular weight increase with reaction time. The final product was also characterized by thermal and solubility analysis.

STUDY DESIGN A finite element parametric analysis to investigate the relative load carrying roles of the shell and centrum in the lumbar vertebral body. OBJECTIVE To address the issue of the structural role of the vertebral shell and clarify some of the contradictions raised by previous studies. SUMMARY OF BACKGROUND DATA A number of experimental and finite element studies have attempted to quantify the relative structural roles of the shell and centrum, but these studies support no consensus on the relative contribution of the shell to vertebral body strength. METHODS The authors developed finite element models to predict the fraction of the total compressive force acting on the lumbar vertebral body that is carried by the shell. Parametric variations were investigated to determine how the fraction of shell force was affected by changes in shell thickness, shell and centrum modulus, centrum anistropy, and loading conditions. RESULTS The fraction of compressive force carried by the shell increased from approximately 0 at the endplate to approximately 0.2 at the mid-transverse plane for a typical case. The shell force was highly sensitive to the degree of anisotropy of the trabecular centrum but was relatively insensitive to changes in shell thickness and the ratio of shell-to-centrum elastic modulus. CONCLUSIONS The conflicting conclusions of previous studies about the structural roles of the vertebral shell and centrum can be explained by differences in their methods. Our findings support the claims that the shell accounts for only approximately 10% of vertebral strength in vivo and that the trabecular centrum is the dominant structural component of the vertebral body.

An experiment was conducted to examine the validity of freely available photo-based 3D scanning software for generating accurate 3D geometry of a vehicle. Currently, 3D vehicle geometry is routinely captured using total station survey equipment, coordinate measuring machines (CMM), laser scanning, or traditional point-based photogrammetry. While these methods produce sufficiently accurate results for collision reconstruction, they have limitations that can affect practical implementation. For example, manual surveying with a total station, CMM or traditional photogrammetry are all limited to the production of coordinate data at discrete, predefined points. In addition, total stations, CMMs and laser scanning devices are also expensive and require a significant amount of time in the field with the vehicle. In contrast, photo-based 3D scanning allows a 3D mesh to be created of a vehicle simply from a series of photographs using a consumergrade digital camera.

Despite the evidence suggesting that between 8% and 55% of manual labourers experience thoracic pain, research on spinal loading during occupational tasks has been almost invariably limited to the lumbar spine. In this study, we determined the ratio of thoracic to lumbar compression force and the relative risk of injury to each region in various postures. Compressive forces on the spine were calculated based on previously reported thoracic and lumbar intradiscal pressures and disc cross-sectional areas. Flexion postures were associated with an approximate doubling in lumbar compression force but only small increases (or even decreases) in thoracic compression. The ratio of thoracic to lumbar compression was above the tolerance ratio (i.e. the ratio of thoracic to lumbar compressive strength) during upright postures and below the tolerance ratio during flexion postures, indicating that upright postures may pose a greater relative risk of injury to the thoracic spine than to the lumbar spine. Practitioner summary: Previously reported thoracic and lumbar in vivo disc pressures during various postures were compared. The ratio of thoracic and lumbar compression increased during upright postures and decreased in flexed postures, indicating that upright postures may pose a greater risk of injury to the thoracic spine than to the lumbar spine.

Fall dynamics have largely been ignored in the study of hip fracture etiology and in the development of hip fracture prevention strategies. In this study, we asked the following questions: (1) What are the ranges of hip impact velocities associated with a sideways fall from standing height? (2) What are the ranges of body configurations at impact? and (3) How do protective reflexes such as muscle activation or using an outstretched hand influence fall kinematics? To answer these questions, we recruited six young healthy athletes who performed voluntary sideways falls on a thick foam mattress. Several categories of falls were investigated: (a) muscle-active vs muscle-relaxed falls; (b) falls from a standing position or from walking; and (c) falls in which an outstretched arm was used to break the fall. Each fall was videotaped at 60 frames s(-1). Fall kinematics parameters were obtained by digitizing markers placed on anatomical points of interest. The mean value for vertical hip impact velocity was 2.75 ms(-1) (+ or - 0.42 ms(-1) [S.D.]). The mean value for trunk angle (the angle between the trunk and the vertical) was 17.3 degrees (+ or - 11.5 degrees [S.D.]). We found a 38 percent reduction in the trunk angle at impact, and a 7 percent reduction in hip impact velocity for relaxed vs muscle-active falls. Finally, regarding the. falls in which an outstretched arm was used, only two out of the six subjects were able to break the fall with their arm or hand. For the remaining subjects hip impact occurred first, followed by contact of the arm or hand.

Recent studies have shown that factors related to fall biomechanics may play as important a role in the etiology of hip fracture as age-related bone loss. Motivated by finite element analyses that showed failure of the proximal femur to be sensitive to loading direction, our objective with the current investigation was to determine experimentally if changes in impact direction affect the failure load of the elderly proximal femur. Thirty-three cadaveric femurs were assigned randomly to three groups of 11 and tested at one of three loading angles, 0 degree, 15 degrees, or 30 degrees, representing a fall on the hip rolled slightly forward, to the side, or rolled slightly backwards, respectively. Femurs were scanned using dual-energy X-ray absorptiometry (DXA) to assess bone mineral density (BMD) and tested to failure in a fall loading configuration at a displacement rate of 100 mm/second. Using an analysis of covariance to adjust for total hip BMD, we found that failure load decreased by 24% as the loading angle changed from 0 degree to 30 degrees. This reduction in failure load is comparable to that associated with about 25 years of age-related bone loss after the age of 65. Therefore, the impact direction associated primarily with a fall is a critical determinant of hip fracture risk that is both independent of bone density and associated with fall biomechanics.

Models of two-dimensional cellular solids are based on idealized unit cells intended to represent the microstructural features of an average cell from a real material. A significant limitation of the unit cell modelling approach is that it does not account for the natural variations in microstructure which are typical of most cellular materials. Our objective was to model one type of microstructural variability—the non-periodic arrangement of cell walls—and to investigate how this variability affected the relations between microstructure and elastic properties in two-dimensional cellular materials (honeycombs). Specifically, we asked: (1) How much variance in the elastic properties does variability in the arrangement of cells walls introduce? (2) Are the relations between microstructural and elastic properties for non-periodic honeycombs the same, on average, as those for periodic honeycombs? (3) Can anisotropy of elastic properties for non-periodic honeycombs be directly related to microstructural anisotropy, as characterized by stereological parameters? We generated models of non-periodic arrays of Voronoi cells with uniform cell wall thickness, and performed finite element analysis (FEA) to determine effective elastic moduli for low density honeycombs (relative densities

Despite our growing understanding of the importance of fall mechanics in the etiology of hip fracture, previous studies have largely ignored the kinematics and dynamics of falls from standing height. Beginning from basic principles, we estimated peak impact force on the greater trochanter in a sideways fall from standing height. Using a one degree-of-freedom impact model, this force is determined by the impact velocity of the hip, the effective mass of that part of the body that is moving prior to impact, and the overall stiffness of the soft tissue overlying the hip. To determine impact velocity and effective mass, three different paradigms of increasing complexity were used: 1) a falling point mass or a rigid bar pivoting at its base; 2) two-link models consisting of a leg segment and a torso; and 3) three-link models including a knee. The total mechanical energy of each model before falling was equated to the total mechanical energy just prior to impact in order to estimate the hip impact velocity. In addition, the configuration of the model just before impact was used to estimate the effective mass. Our model predictions were compared with the results of an earlier experimental study with young subjects falling on a 10-inch thick mattress. Values from literature were used to estimate the soft tissue stiffness. For the models, predicted values for hip impact velocity and effective mass ranged from 2.47 to 4.34 m/s and from 15.9 to 70.0 kg, respectively. Predicted values for the peak force applied to the greater trochanter ranged from 2.90k to 9.99k N. Based on comparisons to the experimental falls, impact velocity and impact force were best predicted by a simple two-link model with the trunk at 45 degrees to the vertical at impact. A three-link model with a quadratic spring incorporated in the knee of the model was the best predictor of effective mass. Using our most accurate model, the peak impact force was 2.90k N for a 5th percentile female and 4.26k N for a 95th percentile female, thereby confirming the widely held perception that “the bigger they are, the harder they fall.”

To address mechanisms involved in wear and permanent deformation of patellofemoral components in total knee arthroplasties, a previously reported knee joint loading model and pressure-sensitive film were used to measure patellofemoral contact areas and pressures in human cadaver knee joints after implantation with six different total knee joint designs. The joints were tested at three different Q angles (physiologic, -10 degrees, and +10 degrees) and four different flexion angles (30 degrees, 60 degrees, 90 degrees, and 120 degrees). Patellofemoral contact areas at normal Q angles ranged from 0.13 to 0.68 cm2 and increased with flexion angle up to 90 degrees. These contact areas differed significantly with flexion angle but not with Q angle. Variations in contact area with type of knee system were only marginally significant (P.04), and post hoc tests showed no significant differences between individual knee designs. Contact pressures at normal Q angle also increased with flexion angle and ranged from 10 MPa to more than 49 MPa. Contact pressures at flexion angles greater than 60 degrees were, for all systems, well in excess of the compressive yield strength of ultrahigh-molecular-weight polyethylene and at least three to four times greater than the recommended maximum compressive stress level of 10 MPa.

Osteoarticular allografts are commonly used in the treatment of segmental bone loss due to a wide resection of tumour. While the use of such grafts has met with considerable clinical success, fractures are a recognized complication of allograft use. Although trabecular bone can play an important structural role in the function of segmental allografts, few data exist on the effects of common storage and sterilization procedures on the mechanical properties of trabecular bone. To this end, we investigated with these experiments the effects of freezing at -20 degrees C, freezing at -70 degrees C, eight freeze-thaw cycles at -20 degrees C, freeze-drying, boiling and autoclaving on the compressive modulus and strength of bovine trabecular bone. Of these treatments, boiling and autoclaving were the only treatments to alter the properties of bovine trabecular bone, resulting in 26 and 58% reductions in strength, respectively. Autoclaving also significantly reduced the compressive modulus by 59%. From these data, freezing at temperatures between -20 and -70 degrees C does not appear to compromise the structural integrity of trabecular bone.

To assess age-related changes in femoral strength as a component in the risk of fracture of the hip, we compared the loads at fracture of the proximal aspects of femora from the cadavera of older and younger individuals, as tested in a fall-loading configuration. To provide a basis for non-invasive in vivo estimates of femoral strength, we also determined the correlations between variables measured with dual-energy x-ray absorptiometry and these loads. Femora from the cadavera of eight older individuals (mean age, seventy-four years) and nine younger individuals (mean age, thirty-three years) were scanned with a Hologic QDR-2000 densitometer to obtain densitometric and geometric information. The femora were then tested mechanically in a loading configuration that simulated a fall on the greater trochanter. The femora from the older group were half as strong as those from the younger group (p < 0.001), and they absorbed one-third as much energy (p < 0.001). The area bone-mineral density of the femoral neck correlated strongly with the load at fracture (r2 = 0.92). CLINICAL RELEVANCE: The prevalence of fracture of the hip increases exponentially with age. 90 percent of those fractures are the result of a simple fall from a standing height.(ABSTRACT TRUNCATED AT 250 WORDS)

We assessed the bone mineral density (BMD) of 16 matched sets of cadaveric proximal femurs and feet using dual-energy x-ray absorptiometry (DXA). We also estimated the femoral neck length from the DXA scans. Quantitative ultrasound densitometry was used to measure the velocity of sound and broadband ultrasound attenuation (BUA) in the calcaneus of each foot. The proximal femurs were then tested to failure in a loading configuration designed to simulate a fall with impact to the greater trochanter. Femoral neck BMD and trochanteric BMD were strongly associated with the femoral failure load (r2 = 0.79 and 0.81, respectively; P0.001), whereas femoral neck length was modestly correlated with femoral failure load (r2 = 0.27, P = 0.04). Calcaneal BMD (r2 = 0.63, P0.001) and BUA (r2 = 0.51, P = 0.002) were also significantly associated with femoral failure load. Given the small sample size, we were unable to detect differences in the strength of the correlations between the independent parameters and femoral failure load. Using linear multiple regression analyses, the strongest predictor of femoral failure load was a combination of femoral neck BMD and femoral neck length (R2 = 0.85, P0.001). Thus, it appears that both femoral and calcaneal bone mineral properties may be useful for identifying those persons at greatest risk for hip fracture.

The rat mandible has been used to study fractures, ridge augmentation, bone defects, and the effects of cryosurgery, all of which affect the structural properties of the mandible. The only previously described mechanical test of the rat mandible used three-point bending with a piece of foam placed between the lingual surface of the mandible and the support. The accuracy and reproducibility of this test were not determined. Two different, three-point bending tests for characterizing the structural properties of the rat mandible have now been evaluated. Acrylic specimens representing three different-sized mandibles were tested in three-point bending with either potted ends or foam interposed between the specimen and the outer load points. Flexural stiffness and failure load were compared against data from a conventional three-point bending test. In addition, five paired halves of 90- and 210-day-old rat mandibles were tested either with interposed foam or with potted ends and the mean coefficients of variation of the stiffness and failure loads were determined. Failure loads of the acrylic specimens with the potted and foam methods were significantly higher (50 and 10%, respectively; p < 0.05) than with the conventional method. The stiffness of the acrylic specimens with the potted and foam methods was significantly different (75% higher and 21% lower, respectively; p < 0.05) than with the conventional test. In addition, there was no difference between the coefficient of variation in stiffness (26 + 11%) or failure load (10 + 4%) measured with the foam test and the comparable values (16 + 11%) and (18 + 13%) in the potted test. Thus it appears that the foam test more accurately reflects the stiffness and failure load of the acrylic specimens than the potted test. Although there was no difference in the coefficients of variation in the potted and foam tests, the technical difficulties associated with potting indicate that the foam test is a better method for testing the structural properties of the rat mandible. The objective of this experiment was to establish a method for accurately measuring the structural properties of the rat mandible and for detecting differences in structural properties between different treatment groups.

The mechanical behavior of damaged trabecular bone may play a role in the etiology of age-related spine fractures since damaged bone exists in and may weaken the elderly vertebral body. To describe some characteristics of damaged trabecular bone, we measured the changes in modulus and strength that occur when bovine trabecular bone is loaded in compression to various strains beyond its elastic range. Twenty-three reduced-section specimens, taken from 17 different bones, were loaded from 0-X-0-9% strain, where X was one of four strains: 1.0% (n = 7), 2.5% (n = 6), 4.0% (n = 5), or 5.5% (n = 5). We found that modulus was reduced for all applied strains, whereas strength was reduced only for strain levels > or = 2.5%; the percentage changes in modulus and strength were independent of Young's modulus but were highly dependent on the magnitude of the applied strains; modulus was always reduced more than strength; and simple statistical models, using knowledge of only the applied strains, predicted well the percentage reductions in modulus (r2 = 0.97) and strength (r2 = 0.74). The modulus reductions reported here are in qualitative agreement with those for cortical bone in tensile loading, supporting the concept that the damage behaviors of cortical and trabecular bone are similar for low strains (< or = 4.0%). In addition, because modulus was always reduced more than strength, damaged trabecular bone may be stress protected in vivo by redistribution of stresses to undamaged bone.

Recombinant human bone morphogenetic protein- (rhBMP-) can be used to enhance the repair of congenital or acquired bone pathologies when formulated in the appropriate carrier. Poly [D, L-(lactide-co-glycolide)] (PLGA) has been shown to be an effective carrier of rhBMP-. We investigated several particle sizes PLGA and several doses of rhBMP- in a rat orthotopic model. We also investigated the effects of a fibrinolytic inhibitory agent, epsilon aminocaproic acid (EACA), on the healing response. Our data indicate that higher doses of rhBMP- resulted in increased failure torque (408 ± 70 N-mm or 60% of the intact value) and higher incidence of union (100%). The induced bone in femurs treated with the smaller particle size PLGA achieved the greatest torsional stiffness and strength. The presence of rhBMP- was necessary for new bone to form, but the presence of EACA did not change these results; the use of the PLGA carrier appeared to increase bone strength and stiffness. In fact, with higher doses of rhBMP- in PLGA, the stiffness of the new bone was equal to that of intact controls (64 ± 20 N-mm/deg [intact femurs] versus 45 ± 10 N-mm/degree [medium dose in small PLGA], 61 ± 17 N-mm/degree [high dose in small PLGA], and 36 ± 11 N-mm/degree [medium dose in large PLGA]; P > .05). In conclusion, PLGA implanted with rhBMP- effectively aided in healing large segmental defects in rat femurs. © 1994 John Wiley & Sons, Inc.

The conflicting conclusions regarding the relationship between the tensile and compressive strengths of trabecular bone remain unexplained. To help resolve this issue, we compared measurements of the tensile (n = 22) and compressive (n = 22) yield strengths, and yield strains, of trabecular bone specimens taken from 38 bovine proximal tibiae. We also studied how these failure properties depended on modulus and apparent density. To enhance accuracy, trabecular orientation was controlled, and each specimen had a reduced section where strains were measured with a miniature extensometer. We found that the mean yield strength was 30% lower for tensile loading. However, the difference between individual values of the tensile and compressive strengths increased linearly with increasing modulus and density, being negligible for low moduli, but substantial for high moduli. By contrast, both the tensile and compressive yield strains were independent of modulus and density, with the yield strain being 30% lower for tensile loading. Thus, the difference between the tensile and compressive strengths of bovine tibial trabecular bone depends on the modulus, but the difference between yield strains does not. This phenomenon may explain in part that conflicting conclusions reached previously on the tensile and compressive strengths of trabecular bone since the mean modulus has varied among different studies. Realizing that our data pertain only directly to bovine tibial trabecular bone for longitudinal loading, our results nevertheless suggest that failure parameters based on strains may provide more powerful and general comparisons of the failure properties for trabecular bone than measures based on stress.

Using a protocol designed to reduce experimental artifacts associated with the conventional compression test for trabecular bone, we performed in vitro mechanical testing on bovine tibial trabecular bone to obtain accurate descriptions of the elastic and yield behaviors. Reduced-section cylindrical specimens were preconditioned for eight tension-compression (+/- 0.5% strain) cycles and then loaded to failure either in tension (n = 15) or compression (n = 14). We found that the pre-yield behavior for every specimen was fully linear, indicating that the initial nonlinear 'toe' is an experimental artifact. Analysis of variance on the moduli indicated that there was no significant difference between the tensile and compressive moduli before preconditioning. However, preconditioning decreased the tensile and compressive moduli on average by 8.8% (p < 0.01) and 5.3% (p < 0.01), respectively, with the decrease in tensile modulus being larger (p < 0.01). These small but significant decreases in modulus suggest that initial yielding involves microstructural damage (as opposed to plastic slip) of individual trabeculae and also indicate that the tensile and/or the compressive yield strain of (bovine tibial) trabecular bone is less than 0.5%. The mean tensile strength was approximately 70% of the mean compressive strength, although this difference in strengths may have been affected by the preconditioning-induced damage. Taken together, these results suggest that there are more similarities between the elastic and yield behaviors of trabecular and cortical bone than had been assumed previously.

Over 50% of all knee injuries involve partial or com plete tear of the anterior cruciate ligament. Surgical reconstruction of this ligament using an isometrically placed bone-patellar tendon-bone autograft is the cur rent technique of choice; however, harvest of patellar tendon as a free graft can lead to increased morbidity. To address this issue, allogenic patellar tendon grafts have been introduced as alternatives to autogenic graft material. The purpose of this study was to exam ine effects of age and strain rate on tensile strength, modulus, and failure mode of bone-patellar tendon- bone allografts from a typical population of tissue do nors. Eighty-two, fresh-frozen, bone-patellar tendon- bone allografts were harvested from 25 different donors, aged 17 to 54. Paired grafts from individual patellar tendons were assigned randomly to tensile testing at either 10% or 100% elongation per second. Tensile strength, modulus, and failure mode were not significantly different for tests conducted at these 2 strain rates. Correlations between tensile strength and age were not significant for tests conducted at either strain rate. Specimens tested at a strain rate of 100% per second exhibited weak but significant negative correlation between modulus and age, with modulus decreasing 25% over the age range examined.

Alteration of morphologic and mechanical properties of trabecular bone in the osteoarthritic proximal tibia may be a contributing factor in tibial component loosening. To explore this issue, the authors performed tissue property measurements, morphologic analysis, and mechanical testing of subchondral, epiphyseal, and metaphyseal trabecular bone specimens retrieved from six human proximal tibias exhibiting a range of medial unicondylar osteoarthritic degeneration. Apparent density in the proximal tibia was altered according to varus misalignment and medial subluxation associated with medial osteoarthritis of the knee. In subchondral bone, a decrease in tissue mineralization contributed to a significant reduction in axial mechanical properties with degenerative disease (P < .0005). In epiphyseal and metaphyseal bone, trabecular thickness and the number of trabeculae increased linearly with volume fraction, providing a power law relationship between axial elastic modulus and apparent density (R2 = .84). Average elastic properties of the tibial epiphysis and metaphysis were not reduced by degenerative disease (P < .05). The results suggest that absolute minimization of tibial resection might not be an optimal strategy for tibial component fixation and that mechanical properties of the tibial resection surface are more homogeneous in planes parallel to the joint surface than in a plane normal to the longitudinal axis of the tibia.

This study assessed the tensile properties of hamstring and patellar tendon anterior cruciate ligament recon structions in older cadaveric knees (age range, 48 to 79 years). Mechanical testing to failure was conducted by translating the tibia anteriorly at 1 mm/sec with the knee in 20° of flexion. The strongest gracilis-semitendinosus graft fixation technique (103% of intact anterior cruciate ligament) had the tendons doubled and secured with soft tissue washers (P < 0.01 ). However, all reconstruc tions using gracilis-semitendinosus grafts were signifi cantly less stiff than the intact anterior cruciate ligament specimens regardless of fixation technique (P< 0.01 ). The highest strength patellar tendon graft fixation tech nique (84% of intact anterior cruciate ligament) was ob tained with a combination interference screw and suture technique. The difference in stiffness between a patellar tendon graft and an intact anterior cruciate ligament was not significant when interference screws were placed at both ends of the graft (P > 0.05). Both types of grafts failed most often on the tibial side. With appropriate fixa tion, both grafts approximated the intact anterior cruci ate ligament in strength, but only patellar tendon grafts secured with interference screws were comparable in stiffness.

A two-dimensional finite element model of an idealized trabecular bone specimen was developed to study trabecular bone damage accumulation during cyclic compressive loading. The specimen was modeled as a two-dimensional honeycomb-like structure made up of an array of hexagonal cells. Each trabecula was modeled as a linearly elastic beam element with the same material properties as cortical bone. Initial microcracks were assumed to exist within the oblique trabeculae and to grow according to the Paris law. Forces and moments were computed in each trabecula and the microcracks were allowed to propagate until fracture occurred. Between cycles, fractured trabeculae were removed from the finite element mesh, and force and moment distributions were calculated for the next cycle. This iterative process was continued until the simulated trabecular bone specimen showed a 10% reduction in modulus. Creep failure was also studied using a single cell analysis, in which a closed-form solution was obtained after prescribing the creep properties of the trabeculae. The results of the crack propagation analysis showed that fractures of only a small number of individual trabeculae can cause a substantial reduction in the modulus of the trabecular bone specimen model. Statistical tests were performed to compare the slopes and intercepts of the S-N curves of our model predictions to those of experimentally derived S-N curves for bovine trabecular bone. There was no significant difference (p0.2 for both slope and intercept) between our model predictions and the experimentally derived S-N curves for the low-stress, high-cycle range. For the high-stress, low-cycle range, the crack propagation model overestimated the fatigue life for a given stress level (for slope, p0.001), while the creep analysis agreed well with the experimental data (for slope, p0.2). These findings suggest that the primary failure mechanism for low-stress, high-cycle fatigue of trabecular bone is crack growth and propagation, while the primary failure mechanism for high-stress, low-cycle fatigue is creep deformation and fracture. Furthermore, our results suggest that the modulus of trabecular bone at the specimen level may be highly sensitive to fractures of individual trabeculae.

Twenty-seven paired human cadaveric knee specimens were used to determine the effect of surgical technique and various interference screw parameters on the pullout strength of patellar tendon femoral bone blocks. The study compared the fixation strength of endoscop ically inserted and conventional "rear-entry" screws of different diameters and lengths. In all tests the most frequent mode of failure was bone block pullout from the interference screw. There was no significant differ ence in fixation strength between 9-mm diameter screws inserted through a conventional rear-entry tech nique and 7-mm diameter screws inserted through an endoscopic technique. There was no significant effect of screw length on fixation strength. The pullout force for 20-mm long screws increased on average 120% when 7-mm diameter screws were compared with 5.5- mm diameter screws. There was no significant effect of increased screw core diameter on fixation strength. There was a weak positive correlation ( r2= 0.45) between screw insertion torque and pullout force. Our measured mean pullout force for the 7-mm endoscopi cally inserted screws of 362 ± 198 N represents 20.1 % of the failure load of the normal young adult anterior cruciate ligament. Our data indicate that properly in serted 7-mm diameter endoscopic interference screws can provide fixation strengths of patellar tendon ante rior cruciate ligament grafts equivalent to those of con ventional 9-mm diameter rear-entry, outside-in screws.

A finite-element model of an isolated elderly human L3 vertebral body was developed to study how material properties and loading conditions influence end-plate and cortical-shell displacements and stresses. The model consisted of an idealized geometric representation of an isolated vertebral body, with a 1-mm-thick end plate and cortical shell. For uniform compression, large tensile stresses occurred all around the cortical shell just below the end plate as a result of bending of the cortical shell as it supported the end plate. Large tensile bending stresses also developed in the inferior surface of the end plate. Equal reductions in both trabecular and cortical bone moduli increased displacements but did not affect peak stresses. A 50% reduction in trabecular bone modulus alone increased peak stresses in the end plate by 74%. Elimination of the cortical shell reduced peak stresses in the end plate by approximately 20%. For nonuniform, anteriorly eccentric compression, peak stresses everywhere changed by less than 11% but moved to the anterior aspect. When material properties were adjusted to represent osteoporosis with disproportionate reductions in trabecular (50% decrease) and cortical (25% decrease) bone moduli, anterior compression increased peak stresses by up to 250% compared to uniform compression. If fractures are initiated in regions of large tensile stresses, the results from this relatively simple model may explain how central end-plate and transverse fractures initiate from uniform compression of the end plate. Furthermore, for anterior compression, disproportionate modulus reductions in trabecular and cortical bone may substantially increase end plate and cortical shell stresses, suggesting a cause of age-related spine fractures.

Transcortical osseous lesions were simulated in thoracic vertebrae to investigate the effects on vertebral strength of defect location, pedicle disruption, and defect size. Alternate vertebrae from 15 thoracic spines were assigned to five defect groups: anterior, posterior, lateral, one pedicle, or both pedicles. The remaining vertebrae served as controls. All vertebrae were tested to failure in combined axial-flexion loading. The intact failure load for each vertebra with a defect was estimated based on the actual failure loads of the control vertebrae from the same spine. The failure loads for vertebrae with transcortical defects (anterior, posterior, lateral) were significantly lower (P = 0.0001) than estimated intact loads; this was not the case for vertebrae with single pedicle disruption (P = 0.90). Relative strengths (defined as actual failure load divided by predicted intact failure load) for the anterior (mean = 0.51), posterior (0.55) and lateral (0.58) defect groups were not significantly different from each other, but were different from the single pedicle defect group (1.09). Relative strength depended only weakly on defect size. Comparison of these results with those of a previous study of simulated defects in the vertebral centrum suggests that transcortical defects result in slightly greater reductions in vertebral strength than defects of comparable size involving only trabecular bone.

We performed a series of uniaxial compression tests on wet bovine trabecular bone to compare both modulus and strength when measured using 2:1 aspect ratio (10 mm long, 5 mm diameter) cylinders (n = 30) and 5 mm cubes (n = 29). We also compared the correlation coefficients in the resulting modulus-density and strength-density regressions and the standard errors of the estimate. When comparing the mean values of modulus and strength for each group, the confounding variations in apparent density were accounted for with an analysis of covariance. The Fisher's Z transformation was used to compare the correlation coefficients statistically. Results from the analysis of covariance indicated that the modulus and strength of the cubes were higher by 36% (p < 0.01) and 18% (p < 0.05), respectively, with respect to the 2:1 cylinder values. The correlation coefficients in the modulus-density and strength-density regressions were not sensitive to the regression model (linear versus power law). However, correlation coefficients for both modulus-density and strength-density regressions were higher (p < 0.05) for the 2:1 cylinders (r = 0.90, modulus; r = 0.94, strength) than for the cubes (r = 0.57, modulus; r = 0.82, strength). In addition, the standard errors of the estimate in both modulus and strength were substantially lower for the 2:1 cylinders. These data indicate that both modulus and strength can depend on the specimen geometry when using conventional compression testing techniques. We conclude, therefore, that inter-study comparisons of modulus and strength may be invalid if these confounding effects of different specimen geometries are not addressed. Our data also indicate that density can better explain the observed variance in modulus and strength when 2:1 cylinders are used as opposed to cubes. Using this phenomenon as a rationale for choosing a standard specimen gometry, we recommend that the 2:1 cylinder be used as a standard specimen in studies designed to determine the effects of various treatments on the uniaxial compressive modulus and strength of trabecular bone.

We studied the fatigue behavior of bovine trabecular bone specimens under stress control using a sinusoidal uniaxial compressive load profile with a frequency of 2 Hz. The stress range was determined from the corresponding initial global platen-to-platen strain range, where the maximum initial strain was between 0.8 and 2.1% and the minimum strain was 0.6%. The local strain distribution was measured on the same type of specimen by affixing glass spheres and photographing them in the unloaded and loaded positions using multiple exposures. The number of cycles to failure (defined as a 5% decrease in secant modulus) was strongly correlated with the initial global maximum strain (r2 = 0.78) and ranged from 20 cycles at 2.1% strain to 400,000 cycles at 0.8% strain. All of the fatigue specimens showed a region of transverse failure approximately 1 mm from the end of the specimen. Microscopic examination of the failure zones revealed two failure modes: a straight transverse brittle-like fracture through the trabeculae, most often found in trabeculae transverse to the loading direction, and buckling-like failure, common in oblique trabeculae, involving bending and splitting. The local strain increased towards the ends of the specimens to a value 2-4 times that in the middle. Modulus degradation with the number of cycles was distinctively different for high-cycle and low-cycle fatigue, suggesting the possibility that both creep and damage accumulation contribute to fatigue failure of trabecular bone.