The rates of fracture at sites with different relative amounts of cortical and trabecular bone (hip, spine, distal radius) have been used to make inferences about the pathomechanics of bone loss and the existence of type I and type II osteoporosis. However, fracture risk is directly related to the ratio of tissue stress to tissue strength, which in turn is dependent not only on tissue composition but also tissue geometry and the direction and magnitude of loading. These three elements determine how the load is distributed within the tissue. As a result, assumptions on the relative structural importance of cortical and trabecular bone, and how these tissues are affected by bone loss, can be inaccurate if based on regional tissue composition and bone density alone. To investigate the structural significance of cortical and trabecular bone in the proximal femur, and how it is affected by bone loss, we determined the stress distributions in a normal and osteoporotic femur resulting from loadings representing: (1) gait; and (2) a fall to the side with impact onto the greater trochanter. A three-dimensional finite element model was generated based on a representative femur selected from a large database of femoral geometries. Stresses were analyzed throughout the femoral neck and intertrochanteric regions. We found that the percentage of total load supported by cortical and trabecular bone was approximately constant for all load cases but differed depending on location. Cortical bone carried 30% of the load at the subcapital region, 50% at the mid-neck, 96% at the base of the neck and 80% at the intertrochanteric region. These values differ from the widely held assumption that cortical bone carries 75% of the load in the femoral neck and 50% of the load at the intertrochanteric region. During gait, the principal stresses were concentrated within the primary compressive system of trabeculae and in the cortical bone of the intertrochanteric region. In contrast, during a fall, the trabecular stresses were concentrated within the primary tensile system of trabeculae with a peak magnitude 4.3 times that present during gait. While the distribution of stress for the osteoporotic femur was similar to the normal, the magnitude of peak stress was increased by between 33% and 45%. These data call into question several assumptions which serve as the basis for theories on the pathomechanics of osteoporosis. In addition, we expect that the insight provided by this analysis will result in the improved development and interpretation of non-invasive techniques for the quantification of in vivo hip fracture risk.

Over 90 percent of the more than 250,000 hip fractures that occur annually in the United States are the result of falls from standing height. Despite this, the stresses associated with femoral fracture from a fall have not been investigated previously. Our objectives were to use three-dimensional finite element models of the proximal femur (with geometries and material properties based directly on quantitative computed tomography) to compare predicted stress distributions for one-legged stance and for a fall to the lateral greater trochanter. We also wished to test the correspondence between model predictions and in vitro strain gage data and failure loads for cadaveric femora subjected to these loading conditions. An additional goal was to use the model predictions to compare the sensitivity of several imaging sites in the proximal femur which are used for the in vivo prediction of hip fracture risk. In this first of two parts, linear finite element models of two unpaired human cadaveric femora were generated. In Part II, the models were extended to include nonlinear material properties for the cortical and trabecular bone. While there was poor correspondence between strain gage data and model predictions, there was excellent agreement between the in vitro failure data and the linear model, especially using a von Mises effective strain failure criterion. Both the onset of structural yielding (within 22 and 4 percent) and the load at fracture (within 8 and 5 percent) were predicted accurately for the two femora tested. For the simulation of one-legged stance, the peak stresses occurred in the primary compressive trabeculae of the subcapital region.(ABSTRACT TRUNCATED AT 250 WORDS)

The objective of this study was to establish the role of loads and prosthesis material properties on the mechanics of the proximal femur after total hip arthroplasty. We developed a three-dimensional finite element model of an intact human femur and the same femur with a conventional collared straight-stem femoral component. Using published data, we defined two sets of loading conditions: one that represented three phases of gait, and one that represented four different extreme loads. The four extreme loads were based on the peak joint contact forces that occur during stair ascent and isometric contraction of various muscle groups. The model was analyzed with three different material properties for the prosthesis, including cobalt-chromium alloy, titanium alloy, and a carbon fiber-reinforced polymer (CFRP) laminate. We assumed that the implant was stable, with rigid bonding, collar contact, and no cement. To address femoral component loosening, we examined the shear stresses at the implant-bone interface; to address adaptive bone remodeling, we examined the principal stresses in the supporting cortical bone relative to those in the intact femur. Our analyses of the various loading conditions demonstrated large out-of-plane bending movements and torsional moments, especially for the load representing stair ascent. Based on stepwise multiple regressions, the maximum shear stresses at the implant-bone interface in the distal region were dependent on the total applied axial force and torsion; the maximum shear stresses in the proximal region were dependent on the axial component of the joint contact force alone. Reduction in the prosthesis stiffness, by substitution of the CFRP material properties, resulted in lower interface shear stresses at the distal end of the stem and higher interface shear stresses at the more proximal sections, consistent with the findings of others. We fit equations, based on composite beam theory, to the maximum implant-bone interface shear stresses and the cortical bone principal stresses as a function of the axial modulus of the prosthesis. These equations can be used to estimate the maximum stresses at the interface and in the cortical bone that would be predicted by similar models, for the same prosthesis constructed of alternative materials, relative to the stresses in the intact femur. The nonlinear nature of these relationships was such that the cortical bone stresses changed more rapidly, as a function of the prosthesis modulus, for lower values of elastic modulus, especially in the more proximal sections.

It has been hypothesized that the histological pattern of fracture healing is controlled at least in part by the local mechanical strains in the interfragmentary region. To test this "interfragmentary strain hypothesis," we applied cyclic bending deformations to tibial osteotomies in 11 sheep. An instrumented flexible plate spanning a 1-mm osteotomy gap was deformed to create a gradient of tissue elongation from 10% under the plate to 100% at the opposite cortex. The cyclic deformations were applied three times per minute, 24 h per day, for 1-5 weeks. However, as a result of tissue differentiation, the bone-plate complex increased in stiffness with healing time, resulting in a marked reduction of the gap deformation at approximately 4 weeks. Fracture healing was evaluated using vascular injection of India ink and conventional histology. A nonlinear three-dimensional finite element model of the interfragmentary tissue at the initial stage of healing was used to predict the complex tissue strains. The ingrowth of vascularized soft tissue into the interfragmentary gap, as well as the subsequent differentiation of this tissue, occurred earlier and to a greater degree in regions of lower strain. In contrast, the proliferation of callus tissue was greatest at the periosteal and endosteal surfaces of the cortex opposite the plate. Direct comparison of the finite element predictions with the histology demonstrated that the spatial distribution of bone resorption at the fracture fragment ends directly corresponded to the locations of elevated tissue strain and stress. However, there was no consistent numerical relationship between the magnitude of these local peak strains and the corresponding volume of cortical bone resorption over the bone cross section.

We carried out weight-bearing video radiological studies on 40 patients with a total knee arthroplasty (TKA), to determine the presence and magnitude of femoral condylar lift-off. Half (20) had posterior-cruciate-retaining (PCR) and half (20) posterior-cruciate-substituting (PS) prostheses. The selected patients had successful arthroplasties with no pain or instability. Each carried out successive weight-bearing knee bends to maximum flexion, and the radiological video tapes were analysed using an interactive model-fitting technique. Femoral lift-off was seen at some increment of knee flexion in 75% of patients (PCR TKA 70%; PS TKA 80%). The mean values for lift-off were 1.2 mm with a PCR TKA and 1.4 mm with a PS TKA. Lift-off occurred mostly laterally with the PCR TKA, and both medially and laterally with the PS TKA. Separation between the femoral condyles and the articular surface of the tibia was recorded at 0°, 30°, 60° and 90° of flexion. Femoral condylar lift-off may contribute to eccentric polyethylene wear, particularly in designs of TKA which have flatter condyles. Coronal conformity is an important consideration in the design of a TKA.

In Part I we reported the results of linear finite element models of the proximal femur generated using geometric and constitutive data collected with quantitative computed tomography. These models demonstrated excellent agreement with in vitro studies when used to predict ultimate failure loads. In Part II, we report our extension of those finite element models to include nonlinear behavior of the trabecular and cortical bone. A highly nonlinear material law, originally designed for representing concrete, was used for trabecular bone, while a bilinear material law was used for cortical bone. We found excellent agreement between the model predictions and in vitro fracture data for both the onset of bone yielding and bone fracture. For bone yielding, the model predictions were within 2 percent for a load which simulated one-legged stance and 1 percent for a load which simulated a fall. For bone fracture, the model predictions were within 1 percent and 17 percent, respectively. The models also demonstrated different fracture mechanisms for the two different loading configurations. For one-legged stance, failure within the primary compressive trabeculae at the subcapital region occurred first, leading to load transfer and, ultimately, failure of the surrounding cortical shell. However, for a fall, failure of the cortical and trabecular bone occurred simultaneously within the intertrochanteric region. These results support our previous findings that the strength of the subcapital region is primarily due to trabecular bone whereas the strength of the intertrochanteric region is primarily due to cortical bone.

The purpose of this investigation was to measure the reduction in bone strength resulting from drill holes in diaphyseal bone and to compare this with finite element and theoretical predictions for stresses in a tubular structure. Fifty-two pairs of canine femora were tested to failure in four-point bending. One bone of each pair was used as the control; the other femora had holes of variable size drilled in the lateral cortex. At a ratio of drill hole diameter to bone diameter of 0.2, the bone retained only 62% of its expected strength. A linear regression between the area fraction (the ratio of the cross-sectional area of the drilled specimen to the control specimen) and the percentage of expected strength yielded a strong positive correlation (R2 = 0.79). The average cross-sectional properties were used as the basis for linear orthotropic and nonlinear elastic-plastic finite element models of idealized geometry. The linear models proved insufficient for prediction of failure loads. The nonlinear models, which accounted for both material plasticity and the stress concentration effects of the defect, yielded good correspondence with the experimental data. While the influence of irregular borders and adaptive remodeling of the bone adjacent to the defect requires further investigation, our results suggest the possibility of prediction of fracture risk based on geometric properties of metastatic lesions. Prophylactic fixation remains a matter of clinical judgement based on the functional demands and expected strength of the affected bones.

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