Joyce H. Keyak

Prediction of femoral fracture load using automated finite element modeling

Hip fracture is an important cause of morbidity and mortality among the elderly. Current methods of assessing a patients risk of hip fracture involve local estimates of bone density (densitometry), and are limited by their inability to account for the complex structural features of the femur. In an effort to improve clinical and research tools for assessing hip fracture risk, this study investigated whether automatically generated, computed tomographic (CT) scan-based finite element (FE) models can be used to estimate femoral fracture load in vitro. Eighteen pairs of femora were examined under two loading conditions one similar to loading during the stance phase of gait, and one simulating impact from a fall. The femora were then mechanically tested to failure and regression analyses between measured fracture load and FE-predicted fracture load were performed. For comparison, densitometry measures were also examined. Significant relationships were found between measured fracture load and FE-predicted fracture load (r = 0.87, stance; r = 0.95, fall; r = 0.97, stance and fall data pooled) and between measured fracture load and densitometry data (r = 0.78, stance; r = 0.91, fall). These results indicate that this sophisticated technique, which is still early in its development, can achieve precision comparable to that of densitometry and can predict femoral fracture load to within -40% to +60% with 95% confidence. Therefore, clinical use of this approach, which would require additional X-ray exposure and expenditure for a CT scan, is not justified at this point. Even so, the potential advantages of this CT/FE technique support further research in this area.

Automated three-dimensional finite element modelling of bone: a new method.

Three-dimensional finite element stress analysis of bone is a key to understanding bone remodelling, assessing fracture risk, and designing prostheses; however, the cost and complexity of predicting the stress field in bone with accuracy has precluded the routine use of this method. A new, automated method of generating patient-specific three-dimensional finite element models of bone is presented--it uses digital computed tomographic (CT) scan data to drive the geometry of the bone and to estimate its inhomogeneous material properties. Cubic elements of a user-specified size are automatically defined and then individually assigned the CT scan-derived material properties. The method is demonstrated by predicting the stress, stain, and strain energy in a human proximal femur in vivo. Three-dimensional loading conditions corresponding to the stance phase of gait were taken from the literature and applied to the model. Maximum principal compressive stresses of 8-23 MPa were computed for the medial femoral neck. Automated generation of additional finite element models with larger numbers of elements was used to verify convergence in strain energy.

Volumetric quantitative computed tomography of the proximal femur : Precision and relation to bone strength

We have developed a three-dimensional computed tomography (CT) scanning and image analysis method for measurement of trabecular and integral bone mineral density (BMD) and geometry in automatically determined femoral-neck and trochanteric subregions of the proximal femur. We measured the correlation of the density and geometry variables to femoral strength assessed in vitro under loading simulating a single-limb condition and a fall to the side. While BMD alone accounted for 48%-77% of the variability in strength for the stance loading configuration, femoral neck cross-sectional area (minCSA) and femoral neck axis length (FNAL) also contributed independently to femoral strength, and a combination of BMD and geometry variables explained 87%-93% of the variance in the data. For the fall loading configuration, trochanteric trabecular BMD alone explained 87% of the variability of strength. The reproducibility in vivo of the technique was assessed in a group of seven postmenopausal women, who underwent repeat scans with repositioning. For trabecular BMD, the precision was 1.1% and 0.6% for the femoral neck and trochanteric subregions, respectively, compared to 3.3% and 1.6% for the corresponding integral envelopes. Thus, trabecular BMD measurements were reproducible and highly correlated to biomechanical strength measurements. These results support further exploration of quantitative CT for assessment of osteoporosis at the proximal femur.

Improved prediction of proximal femoral fracture load using nonlinear finite element models

Hip fracture, which is often due to osteoporosis or other conditions affecting bone strength, can lead to permanent disability, pneumonia, pulmonary embolism, and/or death. Great effort has been directed toward developing noninvasive methods for evaluating proximal femoral strength (fracture load), with the goal of assessing fracture risk. Previously, computed tomographic scan-based, linear finite element (FE) models were used to estimate proximal femoral fracture loads ex vivo in two load configurations, one approximating joint loading during single-limb stance and the other simulating impact from a fall. Measured and computed fracture loads were correlated (stance, r=0.867; fall, r=0.949). However, precision for the stance configuration was insufficient to identify subjects with below average fracture loads reliably. The present study examined whether, for this configuration, nonlinear FE models could be used to identify these subjects. These models were found to predict fracture load within +/-2.0 kN (r=0.962). This level of precision is sufficient to identify 97.5% of femora with fracture loads 1.3 standard deviations below the mean as having below average fracture loads. Accordingly, 20% of subjects with below average fracture loads, i.e. those with the lowest fracture loads and likely to be at greatest risk of fracture, would be correctly identified with at least 97.5% reliability. This FE modeling method will be a powerful tool for studies of hip fracture.

Prediction of femoral fracture load using finite element models: an examination of stress- and strain-based failure theories

Finite element (FE) models are often used to model bone failure. However, no failure theory for bone has been validated at this time. In this study, we examined the performance of nine stress- and strain-based failure theories, six of which could account for differences in tensile and compressive material strengths. The distortion energy, Hoffman and a strain-based Hoffman analog, maximum normal stress, maximum normal strain, maximum shear strain, maximum shear stress (tau(max)), Coulomb-Mohr, and modified Mohr failure theories were evaluated using automatically generated, computed tomographic scan-based FE models of the femur. Eighteen matched pairs of proximal femora were examined in two load configurations, one approximating joint loading during single-limb stance and one simulating impact from a fall. Mechanical testing was performed to assess model and failure theory performance in the context of predicting femoral fracture load. Measured and FE-computed fracture load were significantly correlated for both loading conditions and all failure criteria (p < or = 0.001). The distortion energy and tau(max) failure theories were the most robust of those examined, providing the most consistently strong FE model performance for two very different loading conditions. The more complex failure theories and the strain-based theories examined did not improve performance over the simpler distortion energy and tau(max) theories, and often degraded performance, even when differences between tensile and compressive failure properties were represented. The relatively strong performance of the distortion energy and tau(max) theories supports the hypothesis that shear/distortion is an important failure mode during femoral fracture.

Comparison of in situ and in vitro CT scan-based finite element model predictions of proximal femoral fracture load

Hip fracture is a serious and common injury that can lead to permanent disability, pneumonia, pulmonary embolism, and death. Research to help prevent these fractures is essential. Computed tomographic (CT) scan-based finite element (FE) modeling is a tool that can predict proximal femoral fracture loads in vitro. Because this tool might be used in vivo, this study examined whether FE models generated from CT scans in situ and in vitro yield comparable predictions of proximal femoral fracture load. CT scans of the left proximal femur of two human cadavers were obtained in situ and in vitro, and three-dimensional FE models employing nonlinear mechanical properties were generated from each CT scan. The models were evaluated under single-limb stance-type loading by applying displacements incrementally to the femoral head. The FE-predicted fracture load (F(FE)) was the maximum femoral head reaction force. F(FE) for the in situ-derived models for the two subjects were 5.2 and 13.3% greater than for the in vitro-derived models. These results demonstrate that using CT scan data obtained in situ instead of in vitro to generate FE models can lead to substantially different predicted fracture loads. This effect must be considered when using this technology in vivo.

Predicting proximal femoral strength using structural engineering models.

Hip fracture related to osteoporosis and metastatic disease is a major cause of morbidity and mortality. An accurate and precise method of predicting proximal femoral strength and fracture location would be useful for research and clinical studies of hip fracture. The goals of this study were to develop a structural modeling technique that accurately predicts proximal femoral strength; to evaluate the accuracy and precision of this predicted strength on an independent data set; and to evaluate the ability of this technique to predict fracture location. Fresh human cadaveric proximal femora with and without metastatic lesions were studied using computed tomography scan-based three-dimensional structural models and mechanical testing to failure under single-limb stance-type loading. The models understated proximal femoral strength by an average of 444 N, and the precision of the predicted strength was ± 1900 N. Therefore, the ability to predict hip strength in an individual subject is limited primarily by the level of precision, rather than accuracy. This level of precision is likely to be sufficient for many studies of hip strength. Finally, these models predict fractures involving the subcapital and cervical regions, consistent with most fractures produced experimentally under single-limb stance-type loading.

Validation of an automated method of three-dimensional finite element modelling of bone

This study validated an automated method of finite element modelling of bone from CT scan data. After a fresh-frozen cadaveric femur was modelled, strain gauges were attached to the bone at 11 locations and the femur was mechanically tested by applying a load to the femoral head. Linear regression analysis was used to correlate the strains predicted by the model with the experimentally measured strains. The regression results were significant (P < 0.001), indicating that the strain calculated by the FE model is a valid predictor of the measured strain. Verification of the surface strains also supports the validity of the strains and stresses predicted inside the bone. The present study provides a strong rationale for use of this modelling method as a research tool and in possible clinical applications.

Reduction in proximal femoral strength due to long-duration spaceflight

Loss of bone mass is a well-known medical complication of long-duration spaceflight. However, we do not know how changes in bone density and geometry ultimately combine to affect the strength of the proximal femur as a whole. The goal of this study was to quantify the changes in proximal femoral strength that result from long-duration spaceflight. Pre-and post-flight CT scan-based patient-specific finite element models of the left proximal femur of 13 astronauts who spent 4.3 to 6.5 months on the International Space Station were generated. Loading conditions representing single-limb stance and a fall onto the posterolateral aspect of the greater trochanter were modeled, and proximal femoral strength (F(FE)) was computed. Mean F(FE) decreased from 18.2 times body weight (BW) pre-flight to 15.6 BW post-flight for stance loading and from 3.5 BW pre-flight to 3.1 BW post-flight for fall loading. When normalized for flight duration, F(FE) under stance and fall loading decreased at mean rates of 2.6% (0.6% to 5.0%) per month and 2.0% (0.6% to 3.9%) per month, respectively. These values are notably greater than previously reported reductions in DXA total femoral bone mineral density (0.4 to 1.8% per month). In some subjects, the magnitudes of the reductions in proximal femoral strength were comparable to estimated lifetime losses associated with aging. Although average post-flight proximal femoral strength is greater than forces expected to occur due to falls or normal activities, some subjects have small margins of safety. If proximal femoral strength is not recovered, some crew members may be at increased risk for age-related hip fractures decades after their missions.

Compressive mechanical properties of human cancellous bone after gamma irradiation.

The effect of gamma irradiation on the mechanical properties of human bone was examined. Specimens of cancellous bone were cut from the proximal epiphyseal region of fresh-frozen tibiae and divided into control and irradiated groups according to anatomical region. The irradiated groups were exposed to 10,000, 31,000, 51,000, or 60,000 gray (1.0, 3.1, 5.1, or 6.0 megarad). The specimens were tested in compression to failure to determine failure stress, strain to failure, and elastic modulus. Failure stress and elastic modulus were found to be proportional to the square of the density and were normalized with respect to this property. Significant differences in normalized failure stress (p less than 0.001) and normalized elastic modulus (p = 0.003), when compared with the values for matched control specimens, were found only for the specimens that had been irradiated with 60,000 gray (6.0 megarad).