Comparison of the elastic and yield properties of human femoral trabecular and cortical bone tissue

The ability to determine trabecular bone tissue elastic and failure properties has biological and clinical importance. To date, trabecular tissue yield strains remain unknown due to experimental difficulties, and elastic moduli studies have reported controversial results. We hypothesized that the elastic and tensile and compressive yield properties of trabecular tissue are similar to those of cortical tissue. Effective tissue modulus and yield strains were calibrated for cadaveric human femoral neck specimens taken from 11 donors, using a combination of apparent-level mechanical testing and specimen-specific, high-resolution, nonlinear finite element modeling. The trabecular tissue properties were then compared to measured elastic modulus and tensile yield strain of human femoral diaphyseal cortical bone specimens obtained from a similar cohort of 34 donors. Cortical tissue properties were obtained by statistically eliminating the effects of vascular porosity. Results indicated that mean elastic modulus was 10% lower (p<0.05) for the trabecular tissue (18.0+/-2.8 GPa) than for the cortical tissue (19.9+/-1.8 GPa), and the 0.2% offset tensile yield strain was 15% lower for the trabecular tissue (0.62+/-0.04% vs. 0.73+/-0.05%, p<0.001). The tensile-compressive yield strength asymmetry for the trabecular tissue, 0.62 on average, was similar to values reported in the literature for cortical bone. We conclude that while the elastic modulus and yield strains for trabecular tissue are just slightly lower than those of cortical tissue, because of the cumulative effect of these differences, tissue strength is about 25% greater for cortical bone.

The biomechanics of vertebroplasty. The effect of cement volume on mechanical behavior

STUDY DESIGN

Ex vivo biomechanical study using osteoporotic cadaveric vertebral bodies.

OBJECTIVE

To determine the association between the volume of cement injected during percutaneous vertebroplasty and the restoration of strength and stiffness in osteoporotic vertebral bodies, two investigational cements were studied: Orthocomp (Orthovita, Malvern, PA) and Simplex 20 (Simplex P with 20% by weight barium sulfate content; Stryker-Howmedica-Osteonics, Rutherford, NJ).

SUMMARY OF BACKGROUND DATA

Previous biomechanical studies have shown that injections of 8-10 mL of cement during vertebroplasty restore or increase vertebral body strength and stiffness; however, the dose-response association between cement volume and restoration of strength and stiffness is unknown.

METHODS

Compression fractures were experimentally created in 144 vertebral bodies (T6-L5) obtained from 12 osteoporotic spines harvested from female cadavers. After initial strength and stiffness were determined, the vertebral bodies were stabilized using bipedicular injections of cement totaling 2, 4, 6, or 8 mL and recompressed, after which post-treatment strength and stiffness were measured. Strength and stiffness were considered restored when post-treatment values were not significantly different from initial values.

RESULTS

Strength was restored for all regions when 2 mL of either cement was injected. To restore stiffness with Orthocomp, the thoracic and thoracolumbar regions required 4 mL, but the lumbar region required 6 mL. To restore stiffness with Simplex 20, the thoracic and lumbar regions required 4 mL, but the thoracolumbar region required 8 mL.

CONCLUSION

These data provide guidance on the cement volumes needed to restore biomechanical integrity to compressed osteoporotic vertebral bodies.

Effects of bone cement volume and distribution on vertebral stiffness after vertebroplasty

STUDY DESIGN

The biomechanical behavior of a single lumbar vertebral body after various surgical treatments with acrylic vertebroplasty was parametrically studied using finite-element analysis.

OBJECTIVES

To provide a theoretical framework for understanding and optimizing the biomechanics of vertebroplasty. Specifically, to investigate the effects of volume and distribution of bone cement on stiffness recovery of the vertebral body.

SUMMARY OF BACKGROUND DATA

Vertebroplasty is a treatment that stabilizes a fractured vertebra by addition of bone cement. However, there is currently no information available on the optimal volume and distribution of the filler material in terms of stiffness recovery of the damaged vertebral body.

METHODS

An experimentally calibrated, anatomically accurate finite-element model of an elderly L1 vertebral body was developed. Damage was simulated in each element based on empirical measurements in response to a uniform compressive load. After virtual vertebroplasty (bone cement filling range of 1-7 cm3) on the damaged model, the resulting compressive stiffness of the vertebral body was computed for various spatial distributions of the filling material and different loading conditions.

RESULTS

Vertebral stiffness recovery after vertebroplasty was strongly influenced by the volume fraction of the implanted cement. Only a small amount of bone cement (14% fill or 3.5 cm3) was necessary to restore stiffness of the damaged vertebral body to the predamaged value. Use of a 30% fill increased stiffness by more than 50% compared with the predamaged value. Whereas the unipedicular distributions exhibited a comparative stiffness to the bipedicular or posterolateral cases, it showed a medial-lateral bending motion ("toggle") toward the untreated side when a uniform compressive pressure load was applied.

CONCLUSION

Only a small amount of bone cement ( approximately 15% volume fraction) is needed to restore stiffness to predamage levels, and greater filling can result in substantial increase in stiffness well beyond the intact level. Such overfilling also renders the system more sensitive to the placement of the cement because asymmetric distributions with large fills can promote single-sided load transfer and thus toggle. These results suggest that large fill volumes may not be the most biomechanically optimal configuration, and an improvement might be achieved by use of lower cement volume with symmetric placement.

Mechano-electrochemical properties of articular cartilage: their inhomogeneities and anisotropies

In this chapter, the recent advances in cartilage biomechanics and electromechanics are reviewed and summarized. Our emphasis is on the new experimental techniques in cartilage mechanical testing, new experimental and theoretical findings in cartilage biomechanics and electromechanics, and emerging theories and computational modeling of articular cartilage. The charged nature and depth-dependent inhomogeneity in mechano-electrochemical properties of articular cartilage are examined, and their importance in the normal and/or pathological structure-function relationships with cartilage is discussed, along with their pathophysiological implications. Developments in theoretical and computational models of articular cartilage are summarized, and their application in cartilage biomechanics and biology is reviewed. Future directions in cartilage biomechanics and mechano-biology research are proposed.

Mechanotransduction through growth-factor shedding into the extracellular space

Physical forces elicit biochemical signalling in a diverse array of cells, tissues and organisms, helping to govern fundamental biological processes. Several hypotheses have been advanced that link physical forces to intracellular signalling pathways, but in many cases the molecular mechanisms of mechanotransduction remain elusive. Here we find that compressive stress shrinks the lateral intercellular space surrounding epithelial cells, and triggers cellular signalling via autocrine binding of epidermal growth factor family ligands to the epidermal growth factor receptor. Mathematical analysis predicts that constant rate shedding of autocrine ligands into a collapsing lateral intercellular space leads to increased local ligand concentrations that are sufficient to account for the observed receptor signalling; direct experimental comparison of signalling stimulated by compressive stress versus exogenous soluble ligand supports this prediction. These findings establish a mechanism by which mechanotransduction arises from an autocrine ligand-receptor circuit operating in a dynamically regulated extracellular volume, not requiring induction of force-dependent biochemical processes within the cell or cell membrane.

Effects of Obesity on the Biomechanics of Walking at Different Speeds

PURPOSE

Walking is a recommended form of exercise for the treatment of obesity, but walking may be a critical source of biomechanical loads that link obesity and musculoskeletal pathology, particularly knee osteoarthritis. We hypothesized that compared with normal-weight adults 1) obese adults would have greater absolute ground-reaction forces (GRF) during walking, but their GRF would be reduced at slower walking speeds; and 2) obese adults would have greater sagittal-plane absolute leg-joint moments at a given walking speed, but these moments would be reduced at slower walking speeds.

METHODS

We measured GRF and recorded sagittal-plane kinematics of 20 adults (10 obese and 10 normal weight) as they walked on a level, force-measuring treadmill at six speeds (0.5-1.75 m.s(-1)). We calculated sagittal-plane net muscle moments at the hip, knee, and ankle.

RESULTS

Compared with their normal-weight peers, obese adults had much greater absolute GRF (N), stance-phase sagittal-plane net muscle moments (N.m) and step width (m).

CONCLUSIONS

Greater sagittal-plane knee moments in the obese subjects suggest that they walked with greater knee-joint loads than normal-weight adults. Walking slower reduced GRF and net muscle moments and may be a risk-lowering strategy for obese adults who wish to walk for exercise. When obese subjects walked at 1.0 versus 1.5 m.s(-1), peak sagittal-plane knee moments were 45% less. Obese subjects walking at approximately 1.1 m.s(-1) would have the same absolute peak sagittal-plane knee net muscle moment as normal-weight subjects when they walk at their typical preferred speed of 1.4 m.s(-1).

Tensegrity, cellular biophysics, and the mechanics of living systems

The recent convergence between physics and biology has led many physicists to enter the fields of cell and developmental biology. One of the most exciting areas of interest has been the emerging field of mechanobiology that centers on how cells control their mechanical properties, and how physical forces regulate cellular biochemical responses, a process that is known as mechanotransduction. In this article, we review the central role that tensegrity (tensional integrity) architecture, which depends on tensile prestress for its mechanical stability, plays in biology. We describe how tensional prestress is a critical governor of cell mechanics and function, and how use of tensegrity by cells contributes to mechanotransduction. Theoretical tensegrity models are also described that predict both quantitative and qualitative behaviors of living cells, and these theoretical descriptions are placed in context of other physical models of the cell. In addition, we describe how tensegrity is used at multiple size scales in the hierarchy of life—from individual molecules to whole living organisms—to both stabilize three-dimensional form and to channel forces from the macroscale to the nanoscale, thereby facilitating mechanochemical conversion at the molecular level.

Comparison of biomechanical and biochemical properties of cartilage from human knee and ankle pairs

Cartilage was obtained from eight matched knee (tibiofemoral and femoropatellar) and ankle (talocrural) joints of five different donors (both left and right from donors 14, 22, and 38 years of age, and left only from donors 31 and 45 years of age) within 24 hours of death. All cartilage was graded as normal by the macroscopic visual Collins' scale and the histological Mankin scale. Cylindrical disks of cartilage were harvested from 10 sites within the tibiofemoral and femoropatellar joint surfaces and four sites within the talocrural joint, and uniaxial confined compression measurements were performed to quantify a spectrum of physical properties including the equilibrium modulus, hydraulic permeability, dynamic stiffness, streaming potential, electrokinetic coupling coefficient, and electrical conductivity. Matched specimens from the same 14 sites were used for complementary measurements of biochemical composition and molecular interaction, including water content, hypotonic swelling behavior, and sulfated glycosaminoglycan and collagen contents. In comparison of the top 1-mm slices of talar cartilage with the top 1-mm of tibiofemoral cartilage, the talar cartilage appeared denser with a higher sulfated glycosaminoglycan content, lower water content, higher equilibrium modulus and dynamic stiffness, and lower hydraulic permeability. The equilibrium modulus increased with increasing sulfated glycosaminoglycans per wet weight and decreased with increasing water content for all joint surfaces. Multiple linear regression showed that greater than 80% of the variation in the equilibrium modulus could be accounted for by variations in the biochemical parameters (water content, sulfated glycosaminoglycans/wet weight, and hydroxyproline content/wet weight) for each joint surface. Nonhomogeneous depth-dependent changes in the physical properties and biochemical composition of full-thickness distal femoral cartilage were consistent with previous reports. Since the compressive deformation of cartilage during cyclic loading is confined to the more superficial regions, the differences in properties of the upper regions of the talar compared with tibiofemoral or femoropatellar cartilage may be important in the etiology of osteoarthritis.

Systematic and random errors in compression testing of trabecular bone

We sought to quantify the systematic and random errors associated with end-artifacts in the platens compression test for trabecular bone. Our hypothesis was that while errors may depend on anatomic site, they do not depend on apparent density and therefore have substantial random components. Trabecular bone specimens were first tested nondestructively using newly developed accurate protocols and then were tested again using the platens compression test. Percentage differences in modulus between the techniques (bovine proximal tibia [n = 18] and humerus [n = 17] and human lumbar spine, [n = 9]) were in the range of 4-86%. These differences did not depend on anatomic site (p = 0.21) and were only weakly dependent on apparent density and specimen aspect ratio (r2 < 0.10). The mean percentage difference in modulus was 32.6%, representing the systematic component of the end-artifact error. Neglecting the minor variations explained by density and specimen size (approximately 10%), an upper bound on the random error from end-artifacts in this experiment was taken as the SD of the modulus difference (+/-18.2%). Based on a synthesis of data taken from this study and from the literature, we concluded that the systematic underestimation error in the platens compression test can be only approximated and is in the range of 20-40%; the substantial random error (+/-12.5%) confounds correction, particularly when the sample size is small. These errors should be considered when interpreting results from the platens test, and more accurate testing techniques should be used when such errors are not acceptable.

Biomechanical efficacy of unipedicular versus bipedicular vertebroplasty for the management of osteoporotic compression fractures

STUDY DESIGN

Cadaveric study on the biomechanics of osteoporotic vertebral bodies augmented and not augmented with polymethylmethacrylate cement.

OBJECTIVES

To determine the strength and stiffness of osteoporotic vertebral bodies subjected to compression fractures and 1) not augmented, 2) augmented with unipedicular injection of cement, or 3) augmented with bipedicular injection of cement.

SUMMARY OF BACKGROUND DATA

Percutaneous vertebroplasty is a relatively new method of managing osteoporotic compression fractures, but it lacks biomechanical confirmation.

METHODS

Fresh vertebral bodies (L2-L5) were harvested from 10 osteoporotic spines (T scores range, -3.7 to -8.8) and compressed in a materials testing machine to determine intact strength and stiffness. They were then repaired using a transpedicular injection of cement (unipedicular or bipedicular), or they were unaugmented and recrushed.

RESULTS

Results suggest that unipedicular and bipedicular cement injection restored vertebral body stiffness to intact values, whereas unaugmented vertebral bodies were significantly more compliant than either injected or intact vertebral bodies. Vertebral bodies injected with cement (both bipedicular and unipedicular) were significantly stronger than the intact vertebral bodies, whereas unaugmented vertebral bodies were significantly weaker. There was no significant difference in loss in vertebral body height between any of the augmentation groups.

CONCLUSIONS

This study suggests that unipedicular and bipedicular injection of cement, as used during percutaneous vertebroplasty, increases acute strength and restores stiffness of vertebral bodies with compression fractures.

Intervertebral disc cell death is dependent on the magnitude and duration of spinal loading

STUDY DESIGN

An in vivo study of the toxic consequences of static compressive stress on the intervertebral disc.

OBJECTIVES

To determine whether disc cell death is correlated with the magnitude and duration of spinal compressive loading.

SUMMARY OF BACKGROUND DATA

Static compression in vivo has been demonstrated to induce cell death. Cell death, in turn, has been associated with disc degeneration in humans. There are currently no tolerance criteria for the intervertebral disc that combine both biomechanical and biologic factors, although both have been implicated in cases of accelerated degeneration.

METHODS

Mouse tail discs were loaded in vivo with an external compression device. Compressive stress was applied at one of two magnitudes (0.4 and 0.8 MPa) for 7 days, and at one additional magnitude (1.3 MPa) for 1, 3, and 7 days. Midsagittal sections of the discs were stained for apoptosis using the TdT-dUTP terminal nick-end labeling (TUNEL) reaction. Quantal analysis was used to correlate the extent of cell death to the magnitude and duration of loading.

RESULTS

The probit transformation of the percentage of dying cells was proportional to the sum of the logarithmic transformations of the compressive stress and the time of loading.

CONCLUSIONS

The results of this study demonstrate the feasibility of developing a quantitative correlation between spinal loading and disc degeneration. Such a correlation may be coupled in the future to existing engineering models that predict spinal loading in response to physical exposures and lead to improved definition of the bounds of healthy and unhealthy spinal loading, and ultimately, refined guidelines for low back safety.

The influence of water removal on the strength and toughness of cortical bone

Although the effects of dehydration on the mechanical behavior of cortical bone are known, the underlying mechanisms for such effects are not clear. We hypothesize that the interactions of water with the collagen and mineral phases each have a unique influence on mechanical behavior. To study this, strength, toughness, and stiffness were measured with three-point bend specimens made from the mid-diaphysis of human cadaveric femurs and divided into six test groups: control (hydrated), drying in a vacuum oven at room temperature (21 degrees C) for 30 min and at 21, 50, 70, or 110 degrees C for 4 h. The experimental data indicated that water loss significantly increased with each increase in drying condition. Bone strength increased with a 5% loss of water by weight, which was caused by drying at 21 degrees C for 4 h. With water loss exceeding 9%, caused by higher drying temperatures (> or =70 degrees C), strength actually decreased. Drying at 21 degrees C (irrespective of time in vacuum) significantly decreased bone toughness through a loss of plasticity. However, drying at 70 degrees C and above caused toughness to decrease through decreases in strength and fracture strain. Stiffness linearly increased with an increase in water loss. From an energy perspective, the water-mineral interaction is removed at higher temperatures than the water-collagen interaction. Therefore, we speculate that loss of water in the collagen phase decreases the toughness of bone, whereas loss of water associated with the mineral phase decreases both bone strength and toughness.

Structure and biomechanics of peripheral nerves: nerve responses to physical stresses and implications for physical therapist practice

The structural organization of peripheral nerves enables them to function while tolerating and adapting to stresses placed upon them by postures and movements of the trunk, head, and limbs. They are exposed to combinations of tensile, shear, and compressive stresses that result in nerve excursion, strain, and transverse contraction. The purpose of this appraisal is to review the structural and biomechanical modifications seen in peripheral nerves exposed to various levels of physical stress. We have followed the primary tenet of the Physical Stress Theory presented by Mueller and Maluf (2002), specifically, that the level of physical stress placed upon biological tissue determines the adaptive response of the tissue. A thorough understanding of the biomechanical properties of normal and injured nerves and the stresses placed upon them in daily activities will help guide physical therapists in making diagnoses and decisions regarding interventions.

Effects of degeneration on the biphasic material properties of human nucleus pulposus in confined compression

STUDY DESIGN

The biphasic compressive material properties of normal and degenerate human nucleus pulposus tissue were measured in confined compression.

OBJECTIVES

The objective of this study was to determine the effects of degeneration and age on the mechanical properties of human nucleus pulposus.

SUMMARY OF BACKGROUND DATA

The nucleus pulposus exhibits swelling behavior in proportion to proteoglycan content. In shear, the nucleus exhibits both fluid-like and solid-like properties, suggesting a biphasic nature. To date, biphasic compressive properties of human nucleus pulpous have not been reported.

METHODS

Human nucleus pulposus samples were tested in confined compression. Isometric swelling stress and effective aggregate modulus were measured. Linear biphasic theory was used to determine the permeability of the tissue. Mechanical behavior was correlated with proteoglycan and water content.

RESULTS

Degeneration produced significant decreases in swelling stress (Psw = 0.138 +/- 0.029 MPa nondegenerate, Psw = 0.037 +/- 0.038 MPa degenerate) and effective aggregate modulus (H(A)(eff) = 1.01 +/- 0.43 MPa nondegenerate, H(A)(eff) = 0.44 +/- 0.19 MPa degenerate). Both properties were inversely correlated with proteoglycan content. Permeability increased with degeneration (ka = 0.9 +/- 0.43 x 10(-15) m4/N-s nondegenerate, ka = 1.4 +/- 0.58 x 10(-15) m4/N-s degenerate).

CONCLUSIONS

Swelling is the primary load-bearing mechanism in both nondegenerate and degenerate nucleus pulposus. Knowledge of the biphasic material properties of the nucleus pulposus will aid the development of new treatment strategies for disc degeneration aimed at restoring mechanical function of the intervertebral disc.

A fibril-reinforced poroviscoelastic swelling model for articular cartilage

From a mechanical point of view, the most relevant components of articular cartilage are the tight and highly organized collagen network together with the charged proteoglycans. Due to the fixed charges of the proteoglycans, the cation concentration inside the tissue is higher than in the surrounding synovial fluid. This excess of ion particles leads to an osmotic pressure difference, which causes swelling of the tissue. The fibrillar collagen network resists straining and swelling pressures. This combination makes cartilage a unique, highly hydrated and pressurized tissue, enforced with a strained collagen network. Many theories to explain articular cartilage behavior under loading, expressed in computational models that either include the swelling behavior or the properties of the anisotropic collagen structure, can be found in the literature. The most common tests used to determine the mechanical quality of articular cartilage are those of confined compression, unconfined compression, indentation and swelling. All theories currently available in the literature can explain the cartilage response occurring in some of the above tests, but none of them can explain these for all of the tests. We hypothesized that a model including simultaneous mathematical descriptions of (1) the swelling properties due to the fixed-change densities of the proteoglycans and (2) the anisotropic viscoelastic collagen structure, can explain all these test simultaneously. To study this hypothesis we extended our fibril-reinforced poroviscoelastic finite element model with our biphasic swelling model. We have shown that the newly developed fibril-reinforced poroviscoelastic swelling (FPVES) model for articular cartilage can simultaneously account for the reaction force during swelling, confined compression, indentation and unconfined compression as well as the lateral deformation during unconfined compression. Using this theory it is possible to analyze the link between the collagen network and the swelling properties of articular cartilage.

A generic detailed rigid-body lumbar spine model

The objective of this work is to present a musculo-skeletal model of the lumbar spine, which can be shared and lends itself to investigation in many locations by different researchers. This has the potential for greater reproducibility and subsequent improvement of its quality from the combined effort of different research groups. The model is defined in a text-based, declarative, object-oriented language in the AnyBody Modelling System software. Text-based models will facilitate sharing of the models between different research groups. The necessary data for the model has been taken from the literature. The work resulted in a detailed lumbar spine model with seven rigid segments with 18 degrees-of-freedom and 154 muscles. The model is able to produce a maximum extension moment of 238 Nm around L5/S1. Moreover, a comparison was made with in vivo intradiscal pressure measurements of the L4-5 disc available from the literature. The model is based on inverse dynamics, where the redundancy problem is solved using optimization in order to compute the individual muscle forces and joint reactions. With the presented model it is possible to investigate a range of research questions, because the model is relatively easy to share and modify due to the use of a well-defined and self-contained scripting language. Validation is though still necessary for specific cases.

Biological response of the intervertebral disc to dynamic loading

Disc degeneration is a chronic remodeling process that results in alterations of matrix composition and decreased cellularity. This study tested the hypothesis that dynamic mechanical forces are important regulators in vivo of disc cellularity and matrix synthesis. A murine model of dynamic loading was developed that used an external loading device to cyclically compress a single disc in the tail. Loads alternated at a 50% duty cycle between 0MPa and one of two peak stresses (0.9 or 1.3MPa) at one of two frequencies (0.1 or 0.01Hz) for 6h per day for 7 days. An additional group received static compression at 1.3MPa for 3h/day for 7 days. A control group wore the device with no loading. Sections of treated discs were analyzed for morphology, proteoglycan content, apoptosis, cell areal density, and aggrecan and collagen II gene expression. Dynamic loading induced differential effects that depended on frequency and stress. No significant changes to morphology, proteoglycan content or cell death were found after loading at 0.9MPa, 0.1Hz. Loading at lower frequency and/or higher stress increased proteoglycan content, matrix gene expression and cell death. The results have implications in the prevention of intervertebral disc degeneration, suggesting that loading conditions may be optimized to promote maintenance of normal structure and function.

Novel model to analyze the effect of a large compressive follower pre-load on range of motions in a lumbar spine

A 3-D finite element model (FEM) of the lumbar spine (L1-S1) was used to determine the effect of a large compressive follower pre-load on range of motions (ROM) in all three planes. The follower load modeled in the FEM produced minimal vertebral rotations in all the three planes. The model was validated by comparing the disc compression at all levels in the lumbar spine with the corresponding results obtained by compressing 10 cadevaric lumbar spines (L1-S1) using the follower load technique described by Patwardhan et al. [1999. A follower load increases the load-carrying capacity of the lumbar spine in compression. Spine 24(10), 1003-1009]. Further validation of the model was performed by comparing the lateral bending and torsion response without pre-load and the flexion-extension response without pre-load and with an 800 N follower pre-load with those obtained using cadaver lumbar spines. Following validation, the FEM was subjected to bending moments in all three planes with and without compressive follower pre-loads of up to 1200 N. Disc compression values and the flexion-extension range of motion under 800 N follower pre-load predicted by the FEM compared well with in vitro results. The current model showed that compressive follower pre-load decreased total as well as segmental ROM in flexion-extension by up to 18%, lateral bending by up to 42%, and torsion by up to 26%.

Mechanical evaluation of large-size fourth-generation composite femur and tibia models

Composite analogue bone models provide consistent geometric and structural properties that represent a valuable asset in a range of biomechanical analyses and testing procedures. The objective of this study was to evaluate the diaphyseal structural properties of the large-size Fourth-Generation composite analogue femur and tibia models concentrated on mechanical behaviors under axial compression, bending and torsion. Thirty of each large-size composite analogue models (femora and tibiae) were tested under medial-lateral four-point bending, anterior-posterior four-point bending, axial compression and external rotational torque to evaluate flexural rigidity, axial stiffness, torsional rigidity and ultimate failure strength. The composite femur was tested under torsion at both the femoral neck and the mid-diaphyseal areas. Large-size Fourth-Generation composite replicate bones exhibited intra-specimen variations under 10% for all cases and was also found to perform within the biological range of healthy adult bones (age: <80 years old) range with respect to flexural rigidity (<8%) and torsional rigidity (<12%). The failure modes of these composite models were close to published findings for human bones (four-point bending: butterfly fragment fracture; torsional: spiral fracture; and compression: transverse fracture). The large-size composite analogue femur and tibia are close to ideal replicas for standardization in biomechanical analyses. One advantage of these analogue models is that their variability is significantly lower than that of cadaveric specimens for all loading regimens. Published results vary widely in cadaveric studies, which is likely due to the high anatomic variability among cadaveric specimens. This study evaluated and advanced our overall understanding of the capacity of composite analogue bone models mimic the structural properties of average healthy adult human bones.

Prediction of new clinical vertebral fractures in elderly men using finite element analysis of CT scans

Vertebral strength, as estimated by finite element analysis of computed tomography (CT) scans, has not yet been compared against areal bone mineral density (BMD) by dual-energy X-ray absorptiometry (DXA) for prospectively assessing the risk of new clinical vertebral fractures. To do so, we conducted a case-cohort analysis of 306 men aged 65 years and older, which included 63 men who developed new clinically-identified vertebral fractures and 243 men who did not, all observed over an average of 6.5 years. Nonlinear finite element analysis was performed on the baseline CT scans, blinded to fracture status, to estimate L1 vertebral compressive strength and a load-to-strength ratio. Volumetric BMD by quantitative CT and areal BMD by DXA were also evaluated. We found that, for the risk of new clinical vertebral fracture, the age-adjusted hazard ratio per standard deviation change for areal BMD (3.2; 95% confidence interval [CI], 2.0-5.2) was significantly lower (p < 0.005) than for strength (7.2; 95% CI, 3.6-14.1), numerically lower than for volumetric BMD (5.7; 95% CI, 3.1-10.3), and similar for the load-to-strength ratio (3.0; 95% CI, 2.1-4.3). After also adjusting for race, body mass index (BMI), clinical center, and areal BMD, all these hazard ratios remained highly statistically significant, particularly those for strength (8.5; 95% CI, 3.6-20.1) and volumetric BMD (9.4; 95% CI, 4.1-21.6). The area-under-the-curve for areal BMD (AUC = 0.76) was significantly lower than for strength (AUC = 0.83, p = 0.02), volumetric BMD (AUC = 0.82, p = 0.05), and the load-to-strength ratio (AUC = 0.82, p = 0.05). We conclude that, compared to areal BMD by DXA, vertebral compressive strength and volumetric BMD consistently improved vertebral fracture risk assessment in this cohort of elderly men.

Correlation of bone mineral density with strength and microstructural parameters of cortical bone in vitro

The aim of this study was to evaluate the influence of microstructural parameters, such as porosity and osteon dimensions, on strength. Therefore, the predictive value of bone mineral density (BMD) measured by quantitative computed tomography (QCT) for intracortical porosity and other microstructural parameters, as well as for strength of cortical bone biopsies, was investigated. Femoral cortical bone specimens from the middiaphysis of 23 patients were harvested during total hip replacement while drilling a hole (dia. 4.5 mm) for the relief of the intramedullary pressure. In vitro structural parameters assessed in histological sections as well as BMD determined by quantitative computed tomography were correlated with yield stress, and elastic modulus assessed by a compression test of the same specimens. Significant correlations were found between BMD and all mechanical parameters (elastic modulus: r = 0.69, p < 0.005; yield stress: r = 0.64, p < 0.005). Significant correlations between most structural parameters assessed by histology and yield stress were discovered. Structural parameters related to pore dimensions revealed higher correlation coefficients with yield stress (r = -0.69 for average pore diameter and r = -0.62 for fraction of porous structures, p < 0.005) than parameters related to osteons (r = 0.60 for osteon density and average osteonal area, p < 0.005), whereas elastic modulus was predicted equally well by both types of parameters. Significant correlations were found between BMD and parameters related to porous structures (r = 0.85 for porosity, 0.80 for average pore area, and r = 0.79 for average pore diameter in polynomial regression, p < 0.005). Histologically assessed porosity correlated significantly with parameters describing porous structures and haversian canal dimensions. Our results indicate a relevance of osteon density and fraction of osteonal structures for the mechanical parameters of cortical bone. We consider the measurement of BMD by quantitative computed tomography to be helpful for the estimation of bone strength as well as for the prediction of intracortical porosity and parameters related to porous structures of cortical bone.

Effects of dynamic compressive loading on chondrocyte biosynthesis in self-assembling peptide scaffolds

Dynamic mechanical loading has been reported to affect chondrocyte biosynthesis in both cartilage explant and chondrocyte-seeded constructs. In this study, the effects of dynamic compression on chondrocyte-seeded peptide hydrogels were analyzed for extracellular matrix synthesis and retention over long-term culture. Initial studies were conducted with chondrocyte-seeded agarose hydrogels to explore the effects of various non-continuous loading protocols on chondrocyte biosynthesis. An optimized alternate day loading protocol was identified that increased proteoglycan (PG) synthesis over control cultures maintained in free-swelling conditions. When applied to chondrocyte-seeded peptide hydrogels, alternate day loading stimulated PG synthesis up to two-fold higher than that in free-swelling cultures. While dynamic compression also increased PG loss to the medium throughout the 39-day time course, total PG accumulation in the scaffold was significantly higher than in controls after 16 and 39 days of loading, resulting in an increase in the equilibrium and dynamic compressive stiffness of the constructs. Viable cell densities of dynamically compressed cultures differed from free-swelling controls by less than 20%, demonstrating that changes in PG synthesis were due to an increase in the average biosynthesis per viable cell. Protein synthesis was not greatly affected by loading, demonstrating that dynamic compression differentially regulated the synthesis of PGs. Taken together, these results demonstrate the potential of dynamic compression for stimulating PG synthesis and accumulation for applications to in vitro culture of tissue engineered constructs prior to implantation.

Cartilage interstitial fluid load support in unconfined compression

Under physiological conditions of loading, articular cartilage is subjected to both compressive strains, normal to the articular surface, and tensile strains, tangential to the articular surface. Previous studies have shown that articular cartilage exhibits a much higher modulus in tension than in compression, and theoretical analyses have suggested that this tension-compression nonlinearity enhances the magnitude of interstitial fluid pressurization during loading in unconfined compression, above a theoretical threshold of 33% of the average applied stress. The first hypothesis of this experimental study is that the peak fluid load support in unconfined compression is significantly greater than the 33% theoretical limit predicted for porous permeable tissues modeled with equal moduli in tension and compression. The second hypothesis is that the peak fluid load support is higher at the articular surface side of the tissue samples than near the deep zone, because the disparity between the tensile and compressive moduli is greater at the surface zone. Ten human cartilage samples from six patellofemoral joints, and 10 bovine cartilage specimens from three calf patellofemoral joints were tested in unconfined compression. The peak fluid load support was measured at 79 +/- 11% and 69 +/- 15% at the articular surface and deep zone of human cartilage, respectively, and at 94 +/- 4% and 71 +/- 8% at the articular surface and deep zone of bovine calf cartilage, respectively. Statistical analyses confirmed both hypotheses of this study. These experimental results suggest that the tension-compression nonlinearity of cartilage is an essential functional property of the tissue which makes interstitial fluid pressurization the dominant mechanism of load support in articular cartilage.