0

### TECHNICAL PAPERS: Bone/Orthopedics

J Biomech Eng. 2006;129(2):129-136. doi:10.1115/1.2472367.

Periacetabular bone metastases cause severe pain and functional disability in cancer patients. Percutaneous acetabuloplasty (PCA) is a minimally invasive, image-guided procedure whereby cement is injected into lesion sites. Pain relief and functional restoration have been observed clinically; however, neither the biomechanical consequences of the lesions nor the effectiveness of the PCA technique are well understood. The objective of this study was to investigate how periacetabular lesion size, cortex involvement, and cement modulus affect pelvic bone stresses and strains under single-legged stance loading. Experiments were performed on a male cadaver pelvis under conditions of intact, periacetabular defect, and cement-filling with surface strains recorded at three strain gage locations. The experimental data were then employed to validate three-dimensional finite element models of the same pelvis, developed using computed tomography data. The models demonstrated that increases in cortical stresses were highest along the posterior column of the acetabulum, adjacent to the defect. Cortical stresses were more profoundly affected in the presence of transcortical defects, as compared to those involving only trabecular bone. Cement filling with a modulus of $2.2GPa$ was shown to restore cortical stresses to near intact values, while a decrease in cement modulus due to inclusion of $BaSO4$ reduced the restorative effect. Peak acetabular contact pressures increased less than 15% for all simulated defect conditions; however, the contact stresses were reduced to levels below intact in the presence of either cement filling. These results suggest that periacetabular defects may increase the vulnerability of the pelvis to fracture depending on size and cortical involvement and that PCA filling may lower the risk of periacetabular fractures.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Cell

J Biomech Eng. 2006;129(2):137-147. doi:10.1115/1.2472369.

Mechanics of collagen gels, like that of many tissues, is governed by events occurring on a length scale much smaller than the functional scale of the material. To deal with the challenge of incorporating deterministic micromechanics into a continuous macroscopic model, we have developed an averaging-theory-based modeling framework for collagen gels. The averaging volume, which is constructed around each integration point in a macroscopic finite-element model, is assumed to experience boundary deformations homogeneous with the macroscopic deformation field, and a micromechanical problem is solved to determine the average stress at the integration point. A two-dimensional version was implemented with the microstructure modeled as a network of nonlinear springs, and 500 segments were found to be sufficient to achieve statistical homogeneity. The method was then used to simulate the experiments of Tower (Ann. Biomed. Eng., 30, pp. 1221–1233) who performed uniaxial extension of prealigned collagen gels. The simulation captured many qualitative features of the experiments, including a toe region and the realignment of the fibril network during extension. Finally, the method was applied to an idealized wound model based on the characterization measurements of Bowes (Wound Repair Regen., 7, pp. 179–186). The model consisted of a strongly aligned “wound” region surrounded by a less strongly aligned “healthy” region. The alignment of the fibrils in the wound region led to reduced axial strains, and the alignment of the fibrils in the healthy region, combined with the greater effective stiffness of the wound region, caused rotation of the wound region during uniaxial stretch. Although the microscopic model in this study was relatively crude, the multiscale framework is general and could be employed in conjunction with any microstructural model.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2006;129(2):148-155. doi:10.1115/1.2472370.

In this paper, we study the effects of initial fixed-charge density on the response behavior of pH-sensitive hydrogels subjected to coupled stimuli, namely, solution pH and externally applied electric field. This is the first instance in which a coupled stimuli numerical analysis has been carried out for these polymer gels, which are used as active sensing/actuating elements in advanced biomicroelectromechanical systems devices. In this work, a chemo-electro-mechanical formulation, termed the multi-effect-coupling pH-stimulus (MECpH) model, is first presented. This mathematical model takes into account the ionic species diffusion, electric potential coupling, and large mechanical deformation. In addition, a correlation between the diffusive hydrogen ions and fixed-charge groups on the hydrogel polymeric chains is established based on the Langmuir absorption isotherm, and incorporated accordingly into the MECpH model. To solve the resulting highly nonlinear and highly coupled partial differential equations of this mathematical model, the Hermite-Cloud method, a novel true meshless technique, is employed. To demonstrate the accuracy and robustness the MECpH model, computed numerical results are compared with experimental data available from literature. Following this validation, several numerical studies are carried out to investigate the effects of initial fixed-charge density on the volumetric variations of these pH-stimulus-responsive hydrogels when immersed in buffered solutions.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2006;129(2):156-163. doi:10.1115/1.2472373.

The stiffness and hydraulic permeability of soft contact lenses may influence its clinical performance, e.g., on-eye movement, fitting, and wettability, and may be related to the occurrence of complications; e.g., lesions. It is therefore important to determine these properties in the design of comfortable contact lenses. Micro-indentation provides a nondestructive means of measuring mechanical properties of soft, hydrated contact lenses. However, certain geometrical and material considerations must be taken into account when analyzing output force-displacement $(F-D)$ data. Rather than solely having a solid response, mechanical behavior of hydrogel contact lenses can be described as the coupled interaction between fluid transport through pores and solid matrix deformation. In addition, indentation of thin membranes $(∼100μm)$ requires special consideration of boundary conditions at lens surfaces and at the indenter contact region. In this study, a biphasic finite element model was developed to simulate the micro-indentation of a hydrogel contact lens. The model accounts for a curved, thin hydrogel membrane supported on an impermeable mold. A time-varying boundary condition was implemented to model the contact interface between the impermeable spherical indenter and the lens. Parametric studies varying the indentation velocities and hydraulic permeability show $F-D$ curves have a sensitive region outside of which the force response reaches asymptotic limits governed by either the solid matrix (slow indentation velocity, large permeability) or the fluid transport (high indentation velocity, low permeability). Using these results, biphasic properties (Young’s modulus and hydraulic permeability) were estimated by fitting model results to $F-D$ curves obtained at multiple indentation velocities (1.2 and $20μm∕s$). Fitting to micro-indentation tests of Etafilcon A resulted in an estimated permeability range of $1.0×10−15$ to $5.0×10−15m4∕Ns$ and Young’s modulus range of $130to170kPa$.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Fluids/Heat/Transport

J Biomech Eng. 2006;129(2):164-173. doi:10.1115/1.2472379.

This paper studies two one-dimensional models to estimate the pressure drop in the normal human biliary system for Reynolds number up to 20. Excessive pressure drop during bile emptying and refilling may result in incomplete bile emptying, leading to stasis and subsequent formation of gallbladder stones. The models were developed following the group’s previous work on the cystic duct using numerical simulations. Using these models, the effects of the biliary system geometry, elastic property of the cystic duct, and bile viscosity on the pressure drop can be studied more efficiently than with full numerical approaches. It was found that the maximum pressure drop occurs during bile emptying immediately after a meal, and is greatly influenced by the viscosity of the bile and the geometric configuration of the cystic duct, i.e., patients with more viscous bile or with a cystic duct containing more baffles or a longer length, have the greatest pressure drop. It is found that the most significant parameter is the diameter of the cystic duct; a 1% decrease in the diameter increases the pressure drop by up to 4.3%. The effects of the baffle height ratio and number of baffles on the pressure drop are reflected in the fact that these effectively change the equivalent diameter and length of the cystic duct. The effect of the Young’s modulus on the pressure drop is important only if it is lower than $400Pa$; above this value, a rigid-walled model gives a good estimate of the pressure drop in the system for the parameters studied.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2006;129(2):174-186. doi:10.1115/1.2472380.

Eye diseases, such as Krukenberg’s spindle, hyphema, and hypopyon, are related to the deposition of specific particles such as pigmentary cells, leukocytes, and erythrocytes. These particles are circulated by the aqueous humor (AH) and tend to deposit in regions of low velocities or high resistance. In the present paper, numerical simulations are reported of the AH flow and particle transport, and the particle concentration predictions are qualitatively compared to clinical images. The particle concentration distributions provide an understanding of the likely sources of deposition and the origin of the deposited particles. Pigmentary cells are seen to concentrate in a vertical band on the corneal surface consistent with clinical observations of Krukenberg’s spindle. Leukocytes and erythrocytes are seen to collect at the bottom of the anterior chamber similar to the observations made for hypopyon and hyphema. These results confirm the potential of using numerical calculations in order to obtain a better understanding of the particle transport and deposition patterns in the anterior chamber of the eye.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2006;129(2):187-192. doi:10.1115/1.2472381.

When a stent is implanted in a blocked ureter, the urine passing from the kidney to the bladder must traverse a very complicated flow path. That path consists of two parallel passages, one of which is the bore of the stent and the other is the annular space between the external surface of the stent and the inner wall of the ureter. The flow path is further complicated by the presence of numerous pass-through holes that are deployed along the length of the stent. These holes allow urine to pass between the annulus and the bore. Further complexity in the pattern of the urine flow occurs because the coiled “pig tails,” which hold the stent in place, contain multiple ports for fluid ingress and egress. The fluid flow in a stented ureter has been quantitatively analyzed here for the first time using numerical simulation. The numerical solutions obtained here fully reveal the details of the urine flow throughout the entire stented ureter. It was found that in the absence of blockages, most of the pass-through holes are inactive. Furthermore, only the port in each coiled pig tail that is nearest the stent proper is actively involved in the urine flow. Only in the presence of blockages, which may occur due to encrustation or biofouling, are the numerous pass-through holes activated. The numerical simulations are able to track the urine flow through the pass-through holes as well as adjacent to the blockages. The simulations are also able to provide highly accurate results for the kidney-to-bladder urine flow rate. The simulation method presented here constitutes a powerful new tool for rational design of ureteral stents in the future.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2006;129(2):193-201. doi:10.1115/1.2485780.

We applied a statistical mechanics based microstructural model of pulmonary artery mechanics, developed from our previous studies of rats with pulmonary arterial hypertension (PAH), to patient-specific clinical studies of children with PAH. Our previous animal studies provoked the hypothesis that increased cross-linking density of the molecular chains may be one biological remodeling mechanism by which the PA stiffens in PAH. This study appears to further confirm this hypothesis since varying molecular cross-linking density in the model allows us to simulate the changes in the $P‐D$ loops between normotensive and hypertensive conditions reasonably well. The model was combined with patient-specific three-dimensional vascular anatomy to obtain detailed information on the topography of stresses and strains within the proximal branches of the pulmonary vasculature. The effect of orthotropy on stress∕strain within the main and branch PAs obtained from a patient was explored. This initial study also puts forward important questions that need to be considered before combining the microstructural model with complex patient-specific vascular geometries.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2007;129(2):202-215. doi:10.1115/1.2485961.

The fundamental study of blood flow past a circular cylinder filled with an oxygen source is investigated as a building block for an artificial lung. The Casson constitutive equation is used to describe the shear-thinning and yield stress properties of blood. The presence of hemoglobin is also considered. Far from the cylinder, a pulsatile blood flow in the $x$ direction is prescribed, represented by a time periodic (sinusoidal) component superimposed on a steady velocity. The dimensionless parameters of interest for the characterization of the flow and transport are the steady Reynolds number (Re), Womersley parameter $(α)$, pulsation amplitude $(A)$, and the Schmidt number (Sc). The Hill equation is used to describe the saturation curve of hemoglobin with oxygen. Two different feed-gas mixtures were considered: pure $O2$ and air. The flow and concentration fields were computed for Re=5, 10, and 40, $0≤A≤0.75$, $α=0.25$, 0.4, and Schmidt number, Sc=1000. The Casson fluid properties result in reduced recirculations (when present) downstream of the cylinder as compared to a Newtonian fluid. These vortices oscillate in size and strength as $A$ and $α$ are varied. Hemoglobin enhances mass transport and is especially important for an air feed which is dominated by oxyhemoglobin dispersion near the cylinder. For a pure $O2$ feed, oxygen transport in the plasma dominates near the cylinder. Maximum oxygen transport is achieved by operating near steady flow (small $A$) for both feed-gas mixtures. The time averaged Sherwood number, $Sh̿$, is found to be largely influenced by the steady Reynolds number, increasing as Re increases and decreasing with $A$. Little change is observed with varying $α$ for the ranges investigated. The effect of pulsatility on $Sh̿$ is greater at larger Re. Increasing Re aids transport, but yields a higher cylinder drag force and shear stresses on the cylinder surface which are potentially undesirable.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Joint/Whole Body

J Biomech Eng. 2006;129(2):216-222. doi:10.1115/1.2486008.

We explored how hip joint actuation can be used to control locomotive bifurcations and chaos in a passive dynamic walking model that negotiated a slightly sloped surface $(γ<0.019rad)$. With no hip actuation, our passive dynamic walking model was capable of producing a chaotic locomotive pattern when the ramp angle was $0.01839rad<γ<0.0190rad$. Systematic alterations in hip actuation resulted in rapid transition to any locomotive pattern available in the chaotic attractor and induced stability at ramp angles that were previously considered unstable. Our results detail how chaos can be used as a control scheme for locomotion.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2006;129(2):223-230. doi:10.1115/1.2486060.

Total replacement of the glenohumeral joint provides an effective means for treating a variety of pathologies of the shoulder. However, several studies indicate that the procedure has not yet been entirely optimized. Loosening of the glenoid component remains the most likely cause of implant failure, and generally this is believed to stem from either mechanical failure of the fixation in response to high tensile stresses, or through osteolysis of the surrounding bone stock in response to particulate wear debris. Many computational studies have considered the potential for the former, although only few have attempted to tackle the latter. Using finite-element analysis an investigation, taking into account contact pressures as well as glenohumeral kinematics, has thus been conducted, to assess the potential for polyethylene wear within the artificial shoulder. The relationships between three different aspects of glenohumeral design and the potential for wear have been considered, these being conformity, polyethylene thickness, and fixation type. The results of the current study indicate that the use of conforming designs are likely to produce slightly elevated amounts of wear debris particles when compared with less conforming joints, but that the latter would be more likely to cause material failure of the polyethylene. The volume of wear debris predicted was highly influenced by the rate of loading, however qualitatively it was found that wear predictions were not influenced by the use of different polyethylene thicknesses nor fixation type while the depth of wearing was. With the thinnest polyethylene designs $(2mm)$ the maximum depth of the wear scar was seen to be upwards of 20% higher with a metal-backed fixation as opposed to a cemented design. In all-polyethylene designs peak polymethyl methacrylate tensile stresses were seen to reduce with increasing polyethylene thickness. Irrespective of the rate of loading of the shoulder joint, the current study indicates that it is possible to optimize glenoid component design against abrasive wear through the use of high conformity designs, possessing a polyethylene thickness of at least $6mm$.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2006;129(2):231-239. doi:10.1115/1.2486107.

Joint injuries during sporting activities might be reduced by understanding the extent of the dynamic motion of joints prone to injury during maneuvers performed in the field. Because instrumented spatial linkages (ISLs) have been widely used to measure joint motion, it would be useful to extend the functionality of an ISL to measure joint motion in a dynamic environment. The objectives of the work reported by this paper were to (i) design and construct an ISL that will measure dynamic joint motion in a field environment, (ii) calibrate the ISL and quantify its static measurement error, (iii) quantify dynamic measurement error due to external acceleration, and (iv) measure ankle joint complex rotation during snowboarding maneuvers performed on a snow slope. An “elbow-type” ISL was designed to measure ankle joint complex rotation throughout its range ($±30deg$ for flexion/extension, $±15deg$ for internal/external rotation, and $±15deg$ for inversion/eversion). The ISL was calibrated with a custom six degree-of-freedom calibration device generally useful for calibrating ISLs, and static measurement errors of the ISL also were evaluated. Root-mean-squared errors (RMSEs) were $0.59deg$ for orientation (1.7% full scale) and $1.00mm$ for position (1.7% full scale). A custom dynamic fixture allowed external accelerations $(5g$, $0–50Hz)$ to be applied to the ISL in each of three linear directions. Maximum measurement deviations due to external acceleration were $0.05deg$ in orientation and $0.10mm$ in position, which were negligible in comparison to the static errors. The full functionality of the ISL for measuring joint motion in a field environment was demonstrated by measuring rotations of the ankle joint complex during snowboarding maneuvers performed on a snow slope.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Soft Tissue

J Biomech Eng. 2006;129(2):240-249. doi:10.1115/1.2486179.

Fibrous tissues are characterized by a much higher stiffness in tension than compression. This study uses microstructural modeling to analyze the material symmetry of fibrous tissues undergoing tension and compression, to better understand how material symmetry relates to the distribution of tensed and buckled fibers. The analysis is also used to determine whether the behavior predicted from a microstructural model can be identically described by phenomenological continuum models. The analysis confirms that in the case when all the fibers are in tension in the current configuration, the material symmetry of a fibrous tissue in the corresponding reference configuration is dictated by the symmetry of its fiber angular distribution in that configuration. However, if the strain field exhibits a mix of tensile and compressive principal normal strains, the fibrous tissue is represented by a material body which consists only of those fibers which are in tension; the material symmetry of this body may be deduced from the superposition of the planes of symmetry of the strain and the planes of symmetry of the angular fiber distribution. Thus the material symmetry is dictated by the symmetry of the angular distribution of only those fibers which are in tension. Examples are provided for various fiber angular distribution symmetries. In particular, it is found that a fibrous tissue with isotropic fiber angular distribution exhibits orthotropic symmetry when subjected to a mix of tensile and compressive principal normal strains, with the planes of symmetry normal to the principal directions of the strain. This anisotropy occurs even under infinitesimal strains and is distinct from the anisotropy induced from the finite rotation of fibers. It is also noted that fibrous materials are not stable under all strain states due to the inability of fibers to sustain compression along their axis; this instability can be overcome by the incorporation of a ground matrix. It is concluded that the material response predicted using a microstructural model of the fibers cannot be described exactly by phenomenological continuum models. These results are also applicable to nonbiological fiber–composite materials.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2006;129(2):250-258. doi:10.1115/1.2486225.

A strain energy function for finite deformations is developed that has the capability to describe the nonlinear, anisotropic, and asymmetric mechanical response that is typical of articular cartilage. In particular, the bimodular feature is employed by including strain energy terms that are only mechanically active when the corresponding fiber directions are in tension. Furthermore, the strain energy function is a polyconvex function of the deformation gradient tensor so that it meets material stability criteria. A novel feature of the model is the use of bimodular and polyconvex “strong interaction terms” for the strain invariants of orthotropic materials. Several regression analyses are performed using a hypothetical experimental dataset that captures the anisotropic and asymmetric behavior of articular cartilage. The results suggest that the main advantage of a model employing the strong interaction terms is to provide the capability for modeling anisotropic and asymmetric Poisson’s ratios, as well as axial stress–axial strain responses, in tension and compression for finite deformations.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2006;129(2):259-272. doi:10.1115/1.2540804.

Viscoelastic properties of soft tissues and hydropolymers depend on the strength of molecular bonding forces connecting the polymer matrix and surrounding fluids. The basis for diagnostic imaging is that disease processes alter molecular-scale bonding in ways that vary the measurable stiffness and viscosity of the tissues. This paper reviews linear viscoelastic theory as applied to gelatin hydrogels for the purpose of formulating approaches to molecular-scale interpretation of elasticity imaging in soft biological tissues. Comparing measurements acquired under different geometries, we investigate the limitations of viscoelastic parameters acquired under various imaging conditions. Quasi-static (step-and-hold and low-frequency harmonic) stimuli applied to gels during creep and stress relaxation experiments in confined and unconfined geometries reveal continuous, bimodal distributions of respondance times. Within the linear range of responses, gelatin will behave more like a solid or fluid depending on the stimulus magnitude. Gelatin can be described statistically from a few parameters of low-order rheological models that form the basis of viscoelastic imaging. Unbiased estimates of imaging parameters are obtained only if creep data are acquired for greater than twice the highest retardance time constant and any steady-state viscous response has been eliminated. Elastic strain and retardance time images are found to provide the best combination of contrast and signal strength in gelatin. Retardance times indicate average behavior of fast $(1–10s)$ fluid flows and slow $(50–400s)$ matrix restructuring in response to the mechanical stimulus. Insofar as gelatin mimics other polymers, such as soft biological tissues, elasticity imaging can provide unique insights into complex structural and biochemical features of connectives tissues affected by disease.

Commentary by Dr. Valentin Fuster

### TECHNICAL BRIEFS

J Biomech Eng. 2006;129(2):273-278. doi:10.1115/1.2540836.

Background: Patient-specific computational fluid dynamics (CFD) models derived from medical images often require simplifying assumptions to render the simulations conceptually or computationally tractable. In this study, we investigated the sensitivity of image-based CFD models of the carotid bifurcation to assumptions regarding the blood rheology. Method of Approach: CFD simulations of three different patient-specific models were carried out assuming: a reference high-shear Newtonian viscosity, two different non-Newtonian (shear-thinning) rheology models, and Newtonian viscosities based on characteristic shear rates or, equivalently, assumed hematocrits. Sensitivity of wall shear stress (WSS) and oscillatory shear index (OSI) were contextualized with respect to the reproducibility of the reconstructed geometry, and to assumptions regarding the inlet boundary conditions. Results: Sensitivity of WSS to the various rheological assumptions was roughly $1.0dyn∕cm2$ or 8%, nearly seven times less than that due to geometric uncertainty ($6.7dyn∕cm2$ or 47%), and on the order of that due to inlet boundary condition assumptions. Similar trends were observed regarding OSI sensitivity. Rescaling the Newtonian viscosity based on time-averaged inlet shear rate served to approximate reasonably, if overestimate slightly, non-Newtonian behavior. Conclusions: For image-based CFD simulations of the normal carotid bifurcation, the assumption of constant viscosity at a nominal hematocrit is reasonable in light of currently available levels of geometric precision, thus serving to obviate the need to acquire patient-specific rheological data.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2006;129(2):279-283. doi:10.1115/1.2540860.

Estimating material parameters is an important part in the study of soft tissue mechanics. Computational time can easily run to days, especially when all available experimental data are taken into account. The material parameter estimation procedure is examplified on a set of homogeneous simple shear experiments to estimate the orthotropic constitutive parameters of myocardium. The modification consists of changing the traditional least-squares approach to a weighted least-squares. This objective function resembles a $L2$-norm type integral which is approximated using Gaussian quadrature. This reduces the computational time of the material parameter estimation by two orders of magnitude.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2006;129(2):284-288. doi:10.1115/1.2540892.

Given the tolerance of the right heart circulation to mild regurgitation and gradient, we study the potential of using motionless devices to regulate the pulmonary circulation. In addition, we document the flow performance of two mechanical valves. A motionless diode, a nozzle, a mechanical bileaflet valve, and a tilting disk valve were tested in a pulmonary mock circulatory system over the normal human range of pulmonary vascular resistance (PVR). For the mechanical valves, regurgitant fractions (RFs) and transvalvular pressure gradients were found to be weak functions of PVR. On the low end of normal PVR, the bileaflet and tilting disk valves fluttered and would not fully close. Despite this anomaly, the regurgitant fraction of either valve did not change significantly. The values for RF and transvalvular gradient measured varied from 4 to 7% and $4to7mmHg$, respectively, at $5lpm$ for all tests. The diode valve was able to regulate flow with mild regurgitant fraction and trivial gradient but with values higher than either mechanical valve tested. Regurgitant fraction ranged from 2 to 17% in tests extending from PVR values of $1to4.5mmHg∕lpm$ at $5lpm$ and with concomitant increases in gradient up to $17mmHg$. The regurgitant fraction for the nozzle increased from 2 to 23% over the range of PVR with gradients increasing to $18mmHg$. The significant findings were: (1) the mechanical valves controlled regurgitation at normal physiological cardiac output and PVR even though they failed to close at some normal values of PVR and showed leaflet flutter; and (2) it may be possible to regulate the pulmonary circulation to tolerable levels using a motionless pulmonary valve device.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2006;129(2):289-293. doi:10.1115/1.2540926.

Background: The use of artificial bone analogs in biomechanical testing of orthopaedic fracture fixation devices has increased, particularly due to the recent development of commercially available femurs such as the third generation composite femur that closely reproduce the bulk mechanical behavior of human cadaveric and∕or fresh whole bone. The purpose of this investigation was to measure bone screw pullout forces in composite femurs and determine whether results are comparable to cadaver data from previous literature. Method of Approach: The pullout strengths of 3.5 and $4.5mm$ standard bicortical screws inserted into synthetic third generation composite femurs were measured and compared to existing adult human cadaveric and animal data from the literature. Results: For $3.5mm$ screws, the measured extraction shear stress in synthetic femurs $(23.70–33.99MPa)$ was in the range of adult human femurs and tibias $(24.4–38.8MPa)$. For $4.5mm$ screws, the measured values in synthetic femurs $(26.04–34.76MPa)$ were also similar to adult human specimens $(15.9–38.9MPa)$. Synthetic femur results for extraction stress showed no statistically significant site-to-site effect for 3.5 and $4.5mm$ screws, with one exception. Overall, the $4.5mm$ screws showed statistically higher stress required for extraction than $3.5mm$ screws. Conclusions: The third generation composite femurs provide a satisfactory biomechanical analog to human long-bones at the screw-bone interface. However, it is not known whether these femurs perform similarly to human bone during physiological screw “toggling.”

Commentary by Dr. Valentin Fuster