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TECHNICAL PAPERS

J Biomech Eng. 1995;117(4):373-382. doi:10.1115/1.2794195.

This paper describes the design and accuracy evaluation of a new six degree of freedom load application system for in vitro testing of the human knee joint. External loads of both polarity in all six degrees of freedom can be applied either individually or in any combination while the knee is permitted to move unconstrained in response to applied loads. The flexion/extension degree of freedom permits the full physiological range of motion. In addition to external loads, forces of the three major muscle groups (quadriceps, hamstrings, gastrocnemius) crossing the joint can be developed. Full automation and rapid convergence of loads to programmed values are achieved through a computer which feeds command signals to servo controller/electro-pneumatic servo valves. The servo valves regulate pressure to pneumatic actuators which develop the various loads. Experiments undertaken to quantify the accuracy of both load and displacement measurements reveal that errors particularly in load measurement are effectively controlled through the apparatus design.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1995;117(4):383-389. doi:10.1115/1.2794196.

A joint testing system was designed to transmit a specified motion or force to a joint in all six degrees of freedom (d.o.f.) using a spatial linkage system for position feedback. The precise reproducibility of position provided by this method of position feedback allows determination of in situ ligament forces for external joint loadings. Load on the structure of interest is calculated from six d.o.f. load cell output after the loaded position is reproduced with all other structures removed. In a test of this system, measured loads showed good agreement with applied loads.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1995;117(4):390-396. doi:10.1115/1.2794197.

Uniaxial confined compression and swelling experiments on cylindrical specimens taken either in an axial or in a radial direction from a canine lumbar annulus fibrosus are presented. The loading protocol consisted of a combination of stepwise mechanical and chemical loading. Swelling and consolidation curves of normalized displacement versus square root of normalized time did not show a dependence on site or orientation of the specimen. All stages in which height increases, namely, conditioning, swelling, and desolidation show only slight differences in these normalized curves. Consolidation is initially faster, and later slower. The transport coefficient for axial specimens is higher than for radial specimens, for consolidation e.g., 3.14 ± 1.56 10−10 m2 s−1 and 1.11 ± 0.33 10−10 m2 s−1 respectively, the biphasic aggregate moduli are 1.01 ± 0.31 MPa and 0.66 ± 0.30 MPa, respectively.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1995;117(4):397-401. doi:10.1115/1.2794198.

Extra cellular matrix, which provides physical support to epithelial and endothelial cells and to fibroblasts, also affects a number of important cell biological phenomena, such as cell motility and angiogenesis. Although type I collagen has long been recognized as the primary structural component of the extra cellular matrix, little is known about the physical properties of collagen gels. In this study, we used a servo-controlled linear actuator to impose quick stretches on dilute collagen gels. An axial strain imposed on the gel within few milliseconds resulted in a rapid development of gel tension in the direction of the strain. The gel tension then decayed toward a steady-state value within several seconds. The instantaneous gel stiffness increased and the relaxed gel stiffness decreased with the extent of gel stretching. These rheological parameters were also dependent on the density of the collagen network. Taken together the results indicated that collagen gels possess nonlinear viscoelastic properties.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1995;117(4):402-408. doi:10.1115/1.2794199.

Various studies suggest impact trauma may initially soften cartilage, damage subchondral bone, or a combination thereof. The initial damages are commonly thought due to excessive contact pressures generated on cartilage and the underlying bone. The objective of this research was to develop a small animal model for studying post-traumatic OA and to correlate contact pressures with tissue damage. Blunt insult was graded by dropping a rigid mass onto the hyperflexed hind limb of rabbits. Contact pressure in the patello-femoral joint was measured with pressure sensitive film. One, 3, 6, and 14 days later the animals were euthanized. Damage to cartilage and the underlying bone was assessed visually and in microscopic sections. Indentation experiments were performed on the patellar cartilage with a rigid, flat probe. Contact pressures were nonuniform over the articular surfaces and a high frequency of surface fissures were generated on the lateral facet in severe insults. The appearance of surface fissures correlated better with the magnitude of contact pressure gradients in the damage zone than the magnitude of contact pressures on the facet, per se. Blunt trauma causing surface fissures resulted in a measurable degree of softening in the patellar cartilage, especially close to the defects. Surgical intervention of the joint to insert pressure sensitive film, however, also resulted in significant softening of the cartilage.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1995;117(4):409-413. doi:10.1115/1.2794200.

Recent studies suggest that hip padding systems reduce the incidence of hip fractures during falls. However, no data exist on the force attenuating capacity of hip pads under realistic fall impact conditions, and thus it is difficult to compare the protective merit of various pad designs. Our goal is to design a comfortable hip padding system which reduces femoral impact force in a fall below the mean force required to fracture the elderly cadaveric femur. In pursuit of this objective, we designed and constructed a hip pad testing system consisting of an impact pendulum and surrogate human pelvis. We then developed a hip pad containing a shear-thickening material which allows for shunting of the impact energy away from the femur and into the surrounding soft tissue. Finally, we conducted experiments to assess whether the surrogate pelvis accurately represents the impact behavior of the human female pelvis in a fall, and to determine whether our energy-shunting pad attenuates femoral impact force in a fall more effectively than seven available padding systems. We found the surrogate pelvis accurately represented the human female pelvis in regional variation in soft tissue stiffness, total effective stiffness and damping, and impact force attenuation provided by trochanteric soft tissues. We also found that our padding system attenuated femoral impact force by 65 percent, thereby providing two times the force attenuation of the next best system. Moreover, the energy-shunting pad was the only system capable of lowering femoral impact force well below the mean force required to fracture the elderly femur in a fall loading configuration. These results suggest that the force attenuating potential of hip pads which focus on shunting energy away from the femur is superior to those which rely on absorbing energy in the pad material. While these in-vitro results are encouraging, carefully designed prospective clinical trials will be necessary to determine the efficacy of these approaches to hip fracture prevention.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1995;117(4):414-422. doi:10.1115/1.2794202.

The slit diaphragms of renal glomerular capillaries form an ultrafiltration barrier which may be approximated as a row of cylindrical fibers of macromolecular dimensions. To describe the hindered transport of plasma proteins and other macromolecules through this barrier, we developed an approximate hydrodynamic model for spherical, Brownian particles passing through a row of infinitely long cylinders. The selectivity of the slit diaphragm was assessed by computing concentration profiles for a wide range of molecular sizes for Pe ≤ 1, where Pe is a Peclet number based on the cylinder radius. The sieving coefficient for the slit diaphragm was computed as θSD = CB /CO , where CO was the average concentration at a specified distance upstream from the cylinders (corresponding to the location of the basement membrane), and CB was the concentration far downstream (corresponding to Bowman’s space). The results of previous experimental sieving studies using rats could be accounted for approximately by postulating a wide distribution of spacings between the fibers of the slit diaphragm. Comparing the results for θSD with calculations for a model of the glomerular basement membrane suggests that the slit diaphragm is by far the more size-restrictive part of the overall barrier.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1995;117(4):423-428. doi:10.1115/1.2794203.

The Fontan operation is one in which the right heart is bypassed leaving the left ventricle to drive the blood through both the capillaries and the lungs, making it important to design an operation which is hemodynamically efficient. The object here was to relate the pressure in Fontan connections to its geometry with the aim of increasing the hemodynamically efficiency. From CT or magnetic resonance images, glass models were made of realistic atrio-pulmonary (AP) and cavo-pulmonary (CP) connections in which the right atrium and/or ventricle are bypassed. The glass models were connected to a steady flow loop and flow visualization, pressure and 3 component LDA measurements made. In the AP model the large atrium and curvature of the conduit created swirling patterns, the magnitude of which was similar to the axial velocity. This led to an inefficient flow and a subsequent large pressure loss (780 Pa). In contrast, the CP connection with a small intra-atrial chamber had reduced swirling and a significantly smaller pressure loss (400 Pa at 8 l.min) and was therefore a more efficient connection. There were, however, still pressure losses and it was found that these occurred where there was a large bending of the flow, such as from the superior vena cava to the MPA and from the MPA to the right pulmonary artery.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1995;117(4):429-441. doi:10.1115/1.2794204.

An anatomically correct finite element mesh of the right human nasal cavity was constructed from CAT scans of a healthy adult nose. The steady-state Navier-Stokes and continuity equations were solved numerically to determine the laminar airflow patterns in the nasal cavity at quiet breathing flow rates. In the main nasal passages, the highest inspiratory air speed occurred along the nasal floor (below the inferior turbinate), and a second lower peak occurred in the middle of the airway (between the inferior and middle turbinates and the septum). Nearly 30 percent of the inspired volumetric flow passed below the inferior turbinate and about 10 percent passed through the olfactory airway. Secondary flows were induced by curvature and rapid changes in cross-sectional area of the airways, but the secondary velocities were small in comparison with the axial velocity through most of the main nasal passages. The flow patterns changed very little as total half-nasal flow rate varied between resting breathing rates of 125 m/s and 200 ml/s. During expiration, the peaks in velocity were smaller than inspiration, and the flow was more uniform in the turbinate region. Inspiratory streamline patterns in the model were determined by introducing neutrally buoyant point particles at various locations on the external naris plane, and tracking their path based on the computed flow field. Only the stream from the ventral tip of the naris reached the olfactory airway. The numerically computed velocity field was compared with the experimentally measured velocity field in a large scale (20×) physical model, which was built by scaling up from the same CAT scans. The numerical results showed good agreement with the experimental measurements at different locations in the airways, and confirmed that at resting breathing flow rates, airflow through the nasal cavity is laminar.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1995;117(4):442-447. doi:10.1115/1.2794205.

Flow visualization experiments of Newtonian laminar flow through branching tubes have been performed to identify the shape of the separation surface or flow divider. The influences of Reynolds number, flow fraction into the side branch, and branch geometry on the separation surface shape have been considered. The shapes presented in this paper are formed by the intersection of the separation surface with the cross section of the parent tube. At low Re the separation surfaces are curved in a convex manner, bulging away from the opening of the side branch. Increasing Re causes the surface to become concave. At Re > 194 the surfaces can become closed for Q* > 0.3. The branch angle has no noticeable effect on the separation surface shape. The side to parent branch diameter ratio, Db /Dp , has a strong influence at low Re. As Re increases the diameter effect diminishes. Previous studies have shown no difference between the separation surfaces of T and Y type junctions at low Re. At Re = 194 there is a marked difference between the separation surfaces of T and Y bifurcations.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1995;117(4):448-454. doi:10.1115/1.2794206.

The influence of an eccentrically inserted catheter on the peristaltic pumping in a tube is investigated under long wavelength, low Reynolds number assumptions. The radially asymmetric deformation of the wall arising through an eccentrically inserted catheter is taken into consideration by choosing an appropriate bipolar coordinate system. The effect of the position and size of the catheter on pumping characteristics is studied. The best performance of pumping is noticed at a certain position of the catheter. The size of the catheter, when placed eccentrically, alters the pressure signature significantly inside the bolus, unlike the concentric case discussed by Lykoudis and Roos (1971). Further, the maximum pressure rise in one period of the peristaltic wave is observed to decrease with an increase in the eccentricity.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1995;117(4):455-461. doi:10.1115/1.2794207.

The relationship between the pattern of surface strain and the site of failure in maturing rabbit ligaments was studied in vitro. Bone-medial collateral ligament (MCL)-bone complexes of 24 female New Zealand White rabbits at 3, 6, 9 and 12 months of age (n = 6 rabbits, 12 MCLs per group) were tested in tension to failure. A video dimension analysis (VDA) system was used to map the surface strain at failure across the width and along the length of the medial side of each MCL during testing. Results showed that the highest strains were consistently located at the femoral insertion decreasing towards the midsubstance, with the highest strain occurring in the anterior portion of the MCL immediately adjacent to the femoral insertion. Strains of the complex at failure increased with rabbit maturation. The strain distribution however, did not change dramatically, even though the locations of MCL failure changed from exclusively tibial avulsion in the three month old rabbits to predominantly midsubstance failures in the 12 month old rabbits. In the six month old rabbits, there was a particular dissociation with all MCLs failing near the tibial insertion while femoral strains were apparently the highest. These results suggest two possibilities beyond that of some unknown artifacts of optical strain measurement. First, since failure sites rarely correlated with areas of maximum surface strain in this study, it seems possible that higher strains could exist deeper in the tissue, particularly at the bone-ligament interface of the tibial insertion in immature animals and somewhere within the midsubstance of the MCL in the adult. Secondly, it is possible that the ligament material may be heterogeneous.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1995;117(4):462-465. doi:10.1115/1.2794208.

Based upon the analysis of enzyme-catalyzed reactions occurring in living tissue, a model of thermal injury process is presented in which the fraction of denatured enzyme protein was taken as the indicator of thermal damage degrees. The results from this model describe the dependence of thermal damage on exposure time and temperature elevation.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1995;117(4):466-473. doi:10.1115/1.2794209.

In this paper, the dynamics of quadruped trot, gallop, and bound will be examined using a simple model for the quadruped. The body of the quadruped is modeled as a uniform bar and the legs are modeled by massless springs. It will be shown that symmetry can be used to study the locomotion of this system. Using symmetry, a technique will be developed to obtain periodic solutions for each of the gaits of the quadruped model. These periodic solutions will be computed at various speeds. The energy levels will be compared for each of the gaits. The exchange of energy between its different forms will be shown for different gaits. It will be shown that even without body flexibility, there are significant savings in energy due to gait transition from trot to gallop. The energy levels will be used to predict the trot-gallop transition speed. These results will be compared with the experimental results for horses and dogs.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1995;117(4):474-478. doi:10.1115/1.2794210.

The mid to lower cervical spine is a common site for compression related injury. In the present study, we determined the patterns of localized strain distribution in the anterior aspect of the vertebral body and in the lateral masses of lower cervical three-segment units. Miniature strain gages were mounted to human cadaveric vertebrae. Each preparation was line-loaded using a knife-edge oriented in the coronal plane that was moved incrementally from anterior to posterior to induce compression-flexion or compression-extension loading. Uniform compressive loading and failure runs were also conducted. Failure tests indicated strain shifting to “restabilize” the preparation after failure of a component. Under these various compressive loading vectors, the location which resulted in the least amount of deformation for a given force application (i.e., stiffest axis) was quantified to be in the region between 0.5–1.0 cm anterior to the posterior longitudinal ligament. The location in which line-loading produced no rotation (i.e., balance point) was in this region; it was also close to where the vertebral body strains change from compressive to tensile. Strain values from line loading in this region produced similar strains as recorded under uniform compressive loading, and this was also the region of minimum strain. The region of minimum strain was also more pronounced under higher magnitudes of loading, suggesting that as the maximum load carrying capacity is reached the stiffest axis becomes more well defined.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1995;117(4):479-484. doi:10.1115/1.2794211.

Measured in this study was the effectiveness of two types of retrofits in mitigating shocks in tennis rackets with ideally high grip fixity. The retrofits were a cushioned grip tape and a string implant device. Three types of rackets were investigated: wood, graphite composite, and metal. For low speed ball impact, neither retrofit changed significantly the magnitude and distribution of e, the coefficient of restitution on the racket heads. For moderate ball speeds impacting the rackets along the vertical centerline, three dynamic racket responses were measured: the free vibration damping based on racket head acceleration, the root-mean-square (rms) grip reaction force, and the fast Fourier transform (FFT) of the grip force. These latter experiments showed that the string implant device had a negligible effect on the three dynamic measures of racket response. However, the cushioned grip tape increased racket damping by up to 100 percent, reduced the rms grip force by about 20 percent, and reduced the magnitude of the FFT of this force by about 40 percent.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1995;117(4):485-491. doi:10.1115/1.2794212.

While mechanisms of post-traumatic osteoarthrosis are largely unknown, excessive stresses and strains generated in articular cartilage and the underlying bone may play a role. In this manuscript a technique is described for studying the impact response of a diarthrodial joint. A mathematical model of the rabbit PF joint indicated that contact pressures predicted by a quasi-static plane strain linear elastic model compared well with experimental data when Poisson’s ratio and Young’s modulus of the cartilage were 0.49 and 2 MPa, respectively. This value for the elastic modulus compared well with that obtained from elastic analysis of short-time indentation experiments on cartilage from a previous study. The model analysis also suggested that surface fissuring of patellar cartilage occurs near areas where shear stresses and tensile strains are high. Impact location on the patella significantly influenced the distributions of shear stress along the bone-cartilage interface and tensile strains in the cartilage. These results may help explain some of the mechanisms of initial tissue damage reported elsewhere. Limited experimental data are presented here but the value of such mathematical models for estimation of material properties and for analysis of damage creation is clearly demonstrated.

Commentary by Dr. Valentin Fuster

TECHNICAL BRIEFS

J Biomech Eng. 1995;117(4):492-494. doi:10.1115/1.2794213.

The objective of this note is to reexamine the static behavior of a 2-D channel conveying fluid, when the wall tension becomes small or zero at some point along the channel. In addition to the shear stress exerted by the fluid flow, we take into account restoring forces acting on the wall, such as the bending moment, the transverse shearing force, etc., which have often been neglected in the equation of equilibrium of the tube wall. Numerical results show that zero wall tension does not mean nonexistence of steady solutions. When the wall tension becomes small, it is important to derive the equation of equilibrium by taking into account those terms which have been neglected in comparison with strong effect of the wall tension in physiological vessels.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1995;117(4):495-497. doi:10.1115/1.2794214.

Determining the geometry of articular joint surfaces is critical to the study of human joint mechanics. Photographs of sliced specimens (PSS) is currently one of the most popular methods used to determine surface geometry. This study sought to improve the photographs of sliced specimens method through a new calibration technique that includes a mathematical model and a precise plastic grating (calibration grid). Simulations showed that calibration results for x and y coordinates using the new technique attained a high degree of accuracy that was independent of camera and projector lens quality, degree of parallelity between the projector and digitizing table and selection of the area to be calibrated.

Commentary by Dr. Valentin Fuster

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