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The primary objective of this study is to distinguish between mobile bearing and fixed bearing posterior stabilized knee prostheses in the mechanics performance using the finite element simulation. Quantifying the relative mechanics attributes and survivorship between the mobile bearing and the fixed bearing prosthesis remains in investigation among researchers. In the present study, 3-dimensional computational model of a clinically used mobile bearing PS type knee prosthesis was utilized to develop a finite element and dynamic simulation model. Combination of displacement and force driven knee motion was adapted to simulate a flexion motion from 0° to 135° with neutral, 10°, and 20° internal tibial rotation to represent deep knee bending. Introduction of the secondary moving articulation in the mobile bearing knee prosthesis has been found to maintain relatively low shear stress during deep knee motion with tibial rotation.

Characterizing the relative performance between mobile bearing and fixed bearing knee prosthesis remains seen as a difficult task as the previous short-term and mid-term clinical studies disable to observe any evidence of superiority of one design over another. The aim of the present study is to characterize the mechanics comparison between both designs of prosthesis during deep flexional motion with tibial rotation. Three dimensional (3D) FE model of clinically used mobile bearing posterior stabilized (PS) prosthesis was developed from its CAD data. Explicit finite element model was used to simulate the dynamic loaded deep flexional motion from 0 to 135° with neutral and 10° tibial rotation. Fixed bearing prosthesis was represented by fixing the tibial insert to the tibial component. The fixed bearing design was found relatively sensitive to flexion motion and tibial rotation in terms of contact area and maximum shear stress as compared to the mobile bearing design. Tibial rotation increased the peak value of maximum shear stress up to 58 MPa for the fixed bearing, on the contrary, the mobile bearing maintained the peak value of maximum shear stress at 31 MPa even with tibial axial rotation. The influence of post-cam design was also discussed in this study. The mobile bearing has an ability to maintain conformity and relatively low shear stress during very deep flexion with tibial axial rotation in comparison to the fixed bearing.

The objective of this study is to investigate the effect of the tensile force ratio between the two extensor muscles for the hip joint on the forces acting on the knee joint. We have created a 2D model of lower limbs and have performed some simulations to introduce the forces acting on the knee joint for various daily activities. Our simulation results demonstrated good agreements with the in vivo data available in the literature except that the tangential components of knee joint force became pretty larger than those from the in vivo data at high knee flexion angle. We considered that the discrepancy was attributed to the fact that we had assumed the hamstrings force is equal to the gluteus maximus force in order to solve an indeterminate problem, thereby introducing too large values of the hamstrings force. Then we expected that the above discrepancy could be eliminated if we change the tensile force ratio between the hamstrings and the gluteus maximus basing upon a certain biomechanical criterion. Thus we changed our model so that we could introduce the knee joint forces as a function of the above mentioned tensile force ratio. Simulation was performed for the various tensile ratio values and it was found that the knee joint force was sensitively affected by the tensile ratio and the above discrepancy could be eliminated if the ratio value was appropriately chosen. The subject of our future study is to find out the way how to determine the value of the synergetic muscles' force ratio to best fit the calculated joint force to the in vivo data with reflecting a biological rationality.

The objective of this study was to set forth guidelines for designing such an artificial knee joint that can make deep flexion. To this respect, we performed 2D model simulation for deep squatting in which the conventional posterior stabilized knee prosthesis was fitted. In the simulation we introduced a newly defined slip ratio in order to describe an over slipping phenomenon; the slipping and the rolling directions were mutually reversing. Then, the results demonstrated that the slip ratio was over 1 at high and deep flexion, indicating that the femoral rollback did not occur even though the post and the cam contacted. The relation also indicated that the conventional post-cam mechanism would not work appropriately at high/deep flexion. To overcome the above, and to promote femoral rollback even at high/deep flexion, we proposed a new design of the post-cam mechanism, which we called as a ball-ditch mechanism.

This paper presents the experimental results of photoelasticity for determining the magnitude and distribution of stresses on the polyethylene insert of the knee prostheses designed for high/deep knee flexion. The prostheses used in our experiment were a conventional posterior stabiliser knee, Scorpio NRG (Non-Restricted Geometry, Stryker Co., USA) and CFK (Complete Flexion Knee, Japan Medical Material Co., Japan), which we have developed to enable the patient to make a complete knee flexion (i.e. seiza in Japanese). Prior to the photoelastic experiment, we had introduced the forces exerted on the knee joint during ascending from deep knee flexion. Here we took squatting as an example of deep knee flexion instead of seiza because ascending from seiza contains complex modalities. The introduced data in terms of knee angles and joint forces were applied to the photoelastic models. The results demonstrated that after the knee angle became larger than 90°, the shear stresses on the post and cam of NRG were higher than those on the ball and socket of CFK. We conclude that the design and the configuration of CFK is acceptable at deep knee flexion from a load-bearing viewpoint.

In our previous study, tensile tests on ligament and fascicle from swine hind limbs revealed that the tangent modulus in the stress-strain curve of the ligaments, i.e. stiffness, was higher than that of the fascicles. Since the ligament is a composite of fascicles and ground substance whose stiffness is almost negligible, our finding was contrary to a current notion about the strength of materials that a composite can never be stronger than its constituent elements. To answer this puzzling question, we hypothesized that the fascicles' stiffness is not uniform along the longitudinal axis, and that because the weaker portions are mainly elongated during tensile tests, their stiffness is underestimated. During tensile testing on ligament, the weaker portions of fascicles are reinforced by the stronger portions of the adjoining fascicles. In this study, to confirm that our hypothesis hold, we performed tensile tests on fascicles, thereby finding that fascicles do not elongate uniformly. Next, we developed a continuous elastic model, by taking into account a fascicle's kinematic non-uniformity along its longitudinal direction and mechanical interaction between fascicles and the ground substance. Simulation results demonstrated the inverse characteristics between ligaments and fascicles. Furthermore, the results helped us to understand ligament's failure mechanism.

In this paper, we propose three ideas to improve a kinematic estimation algorithm for total knee arthroplasty. The first is a two-step estimation algorithm that improves estimation accuracy by excluding certain assumptions needed for the pattern matching algorithm reported by Banks and Hodge. The second is incorporating a 3D geometric articulation model into the algorithm to improve estimation accuracy substantially for the depth translation, and to introduce contact points' trajectories between the articular surfaces. The third is an algorithm to process estimation even when the silhouettes of two components overlap. To assess our algorithm's potential for clinical application, we carried out two experiments. First, we used a robot to position the prosthesis. Estimation accuracy was checked by comparing input data to the robot with the estimates from X-ray photographs. Incorporating our articulation model remarkably reduced the error in the depth translation. Next, we performed a clinical assessment by applying the algorithm and articulation model to fluoroscopy images of a patient who had recently had TKA.

The objective of this study is to analyze contact conditions of the posterior stabilizer (PS) type knee prosthesis at level walking and ascending stairs using a 3D mathematical model. Our 3D model made it possible to reproduce comprehensive knee motion, since it included not only the tibiofemoral joint but also the patellofemoral joint. In the model, the forces of major muscles, ligaments and tendons were all taken into account. If we input the data of muscle forces, floor reaction forces and knee flexion angles into the model, then we could determine such variables as the relative positions between the two components, contact points and stresses between the articulating surfaces and valusvalgus and internal-external rotation angles between the tibio-femoral components. Simulation was conducted and the following results were introduced. On the patellofemoral joint, the maximum contact stress at ascending stairs was 2 times higher that that at level walking. On the tibiofemoral joint, the maximum contact stress at ascending stairs was 1.7 times higher that that at level walking. The post/cam contact occurred only at ascending stairs and the contact stress reached as much as 67.1 MPa.

Knee ligaments, especially the cruciate ligaments, are the most commonly injured in accidents or during sports activities. Therefore to measure mechanical property of ligaments can yield significant information not only for preventing its damages but also for repairing or reconstructing it. Previously we performed tensile test on bone-ligament-bone complex and fascicle, then comparing experimental tangent moduli between fascicles and ligaments showed unexpected resultant. Although ligaments are basically composed of collagen fascicles with a tangent modulus is almost negligible, the tangent modulus of ligaments is higher than that of fascicles. To investigate the reason for this, we paid special attention to its hierarchical structure and existence of ground substance between fascicles. To consider this structure, we introduced hypothesis that the fascicles have non-uniformity, and strong and weak portions are reinforced by adjoining fascicles. Furthermore ground substance contributes the interaction between fascicles to increase stiffness of tissues. As a result, tangent modulus of ligaments is higher than fascicles'. To clarify the difference of the tissue's tangent moduli, we developed a two-dimensional continuous model of the ligaments composed of fascicles bundle by taking its hierarchical structure into account. We incorporated a fascicle's transverse anisotropy, kinematic non-uniformity along a fascicle and mechanical interaction between fascicles due to ground substance into the model. Simulation was performed by using Finite Element Method. The results and subsequent stress-strain curves reproduced the above mentioned characteristic and therefore we might clarify the micro failure mechanism of the ligament.

We have developed a technique for estimating 3D motion of knee prosthesis from its 2D perspective projections. As Fourier descriptors were used for compact representation of library templates and contours extracted from the prosthetic X-ray images, the entire silhouette contour of each prosthetic component was required. This caused such a problem as our algorithm did not function when the silhouettes of tibio and femoral components overlapped with each other. Here we planned a novel method to overcome it ; which was processed in two steps. First, the missing part of silhouette contour due to overlap was interpolated using a free-formed curvature such as Bezier. Then the first step position/orientation estimation was performed. In the next step, a clipping window was set in the projective coordinate so as to separate the overlapped silhouette drawn using the first step estimates. After that the localized library whose templates were clipped in shape was prepared and the second step estimation was performed. Computer model simulation demonstrated sufficient accuracies of position/orientation estimation even for overlapped silhouettes ; equivalent to those without overlap.

The object of this study is to introduce contact points' trajectories on the articulating surfaces and kinematics of knee prosthesis under various walking conditions for various types of prosthesis to help to design new prosthesis. Since in vivo measurement of knee prosthetic articulation is almost impossible, we created a 3D mathematical model consisted of the tibio-femoral and patello-femoral compartments of artificial knee joint. As the model could reproduce the comprehensive and sophisticated motion of a knee prosthesis, it is possible to generate and analyze various walking conditions with various types of knee prostheses and hence reproduce joint rotation mismatch, excessive deviation of contact points and a loft-off between the media and lateral sides of the joint.

We have developed a technique for estimating 3D motion of knee prosthesis from its 2D perspective projections. As Fourier descriptors were used for compact representation of library templates and contours extracted from the prosthetic X-ray images, the entire silhouette contour of each prosthetic component was required. This caused such a problem as our algorithm did not function when the silhouettes of tibio and femoral components overlapped with each other. Here we planned a novel method to overcome it; which was processed in two steps. First, the missing part of silhouette contour due to overlap was interpolated using a free-formed curvature such as Bezier. Then the first step position/orientation estimation was performed. In the next step, a clipping window was set in the projective coordinate so as to separate the overlapped silhouette drawn using the first step estimates. After that the localized library whose templates were clipped in shape was prepared and the second step estimation was performed. Computer model simulation demonstrated sufficient accuracies of position/orientation estimation even for overlapped silhouettes; equivalent to those without overlap.

We have developed a technique of pattern matching algorithm for estimating pose/position of each tibial and femoral component respectively from its 2 D perspective projection. Yet our pattern matching method should be objected to assess knee prosthetic kinematics and to introduce the contact points' trajectories between the articulating surfaces in order to check and predict the friction and wear conditions on the articulating surfaces. To this respect, the following two kinds of simulation studies were carried out; One was to estimate relative pose/position between the femoral and the tibial components, and the other was to incorporate the 3 D mathematical model of prosthetic articulation into our pattern matching algorithm. The simulation results demonstrated that the estimation accuracy for a relative motion was equivalent to those for a single component, and the 3 D model could not only introduce the contact points' trajectories between articulating surfaces but also improve the estimation accuracies especially for the z-translation.

The object of this study is to establish in vivo measurement system of strain distribution on the human knee ligaments through two processes ; One is to perform in vivo measurement of ligament's deformation and the other is to introduce strain distribution on the ligament by applying the deformation data to an FEM model. This paper concerns with the first process. To solve technical and ethical problems subject to living body, we planned to use less invasive arthroscopy for a measurement. We applied such theory to the arthroscopic images that reconstructs a 3 D shape of an object from its gray scale image. In the experiment, an industrial endoscope was used for its less aberration instead of the conventional arthroscopy. All parametric data necessary for calculation were introduced from the calibration image. The accuracy of our method was checked from known subjects. After that, the anterior cruciate ligament (ACL) of swine hind leg was used for the measurement. Femur and tibia were set on the specially designed fixture. The pictures of the ACL were taken at various knee flexion angles. Three-dimensional coordinal data were obtained by the level set method from the brightness data on the processed image of the ACL. Then three-dimensional shape of the ACL surface was reconstructed.

For the optimal design of ball bearings, measurement and its elucidation of complex three-dimensional ball motion are very essential. Only the way to detect ball motion in a ball bearing is the magnetic measuring method, developed by Kawakita et al.. However, due to the considerable effect of errors in the measurement, the angular velocity components of ball rotation analyzed by their method lead to problems on the accuracy in analyzing the contact angle and spin moment of the ball. In this report, by applying the magnetic measuring method to the ball bearing under the axial load, the main causes of error affecting the detection output were clarified. Also, using frequency analysis, a simplified method was proposed to analyze the angular velocity components of the ball rotation. Finally, evaluation and examination were carried out about the practicality of our method.

We have developed a technique for estimating 3D motion of knee prosthesis from its 2D perspective projections. Our primary studies of computer model simulation and experiments on the CCD images of prosthesis demonstrated sufficient accuracies of position/orientation estimation for knee prosthesis. Then we changed over our project from the basic analyses and experiments to the countermeasurements against various problems which could possibly happen at clinical application. Among them, the following problems and their solutions are stated in this paper : Miss estimation for an orientations of a tibial compartment due to its symmetry-like shape, effects of image quality on the estimation accuracies. It is also ahown an idea how to improve the estimation accuracy of translation along the direction perpendicular to the image plane.

In this paper, the measurement of 3D motion from 2D perspective projections of knee prosthesis is described. The technique reported by Banks and Hodge was further developed in this study. The estimation was performed in two steps. The first-step estimation was performed on the assumption of orthogonal projection. Then, the second-step estimation was subsequently carried out based upon the perspective projection to accomplish more accurate estimation. The simulation results have demonstrated that the technique archived sufficient accuracies of position/orientation estimation for prosthetic kinematics. Then we applied our algorithm to the CCD images, thereby examining the influences of various artifacts, possibly incorporated through an imaging process, on the estimation accuracies. We found that accuracies in the experiment were influenced mainly by the geometric discrepancies between the prosthesis component and computer generated model and by the spacial inconsistencies between the coordinate axes of the positioner and that of the computer model. However, we verified that our algorithm could achieve proper and consistent estimation even for the CCD images.

Tensile tests were performed on bone-ligament-bone (BLB) units, sections of ligament, and individual collagen fascicles all from the knees of swine hind legs. A universal testing machine was used for the tensile tests of the BLB units. A specially designed test apparatus was used for the tensile tests of ligament sections and fascicles. The strain values were calculated from the elongation values recorded by a video camera. The results showed that the BLB's stiffness was greatest, followed by the fascicles and the ligament sections. The results are contrary to the popular notion that because the ligament is composed of collagen fascicles in a matrix whose stiffness is almost negligible, the ligament should not be stiffer than the fascicles that compose it. The stiffness might have been caused by mechano-chemical interactions between fascicles and matrix, or contributions from the membranous septum that combines fascicles.

Large and highly variable deformations of the ACL cannot be adequately quantified by one-dimensional and/or localized measurements. Since the complex anatomy of the ACL makes uniform loading of all fiber bundles almost impossible, strains on specific portions being tested are considerably altered during knee movement. To observe the ACL's entire surface, we propose a photoelastic coating method. A simulator jig was used to allow a natural motion of the knee whose medial and lateral femoral bone parts were removed in order to expose the ACL for observation. The simulator jig with the knee was mounted on a universal stand which allows tilt and swivel rotations, so that the exposed ACL might be viewed from any direction. Measurements were performed on the strain distributions over the ACL at various knee angles. The panoramic images of the photoelastic fringe patterns yielded significant results. Special attention was paid for insight into the relation between strain distribution and the directions of fiber run.

A complete understanding of joint biomechanics is important in the diagnosis of joint disorders, in the quantitative assessment of treatment outcomes, and in the improvement of prosthetic devices. Yet, there is little data on high flexion activities, despite the fact that these activities are considered not only crucial to people in Asia and the Middle East, but also necessary to people in the West for improving their quality of life. Thus, we planned to invite investigators to contribute original research articles as well as review articles that will stimulate the continuing efforts to develop a thorough knowledge base for biomechanics of the knee joint during demanding activities of daily living. To this end, the special issue has been posted online on the journal's website, in which the following articles are offered for public viewing. “Squatting-related tibiofemoral shear reaction forces and a biomechanical rationale for femoral component loosening” by A. Thambyah and J. Fernandez deduces that the rapid reversal of the tibiofemoral shear force from the posterior to the anterior direction from knee extension to high flexion, respectively, may cause the femoral component loosening of high flexion implants. In this article, the results from biomechanical and finite element analyses clearly explain the relationship between the femoral loosening and frequent squatting. “The effect of malrotation of tibial component of total knee arthroplasty on tibial insert during high flexion using a finite element analysis” by K. Osano et al. pertains to the finite element analysis to study the effect of malrotation of tibial component on tibial insert during squatting maneuver. Three different tibial rotations (15° internal, normal, 15° external) were set as the initial conditions for analyses. Then it was found that when the tibia was internally rotated the insert had such high stress as 160% of normal position. It was concluded that internal malrotation should be avoided as much as possible. “Biomechanical considerations in the design of high-flexion total knee replacements” by C.-K. Cheng et al. is the review article in which the authors share several experiences in the previous literature for biomechanical and kinematic performance of knee prostheses under the specific demand for high flexion of knee joint. For achieving deep knee flexion, some ideas are discussed such as a rounded and convex lateral plateau and a shallower medial plateau on the tibial surface, a rounded postcam contact surface, and an asymmetric curve-on-curve cam shape. “Dynamic finite element analysis of mobile bearing type knee prosthesis under deep flexional motion” by M. A. M. Anuar et al. also pertains to the finite element analysis to distinguish the mechanical performances between mobile bearing and fixed bearing posterior stabilized knee prostheses. Then this study shows the decomposition of multidirectional motion to unidirectional kinematics of femoral-insert and insert-tray articulating interfaces in mobile bearing TKA in maintaining lower stress at tibial condylar. “Modelling and analysis on biomechanical dynamic characteristics of knee flexion movement under squatting” by J. Wang et al. represents the results from the synchronal measurements of both kinematics and contact stresses, respectively, for the tibiofemoral and patellofemoral joints. Dynamic finite element analysis and in vitro experiment were carried out for the range from full extension to squatting of the knee. Comparative study between their results and the literature data is comprehensive. We hope that the above articles are of interest to the reader and more articles would be incorporated in this special issue. Shunji Hirokawa Kazuo Kiguchi Mitsugu Todo Bharat Mody Ashvin Thambyah

Large variable deformations of the ligament cannot be adequately quantified by one-dimensional and/or localized measurements. To obtain accurate measurement of non-uniform strains over the entire surface of anterior cruciate ligament (ACL), we used a photoelastic coating technique and a method that allowed us to photograph an ACL around its longitudinal axis. A cadaver knee was modified to expose its ACL for observation, and the ligament was then coated with a photoelastic material. The knee was locked in a jig that allowed simulation of natural knee motion. The jig containing the knee was then mounted on a stand, which allowed the exposed ACL to be photographed from any angle around its longitudinal axis while set at a chosen degree of knee flexion. The jig itself was rotated on its stand so as to obtain a panoramic view of the ACL at a given knee angle. The obtained images of the photoelastic fringe patterns yielded significant information for understanding how the strain distributions along the fiber bundles change in association with knee motion. From the results we obtained using the photoelastic measuring method, we reached the following conclusions. Reciprocal functioning between the anterior and the posterior bundles from extension to flexion of the knee does occur. Strain distribution is not uniform even along the same bundle. The strain behavior of the ACL under uniaxial tensile test does not duplicate the conditions in which the ACL is damaged during knee motion. The differences in strains on the ACL under active and passive knee motions may not be as large as those reported previously in the literature.

In this study, a constitutive equation for the ACL composite was formulated, and 3-D finite deformations and stress distributions of the ACL were calculated using a finite element method to simulate knee flexion. The mathematical model of the ACL was created by a structurally motivated phenomenological approach. It was assumed that the ACL can be ideally represented as a homogeneous hyperelastic matrix (Mooney–Rivin material) in which two families of densely distributed extensible fibers are embedded; the fibers in one family have a parallel orientation, the other fibers extend radially in eight equally spaced directions. The non-linear stress–strain characteristic exhibited by collagen fibers was represented by a tri-linear curve. Simulation was performed and the results provided some original data; the stress distribution within the ACL body as well as that over the surface, the 3-D defeormations and stress distributions of the ACL viewed from other sides in addition to those from the medial side, and the variations of the stress distribution pattern in the ACL which occured when the tibia was displaced anteriorly or posteriorly.

This paper describes a new structurally-motivated phenomenological approach. Such assumption was made that the ACL can be idealized as being composed of a homogeneous matrix in which two non-interacting families of densely distributed extensible fibers are embedded. Then a constitutive equation for the ACL composite was formulated. Using the finite element method, the visual simulations were performed in terms of three-dimensional change in shape, stress distribution and intraligaments' stresses, as a function of knee angle. Variations in stress due to tibial anterior-posterior displacements were also introduced. All the results were compared and discussed with the previous studies, thereby verifying the validity and usefulness of our model.

This paper describes a new structurally-motivated phenomenological approach. Such assumption was made that the ACL can be idealized as being composed of a homogeneous matrix in which two non-interacting families of densely distributed extensible fibers are embedded. Then a constitutive equation for the ACL composite was formulated. Using the finite element method, the visual simulations were performed in terms of three-dimensional change in shape, stress distribution and intra-ligamentary stresses, as a function of knee angle. Variations in stress due to tibial anterior-posterior displacernents were also introduced. All the results were compared and discussed with the previous studies, thereby verifying the validity and usefulness of our model.

The anterior cruciate ligament (ACL) in the human knee is known to exhibit as large as 10-20% of strain in association with knee flexion, while usual Hookean elastic materials exhibit at most 0.3%. Furthermore the complex ACL in shape deforms in highly variable manners due to the complicated three-dimensional (3-D) movements of the insertions in association with knee flexion. Such large and highly variable deformations of the ACL cannot be adequately quantified by one-dimensional (1-D) and/or localized measurements. In order to measure strains over the entire area of the ACL, the authors employed the photoelastic coating method to analyze stress on the basis of the strains. A specific kind of polyurethane possessing optically high fringe-sensitivity was found to be most suitable for measurement purposes. From the preliminary experiments, it was found that a linear relation with 99% of correlation coefficient between the birefringence order and the strain on the polyurethane film continued up to 45% of strain. This study was done in two steps. In the first step, a study was done using a unique model of a knee joint fabricated in a way which allowed deformation and strain distribution in the model ACL to be directly observed and measured by means of the photoelastic coating method. It was confirmed that the film, having negligible effect on the mechanical properties of some biological soft tissues, closely reflected the strain behavior of the tissue. Thus in the second step, the photoelastic method was employed to measure the strains on an actual ACL during free flexion-extension of the knee. A specially designed apparatus was used to allow a natural motion of the knee. The photoelastic measurements demonstrated the potential utility of the application to measuring strain distributions not only on a model ligament but also on an actual ligament. The measurement of an actual ACL led the following results: the principal strain lines well depicted its fiber directions; a reciprocal function between the anterior and posterior bundles was observed; and there was a zero-strain area on the central and posterior sides near the tibial insertion.

Highly variable deformations of a human anterior cruciate ligament (ACL) cannot be adequately quantified by one-dimensional, localized measurements and/or analyses. This paper describes a new structurally-motivated phenomenological approach. Such assumption was made that the ACL ligament can be idealized as being composed of a homogeneous matrix in which two non-interacting families of densely distributed extensible fibers are embedded. A non-linear stress-strain characteris- tic exhibited by multiple collagen fibers was represented by the multi-linear curve. Then a constitutive equation for the ACL composite was formulated. Using the finite element method, finite deformation and stress distribution of the ACL as a function of knee flexion were introduced through simulation.