Dynamic finite element analysis of mobile bearing type knee prosthesis under deep flexional motion.

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.

Deep flexion-oriented bisurface-type knee joint and its tibial rotation that attributes its high performance of flexion.

In 1989, we developed an artificial knee prosthesis that could accommodate the oriental lifestyle where people would sit more often on the floor than on a chair. The knee had a bisurface feature with an auxiliary joint of a ball and socket at the center of the posterior part aiming at an improved flexional function. The auxiliary joint functions not only to facilitate a rollback movement but also to add a rotational movement. It was investigated whether this knee prosthesis could show an internal rotation of the tibia during flexion such as the physiologic movement of the knee. The internal rotation of the tibia was evaluated for the patients who could sit on legs in the oriental style after total knee arthroplasty. The average flexion angle of patients who achieved this style of sitting was 144.1°, and the average internal rotation was estimated as 14.3° by a pattern matching method using a computer-assisted design system.

Model analysis to investigate the contribution of ground substance to ligament stiffening.

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.

A 3D kinematic estimation of knee prosthesis using X-ray projection images: clinical assessment of the improved algorithm for fluoroscopy images.

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.

A new approach for surface fitting method of articular joint surfaces.

The application of joint contact mechanics requires a precise configuration of the joint surfaces. B-Spline, and NURBS have been widely used to model joint surfaces, but because these formulations use a structured data set provided by a rectangular net first, then a grid, there is a limit to the accuracy of the models they can produce. However new imaging systems such as 3D laser scanners can provide more realistic unstructured data sets. What is needed is a method to manipulate the unstructured data. We created a parametric polynomial function and applied it to unstructured data sets obtained by scanning joint surfaces. We applied our polynomial model to unstructured data sets from an artificial joint, and confirmed that our polynomial produced a smoother and more accurate model than the conventional B-spline method. Next, we applied it to a diarthrodial joint surface containing many ripples, and found that our function's noise filtering characteristics smoothed out existing ripples. Since no formulation was found to be optimal for all applications, we used two formulations to model surfaces with ripples. First, we used our polynomial to describe the global shape of the objective surface. Minute undulations were then specifically approximated with a Fourier series function. Finally, both approximated surfaces were superimposed to reproduce the original surface in a complete fashion.