The mechanics of the knee are complex and dependent on the shape of the articular surfaces and their relative alignment. Insight into how anatomy relates to kinematics can establish biomechanical norms, support the diagnosis and treatment of various pathologies (e.g. patellar maltracking) and inform implant design. Prior studies have used correlations to identify anatomical measures related to specific motions. The objective of this study was to describe relationships between knee anatomy and tibiofemoral (TF) and patellofemoral (PF) kinematics using a statistical shape and function modeling approach. A principal component (PC) analysis was performed on a 20-specimen dataset consisting of shape of the bone and cartilage for the femur, tibia and patella derived from imaging and six-degree-of-freedom TF and PF kinematics from cadaveric testing during a simulated squat. The PC modes characterized links between anatomy and kinematics; the first mode captured scaling and shape changes in the condylar radii and their influence on TF anterior-posterior translation, internal-external rotation, and the location of the femoral lowest point. Subsequent modes described relations in patella shape and alta/baja alignment impacting PF kinematics. The complex interactions described with the data-driven statistical approach provide insight into knee mechanics that is useful clinically and in implant design.
Complications in the patellofemoral (PF) joint of patients with total knee replacements include patellar subluxation and dislocation, and remain a cause for revision. Kinematic measurements to assess these complications and evaluate implant designs require the accuracy of dynamic stereo-radiographic systems with 3D-2D registration techniques. While tibiofemoral kinematics are typically derived by tracking metallic implants, PF kinematic measurements are difficult as the patellar implant is radiotransparent and a representation of the resected patella bone requires either pre-surgical imaging and precise implant placement or post-surgical imaging. Statistical shape models (SSMs), used to characterize anatomic variation, provide an alternative means to obtain the representation of the resected patella for use in kinematic tracking. Using a virtual platform of a stereo-radiographic system, the objectives of this study were to evaluate the ability of an SSM to predict subject-specific 3D implanted patellar geometries from simulated 2D image profiles, and to formulate an effective data collection methodology for PF kinematics by considering accuracy for a variety of patient pose scenarios. An SSM of the patella was developed for 50 subjects and a leave-one-out approach compared SSM-predicted and actual geometries; average 3D errors were 0.45±0.07 mm (mean ± standard deviation), which is comparable to the accuracy of traditional segmentation. Further, initial imaging of the patella in five unique stereo radiographic perspectives yielded the most accurate representation. The ability to predict the remaining patellar geometry of the implanted PF joint with radiographic images and SSM, instead of CT, can reduce radiation exposure and streamline in vivo kinematic evaluations.
The focus of this study is to improve the operation of a biaxial tensile test fixture for use in characterizing hyperelastic materials. The test fixture in this study was constructed based on a design developed by Brieu, et al. [1]. Due to a combination of manufacturing and design issues the original test fixture was not able to provide acceptably accurate stressstrain data. Based on visual inspection of the machine during operation, it was hypothesized that the as-built system over-constrained certain components, which resulted in binding of the specimen grips. This binding made large displacement tests inaccurate and added to the error introduced in obtaining a 1:1 equibiaxial load ratio on a test specimen. A minimum constraint design (MCD) analysis was conducted in order to establish the degree to which the original design was over-constrained, and to ensure that the newly proposed system would function properly. Due to symmetry, the design analysis of the test fixture was simplified to one half of the machine. A modified design of the test fixture was subsequently developed and built to satisfy the minimum constraint design theory. Modifications include an alignment shaft, spherical bushings, and a planar alignment plate. The new fixture design exhibits no binding and an improved biaxial load ratio. Previously, only load ratios of 0.85:1 had been achieved, but the new design achieves the desired equibiaxial loading. Tighter component tolerances have also reduced backlash when reversing from tension to compression during a test, improving the ability to obtain load-displacement data for both the load and unload paths of a specimen.
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