[Show abstract][Hide abstract] ABSTRACT: Kneeling is required during daily living for many patients after total knee replacement (TKR), yet many patients have reported that they cannot kneel due to pain, or avoid kneeling due to discomfort, which critically impacts quality of life and perceived success of the TKR procedure. The objective of this study was to evaluate the effect of component design on patellofemoral (PF) mechanics during a kneeling activity. A computational model to predict natural and implanted PF kinematics and bone strains after kneeling was developed and kinematics were validated with experimental cadaveric studies. PF joint kinematics and patellar bone strains were compared for implants with dome, medialized dome, and anatomic components. Due to the less conforming nature of the designs, change in sagittal plane tilt as a result of kneeling at 90 degrees knee flexion was approximately twice as large for the medialized-dome and dome implants as the natural case or anatomic implant, which may result in additional stretching of the quadriceps. All implanted cases resulted in substantial increases in bone strains compared with the natural knee, but increased strains in different regions. The anatomic patella demonstrated increased strains inferiorly, while the dome and medialized dome showed increases centrally. An understanding of the effect of implant design on patellar mechanics during kneeling may ultimately provide guidance to component designs that reduces the likelihood of knee pain and patellar fracture during kneeling.
Journal of biomechanics 03/2014; · 2.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Fluoroscopic evaluation of total knee arthroplasty (TKA) has reported sudden anterior translation of the femur relative to the tibia (paradoxical anterior motion) for some cruciate-retaining designs. This motion may be tied to abrupt changes in the femoral sagittal radius of curvature characteristic of traditional TKA designs, as the geometry transitions from a large load-bearing distal radius to a smaller posterior radius which can accommodate femoral rollback. It was hypothesized that a gradually reducing radius may attenuate sudden changes in anterior-posterior motion that occur in mid-flexion with traditional discrete-radius designs. A combined experimental and computational approach was employed to test this hypothesis. A previously developed finite element (FE) model of the Kansas knee simulator (KKS), virtually implanted with multiple implant designs, was used to predict the amount of paradoxical anterior femoral slide during a simulated deep knee bend. The model predicted kinematics demonstrated that incorporating a gradually reducing radius in mid-flexion reduced the magnitude of paradoxical anterior translation between 21% and 68%, depending on the conformity of the tibial insert. Subsequently, both a dual-radius design and a modified design incorporating gradually reducing radii were tested in vitro in the KKS for verification. The model-predicted and experimentally observed kinematics exhibited good agreement, while the average experimental kinematics demonstrated an 81% reduction in anterior translation with the modified design. The FE model demonstrated sufficient sensitivity to appropriately differentiate kinematic changes due to subtle changes in implant design, and served as a useful pre-clinical design-phase tool to improve implant kinematics.
Journal of biomechanics 03/2013; · 2.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Complications of the patellofemoral (PF) joint remain a common cause for revision of total knee replacements. PF complications, such as patellar maltracking, subluxation, and implant failure, have been linked to femoral and patellar component alignment. In this study, a dynamic finite element model of an implanted PF joint was applied in conjunction with a probabilistic simulation to establish relationships between alignment parameters and PF kinematics, contact mechanics, and internal stresses. Both traditional sensitivity analysis and a coupled probabilistic and principal component analysis approach were applied to characterize relationships between implant alignment and resulting joint mechanics. Critical alignment parameters, and combinations of parameters, affecting PF mechanics were identified for three patellar designs (dome, modified dome, and anatomic). Femoral internal-external (I-E) alignment was identified as a critical alignment factor for all component designs, influencing medial-lateral contact force and anterior-posterior translation. The anatomic design was sensitive to patellar flexion-extension (F-E) alignment, while the dome, as expected, was less influenced by rotational alignment, and more by translational position. The modified dome was sensitive to a combination of superior-inferior, F-E, and I-E alignments. Understanding the relationships and design-specific dependencies between alignment parameters can aid preoperative planning, and help focus instrumentation design on those alignment parameters of primary concern.
Journal of Orthopaedic Research 07/2012; 30(7):1167-75. · 2.88 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Clinical studies demonstrate substantial variation in kinematic and functional performance within the total knee replacement (TKR) patient population. Some of this variation is due to differences in implant design, surgical technique and component alignment, while some is due to subject-specific differences in joint loading and anatomy that are inherently present within the population. Combined finite element and probabilistic methods were employed to assess the relative contributions of implant design, surgical, and subject-specific factors to overall tibiofemoral (TF) and patellofemoral (PF) joint mechanics, including kinematics, contact mechanics, joint loads, and ligament and quadriceps force during simulated squat, stance-phase gait and stepdown activities. The most influential design, surgical and subject-specific factors were femoral condyle sagittal plane radii, tibial insert superior-inferior (joint line) position and coronal plane alignment, and vertical hip load, respectively. Design factors were the primary contributors to condylar contact mechanics and TF anterior-posterior kinematics; TF ligament forces were dependent on surgical factors; and joint loads and quadriceps force were dependent on subject-specific factors. Understanding which design and surgical factors are most influential to TKR mechanics during activities of daily living, and how robust implant designs and surgical techniques must be in order to adequately accommodate subject-specific variation, will aid in directing design and surgical decisions towards optimal TKR mechanics for the population as a whole.
Journal of biomechanics 06/2012; 45(12):2092-102. · 2.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Validated computational knee simulations are valuable tools for design phase development of knee replacement devices. Recently, a dynamic finite element (FE) model of the Kansas knee simulator was kinematically validated during gait and deep flexion cycles. In order to operate the computational simulator in the same manner as the experiment, a proportional-integral-derivative (PID) controller was interfaced with the FE model to control the quadriceps actuator excursion and produce a target flexion profile regardless of implant geometry or alignment conditions. The controller was also expanded to operate multiple actuators simultaneously in order to produce in vivo loading conditions at the joint during dynamic activities. Subsequently, the fidelity of the computational model was improved through additional muscle representation and inclusion of relative hip-ankle anterior-posterior (A-P) motion. The PID-controlled model was able to successfully recreate in vivo loading conditions (flexion angle, compressive joint load, medial-lateral load distribution or varus-valgus torque, internal-external torque, A-P force) for deep knee bend, chair rise, stance-phase gait and step-down activities.
Computer Methods in Biomechanics and Biomedical Engineering 06/2012; · 1.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In vitro pre-clinical testing of total knee replacement (TKR) devices is a necessary step in the evaluation of new implant designs. Whole joint knee simulators, like the Kansas knee simulator (KKS), provide a controlled and repeatable loading environment for comparative evaluation of component designs or surgical alignment under dynamic conditions. Experimental testing, however, is time and cost prohibitive for design-phase evaluation of tens or hundreds of design variations. Experimentally-verified computational models provide an efficient platform for analysis of multiple components, sizes, and alignment conditions. The purpose of the current study was to develop and verify a computational model of a dynamic, whole joint knee simulator. Experimental internal-external and valgus-varus laxity tests, followed by dynamic deep knee bend and gait simulations in the KKS were performed on three cadaveric specimens. Specimen-specific finite element (FE) models of posterior-stabilized TKR were created from magnetic resonance images and CAD geometry. The laxity data was used to optimize mechanical properties of tibiofemoral soft-tissue structures on a specimen-specific basis. Each specimen was subsequently analyzed in a computational model of the experimental KKS, simulating both dynamic activities. The computational model represented all joints and actuators in the experimental setup, including a proportional-integral-derivative (PID) controller to drive quadriceps actuation. The computational model was verified against six degree-of-freedom patellofemoral (PF) and tibiofemoral (TF) kinematics and actuator loading during both deep knee bend and gait activities, with good agreement in trends and magnitudes between model predictions and experimental kinematics; differences were less than 1.8 mm and 2.2° for PF and TF translations and rotations. The whole joint FE simulator described in this study can be applied to investigate a wide range of clinical and research questions.
Journal of biomechanics 12/2011; 45(3):474-83. · 2.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Evaluating total knee replacement kinematics and contact pressure distributions is an important element of preclinical assessment of implant designs. Although physical testing is essential in the evaluation process, validated computational models can augment these experiments and efficiently evaluate perturbations of the design or surgical variables. The objective of the present study was to perform an initial kinematic verification of a dynamic finite element model of the Kansas knee simulator by comparing predicted tibio- and patellofemoral kinematics with experimental measurements during force-controlled gait simulation. A current semiconstrained, cruciate-retaining, fixed-bearing implant mounted in aluminum fixtures was utilized. An explicit finite element model of the simulator was developed from measured physical properties of the machine, and loading conditions were created from the measured experimental feedback data. The explicit finite element model allows both rigid body and fully deformable solutions to be chosen based on the application of interest. Six degrees-of-freedom kinematics were compared for both tibio- and patellofemoral joints during gait loading, with an average root mean square (rms) translational error of 1.1 mm and rotational rms error of 1.3 deg. Model sensitivity to interface friction and damping present in the experimental joints was also evaluated and served as a secondary goal of this paper. Modifying the metal-polyethylene coefficient of friction from 0.1 to 0.01 varied the patellar flexion-extension and tibiofemoral anterior-posterior predictions by 7 deg and 2 mm, respectively, while other kinematic outputs were largely insensitive.
Journal of Biomechanical Engineering 08/2010; 132(8):081010. · 1.75 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Verified computational models represent an efficient method for studying the relationship between articular geometry, soft-tissue constraint, and patellofemoral (PF) mechanics. The current study was performed to evaluate an explicit finite element (FE) modeling approach for predicting PF kinematics in the natural and implanted knee. Experimental three-dimensional kinematic data were collected on four healthy cadaver specimens in their natural state and after total knee replacement in the Kansas knee simulator during a simulated deep knee bend activity. Specimen-specific FE models were created from medical images and CAD implant geometry, and included soft-tissue structures representing medial-lateral PF ligaments and the quadriceps tendon. Measured quadriceps loads and prescribed tibiofemoral kinematics were used to predict dynamic kinematics of an isolated PF joint between 10 degrees and 110 degrees femoral flexion. Model sensitivity analyses were performed to determine the effect of rigid or deformable patellar representations and perturbed PF ligament mechanical properties (pre-tension and stiffness) on model predictions and computational efficiency. Predicted PF kinematics from the deformable analyses showed average root mean square (RMS) differences for the natural and implanted states of less than 3.1 degrees and 1.7 mm for all rotations and translations. Kinematic predictions with rigid bodies increased average RMS values slightly to 3.7 degrees and 1.9 mm with a five-fold decrease in computational time. Two-fold increases and decreases in PF ligament initial strain and linear stiffness were found to most adversely affect kinematic predictions for flexion, internal-external tilt and inferior-superior translation in both natural and implanted states. The verified models could be used to further investigate the effects of component alignment or soft-tissue variability on natural and implant PF mechanics.
Journal of biomechanics 09/2009; 42(14):2341-8. · 2.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: During knee flexion and extension the ACL and PCL help to coordinate the movement and rotation of the knee by constraining the sliding and rolling motions at the joint. In the natural knee the femur pivots about the medial condyle and the femur tends to roll back on the tibia with increasing flexion . The purpose of this study was to observe if and how rollback occurs in the natural knee using the lowest point (LP) method, and to understand how anterior-posterior (AP) motion is related to flexion angle in the natural knee. A better understanding of natural knee femoral rollback will influence future design of total knee arthroplasties.
[Show abstract][Hide abstract] ABSTRACT: The motion patterns of the human knee joint depend on its passive motion characteristics, which are described by the ligamentious and articular constraints. Since active motions, like walking and squatting are believed to fall within a passive envelope, the basis for the understanding of the knee joint kinematics lies in the description of its passive constraint characteristics . Although several authors studied passive envelope characteristics of a knee, it is not clear from the literature which anatomical structures guide the knee in passive or active motion and how their geometric arrangement produces the unique path of passive knee motion [1–3]. A few mathematical models have been developed to study the structures that guide the passive knee motion [1, 2]. However, their hypotheses were not supported by a sufficiently detailed ligament bundle model, soft tissue properties, ligament insertion-origin sites and their intra-subject variability. To explain the relationship between knee anatomy and its variability with three-dimensional knee motion completely, new methodology must be developed. The objective of the present study was to estimate the effects of variation in knee anatomical factors on the tibiofemoral passive envelope using a multivariate analysis technique, principal component (PC) analysis.
[Show abstract][Hide abstract] ABSTRACT: Many researchers have studied the tibial passive motion, the boundaries of which are defined by various knee ligamentious and bony constraints [1, 2, 3]. The technique has been used in clinical practices and experimental research to assess injury and predict likely surgical outcomes [1, 2]. After total knee replacement surgery (TKR), the implants’ design features and altered ligamentious tension provide the joint constraint and stability. Therefore, the change in passive envelope of motion from the natural condition could be used to observe the altered constraints and stability achieved in TKR knees. The objective of this study was to assess the change in passive envelope of motion after TKR with two implant designs: cruciate retaining and posterior stabilized.
[Show abstract][Hide abstract] ABSTRACT: Experimental testing with cadaveric tissue allows the application of controlled loads and/or motions while still maintaining the inherent variability in the anatomy and soft tissue of the specimens. Multi-axial dynamic loading of tissue allows for experiments to be conducted that simulate conditions approaching physiological. Knee simulators have been used to generate physiological loading on the human knee to study kinematics, soft tissue loading, and joint contact pressure. These machines have been used to investigate injury, surgical outcomes, and prosthetic design. While there are a number of different geometries for knee loading devices, most are based on the Oxford rig design  with a vertical orientation of the leg where the hip is able to translate up and down while allowing flexion at the hip, knee, and ankle. The foot or ankle can have a variety of constraints and degrees of freedom. One of the recent areas of interest in knee biomechanics is the role different structures of the knee play during deep knee flexion activities. This is of particular interest to the orthopedic industry because of the common complaint regarding a feeling of a loss of stability during high flexion activities for post-TKR patients and the prevalence of high-flexion activities in emerging worldwide markets. The objective of this abstract is to describe two knee loading devices that have been used to study knee biomechanics, and most recently high flexion motion, and present some representative data from these tools.
[Show abstract][Hide abstract] ABSTRACT: Many activities of daily living during work, exercise, religious worship, and hobbies require deep knee flexion. Activities such as rising from a low chair or getting into or out of a bath require between 100° and 160° of knee flexion . Other activities such as kneeling or squatting to pick an item off the ground can be difficult with a limited range of motion. Beside deep knee flexion being important for daily living activities, it is essential in non-Western cultures that commonly sit in deep knee-bending positions. In vitro studies looking at knee function often focus solely on the knee joint, ignoring the effect of the muscle, ligament, and tendon constraints of the ankle, and simplifying or neglecting muscle loads. The effects of these assumptions on kinematics are unknown. The purpose of this study was to compare a squatting activity for: 1) whole leg versus knee specimens, and 2) different combinations of quadriceps and hamstrings loading.
[Show abstract][Hide abstract] ABSTRACT: Verified computational models of total knee replacement serve as the primary design-phase tool for parametric analysis of implant geometry. Previously, dynamic finite element models of the Kansas Knee Simulator (KKS) were developed and tibiofemoral (TF) and patellofemoral (PF) kinematic predictions were verified by comparison with experimental measurements [1,2]. In this prior work, the implants were mounted in metallic fixtures to assess the ability of the model to accurately predict the TF and PF kinematics without the additional complexity of variable cadaver specimens and soft-tissue constraint. The next step in the systematic model verification procedure was to verify kinematic predictions with multiple specimen-specific models. Specifically, the objectives of the present study were: 1) to develop an explicit finite element (FE) model of the KKS capable of recreating experimental loading protocols for a deep knee bend activity and 2) to verify predicted six degree-of-freedom (DOF) TF and PF kinematics of two cruciate retaining (CR) and two posterior stabilized (PS) implanted specimen-specific models with deformable, wrapping soft tissue constraint.
[Show abstract][Hide abstract] ABSTRACT: Deep knee flexion is required for many activities of daily living during work, exercise, religious worship, and hobbies. Walker et al.  found that activities such as rising from a low chair or getting into or out of bath require between 100° and 160° of knee flexion. Other activities such as kneeling or squatting to pick an item off the ground can be difficult with a limited range of motion. Beside deep knee flexion being important for daily living activities, it is essential in non-Western cultures that commonly sit in deep knee-bending positions.