Comparison of shear forces and ligament loading in the healthy and ACL-deficient knee during gait

The purpose of this study was to predict and explain the pattern of shear force and ligament loading in the ACL-deficient knee during walking, and to compare these results to similar calculations for the healthy knee. Musculoskeletal modeling and computer simulation were combined to calculate ligament forces in the ACL-deficient knee during walking. Joint angles, ground-reaction forces, and the corresponding lower-extremity muscle forces obtained from a whole-body dynamic optimization simulation of walking were input into a second three-dimensional model of the lower extremity that represented the knee as a six degree-of-freedom spatial joint. Anterior tibial translation (ATT) increased throughout the stance phase of gait when the model ACL was removed. The medial collateral ligament (MCL) was the primary restraint to ATT in the ACL-deficient knee. Peak force in the MCL was three times greater in the ACL-deficient knee than in the ACL-intact knee; however, peak force sustained by the MCL in the ACL-deficient knee was limited by the magnitude of the total anterior shear force applied to the tibia. A decrease in anterior tibial shear force was brought about by a decrease in the patellar tendon angle resulting from the increase in ATT. These results suggest that while the MCL acts as the primary restraint to ATT in the ACL-deficient knee, changes in patellar tendon angle reduce total anterior shear force at the knee.     

Effect of joint laxity on polyethylene wear in total knee replacement

Experimental simulator studies are frequently performed to evaluate wear behavior in total knee replacement. It is vital that the simulation conditions match the physiological situation as closely as possible. To date, few experimental wear studies have examined the effects of joint laxity on wear and joint kinematics and the absence of the anterior cruciate ligament has not been sufficiently taken into account in simulator wear studies.The aim of this study was to investigate different ligament and soft tissue models with respect to wear and kinematics.A virtual soft tissue control system was used to simulate different motion restraints in a force-controlled knee wear simulator.The application of more realistic and sophisticated ligament models that considered the absence of anterior cruciate ligament lead to a significant increase in polyethylene wear (p=0.02) and joint kinematics (p<0.01). We recommend the use of more complex ligament models to appropriately simulate the function of the human knee joint and to evaluate the wear behavior of total knee replacements. A feasible simulation model is presented.     

Computational wear prediction of a total knee replacement from in vivo kinematics

Wear of ultra-high molecular weight polyethylene bearings in total knee replacements remains a major limitation to the longevity of these clinically successful devices. Few design tools are currently available to predict mild wear in implants based on varying kinematics, loads, and material properties. This paper reports the implementation of a computer modeling approach that uses fluoroscopically measured motions as inputs and predicts patient-specific implant damage using computationally efficient dynamic contact and tribological analyses. Multibody dynamic simulations of two activities (gait and stair) with two loading conditions (70-30 and 50-50 medial-lateral load splits) were generated from fluoroscopic data to predict contact pressure and slip velocity time histories for individual elements on the tibial insert surface. These time histories were used in a computational wear analysis to predict the depth of damage due to wear and creep experienced by each element. Predicted damage areas, volumes, and maximum depths were evaluated against a tibial insert retrieved from the same patient who provided the in vivo motions. Overall, the predicted damage was in close agreement with damage observed on the retrieval. The gait and stair simulations separately predicted the correct location of maximum damage on the lateral side, whereas a combination of gait and stair was required to predict the correct location on the medial side. Predicted maximum damage depths were consistent with the retrieval as well. Total computation time for each damage prediction was less than 30 min. Continuing refinement of this approach will provide a robust tool for accurately predicting clinically relevant wear in total knee replacements.     

Material and surface factors influencing backside fretting wear in total knee replacement tibial components

Retrieval studies have shown that the interface between the ultra-high molecular weight polyethylene insert and metal tibial tray of fixed-bearing total knee replacement components can be a source of substantial amounts of wear debris due to fretting micromotion. We assessed fretting wear of polyethylene against metal as a function of metal surface finish, alloy, and micromotion amplitude, using a three-station pin-on-disc fretting wear simulator. Overall, the greatest reduction in polyethylene wear was achieved by highly polishing the metal surface. For example, highly polished titanium alloy surfaces produced nearly 20 times less polyethylene wear compared with blasted titanium alloy, whereas, decreasing the micromotion amplitude from 200 to 50 μm produced approximately four times less polyethylene wear for the same blasted titanium alloy surface. Although the effect of the metal alloy was much smaller than the effect of metal surface roughness or the micromotion amplitude, CoCr discs produced slightly greater polyethylene fretting wear than titanium alloy discs under each condition. The results are essential in design and manufacturing decisions related to fixed-bearing total knee replacements.     

A holistic numerical model to predict strain hardening and damage of UHMWPE under multiple total knee replacement kinematics and experimental validation

Experimental wear testing is an essential step in the evaluation of total knee replacement (TKR) design. Unfortunately, experiments can be prohibitively expensive and time consuming, which has made computational wear simulation a more desirable alternative for screening designs. While previous attempts have demonstrated positive results, few models have fully incorporated the affect of strain hardening (or cross shear), or tested the model under more than one loading condition. The objective of this study was to develop and evaluate the performance of a new holistic TKR damage model, capable of predicting damage caused by wear, including the effects of strain hardening and creep. For the first time, a frictional work-based damage model was compared against multiple sets of experimental TKR wear testing data using different input kinematics. The wear model was tuned using experimental measurements and was then able to accurately predict the volumetric polyethylene wear volume during experiments with different kinematic inputs. The size and shape of the damage patch on the surface of the polyethylene inserts were also accurately predicted under multiple input kinematics. The ability of this model to predict implant damage under multiple loading profiles by accounting for strain hardening makes it ideal for screening new implant designs, since implant kinematics are largely a function of the shape of the components.     

Direct comparison of measured and calculated total knee replacement force envelopes during walking in the presence of normal and abnormal gait patterns

Knee joint forces measured from instrumented implants provide important information for testing the validity of computational models that predict knee joint forces. The purpose of this study was to validate a parametric numerical model for predicting knee joint contact forces against measurements from four subjects with instrumented TKRs during the stance phase of gait. Model sensitivity to abnormal gait patterns was also investigated. The results demonstrated good agreement for three subjects with relatively normal gait patterns, where the difference between the mean measured and calculated forces ranged from 0.05 to 0.45 body weights, and the envelopes of measured and calculated forces (from three walking trials) overlapped. The fourth subject, who had a "quadriceps avoidance" external moment pattern, initially had little overlap between the measured and calculated force envelopes. When additional constraints were added, tailored to the subject's gait pattern, the model predictions improved to complete force envelope overlap. Coefficient of multiple determination analysis indicated that the shape of the measured and calculated force waveforms were similar for all subjects (adjusted coefficient of multiple correlation values between 0.88 and 0.92). The parametric model was accurate in predicting both the magnitude and waveform of the contact force, and the accuracy of model predictions was affected by deviations from normal gait patterns. Equally important, the envelope of forces generated by the range of solutions substantially overlapped with the corresponding measured envelope from multiple gait trials for a given subject, suggesting that the variable strategic processes of in vivo force generation are covered by the solution range of this parametric model.     

Image based weighted center of proximity versus directly measured knee contact location during simulated gait

To understand the mechanical consequences of knee injury requires a detailed analysis of the effect of that injury on joint contact mechanics during activities of daily living. Three-dimensional (3D) knee joint geometric models have been combined with knee joint kinematics to dynamically estimate the location of joint contact during physiological activities—using a weighted center of proximity (WCoP) method. However, the relationship between the estimated WCoP and the actual location of contact has not been defined. The objective of this study was to assess the relationship between knee joint contact location as estimated using the image-based WCoP method, and a directly measured weighted center of contact (WCoC) method during simulated walking. To achieve this goal, we created knee specific models of six human cadaveric knees from magnetic resonance imaging. All knees were then subjected to physiological loads on a knee simulator intended to mimic gait. Knee joint motion was captured using a motion capture system. Knee joint contact stresses were synchronously recorded using a thin electronic sensor throughout gait, and used to compute WCoC for the medial and lateral plateaus of each knee. WCoP was calculated by combining knee kinematics with the MRI-based knee specific model. Both metrics were compared throughout gait using linear regression. The anteroposterior (AP) location of WCoP was significantly correlated with that of WCoC on both tibial plateaus in all specimens (p<0.01, 95% confidence interval of Pearson?s coefficient r>0), but the correlation was not significant in the mediolateral (ML) direction for 4/6 knees (p>0.05). Our study demonstrates that while the location of joint contact obtained from 3D knee joint contact model, using the WCoP method, is significantly correlated with the location of actual contact stresses in the AP direction, that relationship is less certain in the ML direction.     

Measurements of constraint of total knee replacement

Measurement of the constraint of total knee components in a test machine provides an objective method of describing the laxity and stability characteristics of the implant itself, independent of the knee joint into which it would be implanted. A special fixture was designed and fitted to a Bionix multi-channel loading machine. The test consisted of applying a compressive load, applying a cyclic AP force or internal-external torque, and measuring all of the displacements and rotations. Three different commonly-used TKR's showed widely different constraint characteristics. In the cyclic AP test, along with the cyclic AP displacement, displacements and rotations occurred in the other directions. This indicated that all degrees of freedom should be free to move, otherwise anomalous results would be obtained. The paper concludes with recommendations for standardized constraint tests.     

Simulation of a knee joint replacement during a gait cycle using explicit finite element analysis.

The stress distribution within the polyethylene insert of a total knee joint replacement is dependent on the kinematics, which in turn are dependent on the design of the articulating surfaces, the relative position of the components and the tension of the surrounding soft tissues. Implicit finite element analysis techniques have been used previously to examine the polyethylene stresses. However, these have essentially been static analyses and hence ignored the influence of the kinematics. The aim of this work was to use an explicit finite element approach to simulate both the kinematics and the internal stresses within a single analysis. A simulation of a total knee joint replacement subjected to a single gait cycle within a knee wear simulator was performed and the results were compared with experimental data.The predicted kinematics were in close agreement with the experimental data. Various solution-dependent parameters were found to have little influence on the predicted kinematics. The predicted stresses were found to be dependent on the mesh density. This study has shown that an explicit finite element approach is capable of predicting the kinematics and the stresses within a single analysis at relatively low computational cost.     

Three-dimensional dynamic simulation of total knee replacement motion during a step-up task.

A three-dimensional dynamic model of the tibiofemoral and patellofemoral articulations was developed to predict the motions of knee implants during a step-up activity. Patterns of muscle activity, initial joint angles and velocities, and kinematics of the hip and tinkle were measured experimentally and used as inputs to the simulation. Prosthetic knee kinematics were determined by integration of dynamic equations of motion subject to forces generated by muscles, ligaments, and contact at both the tibiofemoral and patellofemoral articulations. The modeling of contacts between implants did not rely upon explicit constraint equations; thus, changes in the number of contact points were allowed without modification to the model formulation. The simulation reproduced experimentally measured flexion-extension angle of the knee (within one standard deviation), but translations at the tibiofemoral articulations were larger during the simulated step-up task than those reported for patients with total knee replacements.     

A theoretical and numerical approach to optimal positioning of the patellar surface replacement in a total knee endoprosthesis

The clinical results of total knee joint arthroplasty with patellar replacement have shown that postoperative problems arise, especially under unfavourable biomechanical conditions. The findings concerning retropatellar contact forces have been obtained by means of different methods, partly through experimental investigations and partly through theoretical considerations. But so far patellar replacement criteria and the resulting changes of the retropatellar contact force were not taken into consideration in other studies. Our mathematical model is based on a mechanical one and the parameter study considers the influence of the height of the patellar surface replacement upon different biomechanical parameters at varying positions. The results suggest that the patellar replacement should therefore be kept as low as possible, thus reducing the retropatellar contact force to a minimum, especially in the extremely stressed flexion areas of up to about 90 degrees.     

Characteristics of sit-to-stand and gait coordination in patients with total hip replacement

Upright posture, standing up from a chair, and gait were analyzed in patients after one-sided total hip replacement and in healthy subjects (control). It was found that the patients predominantly loaded the unoperated leg when they stood quietly or rose from a chair. Subjects’ walking on a 10-m podograph treadmill showed that their walking speed was slower than that of healthy subjects and the swing phase on the side of hip replacement was longer than on the unoperated side. It was assumed that the unequal load on legs during walking, standing, and sit-to-stand performance in patients with total hip replacement was related to the sensory deficit of the artificial joint, leading to the overstrain of the unoperated leg and coxarthrosis in it.     

The development,calibration and validation of a numerical total knee replacement kinematics simulator considering laxity and unconstrained flexion motions

Kinematics testing is essential during the development of total knee replacement (TKR) designs. Although computational analysis cannot replace physical testing, it offers repeatability and consistency at a much lower cost and shorter time, making it an excellent complement to experiments. Previous numerical models have been limited by several factors: the validity of the models is usually only considered for a single TKR design, friction models are typically overly simplified and the determination of simulation parameters is often inadequate, or tedious and expensive. The objective of this study is to develop, calibrate and validate a TKR kinematics simulation considering multiple TKR geometries, an accurate friction model and simulation parameters determined using a systematic optimisation method. The calibrated model was able to predict TKR kinematics for different TKR geometries, and is ideal for screening new implant designs, reducing the number of experiments required at the design stage.     

The development, calibration and validation of a numerical total knee replacement kinematics simulator considering laxity and unconstrained flexion motions

Kinematics testing is essential during the development of total knee replacement (TKR) designs. Although computational analysis cannot replace physical testing, it offers repeatability and consistency at a much lower cost and shorter time, making it an excellent complement to experiments. Previous numerical models have been limited by several factors: the validity of the models is usually only considered for a single TKR design, friction models are typically overly simplified and the determination of simulation parameters is often inadequate, or tedious and expensive. The objective of this study is to develop, calibrate and validate a TKR kinematics simulation considering multiple TKR geometries, an accurate friction model and simulation parameters determined using a systematic optimisation method. The calibrated model was able to predict TKR kinematics for different TKR geometries, and is ideal for screening new implant designs, reducing the number of experiments required at the design stage.     

Mechanics of cranial sutures during simulated cyclic loading

Previous computational and experimental analyses revealed that cranial sutures, fibrous joints between the bones, can reduce the strain experienced by the surrounding skull bones during mastication. This damping effect reflects the importance of including sutures in finite element (FE) analyses of the skull. Using the FE method, the behaviour of three suture morphologies of increasing complexity (butt-ended, moderate interdigitated, and complex interdigitated) during static loading was recently investigated, and the sutures were assumed to have linear elastic properties. In the current study, viscoelastic properties, derived from published experimental results of the nasofrontal suture of young pigs (Sus scrofa), are applied to the three idealised bone-suture models. The effects of suture viscoelasticity on the stress, strain, and strain energy in the models were computed for three different frequencies (corresponding to periods of 1, 10, and 100s) and compared to the results of a static, linear elastic analysis. The range of applied frequencies broadly represents different physiological activities, with the highest frequency simulating mastication and the lowest frequency simulating growth and pressure of the surrounding tissues. Comparing across all three suture morphologies, strain energy and strain in the suture decreased with the increase in suture complexity. For each suture model, the magnitude of strain decreased with an increase in frequency, and the magnitudes were similar for both the elastic and 1s frequency analyses. In addition, a viscous response is less apparent in the higher frequency analyses, indicating that viscous properties are less important to the behaviour of the suture during those analyses. The FE results suggest that implementation of viscoelastic properties may not be necessary for computational studies of skull behaviour during masticatory loading but instead might be more relevant for studies examining lower frequency physiological activities.     

Explicit finite element modeling of total knee replacement mechanics

Joint kinematics and contact mechanics dictate the success of current total knee replacement (TKR) devices. Efficient computer models present an effective way of evaluating these characteristics. Predicted contact stress and area due to articulations at the tibio-femoral and patello-femoral interfaces indicate potential clinical performance. Previous finite element (FE) knee models have generally been used to predict contact stresses and/or areas during static or quasi-static loading conditions. Explicit dynamic FE analyses have recently been used to efficiently predict TKR kinematics and contact mechanics during dynamic loading conditions. The objective of this study was to develop and experimentally validate an explicit FE TKR model that incorporates tibio-femoral and patello-femoral articulations. For computational efficiency, we developed rigid body analyses that can reasonably reproduce the kinematics, contact pressure distribution, and contact area of a fully deformable system. Results from the deformable model showed that the patello-femoral and tibio-femoral kinematics were in good agreement with experimental knee simulator measurements. Kinematic results from the rigid body analyses were nearly identical to those from the fully deformable model, and the contact pressure and contact area correlation was acceptable given the great reduction in analysis time. Component mesh density studied had little effect on the predicted kinematics, particularly for the patellar component, and small effects on the predicted contact pressure and area. These analyses have shown that, at low computational cost, a force-control dynamic simulation of a gait cycle can yield useful and predictable results.     

Influence of kinematics on the wear of a total ankle replacement

Total ankle replacement (TAR) is an alternative to fusion, replacing the degenerated joint with a mechanical motion-preserving alternative. Minimal pre-clinical testing has been reported to date and existing wear testing standards lack definition. Ankle gait is complex, therefore the aim of this study was to investigate the effect on wear of a range of different ankle gait kinematic inputs. Five Zenith (Corin Group) TARs were tested in a modified knee simulator for twelve million cycles (Mc). Different combinations of IR rotation and AP displacement were applied every 2Mc to understand the effects of the individual kinematics. Wear was assessed gravimetrically every Mc and surface profilometry undertaken after each condition. With the initial unidirectional input with no AP displacement the wear rate measured 1.2±0.6 mm³/Mc. The addition of 11° rotation and 9 mm of AP displacement caused a statistically significant increase in the wear rate to 25.8±3.1 mm³/Mc. These inputs seen a significant decrease in the surface roughness at the tibial articulation. Following polishing three displacement values were tested; 0, 4 and 9 mm with no significant difference in wear rate ranging 11.8–15.2 mm³/Mc. TAR wear rates were shown to be highly dependent on the addition of internal/external rotation within the gait profile with multidirectional kinematics proving vital in the accurate wear testing of TARs. Prior to surface polishing wear rates were significantly higher but once in a steady state the AP displacement had no significant effect on the wear.     

Effect of conformity and contact stress on wear in fixed-bearing total knee prostheses

Ultra high molecular weight polyethylene (PE) remains the primary bearing surface of choice in total knee replacements (TKR). Wear is controlled by levels of cross-shear motion and contact stress. The aim of this study was to compare the wear of fixed-bearing total knee replacements with curved and flat inserts and to test the hypothesis that the flat inserts which give higher contact stresses and smaller contact areas would lead to lower levels of surface wear. A low-conforming, high contact stress knee with a low-medium level of cross shear resulted in significantly lower wear rates in comparison to a standard cruciate sacrificing fixed-bearing knee. The low wear solution found in the knee simulator was supported by fundamental studies of wear as a function of pressure and cross shear in the pin on plate system. Current designs of fixed-bearing knees do not offer this low wear solution due to their medium cross shear, moderate conformity and medium contact stress.