Grand challenge competition to predict in vivo knee loads

Impairment of the human neuromusculoskeletal system can lead to significant mobility limitations and decreased quality of life. Computational models that accurately represent the musculoskeletal systems of individual patients could be used to explore different treatment options and optimize clinical outcome. The most significant barrier to model-based treatment design is validation of model-based estimates of in vivo contact and muscle forces. This paper introduces an annual "Grand Challenge Competition to Predict In Vivo Knee Loads" based on a series of comprehensive publicly available in vivo data sets for evaluating musculoskeletal model predictions of contact and muscle forces in the knee. The data sets come from patients implanted with force-measuring tibial prostheses. Following a historical review of musculoskeletal modeling methods used for estimating knee muscle and contact forces, we describe the first two data sets used for the first two competitions and summarize four subsequent data sets to be used for future competitions. These data sets include tibial contact force, video motion, ground reaction, muscle EMG, muscle strength, static and dynamic imaging, and implant geometry data. Competition participants create musculoskeletal models to predict tibial contact forces without having access to the corresponding in vivo measurements. These blinded predictions provide an unbiased evaluation of the capabilities and limitations of musculoskeletal modeling methods. The paper concludes with a discussion of how these unique data sets can be used by the musculoskeletal modeling research community to improve the estimation of in vivo muscle and contact forces and ultimately to help make musculoskeletal models clinically useful.     

Changes in tibiofemoral forces due to variations in muscle activity during walking

Muscles induce large forces in the tibiofemoral joint during walking and thereby influence the health of tissues like articular cartilage and menisci. It is possible to walk with a wide variety of muscle coordination patterns, but the effect of varied muscle coordination on tibiofemoral contact forces remains unclear. The goal of this study was to determine the effect of varied muscle coordination on tibiofemoral contact forces. We developed a musculoskeletal model of a subject walking with an instrumented knee implant. Using an optimization framework, we calculated the tibiofemoral forces resulting from muscle coordination that reproduced the subject's walking dynamics. We performed a large set of optimizations in which we systematically varied the coordination of muscles to determine the influence on tibiofemoral force. Model‐predicted tibiofemoral forces arising with minimum muscle activation matched in vivo forces measured during early stance, but were greater than in vivo forces during late stance. Peak tibiofemoral forces during late stance could be reduced by increasing the activation of the gluteus medius, uniarticular hip flexors, and soleus, and by decreasing the activation of the gastrocnemius and rectus femoris. These results suggest that retraining of muscle coordination could substantially reduce tibiofemoral forces during late stance. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:769–776, 2014.     

Effect of posterior tibial slope on knee biomechanics during functional activity

Treatment of medial compartment knee osteoarthritis with high tibial osteotomy can produce an unintended change in the slope of the tibial plateau in the sagittal plane. The effect of changing posterior tibial slope (PTS) on cruciate ligament forces has not been quantified for knee loading in activities of daily living. The purpose of this study was to determine how changes in PTS affect tibial shear force, anterior tibial translation (ATT), and knee‐ligament loading during daily physical activity. We hypothesized that tibial shear force, ATT, and ACL force all increase as PTS increases. A previously validated computer model was used to calculate ATT, tibial shear force, and cruciate‐ligament forces for the normal knee during three common load‐bearing tasks: standing, squatting, and walking. The model calculations were repeated with PTS altered in 1° increments up to a maximum change in tibial slope of 10°. Tibial shear force and ATT increased as PTS was increased. For standing and walking, ACL force increased as tibial slope was increased; for squatting, PCL force decreased as tibial slope was increased. The effect of changing PTS on ACL force was greatest for walking. The true effect of changing tibial slope on knee‐joint biomechanics may only be evident under physiologic loading conditions which include muscle forces. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 29:223–231, 2011     

Are external knee load and EMG measures accurate indicators of internal knee contact forces during gait?

Mechanical loading is believed to be a critical factor in the development and treatment of knee osteoarthritis. However, the contact forces to which the knee articular surfaces are subjected during daily activities cannot be measured clinically. Thus, the ability to predict internal knee contact forces accurately using external measures (i.e., external knee loads and muscle electromyographic [EMG] signals) would be clinically valuable. We quantified how well external knee load and EMG measures predict internal knee contact forces during gait. A single subject with a force‐measuring tibial prosthesis and post‐operative valgus alignment performed four gait patterns (normal, medial thrust, walking pole, and trunk sway) to induce a wide range of external and internal knee joint loads. Linear regression analyses were performed to assess how much of the variability in internal contact forces was accounted for by variability in the external measures. Though the different gait patterns successfully induced significant changes in the external and internal quantities, changes in external measures were generally weak indicators of changes in total, medial, and lateral contact force. Our results suggest that when total contact force may be changing, caution should be exercised when inferring changes in knee contact forces based on observed changes in external knee load and EMG measures. Advances in musculoskeletal modeling methods may be needed for accurate estimation of in vivo knee contact forces. © 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31: 921–929, 2013     

Effect of component mal‐rotation on knee loading in total knee arthroplasty using multi‐body dynamics modeling under a simulated walking gait

Mal‐rotation of the components in total knee arthorplasty (TKA) is a major cause of postoperative complications, with an increased propensity for implant loosening or wear leading to revision. A musculoskeletal multi‐body dynamics model was used to perform a parametric study of the effects of the rotational mal‐alignments in TKA on the knee loading under a simulated walking gait. The knee contact forces were found to be more sensitive to variations in the varus–valgus rotation of both the tibial and the femoral components and the internal–external rotation of the femoral component in TKA. The varus–valgus mal‐rotation of the tibial or femoral component and the internal–external mal‐rotation of the femoral component with a 5° variation were found to affect the peak medial contact force by 17.8–53.1%, the peak lateral contact force by 35.0–88.4% and the peak total contact force by 5.2–18.7%. Our findings support the clinical observations that a greater than 3° internal mal‐rotation of the femoral component may lead to unsatisfactory pain levels and a greater than 3° varus mal‐rotation of the tibial component may lead to medial bone collapse. These findings determined the quantitative effects of the mal‐rotation of the components in TKA on the contact load. The effect of such mal‐rotation of the components of TKA on the kinematics would be further addressed in future studies. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 33:1287–1296, 2015.     

In vivo medial and lateral tibial loads during dynamic and high flexion activities.

Though asymmetric loading between the medial and lateral compartments of total knee replacements may contribute to implant loosening and failure, the in vivo contact force distribution during dynamic daily activities remains unknown. This study reports in vivo medial and lateral contact forces experienced by a well-aligned knee implant for a variety of activities. In vivo implant motion and total axial load data were collected from a single knee replacement patient performing treadmill gait (hands resting on handlebars), step up/down, lunge, and kneel activities. In vivo motion was measured using video fluoroscopy, while in vivo axial loads were collected simultaneously using an instrumented tibial component. An elastic foundation contact model employing linear and nonlinear polyethylene material properties was constructed to calculate medial and lateral contact forces based on the measured kinematics, total axial loads, and centers of pressure. For all activities, the predicted medial and lateral contact forces were insensitive to the selected material model. The percentage of medial to total contact force ranged from 18 to 60 for gait, 47 to 65 for step up/down, and 55 to 60 for kneel and lunge. At maximum load during the motion cycle, medial force was 1.2 BW for gait and 2.0 BW for step up/down, while the corresponding lateral forces were 1.0 and 1.5 BW, respectively. At mean load in the final static pose, medial force was 0.2 BW for kneel and 0.9 BW for lunge, with corresponding lateral forces of 0.1 and 0.7 BW, respectively. For this patient, a constant load split of 55% medial-45% lateral during loaded activity would be a reasonable approximation for these test conditions.     

Subject‐specific modeling of muscle force and knee contact in total knee arthroplasty

Understanding the mechanical loading environment and resulting joint mechanics for activities of daily living in total knee arthroplasty is essential to continuous improvement in implant design. Although survivorship of these devices is good, a substantial number of patients report dissatisfaction with the outcome of their procedure. Knowledge of in vivo kinematics and joint loading will enable improvement in preclinical assessment and refinement of implant geometry. The purpose of this investigation was to describe the mechanics of total knee arthroplasty during a variety of activities of daily living (gait, walking down stairs, and chair rise/sit). Estimates of muscle forces, tibial contact load, location, and pressure distribution was performed through a combination of mobile fluoroscopy data collection, musculoskeletal modeling, and finite element simulation. For the activities evaluated, joint compressive load was greatest during walking down stairs; however, the highest contact pressure occurred during chair rise/sit. The joint contact moment in the frontal plane was mainly varus for gait and walking down stairs, while it was valgus during chair rise/sit. Excursion of the center of pressure on the tibial component was similar during each activity and between the medial and lateral sides. The main determinants of center of pressure location were internal–external rotation, joint load, and tibial insert conformity. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1576–1587, 2016.     

Sensitivity of knee replacement contact calculations to kinematic measurement errors

The ability to measure in vivo knee kinematics accurately makes it tempting to calculate in vivo contact forces, pressures, and areas directly from kinematic data. However, the sensitivity of contact calculations to kinematic measurement errors has not been adequately investigated. To address this issue, we developed a series of sensitivity analyses derived from a validated in vivo computational simulation of gait. The simulation used an elastic foundation contact model to reproduce in vivo contact force, center of pressure, and fluoroscopic motion data collected from an instrumented knee replacement. Treating each degree of freedom (DOF) in the simulation as motion controlled, we first quantified how errors in measured relative pose of the implant components affected contact calculations. Pose variations of ±0.1 mm or degree over the entire gait cycle changed maximum contact force, pressure, and area by 204, 100, and 117%, respectively. Larger variations of ±0.5 mm or degree changed these same quantities by 1157, 108, and 578%, respectively. In both cases, the largest sensitivities were to errors in superior‐inferior translation and varus‐valgus rotation, with loss of contact occurring on one or both sides. We then quantified how switching the sensitive DOFs from motion to load control affected the sensitivity results. Pose variations of ±0.5 mm or degree in the remaining DOFs changed maximum contact quantities by at most 3%. These results suggest that accuracy on the order of microns and milliradians is needed to estimate contact forces, pressures, and areas directly from in vivo kinematic measurements, and that use of load rather than motion control for the sensitive DOFs may improve the accuracy of in vivo contact calculations. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 26:1173–1179, 2008     

Knee Abduction and Internal Rotation Moments Increase ACL Force During Landing Through the Posterior Slope of the Tibia

The mechanism underlying non‐contact anterior cruciate ligament (ACL) injury is multi‐factorial and still an object of debate. Computational models, in combination with in vivo and cadaveric studies, can provide valuable insight into the contribution of the different factors involved. The goal of this study was to validate four knee finite element models (two males and two females) to kinematic and strain data collected in vitro with an impact‐driven simulator and use them to assess how secondary external knee loads (knee abduction moment [KAM], anterior shear force, and internal rotation torque [ITR]) affect tibiofemoral contact forces and ACL force during impact. Four subject‐specific knee models were developed from specimen computed tomography and magnetic resonance imaging. Patellofemoral and tibiofemoral ligament properties were calibrated to match experimentally measured kinematics and ligament strain. Average root mean square errors and correlations between experimental and model‐predicted knee kinematics were below 1.5 mm and 2°, and above 0.75, respectively. Similar errors and correlations were obtained for ACL strain (< 2% and > 0.9). Model‐predicted ACL forces were highly correlated with the anterior component of the tibiofemoral contact force on the lateral plateau occurring during impact (r = 0.99), which was increased by larger KAM and ITR through the posterior tibial slope and a larger contact force on the lateral side. This study provides a better understanding of the mechanism through which secondary external knee loads increase ACL injury risk during landing. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1730–1742, 2019     

A Validated Forward Solution Dynamics Mathematical Model of the Knee Joint: Can It Be an Effective Alternative for Implant Evaluation?

BackgroundMathematical modeling is among the most common computational tools for assessing total knee arthroplasty (TKA) mechanics of different implant designs and surgical alignments. The main objective of this study is to describe and validate a forward solution mathematical of the knee joint to investigate the effects of TKA design and surgical conditions on TKA outcomes.MethodsA 12-degree of freedom mathematical model of the human knee was developed. This model includes the whole lower extremity of the human body and comprises major muscles and ligaments at the knee joint. The muscle forces are computed using a proportional–integral–derivative controller, and the joint forces are calculated using a contact detection algorithm. The model was validated using telemetric implants and fluoroscopy, and the sensitivity analyses were performed to determine how sensitive the model is to both implant design, which was analyzed by varying medial conformity of the polyethylene, and surgical alignment, which was analyzed by varying the posterior tibial tilt.ResultsThe model predicted the tibiofemoral joint forces with an average accuracy of 0.14× body weight (BW), 0.13× BW, and 0.17× BW root-mean-square errors for lateral, medial, and total tibiofemoral contact forces. With fluoroscopy, the kinematics were validated with an average accuracy of 0.44 mm, 0.62 mm, and 0.77 root-mean-square errors for lateral anteroposterior position, medial anteroposterior position, and axial rotation, respectively. Increasing medial conformity resulted in reducing the paradoxical anterior sliding midflexion. Furthermore, increasing posterior tibial slopes shifted the femoral contact point more posterior on the bearing and reduced the tension in the posterior cruciate ligament.ConclusionA forward solution dynamics model of the knee joint was developed and validated using telemetry devices and fluoroscopy data. The results of this study suggest that a validated mathematical model can be used to predict the effects of component design and surgical conditions on TKA outcomes.     

Greater muscle co‐contraction results in increased tibiofemoral compressive forces in females who have undergone anterior cruciate ligament reconstruction

Individuals who have undergone ACL reconstruction (ACLR) have been shown to have a higher risk of developing knee osteoarthritis (OA). The elevated risk of knee OA may be associated with increased tibiofemoral compressive forces. The primary purpose of this study was to examine whether females with ACLR demonstrate greater tibiofemoral compressive forces, as well as greater muscle co‐contraction and decreased knee flexion during a single‐leg drop‐land task when compared to healthy females. Ten females with ACLR and 10 healthy females (control group) participated. Each participant underwent two data collection sessions: (1) MRI assessment and (2) biomechanical analysis (EMG, kinematics, and kinetics) during a single‐leg drop‐land task. Joint kinematics, EMG, and MRI‐measured muscle volumes and patella tendon orientation were used as input variables into a MRI‐based EMG‐driven knee model to quantify the peak tibiofemoral compressive forces during landing. Peak tibiofemoral compressive forces were significantly higher in the ACLR group when compared to the control group (97.3 ± 8.0 vs. 88.8 ± 9.8 N · kg^?1). The ACLR group also demonstrated significantly greater muscle co‐contraction as well as less knee flexion than the control group. Our findings support the premise that individuals with ACLR demonstrate increased tibiofemoral compression as well as greater muscle co‐contraction and decreased knee flexion during a drop‐land task. Future studies are needed to examine whether correcting abnormal neuromuscular strategies and reducing tibiofemoral compressive forces following ACLR can slow the progression of joint degeneration in this population. © 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 30:2007–2014, 2012     

In vivo contact stresses during activities of daily living after knee arthroplasty

We implanted an electronic knee prosthesis to measure tibial forces in vivo during activities of daily living after total knee arthroplasty. We used tibial forces and knee kinematic data collected in vivo to calculate contact stresses using finite element analysis. The polyethylene insert was modeled as an elastoplastic material, and predicted contact stresses were validated using pressure sensitive sensors. Peak contact stresses generated during walking were similar but about 18% lower than those calculated for International Standards Organization (ISO)‐recommended wear simulation conditions. Stair climbing generated higher contact stresses (32 MPa) than walking (26 MPa). However, both high flexion activities (lunge and kneel) generated even higher contact stresses, with the lunge activity generating the highest stresses (56 MPa). The activities that generated high contact stresses also resulted in high equivalent plastic strain. However, the lunge activity generated dramatically higher plastic equivalent strain than the other activities. In vivo measurement of kinematics, forces, and contact stresses may be used to develop more clinically relevant wear simulator protocols. Contact stresses generated during high flexion activities were substantially higher and were largely due to the reduced contact area in deep flexion rather than due to an increase in contact forces. Our results support the use of “high flexion” designs that improve contact conditions and preserve contact area at high flexion angles. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res     

Effective gait patterns for offloading the medial compartment of the knee

Gait modification offers a noninvasive option for offloading the medial compartment of the knee in patients with knee osteoarthritis. While gait modifications have been proposed based on their ability to reduce the external knee adduction moment, no gait pattern has been proven to reduce medial compartment contact force directly. This study used in vivo contact force data collected from a single subject with a force‐measuring knee replacement to evaluate the effectiveness of two gait patterns at achieving this goal. The first was a “medial thrust” gait pattern that involved medializing the knee during stance phase, while the second was a “walking pole” gait pattern that involved using bilateral walking poles commonly used for hiking. Compared to the subject's normal gait pattern, medial thrust gait produced a 16% reduction and walking pole gait a 27% reduction in medial contact force over stance phase, both of which were statistically significant based on a two‐tailed Mann–Whitney U‐test. While medial thrust gait produced little change in lateral and total contact force over the stance phase, walking pole gait produced significant 11% and 21% reductions, respectively. Medial thrust gait may allow patients with knee osteoarthritis to reduce medial contact force using a normal‐looking walking motion requiring no external equipment, while walking pole gait may allow patients with knee osteoarthritis or a knee replacement to reduce medial, lateral, and total contact force in situations where the use of walking poles is possible. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27: 1016–1021, 2009     

Relative importance of gait vs. joint positioning on hip contact forces after total hip replacement

Implant loosening is a common indication for total hip replacement (THR) revision. High contact forces and implant twisting moments are thought to be associated with implant loosening. Relationships between joint positioning and hip forces, or outcomes, have been investigated through in vivo and in vitro modalities. Relationships between hip forces and gait are less understood, despite repeated findings that gait following a THR does not fully return to normal. We tested the hypothesis that gait parameters would be better predictors of implant force (peak contact forces and peak twisting moment during walking) than joint positioning parameters. Subjects underwent gait analysis, hip force modeling, and measurement of clinical radiographs 1 year after successful THR surgery. Gait parameters were consistently more influential in determining hip forces. Alone, gait explained as much as 67% of the variation in force, compared to a maximum of 33% by joint geometry. Combinations of gait and joint positioning parameters together explained up to 86% of the variation in hip force parameters. Results suggest that gait may provide a valuable postoperatively modifiable target to improve hip loads and potentially reduce the risk for implant loosening. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27:1576–1582, 2009     

The medial–lateral force distribution in the ovine stifle joint during walking

Knowledge of the load distribution in the knee is essential for understanding the interaction between mechanics and biology in both the healthy and diseased joint. While the sheep stifle joint is a predominant model for better understanding regeneration after injury, little is known about the compartmental force distribution between the medial and lateral condyles. By including sheep specific anatomy and gait analyses, we used computational musculoskeletal analyses to estimate the medial–lateral joint contact force distribution in ovine stifle joints during walking by simplifying the system of equations into a 2D problem that was solved directly. Gait analysis was conducted using bone markers in three female Merino‐mix sheep. Joint contact forces were computed with respect to the specific anatomy of the ovine tibia, resulting in low (<0.13 bodyweight) mean anteroposterior shear forces throughout the gait cycle, with mean peak contact forces perpendicular to the tibial plateau of 2.2 times bodyweight. The medial–lateral compartmental load distribution across the tibial condyles was determined and revealed loading predominantly on the medial condyle, bearing approximately 75% of the total load during phases of peak loading. By considering the anatomical characteristics of the ovine stifle joint, together with the dynamic forces during gait, this study provides evidence for predominantly medial loading in sheep, somewhat similar to the distribution reported in man. However, the exact conditions under which the loading in the ovine stifle joint is representative of the human situation will need to be elucidated in further studies. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 29:567–571, 2011     

Investigating the effects of anterior tibial translation on anterior knee force in the porcine model: Is the porcine knee ACL dependent?

This study sought to determine anterior force in the porcine knee during simulated 6‐degree‐of‐freedom (DOF) motion to establish the role of the anterior cruciate ligament (ACL). Using a 6‐DOF robot, a simulated ovine motion was applied to porcine hind limbs while recording the corresponding forces. Since the porcine knee is more lax than the ovine knee, anterior tibial translations were superimposed on the simulated motion in 2 mm increments from 0 mm to 10 mm to find a condition that would load the ACL. Increments through 8 mm increased anterior knee force, while the 10 mm increment decreased the force. Beyond 4 mm, anterior force increases were non‐linear and less than the increases at 2 and 4 mm, which may indicate early structural damage. At 4 mm, the average anterior force was 76.9 ± 10.6 N (mean ± SEM; p < 0.025). The ACL was the primary restraint, accounting for 80–125% of anterior force throughout the range of motion. These results demonstrate the ACL dependence of the porcine knee for the simulated motion, suggesting this model as a candidate for studying ACL function. With reproducible testing conditions that challenge the ACL, this model could be used in developing and screening possible reconstruction strategies. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 29:641–646, 2011     

In situ forces in the anterior cruciate ligament and its bundles in response to anterior tibial loads

The anterior cruciate ligament has a complex fiber anatomy and is not considered to be a uniform structure. Current anterior cruciate ligament reconstructions succeed in stabilizing the knee, but they neither fully restore normal knee kinematics nor reproduce normal ligament, function. To improve the outcome of the reconstruction, it may be necessary to reproduce the complex function of the intact anterior cruciate ligament in the replacement graft. We examined the in situ forces in nine human anterior cruciate ligaments as well as the force distribution between the anteromedial and posterolateral bundles of the ligament in response to applied anterioi tibial loads ranging from 22 to 110 N at knee flexion angles of 0–90°. The analysis was performed using a robotic manipulator in conjunction with a universal force-moment sensor. The in situ forces were determined with no device attached to the ligament, while the knee was permitted to move freely in response to the applied loads. We found that the in situ forces in the anterior cruciate ligament ranged from 12.8 ± 7.3 N under 22 N of anterior tibial load applied at 90° of knee flexion to 110.6 ± 14.8 N under 110 N of applied load at 15° of flexion. The magnitude of the in situ force in the posterolateral bundle was larger than that in the anteromedial bundle at knee flexion angles between 0 and 45°, reaching a maximum of 75.2 ± 18.3 N at 15° of knee flexion under an anterior tibial load of 110 N. The magnitude of the in situ force in the posterolateral bundle was significantly affected by knee flexion angle and anterior tibial load in a fashion remarkably similar to that seen in the anterior cruciate ligament. The magnitude of the in situ force in the anteromedial bundle, in contrast, remained relatively constant, not changing with flexion angle. Significant differences in the direction of the in situ force between the anteromedial bundle and the posterolateral bundle were found only at flexion angles of 0 and 60° and only under applied anterior tibial loads greater than 66 N. We have demonstrated the nonuniformity of the anterior cruciate ligament under unconstrained anterior tibial loads. Our data further suggest that in order for the anterior cruciate ligament replacement graft to reproduce the in situ forces of the normal anterior cruciate ligament, reconstruction techniques should take into account the role of the posterolateral bundle in addition to that of the anteromedial bundle.     

Anti‐gravity treadmills are effective in reducing knee forces

Lower body positive pressure (LBPP) treadmills permit significant unweighting of patients and have the potential to enhance recovery following lower limb surgery. We determined the efficacy of an LBPP treadmill in reducing knee forces in vivo. Subjects, implanted with custom electronic tibial prostheses to measure forces in the knee, were tested on a treadmill housed within a LBPP chamber. Tibiofemoral forces were monitored at treadmill speeds from 1.5 mph (0.67 m/s) to 4.5 mph (2.01 m/s), treadmill incline from ?10° to +10°, and four treadmill chamber pressure settings adjusted to decrease net treadmill reaction force from 100% to 25% of the subject's body weight (BW). The peak axial tibiofemoral force ranged from 5.1 times BW at a treadmill speed of 4.5 mph (2.01 m/s) and a pressure setting of 100% BW to 0.8 times BW at 1.5 mph (0.67 m/s) and a pressure setting of 25% BW. Peak knee forces were significantly correlated with walking speed and treadmill reaction force (R² = 0.77, p = 0.04). The LBPP treadmill might be an effective tool in the rehabilitation of patients following lower‐extremity surgery. The strong correlation between tibiofemoral force and walking speed and treadmill reaction forces allows for more precisely achieving the target knee forces desired during early rehabilitation. © 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31: 672–679, 2013     

In vivo kinematics and kinetics of a bi‐cruciate substituting total knee arthroplasty: A combined fluoroscopic and gait analysis study

After total knee arthroplasty, changes in articular surface geometry, soft tissue treatment, and component alignment can alter normal lower limb function. The guided motion bi‐cruciate substituting prosthesis was designed specifically to restore physiological knee joint motion. We determined whether this design could in vivo normal kinematics and kinetics, not only at the replaced knee, but also throughout both lower limbs. Sixteen patients (4 male, 12 female, mean age of 68.2 years with a range from 58 to 79 years) with primary knee osteoarthritis were implanted with the bi‐cruciate substituting prosthesis. At 6‐month follow‐up, knee joint kinematics was assessed by video‐fluoroscopy during stair‐climbing, chair‐rising/sitting, and step‐up/down. Lower limb overall function was also assessed on the same day by standard gait analysis with simultaneous electromyography during level walking. By video‐fluoroscopy, mean anteroposterior translations between femoral and tibial components during the three motor tasks were 9.7 ± 3.0, 10 ± 2.6, and 6.9 ± 3.5 mm on the medial compartment, and 14.3 ± 3.5, 18.5 ± 3.0, and 13.9 ± 3.8 mm on the lateral compartment, respectively. Axial rotation ranged from 5.6° to 26.2°. Gait analysis revealed restoration of nearly normal walking patterns in most patients. This rare combination of measurements, i.e., accurate rotation‐translation at the replaced knee and complete locomotion patterns at both lower limb joints, suggested that bi‐cruciate substituting arthroplasty can restore physiological knee motion and normal overall function. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27:1569–1575, 2009