Posterior tibial slope influences joint mechanics and soft tissue loading after total knee arthroplasty.

As a solution to restore knee function and reduce pain, the demand for Total Knee Arthroplasty (TKA) has dramatically increased in recent decades. The high rates of dissatisfaction and revision makes it crucially important to understand the relationships between surgical factors and post-surgery knee performance. Tibial implant alignment in the sagittal plane (i.e., posterior tibia slope, PTS) is thought to play a key role in quadriceps muscle forces and contact conditions of the joint, but the underlying mechanisms and potential consequences are poorly understood. To address this biomechanical challenge, we developed a subject-specific musculoskeletal model based on the bone anatomy and precise implantation data provided within the CAMS-Knee datasets. Using the novel COMAK algorithm that concurrently optimizes joint kinematics, together with contact mechanics, and muscle and ligament forces, enabled highly accurate estimations of the knee joint biomechanics (RMSE <0.16 BW of joint contact force) throughout level walking and squatting. Once confirmed for accuracy, this baseline modelling framework was then used to systematically explore the influence of PTS on knee joint biomechanics. Our results indicate that PTS can greatly influence tibio-femoral translations (mainly in the anterior-posterior direction), while also suggesting an elevated risk of patellar mal-tracking and instability. Importantly, however, an increased PTS was found to reduce the maximum tibio-femoral contact force and improve efficiency of the quadriceps muscles, while also reducing the patellofemoral contact force (by approximately 1.5% for each additional degree of PTS during walking). This study presents valuable findings regarding the impact of PTS variations on the biomechanics of the TKA joint and thereby provides potential guidance for surgically optimizing implant alignment in the sagittal plane, tailored to the implant design and the individual deficits of each patient.

Lower-limb internal loading and potential consequences for fracture healing.

Introduction: Mechanical loading is known to determine the course of bone fracture healing. We hypothesise that lower limb long bone loading differs with knee flexion angle during walking and frontal knee alignment, which affects fracture healing success. Materials and methods: Using our musculoskeletal in silico modelling constrained against in vivo data from patients with instrumented knee implants allowed us to assess internal loads in femur and tibia. These internal forces were associated with the clinical outcome of fracture healing in a relevant cohort of 178 extra-articular femur and tibia fractures in patients using a retrospective approach. Results: Mean peak forces differed with femoral compression (1,330-1,936 N at mid-shaft) amounting to about half of tibial compression (2,299-5,224 N). Mean peak bending moments in the frontal plane were greater in the femur (71-130 Nm) than in the tibia (from 26 to 43 Nm), each increasing proximally. Bending in the sagittal plane showed smaller mean peak bending moments in the femur (-38 to 43 Nm) reaching substantially higher values in the tibia (-63 to -175 Nm) with a peak proximally. Peak torsional moments had opposite directions for the femur (-13 to -40 Nm) versus tibia (15-48 Nm) with an increase towards the proximal end in both. Femoral fractures showed significantly lower scores in the modified Radiological Union Scale for Tibia (mRUST) at last follow-up (p < 0.001) compared to tibial fractures. Specifically, compression (r = 0.304), sagittal bending (r = 0.259), and frontal bending (r = -0.318) showed strong associations (p < 0.001) to mRUST at last follow-up. This was not the case for age, body weight, or localisation alone. Discussion: This study showed that moments in femur and tibia tend to decrease towards their distal ends. Tibial load components were influenced by knee flexion angle, especially at push-off, while static frontal alignment played a smaller role. Our results indicate that femur and tibia are loaded differently and thus require adapted fracture fixation considering load components rather than just overall load level.

Periarticular muscle status affects in vivo tibio-femoral joint loads after total knee arthroplasty.

Background: Total knee arthroplasty (TKA) is a highly effective treatment for severe knee osteoarthritis that is increasingly performed in younger, more active patients. As postoperative muscular impairments may negatively affect surgical outcomes and implant longevity, functional muscle recovery gains increasing importance in meeting future patient demands. This study aimed to assess the status of periarticular muscles in the long-term follow-up after TKA and to evaluate its impact on in vivo tibio-femoral joint loads. Methods: A case series was created, with eight patients with knee osteoarthritis. All subjects received an instrumented knee implant in unilateral TKA. Native computed tomography scans, acquired pre and postoperatively, were used to evaluate distal muscle volumes and fatty infiltration. In vivo tibio-femoral joint loads were measured telemetrically during standing, walking, stair climbing and chair rising and were correlated to muscle status. Results: Postoperatively a reduction in fatty infiltration across all periarticular muscles was pronounced. High average peak loads acted in the tibio-femoral joint ranging from 264% during stand-to-sit activities up to 341% body weight (BW) during stair descent. Fatty infiltration of the m. quadriceps femoris and hamstrings were associated with increased tibio-femoral joint contact forces during walking (r = 0.542; 0.412 and 0.766). Conclusion: The findings suggest that a fatty infiltration of periarticular muscles may lead to increased tibio-femoral joint contact forces. However, we only observed weak correlations between these parameters. Improvements in functional mobility and the restoration of a pain-free joint likely explain the observed postoperative reductions in fatty infiltration. Perioperative rehabilitation approaches targeting residual impairments in muscle quality could, contribute to reduced tibio-femoral joint loads and improved long-term outcomes of TKA. However, it has to be pointed out that the study included a small number of patients, which may limit its validity.

Patellar tendon elastic properties derived from in vivo loading and kinematics.

Patellar complications frequently limit the success of total knee arthroplasty. In addition to the musculoskeletal forces themselves, patellar tendon elastic properties are essential for driving patellar loading. Elastic properties reported in the literature exhibit high variability and appear to differ according to the methodologies used. Specifically in total knee arthroplasty patients, only limited knowledge exists on in vivo elastic properties and their corresponding loads. For the first time, we report stiffness, Young's modulus, and forces of the patellar tendon, derived from four patients with telemetric total knee arthroplasties using a combined imaging and measurement approach. To achieve this, synchronous in vivo telemetric assessment of tibio-femoral contact forces and fluoroscopic assessment of knee kinematics, along with full body motion capture and ground reaction forces, fed musculoskeletal multi-body models to quantify patellar tendon loading and elongation. Mechanical patellar tendon properties were calculated during a squat and a sit-stand-sit activity, with resulting tendon stiffness and Young's modulus ranging from 511 to 1166 N/mm and 259 to 504 MPa, respectively. During these activities, the patellar tendon force reached peak values between 1.31 and 2.79 bodyweight, reaching levels of just âˆ¼0.5 bodyweight below the tibio-femoral forces. The results of this study provide valuable input data for mechanical simulations of the patellar tendon and the whole resurfaced knee.

Patient-specific resurfacing implant knee surgery in subjects with early osteoarthritis results in medial pivot and lateral femoral rollback during flexion: a retrospective pilot study.

PURPOSE: Metallic resurfacing implants have been developed for the treatment of early, small, condylar and trochlear osteoarthritis (OA) lesions. They represent an option for patients who do not fulfill the criteria for unicompartmental knee arthroplasty (UKA) or total knee arthroplasty (TKA) or are too old for biological treatment. Although clinical evidence has been collected for different resurfacing types, the in vivo post-operative knee kinematics remain unknown. The present study aims to analyze the knee kinematics in subjects with patient-specific episealer implants. This study hypothesized that patient-specific resurfacing implants would lead to knee kinematics close to healthy knees, resulting in medial pivot and a high degree of femoral rollback during flexion. METHODS: Retrospective study design. Fluoroscopic analysis during unloaded flexion-extension and loaded lunge was conducted at > 12 months post-surgery in ten episealer knees, and compared to ten healthy knees. Pre- and post-operative clinical data of the episealer knees were collected using a visual analog scale (VAS), the EQ 5d Health, and the Knee Injury and Osteoarthritis Outcome Score (KOOS) questionnaires. RESULTS: A consistent medial pivot was observed in both episealer and healthy knees. Non-significant differences were found in the unloaded (p = 0.15) and loaded (p = 0.51) activities. Although lateral rollback was observed in both groups, it was significantly higher for the episealer knees in both the unloaded (p = 0.02) and loaded (p = 0.01) activities. Coupled axial rotation was significantly higher in the unloaded (p = 0.001) but not in the loaded (p = 0.06) activity in the episealer knees. Improved scores were observed at 1-year post-surgery in the episealer subjects for the VAS (p = 0.001), KOOS (p = 0.001) and EQ Health (p = 0.004). CONCLUSION: At 12 month follow-up, a clear physiological knee kinematics pattern of medial pivot, lateral femoral rollback and coupled axial external femoral rotation during flexion was observed in patients treated with an episealer resurfacing procedure. However, higher femoral rollback and axial external rotation in comparison to healthy knees was observed, suggesting possible post-operative muscle weakness and consequent insufficient stabilization at high flexion.

European Society of Biomechanics S.M. Perren Award 2022: Standardized tibio-femoral implant loads and kinematics.

Knowledge of both tibio-femoral kinematics and kinetics is necessary for fully understanding knee joint biomechanics, guiding implant design and testing, and driving and validating computational models. In 2017, the CAMS-Knee datasets were presented, containing synchronized in vivo implant kinematics measured using a moving fluoroscope and tibio-femoral contact loads measured using instrumented implants from six subjects. However, to date, no representative summary of kinematics and kinetics obtained from measurements at the joint level of the same cohort of subjects exists. In this study, we present the CAMS-Knee standardized subject "Stan", whose reference data include tibio-femoral kinematics and loading scenarios from all six subjects for level and downhill walking, stair descent, squat and sit-to-stand-to-sit. Using the peak-preserving averaging method by Bergmann and co-workers, we derived scenarios for generally high (CAMS-HIGH100), peak, and extreme loading. The CAMS-HIGH100 axial forces reached peaks between 3022 and 3856 N (3.08-3.93 body weight) for the five investigated activities. Anterior-posterior forces were about a factor of ten lower. The axial moment around the tibia was highest for level walking and squatting with peaks of 9.4 Nm and 10.5 Nm acting externally. Internal tibial rotations of up to 8.4° were observed during squat and sitting, while the walking activities showed approximately half the internal rotation. The CAMS-HIGH100 loads were comparable to Bergmann and co-workers', but have the additional benefit of synchronized kinematics. Stan's loads are +11 to +56% higher than the ISO 14243 wear testing standard loads, while the kinematics exhibit markedly different curve shapes. Along with the original CAMS-Knee datasets, Stan's data can be requested at cams-knee.orthoload.com.

Uncertainty in Muscle-Tendon Parameters can Greatly Influence the Accuracy of Knee Contact Force Estimates of Musculoskeletal Models.

Understanding the sources of error is critical before models of the musculoskeletal system can be usefully translated. Using in vivo measured tibiofemoral forces, the impact of uncertainty in muscle-tendon parameters on the accuracy of knee contact force estimates of a generic musculoskeletal model was investigated following a probabilistic approach. Population variability was introduced to the routine musculoskeletal modeling framework by perturbing input parameters of the lower limb muscles around their baseline values. Using ground reaction force and skin marker trajectory data collected from six subjects performing body-weight squat, the knee contact force was calculated for the perturbed models. The combined impact of input uncertainties resulted in a considerable variation in the knee contact force estimates (up to 2.1 BW change in the predicted force), especially at larger knee flexion angles, hence explaining up to 70% of the simulation error. Although individual muscle groups exhibited different contributions to the overall error, variation in the maximum isometric force and pathway of the muscles showed the highest impacts on the model outcomes. Importantly, this study highlights parameters that should be personalized in order to achieve the best possible predictions when using generic musculoskeletal models for activities involving deep knee flexion.

Dynamic Knee Joint Line Orientation Is Not Predictive of Tibio-Femoral Load Distribution During Walking.

Some approaches in total knee arthroplasty aim for an oblique joint line to achieve an even medio-lateral load distribution across the condyles during the stance phase of gait. While there is much focus on the angulation of the joint line in static frontal radiographs, precise knowledge of the associated dynamic joint line orientation and the internal joint loading is limited. The aim of this study was to analyze how static alignment in frontal radiographs relates to dynamic alignment and load distribution, based on direct measurements of the internal joint loading and kinematics. A unique and novel combination of telemetrically measured in vivo knee joint loading and simultaneous internal joint kinematics derived from mobile fluoroscopy ("CAMS-Knee dataset") was employed to access the dynamic alignment and internal joint loading in 6 TKA patients during level walking. Static alignment was measured in standard frontal postoperative radiographs while external adduction moments were computed based on ground reaction forces. Both static and dynamic parameters were analyzed to identify correlations using linear and non-linear regression. At peak loading during gait, the joint line was tilted laterally by 4°-7° compared to the static joint line in most patients. This dynamic joint line tilt did not show a strong correlation with the medial force (R 2: 0.17) or with the mediolateral force distribution (pseudo R 2: 0.19). However, the external adduction moment showed a strong correlation with the medial force (R 2: 0.85) and with the mediolateral force distribution (pseudo R 2: 0.78). Alignment measured in static radiographs has only limited predictive power for dynamic kinematics and loading, and even the dynamic orientation of the joint line is not an important factor for the medio-lateral knee load distribution. Preventive and rehabilitative measures should focus on the external knee adduction moment based on the vertical and horizontal components of the ground reaction forces.

Towards planning of osteotomy around the knee with quantitative inclusion of the adduction moment: a biomechanical approach.

PURPOSE: Despite practised for decades, the planning of osteotomy around the knee, commonly using the Mikulicz-Line, is only empirically based, clinical outcome inconsistent and the target angle still controversial. A better target than the angle of frontal-plane static leg alignment might be the external frontal-plane lever arm (EFL) of the knee adduction moment. Hypothetically assessable from frontal-plane-radiograph skeleton dimensions, it might depend on the leg-alignment angle, the hip-centre-to-hip-centre distance, the femur- and tibia-length. METHODS: The target EFL to achieve a medial compartment force ratio of 50% during level-walking was identified by relating in-vivo-measurement data of knee-internal loads from nine subjects with instrumented prostheses to the same subjects' EFLs computed from frontal-plane skeleton dimensions. Adduction moments derived from these calculated EFLs were compared to the subjects' adduction moments measured during gait analysis. RESULTS: Highly significant relationships (0.88 ≤ R2 ≤ 0.90) were found for both the peak adduction moment measured during gait analysis and the medial compartment force ratio measured in vivo to EFL calculated from frontal-plane skeleton dimensions. Both correlations exceed the respective correlations with the leg alignment angle, EFL even predicts the adduction moment's first peak. The guideline EFL for planning osteotomy was identified to 0.349 times the epicondyle distance, hence deducing formulas for individualized target angles and Mikulicz-Line positions based on full-leg radiograph skeleton dimensions. Applied to realistic skeleton geometries, widespread results explain the inconsistency regarding correction recommendations, whereas results for average geometries exactly meet the most-consented "Fujisawa-Point". CONCLUSION: Osteotomy outcome might be improved by planning re-alignment based on the provided formulas exploiting full-leg-radiograph skeleton dimensions.

Biomechanical models: key considerations in study design.

This manuscript summarizes presentations of a symposium on key considerations in design of biomechanical models at the 2019 Basic Science Focus Forum of the Orthopaedic Trauma Association. The first section outlines the most important characteristics of a high-quality biomechanical study. The second section considers choices associated with designing experiments using finite element modeling versus synthetic bones versus human specimens. The third section discusses appropriate selection of experimental protocols and finite element analyses. The fourth section considers the pros and cons of use of biomechanical research for implant design. Finally, the fifth section examines how results from biomechanical studies can be used when clinical evidence is lacking or contradictory. When taken together, these presentations emphasize the critical importance of biomechanical research and the need to carefully consider and optimize models when designing a biomechanical study.

Retention of Posterior Cruciate Ligament Alone May Not Achieve Physiological Knee Joint Kinematics After Total Knee Arthroplasty: A Retrospective Study.

BACKGROUND: The apparently physiological kinematics of the bicruciate-stabilized total knee arthroplasty (BCS TKA) systems have been attributed to the anterior and posterior post-cam mechanism. Although comparisons between TKA designs with either a retained or a sacrificed cruciate ligament have been conducted, we are not aware of any analyses of 2 implants with identical bearing geometry but different cruciate-ligament strategies under equal loading conditions. Knowledge about the kinematic effect of the different cruciate ligament strategies would potentially be valuable to facilitate preoperative planning and decision-making with regard to selecting the most appropriate implant for a patient. METHODS: This retrospective study included 20 patients: 10 treated with a BCS and 10 treated with a cruciate retaining (CR) TKA. Fluoroscopic analyses during high-flexion activities (unloaded flexion-extension and loaded lunge) were conducted at 24 months postsurgery. All patients completed the Knee Society Score, Forgotten Joint Score, and High-Flexion Knee Score questionnaires preoperatively and postoperatively. RESULTS: The BCS cohort showed greater femoral lateral rollback as well as a medial pivot in both activities. In contrast, the CR cohort showed a significant increase in anterior translation on the medial compartment as well as almost absent femoral lateral rollback. Higher clinical scores were observed in the BCS cohort. CONCLUSIONS: At 24 months postsurgery, despite equal bearing geometry, retention of the posterior cruciate ligament in the CR cohort apparently was insufficient to reduce anterior shift. The BCS cohort showed expected knee joint kinematics; however, the kinematics in this cohort could eventually benefit from a smooth transition between the interchanging surfaces. Further investigation should be focused on the surgical technique and its interaction with the TKA design. LEVEL OF EVIDENCE: Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.

Correction to: The Capacity of Generic Musculoskeletal Simulations to Predict Knee Joint Loading Using the CAMS-Knee Datasets.

The article The Capacity of Generic Musculoskeletal Simulations to Predict Knee Joint Loading Using the CAMS-Knee Datasets, written by Zohreh Imani Nejad et al., was originally published electronically on the publisher's internet portal on January 30, 2020 without open access. With the author(s)' decision to opt for Open Choice the copyright of the article changed on February 18, 2020 to © The Author(s) 2020 and the article is forthwith distributed under a Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.

The Capacity of Generic Musculoskeletal Simulations to Predict Knee Joint Loading Using the CAMS-Knee Datasets.

Musculoskeletal models enable non-invasive estimation of knee contact forces (KCFs) during functional movements. However, the redundant nature of the musculoskeletal system and uncertainty in model parameters necessitates that model predictions are critically evaluated. This study compared KCF and muscle activation patterns predicted using a scaled generic model and OpenSim static optimization tool against in vivo measurements from six patients in the CAMS-knee datasets during level walking and squatting. Generally, the total KCFs were under-predicted (RMS: 47.55%BW, R2: 0.92) throughout the gait cycle, but substiantially over-predicted (RMS: 105.7%BW, R2: 0.81) during squatting. To understand the underlying etiology of the errors, muscle activations were compared to electromyography (EMG) signals, and showed good agreement during level walking. For squatting, however, the muscle activations showed large descrepancies especially for the biceps femoris long head. Errors in the predicted KCF and muscle activation patterns were greatest during deep squat. Hence suggesting that the errors mainly originate from muscle represented at the hip and an associated muscle co-contraction at the knee. Furthermore, there were substaintial differences in the ranking of subjects and activities based on peak KCFs in the simulations versus measurements. Thus, future simulation study designs must account for subject-specific uncertainties in musculoskeletal predictions.

Author Correction: Weight Bearing Activities change the Pivot Position after Total Knee Arthroplasty.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Evaluation and validation of 2D biomechanical models of the knee for radiograph-based preoperative planning in total knee arthroplasty.

Thorough preoperative planning in total knee arthroplasty is essential to reduce implant failure by proper implant sizing and alignment. The "gold standard" in conventional preoperative planning is based on anterior-posterior long-leg radiographs. However, the coronal component alignment is still an open discussion in literature, since studies have reported contradictory outcomes on survivorship, indicating that optimal individual alignment goals still need to be defined. Two-dimensional biomechanical models of the knee have the potential to predict joint forces and, therefore, objectify therapy planning. Previously published two-dimensional biomechanical models were evaluated and validated for the first time in this study by comparison of model predictions to corresponding in vivo measurements obtained from telemetric implants for a one- and two-leg stance. Model input parameters were acquired from weight-bearing anterior-posterior long-leg radiographs and statistical assumptions for patient-specific model adaptation. The overall time from initialization to load prediction was in the range of 7-8 minutes per patient for all models. However, no model could accurately predict the correct trend of knee joint forces over patients. Two dimensional biomechanical models of the knee have the potential to improve preoperative planning in total knee arthroplasty by providing additional individual biomechanical information to the surgeon. Although integration into the clinical workflow might be performed with acceptable costs, the models' accuracy is insufficient for the moment. Future work is needed for model optimization and more sophisticated modelling approaches.

Weight Bearing Activities change the Pivot Position after Total Knee Arthroplasty.

The knee joint center of rotation is altered in the absence of the anterior cruciate ligament, which leads to substantially higher variance in kinematic patterns. To overcome this, total knee arthroplasty (TKA) designs with a high congruency in the lateral compartment have been proposed. The purpose of this study was to analyze the influence of a lateral pivot TKA-design on in-vivo knee joint kinematics. Tibiofemoral motion was retrospectively addressed in 10 patients during unloaded flexion-extension and loaded lunge using single plane fluoroscopy. During the unloaded flexion-extension movement, the lateral condyle remained almost stationary with little rollback at maximum flexion. The medial condyle exhibited anterior translation during the whole flexion cycle. During the loaded lunge movement, a higher degree of rollback compared to the unloaded activity was observed on the lateral condyle, whereas the medial condyle remained almost stationary. The results showed a clear lateral pivot during the unloaded activity, reflective of the implant's geometric characteristics, and a change to a medial pivot and a higher lateral rollback during the weight-bearing conditions, revealing the impact of load and muscle force. It remains unclear if the kinematics with a lateral TKA design could be considered as physiological, due to the limited knowledge available on native knee joint kinematics.

Relationship between muscular and bony anatomy in native hips: a theoretical background for approach-specific implant positioning.

INTRODUCTION:: The aim of this study was to analyse the relationship between bony joint orientation and the distribution of hip musculature. METHODS:: The bone anatomy of the hip (femoral antetorsion (AT), acetabular anteversion (AV), and combined anteversion (AV/AT)) and the muscle volume of the gluteal muscles and the tensor fasciae latae were analysed bilaterally using computed tomography data of 49 patients. Muscle force direction (MFD) was determined for each muscle. The total MFD of the hip musculature was calculated and then correlated with the bony anatomy. RESULTS:: The mean AV, AT, and AV/AT were 21.9° ± 5.9°, 7.22° ± 7.4°, and 29.2° ± 9°, respectively. We found the following mean muscle volumes: gluteus maximus: 780 ± 227 cm3, gluteus medius: 322 ± 82 cm3, gluteus minimus: 85 ± 20 cm3, and tensor fasciae latae: 68 ± 22 cm3. The mean MFD was 18.92° ± 1.29°. We found a uniform distribution of the musculature that was not correlated with the bone anatomy. CONCLUSION:: This study highlights the variability in native acetabular and femoral anatomy and that bone hip anatomy does not correlate with the distribution of hip musculature. Although native acetabular anteversion matches the suggested targets for cup insertion, native combined anteversion is not related to current implant insertion targets. Understanding native muscular anatomy and the alterations that occur with different surgical approaches can serve as an explanatory model for THAs that has become unstable despite the components being implanted within the safe zone.

Loading of the hip and knee joints during whole body vibration training.

During whole body vibrations, the total contact force in knee and hip joints consists of a static component plus the vibration-induced dynamic component. In two different cohorts, these forces were measured with instrumented joint implants at different vibration frequencies and amplitudes. For three standing positions on two platforms, the dynamic forces were compared to the static forces, and the total forces were related to the peak forces during walking. A biomechanical model served for estimating muscle force increases from contact force increases. The median static forces were 122% to 168% (knee), resp. 93% to 141% (hip), of the body weight. The same accelerations produced higher dynamic forces for alternating than for parallel foot movements. The dynamic forces individually differed much between 5.3% to 27.5% of the static forces in the same positions. On the Powerplate, they were even close to zero in some subjects. The total forces were always below 79% of the forces during walking. The dynamic forces did not rise proportionally to platform accelerations. During stance (Galileo, 25 Hz, 2 mm), the damping of dynamic forces was only 8% between foot and knee but 54% between knee and hip. The estimated rises in muscle forces due to the vibrations were in the same ranges as the contact force increases. These rises were much smaller than the vibration-induced EMG increases, reported for the same platform accelerations. These small muscle force increases, along with the observation that the peak contact and muscle forces during vibrations remained far below those during walking, indicate that dynamic muscle force amplitudes cannot be the reason for positive effects of whole body vibrations on muscles, bone remodelling or arthritic joints. Positive effects of vibrations must be caused by factors other than raised forces amplitudes.

Impact of antagonistic muscle co-contraction on in vivo knee contact forces.

BACKGROUND: The onset and progression of osteoarthritis, but also the wear and loosening of the components of an artificial joint, are commonly associated with mechanical overloading of the structures. Knowledge of the mechanical forces acting at the joints, together with an understanding of the key factors that can alter them, are critical to develop effective treatments for restoring joint function. While static anatomy is usually the clinical focus, less is known about the impact of dynamic factors, such as individual muscle recruitment, on joint contact forces. METHODS: In this study, instrumented knee implants provided accurate in vivo tibio-femoral contact forces in a unique cohort of 9 patients, which were used as input for subject specific musculoskeletal models, to quantify the individual muscle forces during walking and stair negotiation. RESULTS: Even between patients with a very similar self-selected gait speed, the total tibio-femoral peak forces varied 1.7-fold, but had only weak correlation with static alignment (varus/valgus). In some patients, muscle co-contraction of quadriceps and gastrocnemii during walking added up to 1 bodyweight (~ 50%) to the peak tibio-femoral contact force during late stance. The greatest impact of co-contraction was observed in the late stance phase of stair ascent, with an increase of the peak tibio-femoral contact force by up to 1.7 bodyweight (66%). CONCLUSIONS: Treatment of diseased and failed joints should therefore not only be restricted to anatomical reconstruction of static limb axes alignment. The dynamic activation of muscles, as a key modifier of lower limb biomechanics, should also be taken into account and thus also represents a promising target for restoring function, patient mobility, and preventing future joint failure. TRIAL REGISTRATION: German Clinical Trials Register: ID: DRKS00000606 , date: 05.11.2010.