Loading of the knee joint during activities of daily living measured in vivo in five subjects

Tensile forces on repaired medial meniscal root tears

PURPOSE

The goals of this study were to measure the tensile forces acting on repaired medial meniscal root lesions and to investigate how they depend on femorotibial rotation, flexion, and compressive load.

METHODS

In 6 human cadaveric knees, the posterior medial meniscal root was completely detached and then repaired with a pullout suture. A force transducer was installed such that it measured tensile forces acting on the suture. The resultant tension at the posterior medial meniscal root was measured for flexion angles up to 120° at 2 levels of femorotibial compressive load (100 and 500 N) in neutral, internal, and external rotation of the knee.

RESULTS

Rotation had a highly significant effect on root tension (P < .001). Internal rotation of the femur increased the resultant tension, whereas external rotation decreased it. The tension at the meniscal root was related to the femorotibial load (P < .001). Although no significance was reached, a trend toward higher flexion angles causing more tension was observed. The highest mean tension of 60.1 ± 20.2 N was generated with internal rotation, a 500-N load, and 90° flexion.

CONCLUSIONS

Our study shows in a human in vitro model that motion and weight loading of the knee can generate considerable tensile forces in the posterior medial meniscal root. Internal rotation of the femur increases the resultant tension substantially, whereas external rotation has the opposite effect.

CLINICAL RELEVANCE

The data can potentially aid the surgeon in finding appropriate rehabilitation exercises after a medial meniscal root repair.

Knee joint loading during gait in healthy controls and individuals with knee osteoarthritis

OBJECTIVE

People with knee osteoarthritis (OA) are thought to walk with high loads at the knee which are yet to be quantified using modeling techniques that account for subject specific electromyography (EMG) patterns, kinematics and kinetics. The objective was to estimate medial and lateral loading for people with knee OA and controls using an approach that is sensitive to subject specific muscle activation patterns.

METHODS

Sixteen OA and 12 control (C) subjects walked while kinematic, kinetic and EMG data were collected. Muscle forces were calculated using an EMG-Driven model and loading was calculated by balancing the external moments with internal muscle and contact forces.

RESULTS

OA subjects walked slower and had greater laxity, static and dynamic varus alignment, less flexion and greater knee adduction moment (KAM). Loading [normalized to body weight (BW)] was no different between the groups but OA subjects had greater absolute medial load than controls and maintained a greater %total load on the medial compartment. These patterns were associated with body mass, sagittal and frontal plane moments, static alignment and close to significance for dynamic alignment. Lateral compartment unloading during mid-late stance was observed in 50% of OA subjects.

CONCLUSIONS

Loading for control subjects was similar to data from instrumented prostheses. Knee OA subjects had high medial contact loads in early stance and half of the OA cohort demonstrated lateral compartment lift-off. Results suggest that interventions aimed at reducing BW and dynamic malalignment might be effective in reducing medial compartment loading and establishing normal medio-lateral load sharing patterns.

Implantable sensor technology: measuring bone and joint biomechanics of daily life in vivo

Stresses and strains are major factors influencing growth, remodeling and repair of musculoskeletal tissues. Therefore, knowledge of forces and deformation within bones and joints is critical to gain insight into the complex behavior of these tissues during development, aging, and response to injury and disease. Sensors have been used in vivo to measure strains in bone, intraarticular cartilage contact pressures, and forces in the spine, shoulder, hip, and knee. Implantable sensors have a high impact on several clinical applications, including fracture fixation, spine fixation, and joint arthroplasty. This review summarizes the developments in strain-measurement-based implantable sensor technology for musculoskeletal research.

Loading of the knee joint during ergometer cycling: telemetric in vivo data

STUDY DESIGN

Within-subject, repeated-measures design.

OBJECTIVES

To measure tibiofemoral contact forces during cycling in vivo and to quantify the influences of power, pedaling cadence, and seat height on tibiofemoral contact forces.

BACKGROUND

Cycling is usually classified as a low-demand activity for the knee joint and is therefore recommended for persons with osteoarthritis and rehabilitation programs following knee surgery. However, there are limited data regarding actual joint loading.

METHODS

Instrumented knee implants with telemetric data transmission were used to measure the tibiofemoral contact forces. Data were obtained in 9 subjects, during ergometer cycling and walking, 15 ± 7 months after total knee arthroplasty. Tibiofemoral forces during cycling at power levels between 25 and 120 W, cadences of 40 and 60 rpm, and 2 seat heights were investigated.

RESULTS

Within the examined power range, tibiofemoral forces during cycling were smaller than those during walking. At the moderate condition of 60 W and 40 rpm, peak resultant forces of 119% of body weight were measured during the pedal downstroke. Shear forces ranged from 5% to 7% of body weight. Forces increased linearly with cycling power. Higher cadences led to smaller forces. A lower seat height did not increase the resultant force but caused higher posterior shear forces.

CONCLUSION

Due to the relatively small tibiofemoral forces, cycling with moderate power levels is suited for individuals with osteoarthritis and rehabilitation programs following knee surgery, such as cartilage repair or total knee replacement. The lowest forces can be expected while cycling at a low power level, a high cadence, and a high seat height.

Relative contributions of design, alignment, and loading variability in knee replacement mechanics

Substantial variation in total knee replacement (TKR) outcomes exists within the patient population. Some of this variability is due to differences in the design of the implanted components and variation in surgical alignment, while other variability is due to differences in the applied forces and torques due to anatomic and physiological differences within a patient population. We evaluated the relative contributions of implant design, surgical alignment, and patient-specific loading variability to overall tibiofemoral joint mechanics to provide insight into which measures can be influenced through design and surgical decisions, and which are inherently dependent on variation within the patient population and should be considered in the robustness of the implant design and surgical procedure. Design, surgical, and loading parameters were assessed using probabilistic finite element methods during simulated stance-phase gait and squat activities. Patient-specific loading was found to be the primary contributor to joint loading and kinematics during low flexion, particularly under conditions of high external loads (for instance, the gait cycle with high internal-external torque), while design and surgical factors, particularly femoral posterior radius and posterior slope of the tibial insert became increasingly important in TKR performance in deeper flexion.

Deterioration of stress distribution due to tunnel creation in single-bundle and double-bundle anterior cruciate ligament reconstructions

Bone tunnel enlargement is a common effect associated with knee laxity after anterior cruciate ligament(ACL) reconstruction. Nevertheless, its exact pathomechanism remains controversial. One of the possible reasons could be bone remodeling due to tunnel creation, which changes the stress environment in the joint. The present study aims to characterize the deteriorated stress distribution on the articular surface, which is due to tunnel creation after single-bundle or double-bundle ACL reconstruction. The stress distributions in the knee following ACL reconstruction under the compression, rotation, and valgus torques were calculated using a validated three-dimensional finite element(FE) model. The results indicate that, (a) under compression,von Mises stress is decreased at lateral and posteromedial regions of single/anteromedial (AM) tunnel, whereas it is increased at anterior region of single/AM tunnel in tibial subchondral bone; (b) the concentration of tensile stress is transferred from the articular surface to the location of graft fixation, and tensile stress in subchondral plate is decreased after ACL reconstruction; (c) severe stress concentration occurs between AM and posterolateral tunnels following the double-bundle reconstruction, which may contribute to the tunnel communication after surgery. In summary, the present study affirms that the deterioration of stress distribution occurs near the articular surface, which may cause the collapse of the tunnel wall, and lead to tunnel enlargement.The present study provides an insight into the effect of tunnel creation on articular stress deterioration after single-bundle or double-bundle ACL reconstruction. These findings provide knowledge on the effect of tunnel enlargement after ACL reconstruction in the long term.

Diurnal variations in articular cartilage thickness and strain in the human knee

Due to the biphasic viscoelastic nature of cartilage, joint loading may result in deformations that require times on the order of hours to fully recover. Thus, cartilaginous tissues may exhibit cumulative strain over the course of each day. The goal of this study was to assess the magnitude and spatial distribution of strain in the articular cartilage of the knee with daily activity. Magnetic resonance (MR) images of 10 asymptomatic subjects (six males and four females) with mean age of 29 years were obtained at 8:00 AM and 4:00 PM on the same day using a 3T magnet. These images were used to create 3D models of the femur, tibia, and patella from which cartilage thickness distributions were quantified. Cartilage thickness generally decreased from AM to PM in all areas except the patellofemoral groove and was associated with significant compressive strains in the medial condyle and tibial plateau. From AM to PM, cartilage of the medial tibial plateau exhibited a compressive strain of -5.1±1.0% (mean±SEM) averaged over all locations, while strains in the lateral plateau were slightly lower (-3.1±0.6%). Femoral cartilage showed an average strain of -1.9±0.6%. The findings of this study show that human knee cartilage undergoes diurnal changes in strain that vary with site in the joint. Since abnormal joint loading can be detrimental to cartilage homeostasis, these data provide a baseline for future studies investigating the effects of altered biomechanics on diurnal cartilage strains and cartilage physiology.

ARE TIBIOFEMORAL COMPRESSIVE LOADS TRANSFERRED ONLY VIA CONTACT MECHANISMS?

The tibiofemoral joint is known to bear compressive loads of several body-weights during daily activities. These forces are known to be transferred through the joint via compression of the tibial and femoral surfaces against one another. The menisci are also known to enhance this process by increasing the contact area and decreasing contact stress. However, calculations presented in this paper suggest that the load-bearing capacity of contact mechanisms is seemingly several times smaller than tibiofemoral joint loads. This suggests that probably one or more non-contact load-bearing mechanism(s) exist, and share the load with the already known contact mechanisms.

Muscles that do not cross the knee contribute to the knee adduction moment and tibiofemoral compartment loading during gait

The aims of this study were to evaluate and explain the individual muscle contributions to the medial and lateral knee compartment forces during gait, and to determine whether these quantities could be inferred from their contributions to the external knee adduction moment. Gait data from eight healthy male subjects were used to compute each individual muscle contribution to the external knee adduction moment, the net tibiofemoral joint reaction force, and reaction moment. The individual muscle contributions to the medial and lateral compartment forces were then found using a least-squares approach. While knee-spanning muscles were the primary contributors, non-knee-spanning muscles (e.g., the gluteus medius) also contributed substantially to the medial compartment compressive force. Furthermore, knee-spanning muscles tended to compress both compartments, while most non-knee-spanning muscles tended to compress the medial compartment but unload the lateral compartment. Muscle contributions to the external knee adduction moment, particularly those from knee-spanning muscles, did not accurately reflect their tendencies to compress or unload the medial compartment. This finding may further explain why gait modifications may reduce the knee adduction moment without necessarily decreasing the medial compartment force.

Influence of loading and activity on the primary stability of cementless tibial trays

Several potential advantages exist for cementless tibial fixation including preservation of bone stock and increased longevity of fixation. However, clinical results have been variable, with reports of extensive radiolucent lines, rapid early migration, and aseptic loosening. The primary stability of an implant depends on the micromotion of the bone-implant interface, which depends on the kinematics and kinetics of the replaced joint. Finite element analysis was used to examine the micromotion for different activities (walking, stair ascent, stair descent, stand-to-sit, and deep knee bend) for three commercially available tibial tray designs. Similar trends were observed for all three designs across the range of activities. Stair ascent and descent generated the highest micromotions, closely followed by level gait. Across these activities, the mean peak (maximum) micromotions measured across the entire resected surface ranged from 64 to 78 (186-239) µm for PFC Sigma, 61-72 (199-251) µm for LCS Complete Duofix, and 92-106 (229-264) µm for LCS Complete. The peak micromotions did not necessarily occur at the peak loads. For instance, the peak micromotions for level walking occurred when there were low axial forces, but moderate varus-valgus moments. This highlights the need to examine the whole gait cycle to properly determine the initial stability of tibial tray designs. By exploring a range of activities and interrogating the entire resected surface, it is possible to differentiate between the relative performance of different implant designs.

In vivo measurement of shoulder joint loads during walking with crutches

BACKGROUND

Following surgery or injury of the lower limbs, the use of walking aids like crutches can cause high loads on the shoulder joint. These loads have been calculated so far with computer models but with strongly varying results.

METHODS

Shoulder joint forces and moments were measured during crutch-assisted walking with complete and partial unloading of the lower limbs. Using telemeterized implants in 6 subjects axillary crutches and forearm crutches were compared. A force direction was more in the direction of the long humeral axis, and slightly lower forces were assumed using axillary crutches. Similar force magnitudes as those experienced during previously measured wheelchair weight relief tasks were expected for complete unloading. The friction-induced moment was hypothesized to act mainly around the medio-lateral axis during the swing phase of the body.

FINDINGS

Maximum loads of up 170% of the bodyweight and 0.8% of the bodyweight times meter were measured with large variations among the patients. Higher forces were found in most of the patients using forearm crutches. The hypothesized predominant moment around the medio-lateral axis was only found in some patients. More often, the other two moments had larger magnitudes with the highest values in female patients. The assumed different load direction could only be found during partial unloading.

INTERPRETATION

In general the force magnitudes were in the range of activities of daily living. However, the number of repetitions during long-lasting crutch use could lead to shoulder problems as a long-term consequence. The slightly lower forces with axillary crutches could be caused by loads acting directly from the crutch on the scapula, thus bypassing the glenohumeral joint. The higher bending moments in the female patients could be a sign of lacking muscle strength for centring the humeral head on the glenoid.

Forces acting on the anterior meniscotibial ligaments

PURPOSE

The purpose of this study was to investigate the forces occurring in human anterior meniscotibial attachment structures under various loading conditions.

METHODS

Twelve human knee joints were exposed to eight loading conditions (tibial rotations and varus/valgus stress) using a previously described knee joint simulator. Subsequently, the joints were axially compressed (1,000 N at 0° 30° and 60° knee flexion) using a materials testing machine. Then, we performed a tensile test to failure of the ligaments. Finally, we used the strains that occurred during the loading tests and the force-elongation diagrams obtained from the tensile test to recursively assess the resulting forces.

RESULTS

In the anterior meniscotibial ligaments, we found maximum mean strains of 3.8 ± 2.3% under external moments and 1.5 ± 0.9% for axial compression. With an ultimate load of 454 ± 220 N for the anterolateral meniscotibial ligament and 397 ± 275 N for the anteromedial meniscotibial ligament, we estimated maximum forces of up to 50.2 N for the knee simulator tests and 22.6 N for the axial compression tests.

CONCLUSIONS

The low forces found in the meniscal ligaments suggest that for normal daily activities, meniscal replacement implants and allografts do not require a very rigid fixation at their bony insertions. However, it remains unknown, what level of force occurs in the meniscotibial ligaments under traumatic situations or impact knee loads. Furthermore, the results of the present study could help to optimize meniscal re-fixation and to improve the properties of meniscal replacement materials, such as tissue-engineered artificial menisci. Moreover, the results could be used for the validation of finite element models of the knee joint with the main focus on the meniscus and its biomechanical relevance for tibiofemoral contact pressure.

Accurate internal-external rotation measurement in total knee prostheses: A magnetic solution

In this work we tackled the problem of accurate measurement of internal-external (IE) rotations in the prosthetic knee. We presented a magnetic measurement system to be implanted in the knee prostheses in order to measure IE without soft tissue artifacts. The measurement system consisted of a permanent magnet attached under the tibial plate of the prosthesis and a combination of magnetic sensors in the polyethylene insert. Two different sensor configurations were designed, and five different angle estimators for measurement of IE angles were defined and tested based on several static and dynamic measurements toward a stereophotogrammetry motion capture system. Also a noise analysis was done to see which estimators are less sensitive to measurement noise. One-sensor configuration provided lower power budget with dynamic RMS error of 0.49° and a noise range of ±0.53°. Two-sensor configuration doubles the power consumption but provided slightly lower dynamic RMS error (0.37°) and a noise range of ±0.42°, and offers the possibility of having redundancy in case of damaged sensor.

The role of patient, surgical, and implant design variation in total knee replacement performance

Clinical studies demonstrate substantial variation in kinematic and functional performance within the total knee replacement (TKR) patient population. Some of this variation is due to differences in implant design, surgical technique and component alignment, while some is due to subject-specific differences in joint loading and anatomy that are inherently present within the population. Combined finite element and probabilistic methods were employed to assess the relative contributions of implant design, surgical, and subject-specific factors to overall tibiofemoral (TF) and patellofemoral (PF) joint mechanics, including kinematics, contact mechanics, joint loads, and ligament and quadriceps force during simulated squat, stance-phase gait and stepdown activities. The most influential design, surgical and subject-specific factors were femoral condyle sagittal plane radii, tibial insert superior-inferior (joint line) position and coronal plane alignment, and vertical hip load, respectively. Design factors were the primary contributors to condylar contact mechanics and TF anterior-posterior kinematics; TF ligament forces were dependent on surgical factors; and joint loads and quadriceps force were dependent on subject-specific factors. Understanding which design and surgical factors are most influential to TKR mechanics during activities of daily living, and how robust implant designs and surgical techniques must be in order to adequately accommodate subject-specific variation, will aid in directing design and surgical decisions towards optimal TKR mechanics for the population as a whole.

Evaluating knee replacement mechanics during ADL with PID-controlled dynamic finite element analysis

Validated computational knee simulations are valuable tools for design phase development of knee replacement devices. Recently, a dynamic finite element (FE) model of the Kansas knee simulator was kinematically validated during gait and deep flexion cycles. In order to operate the computational simulator in the same manner as the experiment, a proportional-integral-derivative (PID) controller was interfaced with the FE model to control the quadriceps actuator excursion and produce a target flexion profile regardless of implant geometry or alignment conditions. The controller was also expanded to operate multiple actuators simultaneously in order to produce in vivo loading conditions at the joint during dynamic activities. Subsequently, the fidelity of the computational model was improved through additional muscle representation and inclusion of relative hip-ankle anterior-posterior (A-P) motion. The PID-controlled model was able to successfully recreate in vivo loading conditions (flexion angle, compressive joint load, medial-lateral load distribution or varus-valgus torque, internal-external torque, A-P force) for deep knee bend, chair rise, stance-phase gait and step-down activities.

Influence of limb alignment on mediolateral loading in total knee replacement: in vivo measurements in five patients

BACKGROUND

Malalignment after total knee replacement could cause overloading of the implant bearing as well as of the bone itself, leading to osteolysis and early loosening. To quantify the stresses the implant has to withstand and to define a safe zone of limb alignment, the total contact forces as well as their mediolateral distribution have to be determined. Analytical gait data and mathematical models have been used for this purpose. We performed this study to determine in vivo loads of five patients after implantation of an instrumented tibial baseplate.

METHODS

Five patients with osteoarthritis of the knee received total knee replacement. The tibial component was instrumented with strain gauges for the measurement of three forces and three moments. The signals from the gauges were transferred telemetrically to an external receiver. At twelve months after surgery, postoperative measurements were obtained with the patients walking at a self-selected comfortable speed across a level walkway. Peak axial and medial forces of fifteen to twenty gait cycles were averaged and reported as a percent of individual body weight.

RESULTS

During the stance phase of the gait cycle, two maxima of the axial force occurred. Typical values were 215% of body weight at the first peak and 266% of body weight at the second peak. The medial load share was typically 73% at the first axial force peak and 65% at the second axial force peak. Analysis of inter-individual variations revealed a linear correlation with limb alignment. A deviation of 1° varus from neutral alignment increased the medial load share by 5%.

CONCLUSIONS

Consistent with the results of previous studies, we found that the force transferred by the medial compartment was usually greater than that transferred by the lateral compartment. Concerning the design of total knee replacements, an asymmetric tibial component with a larger medial contact area could possibly reduce peak contact stress on the bone and improve fixation of the implant. Mediolateral load distribution was quantified and correlated with limb alignment, thereby permitting the effects of malalignment after total knee replacement to be estimated.

Effect of partial meniscectomy at the medial posterior horn on tibiofemoral contact mechanics and meniscal hoop strains in human knees

We examined the influence of partial meniscectomy of 10 mm width on 10 human cadaveric knee joints, as it is performed during the treatment of radial tears in the posterior horn of the medial meniscus, on maximum contact pressure, contact area (CA), and meniscal hoop strain in the lateral and medial knee compartments. In case of 0° and 30° flexion angle, 20% and 50% partial meniscectomy did not influence maximum contact pressure and area. Only in case of 60° knee flexion, 50% partial resection increased medial maximum contact pressure and decreased the medial CA statistically significant. However, 100% partial resection increased maximum contact pressure and decreased CA significantly in the meniscectomized medial knee compartment in all tested knee positions. No significant differences were noted for meniscal hoop strain. From a biomechanical point of view, our in vitro study suggests that the medial joint compartment is not in danger of accelerated cartilage degeneration up to a resection limit of 20% meniscal depth and 10 mm width. Contact mechanics are likely to be more sensitive to partial meniscectomy at higher flexion angles, which has to be further investigated.

Implantable sensor technology: from research to clinical practice

For decades, implantable sensors have been used in research to provide comprehensive understanding of the biomechanics of the human musculoskeletal system. These complex sensor systems have improved our understanding of the in vivo environment by yielding in vivo measurements of force, torque, pressure, and temperature. Historically, implants have been modified to be used as vehicles for sensors and telemetry systems. Recently, microfabrication and nanofabrication technology have sufficiently evolved that wireless, passive sensor systems can be incorporated into implants or tissue with minimal or no modification to the host implant. At the same time, sensor technology costs per unit have become less expensive, providing opportunities for use in daily clinical practice. Although diagnostic implantable sensors can be used clinically without significant increases in expense or surgical time, to date, orthopaedic smart implants have been used exclusively as research tools. These implantable sensors can facilitate personalized medicine by providing exquisitely accurate in vivo data unique to each patient.

Comparison of ISO standard and TKR patient axial force profiles during the stance phase of gait

Preclinical endurance testing of total knee replacements (TKRs) is performed using International Organization for Standardization (ISO) load and motion protocols. The standards are based on data from normal subjects and may not sufficiently mimic in vivo implant conditions. In this study, a mathematical model was used to calculate the axial force profile of 30 TKR patients with two current implant types, 22 with NexGen and eight with Miller-Galante II Cruciate-Retaining TKRs, and statistically compare the axial force specified by the ISO standard to the TKR patients. Significant differences were found between the axial forces of both groups of TKR patients and the ISO standard at local maxima and minima points in the first half of stance. The force impulse (area under the axial force curve, representing cumulative loading) was smaller for the ISO standard than the TKR patients, but only for those with NexGen implants. Waveform analysis using the coefficient of multiple correlation showed that the ISO and TKR patient axial force profiles were similar. The combined effect of ISO standard compressive load and motion differences from TKR patients could explain some of the differences between the wear scars on retrieved tibial components and those tested in total joint simulators.

The development of lower limb musculoskeletal models with clinical relevance is dependent upon the fidelity of the mathematical description of the lower limb. Part I: Equations of motion

Contemporary musculoskeletal modelling research is based upon the assumption that such models will evolve into clinical tools that can be used to guide therapeutic interventions. However, there are a number of questions that must be addressed before this becomes a reality. At its heart, musculoskeletal modelling is a process of formulating and then solving the equations of motion that describe the movement of body segments. Both of these steps are challenging. This article argues that traditional approaches to musculoskeletal modelling have been heavily influenced by the need to simplify this process (and in particular the solution process), and that this has to some degree resulted in approaches that are contrary to the principles of classical mechanics. It is suggested that future work is required to understand how these simplifications affect the outputs of musculoskeletal modelling studies. Equally, to increase their clinical relevance, the models of the future should adhere more closely to the classical mechanics on which they are based.

[Anterior meniscotibial ligaments. Forces under various load conditions]

The main biomechanical function of the knee meniscus is to enlarge the contact area of the tibiofemoral joint leading to a reduction in articular cartilage contact stress. The meniscal attachments are essential for converting the axial load into circumferential tension in the meniscal periphery. Consequently, meniscal substitutes need sufficient anchorage to the tibial plateau to adequately restore the biomechanical function of a replaced meniscus. Therefore the aim of the present study was to investigate the loads acting on the anterior meniscotibial attachments under various joint loads.

Compressive tibiofemoral force during crouch gait

Crouch gait, a common walking pattern in individuals with cerebral palsy, is characterized by excessive flexion of the hip and knee. Many subjects with crouch gait experience knee pain, perhaps because of elevated muscle forces and joint loading. The goal of this study was to examine how muscle forces and compressive tibiofemoral force change with the increasing knee flexion associated with crouch gait. Muscle forces and tibiofemoral force were estimated for three unimpaired children and nine children with cerebral palsy who walked with varying degrees of knee flexion. We scaled a generic musculoskeletal model to each subject and used the model to estimate muscle forces and compressive tibiofemoral forces during walking. Mild crouch gait (minimum knee flexion 20-35°) produced a peak compressive tibiofemoral force similar to unimpaired walking; however, severe crouch gait (minimum knee flexion>50°) increased the peak force to greater than 6 times body-weight, more than double the load experienced during unimpaired gait. This increase in compressive tibiofemoral force was primarily due to increases in quadriceps force during crouch gait, which increased quadratically with average stance phase knee flexion (i.e., crouch severity). Increased quadriceps force contributes to larger tibiofemoral and patellofemoral loading which may contribute to knee pain in individuals with crouch gait.

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.

The influence of cyclic concentric and eccentric submaximal muscle loading on cell viability in the rabbit knee joint

BACKGROUND

Cartilage loading is associated with the onset and progression of osteoarthritis and cell death may play an important role in these processes. Although much is known about cell death in joint impact loading, there is no information on joints loaded by muscular contractions. The aim of this study was to evaluate the influence of muscle generated eccentric and concentric submaximal joint loading on chondrocyte viability. We hypothesised that eccentric muscle activation leads to increased cell death rates compared to concentric loading and to controls.

METHODS

16 rabbits received either 50 min of uni-lateral, cyclic eccentric (n=8) or concentric (n=8) knee loading. Muscle activation for these dynamic conditions was equivalent to an activation level that produced 20% of maximum isometric force. Contralateral joints served as unloaded controls. Cell viability was assessed using confocal microscopy.

FINDINGS

Eccentric contractions produced greater knee loading than concentric contractions. Sub-maximal contractions caused a significant increase in cell death in the loaded knees compared to the unloaded controls, and eccentric loading caused significantly more cell death than concentric loading.

INTERPRETATION

Cyclic sub-maximal muscle loading of the knee caused increased chondrocyte death in rabbits. These findings suggest that low levels of joint loading for prolonged periods, as occurs in endurance exercise or physical labour, may cause chondrocyte death, thereby predisposing joints to degeneration.

Role of the anterior intermeniscal ligament in tibiofemoral contact mechanics during axial joint loading

The anterior intermeniscal ligament (AIML) is an anatomically distinct structure that connects the anterior horns of the medial and lateral menisci. We hypothesized that both menisci work together as a unit in converting axial joint loading into circumferential hoop stresses, due to intermeniscal attachments. Therefore, loss of the AIML could lead to increased tibiofemoral contact stress and predispose to arthritic change. In this cadaveric study, we compared tibiofemoral contact pressures on axial loading, before and after sectioning of the AIML. Five fresh frozen human cadaveric knees were mounted on a linear x-y motion table and loaded in extension under axial compression of 1800N (about 2.5 times body weight for a 70kg individual), using a materials testing machine. Tibiofemoral contact pressures before and after sectioning of the AIML were measured using resistive pressure sensors. Contrary to our hypothesis, sectioning of the AIML produced no statistically significant increase in mean contact pressure, peak contact pressure or change in contact area, in either the medial or lateral compartment of the knees. This implies that the menisci work independently in converting axial loads into circumferential hoop stresses, and is probably due to their individual root attachments to the tibia. Based on this study, inadvertent sectioning of the AIML during knee surgery, e.g., arthroscopy, anterograde tibia nailing, anterior cruciate ligament reconstruction, meniscus transplantation and unicondylar knee replacement, is functionally insignificant.

Patellofemoral joint contact forces during activities with high knee flexion

The patellofemoral (PF) joint plays an essential role in knee function, but little is known about the in vivo loading conditions at the joint. We hypothesized that the forces at the PF joint exceed the tibiofemoral (TF) forces during activities with high knee flexion. Motion analysis was performed in two patients with telemetric knee implants during walking, stair climbing, sit-to-stand, and squat. TF and PF forces were calculated using a musculoskeletal model, which was validated against the simultaneously measured in vivo TF forces, with mean errors of 10% and 21% for the two subjects. The in vivo peak TF forces of 2.9-3.4 bodyweight (BW) varied little across activities, while the peak PF forces showed significant variability, ranging from less than 1 BW during walking to more than 3 BW during high flexion activities, exceeding the TF forces. Together with previous in vivo measurements at the hip and knee, the PF forces determined here provide evidence that peak forces across these joints reach values of around 3 BW during high flexion activities, also suggesting that the in vivo loading conditions at the knee can only be fully understood if the forces at the TF and the PF joints are considered together.

Knee joint forces: prediction, measurement, and significance

Knee forces are highly significant in osteoarthritis and in the survival and function of knee arthroplasty. A large number of studies have attempted to estimate forces around the knee during various activities. Several approaches have been used to relate knee kinematics and external forces to internal joint contact forces, the most popular being inverse dynamics, forward dynamics, and static body analyses. Knee forces have also been measured in vivo after knee arthroplasty, which serves as valuable validation of computational predictions. This review summarizes the results of published studies that measured knee forces for various activities. The efficacy of various methods to alter knee force distribution, such as gait modification, orthotics, walking aids, and custom treadmills are analyzed. Current gaps in our knowledge are identified and directions for future research in this area are outlined.

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.

Piezoelectric load measurement model in knee implants

This paper explores the feasibility of a new sensing platform for knee implant diagnostics. The proposed unit measures force and transmits the reading information wirelessly to an external receiving unit. This device is to be located in the tibial tray of the knee implant. The system measures force through the use of piezoelectric elements housed in the insert. At the same time, the piezoelectric material can generate enough energy to transmit the measurements without requiring batteries. Only the modeling of the piezoelectric voltage output is discussed at present. The force measurement can provide useful information about ligament balance while helping in the post-operative physical therapy.

Three-dimensional joint kinematics of ACL-deficient and ACL-reconstructed knees during stair ascent and descent

Mechanical environmental changes in the knee are induced by altered joint kinematics under cyclic loading during activities of daily living after anterior cruciate ligament (ACL) injury. This is considered a risk factor in progressive cartilage degeneration and the early onset of osteoarthritis following ACL injury and even after reconstructive surgery. The purpose of this study was to examine 3D joint kinematics of ACL-deficient and ACL-reconstructed knees to health controls during stair ascent and descent. A 3D optical video motion capture system was used to record coordinate data from reflective markers positioned on subjects as they ascended and descended a custom-built staircase. Spatiotemporal gait and knee joint kinematic variables were calculated and further analyzed. The ACL-deficient knees exhibited a significant extension deficit compared to the ACL-intact controls. A more varus and internally rotated tibial position was also identified in the ACL-deficient knees during both stair ascent and descent. The ACL-reconstructed knees exhibited less abnormality in both spatiotemporal gait parameters and joint kinematics, but these variables were not fully restored to a normal level. The kinematic profiles of the ACL-reconstructed knees were more similar to those of the ACL-deficient knees when compared to the ACL-intact knees. This suggests that the ACL-reconstructed knees had been "under-corrected" rather than "over-corrected" by the reconstructive surgery procedure. Findings from this study may provide more insight with respect to improving ACL reconstruction surgical techniques, which may aid the early progression of cartilage degeneration in ACL-reconstructed knees.

Determination of typical patterns from strongly varying signals

Forces measured in human joints vary considerably when an activity such as walking is carried out by different subjects or when it is repeated. 'Typical' standardised force-time patterns are needed to test and improve joint implants. Mechanically most important for their endurance are the magnitudes and times of force maxima and minima. They should equal the arithmetic means from the single measurements. Similar problems exist when evaluating other strongly varying signals, as in gait analysis. The new method to calculate typical signals (TSs) enhances existing dynamic time warping (DTW) procedures. It allows us to combine any number of signals. The sequence of input signals--used for calculating the TS--has only a minor influence. The accuracy of the method was tested numerically on signals for which the typical patterns could be defined exactly, and also on real joint forces that varied to different extents.

Predicted knee kinematics and kinetics during functional activities using motion capture and musculoskeletal modelling in healthy older people

Knowledge of joint forces and moments is essential for comparisons between healthy people and those with pathological conditions, with observed changes at joints providing basis for a particular intervention. Currently the literature analysing both kinematics and kinetics at the knee has been limited to small samples, typically of young subjects or those who have undergone joint arthroplasty. In this study, we examined tibiofemoral joint (TFJ) kinematics and kinetics during gait, sit-stand-sit, and step-descent in 20 healthy older subjects (aged 53-79 years) using motion capture data and inverse dynamic musculoskeletal models. Mean peak distal-proximal force in the TFJ were 3.1, 1.6, and 3.5 times body weight (N/BW) for gait, sit-stand, and step-descent respectively. There were also significant posterior-anterior forces, with sit-stand activity peaking at 1.6 N/BW. Moments about the TFJ peaked at a mean of 0.07 Nm/BW during the sit-stand activity. One of the most important findings of this study was variability found across the subjects, who spanned a wide age range, showing large standard deviations in all of the activities for both kinematics and kinetics. These data have provided an initial prediction for assessing kinematics and kinetics in the older population. Larger studies are needed to refine the database, in particular to reduce the variability in the results by studying sub-populations, to enable more robust comparisons between healthy and pathological TFJ kinematics and kinetics.

Individual muscle contributions to the axial knee joint contact force during normal walking

Muscles are significant contributors to the high joint forces developed in the knee during human walking. Not only do muscles contribute to the knee joint forces by acting to compress the joint, but they also develop joint forces indirectly through their contributions to the ground reaction forces via dynamic coupling. Thus, muscles can have significant contributions to forces at joints they do not span. However, few studies have investigated how the major lower-limb muscles contribute to the knee joint contact forces during walking. The goal of this study was to use a muscle-actuated forward dynamics simulation of walking to identify how individual muscles contribute to the axial tibio-femoral joint force. The simulation results showed that the vastii muscles are the primary contributors to the axial joint force in early stance while the gastrocnemius is the primary contributor in late stance. The tibio-femoral joint force generated by these muscles was at times greater than the muscle forces themselves. Muscles that do not cross the knee joint (e.g., the gluteus maximus and soleus) also have significant contributions to the tibio-femoral joint force through their contributions to the ground reaction forces. Further, small changes in walking kinematics (e.g., knee flexion angle) can have a significant effect on the magnitude of the knee joint forces. Thus, altering walking mechanics and muscle coordination patterns to utilize muscle groups that perform the same biomechanical function, yet contribute less to the knee joint forces may be an effective way to reduce knee joint loading during walking.