Calibrating joint capsule mechanoreceptors as in vivo soft tissue load cells

A method has been developed whereby the discharge of mechanically sensitive neurons from the cat knee joint capsule can be calibrated and used as load cells. The neurons are located in the upper edge of the capsule which has been previously modeled as a suspension cable and where the loading has been shown to be one dimensional. The calibration procedure relies upon applying known point loads to the cable and measuring its shape. The biomechanical model is then used to compute the cable tension at the neuron location. Results for 20 neurons showed a strong linear relationship between the tension and the frequency of neuronal discharge (r = 0.96, S.D. = 0.05). For 11 of these neurons the in vivo calibration was verified by subsequently excising the posterior capsule and recording from the same neuron while subjecting the cable to measured uniaxial loads. Results showed good agreement between the in vivo and in vitro calibrations. Once calibrated these neurons can be used as load sensors to study in vivo joint loading.     

A biomechanical model of the posterior knee capsule of the cat

A biomechanical model is presented which represents the upper edge of the posterior knee capsule in the cat as a two-segment, vertically loaded catenary suspension cable from which the capsule sheet is suspended. Data are presented which show that the upper edge of the capsule is organized as a cable, which spans the notch between the femoral condyles. When a point load is applied to the cable, measurement of the cable shape allows for calculation of the cable tension and the downward distributed loads acting on the cable. This method was used to measure the in-vivo cable tension and the distributed downward loading acting on the capsule cable. The results show that the lateral side of the posterior joint capsule sustains a higher loading than the medial side.     

Computational modeling of a dynamic knee simulator for reproduction of knee loading

As a first step towards reproducing desired three-dimensional joint loading and motion on a dynamic knee simulator, the goal of this study was to develop and verify a three-dimensional computational model that generated control profiles for the simulator using desired knee loading and motion as model inputs. The developed model was verified by predicting tibio-femoral loading on an instrumented analog knee for given actuator forces and the ability to generate simulator control profiles was demonstrated using a three-dimensional walking profile. The model predicted axial tibia loading for a sagittal-plane dual-limb squat within 1% of measured peak loading. Adding out-of-sagittal-plane forces decreased the accuracy of load prediction. The model generated control profiles to the simulator that produced axial tibia loading within 16% of desired for walking. Discrepancies in predicted and measured quadriceps forces influenced the accuracy of the generated control profiles. Future work will replace the analog knee in both the model and machine with a prosthetic knee.     

Intrinsic innervation of the rat knee joint articular capsule and ligaments

In spite of the practical importance of having a detailed knowledge of knee joint innervation to understand the pathophysiologic aspects, little information is now available concerning the density and pattern of the nerve fibres which are distributed to it. The present study has been designed to investigate the density and distribution of nerve fibres and receptor corpuscles in the knee joint articular capsule, cruciate and collateral ligaments in the rat, using the acetylcholinesterase (AChE) histochemical in toto staining technique. The investigation was performed on male Wistar rats of 3 months of age, some of which had been treated with capsaicin to deplete their afferent 'C' fibres of their content of neuropeptides. AChE-positive nerve fibres and different types of receptor corpuscle endings were found within articular capsule and ligaments. The highest density of AChE-positive nerve fibres was noticeable in the fibular collateral ligament followed by the tibial collateral ligament, the posterior cruciate ligament, the anterior cruciate ligament and the articular capsule. In the articular capsule the number of type I endings was higher than in the ligaments. The opposite is true for the other type of receptor corpuscles found as well as for nerve endings. Capsaicin treatment significantly reduced the density of AChE-positive nerve fibres in knee joint ligaments but did not affect nerve fibres in the articular capsule. Moreover, it caused the disappearance of some kind of receptor corpuscles within the collateral and cruciate ligaments. The above data collectively suggest that the AChE in toto staining technique may represent a good method for investigating joint innervation and that a significant percentage of nerve fibres supplying knee joint ligaments is represented by C fibre afferents.     

Sensitivity of insertion locations on length patterns of anterior cruciate ligament fibers

A technique is demonstrated, employing an instrumented spatial linkage, for the determination of the length patterns of discrete fiber bundles within a ligament under controlled loading conditions. The instrumented spatial linkage was used to measure the three-dimensional joint motion. The linkage was also used as a three-dimensional coordinate digitizer to determine the spatial location of bony landmarks and the ligament's insertion areas. The length of pseudo fiber bundles was determined as the straight line distance between bone attachments. A comparison is presented, showing good agreement, between elongation patterns obtained from this method and those measured using an instrumented fine wire cable fiber. A sensitivity analysis was performed to evaluate the influence of tibial and femoral attachment location on the length pattern of fiber bundles of the anterior cruciate ligament. It was found that the relationship between fiber elongation and knee flexion depended strongly on the fibers femoral attachment location but not on its tibial attachment location.     

The effect of hamstring muscle compensation for anterior laxity in the ACL-deficient knee during gait

The hamstring muscles have been recognized as an important element in compensating for the loss of stability in the ACL-deficient knee, but it is still not clear whether the hamstring muscle force can completely compensate for the loss of ACL, and the consequences of increased hamstring muscle force. A two-dimensional anatomical knee model in the sagittal plane was developed to examine the effect of various levels of hamstring muscle activation on restraining anterior tibial translation in the ACL-deficient knee during level walking. The model included the tibiofemoral and patellofemoral joints, four major ligaments, the medial capsule, and five muscle units surrounding the knee. Simulations were conducted to determine anterior tibial translation and internal joint loading at a single selected position when the knee was under a peak external flexion moment during early stance phase of gait. Incremental hamstring muscle forces were applied to the modeled normal and the ACL-deficient knees. Results of simulations showed that the ACL injury increased the anterior tibial translation by 11.8mm, while 56% of the maximal hamstring muscle force could reduce the anterior translation of the tibia to a normal level during the stance phase of gait. The consequences of increased hamstring muscle force included increased quadriceps muscle force and joint contact force.     

Physical Weight Loading Induces Expression of Tryptophan Hydroxylase 2 in the Brain Stem

Sustaining brain serotonin is essential in mental health. Physical activities can attenuate mental problems by enhancing serotonin signaling. However, such activity is not always possible in disabled individuals or patients with dementia. Knee loading, a form of physical activity, has been found to mimic effects of voluntary exercise. Focusing on serotonergic signaling, we addressed a question: Does local mechanical loading to the skeleton elevate expression of tryptophan hydroxylase 2 (tph2) that is a rate-limiting enzyme for brain serotonin? A 5 min knee loading was applied to mice using 1 N force at 5 Hz for 1,500 cycles. A 5-min treadmill running was used as an exercise (positive) control, and a 90-min tail suspension was used as a stress (negative) control. Expression of tph2 was determined 30 min – 2 h in three brain regions ––frontal cortex (FC), ventromedial hypothalamus (VMH), and brain stem (BS). We demonstrated for the first time that knee loading and treadmill exercise upregulated the mRNA level of tph2 in the BS, while tail suspension downregulated it. The protein level of tph2 in the BS was also upregulated by knee loading and downregulated by tail suspension. Furthermore, the downregulation of tph2 mRNA by tail suspension can be partially suppressed by pre-application of knee loading. The expression of tph2 in the FC and VMH was not significantly altered with knee loading. In this study we provided evidence that peripheral mechanical loading can activate central tph2 expression, suggesting that physical cues may mediate tph2-cathalyzed serotonergic signaling in the brain.     

In vitro measurement of the restraining role of the anterior cruciate ligament during walking and stair ascent

The study aimed to test the hypothesis that the restraining role of the anterior cruciate ligament (ACL) of the knee is significant during the activities of normal walking and stair ascent. The role of the ACL was determined from the effect of ACL excision on tibiofemoral displacement patterns measured in vitro for fresh-frozen knee specimens subjected to simulated knee kinetics of walking (n = 12) and stair ascent (n = 7). The knee kinetics were simulated using a newly developed dynamic simulator able to replicate the sagittal-plane knee kinetics with reasonable accuracy while ensuring unconstrained tibiofemoral kinematics. The displacements were measured using a calibrated six degree-of-freedom electromechanical goniometer. For the simulation of the walking cycle, two types of knee flexion/extension moment patterns were used: the more common "biphasic" pattern, and an extensor muscle force intensive pattern. For both of these patterns, the restraining role of the ACL to tibial anterior translation was found to be significant throughout the stance phase and in the terminal swing phase, when the knee angle was in the range of 4 degrees to 30 degrees. The effect of ACL excision was an increase in tibial anterior translation by 4 mm to 5 mm. For the stair ascent cycle, however, the restraining role of the ACL was significant only during the terminal stance phase, and not during the initial and middle segments of the phase. Although, in these segments, the knee moments were comparable to that in walking, the knee angle was in the range of 60 degrees to 70 degrees. These results have been shown to be consistent with available data on knee mechanics and ACL function measured under static loading conditions.     

Injury tolerance and moment response of the knee joint to combined valgus bending and shear loading

Valgus bending and shearing of the knee have been identified as primary mechanisms of injuries in a lateral loading environment applicable to pedestrian-car collisions. Previous studies have reported on the structural response of the knee joint to pure valgus bending and lateral shearing, as well as the estimated injury thresholds for the knee bending angle and shear displacement based on experimental tests. However, epidemiological studies indicate that most knee injuries are due to the combined effects of bending and shear loading. Therefore, characterization of knee stiffness for combined loading and the associated injury tolerances is necessary for developing vehicle countermeasures to mitigate pedestrian injuries. Isolated knee joint specimens (n=40) from postmortem human subjects were tested in valgus bending at a loading rate representative of a pedestrian-car impact. The effect of lateral shear force combined with the bending moment on the stiffness response and the injury tolerances of the knee was concurrently evaluated. In addition to the knee moment-angle response, the bending angle and shear displacement corresponding to the first instance of primary ligament failure were determined in each test. The failure displacements were subsequently used to estimate an injury threshold function based on a simplified analytical model of the knee. The validity of the determined injury threshold function was subsequently verified using a finite element model. Post-test necropsy of the knees indicated medial collateral ligament injury consistent with the clinical injuries observed in pedestrian victims. The moment-angle response in valgus bending was determined at quasistatic and dynamic loading rates and compared to previously published test data. The peak bending moment values scaled to an average adult male showed no significant change with variation in the superimposed shear load. An injury threshold function for the knee in terms of bending angle and shear displacement was determined by performing regression analysis on the experimental data. The threshold values of the bending angle (16.2 deg) and shear displacement (25.2 mm) estimated from the injury threshold function were in agreement with previously published knee injury threshold data. The continuous knee injury function expressed in terms of bending angle and shear displacement enabled injury prediction for combined loading conditions such as those observed in pedestrian-car collisions.     

Material properties of the human lumbar facet joint capsule

The human facet joint capsule is one of the structures in the lumbar spine that constrains motions of vertebrae during global spine loading (e.g., physiological flexion). Computational models of the spine have not been able to include accurate nonlinear and viscoelastic material properties, as they have not previously been measured. Capsules were tested using a uniaxial ramp-hold protocol or a haversine displacement protocol using a commercially available materials testing device. Plane strain was measured optically. Capsules were tested both parallel and perpendicular to the dominant orientation of the collagen fibers in the capsules. Viscoelastic material properties were determined. Parallel to the dominant orientation of the collagen fibers, the complex modulus of elasticity was E*=1.63MPa, with a storage modulus of E'=1.25MPa and a loss modulus of: E" =0.39MPa. The mean stress relaxation rates for static and dynamic loading were best fit with first-order polynomials: B(epsilon) = 0.1110epsilon-0.0733 and B(epsilon)= -0.1249epsilon + 0.0190, respectively. Perpendicular to the collagen fiber orientation, the viscous and elastic secant moduli were 1.81 and 1.00 MPa, respectively. The mean stress relaxation rate for static loading was best fit with a first-order polynomial: B (epsilon) = -0.04epsilon - 0.06. Capsule strength parallel and perpendicular to collagen fiber orientation was 1.90 and 0.95 MPa, respectively, and extensibility was 0.65 and 0.60, respectively. Poisson's ratio parallel and perpendicular to fiber orientation was 0.299 and 0.488, respectively. The elasticity moduli were nonlinear and anisotropic, and capsule strength was larger aligned parallel to the collagen fibers. The phase lag between stress and strain increased with haversine frequency, but the storage modulus remained large relative to the complex modulus. The stress relaxation rate was strain dependent parallel to the collagen fibers, but was strain independent perpendicularly.     

The effect on patellofemoral joint stability of selective cutting of lateral retinacular and capsular structures

Patient selection for lateral retinacular release (LRR) and its efficacy are controversial. Iatrogenic medial subluxation can occur with inappropriate LRR. The aim of this study was to determine the reduction in patellofemoral stability with progressively more extensive LRR. The force required to displace the patella 10 mm medially and laterally in nine cadaveric knees was measured with and without loading of the quadriceps and iliotibial band. The knee was tested intact, then after progressive release beginning proximal to the patella (PR), the mid-level between the proximal and distal limit of the patella (MR) where the fibres are more transverse, then distally till Gerdy's tubercle (DR) and finally the joint capsule (CR). Both medial and lateral stability decreased with progressive releases, larger for the medial. The MR caused a significant reduction of lateral stability between 30° and 90° of knee flexion. There was an 8% reduction in medial stability at 0° flexion with a complete LRR (DR). A comparable reduction in medial stability in the loaded knee at 20° and 30° flexion was obtained with MR alone, with no further reduction after DR. A capsular release caused a further reduction in medial stability at 0° and 20° and this was marked in the unloaded knee. In extension, the main lateral restraint was the joint capsule. At 30° flexion, the transverse fibres were the main contributor to the lateral restraint.     

A novel method for determining articular cartilage chondrocyte mechanics in vivo

Work relating the mechanical states of articular cartilage chondrocytes to their biosynthetic responses is based on measurements in isolated cells or cells in explant samples removed from their natural in situ environment. Neither the mechanics nor the associated biological responses of chondrocytes have ever been studied in cartilage within a joint of a live animal, and no such measurements have ever been performed using physiologically relevant joint loading through muscular contractions. The purpose of this study was to design and apply a method to study the mechanics of chondrocytes in the exposed but fully intact knee of live animals, which was loaded near-physiologically through muscular contraction. In order to achieve this purpose, we developed an accurate and reliable method based on two-photon laser excitation microscopy. Near-physiological knee joint loading was achieved through controlled electrical activation of the knee extensor muscles that compress the articulating surfaces of the femur, tibia and patella. Accuracy of the system was assessed by inserting micro-beads of known dimensions into the articular cartilage of the mouse knee and comparing the measured volumes and diameters in the principal directions with known values of the beads. Accuracy was best in the plane perpendicular to the optical axis (average error = 1%) while it was slightly worse, but still excellent, along the optical axis (average error = 3%). Reliability of cell volume and shape measurements was 0.5% on average, and 2.9% in the worst-case-scenario. Pilot measurements of chondrocyte deformations upon sub-maximal muscular loading causing a mean articular contact pressure of 1.9 ± 0.2 MPa showed an "instantaneous" decrease in cell height (17 ± 4.5%) and loss of cell volume (22.3 ± 2.4%) that took minutes to recover upon deactivation of the knee extensor muscles.     

Development and validation of a 3-D model to predict knee joint loading during dynamic movement

The purpose of this study was to develop a subject-specific 3-D model of the lower extremity to predict neuromuscular control effects on 3-D knee joint loading during movements that can potentially cause injury to the anterior cruciate ligament (ACL) in the knee. The simulation consisted of a forward dynamic 3-D musculoskeletal model of the lower extremity, scaled to represent a specific subject. Inputs of the model were the initial position and velocity of the skeletal elements, and the muscle stimulation patterns. Outputs of the model were movement and ground reaction forces, as well as resultant 3-D forces and moments acting across the knee joint. An optimization method was established to find muscle stimulation patterns that best reproduced the subject's movement and ground reaction forces during a sidestepping task. The optimized model produced movements and forces that were generally within one standard deviation of the measured subject data. Resultant knee joint loading variables extracted from the optimized model were comparable to those reported in the literature. The ability of the model to successfully predict the subject's response to altered initial conditions was quantified and found acceptable for use of the model to investigate the effect of altered neuromuscular control on knee joint loading during sidestepping. Monte Carlo simulations (N = 100,000) using randomly perturbed initial kinematic conditions, based on the subject's variability, resulted in peak anterior force, valgus torque and internal torque values of 378 N, 94 Nm and 71 Nm, respectively, large enough to cause ACL rupture. We conclude that the procedures described in this paper were successful in creating valid simulations of normal movement, and in simulating injuries that are caused by perturbed neuromuscular control.     

Decreased frontal plane hip joint moments in runners with excessive varus excursion at the knee

Knee varus position and motion have been correlated with increased medial knee loading during gait. The purpose of this study is to determine whether runners with excessive varus excursion (EVE) at the knee demonstrate frontal plane knee and hip kinetics that are different from those of runners with normal varus excursion (NVE). Twelve runners with EVE were compared with 12 NVE subjects using three-dimensional kinematics and kinetics. Frontal plane angles and moments were compared at the knee and hip. Runners with EVE had significantly greater abductor moment of the knee (p = .004) and lower peak abductor moment of the hip (p = .047). Runners with EVE demonstrate knee and hip mechanics thought to be associated with increased medial tibiofemoral loading. Further understanding of how changing hip abductor moments may affect changes in knee abductor moments during running may potentially lead to interventions that augment long-term risk of injury.     

Parameter identification of the human lower limb under dynamic, transient torsional loading

The response of the lower limb to dynamic, transient torsional loading applied at the foot has been measured for a male test subject. The dynamic loading was provided by a computer controlled pneumatic system which applied single haversine (i.e. half cycle of a sine wave) axial moment pulses of variable amplitude (0-100 Nm) and duration (50-600 ms). Potentiometers measured the absolute rotations of the three leg segments. Test variables included rotation direction, weight bearing and joint flexion. Two approaches were explored for specifying parameters (i.e. inertia, damping, stiffness) of a three degree-of-freedom dynamic system model which best duplicated the measured response. One approach involved identification of linear parameters by means of optimization while the other approach entailed estimation. Parameter estimates, which included non-linear, asymmetric stiffness functions, were derived from the literature. The optimization was undertaken so as to identify parameter dependence on test variables. Results indicate that parameter values are influenced by test variables. Results also indicate that the non-linear, estimated model better approximates the experimental data than the linear, identified model. In addition to identifying parameters of a three degree-of-freedom model, parameters were also identified for a single degree-of-freedom model where the motion variable was intended to indicate the rotation of the in vivo knee. It is concluded that the simpler model offers good accuracy in predicting both magnitude and time of occurrence of peak knee axial rotations. Model motion fails to track the measured knee rotation subsequent to the peak, however.     

A dynamic model of the knee and lower limb for simulating rising movements

A two-dimensional dynamical model of the human body was developed and used to simulate muscle and knee-ligament loading during a fast rising movement. The hip, ankle, and toes were each modeled as a simple hinge joint. Relative movements of the femur, tibia, and patella in the sagittal plane were described using a more detailed representation of the knee. The geometry of the model bones was adapted from cadaver data. Eleven elastic elements described the geometric and mechanical properties of the knee ligaments and joint capsule. The patella was assumed to be massless. Smooth hypersurfaces were constructed and used to calculate the position and orientation of the patella during a forward integration of the model. Each hypersurface was formed by applying the principle of static equilibrium to approximate patellofemoral mechanics during the simulation. The model was actuated by 22 musculotendinous units, each unit represented as a three-element muscle in series with tendon. A first-order process was assumed to model muscle excitation-contraction dynamics. Dynamic optimization theory was used to calculate the pattern of muscle excitations that produces a coordinated rising movement from an initial squatting position in minimum time. The calculations support the contention that squatting is a relatively safe exercise for rehabilitation following ACL reconstruction. ACL forces remain less than 20 N for the duration of the task.     

Effects of knee flexion on the structural properties of the rabbit femur-anterior cruciate ligament-tibia complex (FATC)

Many studies have been conducted to determine the biomechanical properties of the anterior cruciate ligament (ACL). The method of holding the femur-ACL-tibia complex (FATC) test specimen, the strain rate applied, the angle of knee flexion and the direction of the applied loads have an important effect on the outcome. It is felt that the tensile properties and strength of the ligament should be measured by applying the tensile force along the axis of the ligament. A versatile clamp was designed to accomplish this purpose. Fifty-seven rabbit knee specimens were tested at angles of flexion of 0 degrees, 30 degrees or 90 degrees. In addition, a comparative study of 25 pairs of rabbit legs were performed, whereby loading was either along the ligament or along the tibial axis. Cyclic hysteresis, ultimate load, energy absorbed, and stiffness were determined. The ultimate load values for the FATC decreased with increased knee flexion for those loaded along the tibial axis, while no such change was detected for FATC tested along the ligament axis. Other structural properties measured followed similar trends. It is concluded that the structural properties of the rabbit FATC change minimally with knee flexion (from 0 to 90 degrees) when loaded along the ligament axis, but decrease significantly with knee flexion when loaded along the axis of the tibia. Therefore, the data obtained in this field of study can be compared only if the direction of loading with respect to the ACL is similar.     

Measurement of strength and loading variables on the knee during Alpine skiing

The study focusses on the prevention of knee injuries during snow skiing. In order to develop a technology of knee injury prevention, both the strength and loading on the knee during skiing activity must be known. This paper reports measurements of variables influencing both knee strength and loading of the joint. The strength variables measured included the degree of activity in six muscles crossing the knee, the knee flexion angle, and the axial load (i.e. weight bearing) transmitted to the knee. Transducers included surface electrodes to monitor electromyogram signals indicating the degree of muscle activity and a goniometer to measure both hip and knee flexion angles. The complete loading on the knee was derived from a dynamometer which measured the six load components at the boot-dynamometer interface. The transducer data were acquired and stored by a compact, battery powered digital data acquisition-controller system. Three male subjects of similar physical size (nominal was 1.8 m and 75 kg) and skiing ability (advanced intermediate to expert) were tested under similar conditions. Each subject skied a total of four slalom runs--one snowplow and three parallel. The total time of each test was 21 s. Example data plots from different types of runs are presented and discussed. Based on observations from the data, necessary performance features for ski bindings offering improved protection from knee ligamentous injuries are defined.     

A new method to measure post-traumatic joint contractures in the rabbit knee

A new device and method to measure rabbit knee joint angles are described. The method was used to measure rabbit knee joint angles in normal specimens and in knee joints with obvious contractures. The custom-designed and manufactured gripping device has two clamps. The femoral clamp sits on a pinion gear that is driven by a rack attached to a materials testing system. A 100 N load cell in series with the rack gives force feedback. The tibial clamp is attached to a rotatory potentiometer. The system allows the knee joint multiple degrees-of-freedom (DOF). There are two independent DOF (compression-distraction and internal-external rotation) and two coupled motions (medial-lateral translation coupled with varus-valgus rotation; anterior-posterior translation coupled with flexion-extension rotation). Knee joint extension-flexion motion is measured, which is a combination of the materials testing system displacement (converted to degrees of motion) and the potentiometer values (calibrated to degrees). Internal frictional forces were determined to be at maximum 2% of measured loading. Two separate experiments were performed to evaluate rabbit knees. First, normal right and left pairs of knees from four New Zealand White (NZW) rabbits were subjected to cyclic loading. An extension torque of 0.2 Nm was applied to each knee. The average change in knee joint extension from the first to the fifth cycle was 1.9 deg +/- 1.5 deg (mean +/- sd) with a total of 49 tests of these eight knees. The maximum extension of the four left knees (tested 23 times) was 14.6 deg +/- 7.1 deg, and of the four right knees (tested 26 times) was 12.0 deg +/- 10.9 deg. There was no significant difference in the maximum extension between normal left and right knees. In the second experiment, nine skeletally mature NZW rabbits had stable fractures of the femoral condyles of the right knee that were immobilized for five, six or 10 weeks. The left knee served as an unoperated control. Loss of knee joint extension (flexion contracture) was demonstrated for the experimental knees using the new methodology where the maximum extension was 35 deg +/- 9 deg, compared to the unoperated knee maximum extension of 11 deg +/- 7 deg, 10 or 12 weeks after the immobilization was discontinued. The custom gripping device coupled to a materials testing machine will serve as a measurement test for future studies characterizing a rabbit knee model of post-traumatic joint contractures.     

Description and error evaluation of an in vitro knee joint testing system

An experimental system for the analysis of knee joint biomechanics is presented. The system provides for the simultaneous recording of ligament forces using buckle transducers and three-dimensional joint motion using an instrumented spatial linkage, as in vitro specimens are subjected to a variety of external loads by a pneumatic loading apparatus with associated force transducers. The system components are described, and results of an evaluation of system errors and accuracy are presented. The experimental setup has been successfully used in the analysis of normal knee ligament mechanics, as well as surgical reconstructions of the anterior cruciate ligament. The system can also be adapted to test other human or animal in vitro joints.