Efficacy of quantifying marker-cluster rigidity in a multi-segment foot model: a Monte-Carlo based global sensitivity analysis and regression model.

Marker-based clinical gait analysis and multi-segment foot models (MSFM) have been successfully used for the diagnosis and clinical management of various lower limb disorders. The accuracy and validity of the kinematics measured depend on the design of the model, as well as on the adherence to its inherent rigid body assumption. This study applies a Monte-Carlo based global sensitivity analysis to evaluate the efficacy of using 'rigid body error (σRBE)' in quantifying the rigidity of a MSFM marker-cluster. A regression model is proposed. It is concluded that σRBE is effective in quantifying rigidity.

Comparison of the hindfoot axes of a multi-segment foot model to the underlying bony anatomy.

Musculoskeletal models used in gait analysis require coordinate systems to be identified for the body segments of interest. It is not obvious how hindfoot (or rearfoot) axes defined by skin-mounted markers relate to the anatomy of the underlying bones. The aim of this study was to compare the marker-based axes of the hindfoot in a multi-segment foot model to the orientations of the talus and calcaneus as characterized by their principal axes of inertia. Twenty adult females with no known foot deformities had radio-opaque markers placed on their feet and ankles at the foot model marker locations. CT images of the feet were acquired as the participants lay supine with their feet in a semi-weight bearing posture. The spatial coordinates of the markers were obtained from the images and used to define the foot model axes. Segmented masks of the tali and calcanei were used to create 3D bone models, from which the principal axes of the bones were obtained. The orientations of the principal axes were either within the range of typical values reported in the imaging literature or differed in ways that could be explained by variations in how the angles were defined. The model hindfoot axis orientations relative to the principal axes of the bones had little bias but were highly variable. Consideration of coronal plane hindfoot alignment as measured clinically and radiographically suggested that the model hindfoot coordinate system represents the posterior calcaneal tuberosity, rather than the calcaneus as a whole.

Marker cluster rigidity in a multi-segment foot model.

Multi-segment foot models (MSFM) are used in gait analysis for the diagnosis and planning of treatment for patients with foot deformities. Like other biomechanical models, MSFMs represent the leg and foot as a series of linked rigid segments, but such a simplification may not be appropriate, particularly for the flexible forefoot. This study investigated the appropriateness of the rigid body assumption on marker clusters used to define the individual segments (tibia, hindfoot, forefoot) of a widely-used MSFM. Rigidity of the marker clusters was quantified using the rigid body error (σRBE) calculated for each frame of a representative gait cycle for 64 normal healthy adults who underwent gait analysis. σRBE is a measure of how well the tracking marker configuration at each frame compares to the arrangement of the same markers in a reference pose. As expected, the patterns of deformation of the three marker clusters differed over the gait cycle. The hindfoot cluster remained relatively undeformed in comparison to the forefoot and tibia clusters. The largest deformations of the forefoot cluster occurred near the beginning and end of the stance phase. The tibia cluster deformed throughout the entire gait cycle, with a pattern similar to that of a typical knee flexion angle graph. The results raise questions about the appropriateness of the rigid-body assumption when applied to MSFMs, particularly in the forefoot region.

Viscoelasticity of Tendons Under Transverse Compression.

Tendons are highly anisotropic and also viscoelastic. For understanding and modeling their 3D deformation, information is needed on their viscoelastic response under off-axis loading. A study was made, therefore, of creep and recovery of bovine digital extensor tendons when subjected to transverse compressive stress of up to ca. 100 kPa. Preconditioned tendons were compression tested between glass plates at increasing creep loads. The creep response was anomalous: the relative rate of creep reduced with the increasing stress. Over each ca. 100 s creep period, the transverse creep deformation of each tendon obeyed a power law dependence on time, with the power law exponent falling from ca. 0.18 to an asymptote of ca. 0.058 with the increasing stress. A possible explanation is stress-driven dehydration, as suggested previously for the similar anomalous behavior of ligaments. Recovery after removal of each creep load was also anomalous. Relative residual strain reduced with the increasing creep stress, but this is explicable in terms of the reducing relative rate of creep. When allowance was made for some adhesion occurring naturally between tendon and the glass plates, the results for a given load were consistent with creep and recovery being related through the Boltzmann superposition principle (BSP). The tendon tissue acted as a pressure-sensitive adhesive (PSA) in contact with the glass plates: explicable in terms of the low transverse shear modulus of the tendons.

Transverse Compression of Tendons.

A study was made of the deformation of tendons when compressed transverse to the fiber-aligned axis. Bovine digital extensor tendons were compression tested between flat rigid plates. The methods included: in situ image-based measurement of tendon cross-sectional shapes, after preconditioning but immediately prior to testing; multiple constant-load creep/recovery tests applied to each tendon at increasing loads; and measurements of the resulting tendon displacements in both transverse directions. In these tests, friction resisted axial stretch of the tendon during compression, giving approximately plane-strain conditions. This, together with the assumption of a form of anisotropic hyperelastic constitutive model proposed previously for tendon, justified modeling the isochronal response of tendon as that of an isotropic, slightly compressible, neo-Hookean solid. Inverse analysis, using finite-element (FE) simulations of the experiments and 10 s isochronal creep displacement data, gave values for Young's modulus and Poisson's ratio of this solid of 0.31 MPa and 0.49, respectively, for an idealized tendon shape and averaged data for all the tendons and E = 0.14 and 0.10 MPa for two specific tendons using their actual measured geometry. The compression load versus displacement curves, as measured and as simulated, showed varying degrees of stiffening with increasing load. This can be attributed mostly to geometrical changes in tendon cross section under load, varying according to the initial 3D shape of the tendon.

The use of turning tasks in clinical gait analysis for children with cerebral palsy.

BACKGROUND: Turning while walking is a crucial component of locomotion that is performed using an outside (step) or inside (spin) limb strategy. The aims of this paper were to determine how children with cerebral palsy perform turning maneuvers and if specific kinematic and kinetic adaptations occur compared to their typically developing peers. METHODS: Motion capture data from twenty-two children with cerebral palsy and fifty-four typically developing children were collected during straight and 90° turning gait trials. Experimental data were used to compute spatio-temporal parameters, margin of stability, ground reaction force impulse, as well as joint kinematics and kinetics. FINDINGS: Both child groups preferred turning using the spin strategy. The group of children with cerebral palsy exhibited the following adaptations during turning gait compared to the typically developing group: stride length was decreased across all phases of the turn with largest effect size for the depart phase (2.02), stride width was reduced during the turn phase, but with a smaller effect size (0.71), and the average margin of stability during the approach phase of turning was reduced (effect size of 0.98). Few overall group differences were found for joint kinematic and kinetic measures; however, in many cases, the intra-subject differences between straight walking and turning gait were larger for the majority of children with cerebral palsy than for the typically developing children. INTERPRETATION: In children with cerebral palsy, turning gait may be a better discriminant of pathology than straight walking and could be used to improve the management of gait abnormalities.

Does increasing applied load lead to contact changes indicative of knee osteoarthritis? A subject-specific FEA study.

This study investigated whether increased loading (representing obesity) in the extended knee and flexed knee led to increased stresses in areas of typical medial and lateral osteoarthritis cartilage lesions, respectively. We created two paired sets of subject-specific finite element models; both sets included models of extended knees and of flexed knees. The first set represented normal loading; the second set represented increased loading. All other variables were held constant. The von Mises stresses and contact areas calculated on the tibial cartilage surfaces of the paired models were then compared. In the extended knee models, applying a larger load led to increased stress in the anterior and central regions of the medial tibial cartilage. These are the typical locations of medial osteoarthritis cartilage lesions. Therefore, the results support that increased loading in the extended knee may result in medial osteoarthritis. In the flexed knee models, applying a larger load increased stress in the anterior and central regions of the lateral tibial cartilage. Lateral osteoarthritis cartilage lesions typically occur centrally and posteriorly. Therefore, these results do not support our hypothesis. Shear stress was increased in areas of typical lateral lesions, however, and should be investigated in future studies.

Muscle contributions to centre of mass acceleration during turning gait in typically developing children: A simulation study.

Turning while walking requires substantial joint kinematic and kinetic adaptations compared to straight walking in order to redirect the body centre of mass (COM) towards the new walking direction. The role of muscles and external forces in controlling and redirecting the COM during turning remains unclear. The aim of this study was to compare the contributors to COM medio-lateral acceleration during 90° pre-planned turns about the inside limb (spin) and straight walking in typically developing children. Simulations of straight walking and turning gait based on experimental motion data were implemented in OpenSim. The contributors to COM global medio-lateral acceleration during the approach (outside limb) and turn (inside limb) stance phase were quantified via an induced acceleration analysis. Changes in medio-lateral COM acceleration occurred during both turning phases, compared to straight walking (p<0.001). During the approach, outside limb plantarflexors (soleus and medial gastrocnemius) contribution to lateral (away from the turn side) COM acceleration was reduced (p<0.001), whereas during the turn, inside limb plantarflexors (soleus and gastrocnemii) contribution to lateral acceleration (towards the turn side) increased (p≤0.013) and abductor (gluteus medius and minimus) contribution medially decreased (p<0.001), compared to straight walking, together helping accelerate the COM towards the new walking direction. Knowledge of the changes in muscle contributions required to modulate the COM position during turning improves our understanding of the control mechanisms of gait and may be used clinically to guide the management of gait disorders in populations with restricted gait ability.

Ground reaction forces and lower-limb joint kinetics of turning gait in typically developing children.

Turning is a common locomotor task essential to daily activity; however, very little is known about the forces and moments responsible for the kinematic adaptations occurring relative to straight-line gait in typically developing children. Thus, the aims of this study were to analyse ground reaction forces (GRFs), ground reaction free vertical torque (TZ), and the lower-limb joint kinetics of 90° outside (step) and inside (spin) limb turns. Step, spin, and straight walking trials from fifty-four typically developing children were analysed. All children were fit with the Plug-in Gait and Oxford Foot Model marker sets while walking over force plates embedded in the walkway. Net internal joint moments and power were computed via a standard inverse dynamics approach. All dependent variables were statistically analysed over the entire curves using the mean difference 95% bootstrap confidence band approach. GRFs were directed medially for step turns and laterally for spin turns during the turning phase. Directions were reversed and magnitudes decreased during the approach phase. Step turns showed reduced ankle power generation, while spin turns showed large TZ. Both strategies required large knee and hip coronal and transverse plane moments during swing. These kinetic differences highlight adaptations required to maintain stability and reorient the body towards the new walking direction during turning. From a clinical perspective, turning gait may better reveal weaknesses and motor control deficits than straight walking in pathological populations, such as children with cerebral palsy, and could potentially be implemented in standard gait analysis sessions.

Spatio-temporal parameters and lower-limb kinematics of turning gait in typically developing children.

Turning is a requirement for most locomotor tasks; however, knowledge of the biomechanical requirements of successful turning is limited. Therefore, the aims of this study were to investigate the spatio-temporal and lower-limb kinematics of 90° turning. Seventeen typically developing children, fitted with full body and multi-segment foot marker sets, having performed both step (outside leg) and spin (inside leg) turning strategies at self-selected velocity, were included in the study. Three turning phases were identified: approach, turn, and depart. Stride velocity and stride length were reduced for both turning strategies for all turning phases (p<0.03 and p<0.01, respectively), while stance time and stride width were increased during only select phases (p<0.05 and p<0.01, respectively) for both turn conditions compared to straight gait. Many spatio-temporal differences between turn conditions and phases were also found (p<0.03). Lower-limb kinematics revealed numerous significant differences mainly in the coronal and transverse planes for the hip, knee, ankle, midfoot, and hallux between conditions (p<0.05). The findings summarized in this study help explain how typically developing children successfully execute turns and provide greater insight into the biomechanics of turning. This knowledge may be applied to a clinical setting to help improve the management of gait disorders in pathological populations, such as children with cerebral palsy.

Identifying gait events without a force plate during running: a comparison of methods.

This paper presents a comparison of four different methods of identifying the times of foot-strike and toe-off during running based on gait marker trajectories. The event times predicted by the methods were compared to those identified using a force plate for both over-ground and treadmill running. The effect of using different threshold values for the detection of gait events using force plate data was also investigated, and as a result, all assessments of event detection accuracy were based on a cut-off value of 10N. The most accurate method of foot-strike detection depended on whether the runner landed with a rear- or a mid-foot strike. For rear-foot-strike running, the best method of identifying foot-strike used the vertical acceleration profile of the posterior heel distal marker and the vertical position profile of the hallux marker. For mid-foot-strike running, the best method of identifying foot-strike used the vertical velocity profile of the mean positions of the posterior heel distal marker and a marker midway between the second and third metatarsal heads. The most accurate method of identifying toe-off did not depend on type of foot-strike and was based on the vertical acceleration and position profiles of the hallux marker.

Subject-specific axes of the ankle joint complex.

The aim of this study was to use a two-axis ankle joint model and an optimisation process (van den Bogert et al., 1994) to calculate and compare the talocrural and subtalar hinge axes for non-weight-bearing ankle motion, weight-bearing ankle motion, and walking in normal, healthy adult subjects and to see which of the first two sets of axes better fit the walking data. Motion data for the foot and shank were collected on eight subjects whilst they performed the activities mentioned. After choosing the best marker sets for motion tracking, a two-hinge ankle joint model was fit to the motion data. Ankle joint ranges of motion were also calculated. It was found that the model fit the experimental data well, with non-weight-bearing motion achieving the best fit. Despite this, the calculated axis orientations were highly variable both between motion types and between subjects. No significant difference between the fit of the non-weight-bearing and weight-bearing models to the walking data was found, which implies that either set of functional axes is adequate for modeling walking; however, the subtalar deviation angle was significantly closer for the weight-bearing activity and walking than for the non-weight-bearing activity and walking, which suggests that it is marginally better to use the weight-bearing functional motions. The results lead to questions about the appropriateness of the two-hinge ankle model for use in applications in which the behaviour of the individual joints of the ankle complex, rather than simply the relative motion of the leg and foot, is important.

The sclerotic line: why it appears under knee replacements (a study based on the Oxford knee).

BACKGROUND: Radiolucent lines and sclerotic margins are often seen on knee radiographs taken a year or longer after knee replacement surgery. Histology has shown that the radiolucent zone is predominantly fibrocartilage and the sclerotic margin is lamellar bone. The reasons for their existence are not clearly understood. METHODS: A three-dimensional finite element model of the medial half of the proximal 75mm of a tibia implanted with a knee replacement was created and run over 365 iterations simulating 1year of in vivo post implant remodelling. After each iteration, new material properties were calculated for all elements of the model using established bone remodelling and tissue differentiation rules. For comparison with patient anteroposterior radiographs, "synthetic anteroposterior radiographs" were generated by reverse calculating radiographic densities from material properties of the model after 365 iterations. Von Mises stress of elements in the bone where the sclerotic line is usually seen were calculated after 365 iterations. These values were compared with the same entities assuming no remodelling. FINDINGS: The mean von Mises stress in the sclerotic region was higher when remodelling was assumed than when not, suggesting that the presence of the soft tissue (radiolucent line) increased the stress in the underlying bone. INTERPRETATION: The sclerotic line is caused by the stiffening of bone due to the relatively larger loads seen by the bone just beneath the soft tissue (radiolucent line) adjoining knee replacements.

Experimental validation of a finite element model of a human cadaveric tibia.

Finite element (FE) models of long bones are widely used to analyze implant designs. Experimental validation has been used to examine the accuracy of FE models of cadaveric femurs; however, although convergence tests have been carried out, no FE models of an intact and implanted human cadaveric tibia have been validated using a range of experimental loading conditions. The aim of the current study was to create FE models of a human cadaveric tibia, both intact and implanted with a unicompartmental knee replacement, and to validate the models against results obtained from a comprehensive set of experiments. Seventeen strain rosettes were attached to a human cadaveric tibia. Surface strains and displacements were measured under 17 loading conditions, which consisted of axial, torsional, and bending loads. The tibia was tested both before and after implantation of the knee replacement. FE models were created based on computed tomography (CT) scans of the cadaveric tibia. The models consisted of ten-node tetrahedral elements and used 600 material properties derived from the CT scans. The experiments were simulated on the models and the results compared to experimental results. Experimental strain measurements were highly repeatable and the measured stiffnesses compared well to published results. For the intact tibia under axial loading, the regression line through a plot of strains predicted by the FE model versus experimentally measured strains had a slope of 1.15, an intercept of 5.5 microstrain, and an R(2) value of 0.98. For the implanted tibia, the comparable regression line had a slope of 1.25, an intercept of 12.3 microstrain, and an R(2) value of 0.97. The root mean square errors were 6.0% and 8.8% for the intact and implanted models under axial loads, respectively. The model produced by the current study provides a tool for simulating mechanical test conditions on a human tibia. This has considerable value in reducing the costs of physical testing by pre-selecting the most appropriate test conditions or most favorable prosthetic designs for final mechanical testing. It can also be used to gain insight into the results of physical testing, by allowing the prediction of those variables difficult or impossible to measure directly.

Simultaneous in vitro measurement of patellofemoral kinematics and forces.

This study involved the development and testing of a system for the simultaneous in vitro measurement of tibiofemoral kinematics and patellofemoral kinematics and forces. Knee motion was tracked using a Vicon 370, and patellofemoral force was measured using a six degree-of-freedom transducer based on the design of Singerman et al. Using this system, twelve knee specimens were tested in supine leg extension under a simulated quadriceps force. The comprehensive set of results corresponds well to the individual results of previous studies. The measurement system will be of value in assessing the effects of total knee arthroplasty on patellar function.

Can functional electric stimulation-assisted rowing reproduce a race-winning rowing stroke?

OBJECTIVE: To compare the ergometer rowing technique of a person with spinal cord injury (SCI), using functional electric stimulation (FES) of his leg muscles, with that of a well-defined group of able-bodied rowers. DESIGN: Whole-body kinematics and kinetics and electric activity of selected muscles were measured during ergometer rowing. SETTING: A hospital-based motion analysis laboratory. PARTICIPANTS: Five male university varsity-level rowers and 1 male rower with SCI. INTERVENTIONS: Eight rowing trials were collected on the university-level rowers, 2 trials each at 20, 24, 28, and 32 strokes/min. The rower with SCI had surface electrodes applied to his medial hamstrings and medial quadriceps muscle bellies. The electrodes were attached to a stimulator that was activated using a button in the ergometer handle. The subject with SCI rowed at a self-selected stroke rate. MAIN OUTCOME MEASURES: Forces at the ergometer handle and foot cradle, 3-dimensional whole-body kinematics, net joint moments, and phasic activity of muscles. RESULTS: Motion of the arms, ankles, and knees of the rower with SCI was similar to those of the university-level rowers; other joint motions and forces applied to the ergometer differed. CONCLUSIONS: FES-assisted rowing in its current implementation cannot reproduce a race-winning rowing stroke. Further development work is required.

Femoral muscle attachment locations in children and adults, and their prediction from clinical measurement.

Accurate representation of children's musculo-skeletal anatomy is becoming increasingly important to biomechanical techniques such as gait analysis. This study used magnetic resonance imaging to examine the locations of the femoral insertions of the psoas, vastus medialis and gastrocnemius muscles in five adults and 17 children (including 7 children with cerebral palsy). The relationship of muscle attachment locations with age and bone geometry was then determined. Scaling techniques and external measurements of parameters such as femoral anteversion/antetorsion were shown to have potential for prediction of the locations of femoral muscle attachment points. It was shown that femoral anteversion can be modelled geometrically as occurring proximal to the lesser trochanter.