Characterisation of dynamic couplings at lower limb residuum/socket interface using 3D motion capture

3D Motion Analysis for the Assessment of Dynamic Coupling in Transtibial Prosthetics: A Proof of Concept

Assessment of coupling between transtibial sockets and users is historically based on clinicians' observations and experience, but can be inaccurate and unreliable. Therefore, we present a proof of concept, for five out of six possible degrees of freedom coupling metric system for a socket, using motion analysis calibrated on a 3D printed limb substitute. The method is compatible with any socket suspension method and does not require prior modifications to the socket. Calibration trials were used to locate the axis of rotation of the knee joint referenced against a marker cluster on the thigh; this allowed for the identification of the limb during test trials despite the entire residuum being obscured from view by the socket. The error in the technique was found to be within 0.7 mm in displacement and 0.7 degrees in rotation, based on the control data. Dynamic testing showed the Inter Quartile Range (IQR) of inter time step variance was <0.5 mm/deg for all metrics. The method can form a basis for objective socket evaluation, improve clinical practice and the quality of life for amputees.

Mechanisms and component design of prosthetic knees: A review from a biomechanical function perspective

Prosthetic knees are state-of-the-art medical devices that use mechanical mechanisms and components to simulate the normal biological knee function for individuals with transfemoral amputation. A large variety of complicated mechanical mechanisms and components have been employed; however, they lack clear relevance to the walking biomechanics of users in the design process. This article aims to bridge this knowledge gap by providing a review of prosthetic knees from a biomechanical perspective and includes stance stability, early-stance flexion and swing resistance, which directly relate the mechanical mechanisms to the perceived walking performance, i.e., fall avoidance, shock absorption, and gait symmetry. The prescription criteria and selection of prosthetic knees depend on the interaction between the user and prosthesis, which includes five functional levels from K0 to K4. Misunderstood functions and the improper adjustment of knee prostheses may lead to reduced stability, restricted stance flexion, and unnatural gait for users. Our review identifies current commercial and recent studied prosthetic knees to provide a new paradigm for prosthetic knee analysis and facilitates the standardization and optimization of prosthetic knee design. This may also enable the design of functional mechanisms and components tailored to regaining lost functions of a specific person, hence providing individualized product design.

Analysis of lower limb prosthetic socket interface based on stress and motion measurements

The study was designed to establish a biomechanical assessment platform for the lower limb residuum/socket interface as a function of duration and speed of movement. The approach exploits an interface sensor which measures multi-directional stresses at the interface. The corresponding interface coupling motion was assessed using a 3D motion capture system. A longitudinal study, involving a trans-femoral amputee, was conducted with nine repeated level walking sessions over a 12-month period. The effect of walking speed on interface biomechanics was also assessed. Interface peak pressures and shear stresses in the range of 55–59 kPa and 12–19 kPa were measured, respectively, over all sessions in the 12 months study period at the posterior-proximal location of the residuum. The peak pressure and longitudinal shear values were found to fluctuate approximately 11% and 40% as against its maximum value, respectively, over 12 months. In addition, up to 12° of angular coupling and up to 28 mm of pistoning were recorded over a gait cycle, which was found to change by 29% and 45% respectively over the study period. The variation in walking speed, by altering self-selected cadence, resulted in changes of pressure and shear stresses at mid-stance of the gait cycle. In particular, as compared with self-selected cadence, for fast speed, peak pressure and peak longitudinal shear stress decreased by 5% and 33%, respectively. For slow speed, peak pressure and peak longitudinal shear stress increased by 7% and 17%, respectively. The corresponding angular and pistoning revealed a variation of up to 29% and 45%, respectively. This biomechanical assessment approach shows promise in the quantitative assessment of interface kinematics and kinetics for lower limb prosthetics, the usage of which could assist the clinical assessment of prosthetic socket fit.

Review of Prosthetics & Orthotics Needs for 21st Century - Vision for 2025

What would we do, if only we had the power to go back?! The best way to consider this is to align and join all the known dots. To think of Prosthetics and Orthotics (P&O) as a system holistically centred around care of the user, identifying all their needs continuously, in their environment and in their lifestyle. This could produce a new value proposition for all multi-disciplinary team members by generating patient-centred therapeutic benefits and clinical outcomes that align all stakeholders in P&O towards using a common narrative, which makes decisions based on data. In this case, data is the outcome, using Standards and Instruments which are validated (e.g. www.amprom.uk) to quantify questions such as: "Have we reduce risk of falls?", "Have we reduced risk of tissue injury?", "Have we reduced risk of low back pain?", "Have we reduced long term risk of osteoarthritis?", etc. If we have, we are assured this will benefit the comfort and confidence for the user. We can have confidence in rehabilitation measured by improved stability and increased activity, and other measures which enable the accurate classification of products and services to match users. A prescription index, based on Outcomes, could, for example, be calculated by a formula which accounts for the percentage reduction in falls probability, a patient satisfaction score, a mobility score and a quality-of-life score, allowing practitioners to base their choices of treatment pathways and component selection. This paper provides both the context for and contributing factors that make the proposing of such an objective Prescription Index an interesting thing to consider when discussing Health Economics in P&O.

An Anatomical Atlas-Based Scaling Study for Quantifying Muscle and Hip Joint Contact Forces in Above and Through-Knee Amputees Using Validated Musculoskeletal Modelling

OBJECTIVE

Customisation of musculoskeletal modelling using magnetic resonance imaging (MRI) significantly improves the model accuracy, but the process is time consuming and computationally intensive. This study hypothesizes that linear scaling to a lower limb amputee model with anthropometric similarity can accurately predict muscle and joint contact forces.

METHODS

An MRI-based anatomical atlas, comprising 18 trans-femoral and through-knee traumatic lower limb amputee models, is developed. Gait data, using a 10-camera motion capture system with two force plates, and surface electromyography (EMG) data were collected. Muscle and hip joint contact forces were quantified using musculoskeletal modelling. The predicted muscle activations from the subject-specific models were validated using EMG recordings. Anthropometry based multiple linear regression models, which minimize errors in force predictions, are presented.

RESULTS

All predictions showed excellent (error interval c = 0-0.15), very good (c = 0.15-0.30) or good (c = 0.30-0.45) similarity to the EMG data, demonstrating accurate computation of muscle activations. The primary predictors of discrepancies in force predictions were differences in pelvis width (p < 0.001), body mass index (BMI, p < 0.001) and stump length to pelvis width ratio (p < 0.001) between the respective individual and underlying dataset.

CONCLUSION

Linear scaling to a model with the most similar pelvis width, BMI and stump length to pelvis width ratio results in modelling outcomes with minimal errors.

SIGNIFICANCE

This study provides robust tools to perform accurate analyses of musculoskeletal mechanics for high-functioning lower limb military amputees, thus facilitating the further understanding and improvement of the amputee's function. The atlas is available in an open source repository.

Analysis of the Relative Motion Between the Socket and Residual Limb in Transtibial Amputees While Wearing a Transverse Rotation Adapter

The coupling between the residual limb and the lower-limb prosthesis is not rigid. As a result, external loading produces movement between the prosthesis and residual limb that can lead to undesirable soft-tissue shear stresses. As these stresses are difficult to measure, limb loading is commonly used as a surrogate. However, the relationship between limb loading and the displacements responsible for those stresses remains unknown. To better understand the limb motion within the socket, an inverse kinematic analysis was performed to estimate the motion between the socket and tibia for 10 individuals with a transtibial amputation performing walking and turning activities at 3 different speeds. The authors estimated the rotational stiffness of the limb-socket body to quantify the limb properties when coupled with the socket and highlight how this approach could help inform prosthetic prescriptions. Results showed that peak transverse displacement had a significant, linear relationship with peak transverse loading. Stiffness of the limb-socket body varied significantly between individuals, activities (walking and turning), and speeds. These results suggest that transverse limb loading can serve as a surrogate for residual-limb shear stress and that the setup of a prosthesis could be individually tailored using standard motion capture and inverse kinematic analyses.

Predictive prosthetic socket design: part 1-population-based evaluation of transtibial prosthetic sockets by FEA-driven surrogate modelling

It has been proposed that finite element analysis can complement clinical decision making for the appropriate design and manufacture of prosthetic sockets for amputees. However, clinical translation has not been achieved, in part due to lengthy solver times and the complexity involved in model development. In this study, a parametric model was created, informed by variation in (i) population-driven residuum shape morphology, (ii) soft tissue compliance and (iii) prosthetic socket design. A Kriging surrogate model was fitted to the response of the analyses across the design space enabling prediction for new residual limb morphologies and socket designs. It was predicted that morphological variability and prosthetic socket design had a substantial effect on socket-limb interfacial pressure and shear conditions as well as sub-dermal soft tissue strains. These relationships were investigated with a higher resolution of anatomical, surgical and design variability than previously reported, with a reduction in computational expense of six orders of magnitude. This enabled real-time predictions (1.6 ms) with error vs the analytical solutions of < 4 kPa in pressure at residuum tip, and < 3% in soft tissue strain. As such, this framework represents a substantial step towards implementation of finite element analysis in the prosthetics clinic.

Influence of the scale reduction in designing sockets for trans-tibial amputees

The development of artificial prosthetic lower limbs aims to improve patient's mobility while avoiding secondary problems resulting from the use of the prostheses themselves. The residual limb is a pressure-sensitive area where skin injuries and pain are more likely to develop. Requirements for adequate prosthetic limbs have now become urgent to improve amputee's quality of life. This study aims to understand how socket design parameters related to geometry can influence pressure distribution in the residual limb. A finite element model was developed to simulate the mechanical loading applied on the residual limb of a below-knee amputee while walking. A sensitivity analysis to socket initial geometry, scaling the socket downward in the horizontal plane, was performed. Recordings include stress levels on the skin and in the residual limb deep soft tissues. Peak stress was reduced by up to 51% with a limited reduction of the socket size. More important scale reduction of the residual limb would lead to possible negative effects, such as stress concentrations in sensitive areas. This result confirms the interest of the prosthetist to develop a well-fitting socket, possibly a little smaller than the residual limb itself, in order to avoid residual limb mobility in the socket that could cause friction and stress concentrations. Non-homogeneous geometrical reductions of the socket should be further investigated.

Microprocessor knees with 'standing support' and articulating, hydraulic ankles improve balance control and inter-limb loading during quiet standing

Introduction

Trans-femoral amputees are at risk of musculoskeletal problems that are in part caused by loading asymmetry during activities, such as prolonged standing, particularly on uneven or sloped ground.

Methods

Four prosthetic conditions were tested; microprocessor knee ‘standing support’ mode activated (ON) and deactivated (OFF), combined with a rigidly attached foot (RA) and with an articulating, hydraulic ankle-foot (HA). Five trans-femoral amputees and five able-bodied controls were measured using a motion capture system and a force plate while standing, facing down a 5° slope. Ground reaction force distributions and centre-of-pressure root-mean-square (COP RMS) were calculated as outcome measures.

Results

Compensatory kinematic adjustments were observed for RA conditions but not for HA conditions. HA-OFF reduced ground reaction force degree-of-asymmetry for all five amputees, compared to RA-OFF. RA-ON reduced ground reaction force degree-of-asymmetry for four amputees, compared to RA-OFF. In terms of balance, the HA conditions reduced the mean inter-limb COP RMS by 24–25% compared to equivalent RA conditions, while ON conditions reduced it by 9–11%, compared to equivalent OFF conditions.

Conclusions

It is important to consider both prosthetic knee and ankle technologies when prescribing devices to trans-femoral amputees. The combination of hydraulic ankle and knee standing support technologies produced outcomes closest to normal biomechanics.

Systematic Experimental Assessment of a 2D-Motion Sensor to Detect Relative Movement between Residual Limb and Prosthetic Socket

A sensor system for measuring the relative movement between prosthetic socket and residual limb based on a 2D-motion sensor is introduced and thoroughly tested experimentally. The quantitative analysis of test rig evaluation is used to identify advantageous sensor settings and liner configurations. Considering these favorable settings, sensor functionality is quantified to errrel=0.52±1.78%. Advancing to convex measurement surfaces, the sensor shows absolute errors of errabs≤1 mm in an observable measurement scenario. The feasibility of measuring gait-induced relative movement with the proposed 2D-motion sensor is shown via a biomechanical plausibility study. Overall, the findings suggest that the proposed sensor system is suitable for investigating the relative movement between residual limb and prosthetic socket in dynamic gait situations.

A combined kinematic and kinetic analysis at the residuum/socket interface of a knee-disarticulation amputee

The bespoke interface between a lower limb residuum and a prosthetic socket is critical for an amputee's comfort and overall rehabilitation outcomes. Analysis of interface kinematics and kinetics is important to gain full understanding of the interface biomechanics, which could aid clinical socket fit, rehabilitation and amputee care. This pilot study aims to investigate the dynamic correlation between kinematic movement and kinetic stresses at the interface during walking tests on different terrains. One male, knee disarticulation amputee participated in the study. He was asked to walk on both a level surface and a 5° ramped surface. The movement between the residuum and the socket was evaluated by the angular and axial couplings, based on the outputs from a 3D motion capture system. The corresponding kinetic stresses at anterior-proximal (AP), posterior-proximal (PP) and anterior-distal (AD) locations of the residuum were measured, using individual stress sensors. Approximately 8° of angular coupling and up to 32 mm of axial coupling were measured when walking on different terrains. The direction of the angular coupling shows strong correlation with the pressure difference between the PP and AP sensors. Higher pressure was obtained at the PP location than the AP location during stance phase, associated with the direction of the angular coupling. A strong correlation between axial coupling length, L, and longitudinal shear was also evident at the PP and AD locations i.e. the shortening of L corresponds to the increase of shear in the proximal direction. Although different terrains did not affect these correlations in principle, interface kinematic and kinetic values suggested that gait changes can induce modifications to the interface biomechanics. It is envisaged that the reported techniques could be potentially used to provide combined kinematics and kinetics for the understanding of biomechanics at the residuum/socket interface, which may play an important role in the clinical assessment of prosthetic component settings, including socket fit quality.

Development of a residuum/socket interface simulator for lower limb prosthetics

Mechanical coupling at the interface between lower limb residua and prosthetic sockets plays an important role in assessing socket fitting and tissue health. However, most research lab-based lower limb prosthetic simulators to-date have implemented a rigid socket coupling. This study describes the fabrication and implementation of a lower limb residuum/socket interface simulator, designed to reproduce the forces and moments present during the key loading phases of amputee walking. An artificial residuum made with model bones encased in silicone was used, mimicking the compliant mechanical loading of a real residuum/socket interface. A 6-degree-of-freedom load cell measured the overall kinetics, having previously been incorporated into an amputee's prosthesis to collect reference data. The developed simulator was compared to a setup where a rigid pylon replaced the artificial residuum. A maximum uniaxial load of 850 N was applied, comparable to the peak vertical ground reaction force component during amputee walking. Load cell outputs from both pylon and residuum setups were compared. During weight acceptance, when including the artificial residuum, compression decreased by 10%, while during push off, sagittal bending and anterior-posterior shear showed a 25% increase and 34% decrease, respectively. Such notable difference by including a compliant residuum further highlighted the need for such an interface simulator. Subsequently, the simulator was adjusted to produce key load cell outputs briefly aligning with those from amputee walking. Force sensing resistors were deployed at load bearing anatomic locations on the residuum/socket interface to measure pressures and were compared to those cited in the literature for similar locations. The development of such a novel simulator provides an objective adjunct, using commonly available mechanical test machines. It could potentially be used to provide further insight into socket design, fit and the complex load transfer mechanics at the residuum/socket interface, as well as to evaluate the structural performance of prostheses.

A pressure and shear sensor system for stress measurement at lower limb residuum/socket interface

A sensor system for measurement of pressure and shear at the lower limb residuum/socket interface is described. The system comprises of a flexible sensor unit and a data acquisition unit with wireless data transmission capability. Static and dynamic performance of the sensor system was characterised using a mechanical test machine. The static calibration results suggest that the developed sensor system presents high linearity (linearity error ≤ 3.8%) and resolution (0.9 kPa for pressure and 0.2 kPa for shear). Dynamic characterisation of the sensor system shows hysteresis error of approximately 15% for pressure and 8% for shear. Subsequently, a pilot amputee walking test was conducted. Three sensors were placed at the residuum/socket interface of a knee disarticulation amputee and simultaneous measurements were obtained during pilot amputee walking test. The pressure and shear peak values as well as their temporal profiles are presented and discussed. In particular, peak pressure and shear of approximately 58 kPa and 27 kPa, respectively, were recorded. Their temporal profiles also provide dynamic coupling information at this critical residuum/socket interface. These preliminary amputee test results suggest strong potential of the developed sensor system for exploitation as an assistive technology to facilitate socket design, socket fit and effective monitoring of lower limb residuum health.