A Framework for Measuring the Time-Varying Shape and Full-Field Deformation of Residual Limbs Using 3-D Digital Image Correlation

A clinical comparison of a digital versus conventional design methodology for transtibial prosthetic interfaces

A transtibial prosthetic interface typically comprises a compliant liner and an outer rigid socket. The preponderance of today's conventional liners are mass produced in standard sizes, and conventional socket design is labor-intensive and artisanal, lacking clear scientific rationale. This work tests the clinical efficacy of a novel, physics-based digital design framework to create custom prosthetic liner-socket interfaces. In this investigation, we hypothesize that the novel digital approach will improve comfort outcomes compared to a conventional method of liner-socket design. The digital design framework generates custom transtibial prosthetic interfaces starting from MRI or CT image scans of the residual limb. The interface design employs FEA to simulate limb deformation under load. Interfaces are fabricated for 9 limbs from 8 amputees (1 bilateral). Testing compares novel and conventional interfaces across four assessments: 5-min walking trial, thermal imaging, 90-s standing pressure trial, and an evaluation questionnaire. Outcome measures include antalgic gait criterion, skin surface pressures, skin temperature changes, and direct questionnaire feedback. Antalgic gait is compared via a repeated measures linear mixed model while the other assessments are compared via a non-parametric Wilcoxon sign-rank test. A statistically significant ([Formula: see text]) decrease in pain is demonstrated when walking on the novel interfaces compared to the conventional. Standing pressure data show a significant decrease in pressure on novel interfaces at the anterior distal tibia ([Formula: see text]), with no significant difference at other measured locations. Thermal results show no statistically significant difference related to skin temperature. Questionnaire feedback shows improved comfort on novel interfaces on posterior and medial sides while standing and the medial side while walking. Study results support the hypothesis that the novel digital approach improves comfort outcomes compared to the evaluated conventional method. The digital interface design methodology also has the potential to provide benefits in design time, repeatability, and cost.

A high precision laser scanning system for measuring shape and volume of transtibial amputee residual limbs: Design and validation

Changes in limb volume and shape among transtibial amputees affects socket fit and comfort. The ability to accurately measure residual limb volume and shape and relate it to comfort could contribute to advances in socket design and overall care. This work designed and validated a novel 3D laser scanner that measures the volume and shape of residual limbs. The system was designed to provide accurate and repeatable scans, minimize scan duration, and account for limb motion during scans. The scanner was first validated using a cylindrical body with a known shape. Mean volumetric errors of 0.17% were found under static conditions, corresponding to a radial spatial resolution of 0.1 mm. Limb scans were also performed on a transtibial amputee and yielded a standard deviation of 8.1 ml (0.7%) across five scans, and a 46 ml (4%) change in limb volume when the socket was doffed after 15 minutes of standing.

Identification of constitutive materials of bi-layer soft tissues from multimodal indentations

The personalisation of finite element models is an important problem in the biomechanical fields where subject-specific analyses are fundamental, particularly in studying soft tissue mechanics. The personalisation includes the choice of the constitutive law of the model's material, as well as the choice of the material parameters. In vivo identification of the material properties of soft tissues is challenging considering the complex behaviour of soft tissues that are, among other things, non-linear hyperelastic and heterogeneous. Hybrid experimental-numerical methods combining in vivo indentations and inverse finite element analyses are common to identify these material parameters. Yet, the uniqueness and the uncertainty of the multi-material hyperelastic model have not been evaluated. This study presents a sensitivity analysis performed on synthetic indentation data to investigate the identification uncertainties of the material parameters in a bi-material thigh phantom. Synthetic numerical data, used to replace experimental measurements, considered several measurement modalities: indenter force and displacement, stereo-camera 3D digital image correlation of the indented surface, and ultrasound B-mode images. A finite element model of the indentation was designed with either Ogden-Moerman or Mooney-Rivlin constitutive laws for both materials. The parameters' identifiability (i.e. the possibility of converging to a unique parameter set within an acceptable margin of error) was assessed with various cost functions formulated using the different synthetic data sets. The results underline the need for multiple experimental modalities to reduce the uncertainty of the identified parameters. Additionally, the experimental error can impede the identification of a unique parameter set, and the cost function depends on the constitutive law. This study highlights the need for sensitivity analyses before the design of the experimental protocol. Such studies can also be used to define the acceptable range of errors in the experimental measurement. Eventually, the impact of the evaluated uncertainty of the identified parameters should be further investigated according to the purpose of the finite element modelling.

Quantifying Bone and Skin Movement in the Residual Limb-Socket Interface of Individuals With Transtibial Limb Loss Using Dynamic Stereo X-Ray: Protocol for a Lower Limb Loss Cadaver and Clinical Study

BACKGROUND

Relative motion between the residual limb and socket in individuals with transtibial limb loss can lead to substantial consequences that limit mobility. Although assessments of the relative motion between the residual limb and socket have been performed, there remains a substantial gap in understanding the complex mechanics of the residual limb-socket interface during dynamic activities that limits the ability to improve socket design. However, dynamic stereo x-ray (DSX) is an advanced imaging technology that can quantify 3D bone movement and skin deformation inside a socket during dynamic activities.

OBJECTIVE

This study aims to develop analytical tools using DSX to quantify the dynamic, in vivo kinematics between the residual limb and socket and the mechanism of residual tissue deformation.

METHODS

A lower limb cadaver study will first be performed to optimize the placement of an array of radiopaque beads and markers on the socket, liner, and skin to simultaneously assess dynamic tibial movement and residual tissue and liner deformation. Five cadaver limbs will be used in an iterative process to develop an optimal marker setup. Stance phase gait will be simulated during each session to induce bone movement and skin and liner deformation. The number, shape, size, and placement of each marker will be evaluated after each session to refine the marker set. Once an optimal marker setup is identified, 21 participants with transtibial limb loss will be fitted with a socket capable of being suspended via both elevated vacuum and traditional suction. Participants will undergo a 4-week acclimation period and then be tested in the DSX system to track tibial, skin, and liner motion under both suspension techniques during 3 activities: treadmill walking at a self-selected speed, at a walking speed 10% faster, and during a step-down movement. The performance of the 2 suspension techniques will be evaluated by quantifying the 3D bone movement of the residual tibia with respect to the socket and quantifying liner and skin deformation at the socket-residuum interface.

RESULTS

This study was funded in October 2021. Cadaver testing began in January 2023. Enrollment began in February 2024. Data collection is expected to conclude in December 2025. The initial dissemination of results is expected in November 2026.

CONCLUSIONS

The successful completion of this study will help develop analytical methods for the accurate assessment of residual limb-socket motion. The results will significantly advance the understanding of the complex biomechanical interactions between the residual limb and the socket, which can aid in evidence-based clinical practice and socket prescription guidelines. This critical foundational information can aid in the development of future socket technology that has the potential to reduce secondary comorbidities that result from complications of poor prosthesis load transmission.

INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID)

DERR1-10.2196/57329.

Stereo Camera Setup for 360° Digital Image Correlation to Reveal Smart Structures of Hakea Fruits

About forty years after its first application, digital image correlation (DIC) has become an established method for measuring surface displacements and deformations of objects under stress. To date, DIC has been used in a variety of in vitro and in vivo studies to biomechanically characterise biological samples in order to reveal biomimetic principles. However, when surfaces of samples strongly deform or twist, they cannot be thoroughly traced. To overcome this challenge, different DIC setups have been developed to provide additional sensor perspectives and, thus, capture larger parts of an object's surface. Herein, we discuss current solutions for this multi-perspective DIC, and we present our own approach to a 360° DIC system based on a single stereo-camera setup. Using this setup, we are able to characterise the desiccation-driven opening mechanism of two woody Hakea fruits over their entire surfaces. Both the breaking mechanism and the actuation of the two valves in predominantly dead plant material are models for smart materials. Based on these results, an evaluation of the setup for 360° DIC regarding its use in deducing biomimetic principles is given. Furthermore, we propose a way to improve and apply the method for future measurements.

Methodology to Investigate Effect of Prosthetic Interface Design on Residual Limb Soft Tissue Deformation

BACKGROUND

Residual limb discomfort and injury is a common experience for people living with lower limb amputation. Frequently, inadequate load distribution between the prosthetic device and the residual limb is the root cause of this issue. To advance our understanding of prosthetic interface fit, tools are needed to evaluate the mechanical interaction at the prosthetic interface, allowing interface designs to be evaluated and optimised.

OBJECTIVE

Present a methodology report designed to facilitate comprehension of the mechanical interaction between the prosthetic interface and the residual limb. As a pilot study, this methodology is used to compare a hands-on and hands-off interface for a single transtibial prosthesis user using secondary Magnetic Resonance Imaging (MRI) data.

METHODOLOGY

MRI data of the residual limb while wearing a prosthetic interface is segmented into a hard tissue and a skin surface model. These models are exported as stereolithography (STL) files. Two methods are used to analyse the interface designs. Firstly, CloudCompare software is used to compute the nearest vertex on the skin surface for every vertex on the compiled internal bony surface for both interface types. Secondly, CloudCompare software is used to compare registered skin surfaces of the residual limb while wearing the hands-on and hands-off interfaces.

FINDINGS

The maximum and minimum nearest distances between the internal bony surface and skin surface were similar between interface types. However, the distribution of nearest distances was different. When comparing the skin surface while wearing both interfaces, where the fit is more compressive can be visualized. For the dataset used in this study, the classic features of a hands-on Patella Tendon Bearing interface and hands-off pressure cast interface could be identified.

CONCLUSION

The methodology presented in this report may give researchers a further tool to better understand how interface designs affect the soft tissues of the residual limb.

Panoramic three-dimensional optical digitization system assisted by a bi-mirror

The digitization of objects' full surfaces finds widespread applications in fields such as virtual reality, art and design, and medical and biological sciences. For the realization of three-dimensional full-surface digitization of objects within complex sceneries, we propose a straightforward, efficient, and robust panoramic three-dimensional optical digitization system. This system contains a laser-based optical three-dimensional measurement system and a bi-mirror. By integrating mirrors into the system, we enable the illumination of the object from all angles using the projected laser beam in a single scanning process. Moreover, the main camera employed in the system can acquire three-dimensional information of the object from several different viewpoints. The rotational scanning method enhances the efficiency and applicability of the three-dimensional scanning process, enabling the acquisition of surface information of large-scale objects. After obtaining the three-dimensional data of the sample from different viewpoints using laser triangulation, mirror reflection transformation was employed to obtain the full-surface three-dimensional data of the object in the global coordinate system. The proposed method has been subjected to precision and validity experiments using samples with different surface characteristics and sizes, resulting in the demonstration of its capability for achieving correct three-dimensional digitization of the entire surface in diverse complex sceneries.

Experimental and numerical analysis of the influence of intramedullary nail position on the cut-out phenomenon

BACKGROUND AND OBJECTIVE

Proximal femur fractures, colloquially known as hip fractures, are a common pathology with increasing incidence in the last years due to the enhanced ageing population. Regarding the extracapsular fracture, the treatment for this pathology consists of a fixation of the fragments using an osteosynthesis device, mainly the intramedullary nail. This repairing method implies several complications, which may include the failure of the fixation device, frequently occurring due to the "cut-out" mechanism. The present work focuses on the study of how the position of the cephalic screw, which should be fixed during surgery, affects the cut-out risk. Through experimental tests and numerical models some variables that can be critical for the cut-out phenomenon are analysed.

METHODS

This study has been carried out through a numerical model based on the finite element method and experimental tests. The digital image correlation technique has been used in experimental tests to measure displacements on the femoral surface with the objective of numerical model validation. Some basic daily activities with different intramedullary nail positions have been analysed through the numerical model, considering variables that can induce the cut-out complication.

RESULTS

The results show how the intramedullary nail position clearly influences the cut-out risk, showing that displacements in the upper, anterior and posterior direction increase the cut-out risk, while displacement in the lower direction endangers the intramedullary nail itself. Thus, the centred position is the one which reduces the cut-out risk.

CONCLUSIONS

This work supposes an improvement in the knowledge of the cut-out phenomenon thanks to the combination of experimental testing and validated numerical models. The effects of different intramedullary nail positions in the femoral head are studied, including a novelty variable as torque, which is critical for the structural integrity of the fixation. The main conclusion of the work is the determination of the central intramedullary nail position as the most favourable one for decreasing the cut-out risk.

Mapping lines of non-extension in persons with lower limb amputation to aid comfort-driven prosthetic socket design

OBJECTIVE

This study aimed to develop a new technique to map the strain field for persons with lower-limb amputations to use for the design of comfortable prostheses.

METHODS

Using a DSLR camera with stenciled 2D markers, we demonstrated a technique to measure skin strain around the residual limb of persons with lower limb amputations. We used open-source software programs to reconstruct a series of cloud points derived from the pictures of the marked residual limb into 3D models, then calculated the minimum, maximum, and non-extension lines from directional strain fields.

RESULTS

A DSLR camera was successful in capturing 2D markers. The maximum mean principal strain was 68% ± 14%, observed around the patella. The minimum compressive mean principal strain of -31% ± 4% was observed posteriorly in the popliteal region of the knee. Although lines of non-extension (LoNE) appear separate in different participants, they are anatomically located in regions that could be generalized for the design of prostheses.

CONCLUSIONS

Marker locations extracted from the video of different poses can be compared to calculate strains from which the position of LoNE can be generated. The use of LoNE could be valuable in reducing discomfort at the socket interface and informing future socket design.

A review of the characterizations of soft tissues used in human body modeling: Scope, limitations, and the path forward

Soft tissue material properties are vital to human body models that evaluate interactions between the human body and its environment. Such models evaluate internal stress/strain responses in soft tissues to investigate issues like pressure injuries. Numerous constitutive models and parameters have been used to represent mechanical behavior of soft tissues in biomechanical models under quasi-static loading. However, researchers reported that generic material properties cannot accurately represent specific target populations due to large inter-individual variability. Two challenges that exist are experimental mechanical characterization and constitutive modeling of biological soft tissues and personalization of constitutive parameters using non-invasive, non-destructive bedside testing methods. It is imperative to understand the scope and appropriate applications for reported material properties. Thus, the goal of this paper was to compile studies from which soft tissue material properties were obtained and categorize them by source of tissue samples, methods used to quantify deformation, and material models used to describe tissues. The collected studies displayed wide ranges of material properties, and factors that affected the properties included whether tissue samples were in vivo or ex vivo, from humans or animals, the body region tested, body position during in vivo studies, deformation measurements, and material models used to describe tissues. Because of the factors that affected reported material properties, it is clear that much progress has been made in understanding soft tissue responses to loading, yet there is a need to broaden the scope of reported soft tissue material properties and better match reported properties to appropriate human body models.

Development of a Hip Joint Socket by Finite-Element-Based Analysis for Mechanical Assessment

This article evaluates a hip joint socket design by finite element method (FEM). The study was based on the needs and characteristics of a patient with an oncological amputation; however, the solution and the presented method may be generalized for patients with similar conditions. The research aimed to solve a generalized problem, taking a typical case from the study area as a reference. Data were collected on the use of the current improving prosthesis-specifically in interaction with its socket-to obtain information on the new approach design: this step constituted the work's starting point, where the problems to be solved in conventional designs were revealed. Currently, the development of this type of support does not consider the functionality and comfort of the patient. Research has reported that 58% of patients with sockets have rejected their use, because they do not fit comfortably and functionally; therefore, patients' low acceptance or rejection of the use of the prosthesis socket has been documented. In this study, different designs were evaluated, based on the FEM as scientific support for the results obtained, for the development of a new ergonomic fit with a 60% increase in patient compliance, that had correct gait performance when correcting postures, improved fit-user interaction, and that presented an esthetic fit that met the usability factor. The validation of the results was carried out through the physical construction of the prototype. The research showed how the finite element method improved the design, analyzing the structural behavioral, and that it could reduce cost and time instead of generating several prototypes.

Multi-Camera Digital Image Correlation in Deformation Measurement of Civil Components with Large Slenderness Ratio and Large Curvature

To address the limitations of conventional stereo-digital image correlation (DIC) on measuring complex objects, a continuous-view multi-camera DIC (MC-DIC) system and its two forms of camera arrangement are introduced. Multiple cameras with certain overlapping field of view are calibrated simultaneously to form an overall system for measuring the continuous full-surface deformation. The bending experiment of coral aggregate concrete beam and the axial compression experiment of timber column are conducted to verify the capability of continuous-view MC-DIC in deformation measurement of civil components with large slenderness ratio and large curvature, respectively. The obtained deformation data maintain good consistency with the displacement transducer and strain gauge. Results indicate that the continuous-view MC-DIC is a reliable 3D full-field measurement approach in civil measurements.

Reliability of three different methods for assessing amputee residuum shape and volume: 3D scanners vs. circumferential measurements

BACKGROUND

Objective and reliable methods are necessary to monitor and manage amputee residuum shape and volume and design prosthetic residuum-prosthesis interfaces. Portable 3D scanners are potential solutions for digitally recording the amputee residuum characteristics.

OBJECTIVE

To investigate test-operator reliability when measuring lower limb residuum shape and volume using two different types of 3D laser-free scanners compared with tape measurements generally adopted in clinic.

STUDY DESIGN

Ten lower limb amputees took part in this study. Residuum volume, cross sectional areas, and perimeter lengths were measured by three different operators on three different occasions using two types of 3D scanners (Artec Eva scanner and OMEGA Scanner 3D) and circumferential measurements.

METHODS

Variance components, intraclass correlation coefficients and intra-rater and inter-rater reliability coefficients were calculated for all measurement conditions.

RESULTS

Residuum volume outputs ranged from 569 to 3115 mL. The factor contributing mostly to the residuum volume error variance was the shape of the residuum (75.85%). Volume intraclass correlation coefficients for both intra-rater and inter-rater reliability exceeded 0.9 for all three conditions. Volume reliability coefficients ranged from 70.68 mL (Artec Eva intra-rater reliability) to 256.85 mL (circumferential measurements inter-rater reliability). Shape relative error reached the highest values for the circumferential measurements (>10% for the cross-sectional areas and >5% for the perimeters).

CONCLUSIONS

The Artec Eva scanner resulted in the lowest test-operator reliability coefficients. However, both investigated scanners are a potential alternative for measuring small and macroscopic changes in residuum characteristics.

Panoramic/Dual-Surface Digital Image Correlation Measurement Using a Single Camera

We propose a cost-effective and simple-to-implement mirror-assisted single-camera panoramic digital image correlation (DIC) method for panoramic/dual-surface profile and deformation measurement. Specifically, two planar mirrors and a single camera attached with a four-mirror adapter are used to capture stereo images of the front and rear surfaces of a test object. These stereo images can be processed by regular stereo-DIC to retrieve shape and kinematics fields of each surface. Further, with the speckle patterns prefabricated on the mirrors, reflection transformation matrices are obtained and applied to transform all reconstructed surfaces into a common world coordinate system. As such, panoramic/dual-surface shape and deformation measurements can be realized. For validation, a high-resolution smartphone camera and an industrial camera were, respectively, used to construct mirror-assisted single-camera panoramic DIC systems. Real experiments, including panoramic shape measurement of an aluminum cylinder, dual-surface shape measurement of an aluminum plate and uniaxial tensile tests of aluminum sheet specimens, were performed, confirming the feasibility and accuracy of the method. Since only a single camera and a few auxiliary reflective mirrors are required, the proposed method provides a cost-effective and convenient way for taking panoramic/dual-surface shape and deformation measurements of regular-sized cylindrical and bar samples.

Reliability and validity of 3D limb scanning for ankle-foot orthosis fitting

BACKGROUND

Recent decreases in the cost of 3D scanners and improved functionality have resulted in increased adoption for ankle-foot orthosis (AFO) fittings, despite limited supporting data. For 3D limb scanning to be a feasible alternative to traditional casting methods, a consistent and accurate representation of limb geometry must be produced at a reasonable cost.

OBJECTIVES

To evaluate the repeatability and validity of multiple lower limb measurements obtained using low-cost 3D limb scanning technology.

STUDY DESIGN

Prospective, randomized, crossover-controlled, cross-sectional, reliability, and validity study.

METHODS

Physical measurements and 3D limb scans were completed for 30 participants. 11 measurements were selected for comparison based on their relevance to AFO fittings. Validity was assessed by comparison of physical and scan-based measures using Pearson's correlation coefficients and root mean square differences. Reliability was assessed using intraclass correlation coefficients and minimal detectable change (MDC) values. Bland-Altman plots were generated for data visualization.

RESULTS

All correlation values were above or equal to 0.80. Most intraclass correlation coefficient values were above 0.95. MDC values for physical and scan-based measurements differed by less than 2.0 mm. Scan MDC values were around or below 4 mm for foot and ankle measures and under 6 mm for circumference and length measures.

CONCLUSIONS

The results of this study demonstrate that low-cost 3D limb scanning can be used to obtain valid and reliable measurements of 3D limb geometry for the purpose of AFO fitting, when collected using the clinically relevant standardized conditions presented here.

The Use of Smartphone Photogrammetry to Digitise Transtibial Sockets: Optimisation of Method and Quantitative Evaluation of Suitability

The fit of a lower limb prosthetic socket is critical for user comfort and the quality of life of lower limb amputees. Sockets are conventionally produced using hand-crafted patient-based casting techniques. Modern digital techniques offer a host of advantages to the process and ultimately lead to improving the lives of amputees. However, commercially available scanning equipment required is often expensive and proprietary. Smartphone photogrammetry could offer a low cost alternative, but there is no widely accepted imaging technique for prosthetic socket digitisation. Therefore, this paper aims to determine an optimal imaging technique for whole socket photogrammetry and evaluate the resultant scan measurement accuracy. A 3D printed transtibial socket was produced to create digital and physical twins, as reference models. The printed socket was photographed from 360 positions and simplified genetic algorithms were used to design a series of experiments, whereby a collection of photos were processed using Autodesk ReCap. The most fit technique was used to assess accuracy. The accuracy of the socket wall volume, surface area and height were 61.63%, 99.61% and 99.90%, respectively, when compared to the digital reference model. The scanned model had a wall thickness ranging from 2.075 mm at the top to 7.758 mm towards the base of the socket, compared to a consistent thickness of 2.025 mm in the control model. The technique selected did not show sufficient accuracy for clinical application due to the degradation of accuracy nearer to the base of the socket interior. However, using an internal wall thickness estimation, scans may be of sufficient accuracy for clinical use; assuming a uniform wall thickness.

Multi-view digital image correlation systems for in vitro testing of arteries from mice to humans

BACKGROUND

Digital image correlation (DIC) methods are increasingly used for non-contact optical assessment of geometry and deformation in soft tissue biomechanics, thus providing the full-field strain estimates needed for robust inverse material characterization. Despite the well-known flexibility and ease of use of DIC, issues related to spatial resolution and depth-of-field remain challenging in studies of quasi-cylindrical biological samples such as arteries.

OBJECTIVE

After demonstrating that standard surrounding multi-view DIC systems are inappropriate for such usage, we submit that both the optical setup and the data analysis need to be specifically designed with respect to the size of the arterial sample of interest. Accordingly, we propose novel and optimized DIC systems for two distinct ranges of arterial diameters: less than 2.5 mm (murine arteries) and greater than 10 mm (human arteries).

METHODS

We designed, set up, and validated a four-camera panoramic-DIC system for testing murine arteries and a multi-biprism DIC system for testing human arteries. Both systems enable dynamic 360-deg measurements with refraction correction over the entire surface of submerged samples in their native geometries.

RESULTS

Illustrative results for 3D shape and full-surface deformation fields were obtained for a mouse infrarenal aorta and a latex cylinder of size similar to the human infrarenal aorta.

CONCLUSION

Results demonstrated the feasibility and accuracy of both proposed methods in providing quantitative information on the regional behavior of arterial samples tested in vitro under physiologically relevant loading.

Digital healthcare technologies: Modern tools to transform prosthetic care

INTRODUCTION

Digital healthcare technologies are transforming the face of prosthetic care. Millions of people with limb loss around the world do not have access to any form of rehabilitative healthcare. However, digital technologies provide a promising solution to augment the range and efficiency of prosthetists.

AREAS COVERED

The goal of this review is to introduce the digital technologies that have the potential to change clinical methods in prosthetic healthcare. Our target audience are researchers who are unfamiliar with the field of prostheses in general, especially with the newest technological developments. This review addresses technologies for: scanning of amputated limbs, limb-to-socket rectification, additive manufacturing of prosthetic sockets, and quantifying patient response to wearing sockets. This review does not address biomechatronic prostheses or biomechanical design practices.

EXPERT OPINION

Digital technologies will enable affordable prostheses to be built on a scale larger than with today's clinical practices. Large technological gaps need to be overcome to enable the mass production and distribution of prostheses digitally. However, recent advances in computational methods and CAD/CAM technologies are bridging this gap faster than ever before. We foresee that these technologies will return mobility and economic opportunity to amputees on a global scale in the near future.

Full-view imaging on dynamic closed surface by curved-to-flat conversion lens

Conventional full-view imaging systems, which often need complicated image processing algorithms to reconstruct full-view images captured by motional/multiple cameras from different views, cannot have good real-time imaging capability. We design curved-to-flat conversion lens (CFCL) based on optic-null medium, which can directly project/image optical patterns from closed object surface onto image plane (e.g., the focal plane of microscopy), and shows good real-time full-view imaging performance. To realize the CFCL, the reduced optic-null medium is designed by subwavelength metal channels filled with homogeneous isotropic dielectrics. Numerical simulation results verify the function of the designed CFCL, which can image various dynamic optical patterns from the closed object surface to the finite-view image plane. The designed CFCL may have many applications in real-timely observing dynamic closed surfaces in full view, e.g., living tissue/cell and soft material's surface.

Residual limb volume fluctuations in transfemoral amputees

This study constitutes the first attempt to systematically quantify residual limb volume fluctuations in transfemoral amputees. The study was carried out on 24 amputees to investigate variations due to prosthesis doffing, physical activity, and testing time. A proper experimental set-up was designed, including a 3D optical scanner to improve precision and acceptability by amputees. The first test session aimed at measuring residual limb volume at 7 time-points, with 10 min intervals, after prosthesis doffing. This allowed for evaluating the time required for volume stabilization after prosthesis removal, for each amputee. In subsequent sessions, 16 residual limb scans in a day for each amputee were captured to evaluate volume fluctuations due to prosthesis removal and physical activity, in two times per day (morning and afternoon). These measurements were repeated in three different days, a week apart from each other, for a total of 48 scans for each amputee. Volume fluctuations over time after prosthesis doffing showed a two-term decay exponential trend (R2 = 0.97), with the highest variation in the initial 10 min and an average stabilization time of 30 min. A statistically significant increase in residual limb volume following both prosthesis removal and physical activity was verified. No differences were observed between measures collected in the morning and in the afternoon.Clinical Trials.gov ID: NCT04709367.

Lower limb prosthetic interfaces: Clinical and technological advancement and potential future direction

The human-prosthesis interface is one of the most complicated challenges facing the field of prosthetics, despite substantive investments in research and development by researchers and clinicians around the world. The journal of the International Society for Prosthetics and Orthotics, Prosthetics and Orthotics International, has contributed substantively to the growing body of knowledge on this topic. In celebrating the 50th anniversary of the International Society for Prosthetics and Orthotics, this narrative review aims to explore how human-prosthesis interfaces have changed over the last five decades; how research has contributed to an understanding of interface mechanics; how clinical practice has been informed as a result; and what might be potential future directions. Studies reporting on comparison, design, manufacturing and evaluation of lower limb prosthetic sockets, and osseointegration were considered. This review demonstrates that, over the last 50 years, clinical research has improved our understanding of socket designs and their effects; however, high-quality research is still needed. In particular, there have been advances in the development of volume and thermal control mechanisms with a few designs having the potential for clinical application. Similarly, advances in sensing technology, soft tissue quantification techniques, computing technology, and additive manufacturing are moving towards enabling automated, data-driven manufacturing of sockets. In people who are unable to use a prosthetic socket, osseointegration provides a functional solution not available 50 years ago. Furthermore, osseointegration has the potential to facilitate neuromuscular integration. Despite these advances, further improvement in mechanical features of implants, and infection control and prevention are needed.

Decoding of facial strains via conformable piezoelectric interfaces

Devices that facilitate nonverbal communication typically require high computational loads or have rigid and bulky form factors that are unsuitable for use on the face or on other curvilinear body surfaces. Here, we report the design and pilot testing of an integrated system for decoding facial strains and for predicting facial kinematics. The system consists of mass-manufacturable, conformable piezoelectric thin films for strain mapping; multiphysics modelling for analysing the nonlinear mechanical interactions between the conformable device and the epidermis; and three-dimensional digital image correlation for reconstructing soft-tissue surfaces under dynamic deformations as well as for informing device design and placement. In healthy individuals and in patients with amyotrophic lateral sclerosis, we show that the piezoelectric thin films, coupled with algorithms for the real-time detection and classification of distinct skin-deformation signatures, enable the reliable decoding of facial movements. The integrated system could be adapted for use in clinical settings as a nonverbal communication technology or for use in the monitoring of neuromuscular conditions.

In vivo panoramic human skin shape and deformation measurement using mirror-assisted multi-view digital image correlation

Panoramic shape and deformation measurements of human skin in vivo may provide important information for biomechanical analysis, exercise guidance and medical diagnosis. This work proposes the application of an advanced mirror-assisted multi-view digital image correlation (DIC) method for dynamic measurements of 360-deg shape and deformation of human body parts in vivo. The main advantage of this method consists in its capabilities to perform full-panoramic non-contact measurements with a single pair of synchronized cameras and two planar mirrors thus representing a lean yet effective alternative to conventional multi-camera DIC systems in 'surrounding' configuration. We demonstrate the capabilities of this method by measuring the full-panoramic shape of a plastic human head, the deformation of a woman face and the principal strain distribution over the full-360-deg surface of a forearm during fist clenching. The applications of this method can be the most disparate but, given the possibility to determine the full-field strains and derived information (e.g. skin tension lines), we envisage a great potential for the study of skin biomechanics in vivo.

A full-field 3D digital image correlation and modelling technique to characterise anterior cruciate ligament mechanics ex vivo

It is limiting to use conventional methods when characterising material properties of complex biological tissues with inhomogeneous and anisotropic structure, such as the anterior cruciate ligament (ACL) in the knee joint. This study aims to develop and utilise a three-dimensional digital image correlation method (3D DIC) for the purpose of determining material properties of femur-ACL-tibia complex across the surface without any contact between the tissue and the loading equipment. A full-field (360° view) 3D DIC test setup consisting of six digital single-lens reflex cameras was developed and ACL specimens from skeletally mature dog knee joints were tested. The six cameras were arranged into three pairs and the cameras within each pair were positioned with 25° in between to obtain the desired stereovision output. The test setup was calibrated twice: first to obtain the intrinsic and extrinsic parameters within camera pairs, and second to align the 3D surfaces from each camera pair in order to generate the full view of the ACLs. Using the undeformed 3D surfaces of the ligaments, ACL-specific finite element models were generated. Longitudinal deformation of ligaments under tensile loads obtained from the 3D DIC, and this was analysed to serve as input for the inverse finite element analysis. As a result, hyperelastic coefficients from the first-order Ogden model that characterise ACL behaviour were determined with a marginal error of <1.5%. This test setup and methodology provides a means to accurately determine inhomogeneous and anisotropic material properties of ACL. The methodology described in this study could be adopted to investigate other biological and cultured tissues with complex structure. STATEMENT OF SIGNIFICANCE: Determining the material properties of soft tissues with complex anatomical structure, such as the anterior cruciate ligament (ACL), is important to better understand their contribution to musculoskeletal biomechanics. Current conventional methods for characterising material properties of the ACL are often limited to a contact measurement approach, however an improved understanding of the mechanics of this complex tissue is vital in terms of preventing injury and developing novel therapies. This article reports the development and utilisation of non-contact optical methodology involving full-field three-dimensional digital image correlation and finite element analysis to accurately investigate material properties of the ACL, in a controlled environment. This technique reduces inaccuracies due to specimen clamping and more importantly considers the inhomogeneous nature of the examined tissue.

Evaluation of the influence of cyclic loading on a laser sintered transtibial prosthetic socket using Digital Image Correlation (DIC)

People with a transtibial amputation worldwide rely on their prosthetic socket to regain their mobility. Patient comfort is largely affected by the weight and strength of these prosthetic sockets. The use of additive manufacturing could give the prosthetist a range of new design possibilities when designing a prosthetic socket. These new design possibilities can in turn lead to improved socket designs and more comfortable prosthetic sockets. This new way of designing and producing prosthetic sockets radically differs from the manual traditional production process. This makes it difficult for prosthetists to understand how all these new design possibilities influence the mechanical properties of the additive manufactured prosthetic socket. Therefore there is a growing need for a method to evaluate the strength and stiffness of newly developed socket designs.We propose a method to evaluate the strength and stiffness of prosthetic sockets. A robotic gait simulator is used to apply realistic kinetics of amputee gait to the tested socket. A Digital Image Correlation (DIC) system is then used to measure the deformation of a prosthetic socket under different loading conditions. This way it is possible to check if plastic deformation will occur in the designed transtibial socket. Furthermore it is possible to assess the effect of cyclic loading on the 3D printed socket.To illustrate the proposed method, a transtibial prosthetic socket was designed using CAD software and produced with laser sintering PA12. DIC measurements were performed on this transtibial socket both before and after it was subjected to a cyclic load of 1 million cycles (mimicking realistic amputee gait).