An MRI-compatible foot-loading device for assessment of internal tissue deformation

Subject-specific material properties of the heel pad: An inverse finite element analysis

Individuals with diabetes are at a higher risk of developing foot ulcers. To better understand internal soft tissue loading and potential treatment options, subject-specific finite element (FE) foot models have been used. However, existing models typically lack subject-specific soft tissue material properties and only utilize subject-specific anatomy. Therefore, this study determined subject-specific hindfoot soft tissue material properties from one non-diabetic and one diabetic subject using inverse FE analysis. Each subject underwent cyclic MRI experiments to simulate physiological gait and to obtain compressive force and three-dimensional soft tissue imaging data at 16 phases along the loading-unloading cycles. The FE models consisted of rigid bones and nearly-incompressible first-order Ogden hyperelastic skin, fat, and muscle (resulting in six independent material parameters). Then, calcaneus and loading platen kinematics were computed from imaging data and prescribed to the FE model. Two analyses were performed for each subject. First, the skin, fat, and muscle layers were lumped into a single generic soft tissue material and optimized to the platen force. Second, the skin, fat, and muscle material properties were individually determined by simultaneously optimizing for platen force, muscle vertical displacement, and skin mediolateral bulging. Our results indicated that compared to the individual without diabetes, the individual with diabetes had stiffer generic soft tissue behavior at high strain and that the only substantially stiffer multi-material layer was fat tissue. Thus, we suggest that this protocol serves as a guideline for exploring differences in non-diabetic and diabetic soft tissue material properties in a larger population.

MR-compatible loading device for assessment of heel pad internal tissue displacements under shearing load

In the last decade, the role of shearing loads has been increasingly suspected to play a determinant impact in the formation of deep pressure ulcers. In vivo observations of such deformations are complex to obtain. Previous studies only provide global measurements of such deformations without getting the quantitative values of the loads that generate these deformations. To study the role that shearing loads have in the etiology of heel pressure ulcers, an MR-compatible device for the application of shearing and normal loads was designed. Magnetic resonance imaging is a key feature that allows to monitor deformations of soft tissues after loading in a non-invasive way. Measuring applied forces in an MR-environment is challenging due to the impossibility to use magnetic materials. In our device, forces are applied through the compression of springs made of polylactide. Shearing and normal loads were applied on the plantar skin of the human heel through a flat plate while acquiring MR images. The device materials did not introduce any imaging artifact and allowed for high quality MR deformation measurements of the internal components of the heel. The obtained subject-specific results are an original data set that can be used in validations for Finite Element analysis and therefore contribute to a better understanding of the factors involved in pressure ulcer development.

An Innovative Weightbearing Device for Weightbearing 3-Dimensional Imaging for Foot and Ankle Surgery Preoperative Planning

Three-dimensional preoperative planning has demonstrated multiple surgical advantages. Currently, we cannot carry out preoperative 3-dimensional planning of foot and ankle orthopedics in most hospitals due to the impossibility of performing weightbearing CT imaging. Our objective is to describe and evaluate an innovative accessible, simple, and effective device that simulates standing while in a supine position, to obtain 3-dimensional images supporting bodyweight load with a conventional CT machine. From a group of 30 volunteers, 10 patients were randomly selected and pressure and its distribution were analyzed while in a standing position in both feet. Differences between both feet were considered normal intrapersonal variability. Subsequently, the right footprint of the same 10 subjects was evaluated in the proposed loading device. Then, their pressures and distribution were compared with respect to standing and with respect to intrapersonal variability. The mean total standing pressure was 93 Kpa (standard deviation [SD] 14.32), which was reduced to 81.95 Kpa (SD 19.54) in the loading device. The load device reduced the pressure by a mean of16% (SD 22% (range -25% to -0.03%). At the hindfoot level, the loading device increased pressure by a mean of 20.59 Kpa, which expressed percentage implies an increase of 14% compared to standing. Due to its easy construction and effectiveness, this is the first device that opens the door of foot and ankle orthopedics in any hospital to 3D preoperative planning and the benefits derived from it.

Magnetic resonance imaging to estimate tissue deformations during penile clamp application: A case series

Background:

Penile clamps provide a means of preventing urinary incontinence in males following radical prostatectomy. In order for the devices to function, significant mechanical loads need to be applied to the penile tissues to close the urethra. However, such loads have the potential to cause damage to the vulnerable skin and underlying soft tissues. Accordingly, the study aimed to estimate the magnitudes of tissue deformations resulting from penile clamp application in three individual cases.

Methods:

Three individuals were recruited who currently use penile clamps to manage urinary incontinence following radical prostatectomy. Magnetic resonance images (MRI) of the penis were taken to produce a series of high contrast coronal and sagittal images both before and during the application of two commercially available clamps, modified for MRI compatibility. Tissue thickness measurements were estimated with the clamps in-situ and normalised to the unloaded baseline state.

Results:

The estimated magnitude of tissue deformations resulting from clamp application ranged between 68% and 84%. There were minimal differences in these deformations between the clamp designs, both of which appeared effective in closing the urethra. Local stress concentrations were observed in the tissues, which were deformed around the shape of the clamp.

Conclusions:

MRI enabled quantification of local tissue deformation during penile clamp application. The results revealed that clamps created large tissue deformations in all three cases, regardless of design. This information could inform the development of new clamp designs and materials to minimise the potential for tissue damage.

Level of evidence:

4

Technologies to monitor the health of loaded skin tissues

There are many situations where the skin and underlying soft tissues are compromised by mechanical loading in the form or pressure, or pressure in combination with shear. If sustained, this can lead to damage in the tissues particularly adjacent to bony prominences, resulting in chronic wounds. An array of bioengineering technologies have been adopted to assess the integrity of loaded soft tissues. This paper aims to review these approaches for the quantification, simulation and early detection of mechanically-induced skin damage. The review considers different measurements at the interface between the skin and support surface/medical device, involving pressure, shear, friction and the local microclimate. The potential of the techniques to monitor the physiological response of the skin to these external stimuli including biophysical measurement devices and sampling of biofluids are critically analysed. In addition, it includes an analysis of medical imaging technologies and computational modelling to provide a means by which tissue deformation can be quantified and thresholds for tissue damage defined. Bioengineering measurement and imaging technologies have provided an insight into the temporal status of loaded skin. Despite the advances in technology, to date, the translation to clinical tools which are robust and cost effective has been limited. There is a need to adapt existing technologies and simulation platforms to enable patients, carers and clinicians to employ appropriate intervention strategies to minimise soft tissue damage.

A preliminary study of patient-specific mechanical properties of diabetic and healthy plantar soft tissue from gated magnetic resonance imaging

Foot loading rate, load magnitude, and the presence of diseases such as diabetes can all affect the mechanical properties of the plantar soft tissues of the human foot. The hydraulic plantar soft tissue reducer instrument was designed to gain insight into which variables are the most significant in determining these properties. It was used with gated magnetic resonance imaging to capture three-dimensional images of feet under dynamic loading conditions. Custom electronics controlled by LabVIEW software simultaneously recorded system pressure, which was then translated to applied force values based on calibration curves. Data were collected for two subjects, one without diabetes (Subject A) and one with diabetes (Subject B). For a 0.2-Hz loading rate, and strains 0.16, 0.18, 0.20, and 0.22, Subject A's average tangential heel pad stiffness was 10 N/mm and Subject B's was 24 N/mm. Maximum test loads were approximately 200 N. Loading rate and load magnitude limitations (both were lower than physiologic values) will continue to be addressed in the next version of the instrument. However, the current hydraulic plantar soft tissue reducer did produce a data set for healthy versus diabetic tissue stiffness that agrees with previous trends. These data are also being used to improve finite element analysis models of the foot as part of a related project.

Demonstration and validation of a new pressure-based MRI-safe pain tolerance device

BACKGROUND

One of the barriers to studying the behavioral and emotional effects of pain using functional Magnetic Resonance Imaging (fMRI) is the absence of a commercially available, MRI-compatible, pressure-based algometer to elicit pain. The present study sought to address this barrier through creation and validation of a novel MRI-safe apparatus capable of delivering incremental, measurable amounts of pressure inside a scanning bore.

NEW METHOD

We introduced an MR-safe device used to administer pressure-based pain. To test against a commercially available, MRI-incompatible algometer (AlgoMed), 199 participants reported their pain tolerance for both devices. A second experiment tested the validity of pressure-based pain in an MRI environment by comparing brain activation with established neural networks for pain. 10 participants performed an identical procedure to test for pain tolerance while being scanned in a 7T MRI scanner.

RESULTS

Results support the validity and reliability of our novel device. In Study 1, pain tolerance with this device was strongly correlated with pain tolerance as measured by a commercially available algometer (r=0.78). In Study 2, this device yielded BOLD activation within the insula (BA 13) and anterior cingulate gyrus (BA 24); as pressure increased, activation in these areas parametrically increased.

COMPARISON WITH EXISTING METHOD

These findings correspond to other studies using thermal, electrical, or mechanical pain applications. Behavioral and functional data demonstrate that this new device is a valid method of administering pressure-related pain in MRI environments.

CONCLUSIONS

Our novel MRI-safe device is a valid instrument to measure and administer pressure-based pain.

The design and validation of a magnetic resonance imaging-compatible device for obtaining mechanical properties of plantar soft tissue via gated acquisition

Changes in the mechanical properties of the plantar soft tissue in people with diabetes may contribute to the formation of plantar ulcers. Such ulcers have been shown to be in the causal pathway for lower extremity amputation. The hydraulic plantar soft tissue reducer (HyPSTER) was designed to measure in vivo, rate-dependent plantar soft tissue compressive force and three-dimensional deformations to help understand, predict, and prevent ulcer formation. These patient-specific values can then be used in an inverse finite element analysis to determine tissue moduli, and subsequently used in a foot model to show regions of high stress under a wide variety of loading conditions. The HyPSTER uses an actuator to drive a magnetic resonance imaging-compatible hydraulic loading platform. Pressure and actuator position were synchronized with gated magnetic resonance imaging acquisition. Achievable loading rates were slower than those found in normal walking because of a water-hammer effect (pressure wave ringing) in the hydraulic system when the actuator direction was changed rapidly. The subsequent verification tests were, therefore, performed at 0.2 Hz. The unloaded displacement accuracy of the system was within 0.31%. Compliance, presumably in the system's plastic components, caused a displacement loss of 5.7 mm during a 20-mm actuator test at 1354 N. This was accounted for with a target to actual calibration curve. The positional accuracy of the HyPSTER during loaded displacement verification tests from 3 to 9 mm against a silicone backstop was 95.9% with a precision of 98.7%. The HyPSTER generated minimal artifact in the magnetic resonance imaging scanner. Careful analysis of the synchronization of the HyPSTER and the magnetic resonance imaging scanner was performed. With some limitations, the HyPSTER provided key functionality in measuring dynamic, patient-specific plantar soft tissue mechanical properties.

A device for characterising the mechanical properties of the plantar soft tissue of the foot

The plantar soft tissue is a highly functional viscoelastic structure involved in transferring load to the human body during walking. A Soft Tissue Response Imaging Device was developed to apply a vertical compression to the plantar soft tissue whilst measuring the mechanical response via a combined load cell and ultrasound imaging arrangement. Accuracy of motion compared to input profiles; validation of the response measured for standard materials in compression; variability of force and displacement measures for consecutive compressive cycles; and implementation in vivo with five healthy participants. Static displacement displayed average error of 0.04 mm (range of 15 mm), and static load displayed average error of 0.15 N (range of 250 N). Validation tests showed acceptable agreement compared to a Houndsfield tensometer for both displacement (CMC > 0.99 RMSE > 0.18 mm) and load (CMC > 0.95 RMSE < 4.86 N). Device motion was highly repeatable for bench-top tests (ICC = 0.99) and participant trials (CMC = 1.00). Soft tissue response was found repeatable for intra (CMC > 0.98) and inter trials (CMC > 0.70). The device has been shown to be capable of implementing complex loading patterns similar to gait, and of capturing the compressive response of the plantar soft tissue for a range of loading conditions in vivo.

An innovative ultrasound foot scanner system for measuring the change in biomechanical properties of plantar tissue from sitting to standing

The present study investigated the reliability of an innovative ultrasound foot scanner system in assessing the thickness and stiffness of plantar soft tissue and the comparison of stiffness and thickness in sitting and standing. Fifteen young healthy individuals were examined. The target sites on the foot sole for investigation included the heel pad, the fifth metatarsal head, the second metatarsal head, the first metatarsal head, and the pulp of the hallux. The test (day 1) and retest (day 2) were performed 1 week apart at the exact time with humidity and temperature of the assessment room under control. The thickness and stiffness of the plantar soft tissue obtained in sitting and standing positions on day 1 were used for comparison. The results showed significant test-retest reliability [intraclass correlation coefficient(3,2)>0.90, P<0.001] at all five sites in both sitting and standing positions. When changing from sitting to standing, the plantar soft tissue became significantly thinner (with decrease ranging from 10 to 14% at various sites) and stiffer (with increase ranging from 123 to 164% at various sites, all P<0.05). The present innovative system is a reliable device for the measurement of the thickness and stiffness of plantar soft tissue in either the sitting or the standing position. The change in positions from sitting to standing resulted in a significant thinning and stiffening of plantar soft tissues. This system could be a potential clinical device to monitor the biomechanical properties of plantar tissue in the elderly or in patients with diseases such as diabetes to estimate the risk of developing foot ulcer or other foot complications.

Optimization of nonlinear hyperelastic coefficients for foot tissues using a magnetic resonance imaging deformation experiment

Accurate prediction of plantar shear stress and internal stress in the soft tissue layers of the foot using finite element models would provide valuable insight into the mechanical etiology of neuropathic foot ulcers. Accurate prediction of the internal stress distribution using finite element models requires that realistic descriptions of the material properties of the soft tissues are incorporated into the model. Our investigation focused on the creation of a novel three-dimensional (3D) finite element model of the forefoot with multiple soft tissue layers (skin, fat pad, and muscle) and the development of an inverse finite element procedure that would allow for the optimization of the nonlinear elastic coefficients used to define the material properties of the skin muscle and fat pad tissue layers of the forefoot based on a Ogden hyperelastic constitutive model. Optimization was achieved by comparing deformations predicted by finite element models to those measured during an experiment in which magnetic resonance imaging (MRI) images were acquired while the plantar surface forefoot was compressed. The optimization procedure was performed for both a model incorporating all three soft tissue layers and one in which all soft tissue layers were modeled as a single layer. The results indicated that the inclusion of multiple tissue layers affected the deformation and stresses predicted by the model. Sensitivity analysis performed on the optimized coefficients indicated that small changes in the coefficient values (±10%) can have rather large impacts on the predicted nominal strain (differences up to 14%) in a given tissue layer.

Effects of foot posture on fifth metatarsal fracture healing: a finite element study

The goal of this study was to evaluate the effects of maintaining different foot postures during healing of proximal fifth metatarsal fractures for each of 3 common fracture types. A 3-dimensional (3D) finite element model of a human foot was developed and 3 loading situations were evaluated, including the following: (1) normal weightbearing, (2) standing with the affected foot in dorsiflexion at the ankle, and (3) standing with the affected foot in eversion. Three different stages of the fracture-healing process were studied, including: stage 1, wherein the material interposed between the fractured edges was the initial connective tissue; stage 2, wherein connective tissue had been replaced by soft callus; and stage 3, wherein soft callus was replaced by mature bone. Thus, 30 3D finite element models were analyzed that took into account fracture type, foot posture, and healing stage. Different foot postures did not statistically significantly affect the peak-developed strains on the fracture site. When the fractured foot was everted or dorsiflexed, it developed a slightly higher strain within the fracture than when it was in the normal weightbearing position. In Jones fractures, eversion of the foot caused further torsional strain and we believe that this position should be avoided during foot immobilization during the treatment of fifth metatarsal base fractures. Tuberosity avulsion fractures and Jones fractures seem to be biomechanically stable fractures, as compared with shaft fractures. Our understanding of the literature and experience indicate that current clinical observations and standard therapeutic options are in accordance with the results that we observed in this investigation, with the exception of Jones fractures.

A novel MRI compatible soft tissue indentor and fibre Bragg grating force sensor

MRI is an ideal method for non-invasive soft tissue mechanical properties investigation. This requires mechanical excitation of the body's tissues and measurement of the corresponding boundary conditions such as soft tissue deformation inside the MRI environment. However, this is technically difficult since load application and measurement of boundary conditions requires MRI compatible actuators and sensors. This paper describes a novel MRI compatible computer controlled soft tissue indentor and optical Fibre Bragg Grating (FBG) force sensor. The high acquisition rate (100Hz) force sensor was calibrated for forces up to 15N and demonstrated a maximum error of 0.043N. Performance and MRI compatibility of the devices was verified using indentation tests on a silicone gel phantom and the upper arm of a volunteer. The computer controlled indentor provided a highly repeatable tissue deformation. Since the indentor and force sensor are composed of non-ferromagnetic materials, they are MRI compatible and no artefacts or temporal SNR reductions were observed. In a phantom study the mean and standard deviation of the temporal SNR levels without the indentor present were 500.18 and 207.08, respectively. With the indentor present the mean and standard deviation were 501.95 and 200.45, respectively. This computer controlled MRI compatible soft tissue indentation system with an integrated force sensor has a broad range of applications and will be used in the future for the non-invasive analysis of the mechanical properties of skeletal muscle tissue.

Biomechanical behaviour of heel pad tissue experimental testing, constitutive formulation, and numerical modelling

This paper deals with the constitutive formulation of heel pad tissue and presents a procedure for identifying constitutive parameters using experimental data, with the aim of developing a computational approach for investigating the actual biomechanical response. The preliminary definition of constitutive parameters was developed using a visco-hyperelastic formulation, considering experimental data from in vitro compression tests on specimens of fat pad tissue and data from in vivo tests to identify the actual trend of tissue stiffness. The discrepancy between model results and experimental data was evaluated on the basis of a specific cost function, adopting a stochastic/deterministic procedure. The parameter evaluation was upgraded by considering experimental tests performed on the fat pad tissues of a cadaveric foot using in situ indentation tests at 0.01 and 350 mm/s strain rates. The constitutive formulation was implemented in a numerical model. The comparison of data from in situ tests and numerical results led to an optimal domain of parameters based on an admissible discrepancy criterion. Numerical results evaluated for different sets of parameters inside the domain are reported and compared with experimental data for a reliability evaluation of the proposed procedure.

An elaborate data set characterizing the mechanical response of the foot

Mechanical properties of the foot are responsible for its normal function and play a role in various clinical problems. Specifically, we are interested in quantification of foot mechanical properties to assist the development of computational models for movement analysis and detailed simulations of tissue deformation. Current available data are specific to a foot region and the loading scenarios are limited to a single direction. A data set that incorporates regional response, to quantify individual function of foot components, as well as the overall response, to illustrate their combined operation, does not exist. Furthermore, the combined three-dimensional loading scenarios while measuring the complete three-dimensional deformation response are lacking. When combined with an anatomical image data set, development of anatomically realistic and mechanically validated models becomes possible. Therefore, the goal of this study was to record and disseminate the mechanical response of a foot specimen, supported by imaging data. Robotic testing was conducted at the rear foot, forefoot, metatarsal heads, and the foot as a whole. Complex foot deformations were induced by single mode loading, e.g., compression, and combined loading, e.g., compression and shear. Small and large indenters were used for heel and metatarsal head loading, an elevated platform was utilized to isolate the rear foot and forefoot, and a full platform compressed the whole foot. Three-dimensional tool movements and reaction loads were recorded simultaneously. Computed tomography scans of the same specimen were collected for anatomical reconstruction a priori. The three-dimensional mechanical response of the specimen was nonlinear and viscoelastic. A low stiffness region was observed starting with contact between the tool and foot regions, increasing with loading. Loading and unloading responses portrayed hysteresis. Loading range ensured capturing the toe and linear regions of the load deformation curves for the dominant loading direction, with the rates approximating those of walking. A large data set was successfully obtained to characterize the overall and the regional mechanical responses of an intact foot specimen under single and combined loads. Medical imaging complemented the mechanical testing data to establish the potential relationship between the anatomical architecture and mechanical responses and to further develop foot models that are mechanically realistic and anatomically consistent. This combined data set has been documented and disseminated in the public domain to promote future development in foot biomechanics.