A. Kristal

Internal mechanical conditions in the soft tissues of a residual limb of a trans-tibial amputee

Most trans-tibial amputation (TTA) patients use a prosthesis to retain upright mobility capabilities. Unfortunately, interaction between the residual limb and the prosthetic socket causes elevated internal strains and stresses in the muscle and fat tissues in the residual limb, which may lead to deep tissue injury (DTI) and other complications. Presently, there is paucity of information on the mechanical conditions in the TTA residual limb during load-bearing. Accordingly, our aim was to characterize the mechanical conditions in the muscle flap of the residual limb of a TTA patient after donning the prosthetic socket and during load-bearing. Knowledge of internal mechanical conditions in the muscle flap can be used to identify the risk for DTI and improve the fitting of the prosthesis. We used a patient-specific modelling approach which involved an MRI scan, interface pressure measurements between the residual limb and the socket of the prosthesis and three-dimensional non-linear large-deformation finite-element (FE) modelling to quantify internal soft tissue strains and stresses in a female TTA patient during static load-bearing. Movement of the truncated tibia and fibula during load-bearing was measured by means of MRI and used as displacement boundary conditions for the FE model. Subsequently, we calculated the internal strains, strain energy density (SED) and stresses in the muscle flap under the truncated bones. Internal strains under the tibia peaked at 85%, 129% and 106% for compression, tension and shear strains, respectively. Internal strains under the fibula peaked at substantially lower values, that is, 19%, 22% and 19% for compression, tension and shear strains, respectively. Strain energy density peaked at the tibial end (104kJ/m(3)). The von Mises stresses peaked at 215kPa around the distal end of the tibia. Stresses under the fibula were at least one order of magnitude lower than the stresses under the tibia. We surmise that our present patient-specific modelling method is an important tool in understanding the etiology of DTI in the residual limbs of TTA patients.

Real-Time Patient-Specific Finite Element Analysis of Internal Stresses in the Soft Tissues of a Residual Limb: A New Tool for Prosthetic Fitting

Fitting of a prosthetic socket is a critical stage in the process of rehabilitation of a trans-tibial amputation (TTA) patient, since a misfit may cause pressure ulcers or a deep tissue injury (DTI: necrosis of the muscle flap under intact skin) in the residual limb. To date, prosthetic fitting typically depends on the subjective skills of the prosthetist, and is not supported by biomedical instrumentation that allows evaluation of the quality of fitting. Specifically, no technology is presently available to provide real-time continuous information on the internal distribution of mechanical stresses in the residual limb during fitting of the prosthesis, or while using it and this severely limits patient evaluations. In this study, a simplified yet clinically oriented patient-specific finite element (FE) model of the residual limb was developed for real-time stress analysis. For this purpose we employed a custom-made FE code that continuously calculates internal stresses in the residual limb, based on boundary conditions acquired in real-time from force sensors, located at the limb-prosthesis interface. Validation of the modeling system was accomplished by means of a synthetic phantom of the residual limb, which allowed simultaneous measurements of interface pressures and internal stresses. Human studies were conducted subsequently in five TTA patients. The dimensions of bones and soft tissues were obtained from X-rays of the residual limb of each patient. An indentation test was performed in order to obtain the effective elastic modulus of the soft tissues of the residual limb. Seven force sensors were placed between the residual limb and the prosthetic liner, and subjects walked on a treadmill during analysis. Generally, stresses under the shinbones were ∼threefold higher than stresses at the soft tissues behind the bones. Usage of a thigh corset decreased the stresses in the residual limb during gait by approximately 80%. Also, the stresses calculated during the trial of a subject who complained about pain and discomfort were the highest, confirming that his socket was not adequately fitted. We conclude that real-time patient-specific FE analysis of internal stresses in deep soft tissues of the residual limb in TTA patients is feasible. This method is promising for improving the fitting of prostheses in the clinical setting and for protecting the residual limb from pressure ulcers and DTI.

Patient-specific analyses of deep tissue loads post transtibial amputation in residual limbs of multiple prosthetic users

Active transtibial amputation (TTA) patients are at risk for developing pressure ulcers (PU) and deep tissue injury (DTI) while using their prosthesis. It is therefore important to obtain knowledge of the mechanical state in the internal soft tissues of the residuum, as well as knowledge of the mechanical state upon its surface. Our aim was to apply patient-specific MRI-based non-linear finite element (FE) models to quantify internal strains in TTA prosthetic users (n=5) during load-bearing. By further employing a strain injury threshold for skeletal muscle, we identified patients susceptible to DTI. The geometrical characteristics of the residuum of the TTA participants varied substantially between patients, e.g. the residuum lengths were 7.6, 8.1, 9.2, 11.5 and 13.3cm. We generally found that internal strains were higher in the bone proximity than in the muscle flap periphery. The highest strains, which in some patients exceeded 50% (engineering strain) for compressive, tensile and shear strains, were found in the shortest residual limbs, i.e. the 7.6 and 8.1cm-long limbs. Correspondingly, the lowest strains were found in the 13.3cm-long residuum, which had the bulkiest muscle flap. Yet, even in the case of a long residuum, about a third of the soft tissue volume at the distal tibial proximity area was occupied by large (>5%) internal compressive, tensile and shear strains. For both patients with shorter residual limbs, the internal principal compressive strains above 5% occupied almost the entire distal tibial proximity area. For a patient whose distal tibial end was flat (non-beveled), internal strains were more uniformly distributed, compared to the strain distributions in the other models, where focal elevated strains accumulated in the bone proximity. We found no muscle strains above the immediate injury threshold, indicating that all patients were not at immediate risk for DTI. Two patients whose residuum fat padding was minimal to none, were the only ones identified as theoretically prone to DTI at long (>3h) continuous weight-bearing periods. We conclude that there is a wide variability in internal mechanical conditions between residual limbs across subjects, which necessitates patient-specific quantitative analyses of internal mechanical states in TTA patients, to assess the mechanical performance of the reconstructed limb and in particular, the individual risk for deep PU or DTI.

Real-time subject-specific analyses of dynamic internal tissue loads in the residual limb of transtibial amputees

Transtibial amputation (TTA) prosthetic-users may risk the integrity of their residuum while trying to maintain everyday activities. Compression of the muscle flap between the truncated bones and the prosthetic socket may cause pressure ulcers and deep tissue injury (DTI). We hypothesize that mechanical stresses in the muscle flap are higher when walking over complex terrains than during plane gait, and so, the residuum could be at risk for DTI when walking over these terrains. Accordingly, we evaluated internal soft tissue stresses in the residuum at the vicinity of the tibia in 18 prosthetic-users (7 vascular, 11 traumatic). For this purpose, we developed a portable monitor that calculated subject-specific internal stresses in the residuum in real-time. Each subject was studied while walking on plane floor, grass, stairs and slope. We found that internal stresses were the highest while subjects descended a slope, during which internal peak and root mean square (RMS) stresses were approximately 40% and 50% greater than in plane gait, respectively. Peak and RMS stresses calculated while descending a slope were approximately 2 times higher for the sub-group of vascular subjects compared to traumatic, but were similar between the two sub-groups for other ambulation tasks. Overall, the present internal stress monitor is a practical tool for real-time evaluation of internal stresses in the residuum of TTA prosthetic-users in the clinical setting or outdoors. Pending integration of appropriate dynamic tissue injury thresholds, the device can be utilized for alerting to the danger of DTI.

Outdoor dynamic subject-specific evaluation of internal stresses in the residual limb: Hydraulic energy-stored prosthetic foot compared to conventional energy-stored prosthetic feet

The prosthetic foot plays an important role in propelling, breaking, balancing and supporting body loads while the amputee ambulates on different grounds. It is therefore important to quantify the effect of the prosthetic foot mechanism on biomechanical parameters, in order to prevent pressure ulcers and deep tissue injury. Our aim was to monitor the internal stresses in the residuum of transtibial amputation (TTA) prosthetic-users ambulating on different terrains, which the amputees encounter during their daily activities, i.e. paved floor, grass, ascending and descending stairs and slope. We specifically aimed to compare between the internal stresses in the TTA residuum of amputees ambulating with a novel hydraulic prosthetic foot compared to conventional energy storage and return (ESR) prosthetic feet. Monitoring of internal stresses was accomplished using a portable subject-specific real-time internal stress monitor. We found significant decrease (p<0.01) in peak internal stresses and in the loading rate of the amputated limb, while walking with the hydraulic foot, compared to walking with ESR feet. The loading rate calculated while ambulating with the hydraulic foot was at least three times lower than the loading rate calculated while ambulating with the ESR foot. Although the average decrease in internal stresses was ≈ 2-fold larger when replacing single-toe ESR feet with the hydraulic foot than when replacing split-toed ESR feet with the hydraulic foot, the differences were statistically insignificant. Our findings suggest that using a hydraulic prosthetic foot may protect the distal tibial end of the TTA residuum from high stresses, therefore preventing pressure-related injury and pain.

MRI Integrated with Computational Methods for Determining Internal Soft Tissue Loads as Related to Chronic Wounds

Excessive and prolonged internal soft tissue loads are one of the main factors responsible for the initiation of internal injuries that may, if ignored or untreated, escalate into chronic wounds. Since internal tissue loads cannot be measured in vivo, computational methods that incorporate the actual anatomy of the living body, are currently the best available resource for acquiring internal mechanical knowledge. In this chapter we discuss various methods that use computational modeling integrated with anatomical data, scanned by magnetic resonance imaging (MRI) in order to determine internal soft tissue loads. Specifically we will elaborate on linear and non-linear finite element (FE) methods and hyperelastic warping.

Patient-Specific Finite Element Models of Transtibial Amputation in Several Prosthetic Users: The Inter-Subject Variability

Active transtibial amputation (TTA) patients are at risk for developing pressure ulcers and deep tissue injury (DTI) while using their prosthesis. It is therefore important to obtain knowledge of the mechanical state in the internal soft tissues of the residuum, as well as the mechanical state upon its surface. For this purpose we employed patient-specific MRI-based non-linear 3D finite element models to quantify the internal mechanical conditions in 3 residual limbs of TTA prosthetic users during load-bearing. The geometrical characteristics of the residuum of the TTA participants varied significantly between patients, e.g. the residuum lengths were 7.6, 9.2 and 13.3cm. We generally found that internal strains were higher in the bone proximity than in the muscle periphery. The highest strains were found in the 7.6cm-long residuum. Correspondingly, the lowest strains were found in the 13.3cm-long residuum, which had the thickest muscle flap. Yet even in the case of a long residuum, a third of the distal tibial proximity area was occupied by internal principal compression strains above 5%. For both patients with shorter residual limbs, the internal principal compression strains above 5% occupied almost the entire distal tibial proximity area. We conclude that the wide viability between residual limbs necessitate quantitative analysis of internal mechanical state in the TTA individual to assess the risk for DTI onset.

Anatomical and Surgical Risk Factors Affecting the Internal Mechanical Conditions in the Transtibial Residuum

Transtibial amputation (TTA) patients face ongoing morphological changes in their residual limb. The residuum volume changes due to weight gain or loss, diurnal edema, and muscle atrophy. Consequently, the TTA prosthetic-user is fitted with a new prosthetic socket approximately every four years. Despite new innovations in socket and liner materials and design, contemporary prosthetics are not yet equipped to confront these changes. The TTA residual limb is therefore subjected to high superficial and internal stresses which may cause injury. Appending the hazardous condition of natural volume change of the residuum is the initial geometrical state of the truncated bones. The primary surgical considerations in TTA are the tibial length, the bevelment of the distal end of the tibia and the location of the surgical scar. These risk factors may significantly affect the well being of the TTA residuum. Previous studies assumed that the criteria for a well-fitted socket were low interface stresses. However, while interface stress measurements may help prevent superficial skin damage, knowledge of the internal stress distribution can prevent the formation of deep tissue injury (DTI) [1]. While superficial pressure ulcers are visually detected, DTI is concealed under the skin and spreads to its surroundings in the soft tissues of the residuum. If this latent wound is ignored, the skin will rupture to reveal a massive injury to skin, fat and muscle tissues, clinically termed as a type IV pressure ulcer. Our purpose was to evaluate the effect of the following risk factors on the internal mechanical condition of the TTA residuum: shorter tibial lengths (thicker muscle flap tissue), milder tibial end bevelments, different mechanical properties of the muscle flap (simulating both variance between patients or flaccid versus contracted muscle) and superficial scarring in inferior and anterior locations on the skin.© 2009 ASME