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 13.3 cm. 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 8.1 cm-long limbs. Correspondingly, the lowest strains were found in the 13.3 cm-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 (>3 h) 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

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

Surgical and Morphological Factors that Affect Internal Mechanical Loads in Soft Tissues of the Transtibial Residuum

The residual limb of transtibial amputation (TTA) prosthetic users is threatened daily by pressure ulcers (PU) and deep tissue injury (DTI) caused mainly by sustained mechanical strains and stresses. Several risk factors dominate the extent of internal tissue loads in the residuum. In this study, we developed a set of three-dimensional finite element (FE) models that were variants of a patient-specific FE model, built from magnetic resonance imaging scans. The set of FE modes was utilized to assess the impact of the following risk factors on the strain/stress distribution in the muscle flap: (i) the tibial length, (ii) the tibial bevelment, (iii) a fibular osteophyte, (iv) the mechanical properties of the muscle, and (v) scarring in different locations and depths. A total of 12 nonlinear FE model configurations, representing variations in these factors, were built and solved. We present herein calculations of compression, tension and shear strains and stresses, von Mises stresses, and strain energy density averaged in critical locations in the muscle flap as well as volumes of concentration of elevated stresses in these areas. Our results overall show higher stresses accumulating in the bone proximity rather than in outlying soft tissues. The longer bone configurations spread the loads toward the external surfaces of the muscle flap. When shortening the truncated bones from 11.2 to 9.2 cm, the von Mises stresses at the distal edges of the bones were relieved considerably (by up to 80%), which indicates a predicted decreased risk for DTI. Decreasing the tibial bevelment mildly, from 52.3° to 37.7° caused propagation of internal stresses from the bone proximity toward the more superficial soft tissues of the residuum, thereby also theoretically reducing the risk for DTI. An osteophyte at the distal fibular end increased the strain and stress distributions directly under the fibula but had little effect (<1%) on stresses at other sites, e.g., under the tibia. Elevation of muscle stiffness (instantaneous shear modulus increase from 8.5 to 16.2 kPa), simulating variation between patients, and muscle flap contraction or spasm, showed the most substantial effect by an acute rise of the von Mises stresses at the bone proximity. The mean von Mises stresses at the bone proximity were approximately twofold higher in the contracted/spastic muscle when compared to the flaccid muscle. Locating a surgical scar in different sites and depths of the residuum had the least influence on the overall loading of the muscle flap (where stresses changed by <7%). Pending further validation by epidemiological PU and DTI risk factor studies, the conclusions of this study can be incorporated as guidelines for TTA surgeons, physical therapists, prosthetists, and the TTA patients themselves to minimize the onset of PU and DTI in this population. Additionally, the present analyses can be used to guide or focus epidemiological research of PU and DTI risk factors in the TTA population

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 approximately 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

Course of soft tissue injuries in the transtibial amputation residuum

A transtibial amputation (ITA) prosthetic user has to deal with daily complications associated with his artificial limb. Though highly progressed, technologically and functionally, the TTA prosthesis is far from ideally replacing an intact limb, mainly due to unique inherent anatomy. After a long and complicated fitting process, the prosthetic limb is often a source of gait abnormalities, limb pain and soft tissue breakdown. While commercial prosthetic accessories and aids are plentiful, tools that analyze the mechanical conditions in the soft tissues of the residual limb during load-bearing are scarce. Therefore, clinicians are not well-equipped with tools that are able to quantify the effectiveness of socket rectification and the use of aids, e.g. liners and thigh corset. In this review we will discuss TTA aetiology and elaborate on the prostheses that are most commonly used today. We will also review gait characteristic and its associated complications. Mainly, we will discuss socket limb interaction and methods for quantifying the internal mechanical conditions in the residual limb for the purpose of preventing deep tissue injuries

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 (104 kJ/m 3). The von Mises stresses peaked at 215 kPa 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 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