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.

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Patients with a spinal cord injury (SCI) are susceptible to deep tissue injury (DTI), a necrosis in excessively deformed muscle tissue overlying bony prominences, which, in wheelchair users, typically occurs in the gluteus muscles under the ischial tuberosities. Recently, we developed a generic real-time, subject-specific finite element (FE) modeling method to provide monitoring of mechanical conditions in deep tissues deformed between bony prominences and external surfaces. We previously employed this method to study internal tissue loads in plantar tissues of the foot [Yarnitzky, G., Z. Yizhar, and A. Gefen. J. Biomech. 39:2673-2689, 2006] and in muscle flaps of residual limbs in patients who underwent transtibial amputation (Portnoy, S., G. Yarnitzky, Z. Yizhar, A. Kristal, U. Oppenheim, I. Siev-Ner, and A. Gefen. Ann. Biomed. Eng. 35:120-135, 2007). The goal of the present study was to adapt the method to study the time-dependent mechanical stresses in glutei of patients with SCI during wheelchair sitting, continuously in real-time, and to compare the trends of internal tissue load data with those of controls. Prior to human studies, the real-time FE model-adapted to study the buttocks during sitting-was validated by comparing its predictions to data from a physical phantom of a buttocks and to non-real-time, commercial FE software. Next, real-time, subject-specific, FE models were built for six participating subjects (3 patients with SCI, 3 controls) based on their individual anatomies from MRI scans. Subjects were asked to sit normally in a wheelchair, on a ROHO cushion, and to watch a 90 min movie. Continuous interface pressure measurements from a pressure mat were used as subject-specific boundary conditions for real-time FE analyses of deep muscle stresses. Highest peaks of compression, shear and von Mises stresses throughout the trial period, and averages of peaks of these stresses were recorded over the trial for each individual. These parameters generally had 3-times to 5-times greater values in patients with SCI compared with controls. Likewise, stress doses, defined as the integration of peak compression stress over time, were approximately 35-times and approximately 50-times greater in the subjects with SCI, the values referring to the highest of all peaks recorded throughout the trial, and to average of peaks over the trial, respectively. We believe that by allowing-for the first time-practical and continuous monitoring of internal tissue loads in patients with motosensory deficits, without any risk or interruption to their lifestyle, and either at the clinical setting or at home, the present method can make a substantial contribution to the prevention of severe pressure ulcers and DTI.

No technology is presently ' available to provide information on internal deformations and stresses in plantar soft tissues during evaluation of the gait pattern, in real-time. Because internal deformations and stresses in the plantar pad are critical factors in foot injuries, this severely limits evaluation o f patients. To allow such analyses, we developed a hierarchal modeling system integrating structural and finite element (FE) foot models. The higher-order part is a two-dimensional model that provides analytical solution of the time-dependent plantar fascia tensile forces during the stance phase. These forces are transferred, together with measured ground reactions to the lower-order level of the modeling system, where they serve as boundary conditions for local FE analysis of deformations and stresses in the plantar pad. Boundary conditions from the higher-order model were validated with respect to previous, more detailed FE models that could not be utilized far real-time analysis. Subsequently, real-time deformations and stresses in the plantar pad were obtained during gait.

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