Journal of Biomechanics 41 (2008) 1897–1909 Internal mechanical conditions in the soft tissues of a residual limb of a trans-tibial amputee S. Portnoy a , Z. Yizhar b , N. Shabshin c , Y. Itzchak c , A. Kristal d , Y. Dotan-Marom d , I. Siev-Ner d , A. Gefen a,Ã a Faculty of Engineering, Department of Biomedical Engineering, Tel Aviv University, Israel b Faculty of Medicine, Department of Physical Therapy, Tel Aviv University, Israel c Department of Diagnostic Imaging, Chaim Sheba Medical Center, Israel d Department of Orthopaedic Rehabilitation, Chaim Sheba Medical Center, Israel Accepted 31 March 2008 Abstract 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. r 2008 Elsevier Ltd. All rights reserved. Keywords: Prosthesis; Deep tissue injury; Pressure ulcer; Patient-specific finite element model; Rehabilitation 1. Introduction During lower-extremity amputations, the posterior shank muscles (gastrocnemius and sometimes the soleus) are folded over the distal ends of the truncated bones, and create a muscular flap that is pressed transversely during load-bearing. The majority of trans-tibial amputation (TTA) patients use prostheses on a daily basis. The unnatural mechanical conditions that result from the interaction between the soft tissues of the residual limb and the prosthetic socket often lead to pain, blisters, edema, pressure ulcers and sometimes flap necrosis and osteomyelitis (Lyon et al., 2000; Mak et al., 2001). The surgically sharpened tibial end can cause critical mechan- ical loads in the tissues underneath, which may eventually lead to flap ulceration (Henrot et al, 2000). It has been long known that tissue loads are higher internally near bony prominences than at the skin surface (Le et al., 1984). However, only recently, a perilous injury condition named ARTICLE IN PRESS www.elsevier.com/locate/jbiomech www.JBiomech.com 0021-9290/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jbiomech.2008.03.035 Ã Corresponding author. Tel.: +972 3 640 8093; fax: +972 3 640 5845. E-mail address: gefen@eng.tau.ac.il (A. Gefen).