Innovative Implementation in Socket Design: Digital Models to Customize the Product

The paper presents an innovative approach based on digital data and computer tools to optimize lower limb socket prosthesis design. The kernel of the approach is a stump's detailed geometric model, with external surface and inner bones. To obtain this model, we integrated RE laser scanning and two medical imaging technologies, Computer Tomography (CT) and Magnetic Resonance Imaging (MRI). The model obtained can not be directly used to build the socket by using Rapid Manufacturing technologies. We demonstrate this assertion by comparing digital model of the limb with the positive plaster cast acquired by an orthopaedic technician during the traditional manual manufacturing process. The comparison evidences some differences concentrated on critical zones, whose deformations strictly depend on technician's manipulation. The analyses of the causes of the mentioned differences can furnish guidelines for physics-based simulations able to reproduce effects obtained by the technician. The paper presents an innovative approach based on digital data and computer tools to optimize lower limb socket prosthesis design; as described in the next section, design and manufacturing of a socket are processes where computer aided methodologies and tools are not intensively used. The main aspects of the methodology we propose are summarised in Figure 1. We consider, as the first time, the problem of the reconstruction of a digital model of the stump; it requires a measurement phase and a following CAD modelling task. Then, physics-based simulations on digital model are necessary to obtain deformed shape of the stump similar as much as possible to that one that stump assume during motion of the patient or during manipulations of orthopaedic technician. Finally, the last two steps concern the design of the socket over the deformed shape of the stump and the manufacturing of the socket using Rapid Prototyping (RP) techniques. In previous years, some researchers have investigated aspects concerning the proposed methodology. To reconstruct the digital model, solutions described in literature have been taken into account and compared, beginning from stump's measurement procedures. Actually, the stump is measured manually, so several measurement protocols have been developed to control and reduce problems of accuracy depending on the instruments used (Geil 2005), on the operators' skills (Vannier 1997), on the measurement conditions and on the status of the patient's stump. Markers on the limb identify anthropometric standard dimensions, usually in correspondence with the articulations (Andriacchi 2000). In the case of trans-tibial amputee, important parameters are stump length (from the under patella support to tibia apex) and femoral-condyle position, these points identify zones with less variations of shape and volume of the skin than the other parts of the stump. Markers are also used as reference for the reconstruction of biomedical images and for human gait analysis (Cappozzo 1996). In the last years, there have been residual limb analyses concerning stump's measurement and interactions with socket (Commean 1998), and its variations during patient's life (Zheng 2001, 2005); these studies evidence how to control modifications of lower limb's morphology, especially for the global limb conformation and skin condition, to guarantee a permanent prosthesis comfort and realize, when necessary, the necessary functional socket adjustments. All these researches highlight how digital-based technologies can help socket process design. Some studies analyze the interface pressure between the residual limb and the prosthetic socket, applying Finite Element Analysis tools to simulate pressure distribution and to define material properties assumptions (Ming 2000, Lee 2004). Recently researches have investigated RP technologies applications both to the production of the positive plaster cast of the stump and to the manufacture the socket (Cheng 1998). Efficiency and velocity of RP technology are a valid support for "custom fit" products, and produces cost reduction even during test evaluation. Technologies such as Stereo Lithography Apparatus (SLA), Selective Laser Sintering (SLS) or