Innovative Implementation in Socket Design: Digital Models to Costumize the Product"
8Pages

Innovative Implementation in Socket Design: Digital Models to Customize the Product U. Cugini1, M. Bertetti1, D. Bonacini1, G. Colombo1, C. Corradini2, G. Magrassi1 1 Politecnico of Milan, Department of Mechanics; 2Orthopaedic and Traumatologic Clinic, University of Milan, Italy umberto.cugini@polimi.it, massimiliano.bertetti@polimi.it, daniele.bonacini@polimi.it, giorgio.colombo@polimi.it, grazia.magrassi@polimi.it, costantino.corradini@unimi.it Abstract 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. Keywords: lower limb prosthesis design, custom socket, 3D digital modelling, reverse engineering. 1. Introduction 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 Proceedings ArtAbilitation 2006: ©ArtAbilitation/SoundScapes ISBN

Fused Depositing Modelling (FDM) produce prototypes with strong mechanical properties compared with 3D Printing, but are much expensive, for what concern hardware, consumer materials and productive process (Freeman 1998, Herbert 2005). The geometric model of the residual limb of the lower leg plays the main role on the approach we are proposing. This model is essential to permit physics-based simulations, detailed design with CAD tools and RP. In this paper we discuss methods and tools to reconstruct a geometric model of the stump and we demonstrate that this geometry is not directly usable...

In this manner the technician acquires the shape of the stump; this is a manipulated model, subjected to deformations especially on fleshy parts, and such a configuration could be different from one technician by another, depending on his/her skill. Through chalk casting on the negative plaster cast, it is possible to obtain the stump model, comparing measures with the ones taken directly on the stump. If the measures are different, the orthopaedic technician files the model till the measures coincide: this last model is used to make socket by lamination. A specific attention, as shown in...