Reverse Engineering and Rapid Prototyping Techniques to Innovate Prosthesis Socket Design Giorgio Colomboª*, Massimiliano Bertettiª, Daniele Bonaciniª, Grazia Magrassiª ª Politecnico di Milano, Dip. Ingegneria Meccanica, via La Masa 34, 20156 Milano, Italy ABSTRACT The paper presents an innovative approach totally based on digital data to optimize lower limb socket prosthesis design. This approach is based on a stump’s detailed geometric model and provides a substitute to plaster cast obtained through the traditional manual methodology with a physical model, realized with Rapid Prototyping technologies; this physical model will be used for the socket lamination. The paper discusses a methodology to reconstruct a 3D geometric model of the stump able to describe with high accuracy and detail the complete structure subdivided into bones, soft tissues, muscular masses and dermis. Some different technologies are used for stump acquisition: non contact laser technique for external geometry, CT and MRI imaging technologies for the internal structure, the first one dedicated to bones geometrical model, the last for soft tissues and muscles. We discuss problems related to 3D geometric reconstruction: the patient and stump positioning for the different acquisitions, markers’ definition on the stump to identify landmarks, alignment’s strategies for the different digital models, in order to define a protocol procedure with a requested accuracy for socket’s realization. Some case-studies illustrate the methodology and the results obtained. Keywords: 3D geometrical model, reverse engineering, rapid prototyping, human body scanning, medical imaging, lower limb prosthesis. 1. INTRODUCTION The paper introduces some important aspects of an innovative methodology which authors are developing, to customize design process of lower limb prosthesis. An important part of prosthesis is the socket; it must adapt to the patient’s morphology because it is the interface between the stump and the prosthesis. Socket’s customization is essential in order to obtain the best adaptability to the patient’s body, guaranteeing a high functional degree and comfort. Until now lower limb prosthesis have been designed and manufactured with handicraft methods, leaving out an adequate functionality and however depending on the skills of the orthopaedic technician, who must find the shape of the residual limb, since this is the first step in the socket design of a lower limb prosthesis. The current design process (Fig.1) is based on a double-step realization of the stump’s plaster cast; in the first step, the technician has to mould manually some plaster bandages pressing on the stump. After the bandages’ solidification, the obtained “mould” is used for the final stump plaster cast. During the first moulding, the technician manipulates the stump, producing deformations especially on fleshy parts, and such a configuration could be different from the resting configuration. Aim of our activity is to propose a computer aided design methodology, based on digital models and numerical simulations in order to obtain the physical mock-up of the stump on which socket’s lamination could be made, applied to the trans-tibial prosthesis case. The proposed methodology has three relevant technical features: first, the reconstruction of a 3D geometrical model of the residual limb, then the numerical simulation of the structural behaviour of the stump, and finally the rapid manufacture of a physical mock-up of the stump. In this paper we discuss the first aspect that is the reconstruction of the 3D digital model of the stump, which replaces plaster cast. For this aim we have used, compared and integrated: • Reverse Engineering1 technologies, for the external surface limb acquisition; • X-ray technique, such as Computer Tomography (CT), for the inner structure acquisition, divided into components (bone structure, muscle tissue, soft tissue and dermis); [email protected]; phone +39 02 23998259 Three-Dimensional Image Capture and Applications VII, edited by Brian D. Corner, Peng Li, Matthew Tocheri, Proc. of SPIE-IS&T Electronic Imaging, SPIE Vol. 6056, 60560P, © 2006 SPIE-IS&T · 0277-786X/06/$1

medical imaging technique, such as Magnetic Resonance Imaging (MRI), used for complete residual limb acquisition that could be a CT substitute since it is a non-invasive operating technique. STUMP MEASUREMENT NEGATIVE PLASTER CAST Taken by the technician with chalk bandages manually modeled POSITIVE PLASTER CAST MODE letting chalk solidify in the negative cast model FILING chalk filing and its measurement analysis SOCKET LAMINATION Fig.1 Current socket process. All these three methods are used to reconstruct the geometric model of the external limb surface, CT and MRI are used also for the bone...

In some research, the Reverse Engineering techniques have been employed in order to estimate shape and dimensions of the human body9,10 or its parts11,12; there are also studies about bone prosthesis applications, usually concerning measurements on physical prototypes of bones artificial joints 13, and of the residual limb plaster cast 14,15. On residual limb prosthesis, studies have detailed residual limb analysis with imaging technologies such as CT16, MRI 17 and ultrasound system18, concerning limb shape and volume changes and the effects of socket’s permanent wearing19,20. In the last years,...

which causes relevant deformations of flashy parts at stump’s apex and gastrocnemio muscle. These deformations may produce problems for socket’s comfort and functionality if not carefully considered into the design process. In order to identify the markers, lead shot with diameter Φ = 2mm have been used: they can be seen both through the laser scanner and through CT, while in the MRI case some round tablets of vitamin E with diameter Φ = 3mm have been employed. To guarantee repeatability of acquisition set, and to have some more fixed parameters for limb configuration, we realized a thermoplastic...