Biomedical Engineering and Technology Innovation Center | Jan - May 2018
India has a large number of trans-femoral (above-knee) amputees who need high-quality yet affordable prostheses to return to near-normal life. Current low-cost prostheses such as ‘Jaipur Leg’ use a plaster replica of the residual limb of patients to fabricate a matching socket. This is a cumbersome manual process that requires skilled technicians, challenges the dignity of patients and often requires rework, being prone to errors. This paper explores modern digital technology to create conforming sockets (thereby better fitment and comfort to patient) as well as higher productivity in prosthesis fabrication. The proposed approach involves 16 measurements on the residual limb to generate 65 parameters, which are used to semi-automatically create a 3D CAD model of the stump followed by 3D printing or CNC machining. The parametric CAD model is compared with 3D scanned model of the plaster stump fabricated using the conventional process. The parameters have been fine-tuned to minimize their number while ensuring the desired accuracy. The results clearly establish the potential of using the proposed approach to obtain sockets with the desired accuracy without using plaster, thereby making it easily scalable to meet the large gap in demand and supply of such prostheses.
Measurement Chart Design
The first step in creating a parametric CAD model of the residual limb is to identify suitable (standard) anatomical landmarks. The measurements between the landmarks, which are specific to a patient, can be used to automatically generate a customized 3D CAD model of the stump, on which the corresponding socket can be fabricated.
The set of measurements and the relations between them were employed to develop a parametric CAD model of the residual limb. For this purpose, 16 key measurements were used to create equations that relate the variables in the limb dimensions. From these 16 measurements, a total of 65 parameters were created, based on which a CAD model is generated from bottom to top: starting with the stump end with a circular cross-section that grows into a rounded quadrilateral cross-section as it approaches the hip region. The limb measurements of patients are entered in an Excel sheet, which is imported into the CAD program (SolidWorks) to generate a patient-specific model of stump, then machined.
The proposed approach was validated on two real-life cases. For each case, the parametric CAD model of the stump was developed based on measurements on residual limb of the patient. The plaster stumps of the same patients were procured from the NGO (RNCT) and 3D scanned using Steinbichler Comet L3D system to produce a triangular mesh of the stumps.
The parametric model as well as the scanned model are converted into STL format, and imported into CloudCompare software for comparison. The two models are superimposed on each other, with the 3D-scanned plaster stump model taken as the reference. The test cases show that the parametric model is congruous with the Plaster-of-Paris stump, with average error less than 12.3%. The deviation was mainly due to patient specific anatomical variations at the weight-bearing regions, non-uniformity of the plaster stump profiling, and some manual errors.
Developments and Insights
The parametric CAD approach improves the overall process of patient-specific socket design and fabrication in terms of time and effort. Since the model is based on a few key parameters, field workers can take these measurements and send them via text messages to prosthesis fabrication centres. The fabricated prostheses can be delivered to the patients. This eliminates the need for patients to travel to the centres. And this enables the process to be scaled up to reach out to many more patients.