Design of an Actuated Volume Compensating SLS Prosthetic Socket

Abstract

Studies have shown residual limb volume can vary −11% to 7% in a single day due to changing activity level or weight. However, volume changes of only 3% to 5% can cause users to have difficulty putting on their prosthetic socket. Many existing volume compensation methods are cumbersome, rely on the amputee to maintain the appropriate pressure level, or allow only for a decrease in limb volume. Automatic compensation for volume gain and loss is therefore needed; however, the complexity of designing such sockets renders a traditional fabrication methods cost prohibitive or technically infeasible. Selective Laser Sintering (SLS), a rapid manufacturing (RM) technology, addresses both of these concerns. SLS is a layer-based RM technology that relies on a high power laser to fuse powder particles into a solid object. Minute detail, directly from a 3D CAD model, is possible and a technique has been established for manufacturing prosthetic sockets with passive compliant regions using SLS. Based on this SLS RM technique, steps toward developing a transtibial Nylon prosthetic socket that automatically adapts to volumetric changes in a residual limb will be described. A design methodology was developed to use RM including concept generation, refinement, and final verification. In concept generation, analogies, such as “Chinese Fingertraps” and balloons, were coupled with a review of socket designs in literature and industry and interviews with prosthetists. Inflation of a bladder integrated into the wall of a SLS socket is one of the promising design concepts generated, but the concept needs further refinement. In order to confidently design an inflatable SLS prosthetic, it is critical to understand the relationship between applied pressure and deflection. A testing specimen—5.08 cm diameter thinwalled membrane—was designed to simulate a bladder integrated into the wall of a SLS socket. Several thicknesses were also used to investigate the effects of this parameter on inflation. Preliminary tests were conducted using compressed air for quantifying pressure vs. displacement. During the tests, leakage through open porosity (due to low density) was detected. Density is strongly related to energy transmitted to the part during sintering. The energy concentration is quantified as the Andrew's Number (AN), the inverse relationship of laser power (LP) to laser scanning speed (SSP) and scan spacing (SS). Therefore, to determine the optimal AN—and therefore increase density—an experiment varying LP and SS (SSP is a manufacturer setting) to determine their effects on apparent density and tensile strength was completed. The optimal AN, 1.63 J/cm2 for Nylon 12 powder, was based on highest apparent density and tensile strength. Using this AN, additional deflection samples were tested. Initial results showed a maximum deflection of 2.1 mm at .145 MPa for a 1.3 mm thick membrane. In comparison, changing the volume of a 3D scan of a patient's residual limb by 6% in a 10.9 cm diameter region on the posterior distal tibia socket end, as recommended by a prosthetist, requires a 5.8 mm displacement. Therefore, early results suggest that a single bladder will not meet deflection requirements, influencing the design of multiple larger regions and use of a more flexible material. Results from these experiments will help eliminate concepts which cannot deflect the necessary amount for the volume change, further refining the concepts towards a solution.