BACKGROUND: 3D printing is becoming more popular across many industries. The first step to safely introducing 3D printed sockets in to prosthetics is to conduct strength testing on these sockets. PURPOSE: This study tests how changing the infill percentage (the percentage of material between the internal and external socket wall) affects the strength of 3D-printed transtibial sockets. METHODS: A Fused Deposition Modelling (FDM) printer was used to print a total of nine transtibial (TT) sockets (three sockets at 30% infill, three sockets at 40% infill, and three sockets at 50%) using polylactic acid (PLA). A strength-testing apparatus measured, in Newtons (N), the maximum load the 3D-printed transtibial sockets could withstand at initial contact of the gait cycle. RESULTS: Based on the specific criteria outlined in this research project, all nine sockets exceeded the 4480N threshold set by ISO Standard 10328. Eight out of nine sockets failed at approximately double the force required with one socket (socket #2) failing at 5360N. Seven out of nine sockets failed at the medial popliteal region and two out of nine sockets failed at lateral mid socket region. Differences in infill percentage from 30%, 40%, 50% did not appear to influence strength of sockets. CONCLUSION: Strength of 3D-printed TT sockets needs rigorous testing to be deemed safe for patient use. More definitive research and a higher number of samples are required to investigate how a larger range of infill percentage can affect strength. Until all the requirements of ISO Standard 10328 are satisfied, the safety of using 3D-printed TT sockets in clinical practice are uncertain. Article PDF Link: https://jps.library.utoronto.ca/index.php/cpoj/article/view/30843/23262 LAYMAN’S ABSTRACT 3D printing is beginning to be used in prosthetics because it has the potential to be less expensive and more customizable to individual needs and styles. Unfortunately, there are companies using this technology to print prosthetic sockets for people without the proper education and training. Before people can start using this new technology safely, testing needs to be done to determine the strength of these 3D printed prosthetic sockets. Our project investigates how strong a 3D printed prosthetic socket is for an amputee below the knee. This is challenging because the entire weight will be put through the socket and it needs to be strong enough that it will not break. There is an international standard that gives instructions and information on testing the strength of a prosthetic socket. Our project will follow a part of these instructions and see how much weight can be put through a socket before it breaks. Our project printed nine identical prosthetic sockets, but the infill percentage of each socket was different. The infill percentage is the amount of material between the walls of an object. We put each socket in a machine and applied a compressive force until it broke and measured that force. Our tests showed the infill percentage did not change the strength of the sockets. They all passed the force measurement given by the international standard. Because our project only tested a part of the standard, there are many more tests that need to be done before the public can start using 3D-printed prosthetic sockets safely. How to Cite: Campbell L, Lau A, Pousett B, Janzen E, Raschke S.U. How infill percentage affects the ultimate strength of 3D-printed transtibial sockets during initial contact. Canadian Prosthetics & Orthotics Journal, Volume 1, Issue 2, No 2, 2018. https://doi.org/10.33137/cpoj.v1i2.30843