ADDITIVE MANUFACTURING is the magazine devoted to industrial applications of 3D printing and digital layering technology. We cover the promise and the challenges of this technology for making functional tooling and end-use production parts.
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MAY 2016 Additive Manufacturing FEATURE / Medical 30 By Christina Fuges We've all heard stories about athletes who have torn the anterior cruciate ligament (ACL) in their knees while playing a sport. It is a serious injury and the recovery process can be long and painful. Just the thought of it makes me cringe. The repair or replacement of that ACL requires the surgeon to frst remove any remnants of the natural ACL where it was attached to the surface of the tibia (shin bone) and femur (thigh bone) then attach a replacement ligament (graft) that accurately mimics the path and positioning of the natural ACL. This is a winding and complicated pathway. To make the process less invasive and more accurate demands a procedure done with precision and a surgical tool ofering control. Such a tool was not readily avail- able until recently, and additive manufacturing made it possible. This new tool is the two-piece Pathfnder System, which consists of the 3D-printed Pathfnder ACL Guide and a fexible guide pin. The system was invented by Dr. Dana Piasecki, an orthopedic surgeon at Ortho- Car olina, an independent academic orthopedics practice in Charlotte, North Carolina, who set out to modify the standard surgical treatment for ACL injuries by using a 3D-printed metal tool. The Pathfnder ACL Guide allows the surgeon to manipulate the guide pin inside the knee without having to open up the knee or contort it to place the graft. It acts as an extension of the surgeon's hand, with the fexibility of the guide pin and the tight grip of the tool providing the surgeon with control over what he or she is trying to do inside the knee. Ninety-fve percent of the guide is the shaft, which is relatively simple. Much of the design emphasis is concentrated in the area from the neck of the device to the tip. "The shape and tolerances here are critical because of the way the tip follows the ligament's normal path to impact the femur at the location and angle that anatomically mimics native ACL positioning," explains John Self, project engineer for Stratasys Direct Manufacturing, which built the device in Inconel 718 using direct metal laser sintering (DMLS). "It also grasps the fexible guide pin, steers it to the proper spot on the femur and holds it during the drilling process." Creating the correct angle for the device tip through tra- ditional manufacturing methods, such as machining it from a block, would require many setups, a lot of angles and undercuts, long machining time, and a higher overall cost. DMLS allowed the complex-geometry tip to be built in a single operation with secondary processing needed, but no need for machining. "Only additive manufacturing can produce this part at an afordable price, and also ofer the freedom to make design changes on the fy," says Jim Duncan, Dr. Piasecki's business partner and CEO of DanaMed Inc., the medical device company founded to develop and produce the Pathfnder System. Method and Material Additive manufacturing played a role in this collaborative project right from the outset when Minneapolis, Minnesota-based Laser Design, a provider of 3D laser scanning equipment and services, was hired to scan Dr. Piasecki's frst concepts and create a 3D CAD model. Once created, Laser Design turned to Stratasys Direct Manufacturing to produce physical, 3D-printed proto- types. Fused deposition modeling (FDM) was used to make 20 iterations of prototypes (in plastic) to prove out initial design concepts. Stratasys Direct then moved on to metal, continuing to refne the part with a focus on the required size, strength, shape and shine: Size. The tip of the device is sized in 5.5- and 7-mm versions to capture the guide pin and reference the back of the femur. DMLS can fabricate the tip accurately while producing the design in one piece. Strength. The device has to be strong enough to hold a spinning guide pin as it drills into the femur without it moving Knee Surgery's Next Step AM made it possible to produce a medical tool that ofers surgeons better control during ACL repair or replacement surgery. The Pathfnder System consists of a 3D-printed guide (seen here) and fexible pin. During ACL surgery, the guide allows the surgeon to manipulate the pin without having to completely open up the knee.