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Metal 3D Printing for Custom Surgical Tools

By Travis Hanson Stratasys Direct Manufacturing

3D printing helps improve ACL operations

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Torn Anterior Cruciate Ligament (ACL) in x-ray view. Repaired ACLs are in danger of re-rupturing.

DanaMedInc.’s Pathfinder ACL Guide is a biocompatible surgical device enabling surgeons to better reconstruct partially or fully torn anterior cruciate ligaments (ACL) and reduce the risk of re-tearing. To manufacture the complex design specially shaped to match the anatomy of the knee, Dr. Dana Piasecki turned to metal 3D printing.

The ACL is one of the most important ligaments connecting the tibia (shin bone) to the femur (thigh bone). In the U.S. around 200,000 people incur partial or full ACL tears every year and out of those that are repaired, many may re-rupture and knees are never the same again. It is a huge disadvantage for runners and professional athletes in particular who constantly subject their knees to significant biomechanical stress. Dr. Piasecki, a distinguished orthopedic surgeon at OrthoCarolina, in Charlotte, N.C., set out to improve the success rate of ACL reconstruction by modifying the standard surgical technique using a 3D printed metal surgical tool.

The New Method

When an ACL is torn it must be surgically removed and reconstructed with a graft using a ligament that is transplanted from the patient or a cadaver. Unfortunately, due to limitations of the surgical technique, in most cases, the graft cannot handle the same stress as the natural ACL once could. A majority of ACL repair surgeries done annually in the U.S. are performed with a surgical technique known as the transtibial technique. The advantage of the transtibial technique is that it is relatively straightforward and is the familiar procedure that most surgeons are trained to perform. The technique is problematic however, in that entering the knee through the front of the tibia often results in the graft being attached to the femur as much as 5-10 mm away from the natural ACL attachment point.

The alternative method, known as the AM portal technique, allows for somewhat better graft placement but is much more difficult to perform and can create complications. Dr. Piasecki started experimenting to find a new technique that would combine the principles of the relatively easier transtibial technique, with the improved graft positioning associated with the AM portal technique to ultimately improve both the ease and outcomes of ACL reconstruction.

“I realized I needed a procedure in which the drill is flexible and could be bent to follow the ligament’s normal path to impact the femur at the location and angle that anatomically mimics native ACL positioning,” Dr. Piasecki said.

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DanaMedInc.’s Pathfinder ACL Guide is a biocompatible surgical device enabling surgeons to better reconstruct partially or fully torn anterior cruciate ligaments (ACL).

Furthermore, the tool design would need to be moved in a non-linear fashion within a small cavity; be strong enough to withstand non-linear forces; be small enough so that the surgeon’s view of the inner knee space is not occluded; have a method for grasping the drill tightly enough to bend and hold it while spinning at high speed; have a provision for detaching the drill from the tool when the drilling was complete; and be ergonomic and easy for the surgeon to grip throughout the procedure.

Manufacturing this complex surgical instrument at a relatively low cost would also pose a challenge. “We are a brand new, self-funded company. The ability to produce low volumes of parts on demand is critical. We needed the freedom to make design changes and produce new parts at a low cost and on the fly,” said Jim Duncan, Dr. Piasecki’s business partner and CEO of DanaMed Inc.

At the same time, the tool would come into contact with the inside of the human body, therefore requiring it to be sterilizable and biocompatible. The maneuvering and drilling around bones would also require a strong and durable material that could maintain shape throughout several use cycles.

Manufacturing the Tool

The Pathfinder featured an organic shape to match the anatomy of knee, including an elongated, slim body section, a slotted groove for capturing and holding a 2.2 mm diameter flexible drill, and for easily disengaging with a simple twist of the tool, a curvilinear head section that would enable the surgeon to reference inner knee surfaces, turn and manipulate easily within the limited space, and rigidly hold position against bending and drilling forces, nitinol steel alloy for strength and biocompatibility, and an ergonomic handle.

There was also a need for a few design variations because even though knee anatomy is consistent from person to person, the left and right knees differ, and children’s knees are considerably smaller. After qualifying the concept with FDM (Fused Deposition Modeling) 3D printing technology, DanaMed manufactured the production parts with Direct Metal Laser Melting (DMLM) in Nickel Alloy 718 through Stratasys Direct Manufacturing. DMLM was the only manufacturing process that could build the intricate geometry in metal at a reasonable price point for low-volume production, and meet the necessary surface finish, oil resistance, and mechanical requirements.

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The Pathfinder featured an organic shape to match the anatomy of knee.

The Response to the Tool

DanaMed registered the Pathfinder ACL Guide with the U.S. Food and Drug Administration, qualifying it as a Class 1 Medical Device certifying biocompatibility for an initial market release. DanaMed is committed to continuous improvement through implementing feedback from doctors during the initial market release to further enhance tool performance.

“Being able to make design changes and 3D print new tools within days was extremely important to helping us perfect the design. We could get feedback from a doctor, make design adjustments and send an updated Pathfinder within a week—something we wouldn’t be able to do with investment casting or injection molding,” said DanaMed’s Duncan. In addition to the advantage of immediately implementing design adjustments at zero cost, 3D printing was about 97 percent cheaper per part when compared to investment casting the desired quantity of tools.

A paper that discusses the clinical results of the Pathfinder ACL Guide has been accepted for publication this year. The paper demonstrates that the AM portal technique with the Pathfinder enables the surgeon to place the ACL replacement ligament with a 98 percent complete overlap on the femur. The paper compares the other approach, transtibial technique. This has a survival rate of 60-80 percent and worse anatomical correctness in its placement. The paper demonstrates that using the Pathfinder is far more accurate. Doctors are already finding the new technique much easier to conduct than traditional methods and achieved a 95 percent success rate in anchoring the graft in the native ACL location.

“It has been essential,” Duncan said. “We could have had a cast product. We never even explored that, because as a start-up company that would have been prohibitively expensive for us. In the early stages, we had the need to make some slight modifications to the design, which was a piece of cake on a 3D printing project. It helped us get into business. 3D printing was the only way we could have done it.”

Read more about metal 3D printing at www.stratasysdirect.com.

Travis Hanson is process manufacturing engineer for Stratasys Direct Manufacturing.

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