By Chuck Seegert, Ph.D.

To repair torn ACLs, Northwestern University researchers are turning to synthetic implants. In an attempt to replace bone-patellar tendon-bone autografts, which are the current standard of care, the team is using braided polymers combined with calcium nanoparticles. In vivo results are promising and show the implants bonding to bone in a way that may translate into human studies.

Tearing the anterior cruciate ligament (ACL) is an all too common injury that affects many people, especially professional athletes. Many cases are treated each year with healthcare costs that total several hundreds of millions of dollars. Repairing this ligament, which is one of the structures that connects the femur to the tibia, is most often done using autografts, but there are many drawbacks that make this solution less than ideal.

The grafts currently in use, called bone-patellar tendon-bone grafts (BPTBs), are taken from the patellar region of the patient’s knee and have a small piece of bone on each side, according to a recent Northwestern University press release. In addition to being an autologous material, which provides biocompatibility, the bone pieces on each end are able to integrate strongly with the femur and tibia on each side of the repair site. Features called tunnels are drilled into the femur and tibia, and then the bone portions of the graft are placed within them. This allows the graft and bone tissues to grow together, thus anchoring the graft in place. As each side becomes more stable, the tendon spans the distance between the femur and tibia, thus replacing the function of the torn ligament.

While this method of repair is one of the most effective available, it suffers from certain shortcomings, according to a recent study published by the Northwestern team in The Journal of Tissue Engineering and Regenerative Medicine. Many times the harvest site where the graft is taken from has long-term discomfort that can lead to knee pain. Using a synthetic material could alleviate the need to take these grafts and allow for the preservation of the knee’s natural biomechanics.

With this in mind, the Northwestern researchers developed a new synthetic design that uses polyester fibers, an antioxidant, porous biomaterial, and calcium nanoparticles, according to the study. The tri-component grafts replicate the mechanical characteristics of the ACL, and the anti-inflammatory material helps reduce reactions from the body. The pores in the anti-inflammatory material are also impregnated with hydroxyapatite to form a nanocomposite that can integrate with bone.

“The engineered ligament is biocompatible and can stabilize the knee, allowing the animal to function,” said Guillermo Ameer, a professor of biomedical engineering at the McCormick School of Engineering and a professor of surgery at the Feinberg School of Medicine. “Most importantly, we may have found a way to integrate an artificial ligament with native bone.”

Initial study results in an animal model showed that, when implanted with the tunneling approach similar to what is used with BPTB surgery, the new implants were able integrate as desired. After six weeks, the animals were bearing weight and functioning well, which indicated that the approach may be useful as an off-the-shelf ACL repair method.

Developing new methods of managing soft tissue orthopedic injuries is an area that some expect to be a significant growth market in the near future. For example, a recent market analysis projects that the soft tissue orthopedic repairs market will reach $8.5 billion by 2019.

Image Credit: Mak-Ham Lam, Daniel TP Fong, Patrick SH Yung, Eric PY Ho, Wood-Yee Chan and Kai-Ming Chan [CC BY-SA 2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons

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