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Braden C. Fleming, PhD


Published 2/1/2013
Terry Stanton

Award-Winning Research May Make ACL Healing Without Reconstruction Possible

Biologic method combines extracellular matrix proteins and autologous blood

Preclinical studies have shown promising results for the use of a “bioenhanced repair” technique for anterior cruciate ligament (ACL) tears as an alternative to ligament reconstruction.

The research, under the direction of Martha M. Murray, MD, and Braden C. Fleming, PhD, received the 2013 Ann Doner Vaughn Kappa Delta Award.

Martha M. Murray, MD

After a series of laboratory studies examining the biology of ACL injury and repair, Dr. Murray, of Boston Children’s Hospital, and Dr. Fleming, of Brown University, developed a technique involving application of an extracellular matrix (ECM)-based scaffold loaded with autologous platelets to a complete ACL rupture. The treatment encouraged both biologic and mechanical healing in large-animal studies.

The next step, reports Dr. Murray, is to conduct preclinical testing required by the U.S. Food and Drug Administration (FDA) to enable the technology to proceed to clinical trials.

Failure to heal
Suture repair of a torn ACL is generally unsuccessful—with a failure rate of 90 percent. As a result, the current standard treatment for an ACL tear is to remove the ligament and replace it with a tendon graft. However, many patients with ACL reconstruction continue to exhibit progressive articular cartilage damage. As many as two thirds may exhibit radiographic evidence of osteoarthritis (OA) 10 to 15 years after surgery.

“Considering that many of our patients sustain these injuries before the age of 16, ACL injuries place young patients at risk for premature posttraumatic OA (PTOA) before age 30, even with our current best methods of treatment,” the authors note.

Dr. Murray and her team wanted to know why a torn ACL does not heal and whether a treatment that would lead to better long-term function could be developed. They designed a series of experiments to define key biologic differences in healing between ligaments such as the medial collateral ligament (MCL) that heal and those such as the ACL that don’t.

They first compared fibroblasts in the ACL with those in the MCL. They found that cells in both injured ligaments have comparable rates of proliferation, that each ligament was able to revascularize after rupture, and that collagen production in each ligament was comparable up to 1 year after injury. But in the injured MCL and other extra-articular ligaments, a provisional scaffold developed—something that was not seen in the ACL.

The synovial fluid that surrounds the ACL washed away the blood clot that forms as an early bridge between the two torn ends of the ligament. As a result, “there was no structure in place to rejoin the two ends of the ligament, no place for surrounding cells to invade and remodel into a functional scar tissue,” said Dr. Murray. The researchers hypothesized that the lack of a provisional scaffold between the two ends of the torn ACL was the key mechanism behind its failure to heal.

An in vivo study in a large animal model supported this hypothesis and indicated that the lack of a scaffold was also associated with a decreased presence of extracellular matrix proteins and cytokines within the ligament wound site.

Building the scaffold
The next challenge was to engineer a substitute provisional scaffold that would be easy to implant and capable of providing the growth factors and enzymes needed to optimize fibroblast and neurovascular ingrowth. They found that platelet-rich plasma (PRP) was useful in stimulating key cellular behaviors.

Braden C. Fleming, PhD
Fig. 1 Schema of bio-enhanced ACL repair. (A) Defect model. (B) Femoral and tibial tunnels (dashed lines) and EndoButton pulled through femoral tunnel and placed on femoral cortex. The EndoButton is loaded with 3 sutures, resulting in 6 free-ending strands (4 red and 2 green). (C) A Kessler suture is placed in the tibial ACL stump, and a collagen scaffold is threaded onto 4 strands (red), pushed into the notch, and saturated with 3 mL of platelet-rich plasma. (D) The 4 suture strands running through the scaffold (red) are passed through the tibial tunnel, while the remaining suture (green) is tied to the tibial Kessler suture, using it as a pulley to reduce and stabilize the tibial ACL stump. (E) The transtibial sutures (red) are tightened and tied over an extracortical button. The free ends of the ACL suture pulley (green) are knotted to secure the reduced ACL in the collagen-platelet composite. Reproduced with permission from Vavken P, Fleming BC, Mastrangelo AN, Machan JT, Murray MM: Biomechanical outcomes after bio-enhanced anterior cruciate ligament repair and anterior cruciate ligament reconstruction are equal in a porcine model. Arthroscopy 2012;28(5):672-680. Larger Image

Using thrombin to activate the PRP resulted in an immediate release of the platelet-derived growth factors, while using collagen as a PRP activator resulted in a more sustained release of growth factors. Keeping the platelets in their physiologic plasma improved the ability of the platelets to stimulate collagen synthesis by ACL fibroblasts.

Looking to translate what they learned from the Petri dish to a living organism, the investigators developed a large-animal model and identified clinically relevant outcome measures. A series of experiments resulted in a technique that combined a tissue-engineered composite scaffold with a suture repair of the ligament—the “bioenhanced repair (Figure 1 )”

Although using the ECM-platelet scaffold stimulated functional healing, using ECM proteins alone or platelets alone did not. Researchers also found that increasing the platelet concentration more than three times greater than normal levels did not improve results.

Use of the ECM-platelet scaffold also significantly improved both yield load and stiffness of the repair tissue; yield load almost doubled and stiffness increased almost 60 percent over that of suture repair alone. A randomized trial in a large-animal model found that the biomechanical outcome of the bioenhanced repair was equivalent to that of ACL reconstruction.

These findings were in young animals, and the investigators next sought to evaluate the effect of age on functional ligament healing. They found that skeletally immature animals healed more quickly and completely than adults, likely because of improved migration and proliferation abilities of younger cells.

Recently, the authors reported that the bioenhanced repair technique may slow or even prevent the development of PTOA after an ACL injury. In their animal study, PTOA was seen in 80 percent of knees treated with a traditional ACL reconstruction 1 year after surgery, but was not seen in the group that underwent bioenhanced repair.

Next step: Trials
According to their paper, the authors believe that their work has proven the following hypotheses:

  1. Early loss of a provisional scaffold (ie, bridge across the wound site) inhibits ACL healing.
  2. Placement of a substitute provisional scaffold can restore functional healing.
  3. Growth factor delivery systems can be specifically designed for use in the joint.

If these findings hold up in clinical trials, a less-invasive method for treating patients with ACL tears would be available. This new technique could also decrease the risk of premature OA after ACL injury, but verifying this will take much longer.

Dr. Murray’s proposal to complete the needed preclinical studies of safety and efficacy in preparation for clinical trials has been accepted into the FDA’s Early Feasibility Pilot program. Trials will begin once these studies are completed and the approved documentation is in place.

Because most of the 400,000 patients with ACL tears in the United States are young and otherwise healthy and active, “there is a need for improved treatment of these injuries,” said Dr. Murray. “We hope to begin to shift some of the focus for research in this area from resection and replacement to repair and regeneration of ACL injuries.”

Disclosure information: Dr. Murray—National Institutes of Health (NIH); Dr. Fleming: NIH; American Journal of Sports Medicine; Journal of Applied Biomechanics.

Terry Stanton is senior science writer for AAOS Now. He can be reached at tstanton@aaos.org

Bottom Line

  • Unlike extra-articular ligaments, the torn ACL does not heal on its own.
  • In vitro studies demonstrated that application of PRP with collagen as an activator resulted in sustained release of growth factors.
  • Studies in animals found that implantation of an extracellular matrix–based scaffold combined with autologous platelets stimulated functional healing.
  • In preclinical in vivo studies, biomechanical results of the bioenhanced repair were similar to those occurring with reconstruction; the reconstruction group had a high incidence of posttraumatic osteoarthritis at 1 year, while the bioenhanced repair group did not.
  • Final preclinical studies required for FDA approval are under way, and with the successful completion of those studies, clinical trials will begin.