The self-healing anterior cruciate ligament in research

“We know the anterior cruciate ligament (ACL) is capable of healing,” explains Associate Professor Stephanie Filbay from the University of Melbourne in Australia. However, there is a lack of research data on its self-healing capabilities and related treatment approaches, such as the new Cross Bracing Protocol (CBP). This non-surgical procedure relies on physiotherapy and the use of limiting braces. In a randomized controlled trial, Professor Filbay is currently investigating CBP with the SecuTec Genu knee brace in comparison to surgical ligament reconstruction – aiming to close knowledge gaps.

Assoc. Prof. Filbay: First, there’s the question of evidence supporting the novel CBP compared to other classic treatment protocols: Our ongoing EMBRACE trial is examining the effectiveness of the CBP as a non-surgical approach with a brace versus surgical ACL reconstruction. Further clinical trials in New Zealand and the UK will investigate CBP compared to rehabilitation alone without a brace. Second, we still don’t know enough about the spectrum of ACL healing. One ACL might appear thick and taut on a follow-up MRI, while another might appear thin and elongated. However, we don’t yet know how these healing trajectories relate to clinical outcomes. And third, we need long-term follow-up data. Theoretically, a non-surgically healed ACL should have a lower risk of developing osteoarthritis of the knee – but this has not yet been studied.

Assoc. Prof. Filbay: It’s a comparative effectiveness superiority randomised controlled trial. We will randomize 180 patients in Australia shortly after their injury using computer-based methods. We are recruiting most of these patients through their physiotherapists and via our website. The surgical group consists of patients receiving early ACL reconstruction within eight weeks of injury, whom we refer to selected surgeons. The bracing group comprises patients who, as part of the CBP, receive a SecuTec Genu hard-frame brace that is fitted and gradually adjusted accordingly. Both groups receive funded physiotherapy consults for up to 12 months post-injury. Our primary endpoint is at 18 months, at which time we collect data on what the patients themselves report about their knee function and how they assess their quality of life, pain, and other symptoms.

Assoc. Prof. Filbay: All patients need to be between 16 and 40 years old, have a complete ACL rupture, and present no later than 20 days after the injury, allowing for acute management. Furthermore, no serious concomitant injuries, such as a displaced meniscus or a posterior cruciate ligament rupture, were permitted, as these would necessitate different treatment protocols. We also excluded patients with severely damaged ACLs.

„If we assume that wearing a range-of-motion limiting knee brace supports healing, CBP could yield even better results than surgery in suitable cases.“

Assoc. Prof. Stephanie Filbay

Assoc. Prof. Filbay: Previous studies show that rehabilitation alone without a brace can be just as effective as surgery. If we assume that wearing a range-of-motion limiting knee brace supports healing, CBP could yield even better results than surgery in suitable cases. That is our hypothesis – the study data will show whether this is the case.

Assoc. Prof. Filbay: This is also part of our analyses and one of the knowledge gaps we want to address. So far, it appears that the severity of the injury is the strongest predictor of the healing quality of the ACL.¹ Age, sex, or the specific injury mechanism, on the other hand, have not yet shown a significant influence. A smaller gap distance between the ruptured ends and comparatively intact, adjacent fibers increase the likelihood of a thick and taut ligament forming – which is visible on a follow-up MRI after three months when the brace is removed. Nevertheless, there are also patients with a larger gap distance who show good healing outcomes, so it is important to determine which factors influence the likelihood of an optimal healing outcome.

Assoc. Prof. Filbay: We don’t explicitly evaluate treatment “success” or “failure,” but rather consider a spectrum of ACL healing in relation to the individual case. We measure patient satisfaction and clinical parameters. A good healing outcome is always one that leads to a stable knee—regardless of the specific treatment. The patient should be able to resume all desired activities, ideally pain-free and without functional limitations. That is the goal.

Assoc. Prof. Filbay: We can assess physical readiness through functional tests such as the single-leg hop test, agility tests, and strength measurements. We assess psychological readiness using validated questionnaires like the ACL-RSI². The major challenge, however, is biological readiness; data on this is still lacking. With surgical grafts, it is known that ligamentization, i.e., the transformation of the graft into a ligament, can take longer than two years. Something similar likely applies to natural ACL healing. So, we are dealing with a continuum – and it is a risk factor for re-rupture if the patient resumes their sport too soon, that is, before biological maturation has restored the ligament’s tensile strength and elasticity.

„A good healing outcome is always one that leads to a stable knee – regardless of the specific treatment. The patient should be able to resume all desired activities, ideally pain-free and without functional limitations.“

Assoc. Prof. Stephanie Filbay

Assoc. Prof. Filbay: Clinicians have told us that, from the patients’ perspective, it is the most comfortable and lightweight brace and is therefore their preferred choice. Wearing comfort is particularly important because the brace is worn for three months during the CBP and fixes the knee in 90-degree flexion for the first four weeks. The SecuTec Genu also features easily interchangeable limitation blocks in the necessary degree settings, allowing for quick and effective implementation of CBP and gradual adjustment of the range of motion.

Assoc. Prof. Filbay: Not the EMBRACE trial, but currently two PhD students and a postdoctoral researcher are analyzing collected data from more than 1,000 patient cases that have undergone various protocols. These include a 6-week protocol and an 8-week protocol for less severe cases, as well as a modified hybrid protocol. There are over 100 cases in each group that we are comparing. This analysis should allow us to make recommendations for individual adjustments or to define further relevant research questions.

Assoc. Prof. Filbay: Based on the data collected from these over 1,000 patient cases, the re-rupture rate is approximately 14 to 15 percent, lowest in patients who had a thick and taut ACL after treatment. However, we must consider this in relation to the patients’ return to sport and the specific sport they return to: After CBP, this rate is thankfully very high – but if 90 percent of patients resume their sport, the re-injury rate is naturally higher than if, for example, only 60 percent return to their sport after surgery. Returning to pivoting and contact sports like soccer also increases the risk further. More clinical trials like the EMBRACE study and long-term data are needed in this area as well.

Assoc. Prof. Filbay: Yes, among others, the Swedish KANON study³. It refuted the assumption that a ruptured ACL necessarily requires surgery and found that rehabilitation alone can yield similarly good results. This study collected data two, five, and eleven years after the injury. We reanalyzed the MRI data and found that after two years, half of the non-surgical patients showed signs of healing, and the ACL can heal over a longer period. These patients also reported better functional stability than other groups. So, there was a positive correlation between the MRI findings and the clinical outcome.⁴

„We have developed a model that accurately predicts who is most likely to achieve an optimal healing outcome 12 weeks after treatment with the CBP.“

Assoc. Prof. Stephanie Filbay

Assoc. Prof. Filbay: We have developed a model that accurately predicts who is most likely to achieve an optimal healing outcome 12 weeks after treatment with the CBP, which we considered to be a thick and taut heal that resembles a normal ACL on MRI. ⁵ However, this shouldn’t be understood as a simple yes/no system. Even with unfavorable prognoses, the ACL can heal surprisingly well in some patients, and even with a high likelihood of healing, surgery may still be necessary. With this predictive model, patients and their treating clinicians can make informed decisions about whether to opt for early ACL surgery, or to try the non-surgical approach with CBP first.

For more information about the EMBRACE study at the University of Melbourne see https://www.embracestudy.com.au.

For more information about the Cross Bracing Protocol, its origins, development, and the extensive clinical experience gained through its application, read our feature article with insights from sports medicine physician Dr. Tom Cross.

¹ Filbay SR, Dowsett M, van Haeringen M, Palmer E., Roger G, Lucas P, et al. A new way of grading severity of ACL rupture on acute MRI to consider potential for non-surgical healing with the Cross Bracing Protocol: ACL Acute Rupture Characteristics for Healing (ACL-ARCH) MRI criteria. Journal of Science and Medicine in Sport 2025. https://doi.org/10.1016/j.jsams.2025.08.017.

² Anterior Cruciate Ligament–Return to Sport after Injury (ACL-RSI): The 12 items of the ACL-RSI scale assess the psychological dimensions of emotions, confidence in one’s performance, and risk appraisal associated with returning to sport following an anterior cruciate ligament injury. Responses are originally recorded using a visual analogue scale (0–100 points); in some applications, numerical rating scales are used (Webster et al. 2008: https://doi.org/10.1016/j.ptsp.2007.09.003).

³ Kooy CEVW, Jakobsen RB, Fenstad AM, et al. Non-operative treatment of anterior cruciate ligament injuries: two-thirds avoid surgery at 2-year follow-up in a nationwide cohort. British Journal of Sports Medicine 2025;59:1743-1751. https://doi.org/10.1136/bjsports-2025-109890.

⁴ Filbay SR, Roemer F, Lohmander LS, Turkiewicz A, Roos EM, Frobell R, Englund M. Evidence of ACL healing on MRI following ACL rupture treated with rehabilitation alone may be associated with better patient-reported outcomes: A secondary analysis of the KANON trial. British Journal of Sports Medicine 2023;57:91-98. https://doi.org/10.1136/bjsports-2022-105473.

⁵ Filbay SR, Koye DN, Sabharwal R. and Cross T. A prediction model for optimal anterior cruciate ligament healing following management with the Cross Bracing Protocol, Osteoarthritis and Cartilage 2026 Vol. 34 Issue 6 Pages 942. DOI: 10.1016/j.joca.2026.03.011.

Pictures: The Stadium Clinic,Sydney, Australia; University of Melbourne; Photoform