The anterior cruciate ligament (ACL) resists anterior translation and rotation, which are essential to knee function for higher demand and athletic activities. Early attempts at ACL repair did not hold up to these higher demands, and ACL reconstruction using tissue grafts has become the gold standard for reliable treatment in those who injured their ACL and desired to return back to higher functional levels.
History of ACL Treatment
Unlike the medial collateral ligament of the knee which is outside the joint and surrounded by soft tissues with good blood supply, the ACL resides within the intra-articular space of the knee joint which is a challenging biological environment that limits the body’s ability to repair and restore the torn tissue when the ACL is injured. Recognition and treatment of ACL tears dates back to the early 20th century, with efforts focused on suturing the torn ends of the ACL back together.(1) These attempts at surgical repair resulted in low success up into the 1970’s.(2) As a result, surgical techniques focused on ACL reconstruction using the central one-third of the patellar tendon as a replacement for the torn ACL became popular in the 1980’s due to its reliability in restoring stability to the knee, and has become recognized as the gold standard technique.(3-6) Use of hamstring tendon, quadriceps tendon, and allograft cadaver tissue have also demonstrated acceptable results dependent on specific patient and athletic populations.(6-8)
Has New Technology Overcome Past Failures with ACL Repair?
ACL reconstruction is not without cost. Anterior knee pain, kneeling pain, prolonged muscle weakness and rehabilitation, and lingering effects from the graft harvest site have been associated with ACL reconstruction surgery(8-10). As a result, efforts with innovating ACL repair surgical techniques have been explored in recent years. Specific ACL injury patterns as classified by Sherman lend themselves to better candidates at attempted repair:(11, 12)
- Proximal Tears
- Type I: Avulsion off the proximal or femoral ACL footprint. This tear pattern preserves the entire length of the ACL which makes it more suitable for attempted repair(12)
- Type II: Proximal intra-substance ligament tear. These tears happen close to the footprint but located within the upper 20% of the ACL fibers near the femoral footprint. These tear patterns also hold reasonable healing potential.(11)
- Mid-Substance Tears
- Type III: These tears occur in the upper 1/3 of the ACL fibers and lead to compromised residual ACL tissue due to both tearing and attenuation. As a result, this tear pattern has been recognized as not suitable for ACL repair
- Distal Tears
- Type IV: These occur in the middle to distal ½ of the ACL fibers near the insertion on the tibial side. When associated with a bony avulsion (tibial eminence fracture), these can be considered for tibial-based repair for younger patients with open physes (growth plates).(13) Otherwise, the location of this tear pattern makes it a poor candidate for ACL repair.
Recent attempts to advance ACL repair surgery can be divided into biomechanical and biological augmentation of the repair constructs. Biomechanical techniques involve placing a high-tensile suture tape placed under tension with the knee in full extension.(14, 15) This serves as an internal brace that takes load off the healing repaired ACL tissues that may provide immediate postoperative stability through the upper range of motion for the knee that can support early mobilization and rehabilitation.(16, 17) Early data suggests suture brace augmentation to ACL repair can result in similar outcomes to ACL reconstruction in short term outcomes.(18) However, at mid-term follow up higher retear rates have been documented in patients who are younger and with high activity levels.(19)
Biological augmentation of ACL repair has included the use of platelet rich plasma (PRP) or mesenchymal stimulating cells (MCS’s) to signal a proliferation and healing response to the surgical repair site. Current evidence demonstrates equivocal results of augmenting ACL repair and reconstruction with PRP, in part to the vast difference in PRP preparation techniques that have been studied.(20-22) Use of bone marrow aspirate concentrated cells (BMAC) may yield faster recovery and graft maturity as noted with imaging studies (MRI) but no difference in outcomes following ACL reconstruction. (23-25) More research is needed to identify when and how to incorporate these types of biological augments to ACL repair surgery.
Novel Approaches and Continued Study
One novel approach to biological augmentation integrates three aspects: repair of torn ACL tissue, suture brace augmentation, and a bio-absorbable scaffold placed on the suture brace that is soaked in whole blood and used to span the site of ACL repair. This technique is known as bridge-enhanced ACL repair (BEAR). The initial trial (BEAR I) was performed in 10 patients which demonstrated the technique was safe and free from significant adverse events as well as demonstrated the potential for similar outcomes to ACL reconstruction.(26, 27) The subsequent BEAR II study compared 35 patients assigned to ACL reconstruction (hamstring autograft) to 65 patients who underwent the BEAR ACL repair technique and followed the patients over two years. Results were encouraging in that outcomes, side-to-side laxity, and reinjury rates were all statistically similar between ACL reconstruction and BEAR ACL repair(28). Some have raised concern that the revision rate, while statistically insignificant, was 14% in the BEAR group vs 6% in the ACL reconstruction group, recognizing that the small size of the ACL reconstruction group may not have provided enough statistical power to identify a true difference. Nevertheless, BEAR ACL repair is now FDA approved, resulting in BEAR ACL surgery being broadly available and marketed to surgeons and patients. The BEAR III study is currently active, to monitor and collect data as the surgery is performed to a large population of patients with the intent to observe outcomes over time, in a variety of patient populations, and with multiple modifications to fixation techniques. In addition, the BEAR-MOON study is designed as a multi-center trial that will compare 200 patients partially randomized to either BEAR ACL repair or patellar-tendon ACL reconstruction in order to compare BEAR to the “gold standard” in treatment of torn ACL’s.(29)
Conclusion
As ACL repair techniques continue to be refined, potential benefits over ACL reconstruction include eliminating graft harvest site morbidity, reduced pain levels, and potential for earlier return of function and return to sport. While great advances have occurred over the last 10 to 15 years that involve surgical technique, stronger mechanical constructs, improved fixation devices, and biological augmentation to the repair site, ACL repair is not yet ready for universal adoption. Not all ACL tear patterns are amenable for ACL repair, and data does not currently exist to support ACL repair in high risk or high demand patient populations such as military members or collegiate, semi-professional, and professional athletes.(30) For those unfortunate enough to sustain an ACL injury, a careful discussion with their orthopaedic surgeon is necessary to weigh the potential risks and benefits to determine if ACL repair or reconstruction is appropriate for their specific demands and risk tolerance. As ACL surgery continues to evolve, sound data and research will help guide appropriate treatment strategies to restore function for patients that sustain ACL injury.
Jonathan Cheah, MD, is an orthopaedic sports medicine surgeon at Santa Clara Valley Medical Center and Clinical Assistant Professor at Stanford University. He is a NCAA Division 1 team physician for San Jose State University.
Lt. Col. R. Judd Robins, MD, is an orthopaedic surgeon at Wright-Patterson Air Force Base Medical Center; Associate Professor of Surgery at Uniformed Services University; and Head Team Physician for the United States Air Force Academy.
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