Can a torn ACL heal?
It absolutely can!
It has been long believed that a completely torn Anterior Cruciate Ligament (ACL) has no chance of healing. The consensus has been that the ACL does not have an adequate blood supply, or due to the intra-articular synovial environment of the knee, ligamentous healing of the ACL was impossible.
These beliefs however, were based on early animal studies in dogs and cats, and it was thus assumed that the human ACL was the same (Alm, 1974; Arnoczky, 1979).
Blood supply to the ACL has been observed and documented in studies in humans as far back as the 1980’s, and spontaneous healing of the ACL has been studied in humans since the mid 1990’s, and even healing of ruptured ACL grafts has been observed! (Arnoczky, 1985; Deie, 1995; Ihara 1996; Scapinelli, 1997; Voloshin 2002).
More recent evidence has shown that the human ACL does in fact have perfusion (oxygenated blood supply) throughout with the proximal 1/3 having the highest relative perfusion (56%) and the middle and distal thirds having 28 and 15.6% relative perfusion respectively. They conclude that this evidence pay indicate the proximal 1/3 of the human ACL may have an increased ability to heal relative to the distal 2/3rds (Lin, 2022).
Sagittal-oblique sections of the ACL, the substance of the ligament, divided into three regions of interest: proximal, middle and distal
.
Takeuchi (2022) observed a significantly higher vascularity rate in torn ACLs in comparison to intact ACLs in the same patients using ultrasound with superb microvascular imaging. This may indicate, like other tissues, the ACLs blood supply increases when injured in order to attempt a healing response.
A study by Nguyen (2013) suggested that the proximal 1/3 of the human ACL does haver a higher intrinsic healing capacity, and is similar to that of the MCL which frequently spontaneously heals. They observed disorganized collagen fibers, increased neovascularization, fat voids, abundance of myofibroblasts and high content of collagen type 3, which are characteristics typical of a spontaneous healing response as seen in the MCL. They also suggest that mechanical tension and cyclical stretching which may occur during walking may increase the number of myofibroblasts which play an important role in wound repair.
Blood Supply to the ACL
Cadaver specimen-anterior view showing: a) The inferior lateral genicular artery (ILG) running at the level of the joint line while providing arterial supply to the periphery of the lateral meniscus. b) The inferior medial genicular (IMG) and the ILG arteries becoming embedded and anastomosing within the infrapatella fat pad. c) The terminal branches of the ILG and IMG arteries supplying the distal portion of the ACL directly.
More and more research evidence is emerging showing that the human ACL does in-fact have a high intrinsic healing capacity, and many can spontaneously heal with healing generally occurring within the first 3 months post-injury (Costa-paz, 2012; Filbay 2022; Ferretti, 2020; Jacobi, 2016; Marangoni, 2018; Pitsillides, 2021; Roe, 2016; Ihara, 2017).
A recent study from Filbay et al. (2022), which examined the non-surgical group from the KANON trial found that at 2 years post-injury 56% of participants had a healed ACL on MRI, and at 5 years, 58% had a healed ACL!.
Pitsillides et al. (2021) concluded that, based on a systematic review including 9 studies and 734 participants
“This systematic review demonstrates the intrinsic healing capacity of the anterior cruciate ligament, indicating a higher healing potential for proximal ruptures.”
Ihara et al. (2017) examined 102 patients with complete ACL tears. At 9 months post-injury, MRI revealed healing of 82% of participants and concluded
“Our findings indicate that the human ACL has a high intrinsic healing capacity”
“placing physiological mechanical stress on the injured ACL as soon as possible after injury as a conservative treatment helps restore the ACL morphologically and mechanically.”
There are no guarantees that anyone’s ACL will spontaneously heal, at present we cannot yet identify individuals who will or will not heal.
Allowing adequate time, avoiding early reinjury or overstressing the ligament, and commencing early graduated rehabilitation may provide the best opportunity for the ACL to heal spontaneously.
What is clear, is that we need to re-evaluate common clinical practice and change the beliefs around the ACL healing potential in humans.
References:
ALM, A. (1974). Vascular anatomy of the patellar and cruciate ligaments: a microangiographic and histologic investigation in the dog. Acta Chir Scand Suppl, 445, 25-35.
Arnoczky, S. P., Rubin, R. M., & Marshall, J. L. (1979). Microvasculature of the cruciate ligaments and its response to injury. An experimental study in dogs. The Journal of bone and joint surgery. American volume, 61(8), 1221–1229.
Arnoczky, S. P. (1985). Blood supply to the anterior cruciate ligament and supporting structures. Orthopedic Clinics of North America, 16(1), 15-28.
Costa-Paz, M., Ayerza, M. A., Tanoira, I., Astoul, J., & Muscolo, D. L. (2012). Spontaneous healing in complete ACL ruptures: a clinical and MRI study. Clinical Orthopaedics and Related Research®, 470(4), 979-985.
Deie M, Ochi M, Ikuta Y. High intrinsic healing potential of human anterior cruciate ligament. Organ culture experiments. Acta Orthop Scand. 1995 Feb;66(1):28-32. doi: 10.3109/17453679508994634. PMID: 7863763.
Filbay, S., Roemer, F., Lohmander, S., Turkiewicz, A., Roos, E. M., Frobell, R., & Englund, M. (2022). 32 Spontaneous healing of the ruptured anterior cruciate ligament: observations from the KANON trial.
Ferretti, A. (2020). To heal or not to heal: the ACL dilemma. Journal of Orthopaedics and Traumatology, 21(1), 1-2.
Ihara, H., Miwa, M., Deya, K., & Torisu, K. (1996). MRI of anterior cruciate ligament healing. Journal of computer assisted tomography, 20(2), 317-321.
Ihara, H., & Kawano, T. (2017). Influence of age on healing capacity of acute tears of the anterior cruciate ligament based on magnetic resonance imaging assessment. Journal of computer assisted tomography, 41(2), 206.
Jacobi, M., Reischl, N., Rönn, K., Magnusson, R. A., Gautier, E., & Jakob, R. P. (2016). Healing of the acutely injured anterior cruciate ligament: functional treatment with the ACL-jack, a dynamic posterior drawer brace. Advances in Orthopedics, 2016.
Lin, K.M., Vermeijden, H.D., Klinger, C.E. et al. Differential regional perfusion of the human anterior cruciate ligament: quantitative magnetic resonance imaging assessment. J EXP ORTOP 9, 50 (2022). https://doi.org/10.1186/s40634-022-00486-8
Marangoni, L., Murillo, B., Bustos, D., Bertiche, P., Bitar, I., & Cabral, D. (2018). Anterior Cruciate Ligament, Acute Tears. Spontaneus Healing, is True or False. Orthopaedic Journal of Sports Medicine, 6(12_suppl5), 2325967118S00204.
Nguyen, D. T., Ramwadhdoebe, T. H., van der Hart, C. P., Blankevoort, L., Tak, P. P., & van Dijk, C. N. (2014). Intrinsic healing response of the human anterior cruciate ligament: an histological study of reattached ACL remnants. Journal of orthopaedic research : official publication of the Orthopaedic Research Society, 32(2), 296–301. https://doi.org/10.1002/jor.22511
Pitsillides, A., Stasinopoulos, D., & Giannakou, K. (2021). Healing potential of the anterior cruciate ligament in terms of fiber continuity after a complete rupture: A systematic review. Journal of Bodywork and Movement Therapies, 28, 246-254.
Roe, J., Salmon, L., Waller, A., Linklater, J., & Pinczewski, L. (2016). Spontaneous healing of the ruptured anterior cruciate ligament: a case series of 21 patients. Orthopaedic Journal of Sports Medicine, 4(7_suppl5), 2325967116S00080
Scapinelli, R. (1997). Vascular anatomy of the human cruciate ligaments and surrounding structures. Clinical Anatomy: The Official Journal of the American Association of Clinical Anatomists and the British Association of Clinical Anatomists, 10(3), 151-162.
Takeuchi, S., Rothrauff, B. B., Kanto, R., Onishi, K., & Fu, F. H. (2022). Superb microvascular imaging (SMI) detects increased vascularity of the torn anterior cruciate ligament. Knee surgery, sports traumatology, arthroscopy : official journal of the ESSKA, 30(1), 93–101. https://doi.org/10.1007/s00167-021-06640-6
Voloshin, I., Bronstein, R. D., & DeHaven, K. E. (2002). Spontaneous healing of a patellar tendon anterior cruciate ligament graft: a case report. The American Journal of Sports Medicine, 30(5), 751-753.
