What Happens if You Leave an ACL Tear Untreated?
One of the most common questions I hear from patients after an anterior cruciate ligament (ACL) injury is whether surgery is really necessary. Some patients are able to walk reasonably well after the initial swelling settles and wonder if they can simply live with a torn ACL.
The answer is not always straightforward. Not every ACL tear requires reconstruction, but it is important to understand what may happen to the knee if the ligament remains torn.
The ACL is one of the major stabilising ligaments of the knee. It prevents the tibia (shin bone) from sliding excessively forward beneath the femur (thigh bone) and also helps control rotational movements of the knee. When the ACL is torn, the knee may feel unstable, particularly during activities involving pivoting, twisting, cutting, or sudden changes in direction.
Immediately after the injury, patients typically experience pain, swelling, and difficulty walking. Over the following weeks, the swelling often subsides and many people regain a surprisingly good range of motion. At this stage, some patients feel that the knee has “healed.” Unfortunately, the torn ACL itself rarely heals sufficiently to restore normal stability.
In the short term, some individuals can function reasonably well despite an ACL-deficient knee. Activities such as walking on level ground, cycling, swimming, and straight-line jogging may still be possible. These individuals are sometimes referred to as “copers” because they are able to compensate for the absent ligament through muscular control and neuromuscular adaptations.
However, not everyone is a coper. Many patients continue to experience episodes of instability or “giving way.” These episodes may occur during sports, climbing stairs, stepping off a curb, or even turning suddenly while walking. Each instability episode places additional stress on other structures within the knee.
The greatest long-term concern is not merely the absence of the ACL itself but the secondary damage that may occur as a result of recurrent instability. Numerous studies have demonstrated that ACL-deficient knees are at increased risk of meniscal injury over time.
The menisci are the shock absorbers of the knee. Repeated episodes of instability can cause tears of the medial or lateral meniscus, particularly the medial meniscus. Unlike the original ACL injury, which is often a single event, meniscal damage may accumulate gradually over years. Once significant meniscal tissue is lost, the knee loses an important protective structure that distributes load across the joint.
Cartilage damage may also develop. Articular cartilage covers the ends of bones and allows smooth, frictionless movement. Recurrent instability and abnormal joint mechanics can accelerate cartilage wear. Over time, this may contribute to the development of osteoarthritis.
For example, there was a patient who ruptured his ACL at age 16 while playing football. At that time, the only evident injury was a torn ACL and the rest of the knee was intact. He was advised surgery but decided on conservative treatment through physiotherapy. He ‘recovered’ well and was greenlit by his physiotherapist to return to competitive football at school National level. He claimed that his knee was stable and he had no issue. However, at about 1 ½ year post-injury, he twisted the same knee again during football. This time a repeated MRI showed extensive damage to the medial and lateral menisci as well as cartilage damage to the medial and lateral femoral condyles. This happened over a period of just 18 months.
Of course not all ACL deficient knees will end up with such serious injuries over the long term. There will be some patients whose knees will remain quite normal despite no ACL. It really depends on the type of activities the individual images in.
Several long-term studies have shown that patients with ACL injuries have a higher risk of developing radiographic osteoarthritis later in life. Interestingly, ACL reconstruction does not completely eliminate this risk, suggesting that the initial injury itself may trigger changes within the joint. Nevertheless, recurrent instability, meniscal injury, and cartilage damage are believed to further increase the likelihood of degenerative changes.
The timeline varies considerably from person to person. Some patients remain relatively functional for many years despite a torn ACL, particularly if they avoid pivoting sports and have strong muscular control. Others develop recurrent instability soon after injury and sustain additional meniscal damage within a relatively short period.
This is why treatment decisions should be individualised. Factors such as age, activity level, occupation, sporting demands, associated injuries, and the presence of instability symptoms all influence management.
For a sedentary individual who does not participate in pivoting sports and does not experience instability, non-operative treatment with physiotherapy may be entirely reasonable. Rehabilitation focuses on restoring strength, balance, and neuromuscular control. Many patients can return to a satisfactory level of function without surgery.
On the other hand, younger patients, athletes, physically demanding workers, and individuals who experience recurrent episodes of giving way are often better served by ACL reconstruction. The goal is not only to restore stability but also to reduce the risk of secondary meniscal and cartilage injury.
The most important message is that the absence of pain does not necessarily mean the knee has recovered. A torn ACL may allow patients to function surprisingly well in everyday life, but the underlying instability can continue to place the knee at risk. Understanding this balance between symptoms, activity demands, and long-term joint health is essential when deciding whether to pursue surgical reconstruction or conservative treatment.
References
- Frobell RB, Roos EM, Roos HP, et al. A randomized trial of treatment for acute anterior cruciate ligament tears. New England Journal of Medicine. 2010;363:331-342.
- Lohmander LS, Englund PM, Dahl LL, Roos EM. The long-term consequence of anterior cruciate ligament and meniscus injuries: osteoarthritis. American Journal of Sports Medicine. 2007;35(10):1756-1769.
- Ajuied A, Wong F, Smith C, et al. Anterior cruciate ligament injury and radiologic progression of knee osteoarthritis: a systematic review and meta-analysis. American Journal of Sports Medicine. 2014;42(9):2242-2252.
- Chalmers PN, Mall NA, Moric M, et al. Does ACL reconstruction alter natural history? A systematic literature review of long-term outcomes. Journal of Bone and Joint Surgery Am. 2014;96(4):292-300.
- Papalia R, Franceschi F, Diaz Balzani L, et al. Unravelling the natural history of ACL rupture without surgery. Knee Surgery, Sports Traumatology, Arthroscopy. 2014;22:2851-2855.
