After an anterior cruciate ligament (ACL) tear, is surgery absolutely necessary? Actually, not necessarily.

This is also a very interesting aspect of ACL research: even with the exact same ACL tear, different people's bodies can produce completely different adaptive outcomes.

With the same ACL tear, why can some people adapt while others cannot?

After an ACL injury, there is a group of people who, without undergoing ligament reconstruction surgery, can still maintain good knee function and even return to a significant level of activity through proper rehabilitation and training. These individuals are commonly referred to as Copers. Another group of people is prone to recurrent knee "giving way," instability, and functional decline, and are referred to as Noncopers.

Research by Fitzgerald, Axe, and Snyder-Mackler established a screening method—including hop tests, daily activity function, subjective knee function, and whether giving-way episodes occur repeatedly after the injury—to determine which ACL-injured individuals might be more suited to adopt a non-surgical rehabilitation strategy first. (Fitzgerald et al., 2000)

This represents a very important point: the degree of ligament tear does not completely determine a person's function. Knee stability is also jointly influenced by muscle strength, neuromuscular control, movement strategies, and other surrounding joint tissues. But what is even more worth pondering is: why, with the same loss of the ACL, can some people's bodies establish new stability strategies, while others experience recurrent instability?

Current research has identified many related factors, but there is still no single mechanism that can fully explain why everyone ultimately develops such vastly different adaptive outcomes.

KANON Trial: Immediate surgery is not always necessary for ACL tears

The KANON Trial is a landmark randomized controlled trial in the field of anterior cruciate ligament (ACL) treatment, which enrolled 121 active patients aged 18–35 with an acute ACL tear sustained no more than 4 weeks prior, comparing two treatment strategies: "early reconstruction plus structured rehabilitation" and "structured rehabilitation first with delayed reconstruction if needed" (Frobell et al., 2010).

The study found that an ACL tear does not mean everyone must immediately undergo reconstruction surgery. Two-year and five-year follow-ups showed that early reconstruction offered no significant clinical advantage over "rehabilitation first, surgery if needed" in multiple patient-reported outcomes for pain, symptoms, daily function, sports function, and knee-related quality of life (Frobell et al., 2010; Frobell et al., 2013).

More importantly, among patients in the "rehabilitation first, then reconstruction as needed" group, a substantial proportion of patients did not end up undergoing ACL reconstruction surgery, showing that some patients, even with an ACL tear, can still rebuild knee function sufficient to cope with daily life and sports demands through rehabilitation, muscle strength, and neuromuscular control (Frobell et al., 2013).

Long-term imaging studies have also not proven that early ACL reconstruction can completely prevent knee joint degeneration; the risk of osteoarthritis after an ACL injury is still influenced by multiple factors, including the original trauma, meniscal injury, subsequent surgery, and long-term loading. (Frobell et al., 2013; Culvenor et al., 2019)

Even more noteworthy is that subsequent research using MRI imaging found that in some patients who did not undergo ACL reconstruction, their native ACL showed signs of re-established continuity on MRI, indicating that the ACL may possess a spontaneous healing capacity that was previously underestimated after a tear. Among patients who received rehabilitation and did not undergo early reconstruction, about one-third showed ACL self-healing; if looking only at those who stuck with rehabilitation and never underwent delayed surgery, the proportion exceeded half (Filbay et al., 2023).

Furthermore, patients with healed ACLs had patient-reported outcomes, such as sport/recreation function and knee-related quality of life, at the two-year follow-up that were even better than those of patients with unhealed ACLs and those who underwent early or delayed ACL reconstruction (Filbay et al., 2023).

Therefore, a major paradigm shift brought by the KANON Trial is: "ACL tear" and "must undergo immediate surgery" cannot be directly equated.

So now we can no longer simply assume: "Torn ACL = surgery is a must."

A more reasonable question should be:

"Can this person's knee joint re-establish sufficient function and stability without ACL reconstruction?"

Of course, whether to undergo surgery must still take into account concomitant meniscus or other ligament injuries, recurrent instability, sports discipline, job demands, and personal goals, rather than using "Coper/Noncoper" as the sole criterion for surgery.

No Surgery: Preserves more of the original tissue environment

If non-surgical treatment is adopted, although the ACL itself is injured, the body does not undergo the interventions required for ACL reconstruction, such as incisions, bone tunnels, graft harvesting, and fixation. Therefore, besides the tissue changes caused by the original injury, the surrounding tissues that have not undergone surgical intervention still retain more of their original structural relationships. At this point, I would like to further introduce my proposed "soft tissue deformation and space" perspective.

The question I am truly concerned with is: After an ACL injury, how much usable movement space is left around this knee joint?

If a person can still maintain good function through muscle, neural control, and other surrounding joint tissues in the absence of an ACL, it shows that the human body does not rely on a single structure to maintain stability. Therefore, the focus of non-surgical recovery should not just be "strengthening the leg." I understand it as: rebuilding the space around the entire knee joint where soft tissues can move, slide, bear force, and control movement.

Surgery: Restores mechanical stability, but also alters the tissue environment

ACL reconstruction surgery can improve the mechanical stability of the knee joint. In the five-year results of the KANON trial, those who underwent early ACL reconstruction indeed showed superior mechanical stability in the Lachman test and pivot-shift test compared to the strategy of rehabilitation first, with surgery if needed (Frobell et al., 2013).

However, surgery is also a new tissue intervention. Skin incisions, graft harvesting, bone tunnels, fixation, and subsequent healing responses all subject the area around the knee joint to new tissue remodeling. One clinically critical issue is arthrofibrosis (joint fibrosis/joint stiffness). Systematic reviews show that arthrofibrosis can indeed occur after ACL reconstruction, leading to limitations in the knee's range of motion for extension or flexion. Therefore, early identification and step-by-step management of range of motion issues are extremely important (Ekhtiari et al., 2017).

So, a successful surgery does not mean the entire knee joint has recovered. Surgery primarily rebuilds the mechanical stability structure provided by the ACL, but muscle, joint range of motion, neural control, and surrounding soft tissues still need to be restored.

Why does the thigh often visibly shrink after ACL surgery?

After ACL injury and reconstruction, quadriceps atrophy and strength deficits are very common problems. Traditionally, we refer to this as muscle atrophy. But if we rethink this from the perspective of "soft tissue space," I would propose another way of understanding it: when muscle volume decreases, it actually means the three-dimensional space originally occupied by tissue in this area is shrinking. Therefore, restoring strength is not just about "building bigger muscles." More importantly, it is about rebuilding muscle mass, strength, neural control, and the ability of muscles and surrounding soft tissues to slide and move during action.

What needs to be restored may not be just a single ligament, but the entire space

Starting from this section, these are extended hypotheses I have proposed from the Soft Tissue Deformation Theory, rather than mechanisms that have been directly proven by current ACL literature.

I would divide knee recovery into two directions:

1. Rebuilding space from the inside

Through strength training, mobility training, and neuromuscular control, restore muscle mass and the mobility of deep soft tissues. Muscle hypertrophy is not just an increase in strength. From a three-dimensional structural perspective, it also means tissue is reoccupying more space.

2. Maintaining tissue mobility from the outside

The skin, subcutaneous tissue, superficial fascia, and deep tissues are not structures completely independent of one another. Therefore, I believe that in addition to exercise, one can also start with external care, mobility, and appropriate stimulation of the skin and superficial soft tissues to try and maintain the movement and sliding environment of the superficial tissues. As for whether "cleansing and nourishing the superficial fascia can further improve joint function after an ACL injury or surgery," this currently remains a direction we propose that requires further experimental validation and cannot be directly regarded as a conclusion of existing ACL research.

The real difference between surgery and non-surgery might not be just a single ligament

Those who do not undergo surgery need to learn how to rebuild the dynamic stability of the knee joint through strength, neuromuscular control, and new movement strategies under the condition of ACL insufficiency; those who undergo surgery regain the mechanical stability structure of the ACL, but must simultaneously face postoperative tissue healing, restricted mobility, quadriceps atrophy, and the process of rebuilding motor control.

Conclusion

Therefore, I believe the question truly worth asking is not just: "The ACL is torn, should I have surgery or not?" but rather, "How much space that can move, be controlled, and be rebuilt is left in this knee joint right now?" This is perhaps why two people who appear to have the same ACL tear can end up with completely different recovery outcomes. What we really need to restore may never have been just a single ligament, but the strength, control, and soft tissue space of the entire knee joint.

Disclaimer

The content of this article is organized based on existing research literature and the author's perspectives on sports science, soft tissue, and physical management. It is for health education and knowledge sharing purposes only, and does not constitute medical diagnosis, treatment advice, or a basis for decision-making on whether to undergo surgery.

The treatment approach after an anterior cruciate ligament (ACL) injury varies depending on the severity of the injury, concomitant injuries, knee stability, age, activity demands, and personal goals. Whether surgery is required and the method of rehabilitation should be comprehensively evaluated by orthopedic surgeons, physical therapists, or related medical professionals based on individual circumstances.

The concepts of "soft tissue deformation," "tissue space," and related physical management concepts mentioned in the text are partly theoretical viewpoints and extended hypotheses proposed by the author, and do not represent established clinical consensus in the current medical community or treatment mechanisms that have been fully validated.