Can an Anterior Cruciate Ligament Tear Heal Without Surgery? Rethinking What We Know About ACL Injuries

For decades, an anterior cruciate ligament (ACL) rupture has been considered a serious knee injury that often leads to surgical reconstruction, particularly in young and active individuals. The traditional view has been relatively straightforward: once torn, the ACL has little capacity to heal, and reconstruction is frequently required to restore knee stability and facilitate a return to sport.

However, emerging evidence is beginning to challenge this assumption.

A secondary analysis from the KANON trial, published in the British Journal of Sports Medicine, reported that a substantial proportion of individuals treated initially with rehabilitation alone demonstrated evidence of ACL healing on MRI several years after injury. These findings have reignited an important discussion within musculoskeletal rehabilitation: can some ACL ruptures heal, and if so, what does this mean for clinical decision-making?

The Traditional Understanding of ACL Injuries

The ACL plays a crucial role in controlling anterior tibial translation and rotational stability of the knee. Historically, the ligament has been considered to possess limited healing potential compared with extra-articular ligaments such as the medial collateral ligament (MCL).

Several anatomical and biological factors have been proposed to explain this:

• The ACL is located within the joint capsule and is exposed to synovial fluid.

• The torn ligament ends may retract and fail to approximate.

• Formation of a stable healing scaffold appears more challenging than in extra-articular ligaments.

As a result, surgical reconstruction became the predominant treatment strategy for many physically active patients, particularly those wishing to return to pivoting or cutting sports.

Yet despite the widespread use of reconstruction, long-term outcomes are not always optimal. Many patients continue to report symptoms, reduced participation in sport, or early signs of knee osteoarthritis regardless of whether surgery has been performed.

This has led researchers and clinicians to ask a fundamental question: is reconstruction always necessary?

What Did the KANON Trial Analysis Find?

The study by Filbay and colleagues analysed MRI scans and patient-reported outcomes from participants enrolled in the KANON trial, one of the most influential randomised controlled trials comparing early ACL reconstruction with rehabilitation and optional delayed surgery.

The researchers examined MRI images obtained two and five years after injury and classified the appearance of the ACL using predefined criteria.

The findings were striking.

Among participants who initially underwent rehabilitation alone and did not subsequently require reconstruction:

• Approximately half demonstrated evidence of ACL continuity on MRI.

• Evidence of healing was observed both at two and five years following injury.

• Individuals whose ACL appeared healed on MRI generally reported better knee-related quality of life and symptoms than those without evidence of healing.

These findings challenge the long-standing belief that a ruptured ACL is incapable of healing.

However, caution is required when interpreting the results.

Does MRI Healing Mean Functional Healing?

One of the most important clinical questions is whether MRI evidence of healing reflects meaningful recovery of ligament function.

The answer remains unclear.

An MRI can demonstrate continuity of ligament fibres, but imaging alone cannot determine:

• Mechanical strength of the healed tissue.

• Ability to resist rotational forces.

• Capacity to tolerate sporting demands.

• Long-term protection against recurrent instability.

In other words, structural healing and functional recovery are not necessarily the same thing.

This distinction is familiar to musculoskeletal clinicians. Structural findings on imaging do not always correlate directly with symptoms, function, or prognosis.

The KANON analysis demonstrated an association between MRI healing and improved patient-reported outcomes. However, it did not establish that healing itself caused those superior outcomes.

What Does This Mean for Physiotherapists?

Perhaps the most important message from this study is not that surgery is unnecessary, but rather that rehabilitation deserves greater recognition as a primary treatment option for many ACL injuries.

High-quality rehabilitation can help patients:

• Restore knee range of motion.

• Improve quadriceps and hamstring strength.

• Develop neuromuscular control.

• Optimise movement strategies.

• Progress safely towards individual activity goals.

Importantly, previous randomised trials have already shown that an initial rehabilitation strategy can produce outcomes comparable to early reconstruction for many patients.

The KANON trial itself demonstrated that a considerable proportion of participants managed successfully without requiring subsequent surgery.

This supports a patient-centred approach in which treatment decisions are based on individual goals, symptoms, knee function, and response to rehabilitation rather than solely on MRI findings.

Is Surgery Still Necessary for Some Patients?

Absolutely.

The emerging evidence regarding ACL healing should not be interpreted as evidence against reconstruction in all cases.

Some individuals continue to experience:

• Persistent episodes of instability.

• Recurrent giving way.

• Difficulty returning to desired sporting activities.

• Secondary meniscal injuries associated with instability.

For these patients, surgical reconstruction may remain an appropriate option.

The challenge for clinicians is identifying which patients are likely to succeed with rehabilitation alone and which may benefit from surgery.

At present, this remains an area of active research.

What Does the Wider Evidence Tell Us?

The KANON findings are not isolated.

In recent years, several observational studies and narrative reviews have reported evidence suggesting that ACL healing may occur more frequently than previously believed. Researchers have proposed that factors such as tear location, tissue approximation, early management strategies, and loading during rehabilitation may influence healing potential.

Nevertheless, important questions remain unanswered:

• Which ACL tears have the greatest capacity to heal?

• What rehabilitation strategies optimise healing?

• Does MRI healing correspond to restoration of normal biomechanics?

• Can healing reduce the long-term risk of osteoarthritis?

• Which patients can safely avoid reconstruction?

Current evidence is promising but far from definitive.

Clinical Implications

For clinicians working in musculoskeletal rehabilitation, the key takeaway is not that surgery should be abandoned.

Rather, this research encourages a shift away from viewing ACL rupture as an injury with only one acceptable treatment pathway.

When discussing management options with patients, it may be appropriate to explain that:

• Some ACL ruptures may demonstrate evidence of healing over time.

• Structured rehabilitation can be highly effective for many individuals.

• Surgery is not automatically required immediately after injury.

• Treatment decisions should be individualised and guided by patient goals, symptoms, knee function, and shared decision-making.

This perspective aligns closely with contemporary evidence-based practice, where clinical decisions are informed not only by imaging findings but also by patient values and functional outcomes.

Conclusion

The study by Filbay and colleagues provides compelling evidence that some ACL ruptures may demonstrate healing on MRI following rehabilitation alone. Furthermore, individuals with evidence of healing reported better patient-reported outcomes than those without apparent healing.

While these findings do not prove that all ACL tears can heal or that surgery is unnecessary, they challenge long-standing assumptions about the biological potential of the ACL and reinforce the value of high-quality rehabilitation as a first-line treatment strategy.

For physiotherapists, this research offers an important reminder: the future of ACL management may lie not in choosing between surgery and rehabilitation, but in understanding which patients are most likely to benefit from each approach.

Reference

Filbay SR, Roemer FW, Lohmander LS, et al. Evidence of ACL healing on MRI following ACL rupture treated with rehabilitation alone may be associated with better patient-reported outcomes: a secondary analysis from the KANON trial. British Journal of Sports Medicine. 2023;57:91–99.

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