Yes, it is possible — but the story is more nuanced than a simple yes or no

Tearing the ACL and not realising it sounds unlikely — yet it happens often enough that clinicians have a term for it: a missed or delayed diagnosis. The reality, though, is not quite 'yes, you can tear it and feel nothing'. It is more specific than that.

For a complete rupture, Cleveland Clinic notes that most people know the exact moment it happens — a sharp pop, sometimes audible to someone standing nearby, followed by swelling that builds over the next one to two hours. UPMC describes a fully asymptomatic complete tear as 'very unusual'. So the genuinely silent complete rupture is rare.

The more common picture is a partial tear, or a complete tear whose symptoms are quickly misread. Up to half of all ACL injuries lack the textbook drama: the pop may be faint or absent, pain may feel more like a twinge than a rupture, and the knee — supported by the PCL, MCL, and LCL — may hold together well enough to walk on. Swelling can appear as deep tightness rather than obvious ballooning, and when it settles within a day or two, the temptation is to file the event away as a minor sprain.

What this means practically is that continued walking, or even returning to light jogging, does not rule out a significant structural injury. The absence of a dramatic incident is not reassurance — it is a reason to look more carefully.

Why partial tears and overuse damage hide so well

The biology offers a partial explanation. Partial degenerative tears caused by repetitive submaximal loading — the kind that accumulates through running mileage, court sport, or habitual pivoting — are currently thought to remain asymptomatic until roughly half the ligament's fibres are destroyed. Below that threshold, the ACL does not generate the signals the nervous system interprets as structural failure; mechanical load is redistributed across the intact fibres and surrounding structures without triggering obvious warning signs.

This stands in contrast to the acute sports-trauma pathway, where force is concentrated in a single moment and the nervous system typically does respond — with a pop, immediate pain, and swelling. The overuse route is more insidious, and, it should be acknowledged, considerably less well studied than the acute sports-injury model.

A case published in Cureus in 2024 illustrates the point concretely. A 36-year-old female runner presented with sharp lateral joint-line pain and no history of trauma. Imaging revealed a 10% thickness ACL tear — a partial, atraumatic injury that had developed through repetitive loading alone. Without a fall or collision to mark the timeline, the presentation was indistinguishable from iliotibial band irritation or a tendinopathy; the underlying structural damage had no obvious moment of onset to report.

Once torn fibres exceed 50%, instability and giving-way become more prominent. Even at that stage, however, persistent pain and swelling — rather than frank mechanical failure — may be the dominant complaint, making the underlying cause easy to overlook on clinical assessment alone.

The subtle signs worth taking seriously

Knowing what to watch for when there was no dramatic moment is where the practical difficulty lies. The functional signals tend to appear first, and they are easy to rationalise. A loss of trust in the knee — an instinctive reluctance to pivot, cut, or land on it, even when pain is low-grade — is one of the more telling signs. Patients often describe an unconscious shift: they favour the other leg on stairs, load it differently when walking downhill, or find themselves pulling out of a movement they would previously have made without thinking. A loose or wobbly sensation on uneven ground, particularly when the knee is slightly bent under load, points in the same direction. These are not dramatic complaints, and on their own they rarely prompt a GP visit; they register as 'my knee feels a bit off' rather than 'something has torn'.

The sensory picture is subtler still. Rather than obvious swelling, many people notice a deep tightness in the joint — a sense of fullness behind the kneecap that feels more like stiffness than injury. A dull ache after light activity that clears with rest reinforces the impression that nothing serious is happening. Taken together, these signals were the presenting complaint in a cohort study of 37 patients whose chronic knee pain had resisted non-operative treatment for months; on arthroscopy, 30 of them — 81% — had an undiagnosed partial ACL tear. That figure comes from a selected population with persistent, unexplained symptoms, so it should not be generalised to everyone who notices a slightly unreliable knee. What it does illustrate is that the injury actively hides behind complaints that feel routine — until assessment reveals otherwise.

Why these injuries are so often misdiagnosed

The difficulty does not end when imaging is arranged. In a 2025 prospective cohort of 203 patients, MRI accurately predicted the arthroscopic tear type in only 35.5% of cases — meaning the scan disagreed with what surgeons found directly in more than six out of ten knees. The predominant error was overclassification: 71% of incorrect readings described the tear as located further down the ligament than arthroscopy actually confirmed.

To make that concrete: a patient who presents with chronic instability, undergoes MRI, and receives a report naming a specific tear pattern may be receiving a description that does not reflect what the ligament looks like inside the joint. The scan is a starting point, not a verdict — and surgical planning that relies solely on MRI without intraoperative assessment of the remnant tissue risks proceeding on a false picture.

Clinical examination carries its own blind spots. In a prospective study, hands-on testing under anaesthetic was 2.23 times more likely to incorrectly identify a partial tear than MRI, suggesting that neither tool, used alone, gives a reliable final answer.

Histology adds a further complication. In partial tears, the bundle of fibres that appears intact on imaging frequently shows neovascularisation and disrupted collagen architecture that is, at a tissue level, indistinguishable from complete rupture. A 'partial' classification on the report may therefore understate the functional severity.

One indirect MRI marker — a patella tendon to anterior tibial cortex angle (PTATA) below 30° on a standard sagittal sequence — has shown a significant association with full-thickness tears and may help when direct findings are equivocal. It is not yet in routine clinical use.

A normal or near-normal scan, in short, does not close the question. Functional instability can persist where imaging appears reassuring, and arthroscopy remains the reference standard when clinical suspicion remains.

When to stop waiting and seek a specialist opinion

Deciding when to act is often the hardest part. A practical threshold: knee symptoms that have not settled within two to three weeks of a sporting incident — even one without a clear mechanism — warrant clinical assessment rather than continued self-management. That window shortens if the knee is giving way, however briefly or infrequently.

A previous label of 'sprain' or 'soft tissue injury' that has not improved with standard physiotherapy deserves re-evaluation. Sprain is frequently the default explanation when no fracture appears on imaging and no dramatic event was reported; as the preceding sections establish, that label can mask a partial ACL injury that conservative care is not adequately addressing.

Functional hesitation is a more specific prompt for escalation. When someone consistently avoids pivoting, cutting, or descending stairs on the affected side — adjusting movement without consciously deciding to — that pattern indicates instability that rehabilitation is unlikely to resolve safely without first establishing a diagnosis.

Recurrent giving way, even minor episodes, carries a particular cost. Untreated ACL laxity is associated with progressive meniscal damage and early-onset osteoarthritis; longstanding partial tears are recognised as a cause of early knee degeneration. The recommendation at this stage is not immediate surgery — it is a specialist assessment to clarify what is present and to map an appropriate pathway before secondary joint damage accumulates.

What a specialist assessment actually involves

Booking an appointment tends to feel like a bigger step than the consultation itself warrants. In practice, a specialist assessment for a suspected ACL injury follows a clear, unhurried sequence — and understanding it in advance can make the decision to attend considerably easier.

The consultation begins with a detailed history: when symptoms first appeared, the mechanism if there was one, how instability manifests — whether it is triggered by specific movements or present more constantly — and how the knee has changed over time. In presentations without a clear traumatic event, this conversation alone often reframes what previous assessments have labelled a sprain.

Hands-on examination follows, using established clinical tests — Lachman, anterior drawer, and the pivot-shift manoeuvre — to assess ligament integrity under controlled conditions. These tests capture functional information that imaging cannot: how the joint behaves under load, rather than how it appears at rest.

Imaging, typically MRI, is then arranged to characterise fibre involvement, identify which bundle is affected, and assess the menisci and articular surface for secondary damage. The scan informs clinical judgement; it does not replace it.

Where imaging remains inconclusive despite a convincing clinical picture, diagnostic arthroscopy can confirm the degree of injury and assess tissue quality directly — information that shapes what comes next in a way that no other tool currently matches.

The outcome of that process is a staged management plan. Many partial tears, and some complete tears in lower-demand patients, are managed conservatively with structured rehabilitation rather than surgery. The assessment's purpose is to reach that decision precisely — not to shortcut to an operation.

For patients in London seeking this kind of structured evaluation, specialist assessment is available at the London Cartilage Clinic on Harley Street — a consultation can be arranged via londoncartilage.com.

1. [1] Is Partial ACL Tear a Cause of Painful Swollen Knees?. (2024). https://doi.org/10.4103/jajs.jajs_96_23 https://doi.org/10.4103/jajs.jajs_96_23
2. [2] Is a partial ACL tear truly partial? A Clinical, Arthroscopic and Histologic Investigation. (2020). https://doi.org/10.1016/j.arthro.2020.02.037 https://doi.org/10.1016/j.arthro.2020.02.037
3. [3] Persistent Lateral Knee Pain From a 10% Thickness ACL Tear in a 36-Year-Old Runner. (2024). https://doi.org/10.7759/cureus.73081 https://doi.org/10.7759/cureus.73081
4. [4] Patella Tendon Anterior Tibial Cortex Angle (PTATA): An Indirect Marker of ACL Tear?. (2025). https://doi.org/10.1016/j.jcot.2025.103065 https://doi.org/10.1016/j.jcot.2025.103065
5. [5] Reliability of Preoperative MRI in the Prediction of ACL Tear Type. (2025). https://doi.org/10.1177/23259671251339491 https://doi.org/10.1177/23259671251339491