By Oxford Orthopaedics
By Oxford Orthopaedics
It happens in motion. Accelerating towards the ball, changing direction to stay with an opponent, sprinting into space, or landing after a jump, and the knee reacts before you can adjust. At the same time, the same disruption can surface during everyday movement, such as stepping down awkwardly, turning quickly, or shifting weight without thinking. Even when you remain upright, the knee no longer feels anchored. Instead, it loses its ability to control load and rotation together, which is why the movement does not resolve cleanly.
As activity settles, that loss of control becomes more apparent. Swelling develops either quickly or over the hours that follow, and confidence in the knee is now dependent on how it is used rather than how much it rests. Some knees remain usable but falter during rotation, deceleration, or descending stairs, while others struggle even with routine actions such as walking or changing direction. These differences are not incidental. They reflect how much of the anterior cruciate ligament (ACL) has been compromised, which is why ACL injuries are assessed along a spectrum rather than approached as a single condition.
Even when the moment of injury feels similar, ACL injuries aren’t all the same. The difference lies in how much of the ligament structure has been affected, and this directly influences how stable the knee feels during movement. For this reason, ACL injuries are described along graded levels rather than treated as a single category.
These grades reflect the extent of fibre disruption within the ligament:
At this level, the ligament fibres have been stretched but remain largely intact. The ACL continues to provide a degree of stability, which is why the knee often feels usable rather than disabled. Discomfort is usually mild and might manifest as a persistent ache rather than sharp pain.
As activity increases, subtle instability can become noticeable. Swelling is often limited, and the knee might feel mostly reliable in straight line movement while feeling less secure during more demanding actions. Because the ligament is not torn, symptoms tend to fluctuate rather than escalate abruptly.
In this instance, the ligament has been stretched and there is a partial tear. The structure of the ACL is compromised, which reduces its ability to stabilise the knee consistently. Compared to milder injuries, swelling is more noticeable and pain is more clearly felt around the joint.
As movement continues, instability becomes harder to ignore. Walking, running, or changing direction might feel uncertain, and the knee might struggle to tolerate load without discomfort. This middle range injury often feels unpredictable, with the knee alternating between periods of relative control and sudden loss of confidence.
At the most severe end of the spectrum, the ligament is completely torn. The ACL can no longer perform its stabilising role, which leads to a marked loss of control within the knee. Pain is often significant at the time of injury, followed by rapid swelling.
As weight bearing resumes, instability becomes pronounced. The knee might buckle or give way even during simple actions, making movement difficult and unreliable. At this stage, the loss of structural support is clear, and the knee’s behaviour reflects a complete breakdown in its ability to manage load and direction.
Because the ACL governs stability and control rather than simple movement, symptoms tend to surface when the knee is asked to manage load, rotation, or sudden change. As outlined earlier, these signs often appear during sport, but they can also emerge during routine daily actions such as turning, descending stairs, or shifting weight, especially as swelling and instability develop. Common symptoms include:
A popping or snapping sensation at the moment of injury
Pain felt immediately after the incident
Pain when standing or placing weight through the affected leg
Swelling that develops within the first 24 hours
Difficulty fully bending or straightening the knee
A feeling of looseness or instability, with the knee giving way during movement
Difficulty walking normally or limping, particularly during direction changes or stairs
ACL injuries are shaped by how the knee responds when control is challenged under speed, load, or imbalance. In these moments, the joint is required to manage forces faster than the surrounding structures can adapt, which leaves the ligament exposed when timing, alignment, or support breaks down. This is why some injuries occur in a single decisive movement, while others emerge when high demand situations push the knee beyond its capacity to stabilise itself. Common causes include:
Awkward or unbalanced landings from a jump
Direct contact or collision that forces the knee out of alignment
Knee hyperextension beyond its normal range
Non-contact loss of control without external impact
Pivoting on a firmly planted foot
Sudden deceleration, cutting, or directional changes while bearing weight
A clear diagnosis usually begins with a specialist assessment to determine whether knee stability has been compromised and to what extent. Because ACL injuries affect control rather than surface structures, the evaluation focuses on how the knee behaves under guided movement, with imaging used to clarify the degree of ligament damage and identify associated injuries.
In Singapore, the following methods are commonly used to diagnose an ACL injury:
Treatment is guided by the severity of ligament damage, the stability of the knee, and how the joint performs during daily movement or sport. The following are the options commonly available in Singapore, with care typically planned by a specialist based on injury severity, activity demands, and overall knee stability.
For less severe injuries, individuals with lower physical demands, or knees that retain functional stability, non-surgical care might be appropriate. These approaches focus on reducing pain and swelling while restoring knee function and improving stability through controlled rehabilitation.
When the ligament is severely damaged or when knee instability interferes with daily movement or sporting activity, surgical reconstruction is usually recommended. A complete ACL tear does not heal on its own, and surgery aims to restore stability rather than repair the torn ligament directly.
ACL injuries can range from mild ligament strain to complete rupture, and early severity doesn’t always indicate how the knee will cope over time. Some knees settle with rehabilitation, while others show persistent limitations once regular movement and activity resume.
As these limitations become apparent, assessment helps determine whether the ligament is providing adequate control or whether ongoing instability is likely to affect longer-term joint function. Review is commonly recommended when:
At this stage, specialist assessment helps clarify whether continued rehabilitation remains appropriate or whether surgical reconstruction offers a clearer path toward restoring function and protecting the knee over time.
Once an ACL injury begins to affect how activity is planned or avoided, the knee gradually defines what feels manageable. Movements are adjusted, loads are reduced, and certain actions are approached with hesitation. As this continues, temporary workarounds often become fixed habits, and the cost of delaying assessment can be repeated buckling of the knee, loss of confidence in movement, and added strain on structures that rely on stability.
Because these changes aren’t always obvious at first, assessment helps clarify whether the knee is coping under current demands or compensating in ways that increase risk over time. At Oxford Cartilage & Sports Centre, care follows established orthopaedic practice and treatment pathways commonly used in Singapore. Dr Francis Wong first reviews findings in detail and then explains how they relate to knee stability, activity demands, and longer-term joint health. From there, the best options are discussed, which might include continued rehabilitation, non-surgical management, or surgical reconstruction when indicated. To speak to our specialist, please contact the clinic to arrange for a consultation.
The ACL sits at the centre of the knee and controls forward movement and rotation between the thigh bone and shin bone. Its role becomes most apparent during turning, deceleration, and changes in direction, where stability is required to keep movement controlled rather than collapsing under load.
Diagnosis usually begins with a physical examination to assess knee stability and movement, often by comparing both knees. Imaging such as an MRI scan is commonly used to confirm the extent of ligament damage and identify associated injuries such as meniscus tears or cartilage injury.
An ACL tear usually occurs when the knee is forced to slow down, pivot, or absorb load suddenly. This is common during sports with stop start or cutting movements, though similar forces can occur during an awkward landing or fall in daily life. The injury results from excessive strain through the ligament rather than impact alone.
A popping sensation, rapid swelling, pain, and difficulty bending or straightening the knee are common signs. Because these symptoms overlap with other knee injuries, it’s best to see a specialist for assessment. A clinical examination and imaging such as an MRI scan help confirm whether the ACL is torn and guide next steps.
Yes. Some individuals are able to walk or continue activity for a period of time despite a torn ACL, particularly when surrounding muscles compensate for lost ligament control. This compensation can mask instability early on. However, it has limits, and over time the risk of knee buckling increases, along with added strain on the meniscus and cartilage.
Not all ACL tears require surgery. When the knee remains stable during daily activity, rehabilitation alone can be sufficient to restore function and control. However, complete tears don’t heal on their own, and when instability persists, further problems often follow over time. For this reason, treatment decisions are guided by knee stability, activity demands, and longer-term joint goals rather than pain alone.
Recovery after ACL reconstruction occurs in stages. Walking without assistance often takes several weeks, while return to sport commonly takes six to nine months. Progress varies between individuals and depends on rehabilitation quality, strength recovery, movement control, and how well stability is restored, rather than time alone.
By Oxford Orthopaedics