ACL Injuries and Knee Instability
Injury to the ACL leading to knee instability, reduced confidence, and altered movement
Anterior cruciate ligament (ACL) injuries and knee instability can affect the way your knee moves, feels, and functions, particularly during activities that involve pivoting, turning, or sudden changes in direction. The ACL is one of the key ligaments that helps stabilise your knee joint, preventing excessive forward movement and rotation of the tibia (shin bone) relative to the femur (thigh bone). When the ACL is injured, this stability can be compromised, leading to a feeling that the knee may give way or lack control, especially during sport or higher-demand activities.
ACL injuries commonly occur during sporting activities, often as a result of sudden twisting, landing awkwardly, or rapid deceleration. They can range from a partial tear to a complete rupture, and may occur in isolation or alongside damage to other structures within the knee, such as the meniscus or cartilage.
Knee instability can develop following an ACL injury, but not all patients experience the same symptoms. Some people are able to return to low-impact activities with appropriate rehabilitation, while others may continue to experience recurrent instability or difficulty trusting the knee.
What is an ACL Injury?
An ACL injury refers to damage to the anterior cruciate ligament (ACL), one of the key ligaments that helps stabilise your knee joint. The ACL connects the thigh bone (femur) to the shin bone (tibia) and plays an important role in controlling forward movement and rotation of the knee.
When the ACL is injured, this stability can be affected. The injury may range from a partial tear, where some of the ligament fibres remain intact, to a complete rupture, where the ligament is fully torn.
ACL injuries most commonly occur during activities that involve sudden changes in direction, pivoting, jumping, or landing. They can also occur through direct impact to the knee, such as during contact sports or accidents.
In some cases, an ACL injury occurs on its own. In others, it may be associated with additional damage to the knee, including the meniscus (cartilage), other ligaments, or joint surfaces.
An ACL injury can lead to symptoms such as pain, swelling, reduced range of motion, and a feeling of instability or the knee “giving way”. The severity of symptoms can vary depending on the extent of the injury and any associated damage.
Causes of ACL Injuries
ACL injuries most commonly occur when the knee is subjected to sudden force, twisting, or instability, particularly during activities that involve pivoting, changing direction, or landing from a jump. In many cases, ACL injuries are non-contact injuries, meaning they occur without direct impact to the knee. These typically happen when the foot is planted and the body changes direction quickly, placing stress on the ligament.
Common causes include:
- Sudden changes in direction while running or turning
- Pivoting on a planted foot, especially during sport
- Landing awkwardly from a jump, particularly if the knee collapses inward
- Rapid deceleration, such as stopping quickly while running
- Poor movement mechanics or muscle control, which can place additional strain on the knee
ACL injuries can also occur through direct contact, such as a blow to the knee during sports like football, rugby, or skiing. This may force the knee into an unnatural position, leading to ligament damage.
Certain factors may increase the risk of ACL injury, including muscle imbalances, reduced core or hip strength, fatigue, and previous knee injury. Anatomical differences and movement patterns can also influence how forces are distributed through the knee. Understanding how ACL injuries occur can help guide both treatment and prevention strategies, particularly for patients looking to return to sport or higher-level physical activity.
Symptoms of ACL Injuries and Knee Instability
Symptoms of an ACL injury can vary depending on the severity of the damage and whether other structures in the knee are affected. Many people notice a sudden change in how their knee feels, moves, and responds to activity.
At the time of injury, it is common to experience a sudden pain in the knee, sometimes accompanied by a “popping” sensation or sound. This is often followed by rapid swelling, which can develop within the first few hours due to bleeding within the joint.
In the days following the injury, you may notice:
- Pain with movement or weight-bearing
- Swelling and stiffness in the knee
- Reduced range of motion, making it difficult to fully bend or straighten the knee
- Tenderness around the joint
A key symptom associated with ACL injuries is knee instability. This may feel like the knee is giving way, shifting, or unable to support your weight, particularly during activities that involve turning, pivoting, or sudden stops.
Some patients may be able to walk in a straight line without significant difficulty but notice instability during sport or uneven ground. Others may experience a more constant sense of lack of confidence in the knee, even during everyday activities.
If symptoms persist, or if instability affects your ability to return to normal activity, further assessment is important. Dr Scott Tulloch will evaluate your knee using clinical examination and imaging, helping determine the extent of the injury and the most appropriate management plan.
Risk Factors for ACL Injuries
A number of factors can increase the likelihood of sustaining an ACL injury, particularly during activities that involve pivoting, jumping, or rapid changes in direction. These risk factors can be related to movement patterns, physical conditioning, and individual anatomy.
Some of the more common risk factors include:
- Participation in high-demand sports, such as football, netball, basketball, skiing, or rugby, where sudden changes in direction are frequent
- Previous ACL injury or knee injury, which may affect joint stability and movement patterns
- Muscle weakness or imbalance, particularly in the quadriceps, hamstrings, and hip muscles, which play an important role in stabilising the knee
- Poor movement mechanics, such as the knee collapsing inward during landing or pivoting
- Reduced neuromuscular control, affecting coordination, balance, and reaction time
- Fatigue, which can impact technique and increase the risk of injury during sport or exercise
Certain anatomical and individual factors may also play a role, including variations in knee alignment, ligament structure, and joint flexibility. In some populations, differences in movement patterns and muscle activation may influence risk.
While not all ACL injuries can be prevented, understanding these risk factors can help guide injury prevention strategies, including targeted strengthening, movement training, and sport-specific conditioning.
Treatment Options for ACL Injuries and Knee Instability
Management of ACL injuries and knee instability is tailored to your symptoms, activity level, lifestyle, and the extent of ligament damage. Treatment may be non-surgical or surgical, depending on how your knee functions and your goals for recovery.
Non-surgical treatment options may be appropriate for some patients, particularly those with lower activity demands or stable knees. These may include:
- Physiotherapy, focusing on strengthening the muscles around the knee, improving balance, and restoring movement
- Activity modification, avoiding high-risk movements such as pivoting or sudden directional changes
- Bracing, in some cases, to provide additional support during activity
- Structured rehabilitation programs, aimed at improving knee control and confidence
Some patients are able to return to day-to-day activities and low-impact exercise without surgery if the knee remains stable and symptoms are manageable.
If the knee continues to feel unstable, or if you wish to return to pivoting sports or higher-demand activities, ACL reconstruction surgery may be considered. This involves replacing the torn ligament with a graft, often taken from your own tendon or a donor source, to restore stability to the joint.
Surgical treatment may also be recommended if there are associated injuries, such as meniscal tears or cartilage damage, which can affect long-term knee health if left untreated.
Following surgery, a structured rehabilitation program is essential to support recovery, restore strength and movement, and guide a safe return to activity.
Dr Scott Tulloch will assess your knee and discuss the most appropriate treatment options based on your individual condition, helping you make an informed decision about your care.
When Should You Seek Help for an ACL Injury?
It is important to seek assessment if you experience a knee injury associated with pain, swelling, or instability, particularly after a twisting movement, sudden change in direction, or awkward landing.
You should consider seeking medical advice if you notice:
- A “popping” sensation at the time of injury
- Rapid swelling of the knee within a few hours
- Difficulty walking or weight-bearing
- Reduced range of motion, including trouble bending or straightening the knee
- A feeling that the knee is giving way, shifting, or unstable
- Ongoing pain or swelling that does not settle with rest
Even if symptoms improve in the days following the injury, underlying damage to the ACL or other knee structures may still be present. Early assessment can help identify the extent of the injury and guide appropriate management.
Prompt evaluation is particularly important if you are looking to return to sport or higher-level activity, or if your knee does not feel stable during everyday movements.