Why Does the Knee Buckle One Year After ACL Reconstruction?

Understanding Functional Instability and Long-Term Recovery

It can be worrying when the knee suddenly “gives way” or buckles — especially after you’ve already undergone an anterior cruciate ligament reconstruction (ACLR) and months of rehabilitation. Many patients assume this means the graft has failed, but in most cases, the cause is not structural. Instead, it’s functional — meaning the problem lies in how the muscles and nerves around the knee are working together.

Functional (Neuromuscular) Instability

This is the most common reason a knee feels unstable one year after ACL reconstruction, even when imaging confirms the graft is intact.

• The graft may be strong and secure, but neuromuscular control and proprioceptive reflex loops (the body’s internal sensors that detect position and movement) can still be immature or fatigued, particularly in the hamstrings and quadriceps.

• After ACL reconstruction, proprioceptive recovery can take up to 18–24 months — long after the graft has biologically healed.

• Episodes of knee buckling often occur under fatigue, during non-sport activities, or due to subconscious imbalance, exactly as many patients describe.

• This is particularly common if rehabilitation has plateaued, or if the athlete returned to sport early without completing targeted reactive neuromuscular and rotational control retraining.

In simple terms — your knee may be strong enough, but your body hasn’t fully relearned how to use it automatically and protectively under stress.

Why Neuromuscular Control Takes Time

The ACL isn’t just a rope that stabilises the knee; it’s also packed with mechanoreceptors — tiny nerve endings that send feedback to the brain about joint position and movement. When the ligament is torn, those signals are lost.

Even after reconstruction, the graft takes time to re-establish those neural connections. Meanwhile, your muscles — especially the hamstrings and quadriceps — have to compensate. If the coordination between them is delayed, the knee may momentarily lose control, causing the feeling of it “giving way.”

This isn’t graft failure — it’s a communication delay between your knee and your brain.

Recognising the Signs of Functional Instability

Patients often describe:

• Sudden, brief “buckling” or “wobble” sensations when walking or pivoting

• Instability under fatigue (after long walks or training sessions)

• Confidence loss when changing direction or stepping down

• No major swelling or pain afterwards

These episodes are unsettling but typically not damaging if the graft is intact. However, they signal that neuromuscular retraining is still incomplete.

How to Manage and Retrain the Knee

Effective management focuses on reactivating and refining the body’s automatic control systems, rather than simply strengthening muscles.

1. Proprioceptive Retraining

Balance and coordination drills — such as single-leg stands, wobble board exercises, and dynamic stability work — retrain the knee’s position sense and improve reflex control.

2. Reactive Neuromuscular Training (RNT)

These are advanced exercises designed to trigger automatic muscle responses. They challenge your stability under unexpected forces, such as resistance bands pulling the knee into rotation, forcing rapid correction.

3. Rotational and Plyometric Control

Jumping, landing, and pivoting exercises help restore dynamic knee stability under real-world conditions, ensuring your muscles react instantly and safely.

4. Strength-Endurance Work

Even strong muscles can fatigue — and fatigue is a major trigger for instability. Gradually increasing endurance in the hamstrings, quadriceps, and gluteal muscles helps maintain control through prolonged activity.

5. Movement Analysis and Feedback

At advanced centres such as MSK Doctors and the London Cartilage Clinic, we use motion capture and force platform analysis to identify subtle movement deficits and tailor personalised rehabilitation programmes.

When to Seek Professional Review

If you experience repeated buckling or loss of confidence in your knee — even one year post-surgery — it’s important to have a clinical and functional assessment.
MRI imaging can confirm graft integrity, but a motion analysis or neuromuscular evaluation often provides deeper insight into why the knee feels unstable.

With the right retraining programme, most patients regain full confidence and stability, preventing long-term complications or secondary injuries.

Key Takeaway

A buckling knee one year after ACL reconstruction rarely means the graft has failed.
Instead, it’s usually a sign that your neuromuscular system is still catching up — learning, adapting, and refining control.
By addressing proprioception, reactive strength, and rotational stability, you can restore confidence, prevent reinjury, and complete the final stage of true functional recovery.