Stiff knee ligaments after surgery: causes, risks and solutions

Why do some patients experience persistent stiff knee ligaments after surgery, even when the graft is anatomically perfect?

Ligament reconstruction is essential to restore knee stability after trauma, but in a significant number of patients, stiff ligaments in knees develops in the weeks or months following the procedure.  

This article explores the full spectrum of causes, types and treatments of stiffness based on the scientific review “Prevention of knee stiffness following ligament reconstruction: Understanding the role of Arthrogenic Muscle Inhibition (AMI)” by Professor Étienne Cavaignac.

Explore other clinical articles by Professor Cavaignac for further insight into ligament reconstruction, robotic surgery, and post-op recovery strategies.

Understanding knee ligament stiffness: definition and clinical forms

When we refer to knee ligaments stiffness, we are usually talking about a post-surgical limitation in joint mobility, either in flexion (bending) or extension (straightening), compared to the opposite side.

A deficit in knee extension is particularly disabling. It alters walking patterns, causes compensatory movement in the hips or lower back and increases the energy cost of locomotion. Flexion deficits, while often less critical in daily life, can severely impair sports performance and squatting.

Knee stiffness following ligament surgery is often classified into:

• Mechanical stiffness: due to fibrous tissue, adhesions or improper graft placement.

• Neuromuscular inhibition, especially arthrogenic muscle inhibition (AMI).

• Mixed forms, where physical and neurological factors coexist.

Ligaments remain stiff following knee replacement surgery: why?

Ligaments can remain stiff following knee replacement surgery. While replacement surgeries primarily address joint degeneration, postoperative stiffness still affects 5–10% of patients. The reasons include:

Intra-articular factors (within the joint)

• Cyclops lesion: a fibrous nodule develops at the tibial insertion of the ACL graft and physically blocks full extension.

• Posterior capsular contracture: the back of the joint stiffens due to immobility or inflammation.

• Adhesions between the graft and surrounding tissues.

• Infrapatellar fat pad fibrosis, which interferes with tendon glide.

• Meniscal remnants or malpositioned tunnels, especially in revision surgeries.

All these conditions physically prevent normal ligament or graft movement, causing stiffness.

Extra-articular causes (outside the joint)

• Hamstring or quadriceps retraction post graft harvesting.

• Skin or subcutaneous adhesions, especially in patients with prior surgery or delayed rehab.

• Inadequate rehabilitation, which fails to counteract postoperative muscle inhibition.

Most notably, AMI plays a major role in persistent stiff knee ligaments.

Stiff knee after ligament injury: the role of arthrogenic muscle inhibition (AMI)

A stiff knee after ligament injury is often not just a mechanical problem but a neuromuscular one, frequently driven by inhibition phenomena such as AMI.

AMI is a reflex neurological mechanism triggered by joint injury, surgery, or swelling. It causes the central nervous system to inhibit voluntary activation of the quadriceps, especially the vastus medialis obliquus (VMO). This leads to a loss of extension and long-term joint dysfunction.

Key facts about AMI:

• Present in over 55% of patients after ACL rupture.

• Results in up to 35% strength loss in the quadriceps.

• Delays or prevents full extension, even if the graft is technically perfect.

• Is reversible in 80% of cases with targeted neuromuscular treatment.

Why is my knee stiff after ligament reconstruction surgery? In many cases, the answer is AMI. Even if your MRI is clear and your ligament appears structurally sound, your nervous system may be actively inhibiting full use of the joint.

How to prevent and treat stiff ligaments in knees

Pre-surgical prevention

Before any ligament reconstruction, especially for the ACL, it is critical to:

• Identify and treat AMI. This requires specific tests (e.g., quadriceps activation ratio) and not just routine clinical exams.

• Begin prehabilitation, especially focused on quadriceps neuromuscular control and full range of motion.

• Delay surgery until full extension and active quadriceps contraction are achieved.

Neglecting these steps increases the risk of developing long-term stiffness.

Immediate post-op protocols

In the first days after surgery, the following is essential:

• Early mobilisation: passive and active movement should start within 24 hours.

• Cryotherapy to limit swelling and thus reduce AMI.

• Neuromuscular re-education: such as biofeedback, electrical stimulation or mirror therapy to re-engage the VMO.

A standard protocol of icing and simple physiotherapy is not sufficient in patients at risk of AMI.

Long-term rehabilitation

At 3 to 6 weeks post-op:

• Full extension should already be recovered.

• Resistance training must begin, particularly targeting the quadriceps.

• Flexion should progress to 120° or more.

If stiffness persists beyond 8 weeks, advanced imaging and neuromuscular testing are indicated.

Surgical interventions

When conservative treatments fail, surgical options include:

• Arthroscopic arthrolysis: removal of adhesions and fibrotic tissue.

• Posterior capsulotomy: for persistent flexion contractures.

• Revision of graft placement, if mechanical causes are found (e.g., misplaced tunnel, impingement).

Surgery should never be considered unless AMI has been ruled out or treated, as it will not resolve neurogenic stiffness.

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Realistic outcomes and patient education

It is important for patients to understand:

• Stiffness is not always the fault of surgical error. It often reflects biological and neurological responses to injury.

• Rehabilitation is not optional: it is as important as the surgery itself.

• Timely intervention is crucial: stiffness that lasts longer than 3 months is far more difficult to reverse.

• Professor Cavaignac’s research shows that most cases of ligament-related stiffness can be avoided with correct identification and management of AMI.

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Conclusion

Stiff knee ligaments after ligament reconstruction or replacement surgery are a multifactorial problem, often caused by overlooked neurological mechanisms like AMI. With precise assessment, early rehab, and patient education, most cases can be prevented or successfully managed.

If you are experiencing persistent stiffness or loss of mobility after knee surgery, you may benefit from a second opinion.  

Trust Professor Etienne Cavaignac to guide you through recovery with a personalised, scientifically proven approach.