A stiff knee can limit movement and cause pain. Discover causes, symptoms, diagnosis and prevention with Prof. Cavaignac.
When your knee is stiff and hurts, even simple daily activities such as walking, climbing stairs or sitting down can become difficult. Knee joint stiffness can result from a wide range of conditions, from post-surgical scarring to joint inflammation. Understanding the underlying reasons is essential to finding the right solution and restoring your mobility.
What are stiff knees?
In medical terms, stiff knees are a permanent and fixed limitation in the knee’s range of motion compared to the healthy side. This can involve:
- Difficulty straightening the knee fully (extension deficit)
- Difficulty bending the knee (flexion deficit)
- A combination of both
The impact on your daily life depends on the degree of restriction. Even a small loss of extension can alter your gait, while a significant loss of flexion can prevent you from climbing stairs or performing sports. Over time, a knee that is stiff can also contribute to muscle weakness, altered biomechanics and early osteoarthritis.
Stiff knee causes and risk factors
There are many stiff knee causes, often linked to a combination of mechanical, inflammatory and neurological factors. Here, we will focus specifically on post-operative knee stiffness, its mechanisms and how it can be managed.
Post-surgical stiffness
- Stiffness after knee replacement is one of the most common scenarios. Scar tissue, or arthrofibrosis, can form inside the joint, restricting movement.
- After ligament reconstruction (e.g. ACL), incorrect tunnel placement, neuromuscular lack of control (such as AMI) can cause knee stiffness and pain.
After a knee replacement surgery, stiff knee is a relatively common complication, particularly in the first weeks following the surgery. It can result from excessive scar tissue formation inside the joint, prolonged swelling, pain that limits movement or inadequate early rehabilitation.
In some cases, stiff knee after knee replacement may also be linked to implant positioning or pre-existing motion loss before surgery. Recognising the signs early is essential, as timely intervention can help restore range of motion and prevent long-term limitations.
After ligament reconstruction: common causes (often combined)
- Intra‑articular: anterior impingement from cyclops scar tissue, meniscal “bucket‑handle” tears, tunnel malposition or graft impingement, joint effusion, adhesions (e.g. infrapatellar contracture), posterior capsular tightness, septic arthritis.
- Extra‑articular: quadriceps or hamstring weakness after graft harvest, posterior chain tightness, painful skin adhesions, complex regional pain syndrome, arthrogenic muscle inhibition (AMI) causing active extension loss.
Stiffness is usually multifactorial so each potential contributor is checked and treated systematically.
Post-traumatic stiffness
After a fracture, ligament injury or direct trauma, the inflammatory response and bleeding inside the joint can lead to adhesions and stiff knee pain.
Arthrogenic Muscle Inhibition (AMI)
Arthrogenic muscle inhibition or AMI is a neurological reflex that reduces the ability of the quadriceps muscle, particularly the vastus medialis oblique (VMO), to contract properly after injury or surgery.
After a knee injury or operation, the nervous system may “protect” the joint by partially shutting down quadriceps activation. While this response can be protective in the very short term, persistent AMI leads to weakness, altered gait and contributes to persistent stiffness in knees and loss of extension.
Clinical studies have shown that AMI is common: it affects up to 48% of patients three weeks after ACL reconstruction and 36% after total knee replacement (according to the study Arthrogenic muscle inhibition: A prevalent issue in knee arthroplasty led by Prof. Cavaignac and his peers). Risk factors after knee replacement include persistent pain more than 15 days post-op, postoperative swelling (hemarthrosis), a preoperative history of long-standing symptoms and being of the female gender.
If not addressed early, AMI can maintain a vicious cycle: reduced muscle activation leads to less movement, which in turn promotes more stiffness. Rehabilitation strategies to reverse AMI include targeted VMO activation, biofeedback, neuromuscular electrical stimulation and specific motor imagery exercises. Identifying and treating AMI promptly is one of the most effective ways to prevent a stiff knee from developing after surgery.
Unlike other causes of knee stiffness, AMI has no surgical treatment; it can only be addressed through targeted rehabilitation strategies such as biofeedback, neuromuscular stimulation and quadriceps reactivation exercises.
If you are wondering “why is my knee stiff?”, it’s important to understand that multiple factors often work together. In such cases, an extensive paraclinical assessment, including imaging and laboratory tests, is often necessary to identify the exact origin of the stiffness and guide the most appropriate treatment.
Recognising the symptoms of a knee pain and stiffness
Patients with knee pain and stiffness may describe:
- Pain and limited motion when moving
- A knee that feels tight and stiff after surgery, especially in the early week
- Swelling, warmth or tenderness around the joint
- Stiffness behind the knee, which may indicate posterior capsular involvement
- Weakness or instability due to reduced muscle activation
The pain can vary from mild discomfort to severe pain that limits daily activities. Over time, untreated stiffness can cause compensatory strain on other joints, such as the hip or lower back.
A feeling of “block” when trying to fully straighten the knee, especially if the hamstrings “grab” and the quadriceps feel “asleep”, is typical of AMI‑related extension loss. Early identification helps prevent chronic stiffness.
How is knee stiffness diagnosed?
Here are the different methods used to diagnose post-op knee stiffness.
Clinical examination
Prof. Cavaignac will:
- Compare your range of motion with the opposite knee
- Assess pain, swelling, and joint stability
- Evaluate muscle strength and quadriceps activation
Functional screening for AMI (done in clinic)
- Hamstring fatigue/relax test (prone): repeated contract/relax to see if the analgesic flexion posture is reversible (Grade 2A).
- Effusion check: if tense haemarthrosis is present, aseptic aspiration both relieves pain and switches off the inhibition.
- Heel‑raise VMO re‑activation: patient learns to contract VMO isometrically until the patella migrates proximally, the heel then lifts and full extension returns.
These steps target the spinal (hamstring reflex), local (effusion‑pain) and supraspinal (VMO re‑education) components of AMI.
Imaging
- X-rays: to identify bony blocks and prosthesis positioning
- MRI: to detect scar tissue, cyclops lesions, meniscal injuries or ligament problems
- CT scan: useful for assessing implant or tunnel placement after surgery
Additional checks
- Ruling out infection with blood tests or joint fluid analysis
- Identifying neurological inhibition such as AMI
A complete diagnosis is essential to determine what causes stiff knees in your situation
Preventing stiffness in the knee
While not all cases can be prevented, several measures reduce the risk of developing stiffness in the knee:
- Early mobilisation after surgery or injury, guided by your healthcare team, is essential, as emphasised in our ACL rehabilitation protocol.
- Effective pain control to allow safe movement
- Targeted physiotherapy focusing on both extension and flexion recovery
- Neuromuscular retraining to address AMI and avoid persistent inhibition
- Avoiding prolonged immobilisation unless medically necessary
If you want to know what to do for stiff knees, prevention and early rehabilitation are always better than waiting until the problem becomes severe.
What helps most
- Early mobilisation with effective analgesia.
- Cryotherapy and structured strengthening (open‑ and closed‑chain quadriceps) plus specific hamstring fatigue drills when reflex over‑activity is present.
- Neuromotor retraining and biofeedback‑guided quadriceps work for persistent VMO inhibition.
What is less useful for AMI
Conventional peripheral electrostimulation alone and simple “cushion‑squeeze” drills do not reverse central inhibition; taping/splinting/heat/soft‑tissue release have minimal or no benefit for AMI.
Crucial principle
When AMI persists, do not plan surgery until the inhibition is resolved, pre‑conditioning the knee this way reduces the risk of postoperative stiffness.
Treatment options: a brief overview
The best approach depends on the cause and severity of your condition.
In some cases, structured rehabilitation is enough. In others, minimally invasive arthroscopic procedures can release scar tissue and restore movement.
To learn more about specific options, recovery timelines such as how long does stiffness last after total knee replacement and surgical innovations, visit our dedicated page: Knee Stiffness Treatment.
Why consult Professor Cavaignac?
With over 1,000 knee surgeries performed each year and recognised expertise in complex cases, Prof. Cavaignac offers comprehensive assessment and personalised treatment plans for stiff knees.
His combination of advanced diagnostic techniques and experience in both sports injuries and joint reconstruction ensures precise care aimed at restoring your mobility and quality of life.
If your knee is stiff and hurts, don’t delay. Book a consultation with Prof. Cavaignac to identify the cause and start your recovery journey.
Contact Professor Cavaignac today to regain comfort, mobility and confidence in your knee.