Skip to main content

8 Reasons You Can’t Straighten Knee

By Last updated on April 17th, 2023April 17th, 2023No Comments

Severe knee injuries can cause loss of full motion in your knee. As a result, you feel unbearable pain when trying to straighten your leg. 

Knee pain has varying causes and intensities. For instance, you may not straighten your knee because you feel like something is blocking it from moving. Another reason could be severe pain and swelling from an injury when you extend your knee. 

The treatment of knee injuries depends on your lifestyle and the intensity of the pain. You can also opt for cell-based therapy as a low-pain treatment for your knee issue.  

Causes of Why You Can’t Straighten Your Knee

Causes of Why You Can’t Straighten Your Knee

Multiple factors can’t restrict your knee movement. The major causes include:

Meniscus Tear

You have two menisci in each knee. The meniscus is a crescent-shaped cushion between your shin bone (tibia) and thigh bone (femur). It absorbs shock and stabilizes your joint. Any injury to the meniscus can result in the inability to knee movement. 

Old age, sports, and stairs climbing are the major causes of meniscus tears. In case of meniscus injury, you’ll feel severe pain and swelling in your knee. As a result, you can’t straighten knee.

ACL Tear

Your knees have an anterior cruciate ligament (ACL) between the femur and tibia to stabilize the movement. The ligament manages the forward and rotational movement of your knee. A trauma or instant pivoting can damage the ACL, resulting in restricted knee movement. 

ACL tears usually occur in athletes playing pivoting sports. The most common symptoms include severe pain, rapid swelling, and failure to straighten the knee.

MCL Injuries

The medial collateral ligament (MCL) is another knee stabilizer on the joint’s inner or medial side. Any hit to the MCL can stretch or tear it down, causing severe pain. Most MCL injuries happen when your knee suffers pressure that causes the tibia to shift outwards.

Like ACL tears, athletes are also prone to MCL injuries, especially those who perform activities that include tackles. 


Multiple severe and mild injuries can cause swelling in your knee. It can restrict your knee movement by causing pain. In extreme cases, swelling may also result in the inability of the knee to straighten. 

Osteoarthritis or Bone Spur

Osteoarthritis is a common form of arthritis, affecting more females than males. It is a joint condition in which the cartilage protecting the knee slowly tears down, resulting in severe pain, swelling, and restricted knee mobility. 

A bone spur or osteophyte is also a reason you can’t straighten the knee. It is when an additional, pointed bone grows along the joint edges due to trauma or arthritis. Bone spurs usually pose no symptoms, but they can restrict your full knee extension in severe cases. 

Quadriceps Tendon Injury

The quadriceps are your front thigh muscles, consisting of four muscles: lateralis, rectus femoris, vastus medius, and intermedius. These muscles ensure your full knee extension or straightening. Repetitive knee injections, steroid use, and old age can cause quadriceps tendon injury

It causes anterior knee pain that may restrict your knee movement.

Patellar Dislocations

Patella or kneecap glides up and down the boney groove of your thigh bone to ensure smooth knee movement. This groove is known as the trochlear groove. When your muscles become weaker or suffer trauma, it results in the dislocation of the kneecap

Patellar dislocations affect more females, particularly between the age group of 10 to 17.

Muscle Imbalance

You can locomote easily when your ligaments, tendons, and muscles move in a synchronized movement. For example, your lower leg muscles work together to enable your knee to extend and flex. 

Any imbalance in this system can result in an injury, affecting your knee mobility. You can’t straighten your knee if your hamstrings become tight or your quadriceps weaken. 

Treatment of Knee Issues

Knee injuries can quickly heal through lifestyle changes and non-invasive methods, such as cell-based therapies. 

Lifestyle Changes

Some lifestyle changes include:

  • Physical Exercises. Physical therapy is an easy way to strengthen your muscles and recover your body balance. Isometric and resistance exercises effectively build your ligaments, tendons, and cartilage. 
  • PRICE. Most specialists suggest PRICE to people with torn muscles. It includes protecting your knee from the possible causes of injuries (sports) or wearing protective gear, resting on the bed to support healing, doing icing and compression therapy to relieve the inflammation and swelling, and elevating your legs to regulate the blood flow. 
  • Proper Nutrition. Your diet should include nutrients and vitamins that accelerate the healing of your inflammation. Calcium, Vitamin C, D, and K, protein, zinc, magnesium, fiber, Omega-3s, and boron help build collagen and cartilage in your knees. 

Cell-Based Therapy and Plasma-Rich Platelet (PRP) Therapy

If your pain doesn’t seem to lower with the above lifestyle changes, you can go for an orthobiologic procedure like cell-based therapies and plasma-rich platelet (PRP therapy. These methods focus on replacing your damaged tissues or cells with healthy ones. 

Depending on your condition, your doctor may suggest one of the below types of cell-based or stem-cell therapy.

  • Minimally Manipulated Adipose Tissue Transplant (MMAT). It includes replacing or transplanting your affected adipose tissues with new ones to rebuild the inflamed areas of your knees. If you need MMAT in several parts of your body, the doctor can perform all of them in the same procedure. This means you won’t need to go through separate procedures for every affected location. 
  • Bone Marrow Concentrate (BMAC). If you can’t straighten the knee due to a bone marrow defect, you may have to go through BMAC. This cell-based therapy replaces the affected cells of your bone marrow with healthier ones.

These procedures are usually performed within 1.5-2 hours. By the end, you may go home on an immediate basis. 

Platelet-Rich Plasma (PRP) therapy focuses on stimulating the growth of healthy tissues. Your body needs platelets to attract healing cells in the blood and produce a sticky web called fibrin. The fibrin accelerates tissue development. 

PRP is a popular orthobiologic treatment used for spine and orthopedic injuries. It typically takes 45 minutes to complete.

How Long Does it Take for Knee Injuries to Heal? 

Every knee injury has a different healing duration. Generally, knee injuries can heal between 4 to 12 weeks, depending on your lifestyle changes and diet. If you exercise regularly, practice PRICE, and take proper anti-inflammatory nutrients, you’re likely to heal from a knee injury faster. 

Dr. Matthew HC Otten

Director of Regenerative Orthopedic and Sports Medicine
Fellowship-trained & Board Certified in Sports medicine
Director Angiography at Harvard Clinical Research Institute
Michigan State University Alumni


View Our Treatments