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Can’t Straighten Knee

8 Common Causes for Immobility in the Knee Joint

It happened abruptly.  You got up from your chair and can’t straighten your knee.  Should you be alarmed?  What does it mean when your knee won’t straighten?  What are the 8 major causes of why you can’t straighten the knee?  When should you be concerned about not being able to straighten the knee?  What are the treatment options?  Let’s dig in.

What Does It Mean When your Knee Won’t Straighten?

Your knee is capable of moving in a number of directions.  The most common are flexion and extension.  Flexion is when you bend your knee and the shin bone moves towards the buttock.  Knee extension is when you straighten out your knee.  The extension is the opposite of flexion.  PT, trainers, and physicians alike measure the degree of knee flexion and extension.  When lying flat on an examination table or hard surface your knee should be able to extend so that there is no angle between the thigh and shin bone.  The inability to straighten the knee is also known as an extension lag and is a reason for concern.

What Are the Major Causes of Why I Can’t Straighten the Knee?

Being unable to straighten your knee can be frightening.  There are multiple factors involved in the movement of the knee.  The major causes of inability to straighten the knee include.

Meniscus Tear

The meniscus is a C-shaped cushion situated between the thigh bone and shin bone.  Each of us has two menisci per knee.  One on the inside (medial)  and one on the outside (lateral) of the knee. The meniscus functions as a shock absorber and joint stabilizer and is susceptible to injury.   Meniscus tears are among the most common knee injuries.  There are many different types of meniscus tears that are classified (1).  Examples include horizontal, vertical, complex, and flap tears.   Risk factors for meniscus tears include advanced age, male gender, work-related kneeling, and climbing stairs (2).  Injury to the meniscus can cause a number of symptoms which include:

  •  Pain
  •  Swelling
  • Inability to straighten the knee

Quadriceps Tendon Injury

The quadriceps muscle is a powerful muscle located in the front of the thigh.  It is actually composed of 4 different muscles which include the rectus femoris, vastis medius, lateralis, and intermedius.  The quadriceps function as a powerful extensor of the knee.  Tendons are thick pieces of connective tissue that connect the muscle to bone.  The quadriceps muscle is susceptible to injury.  Common causes of injury include age over 40 years, steroid use, chronic and repetitive knee injections (3).  Quadriceps tendon injuries typically produce anterior knee pain (4).  It can also prevent you from straightening your knee.

Patella Tendon Injury

The patella tendon is a thick piece of connective tissue that connects the kneecap (patella) to the shin bone.  It is important in stabilizing the knee cap and knee extension.  Injuries occur in athletes who participate in jumping and running sports.  Injury to the patellar tendon can cause patients to complain that they can not straighten their knees.

ACL Injury

The anterior cruciate ligament (ACL) is a key stabilizer of the knee.   The ligament extends from the thigh bone (femur) to the shin bone (tibia) and limits forward and rotational movement of the knee.  Tears in the ACL can occur as a result of trauma, and rapid pivoting and cutting movements.  ACL tears account for 64% of athletic knee injuries in pivoting sports resulting in 120,000- 200,000 ACL reconstructions annually in the United States (5).  There are numerous signs and symptoms of an ACL tear.  Symptoms may vary from patient to patient.  The most common ones are:

  • Pain
  • Rapid swelling
  • inability to straighten the knee

Acute Injury with Swelling

Injury to the knee can cause swelling.   This is also called fluid on the knee.  This swelling can significantly reduce knee movement.  If severe, the swelling can cause the inability to straighten the knee.

Osteoarthritis/Bone Spur

A bone spur is the growth of additional bone along the edges of a joint.  It is also called an osteophyte.  Bone spurs develop as a result of trauma, instability, and arthritis.  If large enough they can prevent the knee from straightening. Bone spurs can cause a physical block to full knee extension.

Patella or Kneecap Dislocations

The kneecap is intended to slide smoothly up and down the boney groove in the thigh bone.  The groove is called the trochlear groove.  If the knee cap becomes dislocated from this boney groove it can cause patients to complain that they can’t straighten the knee.  The annual incidence of primary patellar dislocation is 5.8 per 100 000 in the general population (6).   This occurs most frequently in the 10- to 17-year age group.  Causes of kneecap dislocations include trauma, weak muscles, and the female gender.

Muscle Imbalance

The body is a miraculous system.  It requires synchronized movements between muscles, tendons, and ligaments.  If muscle imbalance occurs the system can break down.  The muscles in the lower legs must work together to allow full flexion and extension of the knee.  If the hamstrings become chronically tight and the quadriceps are weak it may be challenging or difficult to straighten your knee.

When Should I be Concerned About Not Being Able to Straighten Knee?

A single episode where you can’t straighten the knee that is not associated with other symptoms typically does not warrant concern or further evaluation. Multiple episodes of not being able to straighten your knee warrant further evaluation.  Who should evaluate the clicking in your knee?  Bypass your surgeon and schedule a consultation with a physician trained in Interventional Orthopedics. Surgeons provide surgical solutions that are not always in your best interest.  What is Interventional Orthopedics?  It is a board-certified physician in PM& R or anesthesia with advanced orthopedic training with nonsurgical treatment options.  They use orthobiologics to heal injured orthopedic tissue. Orthobioligcs are biological substances naturally found in the body that are used to promote quicker healing of muscle, tendon, and bone injuries. To learn more please click on the video below.

What Are the Treatment Options?

Identifying the underlying cause of why you can not straighten your knee will provide the best clinical results.  Conservative therapy when appropriate is always the first line of treatment.

Meniscus tear

Not all meniscus tears are painful.  In fact, in one study 60% of patients without pain had meniscus tears on MRI.   PRP  is effective in the treatment of meniscus tears.  Steroids should be avoided as they are toxic.  Meniscus surgery has been demonstrated to be no better than PT (7).  In addition, it increases the risk of knee instability and arthritis. To learn more about the treatment of meniscus tears please click on the video below.


Osteoarthritis is the most common form of arthritis.  It is a joint disease where the cartilage that lines and protects the joint gradually wears down resulting in pain, swelling, and restriction in movement.  Approximately 13% of women and 10% of men aged 60 years and older have symptomatic knee osteoarthritis (8).  Risk factors for knee osteoarthritis include obesity, female gender, repetitive knee trauma, muscle weakness, squatting, and meniscus injuries (9).  Loss of cartilage and bone spur formation can both result in you not being able to straighten your knee.

ACL Injuries

The Centeno-Schultz Clinic physicians are experts in the treatment of ligament injuries.  We pioneered the use of bone marrow stem cells in the nonsurgical treatment of ACL tears allowing patients to forgo the risks and complications of traditional ACL graft surgery. The risks of the ACL graft surgery are significant and include a 6x’s risk of re-tear within 24 months post-surgery (7) and development of arthritis (8).  To learn more about the non-surgical treatment of ACL injuries please click on the video below.

Patellar and Quadriceps Tendon Injuries

Patellar and Quadriceps tendon injuries are easily visualized with ultrasound.  The specific type and location of the injury can be identified.  Treatment options include PRP or stem cell injections depending upon the severity of the injury.

Take a deeper dive here:

More Knee Injuries that Can Cause this Symptom

ACL Tears

The Anterior Cruciate Ligament (ACL) is one of four major ligaments in the knee. It is an important stabilizer of the knee and prevents the shin bone (tibia) from sliding in front of the thigh bone (femur). The ACL is susceptible to injury. It is most likely to be injured during activity or by impact. A torn ACL is a common injury for athletes at all levels, but it is most common for people who are active or who experience impact injuries to the knee. ACL injuries can happen to anyone of any age, condition, or ability, and it can be injured in many ways. Examples include abruptly changing direction, slowing down while running, landing incorrectly, or getting struck by someone or some object.

Read More About ACL Tears

Baker’s Cyst

A Baker’s cyst can cause knee pain. A Baker cyst is swelling caused by fluid from the knee joint protruding to the back of the knee. They are NOT a true cyst since it has communication with the synovial sac. They typically arise from degenerative changes or injury to the articular cartilage (arthritis) or meniscus. At the Centeno-Schultz Clinic, we believe that baker’s Cysts are simply a barometer of the health of the knee joint. In a healthy knee, there are absent whereas with injury and degenerative changes they are common. They arise between the tendons of the medial head…

Read More About Baker’s Cyst

Bone Spurs In The Knee

Bone spurs, also known as osteophytes, are abnormal growths that can form along any bony surface in the body. They most frequently occur where tendons and ligaments attach to the bone. As a result, they are more commonly seen in large mobile joints that support weight, such as the hips, spine, ankles, or knees. Surprisingly, most bone spurs are relatively benign but are a clinical sign of instability in the area. Research demonstrates that if a ligament has laxity, the constant strain at the insertion into the bone will elevate the surface of the bone, creating additional bone formation – this is known as a traction osteophyte.

Read More About Bone Spurs In The Knee

Iliotibial Band Syndrome (ITBS)

Also known as “IT Band Syndrome” also known as “ITB Syndrome,” iliotibial band syndrome is a painful medical condition that affects the lateral hip, leg, and knee. It can affect individuals of all ages and most often is caused by repetitive activities like running, cycling, hiking, and walking. Your iliotibial band is a thick band of connective tissue that runs from the outside of your hip down to the outside aspect of your knee. Its principal function is to stabilize the hip and knee. If it becomes tight and dysfunctional, you may experience pain along with this band of tissue due to strain or inflammation. You may also experience pain, limited range of motion in…

Read More About Iliotibial Band Syndrome (ITBS)

Knee Arthritis

In the human body, a joint is simply where 2 ends of bone come together. At the ends of these bones, there is a thick substance called “Hyaline Cartilage” that lines the ends. Hyaline cartilage is extremely slippery which allows the two ends of the bone to slide on top of each other. Then there is a capsule that connects the two ends filled with “synovial fluid” that acts as a further lubricant to make it more slippery! Arthritis in the knee is defined by loss of the hyaline cartilage plus other changes that happen to the bone such as additional bone being laid down (bone spurs/osteophytes). The cartilage layer is worn down to the point of exposing the underlying bone they cover…

Read More About Knee Arthritis

MCL tear

The medial collateral ligament AKA MCL is a thick, strong band of connective tissue on the inside portion of your knee. It connects the top part of the tibia (shin) to the bottom part of the femur (thigh). This is a vital ligament that works along the lateral collateral ligament (LCL), anterior cruciate ligament (ACL), and posterior cruciate ligament (PCL) to bring stability, structure, and movement to the knee. The MCL provides support and stability for the inside (medial) aspect of the knee. MCL tears are a common injury in sports such as football, hockey, and skiing. The ligament can…

Read More About MCL tear

Meniscus Tears

The meniscus is a c-shaped piece of cartilage in the knee that functions as an important shock absorber. It is sandwiched between the thigh and shin bone. There are two menisci per knee. One on the inside portion of the knee (medial) one on the outside aspect (lateral). The knee meniscus is susceptible to injury. The most common injury is a tear in the meniscus. Not all meniscus tears however cause pain. When symptomatic a meniscus tear can cause pain, swelling, and restriction in range of motion. Tears in the knee meniscus can arise from trauma or degeneration. There are many different types of meniscus tears based upon locations….

Read More About Meniscus Tears

Patellar Tendon Tear

The Patellar tendon is the thick connective tissue that starts at the base of the kneecap (Patella) and extends down to the shin.  The is an extension of the Quadriceps tendon (1).  The Quadricep is the large thick muscle that is often referred to as our thigh. The Patellar tendon works together with the Quadriceps muscle to straighten (extend) the knee. It is easy to touch your Patellar tendon as it is immediately below the knee cap. There are many different causes of Patellar tendon tears.  Patellar tendon tears are a common sport injury but can also occur from overuse or a motor vehicle injury…

Read More About Patellar Tendon Tear

Patellofemoral Syndrome

The kneecap is also known as the patella. The thigh bone is the femur. The patellofemoral joint is formed by the kneecap and the grooved surface of the thigh bone. The patella slides up and down in a grooved track in the femur. The groove is more specifically called the trochlear groove. Like a train that travels on a track, under ideal conditions the patella tracks up and down in the trochlear groove. What is Patellofemoral Syndrome? Patellofemoral syndrome is a medical condition characterized by discomfort in the front of the knee and around the patella. Patellofemoral syndrome may also be known as “jumper’s knee” or “runner’s knee.”…

Read More About Patellofemoral Syndrome
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In Conclusion

Flexion and extension are normal knee motions.  There are 7 major causes that prevent your knee from straightening.  These include meniscus tears, quadriceps tendon injury, patellar tendon injury, ACL injury, acute swelling, osteoarthritis, patellar dislocation, and muscle imbalance.  Multiple episodes of inability to straighten your knee warrants concern and evaluation.  Surgeons provide surgical solutions that are not always in your best interest.  A physician trained in Interventional Orthopedics can provide you with the best nonsurgical, non-steroid treatment options. Identifying the underlying cause is critical for the best clinical results.  The Centeno-Schultz Clinic physicians are experts in the treatment of knee injuries.  Treatment options include PRP and stem cell injections.

Skip the Knee Surgery!

Being unable to straighten your knee is a warning sign.  Act now before the problem gets bigger and the options are limited. Schedule a Telemedicine consult with a board-certified physician who can review your history, studies, and provide you with the best nonsurgical treatment option.  Listen to your body as it is warning you!

1. Mordecai SC, Al-Hadithy N, Ware HE, Gupte CM. Treatment of meniscal tears: An evidence based approach. World J Orthop. 2014;5(3):233-241. Published 2014 Jul 18. doi:10.5312/wjo.v5.i3.233.

2.Snoeker BAM, Bakker EWP, Kegel CAT, Lucas C. Risk factors for meniscal tears: a systematic review including meta-analysis. J Orthop Sports Phys Ther 2013;43:352-367.

3.Flanigan D.C., Troyer J., Everhart J.S., Uribe J.W., Wherley E., Yagnik G.P. (2017) Extensor Mechanism Injuries: Quadriceps and Patellar Tendon Ruptures. In: Eltorai A., Eberson C., Daniels A. (eds) Orthopedic Surgery Clerkship. Springer, Cham.

4.King D, Yakubek G, Chughtai M, et al. Quadriceps tendinopathy: a review, part 2-classification, prognosis, and treatment. Ann Transl Med. 2019;7(4):72. doi:10.21037/atm.2019.01.63

5.. Kim S, Bosque J, Meehan JP, Jamali A, Marder R. Increase in outpatient knee arthroscopy in the United States: a comparison of National Surveys of Ambulatory Surgery, 1996 and 2006. J Bone Joint Surg Am. 2011;93(11):994-1000.DOI: 10.2106/JBJS.I.01618.

6.Jain NP, Khan N, Fithian DC. A treatment algorithm for primary patellar dislocations. Sports Health. 2011;3(2):170-174. doi:10.1177/1941738111399237.

7.Collins JE, Losina E, Marx RG, et al. Early Magnetic Resonance Imaging-Based Changes in Patients With Meniscal Tear and Osteoarthritis: Eighteen-Month Data From a Randomized Controlled Trial of Arthroscopic Partial Meniscectomy Versus Physical Therapy. Arthritis Care Res (Hoboken). 2020;72(5):630-640. doi:10.1002/acr.23891

8.Heidari B. Knee osteoarthritis prevalence, risk factors, pathogenesis and features: Part I. Caspian J Intern Med. 2011;2(2):205-212.

9.Yoshimura N, Muraki S, Oka H, Kawaguchi H, Nakamura K, Akune T. Association of knee osteoarthritis with the accumulation of metabolic risk factors such as overweight, hypertension, dyslipidemia, and impaired glucose tolerance in Japanese men and women: the ROAD study. J Rheumatol. 2011;38(5):921-930. doi:10.3899/jrheum.100569.


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