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My Knee Hurts When I Bend It and Straighten It: A Quickstart Guide

Do you have daily knee pain despite having no major trauma or precipitating event?  Has physical therapy, rest and NSAIDs failed to provide relief?  It is very frustrating and can keep you on the sidelines.  To learn more please read below to answer the following questions:  What are the different types of knee pain that are associated with bending and straightening? What are the eight most common causes of knee pain?  What common conditions cause your knee to hurt when you bend it or straighten it?  What treatment options are available?  Let’s dig in. 

My Knee Hurts When I Bend It And Straighten It!

Your knees bend countless times throughout the day.  Running up the stairs, down the hall after kids, and getting into the car.   You straighten the knee as you walk, descend stairs or get into and out of the car.  Bending and straightening the knee is necessary for daily activities. Knee pain with bending or straightening may be a mild, transient irritation or may indicate a more significant problem.  Learn more below and avoid further injury and dysfunction. 

How The Pain In The Knee Appears When Bending or Straightening

Knee pain can vary significantly depending upon many factors including the actual source of the pain,  the severity of the injury, general health and level of activity. Let’s take a deeper look at the various types of knee pain.

Knee Pain in General

Knee pain can be intermittent or constant.  It can be stable, improving, or escalating.  It can be localized in a specific area or radiate down or up the leg.   It  can be dull, sharp, throbbing,  aching or burning.  It can be associated with swelling and restriction in range of motion. 

Pain with Joint Locking

Often times the knee will simply lock creating pain and immobility.  You are walking one minute and the next the knee is stuck.  The pain is abrupt and occurs when attempting to bend or straighten the knee.  An injury to the meniscus or a loose body can cause this type of pain (1). 

Pain Sensation Behind the Knee Cap

Pain can be localized behind the knee.  Most often it is made worse with climbing and descending stairs.  The pain can be dull or sharp and persists while descending from a hike.  Knee pain behind the knee cap is typically due to misalignment of the knee, or cartilage loss. 

Jolting Pain Behind The Knee 

Pain in the back of the knee can stop you in your tracks making bending and straightening very difficult at times.  The pain can be sharp and jolting or a dull ache.  Possible causes of pain in the back of the knee include Baker’s cyst, hamstring tendon irritation or inflammation, ligament instability and posterior horn meniscus injury (2) 

Pain on the Outside of the Knee

Pain on the outside of the knee often times is referred to as lateral knee pain.  It can caused by an injury or repetitive activity. The pain can be intermittent or constant.  There are many causes of lateral knee pain which include osteoarthritis, meniscus injuries, ligament laxity and iliotibial band dysfunction.

Pain on the Inside of the Knee

Pain localized on the inside of the knee is quite common.  It can be constant or intermittent.  Often often it occurs at or slightly below the joint line.  Common causes of medial knee pain include osteoarthritis, medial meniscus injury and inflammation of the low leg tendons.  The later is called pes anserine bursitis. 

Popping Sound

Audible popping sounds in your knee can be quite alarming.  It can be abrupt in onset or occur gradually over time. In most cases it is harmless and is the release of small bubbles in the joint or abnormal movement of ligaments and tendons over boney surfaces. If a popping sound is associated with pain and or swelling there is reason to be concerned.  If persistent a consultation with your doctor is indicated. 

Loss of Strength

Persistent loss of strength in the knee warrants concern.  It can be gradual or abrupt in onset.  It can be due to overuse but other causes include ligament instability, osteoarthritis, meniscus tears, and irritation or injury of the nerves in the low back. The well established connection between low back dysfunction and knee pain has been discussed in a prior blog

What Are The Eight Most Common Causes Of the Knee Pain?

Knee pain can arise from a number of conditions.   It is important to understand where your knee pain is arising from.  This will allow for the best treatment plan.  The most common causes of knee pain include:

Cartilage Injury

Cartlage is a smooth coating on the end of your thigh and shin bone that allow smooth, painless movement of your knee.  It is a flexible connective tissue that cushions your bones against the pressures and forces of daily living.  Unfortunately it is susceptible to injury and degeneration.  The most common causes of cartilage damage are wear and tear, repeitive use, and trauma.  

Ligament Injury

Ligaments are thick pieces of connective tissue that connect bone to bone.  They provide important stability for a joint.  Important ligaments in the knee include the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament ( MCL) and lateral collateral ligament (LCL). Ligaments can be injured due to overuse, injury, aging and medical conditions such as diabetes.  Ligament injury can cause the knee to hurt when you bend or straighten it. 

Meniscus Injury

The meniscus is a c shaped shock absorber sandwiched between the thigh bone (femur) and shin bone (tibia). It is composed of fibrocartilage and serves to absorb the forces of daily living in addition to providing stability to the knee (4). There are two menisci per knee:  one on the inside (medial) and one of the outside (lateral).  Meniscus tears are the most common injury.  The can be degenerative in nature or as a result of a acute injuries.  There are many different types of knee meniscus tears which include radial, horizonal, incomplete, flap, buck handle and complex.  Meniscus tears can cause swelling and dysfunction.  Meniscus tears can also cause the knee to hurt when you bend or straighten it.  

Tendon Injury

Tendons are thick pieces of connective tissue that connect muscle to bone. They serve to stabilize and move a given joint.  There are many tendons in the knee which include the patellar and distal quadriceps tendon.  Tendons are susceptible to injury due to overuse, muscle imbalance, trauma, medications such as certain anti-biotics and medical conditions.  


Proper knee function requires that all the muscles, tendons, ligaments, and nerves work together in a synchronmized and harmonic fashion.  When the system breaks down, knee pain, swelling and dysfunction can occur.  

Nerve Injury

Nerves provide important information to the muscles, tendons and ligaments in the knee. Without this information the knee can not properly function.  Nerve injury can occur locally as is the case with peroneal nerve injuries.  Nerve dysfunction can also occur due to problems in the low back such as disc herniation, disc protrusion, spinal stenosis and lumbar disc slippage.  A previous blog has discussed knee pain due to nerve irritation. 

Bone Injury

There are three principal bones in the knee:  patella, femur and tibia.  Bone can become injured due to trauma.  Acute injuries can lead to bone swelling also known as subchondral edema.  Long term bone injury can result in bone death also referred to as avascular necrosis.  Both injuries can cause significant knee pain that often times is most intense at night.  At the Centeno-Schultz Clinic these boney injuries have been successfully treated with injection of PRP and bone marrow concentrate into the bone (intraosseous).  A previous blog discusses it application and success. 

Referred Pain

Referred pain is pain that is perceived or felt in an area different than where the actual tissue damage is occurring. The classic example is that of an patient who is experiencing an heart attack.  That patient may feel arm pain however the actual tissue damage is occuring in their heart.  An example of referred pain into the knee is a irritation of the the L3 nerve in the low back.  A patient may have no back pain but have pain in the front portion of their knee which is the result of low back nerve irritation. 

Common Conditions That Cause Pain When Bending or Straightening Knee

Pain when bending or straightening the knee can sideline you for the weekend or for months at a time.  There are many different causes of knee pain.  It is important that you identify the specific cause of your pain so that a specific treatment plan can be created. There are 5 major conditions that can affect how the functions like difficulties in bending or straightening the knee:

Knee Arthritis

Arthritis is a medical condition with swelling, tenderness, and stiffness in the knee. There are many different types of arthritis with osteoarthritis being the most common. It affects millions of patients worldwide.  Osteoarthritis is a degenerative joint disease where the cartilage that protects the joint wears down over time.  The result is knee pain when bending or straightening it. 


A bursa is a fluid-filled sac that allows tendons, ligaments, and muscles to easily glide on boney areas.  There are multiple bursae in the knee that allow for pain-free motion.  Bursitis is inflammation of these fluid-filled sacs.  Bursitis can cause significant knee pain and limit bending and straightening of the knee. 

Ligament Injuries

Ligaments are the duct tape that keep your knee joint together and stable.  Trauma and repetitive wear and tear can result in injury or degeneration or tearing of knee ligaments.  Common athletic injuries witnessed virtually every weekend include ACL tears.  The Centeno-Schultz Clinic pioneered a safe alternative to surgery utilizing your own bone marrow concentrate which contains your stem cells.  To learn more please click on the video below. 

Jumper’s Knee

Jumper’s knee is inflammation of the patellar tendon.  It is a very common overuse disorder in athletes who participate in sports that require jumping such as volleyball and basketball. The prevalence varies but can be as high as 22.8% in elite athletes (5). Pain is typically localized on or immediately below the knee cap.  Risk factors for a jumper’s knee include body weight, leg length differences, arch of foot height and quadriceps, and hamstring flexibility (6). 

Meniscus Injuries

Meniscus injuries are a common injury with an incidence of 60 per 100,000 individuals (7). Risk factors include age (older than 60 years), gender (males), work-related squatting and kneeling, and climbing stairs (8).  Meniscus tears can cause a number of different symptoms including pain along the joint line, swelling, clicking, locking, and “giving way” of the knee.  It can also cause knee pain when bending and straightening your knee. 

Baker’s Cyst

A baker’s cyst is a fluid-filled sac located on the backside of your knee.  Its size can vary from small to large creating tightness and a bulge in the back of your knee. Most importantly it is an indication that you have a problem that warrants attention.  A baker’s cyst is an increase in fluid in the knee due to an underlying injury.  Examples of issues that can cause a Baker’s cyst includes osteoarthritis, meniscus tear, ligament tear, instability, and gout. 

Runner’s Knee

Runner’s knee is a clinical condition that involves pain around or behind the kneecap, also known as the patella..  It is typically an overuse injury.  This is not exclusive to runners and can affect non-athletes as well. Knee cap pain can arise from several different conditions including loss of cartilage behind the knee (patellofemoral syndrome), poor tracking of the patella, iliotibial band (ITB) syndrome, and quadriceps weakness.

Common Treatments for Knee Pain When Bending and Straightening

Specific treatment will depend upon the specific type of knee injury and its severity. Not all knee injuries are treated the same. Establishing a proper diagnosis is essential so that treatment and rehabilitation are tailored for your specific condition. When appropriate conservative care should be the first line of treatment. Common treatment options include:

Physical Therapy

Physical therapy is a cornerstone in the treatment of most knee injuries.  Therapy will focus on increasing strength, stability, and range of motion. 

Steroid Injections

When physical therapy, rest, and other conservative treatments fail to provide significant benefits some patients are referred for a steroid injection.  Steroids are powerful anti-inflammatory agents which can reduce swelling and pain.  Unfortunately, they are toxic to most orthopedic tissue and have been demonstrated to damage cartilage, tendons, and ligaments (8). Steroids can cause tendon rupture (9). For this reason, steroids should be avoided. 


Surgery is increasingly popular for knee injuries.  There are many different types of surgeries.  Meniscus “repair” surgery is one of the most common and is poorly understood by most patients.  It is recommended to patients who have sustained a tear in the meniscus.  Rarely is the meniscus repaired?  Rather a portion of the meniscus is simply cut out.  This has a profound long-term impact on the knee including:

  • Increased incidence of knee arthritis
  • Recurrent meniscus tears
  • 2 1/2 times more likley to get a knee replacement.

Most importantly a 2013 study demonstrated that meniscus surgery results could not beat physical therapy (9)

To understand meniscus surgery, its consequences, and alternatives please click on the video below.

Regenerative Options

The Physicians at the Centeno-Schultz Clinic in Broomfield and Denver, Colorado are experts in the treatment of knee injuries.  Treatment options include the use of PRP or Bone Marrow Concentrate.  PRP is rich in growth factors that can decrease inflammation and increase blood flow both of which can accelerate healing.  Bone Marrow Concentrate contains your stem cells which are the body’s powerhouses of healing. All injections are performed under ultrasound guidance and or x-ray guidance.  This ensures accurate placement of the PRP or Bone Marrow Concentrate into the area of knee damage. 

In Conclusion

We bend and straighten our knees hundreds of times a day. 

There are various types of knee pain that can present in different areas.  Examples include behind the kneecap, on the inside, on the outside, and in the back of the knee.

The eight most common causes of knee pain are:

  • Cartilage Injury
  • Ligament Injury
  • Tendon Injury
  • Imbalance
  • Nerve Injury
  • Bone Injury
  • Referred Pain

Common conditions that cause knee pain when bending and straightening are:

  • Knee Arthritis
  • Bursitis
  • Ligament injuries
  • Jumper Knee
  • Meniscus Injuries
  • Baker’s Cyst
  • Runner’s Knee

Our non-surgical knee treatments treat all of these conditions.

Treatment options depend upon the specific type of injury and its severity.  Common treatment options include:

  • Physical Therapy
  • Steroid Injections
  • Surgery
  • Regenerative Options

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8.Snoeker BA, Bakker EW, Kegel CA, Lucas C. Risk factors for meniscal tears: a systematic review including meta-analysis. J Orthop Sports Phys Ther. 2013 Jun;43(6):352-67. doi: 10.2519/jospt.2013.4295. Epub 2013 Apr 29. PMID: 23628788.

9. Katz JN, Brophy RH, Chaisson CE, et al. Surgery versus physical therapy for a meniscal tear and osteoarthritis [published correction appears in N Engl J Med. 2013 Aug 15;369(7):683]. N Engl J Med. 2013;368(18):1675–1684. doi:10.1056/NEJMoa1301408


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