What are the Possible Causes of Knee Pain When Bending My Knee?
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Knee pain can come and go at any time, but some people may experience sudden pain whenever they bend their knee, especially during exercise. There may be several causes of your knee pain whenever you bend or flex the knee.
The Anatomy of the Knee Joint
The knee joint is the largest and most complex joint in the body. It is a hinge joint formed by three bones: the tibia (shinbone), femur (thighbone), and patella (kneecap). The entire joint is surrounded by a knee capsule, comprised of fibrous connective tissue that holds the joint in place, while the synovial membrane within this capsule secretes synovial fluid to lubricate the joint. In front of the femur is the kneecap, which reduces friction between the tendons and the bone while also preventing the tendons from slipping sideways.
The joint’s surfaces are also covered in cartilage, a tough, flexible tissue that protects the bones from friction during movement. The cartilage is also supported by synovial fluid, which can nourish the cartilage, and by two half-moon-shaped menisci, which absorb shock during movement and distribute weight evenly across the joint.
The knee joint is also attached to several ligaments and muscles that stabilise it and support its mobility and flexibility. The main ligaments are the anterior and posterior cruciate ligaments, which support the knee when it is bent, while the medial and lateral collateral ligaments provide rotational stability, preventing side-to-side movement.
As for muscles, the quadriceps muscles at the front of the thigh help extend, or straighten, the knee, while the biceps femoris at the back of the thigh assists with bending. Other muscles support other movements, such as the hamstring muscles that work together to flex your knee and the popliteus, which unlocks the knee at full extension.
How Your Knee Joint Works
The muscles and tendons are responsible for much of the knee’s movement, especially back-and-forth motion (medically known as flexion-extension). They are attached to different parts of your knee joint and to other areas of the connected bones. The quadriceps muscles regulate your knee’s extension and stabilise your patella, while the hamstring muscles support knee flexion.
When the muscles contract, the attached tendon(s) are pulled, moving the bone to flex or extend the knee. Conversely, when the muscles relax, the tendons relax, allowing the bone to move in the opposite direction. As this occurs, the ligaments and other structures help guide proper knee movement and prevent abnormal motion. Meanwhile, the kneecap’s primary role is to protect the knee joint during movement, as it moves with the tibia along the trochlear groove.
A healthy knee can move to 150 degrees (where your calf touches the back of your thigh) from a straightened, neutral position, and may also be turned towards the other leg by about 10 degrees, and outward by approximately 30 degrees. Certain postures or movements may reduce the knee’s range of motion, such as bending your hip and knee simultaneously.
Common Causes of Knee Pain When Bending
We rely on our knee joints daily for various activities involving movement, and there may be instances when knee pain suddenly occurs while flexing or extending the knee. Some of the more common causes may include:
Injury
Direct injury or trauma to the knee joint or its surrounding structures could lead to pain, discomfort, and reduced range of motion when bending the knee. Some common injuries include meniscus tears, ligament injuries (such as an ACL tear), and tendinitis, all of which may arise from sports or work injuries, overuse from intense physical activity, or a medical condition.
Patellofemoral Syndrome
Patellofemoral syndrome is characterised by pain in the front of the knee and around the kneecap. While it may be caused by overuse injuries, it may also stem from patellar misalignment, in which the kneecap moves out of position when the knee bends, irritating nearby soft tissues. This misalignment may result from alignment issues in your legs or from muscular weakness.
Knee Bursitis
Pain or stiffness in the knee joint may also be due to inflammation of the bursae, which are small fluid-filled sacs that cushion surrounding structures near the knee joint. This inflammation can occur due to constant pressure from repeated motions that may irritate the bursae. Other causes may include infection or inflammatory arthritis that could also irritate the bursae.
Osteoarthritis
Osteoarthritis is caused by the gradual wear and tear of the knee joint, leading to stiffness, pain, and inflammation. The condition occurs slowly over time, and symptoms may gradually worsen as your knee joint progressively loses cartilage. Over time, it may cause bone-on-bone contact and contribute to pain, stiffness, and other symptoms whenever you try to move your knee.
Baker’s Cyst
Also known as a popliteal cyst, a Baker’s cyst is a fluid-filled swelling that forms at the back of the knee. This cyst is formed from excess synovial fluid that was overproduced by the knee, which can only drain from the back of your knee joint. Over time, it may result in a buildup of fluid that forms a growth at the back of the knee, causing pain, stiffness, and reduced flexibility in your knee.
Plica Syndrome
This condition affects the plica, a small fold in your synovial membrane. It commonly affects athletes, but others can also develop this syndrome due to an injury to the knee. In most cases, the plica in the middle of your knee (the medial plica) becomes irritated, leading to pain, swelling, and instability.
Chondromalacia Patellae
This condition is characterised by the gradual softening and breakdown of cartilage in your knee joint, specifically the cartilage on the underside of the kneecap. The softened cartilage can slowly crack or shred into fibrous fragments that irritate nearby joint structures, or wear away entirely, leading to painful bone-on-bone contact.
Symptoms to Watch Out For
Knee pain when bending may lead to a number of symptoms, some of which may interfere with your ability to work or play. It helps to look out for certain symptoms that can severely impact your daily life, and to seek prompt medical treatment in such instances.
Pain When Bending the Knee
Under most circumstances, pain when you bend your knee may suggest an injury to your knee joint or its surrounding structures. Rest, medication, and other conservative treatments may be sufficient to manage this pain, but if the pain persists despite treatment, it could suggest an underlying condition or a more substantial injury. The source of the pain may help inform doctors about the potential cause and support them in fine-tuning their diagnosis based on other accompanying symptoms.
Sudden, Sharp Pain While Bending the Knee
Sudden, sharp pain that lingers or disappears when you bend your knee is generally considered a warning sign that suggests an underlying condition or complication. One possible cause may be patellar tendinitis, in which the tendon connecting your kneecap to your shinbone has become inflamed or sustained a tear or rupture. It may be wise to seek medical attention immediately if this occurs.
Grinding, Clicking, or Popping Sounds
There are instances where grinding, clicking, or popping sounds occur when your knee bends, but you do not experience any pain. Occasionally, this may happen due to air bubbles that form in the soft tissue around the knee joint and pop when your knee bends. It may also be due to cartilage rubbing against the soft tissue of the knee joint during movement.
However, if bending your knee also causes these sounds along with pain and discomfort, it could suggest that the knee cartilage has worn down significantly and the bones are now rubbing directly against each other. This may stem from osteoarthritis or chondromalacia patellae, both of which cause a gradual breakdown of cartilage.
Weight-bearing Difficulties
If standing or moving causes difficulties, including a feeling of instability in your knee, it could mean that your knee joint is injured or has an underlying condition. Difficulty with weight-bearing, especially if you regularly maintain a healthy weight, generally points to osteoarthritis or a bone fracture, contributing to pain, swelling, and instability when bending your knee. It warrants an immediate medical examination to determine the exact cause and treat it promptly before it worsens.
Obvious Knee Swelling
Fluid buildup in and around your knee joint could lead to noticeable swelling that may also be accompanied by other symptoms, including pain and discomfort. It may also point to a possible torn ligament, a tear in the meniscus or synovial membrane (which can cause fluid to leak into the knee joint), or other underlying causes, like bursitis.
Signs of Infection
If you suddenly develop a persistent fever (which lasts longer than 24 hours), chills, and nausea, your knee joint may have become infected. Infection can occur when bacteria invade the synovial membrane and contaminate the synovial fluid. This may be due to bacteria entering your body through an open wound on your knee, or from infections elsewhere in the body that spread to the knee.
When to See a Professional to Treat Knee Pain
While knee pain may come and go, if it persists and presents other complications like pain, swelling, and sudden knee instability, it is best to seek immediate medical attention to address the condition before it can worsen. A qualified doctor can perform a thorough medical examination to assess your condition, determine the root cause of your symptoms, and prepare a suitable treatment plan to support your recovery and reduce the risk of side effects or other concerns.
Contact Avant Orthopaedic today to learn more about our chronic knee pain management and treatment options.
Dr. Hope Ang
MBBS (NUS) | MMed (Anaes) | FAMS
Dr Hope Ang is a Consultant Anaesthesiologist with extensive expertise in acute and chronic pain management, dedicated to helping patients regain function, reduce suffering, and improve quality of life through evidence-based, personalised care.
With more than 18 years of clinical experience, Dr Ang specialises in the assessment and treatment of complex chronic pain conditions, including musculoskeletal pain, neuropathic pain, cancer-related pain, and persistent postoperative pain. His approach integrates interventional pain procedures, ultrasound-guided regional techniques, multimodal pharmacological strategies, and close collaboration with multidisciplinary care teams to address both the physical and functional impact of chronic pain.
Dr. Ramesh Subramaniam
MBBS (NUS, SG) | MRCS (Edinburgh, UK) | M Med (Ortho) (NUS, SG) | FRCS (Ortho) (Edinburgh, UK)
Dr Ramesh, a fellowship-trained orthopaedic surgeon, focuses on shoulder, knee, and arthroscopic sports surgery.
Dr Ramesh obtained his MBBS and Master’s in Orthopaedic Surgery from the National University of Singapore and completed Advanced Surgical Training. He earned his Fellowship from the Royal College of Surgeons of Edinburgh in Trauma and Orthopaedics. He was awarded the HMDP scholarship for a clinical fellowship in Shoulder and Knee Surgery in Dublin, followed by the Tan Tock Seng Scholarship for a Sports Fellowship in Knee Surgery in Barcelona. He also completed a Shoulder Surgery Fellowship at Hospital Del Mar and was trained in arthroscopic joint reconstruction and joint replacement surgeries.
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