Knee pain when bending is a common problem. Forces up to seven times body weight can go through the knee as it bends, so it comes as no surprise that it is such a widespread problem.
There are a number of different causes of knee pain when bending, but they are generally all result from a problem with one (or more) of the structures in or around the knee affecting how it moves. This changes how the forces travel through the different parts of the knee and can result in too much pressure going through certain parts, which causes pain. The pain may start suddenly after an injury, or gradually come on over time.
The knee is a hinge joint, which means its main movement is bending and straightening (aka flexion and extension). As the knee moves, a number of things are happening:
1) Muscles: They work in pairs with one relaxing and the other contracting (tightening) to allow the movement e.g. to bend your knee you hamstrings contract and you quads relax
2) Bones: The femur (thigh) and tibia (shin) have to roll, glide and spin to move smoothly. The patella (kneecap) shifts, tilts and rotates as it glides up and down through a groove on the thigh bone – the position and movement of the patella is controlled by the surrounding muscle.
3) Cartilage: The special shock-absorbing cartilage that lines the knee (known as the meniscus) moves and changes shape slightly to prevent it from getting trapped
4) Ligaments: These help control the movement and maintain stability by preventing excessive movement of the bones
These structures all need to work together properly to allow for smooth, pain-free movement. Problems in any of these areas can lead to knee pain when bending. For example, if there is muscle weakness or ligament instability, the bones may move slightly out of position causing them to catch or grind on other structures in the knee.
Knee pain when bending can be split into two types:
1) Weight-bearing: when there is weight going through the knee as you bend it such as when you squat down, go up and down stairs and as you sit down in a chair
2) Non weight-bearing: when there is no weight going through the knee when you bend it e.g. when sitting in a chair and moving the knee or standing but with the leg lifted off the floor as you bend it
The pain tends to be worse when there is weight going through the knee as you bend it due to the compression and pressure on the different structures in the knee. This should come as no surprise when we realise how large the forces going through different parts of the knee. For example, when bending the knee to climb stairs, a force approximately three times body weight goes through the knee. When squatting down the force is even greater at seven times body weight and when jumping, a huge force ten times body weight goes through part of the knee.
So let’s look at the most common causes of knee pain when bending.
Don’t be fooled by the name. This common cause of knee pain when bending is just as likely to affect office workers as runners! It is also (more accurately) know as anterior knee pain or patellofemoral pain syndrome.
What Is Going On: A problem with how the knee cap moves places extra stress and friction on the cartilage as the knee bends and straightens
Causes: Muscle weakness and/or tightness or altered biomechanics such as foot arches and hip angle
Symptoms: Knee pain when bending the knee is the most common symptom and tends to be at the front of the knee, just below and to the sides of the kneecap. It may occur during bending activities such as squatting or coming down stairs, or when you first move the knee after sitting down for prolonged periods e.g. driving or watching TV. Another common feature is a grinding or grating sound as you bend and straighten the knee.
Onset: Symptoms develop gradually
Bursa are small, fluid-filled sacs that site between bone and soft tissues to reduce friction. If there is excessive friction on the bursa they become inflamed. As the knee moves, the pressure on the inflamed bursa which in itself can cause pain, but it can also irritate the surrounding structures causing further pain. There are two bursa in the knee that particularly get irritated by knee flexion.
What Is Going On: Inflammation on the prepatellar bursa found just below the kneecap at the front of the knee
Causes: Prolonged/repetitive kneeling e.g. housework or tradesmen work, a blow to the knee e.g. falling onto the knee
Symptoms: front knee pain when bending and straightening the knee, swelling at the front of the knee
Onset: usually gradual unless there was a direct force through the knee
What Is Going On: Swelling of the semimembranosus bursa at the back of the knee
Causes: Excess fluid in the knee joint seeps back into the bursa and collects resulting in swelling. Usually caused by arthritis but can develop with any condition that causes swelling in the knee joint e.g. a cartilage tear or gout
Symptoms: Stiffness and knee pain when bending and straightening the knee, worse with activity and better with rest. People can often feel a small bulge at the back of their knee that feels like a water balloon
Onset: Symptoms develop gradually
The knee has four ligaments that help control the stability of the knee. The cruciate ligaments sit inside the joint and the collateral ligaments are found either side of the knee
What Is Going On: One of the knee ligaments gets overstretched and tears. There are three different grades of knee sprain depending on how much of the ligament is damaged
Causes: Excessive force through the knee or sudden twisting or the knee
Symptoms: Pain with activity and difficulty bending and straightening the knee. Instability of the knee and swelling. The more severe the injury, the more severe the symptoms
Onset: Immediate or within a couple of days of the injury
The meniscus is a special, extra thick layer of cartilage that lines the knee joint to provide cushioning and protection to the knee. It also helps to transmit the forces through the knee correctly
What Is Going On: Tearing or fraying in the meniscus results in inflammation, and decreased cushioning of the joint. In some instances the torn fragment of cartilage can get stuck in the joint limiting movement
Causes: Wear and tear or sudden twisting of the knee when the foot is fixed
Symptoms: Knee pain when bending especially going up stairs or squatting down. Knee movement may be limited and in some cases the knee gets stuck when the loose fragment gets locked in the joint. The knee will most likely be swollen
Onset: May develop gradually from wear and tear or suddenly after an injury such as twisting the knee when the foot is fixed
Arthritis means “inflammation of a joint” and affects millions of people worldwide. There are over 200 different types of arthritis, but the one that most commonly causes knee pain when bending is osteoarthritis
What Is Going On: Osteoarthritis is where there is thinning and wear and tear of the cartilage lining the knee joint, and the formation of bony lumps in the joint. These result in excessive friction through the knee bones
Causes: There are a multitude of causes of arthritis including previous damage to the knee, genetics, being over-weight and altered biomechanics changing the way the forces travel through the knee
Symptoms: Knee pain, particularly first thing in the morning or after prolonged sitting when you then try and bend your knee. There is often so much knee pain when bending or straightening your knee fully that the joint starts to stiffen. The pain tends to ease with gentle movement but gets worse if you do too much, especially on a bent knee such as stairs or bending down.
Onset: Symptoms develop gradually over time and often come and go. Osteoarthritis most commonly affects people over the age of 65
Other less common causes of knee pain when bending include
1) Patellar Tendonitis: Inflammation and micro-tears in the patellar tendon just below the kneecap, usually after prolonged jumping or kicking movements. The most defining feature of patellar tendonitis is that it is painful if you press on the tendon
2) Iliotibial Band Syndrome (ITBS): Irritation and inflammation of a thick fibrous band on the outer side of the thigh and knee. It usually develops from overuse and most frequently affects runners. The pain tends to be on the outer side of the knee and there may be a popping/grinding sensation as you bend the knee
3) Osgood Schlatters: A common cause of knee pain in teenagers usually associated with sudden growth spurts where the bones grow quicker than the soft tissues leading to excessive tension through the patellar tendon which damages the underlying bone.
The best treatment will depend on the under lying cause of your knee pain when bending. You will find loads more information on the different conditions we have looked at including loads of great treatment advice by using the links above.
Here we have looked at the most common causes of knee pain when bending, but almost any problem with the knee can result in pain as you move it. If none of these is sounding quite right, visit the knee pain diagnosis section for more help work out what is wrong with your knee.