Knee Pain When Bending and Straightening: Possible Causes

The knee is a hinge that bears the body's weight and bends dozens of times a day. Knee pain can be a symptom of many conditions. Below is a review of possible causes and the line beyond which a doctor is needed.

How the knee is structured and why pain occurs during movement

The joint connects three bones: the femur, the tibia, and the patella. Between them lie cartilage and two crescent-shaped menisci, around them are ligaments and synovial fluid, and movement is controlled by muscles and tendons. The patella works as a lever: through it, the quadriceps femoris muscle transmits force to the lower leg. Flexion and extension together provide about 151 degrees, and in the bent position a slight rotation of the lower leg is possible — about 15 degrees. The joint contains synovial bursae, whose number and size are individual. When any of these structures is damaged or altered by disease, the load passes through the affected area, and movement at a certain angle begins to cause pain.

Pain when bending the knee: common causes

If there was no injury, knee pain can also be a symptom of many conditions, and a doctor helps determine the exact cause.

• Osteoarthritis — the most common joint disease: cartilage gradually wears away, pain, stiffness, and mild swelling appear, more often in older people and usually in both knees.
• Bursitis — the bursa becomes inflamed, the skin over it is warm and red, and kneeling and bending increase pain and swelling.
• Tendinitis — pain between the patella and the lower leg, characteristic of repeated running and jumping loads.
• Osgood-Schlatter disease — in adolescents and young adults, the area below the patella is painful and swollen.
• Gout or infectious arthritis — the joint is hot and red, and attacks of very severe pain occur suddenly.

Pain after knee injury: instability and clicking

If pain appeared after an injury, the picture may be different. A tear of a ligament, tendon, or meniscus, or cartilage damage, causes instability: the leg gives way when a person stands up, the knee does not straighten fully, and a click or pop may occur at the moment of injury. Cruciate ligaments tear during extreme movements: the anterior — from sudden twisting or a blow to the back of the bent knee, the posterior — from sudden extension of the lower leg or a direct blow to its front surface. A tear is accompanied by sharp pain and bleeding into the joint cavity: the knee rapidly enlarges, and swelling increases. Sometimes the person does not notice the moment of injury itself, and later a feeling of looseness appears.

Acute pain, clicking, and limited movement: what they mean

Clicking without pain is normal; the joint does not work silently. It is more concerning when the click is painful, the knee locks, gives way, or changes shape — a doctor determines the cause of such symptoms. With a cruciate ligament tear, sharp pain and bleeding into the joint occur. If the knee does not bend fully for weeks, a doctor should also investigate this.

How the cause of knee pain is diagnosed

It all starts with an examination and history: when the pain appeared, what preceded it, which angles of movement are affected. The doctor checks joint stability using techniques that shift the lower leg forward and backward — this is how the "drawer sign" is assessed. Further, X-ray, computed tomography, or magnetic resonance imaging may be used, and in complex cases arthroscopy, when a probe is inserted into the joint cavity and it is examined from the inside. Sometimes contrast is injected into the joint to clarify the picture. Based on the results, the doctor names the cause and decides whether a specialist's help is needed.

When to see a doctor

If the pain does not go away within a few weeks, there is no reason to postpone the visit. Come earlier — with severe pain, inability to move the knee or bear weight on the leg, marked swelling, a change in the shape of the joint, locking and giving way, and also with a high temperature accompanied by redness and heat around the knee: this is how an infection may manifest.

Knee pain by itself is not a reason to call an ambulance; 103/112 — for a severe injury threatening life (for example, a serious accident).

The material is for reference: it describes possible causes but does not replace a doctor's examination.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer