What is the reason why my knee suddenly hurts and I can’t bend it?

Sudden pain in the knee that affects movement and bending is actually quite common in clinical practice, especially with arthritis. Other muscle diseases may also cause this symptom, so the key is to first understand the cause and focus on symptomatic treatment.

1. The initial onset is slow, and the patient first has prodromal symptoms such as fatigue, weight loss, poor appetite, low fever, and numbness and tingling in the hands and feet for several weeks to months. Subsequently, pain and stiffness will occur in a certain joint, and then the joint will swell and become increasingly painful. Initially, one or two joints may be affected, often in a migratory manner. It later develops into symmetrical polyarthritis, and the joint involvement often starts from the small joints at the distal end of the limbs and then affects other joints. The proximal interphalangeal joints are most commonly affected, showing spindle-shaped swelling; followed by the metacarpophalangeal, toe, wrist, knee, elbow, ankle, shoulder and hip joints. Morning joint stiffness and muscle soreness can be relieved after moderate activity.

2. The degree and duration of stiffness are often consistent with the activity of the disease and can be used as an assessment of the activity of the disease. Due to joint swelling and pain and restricted movement, the stiffness and atrophy of the muscles near the joints become increasingly noticeable. Even if the acute inflammation disappears later, the tissues around the joints will become stiff due to the proliferation of fibrous tissue in the joints.

3. As the disease progresses, the patient will have irregular fever, rapid pulse, and significant anemia. The affected joints eventually become stiff and deformed, with the knees, elbows, fingers, and wrists fixed in a flexed position. The fingers often become subluxated outward at the metacarpophalangeal joints, forming a characteristic ulnar deviation deformity, and the patient needs assistance in daily life. Patients with more affected joints have to stay in bed all day, unable to move and in extreme pain.

4. About 10% to 30% of patients develop subcutaneous nodules that are as hard as rubber on the protruding parts of the joints, such as the olecranon process of the upper limbs, the wrists, and the ankles of the lower limbs. Subcutaneous nodules are not easily absorbed, and their appearance often indicates that the disease is in a severely active stage.