Pain within the bend of the knee can be caused by osteoarthritis of the knee, meniscus injury, popliteal cysts, etc. 1. Osteoarthritis of knee joint: degeneration of knee joint cartilage, osteophytes can cause chronic osteoarthritis of knee joint, most of them will be worn out in the medial gap of knee joint, and the local bone is exposed and hardened, which can cause pain in the inner side of knee bend. 2. Meniscus injury: trauma or sports injury can lead to meniscus injury on the medial side of the knee, which can cause pain, bruising, swelling and restriction of functional activities on the medial side of the knee. 3. Popliteal cyst: after knee joint strain, joint inflammation penetrates into the popliteal fossa, which can form a cystic swelling locally, causing pressure on the medial side of the knee, resulting in pain, swelling and joint stiffness in the medial and posterior side of the knee and other parts of the knee. When there is pain in the bend of the knee, one should consult a doctor in a timely manner and take appropriate treatment under the guidance of a specialist, which will help to alleviate the pain and discomfort as soon as possible.