One knee pain, the possible causes are trauma, knee ligament or meniscus injury, osteoarthritis, bone tumor and so on. 1. Trauma: Knee joints are hit by external forces or sprained in a fall, which can cause acute injury to the joint structure and lead to knee pain on one side, and there is a history of obvious trauma. 2. Knee ligament or meniscus injury: Knee ligament or meniscus injury can cause obvious pain in the knee joint, mostly seen in teenagers during strenuous exercise, excessive twisting of the knee joint can cause ligament tear, rupture, meniscus tear, etc.; Chronic wear and tear is mostly seen in manual laborers who need to bear weight or walk for a long time. 3. Osteoarthritis: unilateral knee osteoarthritis is mostly secondary, which can be secondary to intra-articular fracture, dislocation, joint instability caused by ligamentous laxity, and inversion or eversion deformity of the knee joint. 4. Bone tumor: bone tumor is relatively rare in clinic, and those causing persistent pain are mostly malignant, such as osteosarcoma, chondrosarcoma, Ewing’s sarcoma, etc., which are mainly manifested as persistent pain in the knee joints, which is obvious at night, and accompanied by local soft tissue lumps. In addition to the above diseases, the causes of knee pain on one side may also include ankylosing spondylitis, septic arthritis, joint tuberculosis, etc. It is recommended to go to the hospital as soon as possible to find out the causes of the disease and give targeted treatment.