- Torn meniscus
- Torn or damaged cruciate ligament
- Torn pieces of articular cartilage
- Inflamed synovial tissue
- Misalignment of the patella
- Baker's cyst: a fluid-filled cyst that develops at the back of the knee due to the accumulation of synovial fluid. It commonly occurs with knee conditions such as meniscal tear, knee arthritis, and rheumatoid arthritis.
- Certain fractures of the knee bones