Medial Meniscus Tear: The most common cause of medial knee pain, often resulting from a twisting injury or degenerative changes. The meniscus is a cartilage structure that can be damaged, leading to pain and mechanical symptoms.

Ligament Sprain (MCL or LCL): Medial or lateral collateral ligament sprains can cause pain on the respective sides of the knee. These injuries often occur from a valgus (inward) or varus (outward) stress to the knee.
Osteoarthritis: Wear and tear of the joint cartilage can lead to pain in the knee, often more pronounced in weight-bearing areas such as the medial and lateral compartments.
Iliotibial (IT) Band Syndrome: Friction of the iliotibial band against the lateral femoral condyle can cause lateral knee pain, commonly seen in runners or cyclists.
Patellofemoral Pain Syndrome: Pain around or behind the patella, often related to tracking issues of the patella within the femoral groove, can radiate to the medial or lateral aspects of the knee.

Septic Arthritis: An infection within the joint space, which is a medical emergency requiring prompt diagnosis and treatment to prevent joint destruction.
Osteonecrosis (Avascular Necrosis): A condition where the bone tissue dies due to a lack of blood supply, which can cause severe pain and requires early intervention to prevent collapse of the bone.
Tumor (Benign or Malignant): Although rare, tumors can cause knee pain and must be considered, especially if the pain is persistent, worsening, or accompanied by other systemic symptoms.

Plica Syndrome: Irritation of the synovial folds within the knee joint can cause pain and snapping sensations, often mimicking meniscal tears.
Bursitis: Inflammation of the bursae around the knee can cause localized pain and swelling, depending on which bursa is affected.
Neurogenic Pain: Referred pain from nerve compression or damage, such as from a lumbar radiculopathy, can sometimes present as knee pain without a clear orthopedic cause.