Costochondral injury or contusion: This is the most likely diagnosis given the mechanism of injury (being kneed in the chest) and the presentation of rib pain. The force from the knee can cause bruising or injury to the costochondral junctions, leading to pain.

Rib fracture: A direct blow to the chest, such as being kneed, can cause a fracture of one or more ribs. This would be a common concern in the context of trauma to the chest.
Muscle strain: The muscles of the chest wall, including the intercostal muscles, can be strained from the impact, leading to pain.
Soft tissue injury: Bruising or injury to the soft tissues of the chest wall can also occur from a direct blow.

Pneumothorax: Although less likely, a pneumothorax (collapsed lung) is a potentially life-threatening condition that can occur after chest trauma. It's crucial to consider this diagnosis, especially if the patient shows signs of respiratory distress.
Hemothorax: Accumulation of blood in the pleural space can occur after significant chest trauma and is another condition that could be deadly if missed.
Cardiac contusion: A blow to the chest can also injure the heart, leading to a cardiac contusion, which may present with chest pain, arrhythmias, or signs of heart failure.

Sternal fracture: While possible, fractures of the sternum are less common than rib fractures and might require a significant amount of force.
Diaphragmatic rupture: This is a rare but serious condition where the diaphragm is torn, which can lead to abdominal organs herniating into the chest cavity.
Osteochondritis dissecans of the ribs: A rare condition where a piece of cartilage and a thin layer of the bone beneath it become damaged due to inadequate blood supply, potentially caused by trauma.