Critical injury. Fractures that are considered critical would have an immediate affect on a persons life. This could be a fracture in the cervical spine, a fracture that tears a major blood vessel, a rib fracture that punctures a lung or a skull fracture.
Life-threatening. Any fracture that leads to the end-of-life would be a "critical injury". Mostly this includes spine or pelvic injuries. The spine injury is critical because its not only related to paralysis, but it usually is associated with other major injuries. Pelvic fractures are similar, but they also are related to injury of blood vessels that can lead to major hemorrhage (blood loss) and thus end-of-life.
Severity depends. Entirely on location, displacement, joint involvement, bones fractured, compound or closed, and involvement of adjacent blood vessels, nerves, and soft tissue. I don't think it is critical in the sense that you probably will not die from it, but any fracture could be serious. In the lower extremity, amputation is always possible. Ask your doctor for specific recommendations.