A stress fracture is a crack in a bone caused by repeated stresses which are individually insufficient to fracture it. Stress fractures are not full-thickness breaks (although without correct management they may progress to become full-thickness breaks). They may involve the bony cortex only, and show up poorly on X-ray. They are sometimes referred to as hairline fractures.
This is a companion discussion topic for the original article at https://patient.info/doctor/orthopaedics/stress-fractures