A partial or complete fracture which results from the inherent inability of bone to withstand stress applied without violence in a rhythmical, repeated, subthreshold manner is a "stress fracture." Because numerous terms have been used in the medical literature to designate the two types of stress fractures, the following classification is recommended. A "fatigue fracture" occurs when abnormal stress is applied to bone with normal elastic resistance. An "insufficiency fracture" is produced by normal or physiological stress applied to bone with deficient elastic resistance. Fatigue and insufficiency fractures occur most frequently in the weight-bearing bones. The term "pathologic fracture" should be limited to any fracture in bone weakened by tumor.
Pentecost RL, Murray RA, Brindley HH. Fatigue, Insufficiency, and Pathologic Fractures. JAMA. 1964;187(13):1001-1004. doi:10.1001/jama.1964.03060260029006