Fatigue fracture is the failure of a structure as a result of cyclic stresses whose intensity is within the elastic domain and far less than the ultimate tensile or compressive strength of that structure.

Pathologic fracture is a break in a bone as a result of a local or generalized weakness of the bone related to some disease affecting the material properties of the bone (osteoporosis, osteomalacia) or the structural integrity of the bone (osteolytic metastasis destroying part of the bone, screw hole after removal of an osteosynthesis plate, inadequate anatomy of part of the bone).

The fracture resistance of a structure exposed to bending and torsion stresses depends not only on the mechanical properties of its constituting material but also on its shape and design. The mathematical expression of this property is the polar moment of inertia. The polar area moment of inertia of a beam’s cross-sectional area measures the beam’s ability to resist torsion. The larger the polar moment of inertia, the less the beam will twist.

This is applicable to the pars interarticularis of a vertebra. The influence of the shape of the pars on its mechanical resistance is yet to be defined. There must be an optimal anatomical shape and dimension of the pars to provide adequate mechanical resistance to the stresses that are transferred through the pars. Patients with a less appropriate pars anatomy will be more prone to present spondylolisthesis. Only an anatomical study on a wide sample of vertebrae of normal and spondylolytic vertebrae can confirm this hypothesis. The importance of this factor is clearly suggested by the biomechanical studies of Cyron and Hutton (6).

Sports involving repetitive flexion, extension, or rotation of the spine are known to be associated with a high incidence of spondylolysis. These are the throwing sports (26.7%), gymnastics (11% to 17%), rowing (17%), and football, weight lifting, wrestling, diving, tennis, and volleyball (7,8). In a large radiographic study in an adolescent, athletic population, Rossi found a 15% incidence of spondylolysis (9).

Common sense leads us to suspect that the physiopathology of a spondylolysis occurring in a sedentary patient with no history of uncommon demands on his spine is not the same as that observed in the competitive weight lifter or gymnast. In the former group, a predominant “pathological fracture” mechanism is much likelier (material deficiency or/and structural deficiency: inadequate shape of the pars). In the athletic population, the tentative physiopathology of the constitution of pars defect is that of a true fatigue fracture. The repetitive, mechanical stresses on one or both pars of a vertebra imposed by the regular practice of the considered sport leads to a stress-induced reaction according to Wolff’s law. The bone in the pars region reinforces itself in reaction to an increase of the local stresses and strains. This is the stage where a pathological condition may begin, influenced in some patients by some kind of genetically transmitted sensitivity. If the stresses are excessive in intensity and frequency, the bone remodeling and reinforcing capacity may be overrun and unable to compensate for fatigue failure of the bone, and microfractures start to occur. In this early pathological stage, the increased bone metabolism will be demonstrated with single photon emission computed tomography (SPECT) scintigraphy and may be demonstrated with magnetic resonance imaging (MRI) (10). It must be emphasized that the normal increase of metabolism related to Wolff’s law, which occurs before fatigue failure, may also be picked up, although that situation is not pathological. On standard radiographs, the unilateral reinforcement of the pars corresponds to vertebral anisocoria. In this early stage, it seems logical that simply refraining from the causative activity may allow healing as long as there is no underlying pathological condition of the bone (material or structural).