Spondylolysis is a fracture of the pars interarticularis. If the pars defect is bilateral, it may allow slippage of the vertebra, resulting in spondylolisthesis. Mechanical stresses cause spondylolisthesis, which is most common in the lower lumbar spine at levels L4 and L5. These levels carry more load than higher levels of the spine and have greater shear forces due to the lumbar lordosis. Stresses on the pars interarticularis are accentuated during repetitive hyperextension, which results in increased contact of the caudal edge of the L4 inferior articular facet with the L5 pars. This may result in a stress fracture of the pars.

Many more athletes than nonathletes develop a pars defect. Sports that involve repetitive hyperextension and axial loading of the lumbar spine result in repetitive microtrauma to the pars interarticularis. Such sports include gymnastics, wrestling, hurdling, weight lifting, cricket, rowing, pole vaulting, diving, swimming, American football, baseball, tennis, sailing, and volleyball. Gymnasts are four times as likely as controls to develop spondylolysis, and 50% of American football linebackers have a pars defect. In American football blocking, the average compression force at L5 is 7 times body weight, and the peak shear force is 2.6 times body weight.

The lumbar spine has the functions of support, mobility, housing of neurological structures, and control. The orientation of the facets varies for different spinal levels, and this orientation determines how much flexion, extension, lateral flexion, and rotation the body can achieve at a particular vertebral level. The facets and other posterior elements have a load-bearing function to help support the weight of the upper body and anything that it carries, and they are also acted upon by spinal muscle forces. The posterior elements carry approximately 16% of the total load when a person is standing upright but 30% of the total load in hyperextension. During hyperextension, the vertebrae rotate such that the superior articular processes of one vertebra push against the inferior articular processes of the vertebra directly above it. This contact force causes the inferior articular processes to bend upward and tensile stresses to develop on the underside of the pars. During flexion, the inferior and superior articulating facets of adjacent vertebrae attempt to pull away from each other but are held together by the ligaments that encapsulate the joint, which tends to bend the pars forward.

Fatigue failure occurs due to repetitive loads that are less than the load to cause failure after a single load. The fatigue life is dependent on the amplitude of the cyclic stresses and the method by which they are imposed. Fatigue tests of the lumbar neural arch simulated a man walking quickly in a flexed posture with a 50-kg pack on his back. Over 70% of specimens failed at the pars, and specimens simulating persons aged 14 to 30 years failed much sooner (<50,000 cycles) than those simulating middle-aged people (>500,000 cycles with no fracture).

While there is a complex interplay of muscle forces and ligament tension at intervertebral joints, the facet load is the main force acting on the neural arch. Loading of the inferior articular process and bending in the pars cause high stresses because the pars is the narrowest part of the neural arch and thus has the smallest cross-sectional area with which to resist load. With cyclic flexion and hyperextension, or with repeated high-impact forces on the upper body, the pars is repetitively bent in one direction and then the other, and fatigue life may be reached and a crack will initiate at the point of highest principal stress. The crack will propagate across the pars if loading continues.

In addition to occurring in certain athletic groups who subject their upper bodies to high repetitive forces, spondylolysis seems to develop most often in children, adolescents, and young adults. This could be partly due to the fact that the ossification of their neural arches may not be complete and the bone is therefore less stiff and has a lower fatigue life. Or it could be partly due to the fact that the intervertebral discs of young people are more elastic and therefore allow more shear force to reach the facets, increasing the magnitudes of the forces and stresses on the pars. Another cause could be that most young people engage in more frequent strenuous activity than adults, causing the fatigue life of the pars to be reached sooner.