Histomorphic analysis of the development of the pars interarticularis and its association with isthmic spondylolysis

Study Design. Descriptive study analyzing the histologic development of human fetal lumbar spine pars interarticularis. Objectives, To delineate the histologic development of the fetal pars interarticularis and correlate the development of the pars with isthmic spondylolysis. Summary of Background Data. Isthmic spondylolysis is present in 5% to 6% of the population. The incidence at birth is zero but rises sharply to 5% at age 4 to 5 years. The origin of the lesion is unknown; however, the current theory states that isthmic spondylolysis results from a fatigue fracture through a congenitally weak pars interarticularis. No study exists investigating the histologic development of the pars. Methods. Microscopic examination of lumbar spines of human fetuses aged 8-20 weeks' gestation was performed. Specimens were dissected en bloc, decalcified, embedded in paraffin wax, sectioned and fixed on glass;F-slides, and stained. Results. The pars begins to ossify at 12 to 13 weeks' gestation by endochondral ossification. The ossification center originates in the region of the pars in lower lumbar vertebrae, resulting in uneven distribution of trabeculation and cortication in this region. The ossification center arises at the end of the pedicle in upper lumbar segments giving rise to uniform trabeculation through the pars. Conclusions. Uneven distribution of isthmic ossification results in formation of a potential stress riser in the region of the pars in lower lumbar vertebrae, which could be susceptible to fatigue fracture.