The nineteen cases of spondylolysis, in a series of 450 skeletons studied, have been classified into two groups: typical (sixteen cases) and atypical spondylolysis (two cases). One skeleton presented typical and atypical spondylolysis in different vertebrae. In typical spondylolysis, generally found in the lower lumbar vertebrae, the clefts of the isthmus were always located between the downward projecting inferior articular processes of the vertebra above and the upward projecting superior articular processes of the vertebra below. Such clefts appear to be produced by compression of the isthmuses by these articular processes in a mechanism termed a pincers effect. The presence of preceding bone abnormalities or congenital defects of the isthmus would seem to be unnecessary for the production of the lesions. The anatomical factors promoting the development of the condition in some individuals and those preventing its appearance in others are discussed. The participation of other factors, such as erect posture, trauma, age, sex, and race is also analyzed. Skeletons were found in which the development of spondylolysis had stopped before total separation of the vertebral arch had occurred; marked depressions were on the upper and lower surfaces of the isthmuses precisely at their sites of contact with the articular processes of the adjoining vertebrae. The term prespondylolysis has been applied to these cases. In atypical spondylolysis no direct relationship of the clefts to the articular processes of the vertebrae above and below was observed. In these cases the etiology could not be established.
Copyright 1959 by The Journal of Bone and Joint Surgery, Incorporated