Asymmetrical spondylolysis

The objective of this study was to examine examples of spondylolysis in which the pattern of separation was clearly asymmetrical, in order to learn more about the process of bone separation that produces this condition. Although the primary focus was on unilateral complete separation, examples of asymmetry represented by incomplete separation and by complete bilateral separation where the separation sites are in different locations on the two sides were included. Two collections were used, one consisting of Canadian Inuit skeletons curated at the Canadian Museum of Civilization, and the other of 48 individual examples of asymmetrical spondylolysis from sites in a variety of localities curated by several different institutions. The first collection was studied primarily to observe early manifestations of spondylolysis, particularly incomplete separation, while various patterns of asymmetrical complete separation were the focus of the second. The results indicate that asymmetry is part of the earliest osteological picture of spondylolysis, with right-sidedness predominating, a condition perhaps related in some way to handedness. The right-side predominance appears to decrease with age. The ratio of unilateral to bilateral separation ranges from 3-33% in different studies, and a significant number of the unilateral separations have spina bifida occurring in the same vertebra. Overall, the specimens examined here, considered along with clinical cases, nicely illustrate a progression of spondylolysis. A unilateral separation may heal, it may progress to bilateral separation, or it may remain as a permanent condition, producing a pattern of degenerative changes that can include spondylolisthesis. A unilateral healing of bilateral complete separation is likely a rare phenomenon, at least after the separations have reached a certain level of maturity.