New concepts on the pathogenesis and classification of spondylolisthesis

Study Design. A review of the literature in the English language pertaining to the pathogenesis and classification of spondylolisthesis. Objective. To review the morphology and biomechanics of the lumbosacral junction as it relates to spondylolisthesis. To present contemporary theories of the development and progression of spondylolisthesis and an etiology-based classification system. Summary of Background Data. The proper treatment of spondylolisthesis is dependent on recognizing the type of slip and its natural history. Although a number of clinical and radiographic features have been identified as risk factors, their role as primary causative factors or secondary adaptive changes is not clear. In particular, confusion persists over the classification of slips with "isthmic defects." The early identification of spondylolisthesis that will progress to a high grade without intervention remains elusive. Methods. A review of English language literature regarding the pathogenesis and classification of spondylolisthesis. Results. Current literature suggests that spinopelvic parameters, in addition to the morphology and biomechanics of the lumbosacral junction, play a causative role in the development of spondylolisthesis. Progression of developmental slips may be due to growth deficiencies of the anterosuperior sacrum, analogous to Blount's disease at the knee. The Marchetti-Bartolozzi classification system emphasizes the distinction between developmental dysplastic slips and acquired laminar stress fractures, both of which may have isthmic defects. These two types of spondylolisthesis have significantly different natural histories, suggesting the need for different treatment strategies. The Marchetti-Bartolozzi system also allows for the classification of postsurgical, pathologic, and degenerative forms of spondylolisthesis. Conclusions. The morphology of the lumbosacral junction resists high shear and compressive forces. The loss of the posterior restraint through an incompetent bony hook may result in the forward displacement of one vertebra on the subjacent vertebra. The spinopelvic parameters, such as pelvic incidence, may be greater determinants of development and progression than previously appreciated. The Marchetti-Bartolozzi classification system is applicable to all forms of lumbar spondylolisthesis, and seems to be clinically relevant in terms of treatment decisions. Their system emphasizes the distinction between developmental spondylolisthesis with lysis and acquired spondylitic spondylolisthesis, which have been included together in previous classification systems and caused confusion over natural history and treatment. Developmental slips have a greater propensity toward progression, which may be secondary to growth deficiencies of the upper sacrum. Further study is required to confirm these observations.