Introduction: Contrary to the prevailing conviction that lumbar segments affected by lytic spondylolisthesis are unstable, multiple studies have failed to find evidence of increased or abnormal motion at these segments. Affected segments do not exhibit excessive anterior translation: the so-called slip. Previous studies, however, did not use techniques that might reveal abnormalities in the quality of motion, as opposed to its magnitude.

Methods: To determine if features of instability could be detected in the radiographs of patients with spondylolisthesis, a retrospective, cohort study was conducted of the kinematics of the lumbar spine of patients with spondylolisthesis compared with asymptomatic normal subjects. The flexion-extension radiographs of 15 patients with spondylolytic spondylolisthesis were analysed to determine the location of their instantaneous centres of rotation, and their magnitudes of translation and sagittal rotation. Normative data were obtained by applying the same techniques to the radiographs of 20 asymptomatic subjects.

Results: All but one of the 15 patients exhibited at least one segment with abnormal motion. Only one patient had excessive translation at the lytic segment. Four had minor abnormalities affecting either the lytic segment or ones above. Nine patients exhibited major abnormalities. Seven had paradoxical motion at the lytic segment, in which the centre of rotation was located above L5, instead of below, and in which L5 translated backwards, instead of forwards, during flexion. Two patients exhibited axial dropping of L4, instead of horizontal translation, during extension.

Discussion: Not all patients with spondylolisthesis show features of instability. However, a proportion of patients exhibit highly abnormal movements that are consistent with instability. The abnormalities involve movements within normal range but in abnormal directions. Visual inspection of radiographs will not reveal these abnormalities but they can be detected by plotting the instantaneous axes of rotation.

INTRODUCTION: Contrary to the prevailing conviction that lumbar segments affected by lytic spondylolisthesis are unstable, multiple studies have failed to find evidence of increased or abnormal motion at these segments. Affected segments do not exhibit excessive anterior translation: the so-called slip. Previous studies, however, did not use techniques that might reveal abnormalities in the quality of motion, as opposed to its magnitude.

METHODS: To determine if features of instability could be detected in the radiographs of patients with spondylolisthesis, a retrospective, cohort study was conducted of the kinematics of the lumbar spine of patients with spondylolisthesis compared with asymptomatic normal subjects. The flexion-extension radiographs of 15 patients with spondylolytic spondylolisthesis were analysed to determine the location of their instantaneous centres of rotation, and their magnitudes of translation and sagittal rotation. Normative data were obtained by applying the same techniques to the radiographs of 20 asymptomatic subjects.

RESULTS: All but one of the 15 patients exhibited at least one segment with abnormal motion. Only one patient had excessive translation at the lytic segment. Four had minor abnormalities affecting either the lytic segment or ones above. Nine patients exhibited major abnormalities. Seven had paradoxical motion at the lytic segment, in which the centre of rotation was located above L5, instead of below, and in which L5 translated backwards, instead of forwards, during flexion. Two patients exhibited axial dropping of L4, instead of horizontal translation, during extension.

DISCUSSION: Not all patients with spondylolisthesis show features of instability. However, a proportion of patients exhibit highly abnormal movements that are consistent with instability. The abnormalities involve movements within normal range but in abnormal directions. Visual inspection of radiographs will not reveal these abnormalities but they can be detected by plotting the instantaneous axes of rotation.