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IntroductionSeveral studies have attempted to define etiological factors for the development of degenerative spondylolisthesis (DS), and the orientation of lumbar facet angles relative to the transverse plane has been a major focus of interest. The facet joints play a critical role in maintaining stability of the lumbar spine by sharing load in compression and extension, and protecting the disc from excessive shear and rotational forces [1,13,24]. Determined by their spatial orientation, they also guide motion between two adjacent vertebrae. Their oblique orientation in the lumbar spine allows flexion, extension, and lateral bending, but only a small amount of axial rotation [27].It has been shown that in patients with DS, the facet joints are significantly more sagittally oriented, thus allowing the superior vertebra to glide anteriorly. Individuals with sagittally oriented facet joints have therefore been regarded as candidates for DS [23]. However, it is not clear whether this more pronounced sagittal orientation of the facet joint represents a pre-existing morphology or whether it has to be considered a result of secondary remodeling.Facet joint asymmetry (tropism) has been claimed to be related to disc herniation and degenerative disc disease due to rotational instability of the spinal segment [6,8,20]. Whether facet joint tropism plays a role in the development of DS, however, has not been investigated.The present study analyzed facet joint orientation in the transverse plane in patients with DS and compared this to patients with no evidence of ventrolisthesis from three different age groups. The working hypothesis was that if the more sagittal orientation in DS was a pre-existing anatomical feature, it then should be possible to identify a younger patient group being "at risk" for the development of DS later in life. The incidence of facet joint tropism in the different groups was also investigated.Abstract This study analyzed transverse facet joint angulations at the three lower lumbar levels in 132 patients assigned to one of four groups. Group A comprised 23 patients with degenerative spondylolisthesis (DS) at the level L4-5, group B comprised 40 patients above the age of 50 years, group C comprised 38 patients aged between 35 and 50 years, and group D comprised 31 patients under the age of 35 years. Groups B, C, and D had no evidence of DS. Measurements were taken from hard copies of axial MR or CT images. The transverse plane of facet joints was more sagittally oriented in group A than in any other group. This difference was highly significant at the L4-5 level. The incidence of more sagittally oriented L4-5 facet joints was also significantly higher only in group A. The incidence of facet joint tropism, however, was not different in group A. These results support the view that the pronounced sagittal alignment of facet joints in patients with DS represents a secondary remodeling rather than a pre-existing morphology. Facet joint asymmetry does not seem to play a major role in the development of DS.

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There is a significant difference between the insertion loads measured in vivo and those measured in vitro. Additional research is needed to verify whether this method provides an indication of screw fixation quality.