An international, large-scale multicentre study of 33 spine institutes indicates that facet joint tropism—also known as facet joint angulation asymmetry—may be developmental in origin or may be a combination of developmental aspects and secondary changes of degenerative effects. This finding goes against the traditional view that facet joint tropism is purely secondary to remodelling changes as a result of degenerative spondylolisthesis.
Writing in Scoliosis and Spinal Disorders, Dino Samartzis (The University of Hong Kong, China) and others report that there is “still a lack of general understanding” about how facet joint tropism develops, adding that it “remains controversial whether facet joint tropism is a pre-existing developmental phenomena or secondary to progressive remodelling of the joint structure due to degenerative changes”. They explain that the “traditional” view has been that degenerative spondylolisthesis alters kinematics and load distribution, which may lead to secondary changes such as facet joint tropism. They also note that that facet joint tropism may increase degenerative changes (by increasing motion and instability changes), according to an emerging, alternative view. Instead of resulting from degenerative spondylolisthesis, facet joint tropism may in fact play a role in its cause.
The aim of the study, which was undertaken by the AOSpine Asia Pacific Research Collaboration Consortium, was to explore the developmental origins of facet joint tropism. It “addressed the occurrence of facet joint tropism of the lower lumbar spine (L3–S1) in a degenerative spondylolisthesis patient model within the Asia Pacific Region to determine if facet tropism occurred at levels with degenerative spondylolisthesis and at those adjacent to non-degenerative spondylolisthesis segments,” Samartzis et al commented.
In the study, the authors identified 349 patients with single-level degenerative spondylolisthesis. In this population, 34.7% had tropism at one level, 28.7% had tropism at two levels, and 14.3% had tropism at three levels—only 22.3% did not have tropism at any levels. However, tropism was most prevalent in patients with degenerative spondylolisthesis at levels L4–L5 (76.5% of patients).
Samartzis et al report that 50.6% of patients with degenerative spondylolisthesis at levels L4–L5 had tropism, compared with 47.1% of patients with degenerative spondylolisthesis at levels L3–L4 and 31.1% of those with degenerative spondylolisthesis at levels L5–LS1. Furthermore, they found that while tropism was more common at an L4–L5 degenerative spondylolisthesis than at adjacent non-degenerative spondylolisthesis levels, “similar tropism rates were noted at adjacent levels in relation to a L5–S1 degenerative spondylolisthesis and at higher rates at adjacent levels in relation to a L3–L4 degenerative spondylolisthesis.” The authors noted, “In the setting of degenerative spondylolisthesis levels, the immediate adjacent and more distal levels had similar tropism rates between each other”.
According to Samartzis et al, the finding that tropism is present in lumbar levels “with and without” degenerative spondylolisthesis is “contrary to the traditional thought that such facet orientation is secondary to remodelling changes as a result of the degenerative spondylolisthesis”. They added that, instead, their study “lends further credence” to the view that facet joints “directly or indirectly” may play a role in degenerative disc changes. However, the authors noted that their study does not exclude tropism being secondary to degenerative spondylolisthesis. “In fact, in some individuals, this [tropism] could be a combination of developmental and secondary changes,” Samartzis et al stated, concluding that further studies are needed. However, as Samartzis further stated, “If these observations continue to hold up, we may perhaps identified a new imaging biomarker and potential alternative understanding to the drivers of spine degeneration.”