Facet tropism is not associated with lumbar spondylolysis: a computed tomography-based three-dimensional morphometric case–control study

Objective: Facet tropism has been proposed as a morphologic modifier of posterior element loading, but prior evidence in lumbar spondylolysis is inconsistent and often based on 2D measurements. This study aimed to evaluate whether facet tropism differs between patients with lumbar spondylolysis and controls using a CT-based three-dimensional (3D) morphometric workflow with vertebra-anchored reference planes.

Methods: This retrospective case–control study included 39 individuals: 19 with CT-confirmed lumbar spondylolysis and 20 controls (age and sex comparable). Vertebrae L1-L5 were segmented, 3D surface models were generated, and best-fit planes were computed for each superior and inferior facet surface bilaterally. Facet orientation was quantified relative to sagittal, frontal, and transverse planes. Facet tropism (FT) was defined as the absolute right–left difference in facet angles for the same level and facet type; FT presence was descriptively defined as >5, while hypothesis testing used continuous FT values. Group comparisons were performed across L1–L5 and for prespecified lowerlumbar segment metrics (L4-L5 facet-plane mismatch and L5v intra-vertebral mismatch).

Results: Spondylolysis involved L5 in 18/19 patients (4 unilateral, 14 bilateral) and L4 in 1/19. Across L1-L5, facet tropism (FT) did not differ between groups for either superior or inferior facets in any plane (all p > .05). The greatest numerical difference was observed for L5 inferior FT on the sagittal plane (controls 10.38 } 9.40 vs. patients 7.51 } 10.93), but this comparison did not reach statistical significance (p = .092). Lower-lumbar segment-focused mismatch measures also showed no between-group differences (all p > .05). Frontal-plane and sagittal-plane angles showed strong positive correlations in both groups (controls: r = 0.811-0.998; patients: r = 0.851-0.995; all p < .05), whereas transverse-plane parameters showed no meaningful associations.

Conclusion: Using CT-based 3D morphometry, no statistically significant association between facet tropism and lumbar spondylolysis was demonstrated in the present study population, and facet tropism did not discriminate cases from controls across lumbar levels or lowerlumbar segmental mismatch metrics. In this cohort, which was predominantly characterized by L5 involvement and frequent bilateral defects, side-to-side facet asymmetry did not appear to represent a major independent morphologic marker when measured with a standardized 3D framework.

Cite this article as: Karatas ME, .a.an MA, Türkten İ, Pen.e KB, Şakul BU. Facet tropism is not associated with lumbar spondylolysis: a computed tomography– based three-dimensional morphometric case–control study. Acta Orthop Traumatol Turc., 2026; 60(4), 1005, doi: 10.5152/j.aott.2026.261005.