Zygapophyseal Joint Orientation and Facet Tropism and Their Association with Lumbar Disc Prolapse

STUDY DESIGN: Cross-sectional study. PURPOSE: To evaluate the association between zygapophyseal joint angle (ZJA), facet tropism (FT), and lumbar intervertebral disc prolapse (IVDP). OVERVIEW OF LITERATURE: Several studies have shown that FT increases the risk of IVDP and have postulated that a more sagittally oriented zygapophyseal joint provides less mechanical resistance to axial torque, thereby exerting excessive rotational strain on the intervertebral disc, resulting in an annular tear. In contrast, other studies have found no definitive association between FT and IVDP. Therefore, conclusive evidence regarding the role of FT in the pathogenesis of disc prolapse is currently lacking. METHODS: Magnetic resonance imaging scans of 426 patients with single-level lumbar IVDP were analyzed. Right and left ZJAs of the lumbar segments were measured on axial sections. The frequency and severity of FT were determined by calculating the absolute difference between the right and left ZJAs. Patients without IVDP at L4–L5 and L5–S1 served as controls for those with IVDP at L4–L5 and L5–S1, respectively. Chi-square test and t-test were used to compare the severity and frequency of FT between patients with and without IVDP. The receiver operating characteristic analysis was performed to determine the critical FT values that were predictive of IVDP. RESULTS: Patients with IVDP exhibited a higher frequency (L4–L5: 47% vs. 15.08%; L5–S1: 39.62% vs. 22.69%; p=0.001) and severity (L4–L5: 7.85°±3.5° vs. 4.05°±2.62°; L5–S1: 7.30°±3.07° vs. 4.82°±3.29°; p <0.001) of FT than those without IVDP. Critical FT values of 5.7° at L4–L5 and 6° at L5–S1 increased the likelihood of IVDP by a factor of 2.89 and 1.75, respectively. CONCLUSIONS: Our results confirm the existence of a significant association between lumbar IVDP and FT; however, a causal relationship could not be ascertained.