Abstract
PURPOSE: The term "segmental instability" of the lumbar spine is not clearly defined, especially as it relates to degenerative spondylolisthesis (DS) and rotational translation (RT). We investigated whether facet joint effusion on conventional supine MRI indicated increased abnormal motion in DS and RT. METHODS: 160 patients (119 female, 41 male, mean age 68.8 years, range 38.8-89.3 years) who had undergone decompression only or decompression with instrumented fusion for degenerative spondylolisthesis with different degrees of narrowing of the spinal canal were identified retrospectively from our spine surgery database. All had preoperative upright X-rays in AP and lateral views as well as supine MRI. The imaging studies were assessed for the following parameters: percent of slippage, absolute value of facet joint effusion, facet angles, degree of facet degeneration and spinal canal central narrowing, disc height, presence of facet cysts and the presence of rotational translation in the AP X-ray. RESULTS: 40/160 patients showed no facet joint effusion, and in these the difference in the values for the % slip on upright X-ray and % slip on supine MRI was ≤3%. A further 12 patients also showed a difference ≤3%, but had some fluid in the joints (0.44 ± 0.38 mm). In 108 patients, the difference in the % slip measured on X-ray and on MRI was >3% (mean 10.6%, range 4-29%) and was associated with a mean facet effusion size of 2.15 ± 0.85 mm. The extent of effusion correlated significantly with the relative slippage difference between standing and supine positions (r = 0.64, p < 0.001), and the extent of the left/right difference in effusion was associated with the presence of rotational translation (RT 1.31 ± 0.8 mm vs. no-RT 0.23 ± 0.17 mm, p < 0.0001). CONCLUSIONS: Facet joint effusion is clearly correlated with spontaneous reduction of the extent of slippage in the supine position compared to the upright position. Also, the greater the difference in right and left facet effusion, the higher the likelihood of having a RT. Future studies should assess whether analysis of facet joint effusion measured on routine MRI can help in decision-making regarding the optimal surgical treatment to be applied (decompression alone or combined with fusion).