HOW FAR CAN YOU GO LATERALLY DURING LUMBAR SPINE DECOMPRESSION BEFORE CONSIDERING STABILISATION? AN MRI MORPHOMETRIC STUDY

Background: Iatrogenic instability can be produced by lumbar spine decompression surgery not only if decompression extends beyond the lateral border of pars but also if there is insufficient pars left at the end of the procedure resulting in its fracture and hence instability on weight bearing. Generally, it is considered to be safe in preventing iatrogenic instability if half of the facet joint is left intact during decompression surgeries. Purpose: We aimed to answer the following questions. By removing half of the facets can we get adequate decompression of the nerve roots? Is there a difference at different levels in the lower lumbar spine? What is the inclination of the facet joint at each level and how does it affect the stability? Material and Methods: We analysed 200 consecutive magnetic reasonance imaging (MRI) scans of the lumbosacral spine at L3/4, L4/5, and L5/S1 levels. We measured the difference in the distance from midline to the lateral border of the foramen and from midline to the middle of the facet joint at each level on either sides. The angle of the facet joint was also noted. The vertebral body diameters in both the sagittal and coronal plane were noted. Results: At L3/4, the mean distance from the midline to the middle of facet joint was 15.5 +/−1.9 mm (11–23mm), while the mean distance from the midline to the foramen was 21.2 +/− 2.6 mm (13–28mm). At L4/5, the mean distance from the midline to the middle of facet joint was 18.1 +/−2.3 mm (13–25mm), while the mean distance from the midline to the foramen was 23.6 +/− 2.9 mm (16–34mm). At L5/S1, the mean distance from the mid-line to the middle of facet joint was 15.5 +/−1.9 mm (11–23mm), while the mean distance from the midline to the foramen was 26.8 +/− 2.9 mm (20–34mm). The angle of the facet joints at L3/4 is 35.90 +/− 7.40, while at L4/5 it is 43.20 +/− 8.00, and at L5/S1 it is 49.40 +/− 10.10. Conclusion: The distance to the foramen from the level of the middle of the facet joints seem to be between 5–6mm at every level with the lateral border of the foramen being lateral to the middle of the facet joint. Hence, in lumbar spine decompression surgeries, after the mid-line decompression extending up to half of the facet joints, a further undercutting of the facet joints to 5–6mm is therefore required to completely decompress the nerve root in the foramen. The more coronal orientation of the facet joint at L5/S1 conforms better stability than that at L3/4level. Therefore, stabilisation of the spine should be considered if more than 2cm of the posterior elements are removed from midline at L3/4 level.