Interpars - an anatomical examination of the lumbar pars interarticulares with significance for spinal decompression

BACKGROUND: Spine procedures continue to increase significantly. As such, a more precise understanding of the anatomy, especially the pars interarticularis (PI) is critical. Current data characterizing the PI level-by-level is lacking. This study analyzed the average PI width at each level of the lumbar spine in order to elucidate statistically significant PI variations between lumbar levels.

METHODS: The interpars distance, the narrowest distance between the lateral edges of the left and right PI, was measured directly with calipers on 53 complete lumbar specimens and digitally via Fastrack measurements of 30 sets of lumbar vertebrae. For both methods, the mean interpars distances were compared moving down the lumbar spine.

RESULTS: For direct measurements, the average interpars distances increased from L2 to L5. Analysis revealed significant differences across all levels. A significant difference was noted between male and female vertebrae only at L1. For Fastrack measurements, the average interpars distances also increased from L2 to L5. An increase in spinal canal width was observed across all but L1-L2, and an increase in the interpars-to-spinal-canal-width ratio was noted at all levels except L1-L2 and L4-L5.

CONCLUSIONS: The amount of bone in the PI available for surgical removal becomes smaller moving from L5 to L1. There is a larger "margin-for-error" at L4 and L5 when decompressing the spinal canal from one side to the other than there is in the upper lumbar spine. At L1 and L2, de- compressing the entire width of the spinal canal leaves only a millimeter of remaining pars on either side. Care should be taken to use "undercutting techniques" in upper lumbar decompressions to preserve the PI.