There is controversy whether or not the midline structures (spinous processes, inter and supraspinous ligaments) should be preserved or sacrificed (MLP vs MLS) during decompression in the treatment of lumbar spinal stenosis(LSS). MLP operations are popular as they preserve the posterior tension resisting structures. Equally it is increasingly recognized that the facet joints(FJ), partially resected during decompressive procedures for LSS, have importance resisting postoperative spondylolisthesis and instability. This study was performed to examine the effects of MLP or MLS upon FJ morphology.

MRI scans from 7 patients with LSS (L2/3 to L5/ S1), and 4 patients without LSS (L3/4 to L5/S1) were examined and subjected to theoretical decompression with operative plans that performed decompression via a 10mm corridor that either preserved the midline structures via a parasagittal/laminomy (MLP), or sacrificed the midline structures providing an angled corridor for decompression from the opposite side of the table (MLS). The lateral margin of the decompression was the medial border of the pedicle. Cross sectional area (CSA) was determined for the FJ before and after decompression with both MLP and MLS using Image J cross sectional area analysis.

The cross sectional area of the facet joints prior to surgery was 287 mm2 at L3/4, 275 at L4/5, and 284 at L5/S1 in non-LSS pts. In LSS patients the values were 257, 267, 328, and 319mm2 at the levels L2/3 to L5/S1 respectively. MLS reduced the FJ CSA by 6, 4, and 0 % respectively in the non LSS pts (L3/4 to L5/S1), and 14, 9, 11, and 6% in the LSS pts (L2/3 to L5/S1). MLP reduced the FJ CSA by 34, 25 and 17% in non LSS pts (L3/4 to L5/S1), and 57, 43, 39 and 29% in the LSS pts (L2/3 to L5/S1). The differences between the MLP and MLS reductions of FJ CSA were highly significant. Greater relative reductions were seen with MLP when the CSA of the inferior articular process was examined.

This study demonstrates that preservation of the midline significantly reduces the CSA of the facet joint and in particular the IAP. Biomechanical and clinical evidence suggests that the FJs have a greater role in stability of the motion segment in the lumbar spine, and this study suggests that well-intentioned determination to preserve the midline structures may have a deleterious effect upon the anatomical structures responsible for stability of the lumbar spine.