Radiographic evaluation of postoperative bone regrowth after microscopic bilateral decompression via a unilateral approach for degenerative lumbar spondylolisthesis

Object Minimally invasive decompressive surgery using a microscope or endoscope has been widely performed for the treatment of lumbar spinal canal stenosis (LSS). In this study the authors aimed to assess outcomes following microscopic bilateral decompression via a unilateral approach (MBDU) in terms of postoperative bone regrowth and preservation of the facet joints in patients with degenerative lumbar spondylolisthesis (DS) as compared with those in patients with LSS. Methods In the period from May 1998 to February 2007 at the authors' institution, 85 patients underwent MBDU at L4–5. Clinical outcome was evaluated before surgery and at the final follow-up using the Japanese Orthopaedic Association (JOA) score for low-back pain. The following radiographic parameters were assessed at the L4–5 segment before surgery and at the final follow-up: 1) percentage slip on standing lateral radiographs, 2) percentage slip on dynamic radiographs, 3) disc arc on dynamic radiographs, and 4) percentage of facet joint preservation on CT. Bone regrowth on the ventral and dorsal sides of the facet joint on CT were assessed at the final follow-up. Results The cases of 47 patients (23 with DS at L-4 and 24 with LSS at L4–5 without instability) who had a follow-up of at least 2 years were retrospectively reviewed. The improvement ratio in the JOA score, that is, the percentage improvement as indicated by the difference between preoperative and postoperative JOA scores, was not significantly different between patients with DS and LSS. The percentage slip had progressed at the latest follow-up in both groups (1.4% and 1.1%, respectively), and there was no significant difference between the 2 groups. The percentage of facet joint preservation in the DS and LSS groups was 72.8% and 83.4%, respectively, on the approach side and 95.5% and 96.5% on the contralateral side. Facet joint preservation was significantly less on the approach side than on the contralateral side in both groups. The average amount of bone regrowth on the dorsal and ventral sides of the facet joint was 3.4 and 0.9 mm, respectively, in the DS group and 2.0 and 1.0 mm in the LSS group. The difference between the 2 groups was not significant. Facet joint preservation and bone regrowth were not correlated with clinical outcomes. Conclusions Microscopic bilateral decompression via a unilateral approach can prevent postoperative spinal instability because of good preservation of the posterior elements including the facet joints, which is thought to be the main reason for the relatively small amount of bone regrowth after surgery.