Advantages of S2 Ala Screws for L5-S1 Bone Union in Cases of Two-Level L4-S1 Transforaminal Lumbar Interbody Fusion

<p>Introduction: Nonunion in the lumbosacral region should be considered when performing multilevel posterior interbody fusion between multiple vertebrae, including L5-S1.</p><p>Methods: Here, we retrospectively reviewed 70 cases, who underwent initial transforaminal lumbar interbody fusion (TLIF) at L4-S1 levels between April 2011 and December 2020 and could be followed up for more than 1 year after surgery. Cases were divided into two groups-with the S2 ala screw group (S2 group, 42 cases) and without the S2 ala screw group (S1 group, 28 cases). We reviewed demographic data, including gender and age, body mass index, operation time, bleeding volume, flexion and extension angles at L4/L5 and L5/S1 from the available X-ray photos, the Japanese Orthopaedic Association (JOA) scores of lumbar spine diseases at the pre- and postoperative final observation, preoperative paraspinal muscle fat content, bone bridging inside and behind the interbody cage, and cage subsidence of L4/L5 and L5/S1 at the final observation and reoperations.</p><p>Results: There were no substantial differences in gender ratio, operation time, bleeding volume, cage subsidence, and paraspinal muscle fat content between the two groups. The difference in flexion and extension angles at L4/L5 (S1 group: 2.80°; S2 group: 1.34°) and L5/S1 (S1 group: 4.00°; S2 group: 1.98°) was considerably smaller at the final postoperative observation in the S2 group. JOA scores at the final postoperative observation (S1 group: 22.6; S2 group: 25.7) and JOA score improvement rates (S1 group: 56.7%; S2 group: 74.6%) were significantly higher in the S2 group. The reoperation rate for any reason was significantly lower in the S2 group (S1 group: 17.9%; S2 group: 2.4%). There were no major differences in reoperation rate for nonunion at L5/S1 (S1 group: 3.6%; S2 group: 0%). Bone union was defined as "the difference between flexion and extension is less than 2 degrees and the bone bridging inside or behind of the interbody cage," the bone union rate at L5/S1 was considerably higher in the S2 group (S1 group: 28.5%; S2 group: 59.5%). </p><p>Conclusions: In two-level L4-S1 TLIF, postoperative L5-S1 intervertebral mobility was significantly lower, and the bone union rate was significantly higher in the S2 group. Since there is no significant difference in the grade of preoperative paraspinal muscle fat content, these results are unaffected by individual differences in the back muscles. The insertion of the S2 ala screws could reduce L5-S1 intervertebral mobility that caused nonunion in lumbosacral region and increase the bone union rate at L5-S1.</p>