Purpose: A retrospective study comparing the fusion rate and, the incidence of junctional spinal stenosis between a rigid (Wiltse) and a semirigid (Varifix) posterior spinal fusion system.

Material & Methods: 92 patients, mean age 52.3 year old, underwent posterior fusion with semirigid Varifix system (rod diameter 5.0 mm), and 89 patients, mean age 49.8 year old, with rigid Wiltse system (6.5 mm). The mean follow-up was 4.8 years (range 2–9) for Varifix group and 11.7 years (range 9–17) for Wiltse group. Preoperative diagnosis was spinal stenosis (n=56), disc degenerative disease (n=43), degenerative spondylolisthesis (n=37), post-laminectomy instability (n=34), and isthmic spondylolisthesis (n=11). In all patients autologous iliac crest bone graft was used. Spinal fusion was confirmed by A-P, lateral, and flexion-extension radiographic studies, or by direct surgical exploration and observation. Pain intensity was recorded using the Visual Analogue Scale (VAS).

Results: Successful fusion was achieved in 92.4% in the semirigid group and in 93.2% for the rigid group. There was no statistical difference in fusion rate between these two groups (p=0.82). Eight patients with pseudoarthrosis were treated by anterior fusion and 5 by repaired posterior fusion, with a fusion rate of 100%. Postoperative infection was diagnosed in 5 patients (5.4%) in the semirigid group and in 4 patients (4.5%) in the rigid group. They were treated by debridement, irrigation, and intravenous antibiotics. Hardware removal because of pain was performed in 9 patients (9.8%) in the semirigid group, and 17 patients (19.1%) in rigid group. Removal of hardware resulted in improvement in pain in all patients. Junctional spinal stenosis was diagnosed in 2 patients (2.2%) in semirigid group and in 7 patients (7.9%) in rigid group. There was a trend for higher incidence of adjacent level stenosis in rigid group (p=0.07).

Conclusion: Biomechanical studies have shown that the stiffness of spinal construct depends on rod diameter and a decrease in rod rigidity can increase the risk of implant failure. In our study we didn’t find any difference in the fusion rate and in complication rate between these two systems. The increased percentage of the junctional spinal stenosis in rigid group may be explained by the longer follow-up in this group. According to our data the semirigid system may be better tolerated than the rigid system.