Risk of adjacent segment disease after 'topping-off' multi-level lumbar fusions with posterior dynamic stabilisers: an observational cohort study.

Purpose: To determine whether 'topping-off' lumbar fusions, using posterior dynamic stabilising devices (PDSs) with specific biomechanical parameters, reduces the risk of adjacent segment disease (ASD).

Methods: Survival analysis of two non-randomised cohorts, with or without 'topping-off' (T/O or NoT/O), compared the risk of further surgery for ASD following multi-level posterior lumbar interbody fusion (PLIF). The study sample comprised consecutive patients, aged 55 + years, with degenerative pathology at 2, 3 or 4 levels. The NoT/O cohort underwent surgery between August 1993 and September 2019 (n = 425) and the T/O cohort between September 2011 and September 2019 (n = 146). Comparison of ASD risk between cohorts used Cox proportional hazards (CPH) modelling and Kaplan-Meier survivorship analysis.

Results: Analysis was completed on 571 operations across 507 patients. Median follow-up was 63 months (range 0.3-196) and 37 months (range 1.7-98) for the NoT/O and T/O cohorts, respectively. Of 423 patients, 125 (29.6%) patients in the NoT/O cohort underwent further surgery for ASD and 16/145 (11.03%) in the T/O cohort. The hazard ratio (T/O: NoT/O) from the CPH model was 0.42 (95% CL: 0.24-0.74, P = 0.003). Mean annual incidence across the first 5 years was 5.0% in the NoT/O cohort compared with 2.8% in the T/O cohort (P = 0.029). No patient required surgery or developed ASD at a 'topped-off' level. Two patients developed asymptomatic pedicle screw loosening at the level of the PDS device. PROMs were similar between cohorts.

Conclusion: This large, non-randomised, observational study found an approximately 60% reduction in further surgery for ASD with the use of the PDS to 'top-off' PLIF fusions. PDS device-related complications were very low.