2.4. Sample size calculation

The power-calculation was made based on expected rate of fusion, as it was hypothesized that fusion and abnormal motion correlate one to one. Therefore, we hypothesized that ACDA would maintain normal sequence of motion, whereas ACD with fusion does not. Based on available literature at that time, a fusion rate of 15% was assumed for ACDA (reported range of 0–20%) versus a fusion rate of 70–80% for ACD. At an α of 0.05 and power (β) of 80% the number of patients for the previous study to detect a 60% difference in the primary outcome, the presence of a normal sequence of segmental contributions, was 10 per group. The assumptions made above were in line with the percentages that were actually found one year after surgery in this RCT and in line with sample sizes that were previously used in the analysis of this type of cinematographic recording (Boselie et al., 2015, 2017a).