Fig. 3

Histograms of the distribution of patients among the four grades of fusion at the three follow-up timepoints: a 6 months, b 12 months, c 18 months. The raw 0–7 fusion scores were grouped into four grades of fusion: grade 1 (0–2 points) was considered “no fusion”, grade 2 (3–4 points) was considered “instable ankylosis”, grade 3 (5 points) was considered “questionably stable fusion”, and grade 4 (6–7 points) was considered “complete fusion”, as previously illustrated [23]