Inadvertent C2-C3 union after C1-C2 posterior fusion in adults.

Introduction: Some authors pointed out that there were more than a few patients with inadvertent C2-C3 union after C1-C2 posterior fusion, although few detailed studies of C2-C3 union have been reported. The purpose of this study was to clarify whether C2-C3 union accelerated adjacent C3-C4 disc degeneration after C1-C2 posterior fusion and to investigate the related factors for C2-C3 union.

Methods: Sixteen patients with rheumatoid arthritis (RA group) (4 males, 12 females, mean age 60 years, mean follow-up period 4 years and 3 months) and fifteen patients without RA (non-RA group) (11 males, 4 females, mean 52 years, mean follow-up period 3 years and 10 months) who underwent C1-C2 posterior fusion were radiologically assessed. The C2-C3 union was defined as trabecular bone formation at C2-C3 interlamina in lateral radiograph. C3-C4 disc height was measured to evaluate the disc degeneration.

Results: C2-C3 union rate was 56% and 60% in RA group and non-RA group, respectively. In RA group, postoperative C3-C4 disc height was lower (Student's t-test, P = 0.029) and the decrease rate of C3-C4 disc height was higher (Student's t-test, P = 0.015) in patients with C2-C3 union than in patients without C2-C3 union. In non-RA group, the age at operation was older (Student's t-test, P = 0.0007), and the C1-C2 fusion angle (Student's t-test, P = 0.012) was smaller in patients with C2-C3 union than in patients without C2-C3 union.

Conclusions: C2-C3 union after C1-C2 posterior fusion occurred in more than half of both groups. Inadvertent C2-C3 union should be considered a radiological complication and a potential risk factor due to acceleration of C3-C4 disc degeneration in RA.