Importance of the Occipitoaxial Angle and Posterior Occipitocervical Angle in Occipitocervical Fusion

Objective: To observe the effects of occipitoaxial angle (O-C2 angle, OC2A) and posterior occipitocervical angle (POCA) selection on postoperative clinical efficacy and lower cervical curvature in patients with acute acquired atlantoaxial dislocation after occipitocervical fusion (OCF). Methods: A total of 150 healthy subjects without cervical disease (healthy group) were randomly selected based on gender and age. Three spine surgeons measured the OC2A and POCA of the healthy group and averaged the values. A total of 30 patients with an average age of 51.0 years (range, 18-70 years; 16 male and 14 female) with trauma or rheumatoid arthritis (disease group) who underwent occipitocervical fusion (OCF) for atlantoaxial dislocation between January 2012 and June 2016 were reviewed. OC2A, POCA, and cervical spinal angle (CSA) were measured postoperative/soon after surgery and ambulation, and at the final follow-up visit. The preoperative and final follow-up visual analog scale ( VAS), Japanese orthopedics association score (JOA), neck disability index (NDI), and dCSA (change of CSA from postoperative/soon after surgery and ambulation to final follow-up) were recorded. Results: The values of OC2A and POCA in 150 healthy subjects were 14.5 degrees +/- 3.7 degrees and 108.2 degrees +/- 8.1 degrees, respectively, and the 95% confidence interval (CI) were 7.2 degrees-21.8 degrees and 92.3 degrees-124.0 degrees, respectively. There was a negative correlation between OC2A and POCA (r = -0.386, P < 0.001). There were 18 patients (group one) of ideal OC2A and POCA (both within 95% CI of the healthy group) postoperative/soon after surgery and ambulation with a mean follow-up time of 26.3 +/- 20.9 months in disease group. The remaining patients (group two) with a mean follow-up time of 31.3 +/- 21.3 months. There was no statistically significant difference in the baseline data as well as pre-operative outcomes, including VAS score, JOA score, and NDI between the two groups. Likewise, the post-operative outcomes in final follow-up, including VAS and JOA score, had no distinct difference in the two groups. However, NDI (11.0 +/- 2.9) in group two at the final follow-up was significantly higher than that in group one (7.0 +/- 2.3) (P < 0.001). And group two showed statistically greater dCSA (5.9 degrees +/- 7.5 degrees) than group one (-2.3 degrees +/- 6.2 degrees) (P = 0.003). Conclusions: The negative correlation between OC2A and POCA plays an important role in maintaining the biodynamic balance of the occipital-cervical region. OC2A and POCA should be controlled of a normal population in patients with acute acquired atlantoaxial dislocation during OCF, which can further improve the clinical efficacy and prevent loss of lower cervical curvature after surgery.