Is it Suitable to Fix the Occipito-C2 Angle and the Posterior Occipitocervical Angle in a Normal Range During Occipitocervical Fusion?
Authors: Tang Chao , Li Guang Zhou , Kang Min , Liao Ye Hui , Tang Qiang , Zhong De Jun 
Published: March 2020

Study Design:
This is a retrospective study.

Objectives:
The objective of this study was (1) to measure the occipito-C2 angle (OC2A) and the posterior occipitocervical angle (POCA) in a normal population, and (2) to observe the effects of OC2A and POCA selection on postoperative clinical efficacy and lower cervical curvature after occipitocervical fusion (OCF) in patients with basilar invagination (BI) and atlantoaxial fracture and dislocation (AAFD).

Summary of Background Data:
OC2A has received special attention with respect to the clinical efficacy during OCF. However, none of studies have focused on the relationship between OC2A and POCA and have assessed their impact on clinical outcomes in patients with different occipiocervical diseases.

Materials and Methods:
One hundred fifty healthy subjects without any cervical disease (healthy group) were randomly selected based on sex and age. Three spine surgeons measured the OC2A and POCA in the healthy group and averaged the values. Forty-two patients with BI (BI group) and 32 patients with AAFD (AAFD group) who underwent OCF between January 2012 and January 2017 were reviewed. OC2A, POCA, and cervical spinal angle (CSA) were measured preoperatively, postoperatively immediately after surgery and ambulation, and at the final follow-up visit. The preoperative and final follow-up visual analog scale (VAS), Japanese Orthopaedic Association score (JOA), neck disability index (NDI), and the change of CSA from postoperatively immediately after surgery and ambulation to the final follow-up (dCSA) were recorded.

Results:
The values of OC2A and POCA were 14.5±3.7 and 108.2±8.1 degrees in the healthy group, respectively, and the respective 95% confidence intervals were 7.2–21.8 and 92.3–124.0 degrees as the normal range. There was a negative correlation between OC2A and POCA (r=−0.386, P<0.001). The preoperative value of OC2A (5.6±4.3 degrees) in BI group was smaller than that in the healthy group (P<0.05); however, the preoperative value of POCA (123.0±10.4 degrees) in the BI group was larger than that in the healthy group (P<0.05). There was no significantly different of OC2A and POCA between the healthy group and the AAFD group before the operation (P>0.05). The preoperative value of CSA (25.7±9.5 degrees) in the BI group was larger than that in the AAFD group (16.5±5.1 degrees) (P<0.05). In the BI group, 26 patients had ideal OC2A and POCA (both within 95% confidence interval of the healthy group) postoperatively immediately after surgery and ambulation as a subgroup of the ideal angle group. In the remaining patients with BI (nonideal angle group), VAS, JOA, and NDI at the final follow-up were significantly better than those in the ideal angle group (P<0.05). The ideal angle group showed statistically greater dCSA than the nonideal angle group of the BI group (P<0.05). On the contrary, in the AAFD group, the NDI in the ideal angle group (20 patients) was better than that in the nonideal angle group at the final follow-up, and the ideal angle group had a smaller dCSA compared with the nonideal angle group (P<0.05).

Conclusions:
The biomechanical balance of occipitocervical region in patients with occipitocervical diseases with different pathogenesis is different. On the basis of the different types of occipitocervical diseases, an appropriate range of OC2A and POCA should be selected by the surgeon during OCF, which can further improve the clinical efficacy and reduce the loss of the lower cervical curvature after surgery.