Abstract:

BACKGROUND: For patients with basilar invagination under occipitocervical fusion, the unreasonable choice of fixed angle in the occipitocervical region will reduce the clinical efficacy and accelerate the degeneration of subaxial cervical spine

OBJECTIVE: To measure the Occipito-C₂ angle (OC₂A) and the posterior occipitocervical angle (POCA) in healthy subjects, and to analyze the influence of OC2A and POCA selection on the clinical efficacy and the loss of cervical spinal angle (CSA) of subaxial cervical spine in the occipitocervical fusion of basilar invagination.

METHODS: 150 healthy subjects (healthy group) were grouped by gender and age, with 75 males and 75 females divided into five age groups ranging from 20 to 69 years old. OC₂A and POCA of each healthy subject were measured by three spine surgeons. The intraclass correlation coefficient analysis was performed on the two parameters to prove their consistency. The mean of the two parameters and 95% confidence interval (95% CI) were obtained. Clinical data from 42 patients (malformation group) with basilar invagination who underwent occipitocervical fusion from January 2012 to January 2017 were analyzed retrospectively. These patients were divided into ideal angle subgroup and non-ideal angle subgroup, according to whether their OC₂A and POCA immediately after postoperative ambulation were at 95% CI of the healthy group or not. OC₂A, POCA, and CSA angles were measured preoperatively, immediately after postoperative ambulation, and at the final follow-up visit. The preoperative and final follow-up visual analog scale (VAS) score, Japanese Orthopaedic Association (JOA) score, neck disability index (NDI), and the loss of CSA from immediately after postoperative ambulation to the final follow-up (dCSA) were recorded.

RESULTS AND CONCLUSION: The values of OC₂A and POCA were (14.5±3.7)° and (108.2±8.1)° in the healthy group, respectively, and the respective 95% CI values were 7.2°-21.8° and 92.3°-124.0° 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)°] in the malformation group was smaller than that in the healthy group (P < 0.001); however, the preoperative values of POCA [(123.9±10.4)°] and CSA [(25.7±9.5)°] in the malformation group were larger than those in the healthy group (P < 0.001). In the 42 basilar invagination patients, 26 patients (ideal angle subgroup) were fixed in the ideal angle range immediately after postoperative ambulation (95% CI of the healthy group); 16 patients (non-ideal angle subgroup) were not fixed in the 95% CI of the healthy group, of which 14 patients (87.5%) had OC=A lower than 7.2° (lower limit of 95% CI of the healthy group), and 2 patients (12.5%) had POCA higher than 124.0° (upper limit of 95% CI in healthy group). The VAS score, JOA score and NDI of patients in the two subgroups were significantly improved compared with those before operation (P < 0.05). The VAS score and NDI of the ideal angle subgroup were higher than those of the non-ideal angle subgroup, but JOA score was lower than that of the non-ideal angle subgroup (P < 0.05). At the same time, dCSA [(4.0±6.8)°] in the ideal angle subgroup was significantly higher than that in the non-ideal angle subgroup [(-1.6±3.9)°; P < 0.05]. To conclude, the biomechanical balance between the occipitocervical region and subaxial cervical spine in patients with basilar invagination is different from that of normal healthy people. Therefore, it is not recommended to force OC₂A and POCA to be fixed in the normal reference range during occipitocervical fusion for basilar invagination. Fixation of OC₂A and POCA in the normal reference range will reduce the clinical efficacy of patients and accelerate the dCSA.

Key words: occipitocervical angle, posterior occipitocervical angle, occipitocervical fusion, cervical spinal angle, cervical spine function, pain