In the current study, 253 patients were included and randomly assigned to ACD (83 patients), ACDF (85 patients) or ACDA (85 patients). At baseline, X-ray data were available for 228 patients and for 168 patients at 2-year follow-up.
Baseline characteristics are presented in Table 2. The mean age of the study population was 45.2 ± 7.3 years, ranging from 27 to 70 years. There was no difference regarding baseline characteristics between treatment groups. Surgery was most frequently at levels C5–C6 and C6–C7.
Characteristics of cervical sagittal alignment in subgroups
Table 3 demonstrates the characteristics of the cervical sagittal alignment parameters in the different treatment arms. No differences were found regarding sagittal alignment parameters between the three surgical groups neither at baseline nor at 2-year follow-up (P > 0.05). Additionally, it was found that the cervical alignment parameters did not change significantly comparing baseline to post-operative values with the exception of C2–C7 lordosis in the ACDF group (P = 0.048). Irrespective of the surgical method, only C2–C7 lordosis was found to change (increase) significantly over 2 years (from 11.3° to 13.1°, P = 0.023). The other three parameters (OCI, C2–C7 SVA and T1 slope) did not change with a statistical significance. Notably, the angle or slope could be minimally negatively or minimally positively deviating.
Correlation between disc height and cervical sagittal alignment
In the ACD group, there was no correlation between the disc height of the target level and cervical sagittal alignment at baseline (P > 0.05). Likewise, this correlation was absent at 2-year follow-up (P > 0.05). There was a decrease in disc height, but this did not impact overall balance.
Adjacent segment degeneration (ASD)
Preoperatively, the incidence of ASD did not differ in the three groups: 38% in the ACD group (27 patients), 36% (29 patients) in the ACDF group and 29% (22 patients) in the ACDA group (P = 0.428). At 2-year follow-up, ASD increased to 63% of patients in the ACD group (35 patients), and 55% (28 patients) in the ACDF group and to 56% (34 patients) in the ACDA group. Likewise, between three groups, there was no statistically significant difference (P = 0.674).
If ASD was considered to be scored as ‘ASD’ only if disc degeneration and/or the presence of osteophytes was moderate or severe, the incidence of ASD was still comparable in the three treatment arms at baseline: 16% in the ACD group, 14% in the ACDA group and 13% in the ACDA group (P = 0.905). And likewise, 2 years after surgery, the incidence of ASD did not differ between three groups (29% in the ACD group, 26% in the ACDF group and 20% in the ACDA group; P = 0.522).
Furthermore, the progression of ASD was also investigated, comparing follow-up to baseline data. At 2 years after surgery, the proportion of positive ASD progression was comparable in the three treatment arms (33% in the ACD group, 25% in the ACDF group and 31% in the ACDA group; P = 0.693).
Correlation between cervical sagittal alignment and radiological ASD
In order to study the relationship between cervical sagittal alignment parameters and ASD, subjects were dichotomized according to the presence and progression of radiological ASD, irrespective of the surgical method. The average values of sagittal alignment parameters of subjects with and without ASD are shown in Table 4.
At baseline, a higher OCI value was significantly correlated with the presence of ASD (OR 1.05; 95% CI 1.01–1.09; P = 0.009). If patients were dichotomized into mild ASD and ASD, the result was similar (OR 1.05; 95% CI 1.00–1.11; P = 0.044). C2–C7 lordosis, C2–C7 SVA and T1 slope failed to show a correlation with ASD (Table 5).
At 2-year follow-up, again, OCI with higher value was correlated with the presence of ASD (OR 1.08; 95% CI 1.04–1.13; P < 0.001). If patients were dichotomized into mild ASD and ASD, the correlation between higher OCI and ASD was detected as well (OR 1.11; 95% CI 1.06–1.16; P < 0.001). Patients with higher OCI value were likewise correlated with the positive progression of ASD (OR 1.05; 95% CI 1.01–1.09; P = 0.023) (Table 6).
As stated above, no significant changes in mean OCI values existed between baseline and 2-year follow-up. On an individual level, changes were small for the vast majority of patients, but considerate for a minority of patients (Fig. 2). However, no correlation was demonstrated between the change in OCI value and the progression of ASD. Neither was there a correlation between the change in sagittal balance parameter and progression of ASD for the other parameters (Table 7).
Characteristics of clinical outcomes
The clinical outcomes represented by NDI, PCS and MCS were comparable between the three treatment groups, both at baseline and at 2-year follow-up (Table 8). Therefore, the clinical outcomes were studied irrespective of surgical methods. At baseline, the mean NDI was 39.7 ± 15.4, mean PCS was 43.3 ± 13.5, and mean MCS was 59.1 ± 21.5. At 2 years after surgery, the NDI decreased to 16.4 ± 17.1. PCS and MCS increased to 73.9 ± 23.6 and 77.6 ± 21.8, respectively.
Correlation between cervical sagittal alignment and clinical outcomes
At 2-year follow-up, the values of C2–C7 lordosis, C2–C7 SVA, OCI and T1 slope failed to correlate with clinical conditions, and neither was there a correlation of clinical outcome to the changes of these parameters (P > 0.05).
Correlation between cervical sagittal alignments
SVA was significantly correlated with T1 slope (0.45–0.54, P < 0.01) and OCI (0.20–0.37, P < 0.01). C2–C7 lordosis was correlated with T1 slope as well (0.40–0.55, P < 0.01) (Table 9).