Cervical sagittal alignment has become a focus of research in recent years. An imbalance of sagittal alignment may lead to neck pain, deterioration of neurological function, and decreased quality of life [13, 14]. Studying patients’ preoperative cervical sagittal alignment can help to evaluate the severity of the disease, formulate an appropriate preoperative plan, predict the prognosis of the disease, and guide postoperative rehabilitation exercises.
The sagittal alignment changes with aging in the normal population. By stratifying asymptomatic people into different age groups, Scheer et al.  found that the C2–C7 Cobb angle initially decreased and then increased in people aged 20–39, 40–59, and > 60 years. They also found that the C2–C7 SVA and T1 slope were significantly different among the three age groups. These findings indicate that the sagittal alignment of the spine constantly changes throughout life, and when the spine is affected by disease (compared with normal asymptomatic individuals), it is likely to undergo further detrimental changes or even become unbalanced. Xing et al.  found that C2–C7 lordosis and the T1 slope decreased and that the C2–C7 SVA and neck tilt increased in patients with cervical degenerative disease. The C2–C7 Cobb angle can effectively reflect the cervical curvature. When the angle decreases, the muscles at the back of the neck become tensed and pull on bony structures, and spasms may occur in severe cases, resulting in spinal cord compression. In this study, the mean C2–C7 and C5–C7 Cobb angles in the OPLL group were 17.8° ± 8.6° and 10.2° ± 6.3°, respectively, and were better than those in the CSM group (15.3° ± 9.5° and 7.8° ± 5.9°, respectively) (p < 0.05). These results indicate that the cervical lordosis angle is a priority in patients with OPLL, and its optimization is a prerequisite for good results of posterior surgery . For patients with multi-segment OPLL, indirect decompression can be achieved by opening door through the posterior approach to make the spinal cord drift backward, and good surgical results can be obtained. This avoids the more difficult anterior surgery and associated complications.
The cervical ROM is of great significance to patients’ quality of life. Fujimori et al.  found that the loss of ROM was significantly greater in the OPLL group than in the CSM group. Hyun et al.  confirmed that the preoperative ROM was smaller in the OPLL group than in the CSM group, and the decrease in the ROM after the operation was more obvious. The mean ROM of the patients with OPLL in the present study was 33.4° ± 8.0°, which was significantly lower than that of the patients with CSM (40.5° ± 11.1°). This finding indicates that OPLL leads to a significant decrease in ROM in the process of formation and progression of ossification. Meanwhile, the ROM of the continuous, mixed, and segmental types was 29.5° ± 9.1°, 32.7° ± 7.7°, and 36.6° ± 6.6°, respectively. There was a significant difference between the continuous type and segmental type, indicating that the ROM gradually decreases as ossification progresses. Therefore, during postoperative rehabilitation exercises, the impact on ROM due to surgery should be considered in patients with OPLL. Paying attention to the preoperative ROM can provide a basis for formulation of the postoperative rehabilitation plan.
Previous studies have confirmed that the C2–C7 SVA is an important parameter with which to predict the clinical efficacy. Oe et al.  reported that a larger preoperative C2–C7 SVA will affect health-related quality of life. Tang et al.  found that the C2–C7 SVA was positively correlated with the Neck Disability Index score and negatively correlated with the 36-item Short-Form Health Survey score. In the present study, the mean C2–C7 SVA was 21.3 ± 12.8 mm in the OPLL group and 19.4 ± 12.1 mm in the CSM group. There was no significant difference between the two groups. Sakai et al.  observed that the recovery of neurological function was not ideal when the preoperative C2–C7 SVA was > 40 mm. In this study, no patients in the CSM group and two patients in the OPLL group had a C2–C7 SVA of > 40 mm. Therefore, the C2–C7 SVA should be carefully evaluated during preoperative planning to obtain a better clinical efficacy.
The C7 and T1 slopes are important factors in evaluating the cervical sagittal alignment. Knott et al.  proposed the use of T1 slope to predict the whole sagittal alignment. Tamai et al.  confirmed a strong correlation between the C7 slope and T1 slope. The C7 and T1 slopes have thus become research hotspots. In the present study, the mean C7 and T1 slopes in the OPLL group were 22.1° ± 8.3° and 25.6° ± 8.7°, respectively, and those in the CSM group were 21.7° ± 7.6° and 25.3° ± 7.7°, respectively. There was no significant difference between the OPLL and CSM groups. Pearson correlation analysis showed a strong correlation between the C7 slope and T1 slope in the two groups (Pearson correlation coefficients for OPLL and CSM were 0.958 and 0.941, respectively). These results are consistent with previous studies, indicating that the C7 and T1 slopes are stable parameters. Kim et al.  found that patients with a high T1 slope (greater than an average of 26.4°) had larger cervical lordosis but more loss of lordosis after the operation. Kim et al.  confirmed that the improvement rate of the JOA score in patients with a high T1 slope was lower, indicating that the recovery of neurological function was worse. Therefore, for patients with a high T1 slope, the changes in postoperative cervical lordosis and the recovery of neurological function should be considered to formulate appropriate rehabilitation strategies.
Fujiyoshi et al.  proposed the concept of the K-line, arguing that for patients with anterior compression exceeding the K-line, posterior decompression cannot achieve adequate neurological function recovery because of insufficient spinal cord drift. This suggests that the K-line should be considered comprehensively in preoperative surgical planning for patients with OPLL, and reasonable and comprehensive imaging evaluation should be carried out to select surgical approaches that can achieve satisfactory efficacy. Kim et al.  studied the sagittal alignment of 50 patients with cervical spondylosis and found that the K-line tilt was correlated with the T1 slope minus C2–C7 lordosis and strongly correlated with the C2–C7 SVA. Rao et al.  reported that patients with a high K-line tilt (greater than an average of 14.1°) had more loss of lordosis and higher VAS scores after the operation. In the present study, the mean K-line tilt in the OPLL and CSM groups was 9.7° ± 8.2° and 8.5° ± 6.0°, respectively. No significant difference was found between the two groups. Further studies are still needed to explore the significance of the K-line tilt in sagittal alignment.
Through clinical evaluation and examination of the sagittal alignment of patients with multi-segment OPLL, we found that multi-segment OPLL has the following four characteristics. First, multi-segment OPLL mainly involves C3–C6, and the involvement of C4 and C5 is the most common. Second, the disease course is short. The average course of OPLL in this study was 23.4 months, which was considered to be related to the natural course of OPLL. On the one hand, the compression caused by OPLL is hard, persistent compression. Also, the range of compression caused by the ossification in the horizontal and vertical dimensions increases over time. Minor trauma or sudden irritation may significantly aggravate patients’ clinical symptoms. Third, the overall lordosis angle (C2–C7 Cobb angle) and local C5–C7 Cobb angle are good; thus, posterior open-door decompression can obtain good surgical outcome. For patients with special conditions such as excessive local spinal canal invasion, cervical instability, or kyphosis, and other surgical methods such as posterior decompression fixation or posterior combined anterior surgery can be performed accordingly [27, 28]. Finally, the ROM is decreased, and early rehabilitation after surgery should be encouraged to avoid further loss of ROM.