Over-correction of Curvature in Surgical Segments Cause the Non-surgical Curvature Loss in One- and Two-level Anterior Cervical Discectomy and Fusion

Background: In some patients with anterior cervical disecetomy anf fusion (ACDF), the correction degree of cervical curvature after surgery is less than that of the surgical segment curvature, which we dened as non-surgical curvature loss (NSCL). This study aims to elucidate the notication of NSCL and its possible causes. Methods: A retrospective analysis was conducted on 122 patients with one- or two-level ACDF from October 2012 to June 2017. The pre-operative and post-operative Cobb angle of C2-7, surgical segments and non-surgical segments were measured and analyzed by lateral cervical X-ray. Results: All patients were followed up for 22.87±3.15 months. Both one- and two-level ACDF could improve the cervical curvature. In the evaluation of NSCL, it was found that the incidence was 37.50% (27/72) in one-level ACDF and 48.00% (24/50) in two-level ACDF. Furthermore, the NSCL does not spontaneously recover after surgery. By comparing patients with and without NSCL, it was found that patients with NSCL had a signicantly higher correction degree in the surgical segments, while the degree of NSCL was signicantly and positively correlated with the correction degree of the surgical segments. For patients with a slight correction degree of surgical segment, the cervical curvature was well-improved and maintained post-operatively. Conclusion: In summary, the correction of the surgical segment curvature in ACDF should not be overemphasized, because excessive correction may cause NSCL. Even if the surgical segment curvature is only slightly corrected, a good improvement in cervical curvature can still be attained.

(27/72) in one-level ACDF and 48.00% (24/50) in two-level ACDF. Furthermore, the NSCL does not spontaneously recover after surgery. By comparing patients with and without NSCL, it was found that patients with NSCL had a signi cantly higher correction degree in the surgical segments, while the degree of NSCL was signi cantly and positively correlated with the correction degree of the surgical segments.
For patients with a slight correction degree of surgical segment, the cervical curvature was well-improved and maintained post-operatively.
Conclusion: In summary, the correction of the surgical segment curvature in ACDF should not be overemphasized, because excessive correction may cause NSCL. Even if the surgical segment curvature is only slightly corrected, a good improvement in cervical curvature can still be attained.

Background
Cervical disc degenerative disease (CDDD) is a common clinical spinal surgery disease, which has a great impact on the quality of life of middle-aged and elderly patients [1][2][3][4][5]. At present, although this is challenged by many new surgical treatments [5][6][7], anterior cervical discectomy and fusion (ACDF) is still the best choice for the treatment of CDDD [8][9][10]. After decades of clinical application, the clinical effect of ACDF has been fully demonstrated.
In most cases, the focus of ACDF surgery is "decompression.". However, for patients with preoperatively poor curvature, in addition to the decompression of the nerve to relieve symptoms, it is also vital to improve the cervical curvature [11][12][13]. Especially for patients with multi-level ACDF, the improvement and maintenance of the cervical curvature after surgery is crucial for the relief of symptoms.
Theoretically, ACDF surgery will only affect the curvature of the surgical segments, while the curvature of non-surgical segments should not be affected. This means that the "correction degree of the C2-7 Cobb angle -correction degree of the surgical segments" should be close to zero. However, it has been noted that in many patients, it was observed that there's a loss in the non-surgical segment curvature. For these patients, this means that the degree of improvement in cervical curvature is partly compromised. Therefore, the investigators aims to determine the causes for this. Based on this, it was hypothesized that the over-correction of the surgical segment curvature would lead to compensatory curvature loss in the non-surgical segments, as shown in Figure-1. That is, the correction of the surgical segment curvature should be appropriate, but not excessive, which could cause the loss in the non-surgical segment curvature, or even new degeneration.
The present study de ned the phenomenon as "non-surgical curvature loss (NSCL)", and the aim of the present study was to verify the validity of the argument that the "over-correction of the surgical segment curvature can actually lead to NSCL".

Study Design
The present retrospective study was conducted in one large spine center, and a total of 122 patients who suffered from a poor cervical curvature, and were treated on a one-or continuous two-level ACDF between C3-7 with Zero-P spacer from October 2012 to June 2017 were consecutively included into the study. The investigation included the pre-operative data, and the 3-, 12-and 24-month follow-up results. The inclusion criteria for the present study were clearly de ned, as follows: (1) the patient's symptoms and radiological results were consistent with the typical diagnosis of one-or two-level CDDD; (2) the patient's pre-operative C2-7 Cobb angle was less than 10°; (3) the patients were successfully followed up before surgery, and at 3, 12 and 24 months after surgery; (4) no cervical surgery was performed during the follow-up.

Implant and Surgical Technique
One senior spinal surgeon from our team performed all the surgical procedures using the standard ACDF technique. Brie y, after thorough decompression and scraping off of the cartilaginous endplate, the proper Zero-P spacer (Synthes GmbH, Oberdorf, Switzerland) was implanted in the intervertebral space, with arti cial bone grafting. Then, four screws were implanted and locked. The correct position of the implants was identi ed by lateral uoroscopy before the incision was closed.

Radiological Evaluation
Lateral cervical radiograph examination was performed at each follow-up time point. The tube to subject distance was 72 inches, with the radiographic tube centered at the C4-C5 intervertebral disc without magni cation. Lateral radiographs of the cervical spine were taken when the patient was in a comfortable standing position, with the upper extremities positioned naturally at the sides of the trunk, while looking straight forward. All radiograph images were transferred to a computer in DICOM format, and the measurements were performed by two independent observers. After an agreement was reached between the observers, each parameter was independently measured twice by two spine surgeons. The measured items of the cervical sagittal parameters in the radiographs included the Cobb angle of C2-7, surgical segments, and non-surgical segments.

Statistical Analysis
The analysis was conducted using Stata version 13.1 (Stata-Corp LP, College Station, TX, USA).
The level of signi cance was set at P<0.05. All clinical and radiological measurements were taken by two independent and experienced observers. Chi-squared analysis and unpaired t-test were used, respectively, for categorical and continuous data between groups, while paired t-test was used to compare the data of the one group from different time points. A statistical power with a signi cant level (alpha) of 0.05 was calculated using the G-Power software (version 3.1.9.4) when there was a statistical difference [14]. All values were expressed in mean ± standard deviation or percentage.

Patient Characteristics
A total of 122 patients met the inclusion criteria, and agreed to accept one-level (n=72) or two-level (n=50) ACDF. The average follow-up period was 22.87±3.15 months. There were 11 C3/4 segments, 43 C4/5 segments, 89 C5/6 segments, and 29 C6/7 segments. The speci c data is presented in Table-1. By the end of the nal follow-up, no patient needed a second operation for the original CDDD.
Both One-level and Two-level ACDF Could Greatly Improve and Maintain the Cervical Curvature Considering that all patients included in the present study had a poor cervical curvature (C2-7 Cobb angle of <10°) before surgery, the correction of the cervical curvature was particularly important. As shown in observed that the cervical curvature in patients with both one-and two-level ACDF signi cantly improved after surgery. In addition, as shown in Figure-2, no further loss of cervical curvature occurred after the patient's improvement, indicating that the cervical curvature was well-maintained after ACDF.
The Description of "Non-surgery Segment Degradation (NSCL)" By evaluating the cervical curvature of these above patients, it was found that in some patients, there was a similar situation, as shown in Figure-1. Theoretically, the improvement in cervical curvature by ACDF should only be re ected in the surgical segments, that is, there should be a signi cantly positive correlation between the "correction degree of the surgical segments" and "correction degree of the C2-7 Cobb angle". As shown in Figure-3, the results at post-operative three months were analyzed, and it was found that the correlations in both one-and two-level ACDF were relatively poor (R=0.0696, P =0.5612 for one-level group, R=0.0999, P =0.4897 for two-level group,), indicating there's the mismatching. After ACDF surgery, although the surgical segments curvature was corrected, the non-surgical segments curvature degraded, which was de ned as NSCL.
After the evaluation of the cervical curvature in all patients, it was found that NSCL was not rare, with an incidence of 37.50% (27/72) in patients with one-level ACDF and 48.00% (24/50) in patients with two-level ACDF while there was no signi cant difference between the two groups (P=0.2475). Moreover, patients with NSCL were separately followed up, as shown in Table-3 and Figure-4. In addition, both were in the one-and two-level ACDF, and the degree of NSCL at post-operative three months was similar to that at the nal follow-up (P=0.7129 for one-level patients, P=0.6995 for two-level patients). These results suggest that NSCL may persist for a long time after surgery, and will not recover automatically with the extension of time.
The Over-correction of Surgical Segments Is One of the Causes of NSCL As shown in Table-4 and Figure-5, the comparison between patients with NSCL and those without NSCL revealed that the correction of the surgical segment curvature in patients with NSCL was signi cantly higher than that of patients without NSCL in both one-and two-level ACDF. Furthermore, for patients with NSCL, the correlation between the "correction degree of surgical segments" and "degree of NSCL" were analyzed, and both one-and two-level patients presented strong positive correlations. These results suggest that the over-correction of the surgical segment curvature does, to some degree, lead to the development of NSCL.

The Cervical Curvature Can Be Greatly Maintained after Surgery in Patients with Slight Segmental Correction
In the previous results, it was demonstrated that the over-correction of the surgical segment causes the development of NSCL. Can the cervical curvature be improved and maintained if the surgical segments are only slightly correct? In this section, patients with only a slight correction of the surgical segment were separated (<3° in one-level ACDF, <6° in two-level ACDF), and an analysis was conducted. The results revealed that, as shown in Table-5 and Figure-6, the post-operative recovery of the cervical curvature in this group of patients was also very satisfactory, and a relatively stable level was maintained throughout the follow-up period.

Discussion
At present, ACDF is still the rst-line surgical method for the treatment of CDDD, and its surgical effect has been con rmed by clinical application for decades [1][2][3][8][9][10] . The treatment of CDDD focuses on relieving neurological symptoms and reconstructing the spinal structure [15][16][17]. The ways to relieve neurological symptoms are similar, which mainly depend on the operator's operation and experience.
However, the reconstruction of the spinal structure differs from person to person, and there are great differences among patients, which require physicians to treat each patient's unique "personality" separately.
The poor curvature of the cervical spine is a common clinical condition, and for these patients, the "reconstruction of the spinal structure" is really important [18]. The correction of the cervical curvature is mainly achieved by reconstructive surgery. By reviewing the information of patients who have previously received one-or two-level ACDF surgery, patients with poor curvature before surgery were separated, and the changes in cervical curvature after surgery were analyzed. It was found that although ACDF surgery can signi cantly improve the cervical curvature, in part of these patients, the correction degree of the cervical curvature was less than the correction degree of the surgical segments, which means that the curvature of non-surgical segments degraded after the ACDF surgery, and this was de ned NSCL.
Theoretically, there should be a strong positive correlation between the "correction degree of the surgical segments" and "correction degree of the C2-7 Cobb angle", which we didn't see in this study. We believe that NSCL is more or less responsible for this phenomenon and the high incidence suggests that NSCL is a common phenomenon. Furthermore, a separate analysis of the follow-up results for patients with NSCL was conducted, and the results revealed that the NSCL was maintained up to the nal followup, and its degree did not differ from the results at three months. This means that NSCL is persistent, and does not recover over time.
By reviewing the retrospective data, it was found that NSCL often occurs in patients with a high degree of correction in the surgical segments. That is, NSCL may be a compensatory reduction in cervical curvature, and the self-adjustment of the cervical would avoid excessive changes in the cervical curvature.
Accordingly, the correction degree of the surgical segments in patients with and without NSCL were compared, and it was found that there was a signi cant difference between the two groups. Furthermore, it was found that there was a signi cant positive correlation between the "correction degree of surgical segments" and "degree of NSCL" in patients with NSCL. This means that the more the degree of the surgical segment curvature is corrected, the more severe the NSCL would become, con rming that the over-correction of the surgical segment curvature is one of the important causes of NSCL. In addition, it was found that the cervical curvature of patients with only a slight correction of surgical segments was well-improved and maintained after surgery.
In summary, it could be concluded that in ACDF surgery, the correction of the curvature in the surgical segments should not be over-emphasized, because excessive correction may cause NSCL. Furthermore, even if the curvature of the surgical segment is only slightly corrected, this can still attain a good improvement in the cervical curvature. It is noteworthy that ACDF can signi cantly improve the cervical curvature with or without the occurrence of NSCL, but the pathological signi cance of NSCL lies more in the destruction of the non-surgical segment curvature and the possible new degenerative symptoms in the later stage.
The limitations of the present study mainly lie in the quanti cation and de nition of the "slight correction of surgical segments", and the "3°" and "6°" were only determined based on the surgeon's experience. Although positive results were obtained, a more accurate quanti cation result could not be obtained. Hence, this could not provide more speci c guidance for ACDF surgery. Furthermore, the followup time was only 24 months. Therefore, further follow-ups are needed to obtain more objective results.

Conclusion
For patients with poor cervical curvature before surgery, ACDF is a very effective surgical method, which can relieve the symptoms of nerve compression and improve cervical curvature. However, during ACDF surgery, the over-correction in the curvature of the surgical segments may cause the occurrence of NSCL. The presence of NSCL may lead to new CDDD and associated symptoms, which is a condition that needs to be avoided during cervical spine surgery. The results of the present study demonstrate that the curvature of the cervical spine can be well-improved and maintained after surgery, even with the minor correction of the segmental curvature. Therefore, the correction of surgical segment curvature in one-and two-level ACDF surgery should not be over-emphasized.