Effect of preoperative dynamic cervical sagittal alignment on the loss of cervical lordosis after laminoplasty

DOI: https://doi.org/10.21203/rs.3.rs-2440814/v1

Abstract

Purpose Cervical laminoplasty (CLP) is a developed surgical procedure for the treatment of cervical spondylotic myelopathy (CSM), but only a few of those studies focus on preoperative dynamic cervical sagittal alignment and the study of different degrees of loss of cervical lordosis (LCL) is lacking. This study aimed to analyze patients who underwent CLP to investigate the effect of cervical extension and flexion function on different degrees of LCL.

Methods This was a retrospective study of the patients who underwent CLP between January 2019 and December 2020. The cervical lordosis (CL), T1 slope (T1S), cervical sagittal vertical axis (cSVA), CL in flexion (Flex CL), CL in extension (Ext CL), cervical spine range of motion (ROM), cervical spine range of flexion (Flex ROM) and extension (Ext ROM) were measured. The extension ratio (EXR) was defined as 100 × Ext ROM/ROM. LCL was defined as preoperative CL - postoperative CL. Patients were classified into the following three groups according to the LCL: stability group: (LCL ≤ 5°); mild loss group (5° < LCL ≤ 10°); and severe loss group (LCL > 10°). The Japanese Orthopedic Association (JOA) score was used.

Results Seventy-nine patients were enrolled (mean age 62.92 years; 51 men, 28 women) in the study. Among the three groups, cervical extension Ext ROM was the best in the stability group. Compared with the stability group, Flex ROM was significantly higher and the extension ratio (EXR) was significantly lower in the severe loss group. Compared with the severe loss group, JOA recovery rates were better in the stability group. Receiver-operating characteristic curve (ROC) analysis to predict LCL > 10° (area under the curve = 0.808, p < 0.001). The cutoff value for EXR was 16.80%, with sensitivity and specificity of 72.5% and 82.4%, respectively.

Conclusion CLP should be carefully considered for patients with a preoperative low Ext ROM and high Flex ROM, as a significant kyphotic change is likely to develop after surgery. EXR is a useful and simple index to predict significant kyphotic changes.

Introduction

Cervical laminoplasty (CLP) is a developed surgical procedure for the treatment of cervical spondylotic myelopathy (CSM) [1–4]. CLP can preserve the motion of the operated levels and is more suitable for patients undergoing a multi-segmental cervical spinal cord surgery. However, cervical laminoplasty, as posterior non-fusion decompression surgery, can lead to loss of cervical lordosis (LCL).

The alignment of the cervical spine needs to maintain adequate lordosis to provide enough space for the shifting of the spinal cord. Therefore, prevention of LCL and investigation of its risk factors after CLP are necessary. Several studies have reported many preoperative predictors of LCL after laminoplasty [5–20]. and the relationship between static cervical sagittal alignment and kyphotic changes has been well described [5–8, 12, 14, 16, 17, 19–21]. However, only a few of those studies focus on preoperative dynamic cervical sagittal alignment [10, 13, 15] and the study of different degrees of LCL is lacking.

We aimed to retrospectively analyze patients who underwent laminoplasty to investigate the effect of preoperative static and dynamic cervical sagittal alignment on LCL after laminoplasty. We focused on the effect of cervical extension and flexion function on different degrees of LCL to determine potential risk factors for serious post-laminoplasty LCL.

Materials And Methods

Patient enrollment

This study was approved by the IRB of our affiliated institution. We retrospectively reviewed consecutive patients who underwent cervical laminoplasty at our institution between January 2019 and December 2020. Patients were eligible for our study if they met the following inclusion criteria: 1) aged 18 years or older without previous cervical surgery; 2) a lesion involving more than three levels of CSM 3) clinical signs of myelopathy; and 4) at least twelve months of follow-up. Patients with fractures, infections, tumors, combinations with fusion surgery, decompression levels including C2 or thoracic spine levels, and invisible T1 upper endplates were excluded. Finally, seventy-nine patients were enrolled.

Surgical procedures

After the induction of general endotracheal anesthesia, patients were positioned prone on the operating table. The surgeons performed an incision in the back of the neck, and detached the paravertebral muscle from the spinous process and lamina, preserving the facet capsule. All patients underwent open-door laminoplasty with a mini titanium plate system for decompression. One side of the lamina was opened, and the other side served as the hinge.

Radiological parameters

The cervical sagittal alignment parameters were measured on lateral radiographs (Fig. 1): cervical lordosis (CL) was the angle between the C2 lower endplate and the C7 lower endplate; T1 slope (T1S) was the angle between a horizontal plane and a line parallel to the superior T1 endplate; cervical sagittal vertical axis (cSVA) was defined as the horizontal offset from a plumbline dropped from the C2 vertebral body to the posterosuperior corner of the C7 vertebra; CL in flexion (Flex CL) and extension (Ext CL) were measured on radiographs in the flexion and extension positions.

The cervical spine range of motion (ROM) was calculated as Ext CL - Flex CL. Cervical spine range of flexion (Flex ROM) was calculated as CL - Flex CL and cervical spine range of extension (Ext ROM) was calculated as Ext CL - CL. The extension ratio (EXR) was defined as 100 × Ext ROM/ROM. LCL was defined as preoperative CL - postoperative CL. Patients were classified into the following three groups according to the LCL[10, 20]: stability group: (LCL ≤ 5°); mild loss group (5° < LCL ≤ 10°); and severe loss group (LCL > 10°). The flowchart of study is displayed in Fig. 2.

Clinical parameters

The Japanese Orthopedic Association (JOA) score, before surgery and at the 1-year follow-up visit, was used to evaluate clinical outcomes. The recovery rate was calculated as follows: JOA recovery rate = 100× (postoperative JOA - preoperative JOA) / (17 - preoperative JOA).

Statistical analysis

All the data were analyzed using SPSS version 22.0 software (SPSS, Inc, Chicago, IL, USA). Variables were described as mean ± standard deviation and interclass correlation coefficient was used to indicate the measurement consistency between two observers. Pearson correlation analysis was used to analyze the correlation; multiple linear regression model was used to explore the risk factors for LCL. Chi-square test was used to compare categorical data among the groups; T-tests, ANOVA, and Kruskal–Wallis tests were used to assess the differences of radiographic parameters among the groups. A receiver-operating characteristic curve (ROC) analysis was used to determine the optimal cutoff value. P value < 0.05 was considered as evidence of statistical significance.

Results

Seventy-nine patients were enrolled (mean age 62.92 years; 51 men, 28 women) in the study. CL decreased significantly after CLP (pre-, 17.34 ± 10.44 vs. post 12.37 ± 11.42, p < 0.01). The overall demographic, surgery segments, and proximal level are summarized in Table 1.

Table 1

Summary of patient population and LCL correlations (N = 79)

Demographic

    

Sex

Male

51 (64.57%)
 

Female

28 (35.43%)

BMI (kg/m2)

  

25.23 ± 3.87

CL (°)

Pre

17.34 ± 10.44
 

Post

12.37 ± 11.42
 

P value

< 0.01

JOA

Pre

12.66 ± 2.26
 

Post

15.20 ± 1.11
 

recovery rate (%)

57.95 ± 23.28

Surgery segment (n)

    
 

3

29
 

4

36
 

5

14

Proximal level

C3

49
 

C4

30

Correlations between LCL and preoperative parameters

In the correlation analysis, LCL was positively correlated with cervical flexion capacity (r = 0.278, p < 0.05) and negatively correlated with cervical extension capacity (r = -0.456, p < 0.01). No significant correlations were observed between the other evaluated parameters (Table 2).

Table 2

LCL correlations (N = 79)

Parameter

Mean ± SD

Pearson

Age (y)

62.92 ± 9.97

0.081

Follow-up period (months)

19.69 ± 8.72

0.119

Pre CL (°)

17.34 ± 10.44

0.212

Pre T1S (°)

29.35 ± 7.43

0.205

Pre cSVA (mm)

22.21 ± 12.65

-0.175

T1S-CL (°)

12.01 ± 8.41

-0.082

Flex CL (°)

-13.33 ± 8.81

-0.128

Ext CL (°)

26.78 ± 11.65

-0.053

Total ROM (°)

42.09 ± 12.58

0.041

Flex ROM (°)

32.66 ± 12.02

0.278*

Ext ROM (°)

9.43 ± 6.21

-0.456**
*p < 0.05, **p < 0.01statistically significant difference

Risk factors for LCL

Multiple linear regression analysis was conducted by using variables that were found to be significantly correlated with the LCL. The results suggested that LCL decreased by 0.421° (p < 0.001) for each extension CL, and increased by 0.208° (p = 0.042) for flexion CL. LCL could be predicted by using the following regression equation: LCL = 5.507 − 0.421 * Ext ROM + 0.208 * Flex ROM (Table 3).

Table 3

Multiple linear regression model shows correlations between the loss of cervical lordosis and preoperative parameters (N = 79)

Model

Unstandardized coefficients

Standardized coefficient

T

Sig

B

SE

β

(Constant)

5.507

2.562

  

2.105

0.035

Ext ROM (°)

-0.498

0.119

-0.421

-4.184

0.000

Flex ROM (°)

0.127

0.061

0.208

2.069

0.042

Comparison of evaluated parameter variables according to postoperative LCL

Compared with the stability group (LCL ≤ 5°), the preoperative CL was significantly higher while postoperative CL was significantly lower in the severe loss group (LCL > 10°). Among the three groups, cervical extension Ext ROM was the best in the stability group. Compared with the stability group, Flex ROM was significantly higher and the extension ratio (EXR) was significantly lower in the severe loss group. As for clinical symptoms, pre- and postoperative JOA did not significantly differ among the three groups. Compared with the severe loss group, JOA recovery rates were better in the stability group (Table 4).

Table 4

Comparison of evaluated parameters variable according to the postoperative loss of CL

Parameter

Stability group

LCL ≤ 5°

(n = 47)

Mild loss group

5° < LCL ≤ 10°

(n = 15)

Severe loss group

LCL > 10°

(n = 17)

P

Age (y)

62.34 ± 10.49

66.47 ± 7.32

61.41 ± 10.29

p > 0.05

Follow-up period (months)

18.77 ± 7.51

20.88 ± 9.54

20.71 ± 11.03

p > 0.05

Surgery segment

3.89 ± 0.72

3.53 ± 0.64

3.82 ± 0.73

p > 0.05

Proximal level (C3)

28/47

9/17

12/19

p > 0.05

Pre CL (°)

15.92 ± 9.63*

16.93 ± 10.49

23.31 ± 11.42*

p < 0.05

Pre T1S (°)

28.59 ± 6.65

28.79 ± 8.58

31.95 ± 8.28

p > 0.05

Pre cSVA (mm)

24.62 ± 13.33

20.44 ± 9.57

17.11 ± 11.86

p > 0.05

T1S-CL (°)

12.92 ± 8.01

11.85 ± 7.58

9.65 ± 10.09

p > 0.05

Flex CL (°)

-14.95 ± 8.32

-16.76 ± 8.19

-15.08 ± 10.87

p > 0.05

Ext CL (°)

27.37 ± 11.73

24.45 ± 11.32

27.19 ± 12.14

p > 0.05

Post CL (°)

15.29 ± 9.69*

9.40 ± 11.49

6.48 ± 13.52*

p < 0.01

Total ROM (°)

42.32 ± 12.98

41.20 ± 8.64

42.27 ± 14.86

p > 0.05

Flex ROM (°)

30.63 ± 11.43*

33.69 ± 9.80

37.39 ± 14.40*

p < 0.05

Ext ROM (°)

11.69 ± 5.99*@

7.51 ± 5.66*

4.88 ± 4.00@

p < 0.01

EXR (%)

28.25 ± 14.58*

20.65 ± 13.98

11.79 ± 9.22*

p < 0.01

Pre JOA

12.43 ± 2.88

12.87 ± 1.73

13.13 ± 1.00

p > 0.05

Post JOA

15.57 ± 1.13

14.86 ± 1.19

14.50 ± 1.01

p > 0.05

JOA recovery rate (%)

69.23 ± 20.34*

48.18. ± 30.54

35.40 ± 25.01*

p < 0.01

*and @ indicated statistically significant difference between the two groups.

Effectiveness of EXR to predict severe lordosis loss

The ROC curve in Fig. 3 shows good discriminative power of EXR to predict severe lordosis loss after CLP (area under the curve = 0.808, p < 0.001; cutoff value, 16.80%). In the severe loss group, 15/17 patients were subclassified as low EXR (EXR ≤ 16.80%); in the mild loss group, 6/15 patients were subclassified as the low EXR group. In the stability group, only 4/43 patients were subclassified into the low EXR group.

Discussion

CLP has some potential complications, such as LCL, decreased neck motion range, axial neck pain, C5 nerve root palsy, and lamina closure. Among them, LCL is a significant issue. Having sufficient postoperative cervical lordosis is a prerequisite for CLP to obtain the indirect anterior decompression effect. LCL has been reported to be associated with poor outcomes after laminoplasty in many studies[6–8, 16, 22, 23]. Kim et al.[16] and Miyazaki et al.[18] reported that preoperative higher T1S was a risk factor for LCL. T1S-CL[7, 16, 21] and CL/T1S[24] have also been considered as predictors for LCL after CLP. However, there are some studies showing different results regarding the correlation between T1S and LCL [15, 25–27]. Michael et al.[11] and Seo et al.[7] emphasize the importance of cephalad vertebral level and cervical foraminal stenosis in LCL after laminoplasty. Some studies also report other factors for LCL, such as cSVA[14, 16, 21], C7-SVA[19, 20], CGH-C7 SVA[6, 8] and age[5]. In the present study, we evaluated regional static parameters to identify possible risk factors for postoperative kyphotic alignment change. However, no significant correlations were observed between the static parameters and LCL in our study. These static parameters have their own limitations in accounting for LCL after surgery. According to our knowledge, there are no theories with consensus that explain why these static parameters can affect LCL after surgery. Many researchers think that the posterior neck muscular-ligament complex may play an important role in these processes[5, 11, 15–17, 21, 28, 29].

Recently, some studies have shown the relationship between preoperative dynamic cervical sagittal alignment and LCL after CLP. Lee et al[15] reported the extension function of the cervical spine as an indicator to predict kyphotic change after CLP, and showed that significant kyphotic change occurred in patients whose Ext ROM was < 14°. Moreover, some studies have shown that higher Flex ROM results in greater LCL after CLP[10, 17, 28–30]. The present study showed similar results to those studies: preoperative Ext ROM (β = -0.421) and Flex ROM (β = 0.208) were predictors for postoperative LCL. Our study reported a high negative correlation between Ext ROM and LCL, which implies that enough Ext ROM is a highly reliable factor in preventing LCL after CLP. Similar to the results of previous studies[10, 17, 28–30], our study shows the positive correlation between Flex ROM and LCL. Cervical flexion mobility is blocked by degenerative structures, such as bone, ligaments, or muscles. Fujishiro et al.[28] speculated that increased motion in the flexional direction indicates that such structural forces restricting motion toward the kyphotic position are weak. Because of the surgical injury, the equilibrium necessary to maintain cervical sagittal alignment is disrupted and results in a higher prevalence of LCL.

In our study, we discovered that different degrees of postoperative LCL implied different degrees of neurological recovery. Worse JOA recovery rate was reported in patients in the severe loss group compared with the stability group. Similar tendencies were also shown between the stability group and the mild loss group; however, there was no evidence of statistical significance (p > 0.05). Postoperative mild LCL occurred in patients with a low level of Ext ROM and the influences of LCL on postoperative neurological recovery were limited. Preoperative high levels of Flex ROM aggravate postoperative LCL for patients with low Ext ROM, and severe LCL implies poor clinical outcomes (Table 4, Figs. 4 and 5).

Some researchers have speculated that the degree of cervical extension mobility indicates the cervical constriction reservoir[15] and cervical flexion mobility indicates the forces inhibiting cervical kyphosis[28]. Both Flex ROM and Ext ROM were important factors for LCL after CLP. Ono et al.[10] proposed CL ratio (100 × Flex ROM / total ROM) as a novel predictor for the loss of cervical lordosis after laminoplasty and reported the cut off value of CL ratio for predicting postoperative LCL. Compared with Flex ROM, Ext ROM had a greater influence on postoperative LCL in our study. Therefore, we reported EXR (100 × Ext ROM / total ROM) as a predictor, and EXR showed better prediction in severe lordosis loss than Ext ROM or Flex ROM alone. The optimal cutoff value of EXR to discriminate between severe LCL and not severe LCL was 16.8% (Fig. 3). For patients with a preoperative EXR less than 16.8%, more cervical exercises should be encouraged after surgery due to the high prevalence of severe postoperative LCL. Multilevel posterior cervical fusion or anterior cervical fusion surgery can also be considered, if necessary.

This study has several limitations. First, because our study was retrospective, a selection bias may exist. Second, the number of patients was low. Only 17 cases were assigned to the severe loss group. Third, the follow-up period was 1 year. Choi et al.[31] reported that changes in cervical sagittal alignment generally reach a plateau at 6 months after CLP. Thus, the follow-up period was enough to investigate the risks for LCL after CLP. Finally, only the JOA score was used to evaluate clinical outcomes in the present study.

Conclusion

Preoperative dynamic cervical sagittal alignment is a highly useful indicator to predict the LCL after CLP. CLP should be carefully considered for patients with a preoperative low Ext ROM and high Flex ROM, as a significant kyphotic change is likely to develop after surgery. EXR is a useful and simple index to predict significant kyphotic changes.

Abbreviations

CLP: cervical laminoplasty

CSM: cervical spondylotic myelopathy

LCL: loss of cervical lordosis

CL: cervical lordosis 

T1S: T1 slope

cSVA: cervical sagittal vertical axis

Flex CL: cervical lordosis in flexion

Ext CL: cervical lordosis in extension

ROM: cervical spine range of motion

Flex ROM: cervical spine range of flexion

Ext ROM: cervical spine range of extension

EXR: extension ratio

JOA: Japanese Orthopedic Association

ROC: receiver-operating characteristic curve

Declarations

Ethics approval and consent to participate

This study was approved by the Ethical committee of Beijing Xuanwu Hospital (clinical research NO. [2018]086). Informed consent was obtained from all subjects and/or their legal guardian(s). All methods were carried out in accordance with relevant guidelines and regulations.

Consent for publication

Not applicable

Availability of data and materials

The datasets generated and/or analysed during the current study are not publicly available due [This study is part of a series of studies that have not been completely completed] but are available from the corresponding author on reasonable request.

Competing interests

The authors declare that they have no competing interests

Funding

Not applicable

Authors' contributions

Chengxin Liu: Writing, Reviewing, Editing, Methodology and Data Curation. 

Bin Shi: Writing, Reviewing, Data Curation and Supervision.

Wei Wang: Editing, Data Curation and Supervision

Xiangyu Li: Editing and Supervision

Shibao Lu: Supervision.

Acknowledgements

Not applicable

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