Axial pain after anterior cervical discectomy and fusion: risk factor analysis and imaging evaluation

Abstract

Background

Axial pain can occur as a complication following anterior cervical discectomy and fusion (ACDF), which is frequently regarded as a complication of posterior cervical surgery and is often overlooked after ACDF. However, the risk factors of postoperative axial pain (PAP) remain incompletely elucidated. This study aimed to analyze the risk factors and radiological assessment of PAP in patients undergoing ACDF.

Methods

Individuals undergoing ACDF between January 2020 to March 2023 were included. Participants were categorized into two groups by PAP presence. Clinical parameters were collected, and radiographic parameters measured included pre- and postoperative C_2 ~ 7 Cobb angle, C_2 ~ 7 sagittal vertical axis, T₁ slope, intervertebral disc height, facet joint distance (FJD), and interspinous distance. Spinal cord function was evaluated using the Japanese Orthopaedic Association score. Univariate/multivariable analyses identified independent risk factors for PAP after ACDF. Diagnostic performance of postoperative C_2 ~ 7 Cobb angle change and postoperative relative FJD change was evaluated via Receiver Operating Characteristic curve analysis. Optimal cutoff values were determined using the Youden index.

Results

In ACDF patients, PAP prevalence was 15.5% in single-level surgery and 19.8% in multi-level surgery. For single-level ACDF, logistic regression analysis identified postoperative relative FJD change as a PAP risk factor (Area Under the Curve [AUC] = 0.809, optimal cutoff = 27.95%). For multi-level ACDF, postoperative C_2 ~ 7 Cobb angle change was a risk factor (AUC = 0.773, optimal cutoff = 3°).

Conclusions

During ACDF procedures, minimizing FJD in single-level surgery and avoiding excessive C_2 ~ 7 Cobb angle correction in multi-level surgery may reduce PAP prevalence.

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1186/s12891-026-10101-x.

Introduction

Cervical spondylotic myelopathy (CSM) represents a frequent degenerative disorder of the spine [1] and serves as one of the leading etiological factors for numbness and pain affecting the cervical, shoulder and upper extremity regions. Since its first description by Robinson and Smith in 1955 [2], anterior cervical discectomy and fusion (ACDF) has gradually become a mainstream surgical procedure for treating cervical spine diseases and is considered the gold standard for the treatment of cervical disc herniation and cervical spondylosis [3–5]. The relevant complications after ACDF encompass wound infection, postoperative axial pain (PAP), postoperative retropharyngeal hematoma and associated breathing difficulties [6]. Among these, PAP is frequently regarded as a complication of posterior cervical surgery and is often overlooked after ACDF.

PAP has been primarily associated with posterior cervical surgeries since it was first described in 1996 [7], with an prevalence rate ranging from 7% to 58% [8]. In our clinical observations, we identified that some patients undergoing ACDF experienced immediate, persistent and refractory axial pain postoperatively. This prompted us to investigate the underlying causes and their potential association with intraoperative procedures. Indeed, the prevalence of PAP following ACDF has garnered significant attention from numerous scholars. Xu et al. [9] revealed that the prevalence of PAP exceeded 30% among patients receiving single-level ACDF. Cao et al. [10] found that 33.3% of ACDF patients developed PAP. PAP not only significantly reduces the postoperative quality of life but also causes considerable psychological distress to the patients. Therefore, it is crucial to reduce the prevalence of PAP following ACDF. Reported risk factors include hyper-distraction of the cervical vertebrates [11, 12], excessive cervical curvature change [13], loss of cervical mobility [14], decreased neck muscle strength and atrophy [15], prolonged cervical collar use [16], and preoperative neck pain [17]. These studies have different definitions of axial pain post-ACDF. For instance, Nicholas et al. [12] defined PAP as localized in the neck without radiation to the upper limbs, confined to the cervical spine, occipital region and posterior scapular areas. Whereas Hiroyuki et al. [17] thought neck stiffness, nuchal pain and shoulder pain after ACDF is PAP. Furthermore, previous studies included variables such as psychological and many subjective factors, which led to significant differences in the research results. In order to clarify whether there is a correlation between PAP and intraoperative operations, and to better guide the surgical strategy, in this study, we analyzed the risk factors for immediate PAP in ACDF patients using objective imaging data.

According to our clinical observations, most patients with PAP experience discomfort immediately after ACDF. This symptom develops on the first postoperative day after anesthetic resolution and can be effectively relieved by nonsteroidal anti-inflammatory drugs (NSAIDs) in the majority of cases. Therefore, to address this critical clinical challenge, we focused exclusively on immediate PAP and analyzed radiographic parameters to identify modifiable intraoperative risk factors.

Materials and methods

Patient inclusion

In this study, a retrospective analysis was performed on patients who underwent ACDF at the same medical center and treatment group from January 2020 to March 2023. Following strict inclusion and exclusion criteria, data from a total of 235 patients were collected. Institutional Review Board approval (No. [2024]258) was obtained prior to protocol implementation. Informed consent, which was signed by all participants, was obtained prior to their enrollment in the study. The inclusion criteria were: (1) patients presenting with signs and symptoms of spinal cord compression; (2) diagnosis of CSM confirmed by CT or MRI; (3) undergoing ACDF; (4) a postoperative follow-up of at least 12 months. The exclusion criteria were: (1) preoperative presence of varying degrees of axial neck pain; (2) surgical treatment due to trauma, tumor, rheumatism, infection, or ossification of the posterior longitudinal ligament; (3) postoperative intervertebral non-fusion occurred; (4) severe osteoporosis or congenital cervical deformities; (5) cerebral palsy, cerebral infarction, or other systemic diseases; (6) incomplete data, defined as the inability to accurately measure key radiographic parameters due to poor image clarity or anatomical obscuration, or loss to follow-up during the follow-up period.

Surgical procedures

A standard ACDF procedure was performed on each patient by the same surgical team at a single medical center. Routine intravenous induction followed by inhalation anesthesia via video laryngoscopic tracheal intubation was used. Patients were positioned supine with the head slightly turned to the left, and the shoulder and neck elevated to achieve cervical extension. After radiographic localization, a transverse incision of approximately 3–5 cm was made on the right anterior aspect of the cervical spine. The skin and subcutaneous tissues were bluntly dissected, the platysma muscle was incised longitudinally, and the vascular sheath and other tissues were sharply dissected using scissors without cutting the omohyoid muscle, exposing the anterior aspect of the vertebral body. The prevertebral fascia was sharply incised. The intervertebral disc was then removed, exposing the posterior longitudinal ligament, which was typically incised to achieve complete decompression. Finally, an autologous bone-filled cage was used to enhance postoperative bony fusion and placed in the intervertebral space of the operated segment. An anterior cervical plate was placed anteriorly at the responsible segment to ensure spinal stability before bony fusion. To prevent position-induced secondary compression of the cervical spine, intraoperative traction and cervical extension were avoided. The surgery was performed using a spinal surgery microscope (ZEISS, Germany) to assist with decompression.

Radiological assessment

Patients underwent lateral X-rays of the cervical spine at each follow-up preoperatively and postoperatively. Six radiographic parameters were systematically quantified: (1) The C_2 ~ 7 Cobb angle was determined by quantifying the angular dimension formed between the inferior endplates of the C₂ and C₇ cervical vertebrae. (2) The C_2 ~ 7 sagittal vertical axis (C_2 ~ 7 SVA) is radiographically assessed as the linear distance between the C₂ sagittal plumb line and the posterosuperior endplate of the C₇ vertebra. (3) The T₁ slope is defined as the angular value formed between the superior endplate of the T₁ vertebra and the horizontal reference plane. (4) Intervertebral disc height (IDH) is described as the distance between the midpoint of the inferior endplate of the superior vertebra and the midpoint of the superior endplate of the inferior vertebra. (5) Facet joint distance (FJD) quantified as the distance between the midpoint of the upper and lower facet joints. (6) Interspinous distance (ISD), measured as the distance between the midpoint of spinous process of superior vertebra and the inferior vertebra (Fig. 1).

Fig. 1.

Sagittal plane measurement parameters of cervical spine lateral radiograph. C_2~7 Cobb angle: denoted by blue; T₁ Slope: denoted by purple; C_2~7 SVA: C_2~7 sagittal vertical axis, denoted by green; IDH: intervertebral disc height, denoted by yellow; FJD: facet joint distance, denoted by red; ISD: interspinous distance, denoted by brown

Postoperative follow-up and data collection

In strict accordance with the study’s inclusion criteria, we retrieved data from the hospital’s electronic medical records system for enrolled patients, including age at surgery, sex, body mass index (BMI), baseline comorbidities, surgical procedures, operative segments, duration of surgery and imaging data. Patients were followed up preoperatively, postoperatively, and at 3-, 6-, and 12-month intervals.

Neurological function assessment

The Japanese Orthopaedic Association (JOA) scoring system was used to evaluate neurological function. A team of two certified research assistants, blinded to all clinical information, conducted duplicate assessments of the pre- and post-operative JOA scores, and the mean values from both observers were utilized for analysis.

Assessment of PAP

In this study, PAP is defined as persistent dull pain presenting on postoperative day 1 after anesthetic resolution. The pain radiates from the neck to the shoulder or shoulder girdle, accompanied by soreness, stiffness, pressure, and muscle spasms. Although this discomfort is typically alleviated by NSAIDs, a definitive diagnosis is made based on these clinical features after excluding other relevant systemic diseases, independent of medication responsiveness. PAP assessment utilizes the numeric Visual Analog Scale (nVAS). Patients with a nVAS score ranging from 1 to less than 4 were classified as experiencing mild PAP, those with scores from 4 to less than 7 were categorized as having moderate PAP, and patients with an nVAS score of 7 or higher were deemed to have severe PAP.

Statistical analysis

Statistical analyses were conducted with SPSS 26 (IBM, California, USA). The variables with a normal distribution are presented as mean ± standard deviation (SD) and were compared using t-tests. Variables failing to follow a normal distribution were described as median (interquartile range, IQR), with between-group differences examined via the Mann-Whitney U test. Categorical variables were described by frequencies and percentages, with ordered categories compared using Wilcoxon rank-sum test and unordered categories analyzed via chi-square or Fisher’s exact test. A p-value of less than 0.05 was considered statistically significant.

Risk factors were initially screened by univariate logistic regression. We incorporated variables with p < 0.05 and clinically significant indicators (even in cases where p ≥ 0.05) into the multivariable logistic regression model to determine the independent predictive factors of PAP following ACDF. Using Spearman’s rank correlation to detect the correlation between postoperative relative change in IDH and FJD. Diagnostic performance of postoperative C_2 ~ 7 Cobb angle change and postoperative relative change in FJD was assessed through Receiver Operating Characteristic (ROC) curve analysis. The optimal cutoff values were identified using the Youden index.

Results

Single-level ACDF

In total, 129 patients met the inclusion criteria and were followed up regularly for at least one year. Demographic and clinical baseline features of the included patients are presented in Table 1. Patients experiencing moderate PAP constituted the largest proportion (n = 13, 65%), followed by those suffering from severe pain (n = 4, 20%, Table 2). Among these, the C_5/6 segment constituted the highest proportion (55.81%) and exhibited the highest prevalence of PAP (65%, Table 3).

Table 1.

Demographics and baseline characteristics in patients undergoing single-level ACDF

Characteristic	N = 129
Age (year, Median, Q1-Q3)	55 (47, 62)
Sex (n, %)
male	62 (48.06%)
female	67 (51.94%)
BMI (kg/m², Median, Q1-Q3)	23.00 (21.50, 26.60)
Smoke (n, %)
yes	19 (14.73%)
no	110 (85.27%)
Diabetes (n, %)
yes	15 (11.63%)
no	114 (88.37%)
Hypertension (n, %)
yes	25 (19.38%)
no	104 (80.62%)
Operative time (min, Median, Q1-Q3)	58 (53, 62)
Pre C2~7 Cobb Angle (°, Median, Q1-Q3)	10.9 (7.8, 15.1)
Post C2~7 Cobb Angle (°, Median, Q1-Q3)	13 (8, 17)
Post-pre C2~7 Cobb Angle (°, Median, Q1-Q3)	1 (−1, 4)
Pre T1 slope (°, Median, Q1-Q3)	23 (16, 30)
Post T1 slope (°, Median, Q1-Q3)	26 (19, 31)
Post-pre T1 slope (°, Mean±SD)	2 ± 9
Pre C2~7 SVA (mm, Median, Q1-Q3)	21 (13, 30)
Post C2~7 SVA (mm, Mean±SD)	18 ± 10
Post-pre IDH (mm, Median, Q1-Q3)	1.80 (1.00, 2.80)
(Post-pre)/Pre IDH (%, Median, Q1-Q3)	29 (15, 51)
Post-pre FJD (mm, Median, Q1-Q3)	0.68 (0.60, 0.83)
(Post-pre)/Pre FJD (%, Median, Q1-Q3)	22.9 (20.1, 25.7)
Post-pre ISD (mm, Mean±SD)	−1.57 ± 3.19
(Post-pre)/Pre ISD (%, Median, Q1-Q3)	11 (5, 20)
Pre JOA scores (score, Median, Q1-Q3)	10.00 (9.00, 11.00)
Post JOA scores (score, Median, Q1-Q3)	14.00 (13.00, 14.00)

Values are shown as Median (IQR), n (%), or Mean ± SD. Immediate postoperative parameters were shown

Abbreviations: Pre Preoperative, Post Postoperative, Post-pre The postoperative value minus the preoperative value, (Post-pre)/Pre (The postoperative value minus the preoperative value) divided by the preoperative value, BMI Body mass index, C_2~7 SVA C_2~7 sagittal vertical axis, IDH Intervertebral disc height, FJD Facet joint distance, ISD Interspinous distance, JOA Japanese Orthopaedic Association

Table 2.

Postoperative axial pain severity in patients undergoing single-level ACDF

Gradation of Axial Pain	N = 20
Mild (0 < nVAS < 4)	3 (15%)
Moderate (4 ≤ nVAS < 7)	13 (65%)
Severe (7 ≤ nVAS < 10)	4 (20%)

Values are shown as n (%)

Table 3.

The segment in patients undergoing single-level ACDF

Segment	N = 129	Axial symptom, N = 20
C3/4	9 (6.98%)	0 (0%)
C4/5	38 (29.46%)	7 (35%)
C5/6	72 (55.81%)	13 (65%)
C6/7	7 (5.43%)	0 (0%)
C7/T1	3 (2.33%)	0 (0%)

Values are shown as n (%)

Of the total cohort, 20 patients were assigned to the axial pain group and 109 to the non-axial pain group. The postoperative T₁ slope between the two groups was statistically significant (p < 0.05), with a median of 28° (24°−33°) in the axial pain group and 25° (19°−31°) in the non-axial pain group. Additionally, there was a significant difference in postoperative relative change of FJD between the groups (p < 0.001), with a median of 29.0% (24.8%−34.0%) in the axial pain group and 22.3% (19.6%−24.6%) in the non-axial pain group. No significant intergroup differences were detected in age, sex, BMI, smoking status, diabetes, hypertension, operative time, over-correction of lordosis or preoperative variables like C_2 ~ 7 Cobb angle, C_2 ~ 7 SVA, T₁ slope and IDH (Table 4, S1 and Figure S1). Multivariable logistic regression analysis identified that postoperative relative change in FJD was significantly associated with PAP (p < 0.001, odds ratio [OR] = 1.31, 95% confidence interval [CI]: 1.15–1.48). Postoperative T₁ slope had no significant association (p = 0.124) (Table 5).

Table 4.

Univariate analysis of axial pain occurrence after single-level ACDF

Characteristic	Axial pain		p
	Axial symptom, N = 20	No axial symptom, N = 109
Age (year, Median, Q1-Q3)	57 (48, 63)	55 (47, 61)	0.365
Sex (n, %)			0.499
male	11 (55.00%)	51 (46.79%)
female	9 (45.00%)	58 (53.21%)
BMI (kg/m², Median, Q1-Q3)	23.00 (21.00, 24.50)	23.80 (22.00, 27.10)	0.421
Smoke (n, %)			0.174
yes	5 (25.00%)	14 (12.84%)
no	15 (75.00%)	95 (87.16%)
Diabetes (n, %)			>0.999
yes	2 (10.00%)	13 (11.93%)
no	18 (90.00%)	96 (88.07%)
Hypertension (n, %)			0.763
yes	3 (15.00%)	22 (20.18%)
no	17 (85.00%)	87 (79.82%)
Operative time (min, Median, Q1-Q3)	58.5 (52, 63)	57 (53, 61)	0.916
Pre C2~7 Cobb Angle (°, Median, Q1-Q3)	11.2 (8.7, 13.1)	10.9 (7.6, 15.5)	0.817
Post C2~7 Cobb Angle (°, Median, Q1-Q3)	13 (11, 17)	13 (7, 18)	0.843
Post-pre C2~7 Cobb Angle (°, Median, Q1-Q3)	2 (1, 4)	1 (−1, 4)	0.145
Pre T1 slope (°, Median, Q1-Q3)	26 (20, 30)	22 (16, 29)	0.263
Post T1 slope (°, Median, Q1-Q3)	28 (24, 33)	25 (19, 31)	0.041*
Post-pre T1 slope (°, Mean±SD)	4 ± 4	2 ± 10	0.156
Pre C2~7 SVA (mm, Median, Q1-Q3)	21 (13, 29)	21 (13, 30)	0.667
Post C2~7 SVA (mm, Mean±SD)	19 ± 9	18 ± 10	0.663
Post-pre IDH (mm, Median, Q1-Q3)	1.45 (0.58, 3.18)	1.80 (1.10, 2.70)	0.715
(Post-pre)/Pre IDH (%, Median, Q1-Q3)	25 (9, 56)	29 (16, 49)	0.861
Post-pre FJD (mm, Median, Q1-Q3)	0.75 (0.60, 1.00)	0.66 (0.59, 0.81)	0.137
(Post-pre)/Pre FJD (%, Median, Q1-Q3)	29.0 (24.8, 34.0)	22.3 (19.6, 24.6)	<0.001*
Post-pre of ISD (mm, Mean±SD)	−1.80 ± 3.03	−1.52 ± 3.24	0.714
(Post-pre)/Pre ISD (%, Median, Q1-Q3)	14 (5, 20)	11 (5, 20)	0.747
Pre JOA scores (score, Median, Q1-Q3)	9.00 (8.75, 11.00)	10.00 (9.00, 11.00)	0.127
Post JOA scores (score, Median, Q1-Q3)	13.50 (13.00, 14.00)	14.00 (13.00, 14.00)	0.454

Values are shown as Median (IQR), n (%), or Mean ± SD. *p < 0.05 was considered statistically significant. Immediate postoperative parameters were used for analysis

Abbreviations: Pre Preoperative, Post Postoperative, Post-pre The postoperative value minus the preoperative value, (Post-pre)/Pre (The postoperative value minus the preoperative value) divided by the preoperative value, BMI Body mass index, C_2~7 SVA C_2~7 sagittal vertical axis, IDH Intervertebral disc height, FJD Facet joint distance, ISD Interspinous distance, JOA Japanese Orthopaedic Association

Table 5.

Multivariable logistic analysis of axial pain occurrence after single-level ACDF

Characteristic	N	OR	95% CI	p
Post T1 slope (°)	129	1.08	0.98, 1.18	0.124
(Post-pre)/Pre FJD (%)	129	1.31	1.15, 1.48	<0.001*

*p < 0.05 was considered statistically significant. Immediate postoperative parameters were used for analysis

Abbreviations: OR: Odds Ratio, CI Confidence Interval, Post Postoperative, (Post-pre)/Pre (The postoperative value minus the preoperative value) divided by the preoperative value, FJD Facet joint distance

Spearman’s rank correlation revealed no significant correlation between postoperative relative change in IDH and FJD (r_s = 0.056, p > 0.05) (Fig. 2).

Fig. 2.

Correlation analysis between (Post-pre)/Pre IDH and (Post-pre)/Pre FJD. (Post-pre)/Pre: (the postoperative value minus the preoperative value) divided by the preoperative value; IDH: intervertebral disc height; FJD: facet joint distance

Representative cases illustrating the association between postoperative relative change in FJD and PAP are shown in Fig. 3.

Fig. 3.

Typical case presentation. A male patient in his 40 s with cervical spondylotic myelopathy (CSM) underwent single-level ACDF at the C_5/6 level, with cage implantation and autograft fusion (Fig. 3A and B). Immediate postoperative X-ray showed the facet joint distance (FJD) (Fig. 3B) increased by 29.82% compared to preoperative measurements (Fig. 3A). The patient developed moderate axial pain postoperatively (nVAS = 4). Another case involved a 65-year-old male patient with CSM, who similarly underwent single-level ACDF at the C_5/6 level, with cage implantation and autograft fusion (Fig. 3C, Fig. 3D). Immediate postoperative X-ray showed the FJD (Fig. 3D) increased by 8.33% compared to preoperative measurements (Fig. 3C). This patient had a good recovery and did not experience postoperative axial pain

The area under the ROC curve (AUC) for postoperative relative change in FJD and PAP is shown in Fig. 4 (AUC = 0.809). Based on the ROC curve, the optimal Youden index (0.64) corresponds to postoperative relative change in FJD was 27.95% (sensitivity 70.0%, specificity 93.6%).

Fig. 4.

Receiver Operating Characteristic (ROC) curve of (Post-pre)/Pre FJD and PAP. (Post-pre)/Pre: (the postoperative value minus the preoperative value) divided by the preoperative value; FJD: facet joint distance; AUC: the area under the ROC curve; PAP: postoperative axial pain

Multi-level ACDF

We analyzed data from 106 patients with a mean age of 57 who underwent multi-level ACDF (Table 6), including 21 patients with PAP and 85 patients without this symptom. The cohort consisted of 63 male and 43 female participants, demonstrating a mean BMI of 24.2 ± 3.1 kg/m². Patients experiencing moderate PAP constituted the largest proportion (61.90%), followed by those suffering from severe pain (28.57%, Table 7).

Table 6.

Demographics and baseline characteristics in patients undergoing multi-level ACDF

Characteristic	N = 106
Age (year, Median, Q1-Q3)	57 (45, 65)
Sex (n, %)
male	63 (59.43%)
female	43 (40.57%)
BMI (kg/m², Mean±SD)	24.2 ± 3.1
Smoke (n, %)
yes	23 (21.70%)
no	83 (78.30%)
Diabetes (n, %)
yes	11 (10.38%)
no	95 (89.62%)
Hypertension (n, %)
yes	18 (16.98%)
no	88 (83.02%)
Operative time (min, Median, Q1-Q3)	90 (85, 95)
Pre C2~7 Cobb Angle (°, Median, Q1-Q3)	12 (8, 18)
Post C2~7 Cobb Angle (°, Median, Q1-Q3)	17 (11, 23)
Post-pre C2~7 Cobb Angle (°, Median, Q1-Q3)	3 (1, 8)
Pre T1 slope (°, Mean±SD)	23 ± 7
Post T1 slope (°, Mean±SD)	24 ± 6
Pre C2~7 SVA (mm, Median, Q1-Q3)	21 (12, 31)
Post C2~7 SVA (mm, Median, Q1-Q3)	17 (10, 28)
Pre JOA scores (score, Median, Q1-Q3)	10.00 (9.00, 11.00)
Post JOA scores (score, Median, Q1-Q3)	14.00 (13.00, 14.75)

Values are shown as Median (IQR), n (%), or Mean ± SD. Immediate postoperative parameters were shown

Abbreviations: Pre Preoperative, Post Postoperative, Post-pre The postoperative value minus the preoperative value, BMI Body mass index, C_2~7 SVA C_2~7 sagittal vertical axis, JOA Japanese Orthopaedic Association

Table 7.

Postoperative axial pain severity in patients undergoing multi-level ACDF

Gradation of Axial Pain	N = 21
Mild (0 < nVAS < 4)	2 (9.52%)
Moderate (4 ≤ nVAS < 7)	13 (61.90%)
Severe (7 ≤ nVAS < 10)	6 (28.57%)

Values are shown as n (%)

Relative to the non-axial pain group, the axial pain group exhibited significantly larger postoperative C_2 ~ 7 Cobb angle (20° vs. 16°, p < 0.05) and greater postoperative C_2 ~ 7 Cobb angle change (8° vs. 3°, p < 0.001). There were no significant differences in baseline characteristics and other preoperative and postoperative clinical radiographic parameters between the two groups (Table 8). Multivariable logistic regression analysis incorporating postoperative C_2 ~ 7 Cobb angle and postoperative C_2 ~ 7 Cobb angle change was performed. The postoperative C_2 ~ 7 Cobb angle change was significantly associated with PAP (p < 0.01, OR = 1.14, 95% CI: 1.03–1.25). By contrast, the postoperative C_2 ~ 7 Cobb angle showed no statistical significance (p > 0.05) (Table 9).

Table 8.

Univariate analysis of axial pain occurrence after multi-level ACDF

Characteristic	Axial pain		p
	Axial symptom, N = 21	No axial symptom, N = 85
Age (year, Median, Q1-Q3)	56 (45, 69)	57 (46, 63)	0.510
Sex (n, %)			0.797
male	13 (61.90%)	50 (58.82%)
female	8 (38.10%)	35 (41.18%)
BMI (kg/m², Mean±SD)	24.5 ± 3.5	24.1 ± 3.1	0.623
Smoke (n, %)			0.773
yes	5 (23.81%)	18 (21.18%)
no	16 (76.19%)	67 (78.82%)
Diabetes (n, %)			0.453
yes	3 (14.29%)	8 (9.41%)
no	18 (85.71%)	77 (90.59%)
Hypertension (n, %)			0.752
yes	4 (19.05%)	14 (16.47%)
no	17 (80.95%)	71 (83.53%)
Operative time (min, Median, Q1-Q3)	90 (85.5, 94.5)	90 (84, 95)	0.914
Pre C2~7 Cobb Angle (°, Median, Q1-Q3)	11 (8, 13)	12 (8, 19)	0.330
Post C2~7 Cobb Angle (°, Median, Q1-Q3)	20 (18, 23)	16 (10, 23)	0.035*
Post-pre C2~7 Cobb Angle (°, Median, Q1-Q3)	8 (6, 10)	3 (0, 6)	<0.001*
Pre T1 slope (°, Mean±SD)	21 ± 6	23 ± 7	0.121
Post T1 slope (°, Mean±SD)	23 ± 7	24 ± 6	0.530
Pre C2~7 SVA (mm, Median, Q1-Q3)	22 (12, 34)	21 (12, 31)	0.937
Post C2~7 SVA (mm, Median, Q1-Q3)	18 (11, 25)	17 (10, 28)	0.924
Pre JOA scores (score, Median, Q1-Q3)	10.00 (9.00, 11.00)	10.00 (9.00, 11.00)	0.669
Post JOA scores (score, Median, Q1-Q3)	14.00 (13.00, 15.00)	14.00 (13.00, 14.00)	0.535

Values are shown as Median (IQR), n (%), or Mean ± SD; *p < 0.05 was considered statistically significant. Immediate postoperative parameters were used for analysis

Abbreviations: Pre Preoperative, Post Postoperative, Post-pre The postoperative value minus the preoperative value, BMI Body mass index, C_2~7 SVA C_2~7 sagittal vertical axis, JOA Japanese Orthopaedic Association

Table 9.

Multivariable logistic analysis of axial pain occurrence after multi-level ACDF

Characteristic	N	OR	95% CI	p
Post C2~7 Cobb Angle (°)	106	1.00	0.93, 1.07	0.894
Post-pre C2~7 Cobb Angle (°)	106	1.14	1.03, 1.25	0.009*

*p < 0.05 was considered statistically significant. Immediate postoperative parameters were used for analysis

Abbreviations: OR Odds Ratio, CI Confidence Interval, Post Postoperative, Post-pre The postoperative value minus the preoperative value

Representative cases demonstrating the relationship between postoperative C_2~7 Cobb angle change and PAP are shown in Fig. 5.

Fig. 5.

Typical case presentation. A male patient in his 50 s with cervical spondylotic myelopathy (CSM) underwent ACDF at the C_4/5 and C_5/6 levels, with cage implantation and autograft fusion (Fig. 5A and B). Immediate postoperative X-ray showed the C_2~7 Cobb angle (Fig. 5B) increased by 8° compared to preoperative measurements (Fig. 5A). The patient experienced moderate axial pain postoperatively (nVAS = 5). Another case involved a 72-year-old male patient with CSM, who similarly underwent ACDF at the C_4/5 and C_5/6 levels, with cage implantation and autograft fusion (Fig. 5C and D). Immediate postoperative X-ray showed the C_2~7 Cobb angle Fig. 5D) increased by 3° compared to preoperative measurements (Fig. 5C). This patient had a good recovery and did not experience postoperative axial pain

The predictive performance of postoperative C_2 ~ 7 Cobb angle change for PAP was evaluated via ROC curve analysis (AUC = 0.773, Fig. 6). The optimal cutoff value, determined by the highest Youden index (0.505), was associated with a sensitivity of 0.952 and specificity of 0.553.

Fig. 6.

Receiver Operating Characteristic (ROC) curve of post-pre C_2~7 Cobb Angle and PAP. Post-pre: the postoperative value minus the preoperative value; AUC: the area under the ROC curve; PAP: postoperative axial pain

Among patients with PAP following single-level surgery (n = 20), 85% achieved pain resolution within 6 months (45% ≤ 3 months), with only 15% experiencing pain persisting for ≥ 7 months. Multi-level patients (n = 21) showed lower early resolution (23.8% ≤ 3 months) and higher prolonged pain (38.1% ≥ 7 months), indicating multi-level surgery correlates with more persistent axial pain (Table 10).

Table 10.

Duration of axial pain in patients undergoing ACDF

Duration of Axial Pain	Single-level	Multi-level
≤ 3 months	9 (45%)	5 (23.8%)
4 - 6 months	8 (40%)	8 (38.1%)
7 - 12 months	2 (10%)	6 (28.6%)
> 12 months	1 (5%)	2 (9.5%)

Values are shown as n (%)

Discussion

In this retrospective study, we aimed to analyze the risk factors for immediate PAP in ACDF patients using objective imaging data to better guide surgical strategies. Our principal findings demonstrated that the specific risk factors for immediate PAP differ depending on the surgical scope. Specifically, for single-level ACDF, a postoperative relative change in FJD was identified as the independent risk factor, with an optimal cutoff value of 27.95%. Conversely, for multi-level ACDF, the postoperative C_2 ~ 7 Cobb angle change was the primary risk factor, with an optimal cutoff of 3°. These objective radiological findings directly support our hypothesis that the development of immediate PAP is closely associated with specific, modifiable intraoperative structural alterations rather than purely subjective or psychosocial factors.

Previous studies have shown that PAP after ACDF is related to excessive distraction of the intervertebral space [18], and Bai et al. [19] observed that if the height variation of the intervertebral space in the operated segment after ACDF exceeds 10%, the prevalence of PAP significantly increases. Wang et al. [20] believed that excessive or insufficient increase in intervertebral space can increase the prevalence of PAP. However, these conclusions are controversial, as other studies have shown no direct relationship between excessive vertebral distraction and PAP [21, 22]. In this research, we found that PAP shows no correlation with the IDH but is associated with variations in the FJD, consistent with the findings of previous studies by Lee et al. [23] A comprehensive analysis suggests that the following factors may contribute to this discrepancy: Firstly, anatomical studies have shown that ligaments directly connect the facet joint capsules to cervical nerve roots [24]. The radial intraforaminal ligaments existing between the cervical nerve roots and the surrounding tissues function as a protective mechanism for the nerve roots. They situate the nerve roots in the most appropriate position within the intervertebral foramina, provide support to the nerve roots within the foramina, and safeguard them from friction with the surrounding tissues. These ligaments encircling the nerve roots within the intervertebral foramina can, to a certain extent, preserve the morphology of the nerve root sleeves, facilitating the transmission of neuroelectrophysiological signals, the flow of cerebrospinal fluid, and blood circulation within the root sleeves. When the facet joint gap is dilated, it may exert tension on the ligaments that anchor the nerve roots in the intervertebral foramina, potentially inducing PAP. Additionally, patients who underwent single-level ACDF, the C_5/6 segment had the highest prevalence of PAP (65%), followed by the C_4/5 segment (35%). The possible reason for this is the dermatomal overlap between C₅/C₆ innervation and PAP distribution. Secondly, when the facet joint gap is excessively opened, it will stimulate the sensory receptors located on the facet joints, which in turn leads to symptoms such as discomfort or pain in patients. Finally, it could potentially be attributed to the depth of cage placement. In this research, we identified that postoperative relative change in IDH has no correlation with postoperative FJD change. Because the height of intervertebral disc distraction lies in the longitudinal dimension, while the facet joint plane typically assumes a 45° angle and is influenced by both longitudinal and lateral dimensions. When the cage is placed relatively shallow, the effect on the facet joint space is relatively minor. On the contrary, when the cage is placed relatively deep, the influence on the facet joint space is relatively significant. Based on the above analysis, we hold the view that PAP is not related to IDH, but is associated with the change in FJD. These findings carry significant clinical implications for surgical strategies. To prevent the FJD increase from exceeding the 27.95% threshold, especially at high-risk segments such as C_4/5 and C_5/6, surgeons should optimize cage positioning rather than merely restricting cage height. Placing the cage at a relatively shallower (more anterior) position provides adequate anterior column support without excessively wedging open the posterior facet joints. Furthermore, regarding the balance between minimizing facet distraction and achieving indirect decompression, alleviating neural compression remains the absolute clinical priority. In cases involving severe posterior compression, such as ligamentum flavum buckling, sufficient distraction is necessary. However, performing a meticulous direct anterior decompression can reduce the surgical reliance on aggressive indirect distraction, thereby achieving optimal neurological recovery while minimizing the risk of PAP.

Our study found that among patients undergoing multi-level ACDF, a greater increase in postoperative C_2 ~ 7 Cobb angle relative to preoperative values is associated with a higher likelihood of PAP. The greater correction of cervical curvature achieved by ACDF also increases the mechanical load on the cervical posterior column. When the physiological lordosis of the cervical spine decreases or disappears, the pressure on the intervertebral disc increases, the muscle strength of the neck is unbalanced, and fatigue injury is prone to occur. The nerve tissue may be subjected to traction and stimulation, causing discomfort such as upper limb pain, finger numbness, dizziness, etc. A recent study by Xu et al. [25] demonstrated that the spreader-assisted vertebral distraction technique significantly alleviates postoperative cervicalgia in patients undergoing ACDF, this effect may stem from the transient reduction of facet joint load distribution achieved through intraoperative vertebral distraction. Bilateral cervical facet joints contain rich sensory receptors capable of sensing the intensity of physiological stimulation [26], so when pressure on the posterior column increases, the bilateral cervical facet joints that bear most of the pressure will generate pain. Furthermore, the static sagittal balance of a healthy spine constitutes the physiological configuration of muscular structures achieved with maximal functional efficiency [27]. When there is sagittal imbalance in the cervical spine, the imbalance between flexor and extensor muscle strength causes PAP. With the continuation of the disease state for a period of time, the balance between flexor and extensor muscles gradually adapts, resulting in compensatory balance in the disease state, and the greater the correction of cervical curvature achieved by ACDF, the greater the disruption to this compensatory balance, leading to the reappearance of balance between flexor and extensor muscles, causing PAP in patients.

Interestingly, there was no statistically significant difference in the postoperative C_2 ~ 7 Cobb change between the two groups after single-level ACDF. This may be related to the small correction ability of single-level ACDF for cervical curvature. The correction of cervical curvature by multi-level ACDF can be roughly considered as the accumulation of the effects of multiple single-level ACDF in correcting cervical curvature. Therefore, for multi-level ACDF, the postoperative C_2 ~ 7 Cobb change is significant, leading to substantial alterations in cervical curvature and thus causing PAP. In contrast, single-level ACDF demonstrates limited cervical curvature correction. Furthermore, we observed no relationship between spinal cord neurofunctional recovery and PAP, consistent with the findings of other researchers [28–31].

This study has several limitations. Firstly, due to its retrospective nature, certain important radiographic parameters, such as the spinal canal diameter, were not quantitatively measured, which may limit our comprehensive evaluation of spinal stenosis and introduce selection bias. Secondly, subjective perceptions of PAP may vary among different patients in different environments. Thirdly, other postoperative complications that may affect PAP were not considered. Fourthly, for multi-level ACDF, local segmental parameters were excluded from the multivariable analysis due to the methodological challenges of construct-level aggregation, restricting our analysis to global alignment. Fifthly, because the predictive performance (AUC) was evaluated within the same cohort used for model development, the reported ROC thresholds may be subject to optimism bias and statistical overfitting, lacking internal or external validation. Future research should conduct multi-center, large-sample prospective studies with broader radiographic evaluations and independent validation cohorts to control for these factors and reliably determine risk factors associated with PAP.

Conclusion

In this study, we evaluated single-level and multi-level ACDF cohorts separately to identify risk factors for PAP. For single-level ACDF, a postoperative relative increase in FJD exceeding 27.95% was an independent risk factor. For multi-level ACDF, a postoperative C_2 ~ 7 Cobb angle increase of more than 3° was the primary risk factor. Therefore, meticulous control of FJD in single-level procedures and avoiding overcorrection of the C_2 ~ 7 Cobb angle in multi-level procedures may reduce the prevalence of PAP.

Abbreviations

ACDF

Anterior cervical discectomy and fusion

PAP

Postoperative axial pain

FJD

Facet joint distance

AUC

Area Under the Curve

CSM

Cervical spondylotic myelopathy

NSAIDs

Nonsteroidal anti-inflammatory drugs

C_2~7 SVA

C_2 ~ 7 sagittal vertical axis

IDH

Intervertebral disc height

ISD

Interspinous distance

BMI

Body mass index

JOA

Japanese Orthopaedic Association

nVAS

Numeric Visual Analog Scale

SD

Standard deviation

IQR

Interquartile range

ROC

Receiver Operating Characteristic

OR

Odds ratio

CI

Confidence interval

Authors’ contributions

CYT: Writing - original draft, Writing - review & editing, Data curation, Validation, Software. CX: Writing - review & editing, Validation, Methodology, Investigation. CGH: Writing - review & editing, Formal analysis, Resources. GW: Writing - review & editing, Formal analysis, Supervision, Visualization. LCD: Writing - review & editing, Supervision, Funding acquisition, Project administration. PXS: Conceptualization, Writing - review & editing, Supervision, Funding acquisition. DYH: Conceptualization, Writing - review & editing, Supervision, Funding acquisition. All authors read and approved the final manuscript.

Funding

This study was supported by grants from the National Natural Science Foundation of China (grant numbers 82373020, 82203721), the Basic and Applied Basic Research Foundation of Guangdong province (grant numbers 2023A1515010581, 2023A1515011580, 2024A1515030230, 2025A1515011186), Natural Science Research Project of Anhui Provincial Department of Education (grant number 2023AH053409), the Provincial Natural Science Foundation of Anhui (grant number 2208085QH253), China Postdoctoral Science Foundation (grant number 2024T170879, 2022M723045), Anhui Province Postdoctoral Science Foundation (grant number 2023B674), Joint Fund for Medical Artificial Intelligence (grant number, MAI2023Q022).

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

This study was conducted in accordance with the ethical principles of the Declaration of Helsinki. This study was approved by the Clinical Research and Laboratory Animal Ethics Committee of the First Affiliated Hospital of Sun Yat-sen University (No. [2024]258). Informed consent, which was signed by all participants, was obtained prior to their enrollment in the study.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.