In-vivo study of three-dimensional, cervical segmental motion characteristics following anterior cervical fusion with different numbers of fusion levels

doi:10.21203/rs.3.rs-3248409/v1

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In-vivo study of three-dimensional, cervical segmental motion characteristics following anterior cervical fusion with different numbers of fusion levels

https://doi.org/10.21203/rs.3.rs-3248409/v1

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Background

The pathogenesis of adjacent segment degeneration (ASD) and the biomechanical influence following anterior cervical fusion (ACF) surgery with different number of fusion levels remains controversial. The objective of this study was to investigate the effect of different numbers of fusion levels on adjacent segments and overall cervical kinematics.

Methods

From October 2018 to August 2020, patients who underwent ACF at our department were screened for enrollment in the study and divided into three groups based on the number of fusion levels: one-, two- and three-level. All subjects underwent pre- and post-operative cervical spine cone beam computed tomography(CBCT)scans at 7 functional positions. The range of motion (ROM) of the superior and inferior adjacent segments as well as the overall cervical, were measured using a 3D-3D registration technique. Changes in the pre- and post-operative ROMs were calculated for each patient. Clinical outcomes were assessed using the cervical spine stiffness scale, VAS and the JOA Scores.

Results

At the superior and inferior adjacent segment, the ROM of three-level were all greater than one-level during left-right bending and flexion-extension (p＜0.05). The global ROM of three-level were significantly smaller than one-level during flexion-extension (p＜0.05). The postoperative ROM of C4-C5 segment was significantly greater than that of C2-C3 (p＜0.05) at flexion-extension. The post-operative VAS、JOA scores showed no significant differences between the three groups(p > 0.05). But the post-operative cervical spine stiffness scores of the three-level was higher than that of the one-level (p＜0.05).

Conclusion

Three-level ACF had increased compensatory motion at the superior or inferior adjacent segments than one-level group at last follow-up and were probably to develop adjacent segment degeneration, which is more common in C4-C5 segment. Compared with the one-level fusion, the global ROM of the three-level decreased slightly and had little effect on the postoperative function of cervical spine.

Anterior cervical fusion

Adjacent segment degeneration

CBCT

3D-3D Registration Technology

Six degrees of freedom

Anterior cervical fusion (ACF) surgery was first reported in 1950s[1].With the fusion rate of up to 95%, this surgical technique can provide satisfactory post-operative neural decompression and good stabilization[1, 2].Despite the generally positive clinical outcomes after ACF surgery, adjacent segment degeneration (ASD) remains an important problem that should not be ignored after fusion surgery[3–5].This degeneration is defined as the appearance of new degenerative changes at spinal levels adjacent to the fusion segment ,which is often clinically silent initially[1, 6].

Many factors are involved in the development of ASD. Biomechanical studies have shown increased motion and intradiscal pressure at adjacent segments after spine fusion[7–9].Even so, there also have been extensive debates concerning whether ASD is caused by natural degeneration or aggravated by cervical surgery, whether the degeneration of adjacent segments related to the number of fused segments, and whether loss of ROM at the fused segments could be compensated for by increases in unfused adjacent segments[7, 10].

Some studies show that, as the number of fused segments increases, the mobilities of the adjacent segments increase that could lead to accelerated disc degeneration[11–13]. Conversely, there are also others reported that ASD might be a sequela of the natural aging of patients and a natural progression of cervical disc degeneration, and the number of fused levels has no effect on the occurrence of ASD[5, 8, 14].Consequently, it is clinical significance to identify the correlation between the number of fusion levels and the development of adjacent segment degeneration.

To better understand the cervical spine kinematics after ACF and whether the development of ASD is related to the number of fusion levels from a biomechanical perspective, we designed the current study based on combined cone beam computed tomography (CBCT) and 3D-3D image registration technology to explore the effect of ACF surgery itself on ASD at last follow-up by comparing the influence of single-, two- and three-level fusions on the cervical global ROM and adjacent segmental kinematics.

Patient recruitment

This study protocol was approved by the hospital’s research and ethics committee. All patients provided written, informed consent before participating in the study. Inclusion criteria consisted of patients ≥ 18 years of age with cervical radiculopathy and myelopathy who conservative treatment fails. Exclusion criteria included patients with other type cervical spondylosis; severe posterior longitudinal ligament ossification; previous cervical spine surgery and cervical vertebra deformity. All fusions were at levels between C3 and C7. Patient demographic data, including age, sex, BMI, operation level, pre-operative Pfirrmann grades of adjacent discs and postoperative plate-to-disc distance (PDD) were collected from all patients (Table 1). PDD refers to the distances between the tips of the caudal and cranial adjacent disc were measured on the postoperative lateral X-ray film of the cervical spine. Clinical outcomes were assessed using Visual Analogue Scale (VAS) and the Japanese Orthopedic Association Scores (JOA) as well as the cervical spine stiffness self-test score, which defined as: 0 points representing no stiffness, and 1–3 points, 4–6 points and 7–10 points representing mild, moderate, and severe stiffness, respectively. The outcome measures were collected pre-operatively and post-operatively (Table 2).

Table 1

Patient demographics for the three fusion-level groups.
	One-Level	Two-Levels	Three-Levels
Number of patients	19	17	16
Age	50.1 ± 7.5	50.4 ± 6.1	51.2 ± 9.9
Gender	10M/9F	8M/9F	8M/8F
BMI^a(kg/m²)	22.3 ± 2.5	22.4 ± 2.1	22.3 ± 1.6
Operation level
C3-C4	3	-	-
C4-C5	6	-	-
C5-C6	10	-	-
C3-C4, C4-C5	-	6	-
C4-C5, C5-C6	-	10	-
C5-C6, C6-C7	-	1	-
C3-C4, C4-C5, C5-C6	-	-	14
C4-C5, C5-C6, C6-C7	-	-	2
Superior adjacent segment PPCDD^b	3.4 ± 0.6	3.4 ± 0.5	3.5 ± 0.6
Inferior adjacent segment PPCDD	3.3 ± 0.5	3.4 ± 0.6	3.3 ± 0.5
Superior PDD^c	6.2 ± 0.8	6.2 ± 1.7	7.9 ± 2.4
Inferior PDD	8.2 ± 2.5	6.5 ± 1.6	6.1 ± 0.6
^aBMI: body mass index. ^bPCDD: Pre-operative Pfirrmann Classification of Disc Degeneration. ^cPDD: plate-to-disc distance

Table 2

The pre and postoperative clinical outcomes in the three fusion-level groups.
	Preoperative			Postoperative
	One-Level	Two-Levels	Three-Levels	One-Level	Two-Levels	Three-Levels
Stiffness	2.5 ± 1.3	2.9 ± 1.8	3.6 ± 1.6	0.4 ± 0.7(*)	1.1 ± 1.2	1.6 ± 1.3(*)
VAS^a	3.2 ± 1.6	2.5 ± 2.2	3.2 ± 1.8	1.1 ± 1.3	1.4 ± 1.2	1.5 ± 1.6
JOA^b	12 ± 2.5	12.7 ± 1.9	12.5 ± 2.5	15.3 ± 1.5	15.4 ± 1.4	15.2 ± 1.4
^aVAS: Visual Analogue Scale. ^bJOA: Japanese Orthopaedic Association Scores.
* Statistical significance between group comparisons (p = 0.046).

3D-CBCT modeling and six degrees of freedom (6DOF) coordinate systems

Computed tomography scans of each subject were taken using CBCT (I-CAT, KaVo3DeXam, PA) with the following parameters: 0.2 mm slice thickness, 0.25 mm pixel size, 20.27 mA tube current, 120 kV voltage, and a rotation (scan) time of 14.7 seconds. Participants underwent CBCT in seven positions (neutral, maximum head left-right rotation, maximum head flexion-extension, and maximum head lateral bending left-right). Patients sitting upright with their hip fixed and kept their shoulders immobile while turning their head and cervical spine to reach the maximum ROM in each of the 7 positions (Fig. 1). During scanning, patients wore a lead apron to protect their head and chest from radiation. The CBCT images were transferred to a computer workstation in DICOM format. A 3D model of each vertebral segment at each position was constructed using Mimics solid modeling software (Version 20.0, Materialize, Leuven, Belgium) (Fig. 2). The same modeling method was used to process the post-operative CBCT images, the surgically fused segments were treated as one piece (Fig. 3).

To precisely describe the 6DOF kinematics of the cervical spine, local vertebral coordinate systems were established in the neutral position for each subject (Fig. 4)[15]. For the local coordinate system of C1, the origin was located at the postero-inferior border of the anterior arch of C1. The z-axis of C1 was parallel to the line of the postero-inferior border of the anterior arch and the antero-inferior border of the posterior arch, with the anterior plane defined as positive. The y-axis was defined perpendicular to the z-axis, with superior being positive. The x-axis pointed to the left. The coordinate systems of C2 to C7 were also defined such that the origin was located at the most inferior point on the posterior wall of the vertebral body in the mid-sagittal plane. The z-axis was defined as the line connecting the anterior and posterior points in the inferior plane of the vertebral body, with anterior considered positive. The y-axis was defined as perpendicular to the z-axis, with the superior plane being positive. The positive x-axis was directed to the left.

3D–3D registration of functional positions and kinematics analysis

At each functional position, a 3D-3D registration technique was used to superimpose caudal segments on their corresponding models at the neutral position using Fluo-Motion software v2.0 (Innmotion Inc., Shanghai, China). The accuracy of this method for 3D-3D registration was smaller than 0.1 mm. Using this technique, the rotational matrix was obtained between the neutral position and each functional position for each caudal segment. The corresponding cephalad segment was translated and rotated in the same way to obtain the segment ROM when compared to its neutral position (Fig. 5) [16].

The 6DOF segmental ROM characteristics were calculated based on the corresponding vertebral functional positions with respect to the neutral position. Segmental motion characteristics at the superior and inferior adjacent segments, and overall cervical, were described in 6DOFs using Euler angles, with the sequence of flexion-extension (Flex-Ext), left-right twisting (Twist L-R), left-right bending (Bend L-R), and translations along the three axes. The maximum ROMs of forward flexion to extension, left-to-right twisting, and left bend to right bend were specifically analyzed for each segment from C1 to C7. Changes in the pre- and post-operative ROMs were calculated for each patient.

Statistical Analysis

Continuous variables were measured as mean ± standard deviations. A repeated-measures analysis of variance (Two-way ANOVA) with LSD post-hoc tests were conducted to compare patient demographics, clinical outcomes, and the ROMs of superior or inferior adjacent segments and overall cervical between the three groups. The analysis was conducted using SPSS 24.0 statistical software (IBM Corp., Armonk, NY, USA). Statistical significance was determined at the p < 0.05.

Patient demographics and clinical outcome

A total of 58 patients underwent for ACF surgery at our clinic from October 2018 to August 2020 were prospectively recruited for this study. Six patients were lost to follow-up, 52 patients (one-level: n=19;two-level༚n༝17༛three-level༚n༝16) were eventually enrolled, and the follow-up rate was 90%. The mean age of the 60 patients analyzed was 51.5 years (rang 41–69 years). The average follow-up duration was 12.4 months. No significant differences were found for age, gender, BMI, or the pre-operative Pfirrmann Grades of the adjacent discs among the three groups (Table 1). The pre-operative VAS, JOA scores and cervical spine stiffness scores were not significantly different among the three groups. Post-operatively, the mean stiffness score for the three-level group was 1.6 ± 1.3, which was significantly greater than that of the one-level group (0.4 ± 0.7, p = 0.046). However, the post-operative VAS and JOA scores showed no significant differences between the three patient groups. But the JOA and VAS scores of the post-operative in the three groups were significantly improved compared with the pre-operative (Table 2).

ROM changes at the superior adjacent segment

During left–right bending, the primary ROM in the three-level group was 3.7 ± 1.3°, which was significantly greater than in the one-level group (1.7 ± 1.4°, p = 0.029). Flexion-extension, the primary ROM was significantly greater in the three-level group compared with the one-level group (2.7 ± 2.3° vs. 4.8 ± 1.9°, p = 0.026). During left-right twisting, the primary ROMs in the three groups were 1.1 ± 0.6° vs. 2.3 ± 1.6° vs. 2.2 ± 1.3°, respectively (p＞0.05) (Fig. 6A). There were no significant differences observed between the three groups for coupled translations and rotations (Table 3).

Table 3

ROM changes of 6DOF at the superior adjacent segment during different activities.
	Translation (mm)			Rotation (degree)
	X	Y	Z	α	β	γ
Twisting left and right
one-level	0.2 ± 0.2	0.1 ± 0.1	0.2 ± 0.1	0.7 ± 0.5	1.1 ± 0.6	1.1 ± 1.2
two-levels	0.4 ± 0.4	0.1 ± 0.1	0.2 ± 0.2	1.1 ± 0.9	2.3 ± 1.6	3.0 ± 1.5
three-levels	0.4 ± 0.3	0.2 ± 0.1	0.3 ± 0.3	0.8 ± 0.7	2.2 ± 1.3	3.8 ± 3.0
Bending left and right
one-level	0.4 ± 0.4	0.2 ± 0.2	0.1 ± 0.1	1.1 ± 1.1	1.2 ± 1.0	1.7 ± 1.4(*^a)
two-levels	0.4 ± 0.3	0.2 ± 0.1	0.3 ± 0.2	1.4 ± 1.1	2.2 ± 1.9	3.0 ± 1.7
three-levels	0.4 ± 0.3	0.2 ± 0.2	0.2 ± 0.3	1.1 ± 0.8	2.8 ± 2.4	3.7 ± 1.3(*^a)
Flexion and extension
one-level	0.2 ± 0.2	0.6 ± 0.4	0.7 ± 0.6	2.7 ± 2.3(*^b)	0.7 ± 0.4	1.0 ± 0.7
two-levels	0.5 ± 0.7	0.5 ± 0.3	0.8 ± 0.5	4.1 ± 1.8	0.6 ± 0.4	0.8 ± 0.4
three-levels	0.2 ± 0.2	0.5 ± 0.5	1.1 ± 0.9	4.8 ± 1.9(*^b)	0.6 ± 0.5	1.0 ± 0.7
Values are presented as mean ± SD; The ranges of primary rotations are bolded and italicized.
X represents lateral translation, Y represents the supero-inferior translation, and Z represents antero-posterior translation. α represents flexion extension, β represents axial rotation, γ represents lateral bending.
* Statistical significance between group comparisons (p^a =0.029 and p^b =0.026).

ROM changes at the inferior adjacent segment

During left-right bending, the primary ROM in the three-level group was 2.9 ± 1.5°, which was significantly greater than in the one-level group (1.3 ± 0.8°, p = 0.016). The ROMs of the three-level group were significantly greater than the one-level group at flexion-extension (4.3 ± 0.7° vs. 2.7 ± 1.0°, p = 0.019). No significant differences was observed in left-right twisting between the three groups(0.6 ± 0.7° vs. 1.7 ± 1.4° vs. 1.2 ± 0.7°, p＞0.05) (Fig. 6B). The coupled translations and rotations in the three groups showed no significant differences (Table 4).

Table 4

ROM changes of 6DOF at the inferior adjacent segment during different activities.
	Translation (mm)			Rotation (degree)
	X	Y	Z	α	β	γ
Twisting left and right
one-level	0.3 ± 0.2	0.1 ± 0.1	0.1 ± 0.1	0.8 ± 0.7	0.6 ± 0.7	1.3 ± 1.3(*)
two-levels	0.4 ± 0.4	0.1 ± 0.2	0.2 ± 0.1	0.9 ± 0.9	1.7 ± 1.4	1.7 ± 0.9
three-levels	0.6 ± 0.6	0.2 ± 0.1	0.1 ± 0.1	0.5 ± 0.8	1.2 ± 0.7	2.1 ± 1.1
Bending left and right
one-level	0.2 ± 0.2	0.1 ± 0.1	0.1 ± 0.1	0.6 ± 0.5	0.5 ± 0.3	1.3 ± 0.8(*^a)
two-levels	0.5 ± 0.4	0.3 ± 0.4	0.4 ± 0.3	1.2 ± 1.2	1.4 ± 1.3	2.5 ± 1.4
three-levels	1.0 ± 0.9	0.3 ± 0.1	0.3 ± 0.2	1.2 ± 0.9	0.8 ± 0.6	2.9 ± 1.5(*^a)
Flexion and extension
one-level	0.2 ± 0.2	0.4 ± 0.2	0.3 ± 0.3	2.7 ± 1.0(*^b)	0.3 ± 0.1	0.7 ± 0.6
two-levels	0.5 ± 0.6	0.6 ± 0.3	0.3 ± 0.2	3.0 ± 1.7	0.5 ± 0.6	0.8 ± 0.9
three-levels	0.3 ± 0.3	1.5 ± 0.9	0.4 ± 0.3	4.3 ± 0.7(*^b)	0.4 ± 0.3	1.0 ± 0.9
Values are presented as mean ± SD; The ranges of primary rotations are bolded and italicized.
X represents lateral translation, Y represents the supero-inferior translation, and Z represents antero-posterior translation. α represents flexion extension, β represents axial rotation, γ represents lateral bending.
* Statistical significance between group comparisons (p^a=0.016 and p^b =0.019).

ROM changes of the overall cervical spine

During flexion-extension, compared with one-level fusion (20.6 ± 5.9°), the cervical global ROM of the three-level (12.3 ± 4.5°) was reduced by 67.5% (p = 0.032). For left–right twisting and bending, the global ROM in the one-, two-, and three-level groups were not significantly different each other (20.0 ± 7.5° vs. 19.4 ± 9.9° vs. 18.6 ± 7.2°, respectively; p > 0.05 and 20.9 ± 6.9° vs. 18.1 ± 7.4° vs. 17.3 ± 6.2°, respectively; p > 0.05). (Fig. 6C). There were no differences between the three groups for the coupled translations and rotations (Table 5).

Table 5

ROM changes of 6DOF at the overall cervical spine during different activities.
	Translation (mm)			Rotation (degree)
	X	Y	Z	α	β	γ
Twisting left and right
one-level	4.9 ± 3.9	0.8 ± 0.6	2.8 ± 2.5	3.3 ± 2.4	20.0 ± 7.5	3.2 ± 2.2
two-levels	6.6 ± 5.9	0.8 ± 0.8	2.3 ± 1.8	2.3 ± 2.1	19.4 ± 9.9	3.6 ± 3.2
three-levels	7.8 ± 3.8	1.0 ± 0.3	2.5 ± 2.3	3.8 ± 2.7	18.6 ± 7.2	5.2 ± 3.4
Bending left and right
one-level	8.0 ± 4.9	0.8 ± 0.4	1.9 ± 1.6	4.8 ± 2.9	6.7 ± 3.6	20.9 ± 6.9
two-levels	12.0 ± 6.1	2.5 ± 1.9	6.1 ± 4.9	5.1 ± 5.1	6.6 ± 5.0	18.1 ± 7.4
three-levels	12.2 ± 6.5	0.7 ± 0.9	1.8 ± 1.6	1.8 ± 1.5	8.2 ± 6.3	17.3 ± 6.2
Flexion and extension
one-level	1.5 ± 0.8	3.2 ± 1.7	12.8 ± 8.3	*20.6 ± 5.9()**	2.0 ± 0.7	1.5 ± 1.1
two-levels	2.4 ± 1.7	4.6 ± 3.0	14.2 ± 7.1	16.5 ± 8.3	3.2 ± 1.8	1.8 ± 1.4
three-levels	2.3 ± 1.5	3.0 ± 2.1	15.8 ± 7.2	*12.3 ± 4.5()**	3.3 ± 1.7	3.2 ± 1.7
Values are presented as mean ± SD; The ranges of primary rotations are bolded and italicized.
X represents lateral translation, Y represents the supero-inferior translation, and Z represents antero-posterior translation. α represents flexion extension, β represents axial rotation, γ represents lateral bending.
* Statistical significance between group comparisons (p = 0.032).

ROM changes at the different adjacent segment.

For flexion-extension, the average primary rotations of C4-C5 showed a greater ROM compared to other segments, but this difference was only observed between C4-C5 and C2-C3 segments (4.8 ± 0.9° vs. 2.5 ± 1.5°, p = 0.018). While left-right twisting and bending activities, no significant differences in primary rotations were found between the different adjacent segments (p > 0.05) (Fig. 6D). There were no significant differences in coupled motions between adjacent segments (Table 6).

Table 6

ROM changes of 6DOF at the different adjacent segment during different activities.
	Translation (mm)			Rotation (degree)
	X	Y	Z	α	β	γ
Twisting left and right
C2-C3	0.3 ± 0.2	0.1 ± 0.1	0.1 ± 0.1	0.8 ± 0.5	1.6 ± 1.0	1.7 ± 1.8
C3-C4	0.4 ± 0.4	0.1 ± 0.1	0.2 ± 0.2	1.1 ± 1.0	2.1 ± 1.3	1.3 ± 1.9
C4-C5	0.3 ± 0.3	0.1 ± 0.1	0.2 ± 0.1	0.9 ± 0.6	2.5 ± 2.2	1.4 ± 1.0
C5-C6	0.8 ± 0.5	0.1 ± 0.1	0.2 ± 0.1	1.1 ± 0.5	2.2 ± 1.3	1.8 ± 1.6
C6-C7	0.4 ± 0.5	0.1 ± 0.2	0.2 ± 0.1	0.9 ± 0.9	1.9 ± 1.4	1.8 ± 1.2
Bending left and right
C2-C3	0.5 ± 0.3	0.1 ± 0.1	0.2 ± 0.1	1.1 ± 0.8	1.6 ± 2.3	2.4 ± 1.4
C3-C4	0.5 ± 0.4	0.3 ± 0.1	0.3 ± 0.3	1.3 ± 1.6	1.3 ± 0.9	2.8 ± 1.9
C4-C5	0.4 ± 0.5	0.2 ± 0.1	0.2 ± 0.1	0.9 ± 0.6	1.1 ± 1.3	3.3 ± 1.9
C5-C6	0.3 ± 0.3	0.1 ± 0.1	0.2 ± 0.3	0.9 ± 0.6	1.4 ± 0.3	2.7 ± 1.5
C6-C7	0.6 ± 0.8	0.3 ± 0.3	0.3 ± 0.2	1.1 ± 1.2	1.2 ± 0.6	3.1 ± 1.9
Flexion and extension
C2-C3	0.2 ± 0.3	0.4 ± 0.3	0.7 ± 0.7	2.5 ± 1.5(*)	0.7 ± 0.5	1.1 ± 0.7
C3-C4	0.6 ± 0.9	0.5 ± 0.4	0.8 ± 0.6	4.1 ± 1.6	0.7 ± 0.6	1.0 ± 0.6
C4-C5	0.2 ± 0.1	0.7 ± 0.4	0.8 ± 0.6	4.8 ± 0.9(*)	0.5 ± 0.4	0.7 ± 0.8
C5-C6	0.5 ± 0.7	0.4 ± 0.2	0.4 ± 0.3	3.4 ± 1.0	1.0 ± 0.9	0.6 ± 0.3
C6-C7	0.3 ± 0.5	0.7 ± 0.5	0.4 ± 0.3	3.9 ± 1.8	0.6 ± 0.2	0.9 ± 0.8
Values are presented as mean ± SD; The ranges of primary rotations are bolded and italicized.
X represents lateral translation, Y represents the superior-inferior translation, and Z represents antero-posterior translation. α represents flexion extension, β represents axial rotation, γ represents lateral bending.
* Statistical significance between group comparisons (p = 0.018).

This study investigated the effect of one-, two- and three-level ACF on adjacent segmental kinematics and global ROM by using the technology of CBCT and 3D-3D registration. We found that in the superior and inferior adjacent segments, the primary ROM changes of left–right bending in the three-level group were significantly increased by 117.6% and 77.8% respectively, compared with the one-level group, the primary ROM changes of flexion and extension in the three-level group increased significantly by 123.1% and 59.3% respectively, compared with the one-level group. The findings proved that three-level fused could result in greater movement changes in the adjacent segments than one-level and were more likely to develop adjacent disc degeneration.

Some research found the similar results, Shin et al.[17]evaluated the motion changes of adjacent segments using radiological measurements and reported that compared with a one-level group, the primary ROM of three-level fusion significant increase in the upper and lower adjacent segments by 156.0% and 176.1%, respectively. According to a cadaveric biomechanical study[18], also found that compared with the single-level fused, the ROM in two-level upper adjacent segments increased by 31.3% and 33.8% in lower adjacent segment. Contrary to our findings, in our study, the increase in mobility of the upper adjacent segment was greater than that of the lower adjacent segment. In addition, the finite element analysis confirmed that mechanical load strain of the adjacent segment is much higher after multilevel fusions than single-level fusion[19].

Conversely, some scholars believed that adjacent segment degeneration is the result of natural degenerative process, not caused by spinal fusion itself[20–22]. Basques et al.[23]reported that compared with one-level fusion, multilevel cervical anterior fusions did not increase the risk of ASD. A similar conclusion was drawn by van Eck et al. [8]Furthermore, some authors found that the incidence of single-level ASD was higher than that in a multi-level group. For example, in the study of Bydon et al.[24], They found that the incidence of ASD was 47.2% in the one-level group and 34.2% in the multi-level group. The authors believed that one possible reason for this discrepancy may be attributable to the fact that the sample size of multilevel fusion group was small in their study.

Several local factors related to adjacent segment degeneration have been reported including the pre-existing asymptomatic disc degeneration at the adjacent segments, postoperative plate-to-disc distance less than 5mm and the number of the fusion level[22, 25, 26]. Moreover, other general factors which may be associated with disc degeneration including gender and age may play role in the development of adjacent segment degeneration[25]. Through the analysis of these factors, the only statistically significant relation observed in our study was the number of fused levels. Which demonstrated that the number of fusion segments is associated with the kinematics of the adjacent segments, implying that more attention should be paid to adjacent segments degeneration in the treatment of patients with multilevel fusions.

Few studies have reported the in vivo 6DOF kinematics of the adjacent segment following the ACF surgery. There is a lack of kinematic support for the stability of cervical spine after different fusion levels. We found that there was no obvious divergence in all directions of translation between the three groups at the seven functional positions, suggesting that multilevel fusion was not significantly instabilities compared to single level at last follow-up. Of course, more additional biomechanical studies are needed to confirm this speculation.

In current study, we also measured the ROM changes in postoperative different adjacent segments. And the data show that the most ROM changes was C4-C5. These mean that adjacent segment degeneration is most likely to occur at C4-C5. Some research found the similar results, Matsumoto et al[27]. investigated adjacent segment degeneration using MRI and reported that the C4-C5 segment is most prone to ASD. This result was also supported by the study of van Eck et al.[8] By contrast, Komura et al.[28] reported that ASD was more likely to occur at the C5-C6 segment. One of the reasons that the adjacent segment degeneration after ACF fusion occur at C4-C5 instead of C5-C6 may be that the C5–C6 segment had been frequently degenerated before surgery and was included into fusion (accounting for 75.5% in present study), leaving C4-C5 as unfused adjacent segments could lead to a greater ROM.

Regarding the global ROM of cervical spine, compared with one-level fusion, the cervical global ROM of the three-level was significantly reduced by 40.3% in flexion-extension. However, this phenomenon was not observed in left–right twisting and bending. In previous study on normal cervical spine mobility, the lower cervical spine is said to primarily function in flexion and extension, with C5-C6 being the most active[29, 30]. Three-level fusion surgery usually include C5-C6 segment in our study. Therefore, the flexion-extension ROM of three-level group would be significantly reduced compared to the other two positions. This is in consistent with previous findings, Chien et al.[31]reported that the entire cervical spine sagittal ROM of the multi-level fusion was 45.9% lower than that of the one-level. Shin et al[17].also found global cervical ROM in patients undergoing multilevel fusion reduced at 6 months. By contrast, Landers et al.[32] reported that the global cervical ROM improved significantly in all three motion planes 6 months post-operation, regardless of the number of levels fused. And Hilibrand et al.[9]also concluded that there was no correlation between post-operative global cervical motion and the number of operated levels. One explanation for this discrepancy may be due to the different ROM acquisition methods.

In current study, the post-operative VAS and JOA scores in three groups improved compared with those pre-operation. But there was no significant difference in the postoperative VAS and JOA scores between the three groups, which indicated that three-level fusion had little effect on the postoperative function of cervical spine. These similar to the results of Chien et al.[31]. Furthermore, we found that the post-operative cervical spine stiffness score in the three-level group was significantly higher than in the one-level group (p = 0.046), and patients with multilevel fusion had stronger subjective feelings of stiffness. Similar results were observed by Prasarn et al.[18]. This means that as the level of fusion increases, the force required for the patient's cervical spine to reach the same range of motion increases. In the long run, this may cause strain on the neck muscles, and correspondingly increase the load on the bone-ligament complex, resulting in hyperplasia and degeneration of bone and ligament.

Our study has several limitations. First, the follow-up period was only about 12.4 months follow-up time. However short-term measurements can better illustrate the actual impact of the different numbers of fusion levels on the adjacent segment after ACF surgery. Because the compensatory motions at the adjacent segment accompanied with spondyloarthritis and adjacent segment nature degeneration may result in the reduction of the ROM. Second, the sample size was relatively small. However, our results were still statistically significant (There were pre- and postoperative self-controls). Long-term studies are also very necessary, combined with short-term studies to better understand the impact of ACF surgery on adjacent segments degeneration. We will continue to include more patients in future studies.

We found that three-level ACF had increased compensatory motions at the unfused adjacent segments and were more likely to develop adjacent disc degeneration, which is more common in C4-C5 segment. Compared with the one-level fusion, the global ROMs of the three-level decreased slightly and had little effect on the postoperative function of cervical spine. These findings may serve as a reference for dynamic motion in adjacent segments of multilevel fusions and assist surgical decision-making for multilevel cervical degenerative diseases.

Ethics approval and consent to participate

This study was approved by the ethical review committee of The First Affiliated Hospital of Nanchang. All procedures were performed in accordance with relevant guidelines. University before the initiation of the study and a signed informed consent form was obtained from each subject.

Consent to publich

All involved subjects and the authors listed have approved the publication of the manuscript.

Availability of data and materials

The datasets supporting the conclusions of this article are included within the article. The raw data can be requested from the corresponding author on reasonable request.

Competing interests

The authors declare that they have no competing interests.

Funding

This work was supported by the Natural Science Foundation of China [grant number:81960408,82260445], Training plan for academic and technical leaders of major disciplines in Jiangxi Province [grant number: 20204BCJL22047],Clinical Cultivation Project of The First Affiliated Hospital of Nanchang University (No.YFYLCYJPY202202), [grant number: YC2022-s199].

Authors’ contributions

YZ and ZZ performed all the researches and wrote the manuscript. SL, GC, CL, JY and JL participated in the collection of researchful data. GL and SW guided the entire process of the research. ZW conceived and designed the study. All authors read and approved the final manuscript.

Acknowledgements

Not applicable.

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No competing interests reported.

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In-vivo study of three-dimensional, cervical segmental motion characteristics following anterior cervical fusion with different numbers of fusion levels

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Abstract

Figures

Introduction

Materials and Methods

Patient recruitment

3D-CBCT modeling and six degrees of freedom (6DOF) coordinate systems

3D–3D registration of functional positions and kinematics analysis

Statistical Analysis

Results

Patient demographics and clinical outcome

ROM changes at the superior adjacent segment

ROM changes at the inferior adjacent segment

ROM changes of the overall cervical spine

Discussion

Conclusion

Declarations

References

Additional Declarations

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