Abstract
Background
Anterior cervical discectomy and fusion (ACDF) has been regarded as a standard procedure to treat Cervical degenerative disc disease (CDDD). Anterior bone loss (ABL), observed at the anterior vertebral body of the operative segment, usually considered to occur only after cervical disc replacement. However, some research showed ABL appears to be similarly prevalent following ACDF. Despite this recognition, the precise mechanisms and implications of ABL on surgical efficacy remain uncertain.
Methods
A total of 90 patients who underwent single-level ACDF using Zero-P with a minimum follow-up of at least one year were retrospectively reviewed. ABL was measured and classified into four grades according to Kieser’s methods. According to that, the patients were grouped into none-mild ABL group and moderate-severe ABL group. Baseline data, clinical evaluation, and radiological parameters were recorded and compared.
Results
Of the 90 patients, 57 (63.3%) developed ABL postoperatively. Among them, 16 cases (28.06%) were mild, 26 cases (45.64%) were moderate, and 15 cases (26.30%) were severe. Univariate Analysis (P = 0.008) and Logistic Regression (P = 0.006) revealed significant differences in body mass index (BMI) between the moderate-severe ABL group and none-mild group. Although a worse muscle condition was found in the former, it did not meet the significant criteria (P = 0.164). A more severe ABL did not affect either clinical outcomes or radiological parameters. However, this can result in a higher incidence of implant subsidence and may accelerate the degeneration of the adjacent caudal segment.
Conclusion
ABL should be considered a common phenomenon after ACDF and reflects the degree to which the changed biomechanical condition changes. BMI was an independent influencing factor for the occurrence of moderate-severe ABL. ABL does not affect clinical outcomes but is associated with implant subsidence and accelerated degeneration of the adjacent caudal disc.
Keywords: Anterior cervical discectomy and fusion, ACDF, Bone loss, Subsidence, Adjacent segment degeneration
Introduction
Anterior cervical discectomy and fusion (ACDF) has been regarded as a standard procedure to treat Cervical degenerative disc disease (CDDD). Although this technique is widely used, some complications such as pseudarthrosis, implant subsidence, and adjacent segment degeneration, may impair surgical efficacy.
Anterior bone loss (ABL), observed at the anterior vertebral body of the operative segment, usually considered to occur only after cervical disc replacement (CDR). The prevalence of ABL varies from 8 to 63.7% and differs greatly among different types of artificial cervical discs [1–3]. The clinical characteristics of it, as a non-progressive process confined within 1 year, have been reported in several studies [4, 5]. In addition, severe ABL was observed to be associated with persistent pain, segmental kyphosis, and heterotopic ossification in some cases [6]. Moreover, ABL may expose the anterior portion of the implant and result in implant subsidence [6]. However, as research progresses, more researchers prone to regard it as a common phenomenon rather than a complication because of its non-significant clinical or radiological effects in most patients [7, 8].
ABL is equally common after ACDF according to our clinical observation and research, but the mechanism and effect on surgical efficacy are still unclear [5]. Therefore, in this retrospective study, we investigated the factors influencing the occurrence of it, as well as its potential impact on implant subsidence and adjacent segment degeneration. It will help clinicians prevent and deal with it well.
Methods
Patient population
This retrospective study, which was approved by the Institutional Review Board, included patients who underwent single-level ACDF using Zero-P (Zero-P; Synthes GmbH, Zuchwil, Switzerland) at a single-center from January 2010 to December 2020 in our hospital. Patients were included if they were diagnosed as C4-7 single-level CDDD causing symptomatic radiculopathy or myelopathy and refractory to conservative treatment for at least 6 weeks. Exclusion criteria included: (1) previous surgery operated on cervical spine; (2) follow-up time was no more than 12 months; (3) radiological material was insufficient; (4) other cervical disease like infection, tumor, and fracture; (5) multilevel or hybrid surgery.
Surgical technique
All operations were conducted by the same experienced surgeon utilizing the classical Smith–Robinson approach [9]. Complete decompression was achieved after the removal of the herniated disc, posterior longitudinal ligament, and osteophytes. Subsequently, discectomy and decompression were performed, followed by preparation of the endplates. An appropriate size Zero-P was selected and inserted into the intervertebral space. Copious irrigation and verification of proper position of implant were necessary before incision was closed. Drainage was put according to the intraoperative situation. All patients were instructed to wear a soft cervical collar for at least 3 months.
Clinical outcomes
Clinical outcomes were assessed using the Japanese Orthopaedic Association (JOA) score, Neck Disability Index (NDI) functional score, and visual analog scale (VAS) score. The JOA score was used to evaluate the neurological status of patients with myelopathy, the NDI score was used to assess the function of the neck, and the VAS score was used to assess the neck and arm pain. The data were collected preoperatively, at 1-week postoperatively, and at the last postoperative follow-up.
Radiological evaluation
The C2-7 curvature (Cobb C), C2-7 range of motion (ROM), and length of cranial and caudal endplates were collected preoperatively, immediately postoperatively, and at the last follow-up (Fig. 1). The Functional spinal unit (FSU) angle was measured preoperatively and at the last follow-up. We got the FSU-ROM by using the degree to which the FSU was in the hyperextension position minus the degree to which it was in the hyperflexion position. The anterior disc height (ADH) and posterior disc height (PDH) were collected immediately after surgery and at the last follow-up. Subsidence was defined as an ADH and/or PDH descent > 2 mm from immediate postoperative to final follow-up. ABL was measured as the ratio of (a-b) to a in the lateral X-ray and classified according to Kieser’s methods (Fig. 2; Table 1) [10]. If a difference in grade between the cranial and caudal endplates was found, the higher was used. The Goutallier classification of cervical paraspinal muscle (CPM) was graded on a 0–4 scale based on a qualitative assessment of fat atrophy of the muscle multifidus belly (Fig. 3). All measurements were conducted on an axial T2 weight section from MRI, at the C5/6 level, which was a common practice in CPM studies [11]. All patients’ imaging were analyzed by two reviewers independently. For qualitative data, if there was a disagreement between two reviewers, the issue would be referred to a third, more experienced surgeon for resolution. For angle-related data, if the difference between two reviewers was less than one degree, the average of their measurements was used as the final result. If the difference exceeded one degree, the measurement would be repeated until the error meets the standard. For data of disc height, the threshold was one millimeter.
Fig. 1.
Measurements of the radiographic parameters. (1) red line: cobb C; (2) yellow line: ADH and PDH; (3) white line: functional spine unit (FSU) angle
Fig. 2.
Anterior bone loss was calculated as (a-b)/a in the lateral X-ray. a was defined as the endplate length immediately after surgery, while b was defined as the endplate length at the last follow-up
Table 1.
Classification of ABL after anterior cervical discectomy and fusion
| Grade | Radiographic findings |
|---|---|
| None | ABL accounts for ≤ 1% of the length of subchondral vertebral body |
| Mild | ABL accounts for > 1% or ≤ 5% of the length of subchondral vertebral body |
| Moderate | ABL accounts for > 5% or ≤ 10% of the length of subchondral vertebral body |
| Severe | ABL accounts for > 10% of the length of subchondral vertebral body |
Fig. 3.
T2-weighted axial MRI section demonstrating fatty infiltration of muscle multifidus belly at C5/6. Goutalier grade 0 (A). Goutalier grade 1 (B); Goutalier grade 2 (C). Goutalier grade 3 (D). Goutalier grade 4 (E)
The degree of degeneration of the index disc was evaluated postoperatively and at the last follow-up via neutral lateral radiographs using a quantitative scoring system proposed by Walraevens et al. (Table 2) [12]. Height loss was defined as the ratio of the middle disc height to the height at an adjacent level. Anterior osteophytes were defined as the ratio of its length to the anteroposterior (AP) diameter of the middle of the corresponding vertebral body. Endplate sclerosis was classified according to the degree of detectability.
Table 2.
Scoring system for cervical disc degeneration based on neutral lateral radiographs
| Items | Points | |
|---|---|---|
| 1. Height loss | 0% | 0 |
| ≤ 25% | 1 | |
| > 25%—≤ 50% | 2 | |
| > 50%—≤ 75% | 3 | |
| > 75% | 4 | |
| 2. Anterior osteophytes | No osteophytes | 0 |
| ≤ 1/8 AP diameter | 1 | |
| > 1/8—≤ 1/4 AP diameter | 2 | |
| > 1/4 AP diameter | 3 | |
| 3. Endplate sclerosis | No sclerosis | 0 |
| Detectable | 1 | |
| Definite | 2 | |
| Overall degree of disc degeneration = 1 + 2 + 3 | 0 points (no degeneration) | |
| 1–3 points (mild degeneration) | ||
| 4–6 points (moderate degeneration) | ||
| 7–9 points (severe degeneration) | ||
Statistical analysis
All the statistical analyses were conducted using SPSS software version 26.0 (SPSS Inc., Chicago, IL, USA). A P value less than 0.05 was considered statistically significant. The results were presented as mean ± standard deviation (SD) when the data satisfied the criteria for normality. Otherwise, the results were presented as the median (interquartile range). The independent t test or the nonparametric Mann–Whitney U test was used to compare quantitative data between two groups, depending on whether the data were normally distributed. The nominal variables were assessed using the Pearson chi-square test or Fisher exact test. The ordinal variables were assessed using the Rank Sum test. Repeated Measures ANOVA and Wilcoxon matched-pairs signed rank test were used to analyze changes between preoperative and postoperative parameters. Logistic regression analysis was used to identify risk factors for moderate-severe ABL.
Results
Demographic data
A total of 90 patients who underwent single-level ACDF were included in our research. 63.3% (57/90) of them were diagnosed with ABL, including 16 mild cases, 26 moderate cases, and 15 severe cases (Figs. 4 and 5). Considering the measurement error, it’s difficult to distinct none ABL and mild ABL. In addition, patients with moderate ABL and severe ABL had similar clinical manifestations. Then patients with none and mild ABL were assigned to the none-mild ABL group (49/90, 54.4%), and patients with moderate and severe ABL were assigned to the moderate-severe ABL group (41/90, 45.6%). Univariate analysis revealed that the BMI of the none-mild group was greater than that of the moderate-severe group (P = 0.008). In addition, there were no significant differences in age, gender, bone metabolism related indicators, surgery related indicators or follow-up time, as shown in Table 3. After controlling for age, T-value, and follow-up time, Logistic regression analysis revealed that patients with a higher BMI had a significantly lower incidence of moderate-severe ABL (OR = 0.825, 95% CI: 0.710–0.959) (Table 4). However, there was no difference between nutritional indicators. Although the moderate-severe ABL group seemed to have worse muscle conditions, there was no significant difference between the two groups (Table 5).
Fig. 4.
Zero-P was properly inserted immediately after surgery (A). Severe bone loss at both the superior and inferior endplates accompanied by subsidence was observed at the postoperative follow-up time (B–F)
Fig. 5.
Zero-P was properly inserted immediately after surgery (A). No bone loss occured at any follow-up time (B–F); however, anterior osteophytes in both adjacent segments were observed at the last follow-up time (F)
Table 3.
Baseline demographic characteristics
| Characteristic | Moderate-severe ABL | None-mild ABL | P value |
|---|---|---|---|
| No. of patients, n | 41 | 49 | – |
| Gender (M/F)‡ | 18/23 | 24/25 | 0.631 |
| Age (y)† | 47.61 ± 12.07 | 50.76 ± 10.53 | 0.190 |
BMI (kg/ )†
|
22.32 ± 4.52 | 24.56 ± 3.18 | 0.008 |
| T valuea † | -0.38 ± 0.92 | -0.26 ± 1.16 | 0.625 |
| Ca (mmol/L)† | 2.26 ± 0.13 | 2.21 ± 0.10 | 0.777 |
| P (mmol/L)† | 1.09 ± 0.18 | 1.07 ± 0.17 | 0.544 |
| ALP (mmol/L)† | 66.92 ± 17.49 | 66.85 ± 14.71 | 0.982 |
| Smoking (Y/N)‡ | 11/30 | 15/34 | 0.693 |
| Alcohol use (Y/N)‡ | 6/35 | 9/40 | 0.636 |
| Levels‡ | 0.099 | ||
| C4-5 | 3 | 2 | |
| C5-6 | 38 | 42 | |
| C6-7 | 0 | 5 | |
| Cage Height (mm)‡ | 0.094 | ||
| 5 | 2 | 1 | |
| 6 | 5 | 1 | |
| 7 | 17 | 28 | |
| 8 | 15 | 19 | |
| 9 | 2 | 0 | |
| Operation time (min)† | 110.43 ± 29.07 | 107.67 ± 20.06 | 0.596 |
| Blood loss (ml)* | 50(70) | 50(75) | 0.915b |
| Follow-up time (m)† | 29.65 ± 26.38 | 25.71 ± 19.32 | 0.416 |
†: Independent t test
‡: Chi-square test
*: Mann-Whitney test
a: T value was measured on the femoral neck using dual-energy X-ray absorptiometry scans
b: Z value: -0.107
Table 4.
Logistic regression analysis outcomes for moderate-severe ABL
| Variable | β | Standard error | Wald χ2 | OR (95%CI) | P value |
|---|---|---|---|---|---|
| Age | 0.011 | 0.025 | 0.201 | 1.011(0.963,1.061) | 0.554 |
| T-value | 0.170 | 0.284 | 6.780 | 1.185(0.679,1.692) | 0.652 |
| BMI | -0.192 | 0.076 | 6.323 | 0.825(0.710,0.959) | 0.006 |
| Follow-up time | 0.022 | 0.012 | 3.359 | 1.012(0.988.1.036) | 0.127 |
Method: backward LR
Table 5.
Nutritional conditions and the fatty infiltration into cervical paraspinal muscle
| Characteristic | Moderate-severe ABL | None-mild ABL | P value |
|---|---|---|---|
| Hemoglobin (g/L)† | 138.66 ± 14.90 | 138.12 ± 18.02 | 0.880 |
| Total protein (g/L)† | 70.76 ± 6.41 | 70.46 ± 4.84 | 0.805 |
| Albumin (g/L)† | 44.68 ± 4.75 | 45.29 ± 3.59 | 0.491 |
| Goutallier classification* | 0.164 | ||
| 0 | 14 | 7 | |
| 1 | 20 | 18 | |
| 2 | 11 | 9 | |
| 3 | 4 | 6 | |
| 4 | 0 | 1 |
†: Independent t test
*: Mann-Whitney test
Clinical outcomes
Clinical outcomes, including JOA, VAS and NDI scores, were significantly improved at postoperative follow-up than preoperatively in both groups. Similarly, improvements in the JOA, NDI and VAS scores were noted at the last follow-up compared with the postoperative values in both groups. However, no significant difference was found between the two groups at any follow-up period (Table 6).
Table 6.
Comparison of clinical outcomes between none-mild ABL and moderate-severe ABL
| Variables | JOA | VAS | NDI | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Pre | Post | Last | Pre | Post | Last | Pre | Post | Last | |
| Moderate-severe ABL | 9.80 ± 1.32 | 12.46 ± 1.36* | 15.26 ± 1.34# | 7.48 ± 0.77 | 3.87 ± 0.74* | 2.02 ± 0.47# | 32.15 ± 1.58 | 19.53 ± 1.43* | 7.75 ± 0.88# |
| None-mild ABL | 9.69 ± 1.06 | 12.32 ± 1.31* | 15.16 ± 1.23# | 7.51 ± 0.89 | 3.65 ± 0.75* | 1.97 ± 0.55# | 31.91 ± 1.81 | 19.65 ± 1.39* | 7.69 ± 1.02# |
| P value | 0.661 | 0.629 | 0.702 | 0.899 | 0.160 | 0.682 | 0.532 | 0.698 | 0.761 |
* Compared with preoperative value, P < 0.05
# Compared with the value at 1-week postoperatively, P < 0.05
Radiological outcomes
Table 7 shows the basic descriptive statistics on the plain films. Cobb C was increased immediately after the operation and at the last follow-up in both groups, compared with the preoperative values. (Table 7). However, the C2-7 ROM and FSU-ROM of the operated segment significantly decreased at the last follow-up. Surprisingly, the FSU-ROM of the cranial and caudal segments did not change significantly. Immediately after the operation, the cranial and caudal endplate lengths were slightly greater in the moderate-severe ABL group (P = 0.006,0.006). However, at the last follow-up, the endplate lengths were shorter in the moderate-severe ABL group (p = 0.004,0.004). Similarly, the ADH and PDH were also slightly higher in the moderate-severe ABL group (P = 0.070, 0.051). However, at the last follow-up, there were no differences between the two groups. Furthermore, we found that compared with immediately after surgery, there were significant decreases in the ADH and PDH in both groups at the last follow-up. The reduction was more pronounced in the moderate-severe ABL group.
Table 7.
Comparison of radiographic parameters between the two groups
| Item | Moderate-severe ABL | None-mild ABL | P value |
|---|---|---|---|
| Cobb C (°) | |||
| Pre | 10.07 ± 10.02 | 12.34 ± 10.87 | 0.308 |
| Post | 14.01 ± 9.10* | 16.47 ± 8.78* | 0.196 |
| Last | 14.01 ± 11.02* | 15.26 ± 10.10* | 0.574 |
| C2-7 ROM (°) | |||
| Pre | 45.99 ± 17.09 | 49.40 ± 13.82 | 0.301 |
| Last | 39.26 ± 11.86* | 42.44 ± 9.24* | 0.256 |
| FSU-ROM (°) | |||
| Surgery level | |||
| Pre | 8.91 ± 3.35 | 9.42 ± 4.23 | 0.161 |
| Last | 2.66 ± 1.94* | 2.46 ± 1.92* | 0.692 |
| Cranial Level | |||
| Pre | 10.00 ± 4.00 | 11.64 ± 4.96 | 0.179 |
| Last | 11.16 ± 4.59 | 10.64 ± 4.19 | 0.653 |
| Caudal Level | |||
| Pre | 9.39 ± 3.67 | 10.20 ± 4.43 | 0.461 |
| Last | 9.93 ± 5.46 | 8.87 ± 3.68 | 0.380 |
| Upper endplates length (cm) | |||
| Pre | 2.06 ± 0.17 | 2.05 ± 0.17 | 0.905 |
| Post | 2.04 ± 0.24 | 1.93 ± 0.16* | 0.006 |
| Last | 1.83 ± 0.16# | 1.92 ± 0.16 | 0.004 |
| Lower endplates length (cm) | |||
| Pre | 2.02 ± 0.16 | 1.99 ± 0.17 | 0.744 |
| Post | 1.99 ± 0.21 | 1.93 ± 0.15* | 0.006 |
| Last | 1.82 ± 0.18# | 1.89 ± 0.13# | 0.004 |
| Anterior disc height (cm) | |||
| Post | 0.97 ± 0.16 | 0.92 ± 0.10 | 0.070 |
| Last | 0.84 ± 0.14# | 0.87 ± 0.10# | 0.278 |
| Posterior disc height (cm) | |||
| Post | 0.81 ± 0.12 | 0.77 ± 0.08 | 0.051 |
| Last | 0.73 ± 0.12# | 0.74 ± 0.08# | 0.756 |
* Compared with preoperative value, P < 0.05
# Compared with the value at immediate after operation, P < 0.05
Complications
As shown in Table 8, implant subsidence occurred in 15 cases (15/90, 16.67%), including 11 cases in the moderate-severe ABL group and 4 cases in the none-mild ABL group; the difference between the two groups was significant (P = 0.019). In addition, our study revealed increased adjacent disc degeneration scores in each group at the last follow-up compared with preoperative assessments. At the last follow-up, the caudal segment disc degeneration score was significantly greater in the moderate-severe ABL group (P = 0.003), while the cranial segment disc revealed no difference. Further exploration through univariate analysis revealed that the degree of endplate sclerosis in both the cranial and caudal segment discs was more severe in the moderate-severe ABL group (P = 0.026; 0.001).
Table 8.
Comparison of the incidence of subsidence and the degeneration of adjacent segments
| Items | moderate-severe ABL | none-mild ABL | P value* | |
|---|---|---|---|---|
| subsidence | 11 | 4 | 0.019 | |
| Post-op | ||||
| Overall degree of cranial disc degeneration | 0.46 | 0.59 | 0.347 | |
| Mean height loss score | 0.27 | 0.31 | 0.695 | |
| Mean anterior osteophyte score | 0.17 | 0.22 | 0.494 | |
| Mean endplate sclerosis score | 0.02 | 0.06 | 0.401 | |
| Overall degree of caudal disc degeneration | 0.49 | 0.53 | 0.865 | |
| Mean height loss score | 0.10 | 0.16 | 0.312 | |
| Mean anterior osteophyte score | 0.32 | 0.27 | 0.524 | |
| Mean endplate sclerosis score | 0.07 | 0.10 | 0.861 | |
| Last | ||||
| Overall degree of superior disc degeneration | 1.46# | 1.18# | 0.374 | |
| Mean height loss score | 0.56# | 0.49# | 0.609 | |
| Mean anterior osteophyte score | 0.51# | 0.53# | 0.601 | |
| Mean endplate sclerosis score | 0.39# | 0.16# | 0.026 | |
| Overall degree of inferior disc degeneration | 1.88# | 1.16# | 0.003 | |
| Mean height loss score | 0.42# | 0.29# | 0.326 | |
| Mean anterior osteophyte score | 0.78# | 0.63# | 0.232 | |
| Mean endplate sclerosis score | 0.68# | 0.24# | 0.001 |
*: Mann-Whitney test
#: Wilcoxon matched-pairs signed rank test, compared with the value at immediate after operation, P < 0.05
Discussion
ABL, which has been previously mentioned in several reports, is usually thought to occur only after CDR [3, 6]. Nonetheless, recent research has indicated that it is also deemed normal following ACDF [5]. Unfortunately, the underlying mechanism and impact of this process on surgical efficacy have not been elucidated. In this study, we analyzed potential factors influencing the occurrence of ABL after ACDF and its effect on surgical efficacy. These findings broaden our understanding of the topic, offering more comprehensive insight into its mechanism and effect.
In this study, the incidence of ABL in ACDF was 63.3% of that in Zero-P, while the incidence of moderate-severe ABL was 45.56%. Previous research revealed that the majority of ABL after ACDF occurred within 3 months, all were confined within 1 year, and no further ABL was noticed after 12 months [5]. Prior studies have proposed several presumptions such as stress shielding effects, micromovement between the interface of the implant, and intraoperative endplate preparation, to explain this phenomenon [3, 10]. But from our perspective, changed biomechanical environment due to various factors may be the answer. A finite element analysis of a single-segment ACDF procedure showed that the stresses were concentrated in the anterior region of the cage at 11.1–17.4 MPa [13]. The other demonstrated that the postoperative stresses were concentrated on the cage after the operation, and the stresses were significantly reduced and distributed more uniformly when the space was completely fused [14]. This may elucidate the mechanism, suggesting that altered stress distribution indeed occurs in the early stages, potentially leading to ABL. However, this resorption diminishes gradually due to the shift in stress from the implant to a broader area within the fusion process. Besides, prior studies suggested that insufficient endplate coverage could lead to uneven stress distribution. Inappropriate increment of intervertebral height would further increase the stress of implant-endplate interface in the scenario of insufficient endplate coverage [15, 16]. Through our previous clinical experience, we would like to prepare endplates of target level as much as possible. This process eases insertion process and can offset the mismatch of the interface between the vertebral body and the implant. Our study revealed that the lengths of the endplates were greater in the moderate-severe ABL group postoperatively. This may contribute to an increased mismatch at the implant-endplate interface [17]. Such mismatches not only alter the stability and stress distribution but also increase the likelihood of micromovement, potentially leading to ABL [18]. In addition, the ADH and PDH were slightly higher in the moderate-severe ABL group, which may further exacerbate this process [17]. Interestingly, this phenomenon seems to challenge the hypothesis that ABL is attributed to vascular damage during endplate milling [10]. More milling would theoretically inflict more substantial vascular damage, leading to a more severe ABL. Besides, we found that BMI, rather than age, bone-metabolism related indicators, or surgery related items, was an independent factor influencing the occurrence of ABL. Since the cervical spine is subjected to a lower axial load, we speculated that this difference may be primarily related to CPM conditions. When the CPM are atrophied, it is difficult for them to function as stabilizers in an abnormal mechanical environment [11, 19]. Although the fatty infiltration condition seemed worse in the moderate-severe ABL group, it did not meet the significant criteria. As expected, both BMI and ABL were influenced by multiple factors. Identifying a specific index that could explain this perfectly seems impossible.
We observed a strong association between implant subsidence and moderate-severe ABL. It is readily understandable that such subsidence could expose and uncover the implant, consequently resulting in a loss of support at the anterior part.
Alterations in the biomechanical environment are widely recognized to influence disc degeneration. Hence, it is unsurprising that a more severe ABL, reflecting a greater change in the environment, is associated with a more rapid degenerative process. Our study indicated accelerated degeneration in both adjacent segments at the last follow-up, compared with the postoperative scores. Furthermore, the degree of degeneration was more severe at the caudal segment in the moderate-severe ABL group. A recent study has indicated that for low-level fusions (C5-C6, C6-C7), there is an increase in intradiscal pressure in both adjacent segments, which induces biochemical changes within it, with a more pronounced increase in the caudal disc [20, 21]. Diminished nutrients and failure to remove waste products adequately from the disc can lead to increased lactate levels and decreased pH, which can impair metabolism and lead to cell death. Additionally, increased compressive forces over time have been reported to increase the presence of collagen Type 1 and to decrease proteoglycans, chondroitin sulphate, and collagen Type 2. All these changes lead to disc degeneration [22]. Totally, factors contributing to the development of ABL can cause spinal instability and implant micromovement, leading to abnormal changes in intradiscal pressure and ultimately accelerating disc degeneration [23, 24]. Besides, Micromovement-induced periosteal inflammation may lead to accelerated ossification of the endplates, altering mechanical properties and impairing diffusion and nutrient supply, thus accelerating this process [25].
For the first time, our present study elucidates the influencing factors for the ABL after ACDF, along with its impact. When we observe this phenomenon during early follow-up, we would not worry about the effectiveness of the surgery. Instead, a comprehensive evaluation is warranted to determine the optimal timing for initiating rehabilitation of cervical spine function. Timely exercise can aid in restoring the biomechanical environment. Furthermore, the duration of neck brace usage should be investigated; prolonged wearing may worsen muscle conditions, ultimately leading to an unfavorable biomechanical environment. However, there are several limitations in our study. First, retrospective observation may inevitably lead to bias. Besides, whether this phenomenon is relevant to different devices is unknown. Second, the number of patients and length of follow-up were relatively insufficient. In addition, direct evidence of biomechanical changes after ACDF should be further explored.
Conclusion
ABL should be considered a common phenomenon after ACDF and reflects the degree to which the biomechanical environment changes. We should not worry about clinical outcomes but should regard it as a sign of post-operative management for preventing adjacent segment degeneration.
Acknowledgements
We are very grateful to those who helped in the study process.
Abbreviations
- CDR
Cervical disc replacement
- ACDF
Anterior cervical discectomy and fusion
- JOA
Japanese orthopedic association
- NDI
Neck disability index
- VAS
Visual analog scale
- Cobb C
Overall cobb angle
- ROM
Range of motion
- FSU
Functional spinal unit
- CDDD
Cervical degenerative disc disease
- CPM
Cervical paraspinal muscle
- ASD
Adjacent segment degeneration
- CT
Computed tomography
- MRI
Magnetic resonance imaging
Author contributions
RZ and CYL contributed equally to this work and share the first authorship. RZ conducted the design of the study and drafted the manuscript with the help from CYL, YZJ, and JBH. TKW and CYL helped in the statistical analysis. RZ, YZJ, and ZHP conducted the interpretation of data. TKW, AYY, and HL contributed to the revision. All authors have read and approved the final manuscript.
Funding
This study was supported by the Cadre Health Research Project of Sichuan Province(ZH2023-105), National Natural Science Foundation of China (82302785), and the 1.3.5 project for Postdoctoral Foundation of West China Hospital of Sichuan University (2023HXBH080). No relevant financial activities outside of the submitted work.
Data availability
Datasets are available from the corresponding author on reasonable request.
Declarations
Ethical approval and consent to participate
All participants provided signed, informed consent prior to study participation. This study was permitted by Ethics Committee on Biomedical Research, West China Hospital of Sichuan University (2019,946) and in accordance with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.
Consent for publication
Informed consent was obtained from all individual participants included in this study.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rui Zong and Chuan-yu Liu contributed equally to this work and share the first authorship.
Contributor Information
Hao Liu, Email: dr.liuhao6304@hotmail.com.
An-yun Yu, Email: 839878806@qq.com.
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Associated Data
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Data Availability Statement
Datasets are available from the corresponding author on reasonable request.