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INTRODUCTION
Cervical spondylotic myelopathy (CSM) is one of the most common spinal disorders treated by spinal surgeons worldwide. CSM often presents in the elderly with several signs and symptoms, including neck pain, radiculopathy, or myelopathy [1,28].
Posterior cervical spinal fusion with decompression is one of the possible treatment options in patients with CSM who have conditions such as spondylosis, spinal stenosis, and degenerative disc disease [24]. The development of distal junctional kyphosis (DJK) after cervical fusion surgery can be one of the possible concerns just like the development of proximal junctional kyphosis after thoracolumbar fusion surgery.
DJK may lead to fixation failure, disc degeneration, distal level fractures, or spondylolisthesis, resulting in pain, myelopathy, or deformities [17]. The development of proximal junctional kyphosis and its characteristics and incidence following thoracolumbar fusion surgery have been well-known. However, the development of DJK following cervical spinal fusion surgery has not been studied or reported [4,7,18,30]. In this study, the development and risk factors of DJK after posterior cervical spinal fusion surgery were analyzed.
MATERIALS AND METHODS
Study population
Approval for this study was obtained from the participating center’s Institutional Review Board of Gangneung Asan Hospital (IRB No. GNAH 2020-12-009). We retrospectively analyzed the clinical and radiographic outcomes of 64 patients aged ≥18 years (51 males and 13 females) who had multilevel CSM and underwent single-stage multilevel (3–6 levels) posterior cervical fusion surgery. The surgeries were performed by a single spine surgeon between January 2012 and December 2017. The surgical level was from C2 to C7. The Neck Disability Index (NDI) score and Visual analog score (VAS) of the arm and neck were used to measure clinical results. We also compared preoperative and postoperative (last follow-up) values.
Demographic data were collected for all patients, including age, sex, underlying disease, smoking history, body mass index (BMI), surgical details, follow-up information, electronic medical records, and radiographic image reviews. Exclusion criteria were previous cervical spine surgery, anterior surgery, follow-up less than 24 months, and surgery for other reasons.
We divided patients into DJK and non-DJK groups according to the presence of DJK at the last follow-up. DJK was defined as the changes of an angle of -10° or less at the distal disc level from the end of the fusion construct between baseline and final follow up [23].
Surgical procedure
We performed cervical pedicle screw (CPS) insertion in all posterior cervical spinal fusion patients. The safety and efficacy of subaxial CPS placement have been validated multiple times [11,20,22]. Although the CPS placement with a freehand technique had been primarily considered, some lateral mass screw conversions or skipping of screw were also done [8,31].
Following screw insertion and decompression, we extended the patient’s head position using remote-controlled table head segments to achieve ideal lordosis from 15–30 degrees [1].
Radiologic evaluation
Cervical lordosis (CL) from C2 to C7, the T1 slope, the segmental angle index level, and the C2-7 sagittal vertical axis (SVA) were evaluated via lateral cervical X-ray (Fig. 1). Radiographic parameters were measured before surgery, immediately after surgery, and at the last follow-up visit, and the changes in those parameters were analyzed. All radiographs were prepared using a photo archiving and communication system (PetaVision for Clinics version 2.0; Asan Medical Center, Seoul, Korea).
All radiographic parameters were compared between the two groups.
Statistical analysis
Data are expressed as percentages and presented as mean± standard deviation. Student’s t-test was used to analyze the differences between continuous variables. p-values less than 0.05 were considered statistically significant, and multivariate logistic regression analysis was performed to identify independent factors that help predict the development of DJK after posterior cervical spinal fusion. Statistical Package for the Social Sciences (SPSS) Statistics for Windows, version 17.0 (IBM, Armonk, NY, USA), was used for all statistical analyses.
RESULTS
Demographic parameters
A total of 64 patients with multilevel CSM were included in this study, where 13 developed DJK and 51 did not. Of the 13 patients with DJK, three were asymptomatic and 10 experienced neck pain around the operated site (VAS score ≥4) after surgery (Table 1). Only one among the 10 patients underwent revisional anterior cervical discectomy and fusion surgery to resolve radiculopathy caused by distal junctional problem. This patient, who underwent posterior C3-4-5-6 levels fusion surgery before 2 years, complained of radiculopathy caused by disc degeneration, stenosis, and kyphosis at C6-7 level.
The mean patient age was 60.85±10.18 years, and the mean follow-up period was 35.09±11.13 months. No significant difference between DJK and non-DJK groups was observed in sex, age, underlying diseases, BMI, or smoking history (Table 2).
Clinical outcomes
The clinical outcomes of all the patients showed improvement. There were no significant differences in the VAS scores of the arm or neck or in the NDI between the two groups before and after surgery. Higher NDI and VAS values were observed at the last follow-up in the DJK group than in the non-DJK group. A significant difference was observed in the VAS scores of the neck (p=0.01) (Table 3).
Radiographic outcomes
Significant differences were found in the T1 slope and C2-7 SVA (p=0.03 and p<0.01; respectively) (Table 4). Immediately after surgery, no significant difference in the T1 slope and C2-7 SVA was observed. However, at the last follow-up, a significant difference was observed in the C2-7 SVA (DJK group, 30.59±13.88; non-DJK group, 21.57±8.86; p<0.01).
In all the patients, the cervical spinal radiological parameters tended to return to their preoperative values (Figs. 2 and 3).
When examining the changes between radiological parameters before surgery, immediately after surgery, and at the last follow-up, the changes in T1 slope, CL, and segmental angle before and immediately after surgery were significantly larger in the DJK group (Table 5).
The changes in CL, and T1-Cl mismatch between immediately after surgery and the last follow-up were significantly larger in the DJK group (Table 6). Excessively lordotic segmental angle resulted in higher T1 slope, C2-7 SVA and CL changes in the immediately postoperative state. At the last follow-up, the T1 slope and C2-7 SVA return to the preoperative value, but the segmental lordosis (SL) did not return to the preoperative value. If the adjacent segment compensates for the alignment change of the index level appropriately, it becomes a non-DJK group, and if it is not compensated pathologically, DJK occurs (Fig. 4).
From the multiple logistic regression analysis of DJK occurrence, preoperative higher C2-7 SVA and preoperative lower T1 slope were identified as independent risk factors (p=0.03 and p<0.01, respectively) (Table 7).
DISCUSSION
We investigated the risk factors for DJK development after posterior decompression and fusion surgery. The T1 slope and CL displayed a tendency to return to their preoperative levels. Although many studies have been published on the relationship between the T1 slope and CL, to date [10,14,15], there is no previous report on the gradual alignment change after posterior cervical spinal fusion surgery resulting in DJK development.
The return of the T1 slope and CL to preoperative levels may have resulted from patients tending to retain their preoperative spinopelvic alignments, which is due to the preservation of the preoperative T1 slope [2,5]. This suggests that the T1 slope is the key parameter, with the CL depending on its value. Immediately after surgery, an increase in SL was associated with increases in the CL and T1 slope. During follow-up, because of the tendency to regain the preoperative T1 slope, the T1 slope decreased, followed by a decline in CL which inevitably related with DJK. Among the immediate postoperative parameters, only SL is amenable to surgical manipulation. The CL and T1 slope appeared to be dependent parameters following the SL changes. If greater SL changes are made during surgery, larger CL and T1 slope changes should also follow. However, the SL values did not return to preoperative levels despite gradual decreases after surgery. This observation may have been associated with the stiffness of the instrumentation (Fig. 3).
The multiple logistic regression analysis in this study showed that the preoperative C2-7 SVA and T1 slope are independent risk factors for DJK (p<0.01 and p=0.03, respectively). In another study [23], preoperative cervical kyphosis (<-12°), preoperative cervical SVA (>56.3 mm), and preoperative CL-T1 slope (>36.4°) were analyzed as DJK risk predictors. It was also noted that patients with misalignment exceeding these thresholds had a 5–6 fold increased risk of DJK due to increased shear stress in the distal structures as cervical kyphosis increases the slope of the transition segment.
These findings are similar to those in the lumbar deformity literature on the incidence of proximal junction kyphosis. Kim and Iyer [13] showed that in SVA, thoracic kyphosis, lumbar lordosis, sacral inclination, and pelvic retraction, poorer preoperative measures were found in the proximal junctional kyphosis. It has been described as leading to kyphosis.
Neither age, sex, BMI, underlying disease, nor smoking significantly increased the risk of DJK. Although many studies have mentioned the relationship between smoking and cervical disc degeneration [16,26], our result didn’t show any significance.
We found in this study that four of the 28 patients (14%) completed the post-fusion surgery at C6 and nine of the 31 patients (29%) ended up at C7. It has been found that DJK occurs at the cervicothoracic junction (Table 1). Several studies [21,25] have recommended cervicothoracic junction crossing fusion surgery to reduce the revision surgery rate due to adjustment segment disease in posterior fusion surgery involving the cervicothoracic junction. However, Truumees et al. [29] recommend it only for smokers or patients at high risk of pseudoarthrosis and to avoid in medically frail patients. Also, Kennamer et al. [9] mentioned that there is no particular difference in the revision rate as a result. Even in long instrumented spinal fusion surgery of the thoracolumbar spine, some studies recommend extension of the uppermost instrumented vertebrae to the upper thoracic level or at least above T10 to avoid proximal junctional kyphosis [19,27]. However, other studies have reported that the risk of proximal junctional kyphosis was not increased with uppermost instrumented vertebrae at the thoracolumbar junction level compared to uppermost instrumented vertebrae above the thoracolumbar junction level [3,6,12]. Consequently, extension of the fusion to the thoracic level for DJK prevention remains controversial.
In 2018, Passias et al. [23] found that the presence of neurological symptoms was the most appropriate predictor of DJK. In our study, there was no difference between the two groups immediately after surgery in terms of clinical outcomes, but worse results were observed in the DJK group in the VAS neck score at the last follow-up.
There are some limitations to this study. First, this was a retrospective study. Second, the patients were examined only with X-ray imaging and not with computed tomography or magnetic resonance imaging. Finally, these results were derived from a small sample of patients. Studies with larger cohorts are needed. In addition, many patients who showed improvement were excluded because they did not meet the follow-up requirement, which may have resulted in selection bias.
CONCLUSION
In our study, risk factors of DJK in patients who developed DJK after posterior cervical fusion were analyzed. In patients with lower T1 slope and large C2-7 SVA, misalignment occurred due to excessive segment angle change during surgery, eventually resulting in DJK at last follow-up. In the clinical results, it was confirmed that the clinical results deteriorated again in the DJK group at the last follow-up. It is recommended to refrain from excessive SL changes during multilevel cervical post fusion surgery, especially in patients with a small preoperative T1 slope and a large SVA value.
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