Mechanisms of compensatory for cervical lordosis changes after laminectomy with fusion

Cervical spondylotic myelopathy (CSM) is a common disease in older adults, that leads to cervical spinal cord function impairment. Surgery should be considered for patients who are refractory to conservative treatment. Laminectomy with fusion (LCF), a posterior decompression method, is a common procedure for multi-level CSM. However, C2-C7 Cobb (C2-7) angle reduction or cervical lordosis (CL) loss occurs commonly after LCF [1]. Even cervical positive sagittal malignment leading by severe CL loss is associated with poor outcomes [2].

To CL loss and cervical positive sagittal malignment, the cranial and caudal levels must compensate to maintain cervical sagittal balance. Nori et al. [3] studied patients with selective laminectomy and found that postoperative lower cervical kyphotic changes were compensated for by upper cervical lordotic changes. Ikeda et al. [4] found that the occipito-C2 (O-C2) angle increased and T1 slope (T1S) decreased as the compensatory mechanism for kyphotic change after anterior cervical corpectomy and fusion. Nevertheless, the compensatory mechanisms for cervical sagittal balance after LCF remain unclear to the best of our knowledge.

Therefore, in this study, we investigated the influence of O-C2 changes (ΔOC2) and T1S changes (ΔT1S) on the cervical sagittal balance after 4 or 5-level (C3-7 or C3-6) LCF to clarify the compensatory mechanisms.

There was a tendency of decreased CL in patients who underwent the posterior LCF with CSM. Several authors previously demonstrated loss of CL following a posterior surgical approach. Roguski et al. [5] found that patients on average lost 4.2 degrees of lordosis and had a mean postoperative Cobb angle of − 3.4 ± 16.3 degrees. Cabraja et al. [6] found that patients lost an average of 6.5 degrees of lordosis, with a mean postoperative C2-7 of − 6.6 ± 13.3 degrees. Lee et al. [7] found that a mean C2-7 changed from − 10 ± 11.6 degrees preoperatively to − 5.1 ± 12.0 postoperatively after extensive laminectomy with fusion. In the present study, C2-7 significantly decreased in Groups D and I. The average postoperative change of the C2–C7 was − 4.94 ± 6.48° in Group M, − 5.36 ± 7.40° in Group I.

Although cervical lordosis changed after laminectomy, CL was not the main parameter to analyze the cervical sagittal balance [8]; cSVA is an important parameter associated with clinical symptoms and used evaluate cervical balance [2, 9]. To maintain the sagittal balance of the cervical spine, compensatory behaviors were maintained in the cSVA in Group M. For patients with increased cSVA, three patients had postoperative cSVA > 40 mm. Three patients had relatively poor outcomes with recovery rates no more than 60%. There was no significant difference in recovery rate between Groups I and D. We believe that maintaining cervical sagittal balance is critical for achieving excellent clinical outcomes. Sielatycki et al. [9] found that patients with kyphosis cervical spine who underwent LCF had improved clinical outcomes associated with creating more lordosis and decreasing SVA. However, they did not find such an association in patients with lordosis and stated that any amount of lordosis might be sufficient for LCF.

Lee et al. [10] introduced the term T1 slope in 2012. Since then, studies have concentrated on the preoperative relationship between T1S and CL to predict the postoperative change of CL [11, 12]. Other studies demonstrated that the T1 (C7) slope was associated with factors of cervical sagittal alignment, including C2–C7 lordosis and C2–C7 SVA [13, 14]. Kennamer et al. [15] in a recent study of cervical alignment with posterior cervical fusions, demonstrated a relationship between increased SVA and increasing T1 slope and worsened clinical outcomes. Hyun et al. [11] obtained a similar result, and the authors suggested that this phenomenon could indicate a compensatory mechanism to regulate the angle of gaze. Thus, when the cSVA changes, a corrective change in T1S ensues. Nevertheless, it is unknown whether a separate change of T1S was sufficient to affect cSVA, especially under the condition of the loss of cervical lordosis. The T1 vertebra is the foundation of the cervical spine. Because C7/T1 is a transition zone, this level is exposed to unique biomechanical forces, particularly flexion or translation stress. When the C2-7 changes, T1S can be altered by changing thoracic kyphosis or other factors to adjust cervical sagittal balance and preserve horizontal gaze [16, 17]. Hyun et al. [11] showed a mean T1S change from 25.7 ± 6.9 degrees preoperatively to 23.2 ± 8.0 degrees postoperatively. In the present study, in Group M, ∆TIS was significantly smaller than Group I (− 6.94 ± 6.05°, − 0.77 ± 4.23° respectively). Thus, in Group M, but not in Group I, an increase in cSVA was avoided by a sufficient reduction in TIS. Using stepwise multiple linear regression analysis, we found that ΔcSVA was affected primarily by ∆TIS. Therefore, the decrease of TIS should be the first and foremost compensation for the change of cSVA and the loss of C2-7. When the T1S could not decrease sufficiently or increase, compensation could not be achieved. Next, O-C2 change could participate in compensation to achieve gaze horizon. Li et al. [18] studied patients with lumbar degenerative disease and found that thoracic hypokyphosis compensation prevented T1 slope increase; thus, T1S change was related to thoracic kyphosis change. However, patients without sufficient T1S compensation might have reasons that limit thoracic kyphosis change, including spondylitis, diffuse idiopathic skeletal hyperostosis, sarcopenia, and debility, and these patients are challenging to maintain cervical sagittal alignment balance.

Correlation analysis revealed a negative correlation between the C2–7 and ΔO-C2 neutral angle postoperative changes. In the present study, postoperative O-C2 angle and ΔO-C2 angle in Group I were significantly larger than in Group M. This finding suggests that the O-C2 increased to compensate for the balance adjustment and horizontal gaze in Group I. In addition, there was a significant positive correlation between ∆cSVA and ∆O-C2. As the ∆cSVA increased, so did the ∆O-C2. Thus, the forward tilt of the C2–C7 segments could be compensated for by the hyperextension of the O-C2 segments after LCF to insure horizontal gaze. Woodroffe et al. [19] found that the inclusion of C2 in the fusion construct resulted in increased sagittal balance, increasing the SVA and T1S. This finding indicated that when the O-C2 change was eliminated, the cervical sagittal balance would be affected. According to this analysis, the O-C2 change was the compensatory mechanism for adjusting cervical sagittal balance after LCF when TIS change would not preserve or reduce the original cSVA.

We summarized the compensatory mechanisms in Fig. 3. O-C2 angle and T1S would compensate for loss of CL. O-C2 compensation is the supplement of T1S compensation in cervical alignment balance adjustment.

This was a retrospective cohort study; thus, several limitations should be noted. First, the number of cases in this study was small, especially in Group M, and there were not many cases to confirm our findings. Second, the level of LCF was not uniform. Third, the radiographic follow-up time was relatively short. Finally, this study was a single-center follow-up analysis; future studies should include several centers.

The decrease of TIS should be the first and foremost compensation for the loss of lordosis in C2-7 segments after the posterior laminectomy with fusion. When the change of T1S alone could not prevent the deterioration of cervical sagittal balance, further increase in the O-C2 segment happened.