Recently, several studies have been published regarding the preoperative parameters of cervical sagittal balance, including the C2-C7 cervical angle (CA), sagittal vertical axis (SVA), and T1 slope (T1S)^1,18,30). These parameters are of significance in predicting the changes that occur when the balance between gravity and the force of extension from the posterior neck muscle is disturbed. Thus, these factors have been mainly studied in laminoplasty, which severely invades the posterior neck muscular-ligament complex^{5,15,16,22,26)}. Because anterior cervical discectomy and fusion (ACDF) does not damage the posterior neck muscular-ligament complex, investigations regarding the influence of preoperative parameters are limited; however, several studies have focused on the preoperative C2-C7 CA^9,10,12,32). Most recently, Kwon et al.¹⁹⁾ approached post-ACDF cervical alignment using T1S and SVA.

Despite the safety and effectiveness of ACDF^31), the use of a stand-alone cage has been proven to be associated with relatively high subsidence rates and subsequent local kyphosis at the index level⁷⁾. Various factors including cage type and location, distractive force, and endplate preparation may affect subsidence. In the present study, we considered axial loading as an important factor of subsidence, and focused on axial loading in the neutral cervical position, as this is most frequently imposed on the cage. We assumed that the axial loading differs depending on the cervical alignment. The purpose of the present study was to evaluate the effect of preoperative parameters on postoperative cervical alignment as well as subsidence and pseudarthrosis.

Cervical alignment has a significant effect on postoperative clinical outcomes^11,14). In particular, studies on kyphosis after laminoplasty or long-level posterior fusion with accompanying injuries in the posterior neck muscles are currently in progress^5,13,16,29). However, to our knowledge, only one study has investigated the effect of ACDF on cervical sagittal parameters¹⁹⁾.

The reported factors affecting cervical alignment after ACDF include preoperative alignment^2,20), intra-operative segmental distraction^6), surgical level (single or multiple)^9), and cage characteristics^3,8,27). In addition, postoperative subsidence causes unexpected changes in cervical alignment. Pseudarthrosis indicates that the cervical alignment changes can proceed further¹⁷⁾. In addition to these factors, age, BMI, smoking history, low bone mineral density^4), endplate preparation^21), and cage size and location²⁵⁾ are known risk factors for subsidence. However, few studies exist regarding factors such as preoperative T1S and SVA.

The conclusions drawn in previous studies regarding T1S and SVA can be summarized as follows: T1S is an individual intrinsic value⁵⁾ determined by the thoracolumbar alignment^1), and compensated by cervical lordosis and SVA change. However, after surgeries, such as laminoplasty, with accompanying posterior neck muscle injury, this compensation becomes insufficient, resulting in kyphosis. In the present study, we observed changes in preoperative parameters that were similar to those of the previous study¹⁹⁾. In addition, as SVA increases, the CA ROM and SA ROM decrease. It is considered that motion is additionally reduced because the posterior neck muscle extension is already being used to maintain alignment.

Nolan and Sherk²³⁾ reported that the semispinalis cervicis and capitis muscles, the interspinous and supraspinous ligaments, which constitute the cervical spinous process ligament-muscle complex (SPLMC), are important factors in maintaining cervical sagittal dynamic and static balance. From a statistical point of view (Fig. 5), as the T1S increases, the cervical spine tends to fall forward (F×sin θ). To maintain cervical spinal balance, the SPLMC generates force (Fm) to counteract F×sin θ. However, because Fm acts on the spinous process axis, F×sin θ does not coincide with the axis. Therefore, while part of Fm is used to counteract F×sin θ, the rest of the force acts on a type of lever. This lever consists of the fulcrum (facet), effort side (spinous process), and load side (vertebral body). The force acting on the spinous process is converted to the opposite direction on the loading side, and this force (F_lever) is thought to reduce the loading on the cage. Furthermore, in the linear regression analysis of subsidence and T1S, conversion of T1S to cos shows a much higher significance (R²=0.737, p<0.001) compared to that for the angle alone (R²=0.123, p=0.009). In addition, the tendency of CA to increase as T1S increases is also explained by this force (F_lever), which induces lordosis by extending each segment. After laminoplasty, the ability to counteract F×sin θ is decreased because of the fibrotic change in the SPLMC. When laminoplasty is performed in patients with a large T1S, SPLMC cannot counter F×sin θ, resulting in SVA increases. However the insufficiency of the counteracting F×sin θ cannot explain kyphosis. We hypothesized that a decrease in F_lever, which acts to extend each segment, is the cause of kyphosis.

Pseudarthrosis was not affected by any preoperative factors in the present study; however it was affected by age. In addition, there was no association with subsidence in the non-S group (10/35 [28.6%]) or in the S group (6/20 [30.0%]). However, the pseudarthrosis incidence rates were 5/25 (20%) in the T1S<28° group and 11/30 (36.7%) in the T1S≥28° group. These results, although not statistically significant, suggest that further evaluation in larger-scale studies is needed.

This study had several limitations. We analyzed only ACDF with a stand-alone cage; cases with an anterior plate or anchoring cage were excluded due to differences in the static mechanism features. Second, the present study included two-segment ACDF and was not limited to a single level (e.g., only C5-6). Even with this limitation, we obtained high statistical significance. Nevertheless, for a more accurate mechanical analysis, one-segment single-level ACDF studies are required. Third, the static analysis was limited to the force acting on the sagittal plane of the cervical vertebrae, and did not include various actual forces; future cadavaric and fine element analysis studies are required. Finally, the study was small-sized and had a relatively short-term follow-up; differences in clinical outcomes according to fusion or subsidence could not be determined. The impact of subsidence on these factors is still controversial; however, a recent report suggests that sub subsidence is associated with poor long-term clinical outcomes¹⁷⁾. Thus, minimizing subsidence is of importance and future large-scale, long-term studies are needed.

The present results have significant clinical implications. First, efforts to reduce subsidence in patients with low T1S should be taken. Although this study did not demonstrate a clinical difference, a recent long-term study noted that subsidence groups reported worse long-term outcomes and mal-alignment¹⁷⁾. To reduce subsidence, a plate that disperses the loading on the cage may be applied; a more careful endplate preparation or lower-elasticity graft such as an allograft application may also be considered. Second, the effect of indirect decompression by the cage insertion will be decreased due to high degrees of subsidence. Thus, we should consider sufficient direct decompression to prevent symptom recurrence according to subsidence in low TIS patients.

Lower T1 sagittal slope could be a risk factor of subsidence following ACDF. According to the ROC curve analysis, a preoperative T1 sagittal slope <28° is likely to predict the development of subsidence. Surgeons need to be aware of this risk factor, which can be detected on preoperative plain radiographs, and should consider various supportive procedures to reduce the subsidence rate in such cases.