Risk Factors and Radiologic Changes in Subsidence after Single-Level Anterior Cervical Corpectomy: A Minimum Follow-Up of 2 Years

Degenerative cervical stenosis is a disease characterized by degenerative changes in the cervical spine that occur with aging, narrowing of the spinal canal and instability of the spine. If the motion segments continue to exhibit instability due to multilevel degenerative cervical stenosis and spinal cord compression continues, cervical spondylotic myelopathy with radiculopathy may occur. Once cervical spondylotic myelopathy is diagnosed, surgery should be performed in a timely manner to decompress the spinal cord, stabilize the spine and improve neural function.6, 17) Anterior cervical corpectomy and fusion (ACCF) is widely used for the treatment of multilevel degenerative cervical stenosis. This surgery is safe and highly effective in treating cervical degenerative disease. Many previous studies have shown good clinical results for this procedure because it allows direct decompression of the neural elements and immediate stabilization of the affected motion segments.2, 3, 5, 19, 22) ACCF has been confirmed to be beneficial for solid fusion and the restoration of cervical alignment. The materials used for solid fusion and stabilization include tricortical autogenous iliac bone grafts, autogenous or allogenous fibular grafts, and titanium mesh cages.12)

However, ACCF requires more operative time and leads to more blood loss than does anterior cervical discectomy and fusion (ACDF). Moreover, ACDF maintained better cervical lordosis and segment height than ACCF.9) These differences in results are mainly caused by the occurrence of graft subsidence over time. In addition to autogenous and allogenous fibular strut grafts, subsidence of the vertebral body occurs in titanium mesh cages.7) Subsidence is caused by the concentration of stress on the implant, which causes cage and plate failure as well as nonfusion and cervical kyphosis worsening. In addition, anterior cervical corpectomy using a titanium mesh cage sometimes results in delayed nonunion and thus changes in cervical alignment, and patients may require revision surgery.7, 9) By reviewing the follow-up results of patients who underwent single-level corpectomy 2 years prior, we investigated the radiologic and clinical outcomes of cervical corpectomy and the risk factors for subsidence.

Subsidence occurred in 20 of the 74 patients (27.0%). The mean age was 57.73±10.59 years, and there were 54 males and 20 females. The corpectomy site was C4 in 15 patients, C5 in 29 patients and C6 in 30 patients.

The subsidence group and the nonsubsidence group were compared. There were no significant differences in age, sex, the corpectomy level or the presence of OPLL between the 2 groups (TABLE 1).

TABLE 1
Parameters for the patients who did and did not exhibit subsidence after single-level cervical corpectomy

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Cervical parameters

The preoperative, postoperative, and pre-postoperative changes in the cervical parameters of the subsidence group and the nonsubsidence group were assessed. The preoperative T1 slope (p=0.020), change in T1 slope (p=0.026), and postoperative C27 Cobb angle (p=0.029) differed significantly between the 2 groups (TABLE 2). The preoperative T1 slope was 23.82°±7.37° in the nonsubsidence group and 28.46°±7.67° in the subsidence group (p=0.020). The change in the T1 slope was 2.26°±5.88° in the nonsubsidence group and −1.15°±5.23° in the subsidence group (p=0.026). In addition, the postoperative C27 Cobb angle was 8.94±8.92 mm in the nonsubsidence group and 14.10±8.62 mm in the subsidence group (p=0.029) (TABLE 2). We performed multivariate logistic regression analysis of the statistically significant factors. The preoperative T1 slope was a statistically significant factor (p=0.026) (TABLE 3).

TABLE 2
Cervical parameters of the subsidence and nonsubsidence groups

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TABLE 3
Multivariate analysis logistic regression analysis of risk factors for subsidence after single-level anterior cervical corpectomy

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In addition, the changes within the subsidence and nonsubsidence groups over time were assessed. In the subsidence group, the segmental angle and ACH decreased significantly from preoperatively to immediately after surgery and at 24 months postoperatively. In the subsidence group, the ACH significantly decreased from preoperatively to 24 months postoperatively, but the PCH did not decrease at 24 months postoperatively. That is, the ACH decreased more than the PCH did. The nonsubsidence group did not show changes over time in any of the cervical parameters (FIGURE 2).

FIGURE 2
Cervical parameters were assessed preoperatively, immediately postoperatively, 6 months postoperatively, 12 months postoperatively, and 24 months postoperatively. (A) C27 Cobb angle, (B) segmental angle, (C) ACH, (D) PCH, (E) C27 sagittal vertical angle, (F) T1 slope, (G) thoracic inlet angle, (H) neck tilt.
ACH: anterior corpectomy height, PCH: posterior corpectomy height.

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Clinical outcomes

In the nonsubsidence group, the neck and arm VAS scores improved from 6.85±1.14 and 7.30±1.22 preoperatively to 2.07±0.86 and 1.77±0.72 postoperatively (p<0.001). In the subsidence group, the neck and arm VAS scores improved from 7.14±1.42 and 6.68±1.29 preoperatively to 1.95±0.86 and 1.64±0.69 postoperatively (p<0.001). In addition, there were no significant differences in neck and arm VAS scores between the nonsubsidence and subsidence groups (p>0.05) (TABLE 4).

TABLE 4
Clinical outcomes of the subsidence and nonsubsidence groups

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Cervical corpectomy can be used to treat almost any pathology that causes spinal compression, such as osteophytes, discs, and ossified posterior longitudinal ligaments8, 18); after corpectomy, the anterior column may be supported using structural autogenous and allogenous grafts and titanium mesh cages.12) This surgery can be used to relieve symptoms associated with cervical spondylotic myelopathy and preserve structural stability.

Cervical corpectomy is a useful surgical method, but the incidence of complications such as respiratory distress and graft subsidence, dislodgement, and migration with screw pullout is relatively high. A failure rate of 71% has been reported for 3-level fixed plated reconstruction, and a high early failure rate of 75% has been reported for the use of titanium mesh cages and fixed anterior plate reconstruction in multilevel corpectomy.4, 21, 23) Many studies have been conducted on the complications after cervical corpectomy, especially subsidence, which increases the concentration of stress on the implants and leads to anterior plate and instrument failure, which is likely to cause and worsen neurological symptoms.7, 26) One study retrospectively reviewed the outcomes of anterior cervical corpectomy and titanium mesh cage fusion. The subsidence rate of cervical corpectomy was reported to be 30.8% (4.2±2.1 mm). Thus, subsidence after corpectomy and mesh cage fusion is unavoidable and an important complication.19) Many factors affecting subsidence after surgery have been studied. Many of these factors are related to aging, especially in postmenopausal women with osteoporosis, reduced bone quality of the vertebrae and a reduced thickness of the endplate.10, 19)

However, few studies have studied cervical parameters related to sagittal balance and radiologic features. Thus, we studied the risk factors and radiological changes in subsidence by reviewing 2 years of follow-up data of patients who underwent single-level anterior cervical corpectomy. The larger the T1 slope and C27 Cobb angle are after surgery, the higher the subsidence rate.

There are several factors that may explain the results. Corpectomy constructs lead to large loads on the bone grafts and endplates because of the limited contact area between the mesh cage and endplates. Biomechanical studies have suggested that the lever arm at the distal end of the plate increases with the length of the plate, and the constructs with long, fixed moment arms tend to exert far more load on the caudal screw bone interface than do rostal contracts, which frequently result in construct failure at the caudal ends of the construct.24, 25, 27)

Titanium mesh cages have several advantages and disadvantages.12, 13) First, the spike is fixed to the upper and lower vertebral bodies, enhancing graft stability. However, the spike easily penetrates into the vertebral body and causes subsidence. In particular, subsidence often occurs in multilevel corpectomy. In addition, the titanium mesh cage is filled with an autogenous cancellous bone graft to promote early fusion.5) In other words, extrusion, migration, or collapse of the cage is possible, but a loss of lordosis may occur due to subsidence after surgery.26) However, subsidence occurred in our study, resulting in a loss of lordosis, which is consistent with the results of other studies, but the presence of subsidence did not affect the clinical outcome.

Brodke et al.1) found that the static plate decreases due to a low load-sharing capacity after 10% subsidence occurs. Early reconstruction failure occurred in 3 out of 6 people when the lordotic angle was more than 15 degrees. Hyperlordosis of the cervical spine yields high shear stress at the bottom of the fused segment, which can lead to anterior slippage of the construct.21) Herrmann and Geisler11) confirmed that device failure and pseudoarthrosis occur at the end of long-segment constructs rather than at the top of the fused segment. The authors speculated that the long lever arm of the anterior plate transmits unacceptably high forces to the inferior level of the fusion area. In particular, patients with continuous micromotion and hyperlordotic alignment in the cervical spine, which can lead to reconstruction failure, were found to have increased levels of shear stress in the bottom of the fusion area.11, 20, 21)

The risk factors for subsidence are an older patient age, more corpectomy levels, severe osteoporosis, excessive endplate removal and intraoperative overdistraction.15) In addition, metal attributes and the shape of the graft are also important risk factors.28) T1 slope is one of the sagittal spine parameters and is an important parameter for cervical sagittal balance. To date, many normal or physiological parameters of cervical alignment remain controversial. Because cervical vertebrae are the most mobile part of the spine, unlike thoracic vertebrae, normative values for the sagittal balance have a large variation. The average reported T1 slope is 25.7 to 33. degrees, which can be affected by aging or posture.14) As shown in our study, an increased C27 Cobb angle and large T1 slope cause an empty space in the vertebral body graft, which is believed to induce subsidence.

The limitations of this study include its retrospective nature and the fact that only a small number of cases were included. In addition, the study did not accurately present how much T1 slope affects subsidence occurrences, and there are limitations that do not include an analysis of factors that would otherwise have affected. Therefore, based on the results of our study, a prospective study is needed to further evaluate the risk factors for and radiologic changes in subsidence after anterior cervical corpectomy.

Subsidence gradually occurred after single-level anterior cervical corpectomy for up to 24 months. However, it did not affect the radiologic and clinical outcomes because subsidence occurred simultaneously in front and back of the cage. When the T1 slope was large and the C27 Cobb angle change was severe, more subsidence occurred after anterior cervical corpectomy. If a patient exhibits the above cervical parameters after surgery, the patients' symptoms need to be carefully examined and evaluated.