ACDF is one of the most commonly performed spinal procedures. It allows for direct decompression and is commonly used to treat cervical intervertebral disc herniation and ideally to resolve spinal cord compression. The approach also maintains the cervical alignment and stabilizes the spine column. However, the anterior approach is complex and associated with a high risk of complications [11]. C5 palsy after ACDF is an uncommon but relatively serious complication. In the present study, we analyzed the radiographic characteristics of patients who developed C5 palsy after ACDF for cervical degenerative disease.
C5 palsy is quantified and diagnosed by the patient’s manual muscle test score (range of 0–5). Its presence is generally defined as a reduction of at least 1 point in this score compared with the preoperative score [12]. Multiple theories for the etiology of C5 palsy have been proposed. Satomi et al. considered that intraoperative iatrogenic trauma is the main cause of postoperative motor paresis of the upper extremities; such a theory could explain the cases of palsy that develop immediately following surgery, but not those that appear several days following surgery[13]. Tsuzuki et al. proposed nerve root traction caused by consecutive shifting of the cord following decompression[14]. Other authors have considered that the anterior horn of the spinal cord undergoes damage from reperfusion injury after decompression surgery [15]. Similarly, Komagata et al. proposed that C5 palsy is caused by ischemia as a result of a decreased blood supply from the radicular artery, but this has not been substantiated on MRI[16]. However, Saunders postulated that the C5 root may be uniquely sensitive to inadequate foraminal decompression[17]. Notably, all of these are assumptions regarding C5 palsy development and do not have significant supportive evidence.
In the present study, radiographic data from patients who developed C5 nerve root palsy were independently reviewed to determine the predictive factors that may result in this complication. We found that the APD was significantly smaller in patients who developed C5 nerve root palsy than in patients who did not (P = 0.001). The APD was a significant predictor of the development of C5 nerve root palsy. This result is similar to the findings reported by Blizzard, who showed a strong correlation between the preoperative APD and C5 nerve root palsy[18]. The palsy more regularly appears at C5 because the C5 root is shorter and the angle from the cord is more obtuse than at other levels. The inherent nature of decompression at C5, whether anterior or posterior, may have similar effects on the spinal cord or spinal canal that contribute to the development of the palsy.
The change in the preoperative to postoperative cervical alignment determined the rate of the change in cervical curvature. Takemitsu et al. reported that the cervical curvature of patients who developed C5 palsy underwent an obvious change, and they considered that correction of the cervical alignment by posterior instrumentation might have caused excessive posterior shifting of the spinal cord and iatrogenic foraminal stenosis[19]. In the present study, patients with postoperative C5 palsy had a significantly greater amount of segmental lordosis. We consider that increased cervical curvature plays an important role in the development of postoperative C5 palsy. Chen et al. suggested that C5 palsy may also be correlated with excessive reconstruction of cervical curvature[20]. Hojo et al. found that the change in cervical curvature in patients with C5 palsy was significantly larger than in patients without C5 palsy, whereas inadequate lordosis may not reduce ventral spinal cord compression caused by herniated discs or ossification of the posterior longitudinal ligament [21]. Therefore, we propose that avoiding excessive reconstruction of cervical curvature during surgical decompression procedures may reduce the incidence of C5 nerve root palsy.
In several studies, C5 palsy was suggested to be associated with preexisting stenosis of the C4–5 intervertebral foramen. C4–5 intervertebral foraminal stenosis on preoperative CT scans has been proposed as a predisposing factor 6. Some authors have observed significant narrowing of the C4–5 foramen on preoperative CT axial images between patients with and without palsy [22, 23]. The nerve root traction hypothesis helps explain C5 palsy occurrence in the setting of foraminal stenosis and derotation postdecompression[24, 25]. However, our findings revealed no significant difference in preoperative C4–5 foraminal stenosis in patients with C5 palsy, and we found that foraminal stenosis was not correlated with the incidence of C5 palsy. One possible explanation for these results is that our study was underpowered because of the small number of patients with postoperative C5 palsy (n = 21).
Although the current findings are in agreement with those of previous studies, our study had several limitations. First, the sample size was relatively small. Second, this was a purely radiographic study, and no attempt was made to correlate the results with clinical outcomes. Finally, as with any retrospective study, selection and measurement biases may be present. Therefore, future studies should be prospective, randomized, controlled, and of longer duration.