PC5P is a well-known postoperative complication that has been reported in many studies. The incidence of PC5P is reported to be about 0-30%, and the difference in incidence varies greatly depending on the surgical approach. [1, 2, 3]
The prognosis for PC5P has been reported to be relatively good, with an onset time mostly within a few hours to a few weeks after surgery, and spontaneous full recovery was achieved within the first 6 months in most cases. [2, 3, 5, 6, 10] However, even if the prognosis of PC5P is good, dysfunction due to PC5P may affect postoperative rehabilitation or quality of life and adversely affect the overall prognosis. Therefore, it is important to understand the etiology of PC5P and prevent it in advance.
Risk factors for PC5P include: 1) iatrogenic direct nerve root injury or intraoperative traction [16, 17]; 2) surgical strategies such as multiple segmental operation with/or posterior approach leading to extensive spinal cord shift [2, 5]; and 3) shoulder traction for positioning [10, 11, 12, 13].
Since intraoperative nerve injuries can be avoided with meticulous care of the surgeon, they are modifiable risk factors for PC5P. However, the selection of surgical strategies depends on each patient’s pathology, and modifications may be limited or not possible. The overall incidence of PC5P for anterior approaches was 4.3% and that for dorsal approaches was 10.9%. According to these results, PC5P occurred more in posterior surgery. 
This is due to the anatomical features of the C5 nerve root. C5 dorsal rootlets were originated horizontally and shorter compared to other cervical rootlets. [18, 19] Therefore, C5 ventral rootlets appeared to become taut and easily injured after dorsal decompression. Similarly, PC5P also occurred after anterior surgery due to the anatomical features described above. However, “cord shift” theory focuses mainly on the posterior approach, and the relationship of anterior surgery has not been explained completely. Therefore, other risk factors of unexpected PC5P should be considered.
Excessive shoulder traction could affect both anterior and posterior surgery. Therefore, it may be a risk factor in anterior surgery. In some studies on intraoperative neuromonitoring (ION), loss of somatosensory evoked potential (SSEP) during anterior surgery returned after releasing shoulder traction. [11, 12] These studies suggest that preoperative shoulder traction may affect PC5P.
Generally, shoulder traction is used for fluoroscopic imaging of the lower cervical spine [10, 13]. Since there are no standardized criteria for traction forces, we had concerns about how much traction force should be applied. Woodroffe applied 10, 20, and 30 lb in five subjects, and then used MRI to determine the angles between the vertical spinal axis, the C5 nerve root, and the upper trunk. The angle between the C5 root and the upper trunk increased with higher weight and it was statistically significant. Therefore, Woodroffe demonstrated that shoulder traction could lead to C5 nerve root tension with subsequent injury and palsy. However, since the standards of 10, 20, and 30 lb used in that study did not reflect the traction force applied in actual operations, we determined the traction forces based on the pre-investigated values from our institute. These values were measured from anesthetized patients who underwent anterior surgery.
Although we have other concerns about nerve dysfunction according to the length of cervical root elongation, there are no precise data regarding that. Some reports of peripheral nerve injuries have shown that the thresholds for nerve rupture and dysfunction are different for each nerve. [22, 23] We reviewed studies about the ulnar nerve elongation for reference data of stretched length. These studies showed increased length and strain with elbow movement, but did not provide a detailed analysis of the dysfunction associated with increased length. [24, 25] Therefore, it is not possible to know exactly how the increased length in this study affected PC5P.
However, there are some studies that can infer this. When the abduction extension cervical nerve root stress test was performed on radiculopathy patients, pain aggravation or paresthesia occurred, and under the same conditions, the cadaver nerve root was displaced by about 2 to 6 mm.  The results of our study showed an increase from about 2 to 6.5 mm in the 8, 10, and 15 kg traction forces, which is similar to the traction forces applied in actual surgery. Therefore, we suggest that the stretched nerve root observed in our study could cause neuropathic symptoms. In addition, since this tension was maintained during surgery, patients may progress to PC5P.
In another cadaveric study, the intradural length during shoulder traction was observed after dissecting the dural sac, and the study concluded that shoulder depression could be a risk factor for PC5P.  It also reported that intradural movement at the “gutter level” (the transition from inside the foramen to outside the foramen) was much more dramatic. However, that study observed the entire rootlet length regardless of the gutter level. Therefore, we observed the cervical nerve root movement grossly passed by the gutter level.
Moreover, it has been reported that the main etiology of PC5P is impairment of the C5 nerve root induced by existing C4-5 foraminal stenosis. Therefore, we propose that the nerve root is more easily pinched in the gutter level if there is foraminal stenosis of C4-5. Also, most patients who underwent surgery present myeloradiculopathy, and they have both foraminal stenosis and central stenosis. In this case, the nerve root is more pinched at the gutter level, since the space of intradural rootlets is reduced. Therefore, C5 nerve root elongates more in the extraforaminal zone (passed by the gutter level), and this leads to PC5P (Figure 5). So we recommend that patients with foraminal stenosis and central stenosis are treated more gently and carefully during shoulder traction.
We have some limitation of this study. C6 extended length measured in second experiment was similar to that of the C5 nerve root. It can be used for subsequent comparative study in the future, but for now, it is insufficient to explain the significance. Secondly, study was limited in that it did not have a control group for comparison. Also, only one male cadaver is not enough to explain the effects of shoulder traction. Therefore, a serial cadaver study with a control group is needed in the future. In addition, our results could differ from real patient results, as we dissected and cut structures, such as the soft tissue, carotid artery, and SCM. Further studies are needed to overcome the limitations above.