By retrospective radiological analysis of early postoperative magnetic resonance (MR) pictures, the researchers were able to determine the effects of posterior cervical muscle enlargement on C5 palsy (C5P). Cervical laminoplasty was a well-established procedure, however, the risk of C5P following surgery was yet unknown. The medial branches and cervical nerves may be stretched as a result of lateral stretching and postoperative swelling of the multifidus muscle, according to studies. The C5 nerves of 107 consecutive individuals who had laminoplasty were investigated in total. As parameters, the researchers looked at their demographic and surgical information, as well as radiography and MR images, which included the axial cross-sectional area (CSA) of the posterior muscles. Patients and C5 nerves were split into two groups: C5P and non-C5P. The parameters and changes in the two groups were comparable to determine if there were any relationships with C5P. Age, sex, smoking history, diabetes mellitus, and preoperative Japanese Orthopedic Association ratings were not substantially different between the groups in demographic data. Only the C5P group’s body mass index (BMI) was considerably higher. The number of laminoplasty, operative time, decompression trough width, cervical sagittal alignment, preoperative spinal cord rotation, and posterior shift of the spinal cord were not significantly different in the C5P group in terms of surgical and imaging data, but the multifidus CSA change ratio was significantly higher in the C5P group. The multifidus CSA change ratio and BMI were found to be significant independent variables in multiple logistic regression analyses. Multifidus edema is linked to C5P, presumably due to traction on the C5 nerve via the cervical dorsal rami’s medial branches. The medial branch of the dorsal rami was the shortest and may have the most traction force. As a result, a moderate manipulation of the deep posterior muscles during surgery could be a viable C5P countermeasure.