Dynamic factors are important contributors to neurologic deficits in cervical spondylotic myelopathy (CSM) patients. Between 2005 and 2009, we retrospectively investigated 72 patients with CSM with or without lower cervical instability for their neurologic status after low-energy trauma and surgery. Patients were divided into two groups: the instability group and the stability group. The neurologic status was evaluated using the Japanese Orthopaedic Association (JOA) scoring system. The incidence of neurologic deterioration after trauma was higher in patients with lower cervical instability than in those without (p<0.05). Patients in the instability group had a lower preoperative JOA score (p<0.05) and experienced less post-surgery improvement in neurologic function than those in the stability group (p<0.01). Even a minor trauma to the neck can lead to irreversible spinal cord injury for patients with CSM with cervical instability. Eliminating local instability through surgical and non-surgical methods is necessary for such patients before decompression and fusion surgery.