A 47-year-old female presents to the clinic with neck pain. She states the pain has been ongoing for almost a year. She has tried over-the-counter medications which only provided mild relief. She has also tried massage therapy which she says relieved her pain, however it returned soon after. Osteopathic structural examination of the cervical spine reveals that C4 is restricted in translation to the left. The restriction seems to improve upon flexion of the neck. Which of the following is the most likely diagnosis given the physical exam findings?