Both health care providers and lay people may manipulate the spine. The health care provider group most likely to employ spinal manipulation is chiropractic. A recent (2017) in-depth analysis indicates that there are approximately 70,000 practicing chiropractors in the United States, the majority of whom use spinal manipulation in their clinical practice. There are eighteen government accredited chiropractic colleges located throughout the United States. Many of them have achieved university status, granting academic degrees in addition to chiropractic degrees (DC degree).
The lay press and occasionally peer-reviewed journals claim that cervical spine manipulation may cause injury to a cervical artery. This claim is exceptionally controversial because proof of such an occurrence is impossible and the alleged incidence is so incredibly rare that definitive conclusions are also impossible.
The most recent credible evidence (March 2019) estimates the claim of cervical artery injury to be as few as 1 per 8.1 million chiropractic office visits and 1 per 5.9 million cervical manipulations by practicing chiropractors. Assuming that a typical chiropractor has 20 patient visits per day, 100 patient visits per week, 5,000 patient visits per year, and a 40-year career, this would total approximately 200,000 patient visits. Such a chiropractor would have to be in clinical practice, at that volume, for 1,200 years before statistically seeing a single alleged vascular injury event.
Although cervical artery dissection primarily occurs spontaneously, physical trauma to the neck, especially traumas involving hyperextension and rotation, has been suspected to trigger them. This is especially noted with whiplash-type injury mechanisms.
Despite this incredibly low incidence of risk, students in chiropractic college are taught about such risks out of an abundance of caution; chiropractors are licensed health care professionals. The cervical arteries at risk are the internal carotid artery and the vertebral artery.
The artery that has the greatest risk for injury is the vertebral artery. This is because the vertebral artery ascends in the cervical vertebrae, in a hole called the foramen transversarium.
The atlas-axis (C1-C2) vertebral articulation of the cervical spine is mechanically unique. It is designed for the function of rotational motion. Theoretically, such rotation places the vertebral artery at increased risk of tractional stress, increasing the potential for injury. Students in chiropractic college are taught not to apply a rotational manipulative thrust of C1 on C2. Furthermore, biomechanical studies indicate that the vertebral artery will experience additional tractional stress if the upper cervical spine is in extension. Chiropractic college students are taught not to use the combination of C1—C2 rotation—extension—thrust manipulations (adjustments)