A cadaveric study using an artificially created unstable cervical spine injury has shown marked displacement of the vertebrae when cervical collars were applied, and when the bodies were moved in a way that simulated normal transfer and log-rolling. There was no comparison with a no-collar situation, so we can’t say from this that collars are necessarily bad, just that they’re no good in this cadaveric model. I like this statement by the authors:
“A variety of collars, backboards, and other equipment and techniques are being used in an attempt to achieve spine stabilization, largely without any validation of efficacy when used in the presence of a severe cervical injury. Randomized, prospective clinical trial designs are challenging in this domain theless, basic cadaver studies can provide valuable insight into potential clinical efficacy.”

Even more musical to my ears is the editorial commentary by neurosurgery professor Richard L. Saunders, MD:
“…the more compelling question is whether there is a place for collars in emergent protection of the injured cervical spine or are they simply a gimcrack***?
The incidence of second injuries to the spinal cord in the extraction of accident victims under the best of EMT performance is not known and would be difficult to determine. However, in an effort to minimize that incidence, paramedical gospel is the application of a cervical collar, maintaining the neck in in-line and in a neutral position. By definition, this gospel implies the deliberate movement of the neck to apply an orthotic known to be nonprotective. Furthermore, the neutral and in-line admonition implies that the patient’s neck position can be safely adjusted to “look better” without a shred of evidence that this might be a safer strategy than avoiding any unnecessary neck movement whatsoever….
…In a conclusion common to many small study reports, the authors recommend that more work should be done in this area. In my opinion that might be best in refinements of extraction methods with an eye to only that neck movement necessary to resuscitation, collar be damned.”
Motion Within the Unstable Cervical Spine During Patient Maneuvering: The Neck Pivot-Shift Phenomenon
J Trauma. 2011 Jan;70(1):247-50
*** I confess never to have encountered this word before. According to the freedictionary.com, a gimcrack is ‘A cheap and showy object of little or no use; a gewgaw‘. Now, WTF is a gewgaw?!?!