Cervical spine (neck) injuries can be hard to classify and describe. Damage can occur to the bones, joints, ligaments, and discs. In addition the injury can affect the nerves and/or spinal cord causing a neurologic problem.
Anything that can change the alignment and function of these various parts can result in an unstable spine. According to spine experts, stability is the ability of the spine to hold up under loads keeping each segment in place. The vertebra stay lined up in their normal positions at rest and during neck movement. There should be no damage or irritation to the nerves or spinal cord due to structural changes.
Today's technology has made it possible to take X-rays while the spine is moving. Any obvious loss of form or structure will show up as an unstable segment. In your daughter's case, swelling from an injury, fracture(s), or damage to the ligaments can make for an unstable spine until healing occurs. Then the spine can be re-evaluated under conditions of normal motion. If all goes well, everything will move as it should under normal loads. In other words, she will have a stable cervical spine.