There are only two ways in which the dens can end up displaced that far anteriorly. One is if there is a fracture at the base of the dens, and the other is if the patient has a congenital process called os odontoideum.
The tip of the dens is formed from a separate ossification center than the rest of the dens. Sometimes this ossification center does not unite with the rest of the dens and os odontoideum is the name applied to this. There is some controversy about whether os odontoideum is actually a real entity. Some authorities feel that there is no such thing, and that all cases that are called os odontoideum are really fractures of the dens that somebody missed. No matter how you feel about os odontoideum, this patient is unlikely to have one, since the ununited ossicle seen in os odontoideum usually has a smooth, sclerotic margin all the way around. In addition, os odontoideum is usually associated with hypertrophy of the anterior arch of C-1, which is not seen here.
That pretty well leaves fracture as the leading consideration in this case. Did you notice that when the patient was positioned on his side for the tomogram that the dens fragment dropped back into normal alignment? An actual fracture line is not well seen on the tomogram, but a discontinuity is distinctly seen at the anterior margin of the dens (rightmost arrow). One can also see a lucent lesion within the body of the dens with a generally well-defined margin (radial arrows).
As soon as these films dropped out of the processor and the radiologist noted the unstable dens fracture, he made his way with some haste for the radiology suite to apply a cervical collar to the patient. As soon as he entered the room, the patient asked, "Say, is my breast cancer acting up again?" Breast cancer in men is not very common (thank goodness!) but is particularly virulent when it does arise. This gentleman had been first diagnosed seven years prior to this and did well following treatment until he suffered a femoral metastasis 3 years later. After that was treated, he did well until the current episode of neck pain.
A subtle a priori clue to this case from the history was that the patient woke up with the neck pain, indicating that this fracture was not the result of any significant trauma. Anytime that one sees a fracture with no significant history of trauma, it should raise a red flag in one's brain and bring the following differential to mind:
Osteoporosis as a cause for a dens fracture is possible in this 74 year old man. However, it would be prudent to treat this as a pathological fracture until proven otherwise. The tomographic demonstration of the lucent lesion clinches the diagnosis of a pathological fracture.
There are several useful lessons to be learned from this case. One of the most important is the value of a decent history when looking at a set of films. The history was just barely sufficient to make one consider the possibility of a pathological fracture if one was on one's toes. However, what if the history had been more complete and actually read:
74 y/o male with a history of breast carcinoma who has been treated for a prior metastasis to his femur. Patient has no history of significant trauma, but awoke with significant neck pain. Please rule out a pathological fracture to the cervical spine.
This would certainly have made life a lot easier for the radiologist, wouldn't it?
Another lesson here is that if you can't tell what is going on, get another view. This is a useful general principle, and may simply take the form of another plain radiograph from another angle. One can also sometimes profit from stress views, views obtained later in time (10 -14 days later can be helpful in showing subtle fractures),views of an opposite extremity for comparison, or even other imaging methods, such as tomography, CT, MR, ultrasound, or radionuclide imaging. In this case, other plain films didn't help, so we went to tomography to make the diagnosis.