Dynamic films: How to take and read them

by Jean-Yves Maigne, MD

Standard technique for the sitting film. The footrest helps the patient to feel more stable and to reproduce the position on a real seat 'as the stool is too high). The back is upright. If the pain is not experienced in this position, the patient is asked to slowly extend it. For a better balance, he/she can grasp a handdle with the left hand. When the familiar pain is felt, the film is taken. If, despite a sufficient period of time (5 minutes), there is still no pain, the patient is asked to sit in the same position which elicits it. In this latter case, it is rare to find an abnormality.

An important point is to focus the radiograph on the coccyx itself. There is no point in taking the lumbar spine on the same film.

Here is a normal standard (on the left) and dynamic (on the right) radiograph. The mobility of the coccyx can be assessed by superimposing the two films and matching the sacrum (see below).

Left: she superimposition of the two radiographs is shown here on the left. The black arrow indicates the first mobile joint, which is to be considered for the measurement of mobility (here: 34° of flexion, see the full case).

On the right, the sketch shows a case where the first mobile joint is the sacrococcygeal disc. Thus, the angle of mobility is B-A-C.

Note also that it is necessary to pivote the second film to match the sacrum. This angle is exactly the pelvic tilt (or sagittal pelvic rotation) when you pass from seating to standing up (see below). It is easy to demonstrate that obese patients (at least these with coccydynia) have a weaker pelvic rotation than slimmer patients. The same apply to patients with low back pain or with a history of lumbar surgical fusion. This is not without consequence on the lesions: a luxation is much more frequent in obese patients.

Different angles can be measured. A: standing radiograph. B: sitting radiograph showing flexion of the coccyx (dotted line). Right : Superposition of the 2 radiographs matching the sacrum, obtained by pivoting the standing film A through an angle representing sagittal pelvic rotation (angle 1). Angle 2 is the angle at which the coccyx strikes the seat surface (angle of incidence). Angle 3 indicates the coccygeal mobility. Another angle, not indicated here, is the "sacrococcygeal angle", the angle between the sacrum (in fact, the vertical axis of the last sacral vertebra but one) and the coccyx.

My studies show that the direction of coccygeal movement is strongly influenced by the amount of pelvic sagittal rotation and by the angle of incidence. The patients with pronounced pelvic rotation (mean >40°) have a small angle of incidence (i.e. their coccyx hit the seat quite horizontaly) and forward moving coccyges, whereas those with a lesser degree of pelvic rotation (mean: <30°) have a pronounced angle of incidence (i.e. their coccyx hit the seat more verticaly) and backward moving coccyges. Also, the first have straight coccyges (sacrococcygeal angle > 150°) and the second more curved ones. The Body Mass Index is also strongly associated with the angles. See my paper (pdf)