Any vertebra may be posterior: the Atlas is rarely so as a whole, and never unless the Axis is also displaced backward; the Cervical and Dorsal regions present frequent variations of this sort, which must not, however, be confused with long, prominent, or overdeveloped spinous processes; the Sacrum may be posterior to the ilium on one side, or to both ilia.
Occipital Subluxations
Mention should be made here of a form of subluxation not strictly vertebral—displacement between the condyles of the occipital bone and the lateral masses of the Atlas. This occurs when the head has been moved too violently upon the Atlas so as to cause an immediate nerve irritation and muscle tension sufficient to hold it in its abnormal position. The Cervicals may be quite normal below the Atlas though this, of course, is not the rule. Correction of occipital subluxations is made by applying force to the Atlas and to the skull, sometimes by holding Atlas and rotating the skull.
Age of Subluxations
The relative age of subluxations may be determined, within rather wide limits, it is true, by a study of the form of the spinous process. Newly acquired subluxations are sharply defined, having noticeable edges on the spinous process. In time they tend to become rounded and blunt and appear to cover more surface, just as the mountain range which, when first upheaved, is sharp and rugged, gradually rounds into regular curves through the work of the elements.
In this way Nature protects the subluxated vertebra from further contact with the environment surrounding man, the rounded process offering less opportunity for a blow or shock to affect it.
Changes in Shape
Bone diseases such as rachitis osteomalacia, etc., and especially Potts’ Disease, or spinal caries, make marked changes in the shape of vertebrae. Also a subluxated vertebra may gradually assume a shape suited to the abnormal position it occupies, the commonest change being the assumption of a wedge shape by the centrum. This is a great obstacle to adjustment, as the abnormal shape of the vertebra makes it tend to settle after each movement into the old abnormal position.
There are few spines without some more or less misshapen vertebrae.