Something to consider. 

 

In the classic Grostic headpiece placement, if the spinous process of the axis is listed on the same side as atlas laterality, it is termed an "inferior spinous.” According to the Grostic rule, this requires that the headpiece be set so that the external occipital protuberance (E.O.P,) is 1/4" to 3/8" below the spinous process of C7. (Usually referred to as "vertebral prominence,” or "V.P.") 

 

But then the question must be asked: What effect does this change in the horizontal attitude of the atlas and the cervical spine have on the relationship of the atlas and the cervical spine to the calculated vector – the "height factor" of the adjustment? 

 

The drawings below illustrate this point. We have used a distance of 5" between E.O.P. and V.P., which seems to be the average. Of course, this distance varies with the length of the patient's C-spine. The shorter the C-spine, the greater the angle, and visa-versa.  

 

 

 

Drawing #1 shows the E.O.P. set horizontally in line with V.P.  
 

As is shown in the above drawing, the angle of the atlas and the cervical spine has been changed by 4.3 degrees, which converted to inches in the Grostic technique would amount to a variation of slightly more than 2 inches in the height factor. As a result of this change in the attitude of the atlas and the C-spine relative to the applied force, the true angle of the height factor of the adjustment has been lowered by slightly more than 2 inches.  

 

 

Drawing #3 shows the angle of deviation from the horizontal when the headpiece is set for a superior spinous. 
 

Therefore, if the calculated height factor were H 2 in either of these cases, the headpiece setting would create a 100% error. This would cause the actual angle of the vector with reference to the atlas and C-spine to be lowered by 2 inches in the case of the inferior headpiece setting, and raised by 2 inches in the case of the superior headpiece setting.  

 

If the calculated vector were H1, the difference between a straight horizontal head-piece setting and an inferior headpiece setting would create a 200% error in the actual angle of the height factor of the adjustment relative to the atlas and the C. spine. 

 

Looking at this another way: The difference in listing a spinous as inferior 1 degree and listing it as superior 1 degree will cause roughly a 4" difference between the calculated "height factor" and the actual applied force. 

 

One does not have to be a mathematician to extrapolate the enormous unaccounted for variables involved in the above axioms.