What is the hinge axis?  Any three dimensional object that rotates does so around a central axis.  In the case of the mandible we have two condyles joined by a rigid body (the mandible).  Location of the hinge axis of the mandible can be used diagnostically (electronic Condylography, and hinge axis transfer of study casts to an articulator).  This allows accurate study  of a patient’s mandibular movements.  In addition, there are many advantages in more complex cases where prosthetics are fabricated at an increased vertical dimension (e.g. cases with wear).  When verticalization is done around the true hinge axis subsequent restorations are formed which can be transferred to the mouth with minimal adjustments.  (REF Bosman 1974). 

Similarly, transfer of the  maxillary and mandibular casts in an accurate three dimensional relationship allows fabrication without excursive interferences.  If a clinician uses an anatomical facebow (earbow) or chooses not to use an articulator at all he or she risks the creation of significant interferences, reduction of function and iatrogenic harm. 

So, how is the hinge axis located?  The gold standard is a mechanical technique using metal styli attached to a lower bow fixed to the mandible which traces movement on paper flags over the TM joints. The true hinge axis is found by adjusting the styli to a point where only rotation is observed.  This is accomplished at a minimal opening of less than 15mm interincisal distance to  limit translation.  

Numerous  electronic instruments exist that  can also be used with mixed results depending on the methodology.  At VieSID we use Electronic Condylography which is supported by the EU as a level 1 accurate medical device.  Hinge axis is determined manually then confirmed electronically.