Hinge axis determination of the temporomandibular joint and its interpretation: what do we really measure?

Terminal hinge axis (THA) determination is recommended in some clinical situations and for some diagnostic purposes. Different methods are described and are available for this task. In particular, circle fitting or iterative trace recording methods, in the conventional process known as the pantographic or Lauritzen method, have mostly been applied in clinical settings and have even been used as exact reference measures in many studies. The aim of this study was to investigate whether the conditions for THA determination by these methods principally allow for the differentiation between a pure rotational movement and a combined translational and rotational movement. A further question relates to how large the uncertainties are if an additional translational movement is present during the first phase of mouth opening or closing. These questions were investigated by an exact simulation. The methods under consideration are seen to be able to detect a pure rotational movement, if one or both of the following conditions are met: a) the traces recorded around the rotational center perform a circlelike motion, and b) a point or pin on the rotational center remains at rest during movement. It can be proven that in the relevant clinical situation these conditions also hold in the case of a combination of translational and rotational movement. Furthermore, small translations of 1.1 mm lead to a deviation of the THA of around ± 6.7 mm, and a translation of 2.2 mm to an uncertainty of even ± 13.5 mm. The significance of these results suggests that the commonly used methods for THA determination should be reevaluated, and the literature on this topic should be carefully scrutinized.