Transverse Hinge Axis

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Transverse Hinge Axis

 

Article by  Gianni Frisardi · Flavio Frisardi

 

This chapter builds on previous critiques of the disconnection between clinical practice and the bioengineering foundations of diagnostic tools. Despite their limitations, devices like the Sirognathograph and Kinesiograph K7 remain relevant for understanding mandibular movements. However, their inability to capture rotational dynamics leads to errors in representing mandibular kinematics, particularly the transverse hinge axis ($_{t}HA$). The analysis emphasizes the clinical relevance of accurately determining $_{t}HA$, especially for prosthetic rehabilitation, where even minor localization errors can compromise treatment quality.

The RDC excluded kinematic replicators from TMD diagnostics due to their low clinical validity. However, this chapter argues that these tools retain value in understanding mandibular kinematics, particularly in capturing the rototranslational dynamics around $_{t}HA$. The discussion introduces the paraocclusal clutch, which avoids vertical interferences, contrasting it with the occlusal clutch, which can lead to occlusal recording errors. Simulations demonstrate how tHA mislocalization impacts dental occlusion, particularly when inclined dental cusps exacerbate occlusal discrepancies.

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  Figure 2: Simulation of angular discrepancies due to hinge axis mislocalization.

Impact of Mislocalized tHA: With flat dental cusps and a mandibular opening of 0 mm, cuspal errors are negligible. With a 3 mm mandibular opening, errors range from 0 mm (exact localization) to 1 mm (mislocalization of 10 mm). Inclined cusps introduce significant errors even at 0 mm opening, with vertical cuspal errors reaching 0.87 mm for a 10 mm mislocalization. Significance of Rototranslational Dynamics: Misrepresentations of mandibular dynamics result in occlusal discrepancies that compromise masticatory efficiency and patient comfort.

The RDC's decision to exclude kinematic tools like the Sirognathograph is challenged by evidence showing their importance in capturing mandibular dynamics. Accurate determination of $_{t}HA$ is essential for precise occlusal alignments, particularly in prosthetic rehabilitation. The simulations demonstrate that neglecting angular dynamics leads to significant errors, affecting both diagnostic accuracy and treatment outcomes.

This chapter underscores the need for advanced kinematic tools to enhance clinical practice. By integrating these tools, clinicians can ensure more accurate diagnostics and superior prosthetic treatments, minimizing the risk of occlusal errors and improving patient outcomes. The discussion sets the stage for exploring other critical axes, such as the vertical ($_{a}HA$) and sagittal ($_{c}HA$), in subsequent chapters.

«But is understanding $_{t}HA$ dynamics enough?»
(No, the mandibular kinematic phenomenon requires a comprehensive understanding of all six degrees of freedom. These include the vertical and sagittal axes, topics that will be addressed in upcoming chapters.)

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