Читать книгу Practical Procedures in Dental Occlusion - Ziad Al-Ani - Страница 18

Situated close to the collagen fibres and in between like a sandwich within the periodontal ligament and described as Ruffini‐like by Byers (1985) and Lambrichts (1992) in humans. The main periodontal Ruffini nerve endings have been classified as type 1 and 2. Type 1 shows lamellar terminal Schwann cells and expanded axon terminals with axonal spines which penetrate surrounding tissue; type 2 is characterised by lesser branched Ruffini endings with fewer axonal spines, less basal lamina and fewer Schwann cells. Both of these receptor types are present in the periodontal ligament (Maeda et al. 1999).These are crucial for force interpretation and control of mastication and therefore it would be assumed that the different teeth (incisors–molars) would have differing sensitivity thresholds and this has been shown to be correct by Johnsen et al. (2007). This is called the interocclusal tactility threshold (ITT) as discussed by Herren (1988) and this study uses foils of varying thickness between healthy teeth. The smallest ITT recorded is between 8 and 60 μm but during chewing the threshold increases by a factor of 5 (range 2.5–8×) and the occlusal perception is reduced due to descending inhibition (sensory gating), meaning there is a filtering out of irrelevant information which allows for enhanced detection of unexpected stimuli. There are also interindividual variations which are ascribed to differing attentional degrees in the higher brain centres and this will be linked to adaptability and neuroplasticity (capability to change and adapt to new demands).

The direction of force also shows that the PDMRs respond more when the forces are axial, i.e. in the direction in which they function best – ‘Directional bias may reflect functional adaptation’ (Sato 1988). When force is applied, the collagen fibrils are compressed which fires the mechanoreceptors. The use of the split and hold tests by Trulsson and Johansson (1996) also determined the amount of force required through a positive feedback loop, meaning the harder the food, the more force is applied through the initial feedback from the mechanoreceptors. A recent systematic review by Piancino et al. (2017) summarises studies that looked at this in greater detail.

What happens, then, when we lose teeth and the PDMRs are lost? Several studies such as Svensson and Trulsson (2011) and Svensson et al. (2013) have shown higher biting and food holding forces, indicating that optimal restoration design is fundamental.