作者:谭建国教授
前文我们谈到了能够简洁明了地进行前牙美学修复设计的前牙美学四要素, 包括上中切牙切缘的位置、上中切牙临床牙冠的宽长比、上中切牙龈缘的位置和上前牙的宽度比例。其中上中切牙切缘的位置的确定是关键中的关键,它的确定不仅仅和前牙美学有关系,还会对咬合产生重要的影响。因此我们称这个“Edge”为“On the Edge of Esthetics and Function”。
那么如何确定上中切牙切缘的位置呢?上中切牙切缘的位置至少要从切龈向和唇舌向两维去考虑确定。我们先从美学的角度考虑上中切牙切龈向的位置关系。上中切牙切龈向的位置主要根据休息位时上唇唇下上中切牙的暴露量确定,因为只有暴露的上中切牙才是影响美观的,而休息位时最有美学代表性的生理性位置。关于此研究很多,规律性的结论是:
1. 休息位时上中切牙唇下暴露量为2-4mm
2. 女性上中切牙的唇下暴露量大于男性
3. 随着年龄的增高,上中切牙的唇下暴露量减少。
上中切牙的唇下暴露量不仅仅与牙、牙槽骨和颌骨因素有关,很重要地还受上唇的影响,特别是上唇的长度。中青年上唇的长度男性为22-24mm,女性平均为20-22mm。女性比男性平均短2mm。随着年龄的增长,每过十年上唇长度平均增长1mm。因此,休息位时上中切牙的唇下暴露量与性别和年龄都密切相关。
从美学角度根据休息位时上中切牙唇下暴露量而确定的上中切牙切缘的位置还要和后牙合平面协调,前牙切平面必须和后牙合平面保持一致。因此,上中切牙切缘位置的确定是一个综合因素决定的。
上中切牙切缘唇舌向位置的确定要根据上中切牙解剖上的生理性唇倾度、上下前牙正常的覆合覆盖关系,以及前导的设计等多个影响因素。
总之,无论是从切龈向还是唇舌向,上中切牙切缘位置的改变,不仅影响前牙的美学,还会影响口腔的咬合和发音等功能运动。
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