(1) Model observation is the observation of the surface of a plaster model of a tooth defect using a model observer to determine the inter-parallelism between the axial surfaces of each of the remaining natural teeth, as well as their inter-parallelism with the surfaces of the alveolar ridge. Model observation is divided into two processes: diagnostic model observation and working model observation.
(2) The model observer is mainly composed of three parts: the observation frame, the observation table and the analysis and measurement tool.
1) Observation frame: includes a base platform, a vertical support arm fixed with the base, a horizontal arm fixedly connected with the vertical arm or horizontally rotatable, a vertical measurement arm that can be lifted and moved, and a card head located at the end of the vertical measurement arm for fixing the analysis tool.
2) Observatory: the upper part is placed and fixed model, there is a steering joint ball at the bottom of the base, which can be adjusted and fixed model tilted in different directions, i.e., change the relative angle between the model and the vertical measuring rod.
3) Analysis tool: fixed at the lower end of the vertical measuring arm and in the same direction, used to measure the model of the natural tooth axis surface and alveolar ridge organization of the inverted concave. It includes an analyzing rod, a tracing lead core, an inverted concavity ruler (with a circle of lateral protrusions at the end, and three kinds of protrusion widths: 0.25mm, 0.5mm, and 0.75mm), and a shaping wax cutter.
2. The use of the model observation
(1) the division of inverted concave area and non-inverted concave area
Observation line, also known as the wire, is a model observer tracing lead core along the axis of the crown of the tooth and the hard and soft tissues of the most protruding point of the line drawn. The line of observation (dentition) is the non-inverted area, and the gingival side of the line of observation is the inverted area.
Inversion can be divided into tooth inversion and tissue inversion. A dental impaction is the area on the crown between the observation line and the gingiva, and a tissue impaction is the area below the tissue protrusion.
(2) When the analyzing rod is in contact with the observation line, the vertical distance from a point in the inverted area to the analyzing rod is called the depth of the inversion at this point, and the depth of the inversion is different at different locations in the inverted area.
(3) Inverted concave can be divided into available inverted concave and unfavorable inverted concave. The available recess refers to the denture retaining device of removable partial denture to obtain the denture retaining into the abutment teeth recessed. An unfavorable impingement is a tooth or tissue impingement that cannot be accessed from any part of the denture without affecting the insertion and removal of the denture.
The vertical measuring arm represents the direction of insertion of the removable partial denture? The seating channel. In other words, the position of the observation line, i.e. the position and depth of the impaction, is related to the orientation of the denture seating channel. The position and depth of the tooth and tissue impaction can be changed by changing the direction of the denture seating channel.
3. Application of model observation
(1) Select and determine the denture seating channel.
The following two methods are often used to determine the seating channel:
①Average inversion (even concave method): for cases with many gaps and large inversions, the vertical seating channel should be used to average the inversion of each abutment tooth. The direction of the denture seating channel is basically the direction of the angular bisector of the intersection angle of the long axes of the abutment teeth at the front and back ends of the gap and the abutment teeth on both sides of the arch. If the long axis of the abutment teeth are parallel to each other, the direction of the seating channel and the direction of the long axis of the abutment teeth.
② Adjustment of inverted concave (concave method): is to make the gap between the two ends of the abutment teeth of the inverted concave properly concentrated in one end of the abutment teeth, resulting in a favorable inverted concave. Denture tilt in place, can be used to enhance the locking effect of denture retention.
Most of the missing anterior teeth are tilted backward, and the denture is inclined from front to back, which not only eliminates the inversion of the alveolar ridge labial tissue and reduces the gap between the artificial anterior teeth and the distal neighboring teeth for the benefit of aesthetics, but also concentrates the inversion of retention in the posterior abutment teeth, which is conducive to the placement of retention clips.
The model is generally tilted forward and the denture is positioned diagonally from back to front in the case of free missing posterior teeth.
(2) Determine the position of the guide plane of the remaining teeth.
(3) Determine the hard and soft tissue inversions. This includes the location and depth of available retentive inversions on the abutment teeth and unfavorable inversions that should be adjusted, avoided, or eliminated.
(4) Assist in the development of a restorative treatment plan. Denture design, restorative preparation of tissue morphology modification, residual teeth shape grinding, restoration, abutment preparation should be based on model observation.
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