Describe/explain the position, extension and angulation of the mandibular acrylic block and its occlusal planes of a Twin Block appliance for the correction of a Class II division I malocclusion with a deep overbite.
Answer:
The position of the occlusal inclined plane is determined by the mandibular block and is important in the treatment of a deep overbite. The inclined plane should be clear of the mesial surface contact with the mandibular molar. This would allow the mandibular molar to erupt unobstructed, thereby reducing the overbite.
The inclined plane on the mandibular acrylic block is angled from the mesial surface of the 2nd premolar or deciduous molar at 70 degrees to the occlusal plane. In cases where the mandibular forward protrusions are inconsistent, the angulation may be reduced to 45 degrees in order for the block to occlude correctly.
The inclined block should extend distally to the buccal cusp of the mandibular 2nd premolar or deciduous molar, stopping short of the distal marginal ridge (this allows the leading edge of the inclined plane of the maxillary appliance to be positioned mesial to the mandibular 1st molar, thereby not obstructing eruption).
Buccolingually the mandibular block covers the occlusal surfaces of the mandibular premolar or deciduous molars, to occlude with the inclined plane on the maxillary twin block. The flat occlusal bite block passes forward over the 1st premolar, and becomes thinner buccolingually in the mandibular canine region. The thickness of the block can be reduced in that area in order to improve speech by allowing the tongue freedom of movement in the phonetic area. In order to avoid breakage, the lingual flange in the midline should be of adequate thickness.
Clark, W.J. 2002. Appliance design and construction. Twin block functional therapy: applications in dentofacial orthopaedics. 2nd ed. London: Mosby. p80.
Hi Cheryl, also just remember that when constructing the blocks a decision must be made concerning the angulation of the blocks in relation to the line of the arch. There are 2 alternatives. First, the blocks may be aligned in each quadrant at right angles to the line of the arch in the same pattern as the teethare aligned. Alternatively, the lower blocks may be aligned at right angles to the midline bisecting the arch. The upper blocks would be constructed to match this angulation.
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2. Provide a full description of the position, extension and angle of the inclined plane of the mandibular acrylic block with reference to treating or correcting a deep overbite condition.
Twin Blocks are simple bite-blocks that effectively modify the occlusal inclined plane using upper and lower bite blocks which engage on occlusal inclined planes.. Wearing bite-blocks is rather like wearing dentures and patients can eat comfortably with the appliances in place. The upper and lower bite blocks engaged mesial to the 1st permanent molar at 90° to the occlusal plane, when the mandible postured forward. This positioned the incisors edge-to-edge with 2 mm vertical separation to hold the incisors out of occlusion. The patient had to make a positive effort to posture the mandible forward to occlude the bite blocks in
protrusive bite.
The inclined plane on the lower bite-block is angled from the mesial surface of the second pre-molar or second deciduous molar at 70° to the occlusal plane. This places the leading edge of the inclined plane on the upper appliance mesial to the lower first permanent molar, thus keeping a provision for the unhindered eruption of the lower 1st permanent molar. Mesially, the lower bite block extends up to the canine region with a flat occlusal surface.
Twin Block appliance acrylic extends to cover the incisal edges of lower incisors. This helps to avoid tipping of the lower incisors and improve in retention. Torquing springs were used to control position of the upper incisors.