Laboratory Construction Procedure
Self Cure Technique
1. Wire Work is carried out as normal ensuring that wires crossing the occlusal surface are either adapted into embrasures or do not impede the placement of the screws.
2. There should be at least 6mm of free space between the cusps of the upper and lower pre-molars. If the working bite gives a smaller space then the articulator should be opened until sufficient space is achieved. This will not affect the finished result.
3. For severe Class 2 Div 2 cases where a working bite cannot be taken due to a deep anterior overbite, the models may be articulated in RCP and then opened up on the articulator to give clearance for the screws and the lower model slid forward on the articulator fork to give the start position. Advancement can then follow the proclination and alignment of the upper incisors. (NB Screw positioning can sometimes be difficult to judge in these cases due to the difference in arch widths in this position)
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| 4. The screws and housings should be prepared for use by applying a thin layer of wax around the head of the screw and filling in the socket on the head with wax (figs 1 & 2). It is helpful if the exposed end of the screw in the housing is also lightly waxed over (fig 3). | ||
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5. A wax dam should be formed to the correct profile of the finished upper blocks (fig 4 ) | |
| 6. The wax dam is softened and the support arms on the side of the screw are pushed gently into the softened wax. | ||
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| 7. The resulting raised points of wax are melted with the end of a heated knife or similar and the ends of the screw support arms are pushed through to give a visual screw position and alignment as shown in figs 5 & 6 with the working face of the screw heads aligned with the inclined anterior edge of the dam. | ||
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8. After the screws have initially been aligned by eye, the arms are then lightly waxed into position. The alignment rods are then attached to the arms to accurately check the positioning (fig 7). Any adjustments to the screw position should be done by moving the screw and housing bodily in the locally softened wax dam and not by pushing on the alignment rods. | |
| 9. When the correct positions have been achieved the screw support arms are waxed firmly to the dams and the rods are carefully removed. | ||
| 10. The upper appliance can then be sprayed up in the normal manner using a high quality hard acrylic and the blocks shaped taking care to leave the screw heads exposed from the surrounding acrylic. It is essential to ensure that no voids are created around the screw housings during build up. | ||
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11. After processing, the screw support arms are trimmed flush with the buccal aspect of the upper block and the appliance is finished in the normal way (fig 8). Care must be taken not to damage the screw heads when trimming and polishing. Check for free movement of screws at this stage using the screwdriver provided in the kit and then tighten down. | |
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12. The upper appliance is then placed back on the model and wax ramps built up using the screw heads to indicate the correct angle and the working face of the screw head is left exposed (fig 9). These ramps will determine the finished angle of the lower block face. | |
| 13. A suitable separator is then applied and the lower appliance can be sprayed up and the blocks formed against the upper in the usual manner and then finished. The screwheads may be lightly polished at final finishing. | ||
Screw Alignment |
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1. If a midline expansion screw is fitted the advancement screws will need to follow the line of the upper arch when viewed occlusally to compensate for the divergence of the heads when the midline screw is opened (fig 10) 2. In severe Class 2 Div 2 cases starting just forward of RCP it may be necessary to position the screws more palatally to compensate for the difference in arch widths. It can also be necessary to make the lower block faces wider than normal to ensure maximum contact throughout the treatment. 3. If no midline screw is fitted then the screws should lie as near parallel as possible with the palatal midline so that the heads remain in contact with the centre of the lower block faces after advancement. 4. The recommended maximum convergence angle is determined from the length of the alignment rods. |
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5. The screws should follow an average of the lower posterior occlusal plane when viewed from the buccal aspect and both screws must be in the same horizontal plane in all cases (figs 11 & 12). 6. It is not essential that both screws are at exactly the same angle relative to the midline but this should be done where possible. 7. In close bite and late mixed dentition cases when the occlusal planes tend to be flat and parallel, the screws should be aligned to avoid collision with the cusps of the upper first premolars/canines on advancement. 8. Screw heads should remain in contact with the full depth of the lower block face even after advancement. |
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Heat Cure Technique |
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| 1. Wire work and bite opening are the same as for the self-curing technique. | ||
| 2. The screws are lightly waxed onto the occlusal surface of the upper model and alignment checked and adjusted as described in self-cure section. | ||
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3. The wax blocks are then formed around the screws leaving just the heads and the support arms exposed (fig 13). | |
| 4. The lower block face angle can be determined from the working face of the screw heads and the lower appliance waxed up accordingly leaving a slight indentation at the point of contact of the working face of the screw. | ||
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5. Invest in the usual way taking care to ensure that the screwheads are filled with plaster and fully embedded when topping the flask and then boil out leaving screws retained in counter half of flask by the head and the housing extensions (fig 14) | |
| 6. After applying mould sealant, acrylic should be carefully packed around the screw housings before packing the remainder of the moulds in the usual way. Use soft dough in warm flasks. De-flask and finish upper as for self-cure. Finish lower in usual way. | ||