The models are placed into the wax bite provided and articulated in the usual way. Here, a single hinge articulator has been used.

The models are prepared by removing any anomalies, using an Ash 5 / 9 or similar and blocking out any major undercuts with wax if necessary.

The wire work is then fabricated for both upper & lower appliances & the mid-line screw is secured in place in the usual way.

Upper wire work - Labial bow 3/3 in 0.8mm S.S wire, Adams clasps 6/6 in 0.7mm S.S wire.

Lower wire work - Double Adams clasps 54/45 in 0.7mm S.S wire, 'C' clasps 21/12 in 0.8mm S.S wire, Ball ended clasps distal 4/4 in 0.8mm S.S wire.

The models are then placed back on to the articulator.

Wax 'shuttering' is attached & sealed to the upper model, angling from the mesial of the upper 5s, down to the mesial of the lower 6s. These enable the technician to spray the monomer / polymer (acrylic) up against the 'shuttering' to create the upper blocks. The anterior and posterior of the model is also blocked out as shown.

The upper model is then soaked in cold water for approx. 10 mins.

Monomer & Polymer (acrylic) are applied to the upper model as shown. Building the blocks in the desired areas and ensuring the acrylic flows under the mid-line screw.

Once the blocks are the required height, the upper & lower models are placed back on to the articulator (It is sometimes a good idea to wet the lower model briefly with cold water prior to doing this as it will prevent any acrylic 'sticking' to it) and the upper is gently closed into the lower model, allowing the lower posterior teeth to contact the acrylic blocks.

The upper model is then again removed from the articulator & placed into a hydroflask and polymerized (cured) according to the manufacturer's instructions.

Once cured, the upper model / appliance is removed from the hydroflask and the wax removed by applying boiling water. The upper appliance is then ready for trimming.

Firstly, the upper part of plastic positioning segment of the mid-line screw is removed (pulled out) using pliers.

- A selection of burs that may be used to trim the appliance.

The excess acrylic is trimmed away, gradually shaping the appliance. The blocks should be straight and even in the buccal and palatal areas and rounded off posteriorly. The crucial area is the angle of the interface, which should be approx. 70 degrees and on this particular appliance, running from the mesial of the upper 5s, down to the mesial of the lower 6s. Also, the slope should be trimmed at 90 degrees to the mid-line to prevent the lower appliance slipping back once the mid-line screw is activated. The appliance is then smoothed using the appropriate bur.

The mid-line is then marked on the appliance and then cut through using a fine blade saw. The mid-line screw is then opened by a couple of turns to allow polishing of the appliance.

The upper appliance can now be polished using pumice and a polishing block on a lathe in the usual way.

A separator of clear adhesive tape is then placed over both blocks as shown to prevent the lower appliance 'sticking' to the upper once the lower acrylic is sprayed and cured.

Wax 'shuttering' is again used & attached & sealed to the lower model, angling the wax so it follows the anglulation of the upper interface. The labial and lingual section of the lower model are also blocked out.

The lower model is then soaked in cold water for approx 10 mins.

The lower appliance is sprayed up is the same way as the upper appliance. Slowly building up both blocks to the required height, then cover the wires in the lingual area between the blocks. Placing both models onto the articulator, gently close the upper appliance into the lower appliance. Ensuring full coverage of the interface by the lower blocks.

Both models remain on the articulator and may be held together by an elastic band in necessary and placed once again into a hydroflask for 10 mins. (A larger hydroflask has been used here to allow the whole appliance and articulator to remain together).

Once cured, wax on the lower appliance is then removed in the same way as the upper and the cello tape is removed from the upper appliance. The lower appliance is then ready for trimming.

The lower appliance is trimmed & polished in the same way as the upper appliance. Ensuring the blocks are the required size as requested by the clinician.

The finished Twin Block Appliance.