Double Arm Swing-lock

Fig 1: jaw relationship, left-hand side

Fig 2: jaw relationship, right-hand side

Fig 3: jaw relationship, front

Fig 4: swing-lock components from Swing-lock Inc.

Fig 5: right-hand side hinge components incorporated into wax pattern.

Fig 6: left-hand side hinge components incorporated into wax pattern.

Fig 7: locking bar incorporated into wax pattern.

Fig 8: the completed wax pattern ready for investing

Fig 9: final casting ready for finishing.

Fig 10: finished framework with arms open

Fig 11: finished framework with arms locked

Fig 12: finished framework on master model

Fig 13: finished framework on master model, top view

Fig 14: acrylic wax-up ready for processing

Fig 15: acrylic wax-up, right-hand side

Fig 16: finished appliance

Fig 17: the finished appliance, front view

Fig 18: the finished appliance, left quarter view

Fig 19: the finished appliance, right quarter view

Fig 20: the finished appliance with arms open

This case was first presented to Steve in 1993. The patient has a class 3 bite, as seen in figs 1,2 and 3, and wanted a denture made to present a class 1 appearance. Previous attempts had failed because the anterior teeth needed to be set so far forward of the ridge that they destabilised the whole appliance even with clasps. The challenge was to manufacture an upper denture that would be at once cosmetically pleasing, retentive and stable.

His extensive prior experience with swinglock cases suggested to Steve that he could achieve the desired result using this technique, but a problem was presented by the required span of the hinged bar. The patient had only five remaining teeth - the right 5,6 and 7 and the left 4 and 7 - which meant the bar would be required to span the entire arch from left to right 7 if all of the available undercuts were to be exploited.

It was apparent that the conventional one-hinge, one-lock approach would not work so it was decided that two two hinges would be used, both emanating from the distal region of the 7s. This resulted in two hinged bars, which carried the locking elements with a wrap-over effect just right of the incisal papilla. Each hinged element would engage the deepest undercuts available from the standing teeth and also the tissue undercuts buccally. The major connector would be a full plate as the patient was used to this and it provided a stable platform for the attachment of the moving parts. The anterior teeth were to be gum fitted to the plate and the buccal gum work attached to both of the hinged bars, forming a cosmetic join between them and around the gingival margins of the anterior teeth. This design was untested and in Steve's experience unprecedented. It was also apparent that this device would be quite a mouthful, so now the questions were: would it work and could or would the patient tolerate it?

As this would be a complicated case, establishing the correct aesthetic appearance in advance was crucial, so a wax try-in was first prepared. After the try-in appointment, the next step was to cast the red plastic swing-lock parts in cobalt chromium alloy (Fig 4). These were then incorporated into the wax pattern as shown in figs 5,6 and 7. The positioning of these parts is critical to the eventual function of the appliance so great care is taken to ensure that they are parallel relative to each other and that the locking elements exactly coincide on closing. The wax pattern was completed using the wax setup as a guide for correct retention pin placement and sprued ready for investing as in fig 8. After casting it was devested and sandblasted fig 9 After the removal of the sprues the casting was trimmed and the moving hinge and lock elements checked for movement. The whole case relies on a thin layer of oxides formed on the surface of the pre-cast swinglock parts that builds up during the burn-out process. This layer prevents a weld from forming between the components and the new material. It is always a tense moment when the newly cast framework is devested, but in this instance everything worked perfectly (figs 10 and 11). After finishing polishing (figs 12 and 13) the teeth were transferred from the original wax setup onto the cobalt chrome base (figs 14 and 15). This was then returned to the surgery for a further trial. At this point it was already known that the appearance was right, but it was still unknown how the patient would react to the framework. It was a great relief to be informed of the patient's favourable response.

Duplicate models were made for the processing, which, as with all the previous stages, was not straightforward. The opposing partial lower denture that was being manufactured alongside this appliance was of a conventional design and was simply processed in injection-moulded high-impact acrylic. The upper, however, required much more careful treatment. It was decided to inject the gum-fitted teeth and the left 5,6 saddle in the same high-impact resin as the lower denture and use a pourable shade-compatible resin for the hinged bars in a separate process. After processing and finishing the main framework, the cosmetic gum work on the buccal bars and around the gingival margins of the anterior gum fitted teeth were profiled, the left side being designed to overlap the right. To keep the wax from sticking to the teeth and itself, a separating agent was used. Silicone lab putty was then used to make an index with vents into which the acrylic was poured right-hand side first. Acrylic resin chemically bonds to other acrylic surfaces, so a way had to be found of preventing the moving parts from bonding together. The normal techniques of using painted compounds or petroleum jelly were judged to be insufficient, so instead, very thin tin foil was applied and burnished over the whole of the buccal aspect of the denture teeth. This would prevent the acrylic from bonding and at the same time give a very close fit . This process was used with the left-hand bar to separate it from the previously cured right-hand bar and teeth. After careful polishing the case was placed back on the mounted master cast and checked for fit and any minor bite discrepencies. Figs 16 to 20 show the complete finished dentures prior to dispatch.

This case is now into its second incarnation. The first appliance was showing signs of wear, but after more than 16 years it had done remarkably well. The patient must be meticulous over their oral hygiene because in that time the natural dentition has remained unchanged, enabling an exact copy to be made. Although the design is a little unusual it has stood the test of time and is still in use after 20 years.

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