With the advancement of new laser technology, the expansion of laser welding applications in all areas of the laboratory has become a real asset. In particular, the use of laser welding has greatly enhanced our ability to easily make removable partial denture repairs that are of a higher quality than those accomplished with conventional techniques. There's no need to invest, reapply acrylic or solder; it saves us time and our clients have come to rely on the consistent quality achieved with this technology.
In the past, laser welding had fewer applications because technicians had limited control of the laser beam. In earlier models, we could only adjust the voltage and duration of the pulse length, which allowed us to control the diameter and depth of the weld. Problems such as warpage of the framework occurred.
However, the latest models offer more control over the focal point of the laser beam. This results in a higher density and better quality weld, providing a purer and stronger fusion of the two metals and reducing the tendency for warpage. While previous models only delivered one pulse per press on the foot pedal, newer models include a multi-burst pulse mode control, which allows the operator to select the number of pulses delivered upon each press, creating an overlapping weld that is stronger and more esthetic. Greater control of the laser beam also allows welding closer to acrylic and metal junctions without burning the acrylic.
Following are some techniques to make common partial denture repairs using laser welding technology.
Retentive loop addition
One of the most common partial denture repairs is to replace or add a new tooth to a pre-existing partial due to tooth loss. Laser welding a wrought wire loop to the major connector for retention of an acrylic tooth saves time; in fact, this reliable technique allows repairs to be accomplished in one day.
- Be sure the dentist takes a pick-up impression of the partial denture (preferably alginate). Pour the impression with a good quality die stone and be sure that the partial does not vibrate loose while pouring.
- Allow gypsum to set completely, then gently separate the impression.
- Use a three-prong plier to bend a piece of 18-gauge stainless steel wrought wire into a loop that fits the edentulous area and overlaps the major connector by 2 to 3mm with intimate metal-to-metal contact. Be certain to thoroughly clean the major connector prior to laser welding.
- Set the welding unit at 230v x 12Ms x 2.5Hz x 1mm depth. Use the multi-pulse mode if your welder is equipped with that feature; otherwise, 20 to 30 single pulses are required to achieve an adequate weld.
- Finish the completed weld with a fine stone bur and steam clean. The partial denture is now ready to accept the additional denture tooth.
Metal backings
To replace or add a new anterior tooth to a pre-existing partial in which there is insufficient interocclusal space, a metal backing may be required. This backing can be laser-welded to the framework with a "wax and pull" technique. An opposing cast is needed from the dentist, as well as a pickup impression of the partial denture.
- Prepare the partial denture cast as described above in the retentive loop addition. Mount the casts on a semi-adjustable articulator.
- Remove the partial from the cast and groove the major connector in the area that will accept the overlap of the backing. Block out interproximal areas as necessary.
- Lubricate the area of the cast to be waxed with petroleum jelly. Wax up the backing in a traditional manner, overlapping the major connector in the grooved area. This overlap provides a strong foundation for the laser-welded joint.
- Sprue the waxed backing with 8-gauge sprue wax and gently "tease" the wax pattern off the cast using the sprue as a handle.
- Invest and burn out and cast the component with a compatible chromium-cobalt metal.
- Devest, sandblast and electropolish for 10-15 minutes. Finish and polish the backing except the underside of the overlap. Leave this area unpolished to achieve a flush fit of the major connector.
- Set the welder at 240V x 15Ms x 3.5Hz x 1mm. Use the multi-pulse mode to obtain maximum overlapping of the welds. Reseat the partial onto the cast and equilibrate as needed until the bite is correct.
- Finish the welded area with a fine stone and polish to a high luster. Add the facing tooth in the usual manner.
Clasp repair
In many cases, it's common for the buccal clasp of a "back-to-back" Akers clasp to break off, leaving only the rest and the lingual clasp in place. The "wax and pull" technique works well for this repair also, and it can be accomplished in a single day since it eliminates the need to cut off the major connector, block out and fabricate a refractory model, as well as the risk of burning acrylic during the soldering procedure.
- Prepare the partial denture cast as previously detailed. Remove the existing partial from the model and place a groove in the rest area of the metal framework to provide an overlap for a new back-to-back clasp.
- Survey the model to locate the .1mm undercuts.
- Lubricate the model and wax the new clasp with medium inlay wax, overlaying the groove in the rest area. Do not use a preformed pattern because it is too flimsy to properly pull from the model.
- Sprue with a 10-gauge wax. Pull, invest, burnout and cast the component.
- Finish the casting as usual, leaving the underside of the overlap untouched except for eliminating any bubbles resulting from investing.
- Set the welder at 220V x 11Ms x 2.0Hz x 0.90mm and weld the new assembly to the pre-existing partial at the overlapped area.
- Finish the welded area with a fine stone and polish to a high shine.
Porosity
Laser welding to fill in porosity and other voids in a framework eliminates the need to invest and solder, which are time consuming and can result in warpage.
- Set the welder at 220V x 11Ms x 2.0Hz x 0.90mm. A single-pulse mode is easier to control in this situation.
- Use laser welding wire to run a 360° bead around the periphery of the porosity pit. Starting from the outside of the pit, slowly work toward the center until completely filled. Repeat this process until all porosities are filled.
- Finish with a fine stone and bring to high polish.
