How to Fabricate An Acrylic Resin RPD With Tooth-Shaded Clasps
Posted Aug 07, 2013 in Technical
While the Nesbit or unilateral/bilateral RPDs for temporary replacement of missing dentition have been around for many years, they've typically been made with nylon-based, acetal resin or wrought wire retention arms. However, little attention has been given to using tooth-colored acrylic for the clasps and pink PMM acrylic resin for the base, which renders a more esthetically appealing removable prosthesis.
Here's my technique for fabricating an all-acrylic, hi-impact, heat-cured Nesbit with tooth-shade clasps engineered to .01 retention. You can also use this technique to fabricate a multi-unit replacement, cross-arch acrylic partial that's retained in the same manner.
Bead the master model.
Design the Nesbit and block it out to create a .01 retention ledge at the retentive tip of the clasp; everything else on the tooth is blocked out to .00. (Note: because of the .01 retention ledge, this prosthesis cannot be injected.)
Design the acrylic resin clasps to be about 25% larger than conventional metal (cast) clasps and then carefully reduce until they are approximately 5-10% larger than conventional clasps. Finish and polish; the finished size is less than acetal resin or nylon base resin by design.
Duplicate the master model with the colloid or impression technique. Accuracy is paramount; the duplicate model must be as dimensionally accurate as the master model.
Pour and recover the new duplicate model.
Mount or remount on an articulator.
Set the teeth in place and wax the Nesbit to the desired size and configuration. While an upper Nesbit should be beaded for a slight seal in the palate, a lower should not.
Extend the Nesbit base one tooth beyond the last abutment at each mesial and distal extension.
Wax the desired lingual contours. Wax in the desired clasp forms on the buccal arms and extend into the interproximal. Ensure enough bulk and extension for proper contour, size and function.
Invest in a Hanau flask. Boil out, cool, add separator and trial pack the pink acrylic normally (I used Fricke Dental's Hi-Impact, Special fibered, #1 Hi-I heat-cured, LW). Make sure the acrylic is firm.
Mix the tooth-shade acrylic--I used Fricke Dental's Vitacrylic self-curing powder/heat-curing liquid--in a separate mixing jar, cover and allow to firm up. Although the tooth-shade resin is not high impact, the use of Hi-I liquid, the denture base monomer and heat processing adds excellent resiliency to the clasp arms to perform at .01 retention!
Use a Bard-Parker or sharp knife and cut away the soft pink acrylic clasps at the junction of the pink acrylic base and the tooth-shaded acrylic clasp.
Remove any pink occlusal rests that may have been designed or incorporated in the abutment teeth.
Approximate the amount of tooth-shaded acrylic needed to completely fill the void. Put the mixed tooth-colored acrylic in place. If you have overextended the tooth-shaded acrylic into the pink, cut it away and replace with pink acrylic. Accuracy is important. Your last trial pack should ensure that the clasps are tooth colored and the base is pink colored so continue to adjust as needed to ensure this. It's easier to do this now than after it's cured. Check to make sure your separator has not torn away.
Carefully close the flask, place it in a compress and cure at 165 °F for nine hours. Alternate cures may be used.
Cooling time is important. Be sure the flask is cool to the touch before opening. Open carefully and remove the Nesbit.
Clean and de-bur the partial and trim all rough edges. Using an air chisel is not recommended.
Carefully finish the partial, ensuring that the ledges of the clasp contact and the beads are all retained during the finishing steps. (Do not hold the partial by the clasps during finishing.)
Contour the clasps to the desired shape, proportion and size using a fine-cut bur or finishing stone. Use clean, even strokes so as to not gouge the clasp arms. Contour the clasp arm to ensure the "ledge" is the actual extent of the clasp, proportioned and tapered.
Fit the partial carefully to the "boiled off" master model before polishing and check the bite and fit resistance; smooth and correct any discrepancies.
Carefully finish and polish using slow speeds. Fine-cut arbor bands and rubber points are recommended to ensure a quality finish. Use fine pumice on a medium brush-wheel. Polish with an Abbott brush with Tripoli polishing compound and then a rag-wheel (slow) with hi-gloss polish.
Clean ultrasonically, disinfect and bag.
Don't deliver on the master model. Inform the dentist that the Nesbit has been pre-fit to the model and that the clasps are not adjustable; they are fixed at .01 retention. In addition, advise the client and the patient that the clasps are not to be used to insert or remove the Nesbit; instead, grasp the most mesial replacement tooth and put the restoration gently in place.
With proper hygiene, use and care, the Nesbit should remain physically sound for two to three years. However, because the Nesbit is entirely tissue supported, wearing it in excess of the recommended time could result in permanent tissue and surrounding tooth damage or loss.
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