November 8, 2021
by Dr. Goth Siu, BHSc, DMD, MS, Cert. Prostho., FRCD(C), FACP; Yuzo Matsumura, RDT, MDC
Replacing a full set of teeth with dental implants, especially in the mandible, has been well researched and is often a life-changing treatment for many patients. This treatment involves the extraction of failing teeth, immediate implant placement, immediate loading of the implants with a fixed provisional prosthesis, followed by the fabrication of a definitive prosthesis after osseointegration. The final prosthesis is usually fabricated with denture teeth and heat cured acrylic supported by a titanium bar. (Fig. 1)
The survival rate of mandibular dental implants supporting a full arch fixed restoration over a ten-year period is reported to be around 97%.1 The success of the prosthesis is equally important. Unfortunately, it was reported that the cumulative rate of “prosthesis free of complications” to be only 8.6% at 10 years.2 The most common prosthetic complication reported was chipping and fracture of the veneering material (such as the denture acrylic teeth) with a rate of 66.6% in ten years.2,3,4 Bozini et al. (2011) reported that by 15 years, over 50% of studied prostheses demonstrated fracture or wear of the veneering material.5 (Fig. 2) If fractures arise, both patient and dentist suffer financial, emotional and time burdens. Since this type of full arch treatment are usually high in cost and often viewed by the patient as an end-stage solution to a lifetime of dental problems, any complications can put strain on the doctor-dentist relationship. Therefore, the material and design of the prosthesis is of vital importance to ensure a positive long-term outcome for the patient while minimizing time and cost to the dentist from managing complications.
Due to common fractures of acrylic teeth, stronger materials such as zirconia have been developed and used. However, there are certain clinical situations where acrylic and a titanium bar are the materials of choice. One such scenario is when both arches are restored with implant fixed prosthetics. The maxillary arch can be made in zirconia (with or without layered porcelain) for strength and aesthetics, while the mandibular arch can be fabricated in a softer acrylic supported by a titanium bar. The benefits of this difference in materials is controlling the wear to primarily the lower arch which is less aesthetically apparent and easier to repair. Another benefit is the sound during mastication; some patients do not like the high pitch sound created by two opposing zirconia prostheses.
The conventional method to fabricate an acrylic and titanium definitive prosthesis is by setting denture teeth and processing heat-cured acrylic to wrap around a titanium bar. There are many bar designs that can be used, however, the most common complications are related to the fracture of the overlying acrylic teeth. Moreover, when the teeth are worn, the dentist is required to re-establish vertical dimension, capture jaw relations, reposition teeth, do a try-in, and then deliver the refurbished prosthesis. With such high reported complication rates of acrylic fractures and wear, there should be a way to utilize the same materials in a more efficient way.
With advances in digital technology, an alternative method can be done to minimize risk of fracture of the acrylic and greatly reduce the appointments to renew a worn prosthesis. This is accomplished by milling a monolithic polymethyl methacrylate (PMMA) superstructure and cementing it onto a uniquely designed titanium bar that provides proper support for the occlusal load. A milled PMMA prosthesis is advantageous when compared to processed acrylic and denture teeth.
Milled PMMA has reduced risk of fracture since it is monolithic and less porous than processed acrylic. There is no shrinkage during the fabrication process and thus less internal stresses. Lastly, when the prosthesis is worn, an exact replica of the original design can be easily milled out again without the need to undergo all the previous steps.
Since the prosthesis is milled, it must be cemented onto a titanium bar. Therefore, the titanium bar is required to have a path of insertion and adequate retention form. (Fig. 3) A vertical element with a 6-degree taper in designed in the bar. The height of this element should also be adequate to prevent rotation of the PMMA superstructure, much like walls of a crown preparation providing retention and resistance form. To further reduce fracture risk, the titanium should span almost the entire intaglio surface so the overlying PMMA is completely supported by the titanium, especially under areas of occlusal load. (Fig. 4) If the soft tissue is not entirely flat, the soft tissue model should be adjusted prior to scanning to ensure the ideal titanium intaglio surface. The most buccal aspect of the titanium should be cut back around 2mm so metal does not show from the frontal aspect for maximum aesthetics. Lastly, retention elements such as holes can be placed into bar so that when the PMMA is cemented onto the bar with cold-cured PMMA, it provides mechanical locking. (Figs. 5-8)
Following cementation of the PMMA to the bar, pink composite is sculped and bonded to create the gingival aspect of the prosthesis. (Figs. 9-11) With many pink shades available in composite, gingival aesthetics can be superior to pink denture acrylic. It can also match pink porcelain in situations where the maxillary prosthesis is fabricated in zirconia and porcelain. (Fig. 12)
The clinical and laboratory steps are as follows:
With a high complication rate reported in the literature related to fracture and wear of the acrylic, practitioners and laboratories should utilize the latest materials and technologies available to combat these issues. Instead of using denture teeth and heat-cured PMMA around a titanium bar, milled PMMA cemented on a titanium bar is an excellent alternative. Milled PMMA is monolithic, higher strength, offers similar or enhanced aesthetics, and can easily be remade when the prosthesis is worn. The only disadvantage is the time needed to rethink and relearn the laboratory steps in the fabrication process. Given that CAD/CAM technology is here to stay, this should not be a limiting factor.
Oral Health welcomes this original article.
Declaration: The authors declare no financial interest in the companies mentioned.
About the Authors
Dr. Siu completed his Doctor of Dental Medicine degree at the University of Pennsylvania in Philadelphia. He then completed a specialty residency program in Prosthodontics at the University of Illinois at Chicago. Dr. Siu is board certified in Prosthodontics in Canada and the US. He maintains a private practise limited to prosthodontics in Toronto with an emphasize in aesthetic dentistry.
Yuzo was trained in Japan and completed a postgraduate Master Dental Ceramics program. In 2015 he became the owner and president of Elephant Dental Studio, a high-quality oriented lab in Toronto focusing on complex and interesting cases with Prosthodontists across Canada and the United States.