The Fixed/Removable Interface: A Case Report

Case report

An 86-year-old female was referred by her general dental practitioner (GDP) to the Restorative Dentistry department at UCL Eastman Dental Hospital to provide her with more stable dentures. The patient was complaining that ‘my bite is not right’, ‘I cannot eat properly’ and ‘my front tooth is loose.’ She admitted parafunction when stressed, but had no associated pain. There was no relevant medical history and she was a highly motivated, 6-monthly attendee to her GDP. She had a history of tooth wear on her lower anterior teeth, which had been previously restored with direct composite resin.

Extra- and intra-oral soft tissue examination was unremarkable and intraorally, she had good oral hygiene with scores of zero on her BPE.

Dental examination revealed a Class II division 1 incisor relationship with the right lateral excursive guidance solely on UR1. This tooth was an abutment for a conventional cantilever bridge replacing UL1 and was grade 1 mobile, but with no periodontal pocketing. The patient was partially dentate with Kennedy Class 2 Modification 1 in both maxillary and mandibular arches. Recurrent caries on UR7, a perforated provisional crown on UL3, marginal chipping of the composite resin restoration on UL4 and a severely mesial and lingually tilted LL8 were noted (Figure 1).

All teeth responded positively to pulp testing with Endo-Frost (Coltène, Altstätten, Switzerland). Radiographic examination revealed generalized 20% horizontal bone loss with 30% bone loss on the distal aspect of UL4. The UR2 and UR3 root had been extracted by her GDP prior to referral. The UR1 abutment showed a funnel-type widening of the coronal periodontal ligament space, typical in cases of occlusal trauma (Figure 2).

The patients was diagnosed with the following:

Treatment

First, stabilization and individual tooth assessment was carried out. The restorations in UR7, UR1, and UL2–UL4 were removed to assess the underlying tooth structure (caries, cracks, clinical crown height and dentine volume). All teeth were deemed restorable, and so the UR7 was restored with an amalgam core, while UL4 was restored with a direct composite resin overlay to provide additional cuspal protection and appropriate buccal bulbosity for a possible future clasp.

After stabilization, study casts were articulated on a Denar Mark II semi-adjustable articulator with the aid of a facebow and jaw registration in centric relation (CR). The primary casts were also surveyed at this stage to identify the natural undercuts and guide planes. This step was required to determine the path of insertion for the RPD, and aid in the RPD design.

A diagnostic denture tooth set-up and wax-up of the anterior teeth was carried out. There was a 1-mm anterior increase in the occluso-vertical dimension (OVD) to allow space for restorations, and for aesthetic reasons, because the existing overbite was deep. The new occlusal scheme was designed to allow for left canine guidance, right group function and shared protrusive guidance between the UR1 and UL2. A denture tooth and bisacryl composite try-in was carried out to verify aesthetics, phonetics and function (Figure 3). Figure 4 shows the intended denture design. Owing to the position of LL8 and the abundance of occlusal space available, full framework coverage of this tooth was carried out to maximize tooth support.

The UR7, UR1/UL2 and UL3 were prepared for a conventional gold onlay, three-unit bridge and single crown, respectively.

Provisional acrylic crown and bridge units were fitted according to the diagnostic wax-up, followed by upper and lower provisional acrylic RPDs at the new OVD and occlusal scheme (Figure 5). The provisional phase allowed the assessment of the occlusion, function, aesthetics and phonetics. The patient was re-assessed for these parameters over a period of 3 months, including the mobility of UR1, all of which had improved, and the patient was happy to proceed to the definitive phase.

The gold onlay on UR7 was designed with mesial and distal rest seats and a mesial guide plane. The three-unit bridge (UR1–UL2) and single crown (UL3) were made of porcelain fused to noble metal alloy (PFM) and were designed with cingulum rest seats and mesial, distal and palatal guide planes (Figure 6). A 0.5-mm disto-buccal bulbosity was designed on the UL3 for a potential future clasp should the UL4 be lost. It is important to note that when planning PFMs as denture abutments, the crown features that will engage with the denture must always be metal in order to reduce the risk of porcelain fracture, and, perhaps, wear of the metal framework by the abrasive porcelain. Therefore, PFM is preferable to all-ceramic units in such cases.

Once all the fixed prostheses had been cemented, the master impressions were made for the CoCr RPDs. Impressions were made in custom-made trays, using medium and light body addition cured silicone in a selective-pressure technique, which allowed for the differential displaceability of teeth and mucosa. An altered cast approach was further used to capture the lower right free end saddle. Owing to the abundance of occlusal space, a heat-cured acrylic tooth was added to the framework coverage of LL8 using CoCr beads cast into the framework, which allowed for mechanical retention of the acrylic tooth (Figure 7).

Re-assessment was required after 2 days, 2 weeks and 1 month before discharging back to her GDP with a maintenance protocol and plans for future failure, which had been incorporated in the denture design.

Discussion

The use of CoCr RPDs was not only predicated on their versatility of design and usage of abutment teeth, but also on the abundance of long-term clinical data showing their success compared with acrylic dentures.^3,4,5 Indeed, Bergman et al found that 65% of CoCr RPDs were still in service after 25 years.³

While attention to biomechanical principles in RPD design is fundamental, the consensus is that long-term success also requires an emphasis on regular recalls, oral hygiene and prophylaxis.⁶ Clinical experience realizes that, despite all measures of planning and meticulous execution in the most motivated of patients, complications can still occur because of the dynamic nature of the oral cavity.

Complications with RPDs are more commonly caries, periodontal disease, fracture or loss of abutment teeth, and less commonly wear, distortion or fracture of RPD components.^6,7,8 CoCr RPDs should be designed to minimize these risks, and to plan for their management. The discharge maintenance protocol advised regular recall and prophylaxis, and advised on how to manage specific contingencies, some of which are discussed later.⁶

The patient's main concern was the lack of stability of her current RPDs and the absence of an even, comfortable occlusion. This was likely to be the primary factor in her occlusal trauma.

Therefore, the aims of treatment were to:

To achieve these aims, there was a need to make strategic use of the abutment teeth as described. Indeed, a 10-year retrospective clinical study showed greater CoCr RPD success rates with greater number of abutments.⁹ As mentioned, this should be tempered with the need to keep denture designs hygienic and conducive to oral hygiene measures; ‘over-engineered’ denture designs are unlikely to achieve this.⁶ Finally, the resultant occlusal stability was a key feature in the patient's uneventful adaptation to the new occlusal scheme.¹⁰

Splinting UR1 and UL2 in a three-unit bridge reduced the mobility of UR1, providing functional comfort, and allowed for the UL1 pontic and UL2 abutment to provide additional tooth support owing to the incorporation of cingulum rest seats. Indeed, literature reviews and in vitro studies have found a reduction in RPD abutment stress when splinting.^6,11,12 An alternative option would have been to provide single crowns on both UR1 and UL2, while replacing UL1 with a denture tooth. However, this would not have provided the necessary splinting of UR1 and would have presented an additional challenge for matching alternative porcelain crowns (UR1 and UL2) with denture teeth (UR2 and UL1). Our approach therefore gave a more functional and aesthetic result.

If the UL2 abutment was to fracture in the future, the bridge could be sectioned at the UR1/UL1 junction, the UL2 extracted and both UL1 and UL2 added to the RPD. The RPD design allowed for such eventualities because the CoCr baseplate extended over the palatal surfaces of the anterior abutments.

The 0.5-mm disto-buccal bulbosity incorporated into the UL3 PFM crown allowed for a stainless steel, gingivally approaching clasp to be added to the denture, to engage this tooth if the UL4 is extracted in the future. The bulbosity was placed distal of the midline in order to make the future clasp less visible from the front.

The treatment carried out was conservative with minimal tooth preparation. However, it may be suggested that a composite resin overlay may have been sufficient for UR7, rather than the gold onlay, because it only opposed a denture tooth. In this case, a gold onlay was deemed more appropriate because it had a more conservative preparation, a finer polish than composite resin, and would not undergo the same surface degradation as composite resin over time. Being arguably the most strategic abutment in the upper arch, a laboratory-fabricated restoration would provide the best long-term outcome for the patient.¹³

Although some studies dispute the clinical benefit of the altered cast technique for distal extension saddles,^14,15 in this case, it provided the opportunity to make a more accurate impression of the distal extension saddle. This allowed for maximal extension over the primary support areas, allowing for optimal distribution of occlusal forces and, therefore, a reduction in the rate of bone resorption.^16,17

The root-filled UR5 could have been used to house a radicular precision attachment contributing to increased support and retention. An example of this can be found in Figure 8. However, this was unnecessary because the UR5 root was already providing some degree of support, and it was situated in the middle of a bounded saddle that was already providing adequate retention. The adjacent abutments (UR7 and UR1) had good prognoses, and there was no reason to expect that they would be extracted in the future. The need for an endodontic re-treatment to provide a precision attachment giving questionable benefit was not deemed to be the best use of the patient's time. A more appropriate scenario for their use would be as in Figure 8a, where there is an insufficient number or spread of abutments to provide the necessary retention and support for an RPD. A contemporary review has detailed attachment types, abutment selection and treatment planning of precision attachments.^18,19

Conclusion

CoCr RPDs rely on abutment teeth to provide the necessary support, retention and stability for optimal function. While abutment teeth can be used in various ways, this case report highlights the benefit and versatility of indirect restorations. Other treatment challenges, such as management of occlusion, aesthetics and planning for failure, were also discussed.

The report emphasizes the need for comprehensive diagnoses, prognostication and treatment planning, followed by meticulous execution of provisional restorations, and a period of review before delivering the definitive restorations. This staged approach allows for challenges to be realized and corrected during the provisional stage, providing a more predictable and pleasing outcome.