A combined digital-conventional workflow to fabricate a removable partial denture for a patient with a severe gag reflex

Case report

The patient is a 61-year-old female with no relevant medical history who presented to the senior house officer restorative clinic seeking a fixed prosthodontic replacement for her recently extracted upper right lateral incisor tooth. She was also troubled by her lack of posterior teeth, both from an aesthetic as well as a functional perspective. The upper right lateral incisor tooth was lost to a non-restorable crown-root fracture 6 months prior to presentation. A provisional, PMMA FPD had been fabricated to restore this site prior to referral to the senior house officer restorative clinic. On intra-oral examination, a Class II division 2 incisor relationship, complicated by deep vertical overlap and a lack of posterior support, was noted. The maxilla was a Kennedy II mod II arch, whilst the mandible was a Kennedy I arch with severely atrophied posterior ridge form. The periodontal status was stable, oral hygiene was fair, however, there was no evidence of periodontal pocketing beyond 2 mm and there was no evidence of bleeding on probing. (Figure 1a–c).

On presentation to the operator, the patient reported profound difficulty with the severity of her gag reflex during dental procedures, particularly impression-taking procedures with conventional elastomeric impression materials. The gag reflex was so strong that several episodes of vomiting had occurred during dental procedures in the past. In spite of these difficulties, the patient was determined to seek a treatment solution which would enable her in wearing a partial denture in recognition of the necessity of posterior teeth in protecting the remaining anterior dentition.¹⁵ More complex treatment options, involving fixed rehabilitation with dental implants and bone augmentation, were rejected by the patient, and the combined fixed and removable partial denture treatment option was accepted as her preferred option.

The aims of treatment were to restore the maxillary labial segment with a 3-unit FPD and single unit crown and then to restore the posterior dentition with an RPD, using a staged approach to overcome the profound gag reflex and assess the patient's tolerance of a maxillary partial denture.

Clinical and laboratory procedures

Tooth preparations in the maxillary labial segment were refined in order to achieve adequate retention and resistance form with supragingival margin placement (Figure 2a), and a virtual mastercast was fabricated by chairside digitization using a 3Shape Trios^® (3Shape, Copenhagen, Denmark) intra-oral scanner (Figure 2b and c). At the dental laboratory, uniformly cut-back copings were prepared by computer-aided-design (Figure 3a and b) and were machine fabricated in cobalt-chromium using the selective laser melting (SLM) additive manufacturing technique (Figure 3c and d). The crown and FPD were surveyed and incorporated cingulum rest seats and a guide-plane on the distal surface of the UR3. A metal try-in was carried out to verify the fit and occlusal relationship of the copings (Figure 4a and b), following which aluminous porcelain was hand-stacked at the dental laboratory (Figure 5a and b). The definitive metal-ceramic units were cemented with glass ionomer cement (Ketac^™ Cem, 3M ESPE, Minnesota, USA).

With no improvement in the patient's profound gag reflex, a novel approach was used for fabrication of the cobalt-chromium framed RPD, whilst avoiding slow-setting conventional elastomeric impression procedures. The maxillary denture-bearing area, including the dentition and mucosa, was recorded by chairside digitization using a 3Shape Trios^® intra-oral scanner. From this intra-oral scan, a partial denture framework was prepared by CAD (Figure 6a) and, again, was machine-fabricated in cobalt-chromium by SLM (Figure 6b). The cobalt-chromium partial denture framework was tried in the patient's mouth. Several cycles of fit-check adjustments were required to permit full and accurate seating of the framework until a clinically acceptable framework fit was achieved by this method. The patient tolerated the full seating of the framework well, with limited exacerbation of her gag reflex.

In order to capture the mucosal aspect of the denture-bearing area accurately, a conventional functional impression was made in zinc oxide eugenol (Kelly's^® Z.O.E. Impression Paste, Waterpik, Colorado, USA) using a custom tray attached to the partial denture framework (Figure 7a and b). At this point, it was necessary to fabricate a conventional gypsum model of the maxilla to permit heat processing of PMMA to the partial denture framework. As such, the framework, complete with custom impression of the denture-bearing mucosa, was picked up into a fast-setting alginate impression, and poured immediately to form a conventional denture mastercast. Denture teeth and PMMA were heat-processed to the cobalt-chromium framework using the flask, pack and press technique (Figure 8a, b and c). The RPD was definitively fitted and the patient has adapted well to her new prosthesis (Figure 9a, b and c). Given that this combined digital and conventional workflow has been successful in fabricating the maxillary removable prosthesis, and given the adaptation of the patient to this removable prosthesis in the context of her gag reflex, the next stage of the treatment plan is to begin fabrication of a mandibular RPD using a similar workflow.

Discussion

Patient groups

Conventional elastomeric impression materials take several minutes to set in the mouth. This can be a very long, unpleasant and, in rare instances, dangerous experience for patients. Groups affected include those with a profound gag-reflex, but also patients with learning impairments, patients with temporomandibular joint disorders, children, and those who are at risk of aspiration. Bateman and Saha¹⁶ reported a case of a 72-year-old man following aspiration of dental impression material. The patient required hospitalization and passed away 33 days later in respiratory arrest. They also identified three other cases in the literature reporting aspiration of impression material.^17–19 For patients with dysphagia, reducing reliance on flowable impression materials in the mouth may reduce aspiration risk for those susceptible.

By using intra-oral scanners, the use of traditional materials can be reduced. In this regard, without physical material in contact with the oral and pharyngeal tissues, the potential for gag-reflex activation may be reduced. This should be true if the reason for gagging was caused by a physical stimulus and not a mental trigger. Dickinson and Fiske developed a classification in 2005, including the causes of gagging. They included a ‘Gagging Prevention Index’, which was graded by the level of treatment that was possible to carry out, taking into account the severity of gagging experienced.^20,21 The use of intra-oral scanners have the potential to reduce the severity on this scale, without changing to a compromised alternative treatment plan.

For patients with temporomandibular joint disorders, this method allows for breaks in the scan process in an effort to reduce symptoms of fatigue on wide mouth opening.

Challenges with chairside digitization

Chairside digitization presents challenges when trying to obtain highly accurate cross-arch dental prostheses.²² Error accumulation that occurs during the stitching of multiple 3D images over a broad surface area across the arch can lead to clinically relevant levels of inaccuracy. A second cause of error in digital acquisition of the dentition is the occlusal relationship in both static and dynamic relations. This is compounded by partially dentate patients that fall into the Kennedy Class I and II categories. This case required model generation and accurate interocclusal record techniques involving a semi-adjustable articulator. The complexities of this case required both accurate articulator mounting, and accurate dynamic lateral and protrusive replication, to ensure that the restorations conformed to the patient's existing occlusal parameters, and canine and protrusive guidance were controlled. A re-organized occlusal approach can present with even more profound challenges for a purely digital workflow.²²

Cobalt-chromium frameworks for RPDs previously had high financial and time costs due to the casting of the metal alloys. It is likely, as these alternative methods become more established, manufacturing time and costs will reduce. The initial setting-up costs for the chairside digital equipment is a potential barrier to some practitioners at this time.

Removable partial dentures with distal extensions should incorporate maximum functional extension of the denture-bearing area. Intra-oral scanners do not operate in the same way that impression materials do to achieve this, such as border moulding to achieve a functional impression. The intra-oral scanner will only capture a snapshot of the tissues in their retracted state, which is not truly based on functional movements. The argument can also be made that scanners lack the muco-compressive properties of impression material. The clinical impact of this is not fully understood at this moment in time.

Lastly, in a digital workflow when adding acrylic to the metal framework, the options are limited. As no mastercast is required for the SLM method of framework fabrication, one is only supplied if requested, and this comes in the form of a 3D printed resin model fabricated from the scan. The resin model prohibits the use of heat-cured PMMA as the technique of flask, pack and press is not compatible with this. The resin model cannot be destroyed to retrieve the processed denture as predictably as a stone model can be. Currently, in fully digital workflows, cold-cure alternatives are used which can have compromised material properties compared to pressure-moulded, heat-activated resins, such as reduced transverse strength and increased porosity.²³ The described method in this report suggests an alternative way to simply convert to conventional techniques at a relevant stage to take advantage of the benefits of heat-cured PMMA.

Conclusion

Although patients with a profound gag reflex may initially reject a removable prosthesis, a carefully designed cobalt-chromium RPD may be a suitable option, as it was in this case. By using a surveyed crown and FPD, it maximized the success of the removable prosthesis. This permitted the patient to still have a restored UR2 should she not have tolerated the RPD. Other natural tooth preparations were minimal and the intra-oral scans were not invasive or traumatizing to the patient. The necessary conventional techniques were more easily controlled by the use of customized trays and control of material setting properties by temperature and technique manipulation. As can be seen in Figure 8, the metal was thin and streamlined and the palatal ring major connector minimized the chances of activating the gag-reflex. Try-in stage of the framework is likely to be a good indicator, if the patient will tolerate the prosthesis. In summary, cobalt-chromium RPDs may be one option to be considered in a partially dentate patient with a history of a gag reflex if the prosthesis is designed well, the patient is willing, and an appropriate technique is used.