From Volume 41, Issue 1, January 2014 | Pages 62-67
The clinical and laboratory steps involved in rehabilitating the maxillary arch following the loss of several teeth due to periodontal disease are outlined in this case report. This article illustrates the use of a laboratory based CAD/CAM system (Sirona In-Lab) and a copy milling technique in the fabrication of a fixed-movable bridge, high strength, all-ceramic, cross-arch bridge.
Patients' attitudes towards and expectations from dentistry have risen dramatically over the past few decades.1 This has highlighted the importance of attention to detail during the planning and execution of a course of dental treatment. This has increased the pressure on the clinician and technician and the interface between them.
Periodontal disease is a leading cause of tooth loss and the prosthetic management has conventionally been in the form of removable partial dentures.2 However, both clinicians and patients are showing an increasing tendency to approach this issue with fixed bridgework. Since Nyman et al published their seminal work in the 1970s,3 the prosthetic management of periodontitis has included bridgework acting as a fixed splint for periodontally involved teeth. A fixed-movable approach in fixed bridgework is sometimes necessary to allow for non-parallel paths of insertion, abutments with questionable prognoses and to incorporate attachments for the retention of removable prostheses.
High strength ceramics have been successfully used for several years for the fabrication of single crowns. Current evidence suggests that the 5-year survival on anterior teeth of all-ceramic single crowns (93%) is similar to metal ceramic crowns (95%), while on posterior teeth, all-ceramic single crowns made from densely sintered alumina and reinforced glass-ceramics showed 5-year survival similar to metal ceramic crowns.4 However, while considering fixed bridgework, the 5-year survival evidence suggests that all-ceramic prostheses (89%) show statistically significantly lower survival rates compared with metal ceramic bridgework (95%).5 Yttria-stabilized polycrystalline tetragonal zirconia (Y-TZP) ceramics have been shown to be capable of withstanding occlusal loads and are suitable framework materials for all ceramic bridges.6,7 However, the mode of failure for zirconia bridgework appeared to be similar to metal-ceramic bridgework. This was due to biological reasons, such as marginal caries and loss of vitality of abutments, rather than technical complications, such as framework fracture, delamination, etc.5
Fixed-movable approaches have hitherto been limited to fixed prostheses with a metal substructure as currently available CAD/CAM systems have limited provisions to design and machine attachments.8 This has therefore led to the utilization of alternative techniques to enable the design of fixed-movable bridges entirely in zirconia.
A 35-year-old female patient, with a history of aggressive periodontitis, had lost several teeth over the past 18 years. She had been receiving active periodontal treatment and supportive therapy regularly and her periodontal status had been stable for the past 4 years, except around her maxillary left first molar (UL6), which showed advanced bone loss and had a poor prognosis. The UL6 was asymptomatic and did not show any signs of active disease but had limited value as a prosthetic abutment owing to the associated advanced bone loss. The other teeth were periodontally stable, with pocketing of <4 mm at all sites and no mobility. The maxillary right second premolar (UR5) had been previously endodontically treated but had lost its coronal restoration. The position of the midline in the maxillary arch was suboptimal owing to increased proclination of the left central incisor (UL1) and migration across the midline (Figure 1).
The patient wore acrylic partial dentures to replace her missing teeth in both arches but was increasingly unhappy owing to the poor aesthetics and found it particularly difficult to tolerate the maxillary prostheses. Radiographically, maxillary teeth showed greater periodontal bone loss compared to the mandibular dentition (Figure 2) and the patient showed greater tolerance of her lower prosthesis. Her aesthetic expectations were very high and she had a moderately high lip line.
The treatment options available to her were:
Though the advantages of constructing new, better designed, removable partial dentures were explained, the patient indicated that she did not wish to consider the removable option, especially in the maxilla, as she found her current denture difficult to tolerate and frequently used denture fixatives. The implant option was considered and eliminated, as she did not wish to undergo surgery.
Adhesive bridgework was considered but not pursued as it would not have been possible to alter the appearance of teeth which had drifted, especially the UL1. The patient did not wish to undergo orthodontic treatment. Therefore, a decision was made to pursue the fixed bridge option.
The agreed plan of treatment was as follows:
Initial diagnostic data were collected via pre-operative photographs, alginate impressions, facebow record and jaw relations. This was followed by the creation of a diagnostic wax-up on the study models. The maxillary left first molar (UL6) was extracted and healing was uneventful. The maxillary right second premolar (UR5) was endodontically re-treated and built up using a fibre post (ParaPost® Fiber Lux, Coltene Whaledent Ltd, UK) and composite resin (Esthet.X, Dentsply, UK). Oral hygiene education was reinforced.
The maxillary teeth were prepared over two appointments. However, the maxillary right second molar (UR7) was not prepared, as the patient did not wish to undergo any further tooth preparation. An impression was made in an acrylic special tray with silicone (Aquasil Ultra Monophase Reg Set, Aquasil Ultra LV Reg Set, Dentsply Ltd). A temporary bridge was made using a stent prepared from the diagnostic wax-up with ProTemp™ (ProTemp™ 4 Temporization Material, 3M ESPE) and was cemented with Temp Bond (Kerr Corporation, Sybron Dental Specialties).
A fixed-movable approach was adopted in order to provide a degree of serviceability, accounting for the possibility that individual abutments may fail or require further treatment and also because the labial position of the maxillary left central incisor would not allow a common path of insertion over all abutments.
In the mandibular arch, a similar impression was made in an acrylic special tray in order to construct the cobalt chromium framework for the partial denture.
Acrylic copings (Figure 3a) were made and then linked to fabricate the proposed framework in order to confirm path of insertion and stability capabilities. A total of four bridge sections completed the bridge design (Figure 3b). With pins, conventionally used in working model fabrication, a novel slide attachment (Figures 3c–f) was made for try-in purposes.
A full, anatomically correct, wax preview was prepared on the plastic framework in accordance with the bite registration. This was done to permit verification of the aesthetics and occlusion, in addition to the fit of the plastic framework at the try-in stage (Figure 4). The cobalt chromium framework for the mandibular partial denture was cast and wax registration rims were made on the framework.
The fully waxed-up, acrylic framework was tried-in and assessed for its path of insertion, accuracy, functional ability and aesthetic characteristics. At this stage, it was noted that the midline was incorrect and there were premature occlusal contacts (Figure 5). Adjustments were made and a new occlusal record against the new mandibular framework was registered.
The waxed-up framework was cut back to create space for the veneering material and was subsequently milled using the Sirona In-Lab system (City Ceramics, Cornwall) (Figure 6a). As the bridge design was in four separate sections, each section was designed and milled in the same order as they were to be inserted in the mouth. The Sirona In-Lab software has no capacity for milling slide attachments, therefore a unique approach was taken. The attachments were fashioned as traditional pontics within the CAD software and were subsequently manually milled with zirconia in the hard state (Figure 6b). The crystal phase of zirconia is affected by extreme heat,9 therefore, to preserve the stability of the material, copious water irrigation was used to keep the framework cool during the milling procedures.
The thickness of the zirconia frameworks in the anterior section was found to be deficient in the pontic region of the maxillary right lateral incisor (UR2) owing to the size limits of the YZ block. In other words, the required framework size was too big for the largest zirconia block.
This resulted in a thick layer of veneering porcelain, which fractured during firing. As the size limits of the Vita YZ block had been reached, the ZirkonZahn pantograph technique (Precident Dental Laboratory) was used to fabricate this section. The anterior section was therefore re-designed and a sintered zirconia framework, exhibiting the correct ceramic support in the anterior region, was obtained. The completed zirconia frameworks (Figure 7) were verified on the master model and then veneered.
The bridgework was tried in and cemented using a resin cement (RelyX Unicem, 3M ESPE, UK). A pleasing aesthetic result was obtained; an outcome that the patient was happy with (Figure 8). The anterior aesthetics were never going to have ideal symmetry, but a superior smile was obtained. At this stage, the mandibular partial denture was tried-in and the occlusion and aesthetic appearance was evaluated. The partial denture was then processed and delivered at a subsequent appointment.
In an effort to design a semi-fixed, all-ceramic bridge, three different techniques were employed, namely:
Though complicated, this method has permitted the fabrication of a high-strength, all-ceramic, semi-fixed prostheses with excellent functional and aesthetic properties. As CAD/CAM techniques are constantly evolving, newer systems are likely to incorporate attachments within their software permutations, thus greatly simplifying the fabrication. However, there is some evidence to suggest that CAD/CAM machining reduces the strength of zirconia, which may be further reduced by different surface treatments.10
Replacement of missing teeth in patients with periodontal disease using fixed bridgework has remained a topic of significant debate. It must be emphasized that the patient's periodontal status must be stable and the maintenance of immaculate oral hygiene is crucial in ensuring the long-term success of these prostheses.
It is important to note that there is no evidence currently available for fixed-movable all-ceramic bridgework. Therefore, predicting the longevity of these bridges is difficult. Additionally, this form of oral rehabilitation involved significant tooth preparation, resulting in a high biological cost, primarily associated with the potential loss of vitality of the abutment teeth.11
A challenging clinical case was effectively and aesthetically managed using innovative laboratory stages. As there is limited provision in currently available CAD software to design attachments, the sintered zirconia was hand-milled. In the anterior section, the largest CAD/CAM Vita block was still too small for the desired framework design, which caused the veneering ceramic to fracture owing to its increased thickness. A copy milling technique, which employs larger blanks had to be used to remake the framework, resulting in a satisfactory result.
