October 8, 2019
by Valentin Dabuleanu, BSc, DDS, MSc, FRCD(C); Tudor Dabuleanu, DDS
Modern implant dentistry has been shown to yield excellent well-documented long-term results, with 10-year success and survival rates above 95%.1 Dentists now have the choice of using an array of preoperative planning tools, both conventional and computer-aided, to help them assess potential rehabilitative treatment solutions for their patients. Diagnostic wax-ups have been a critical tool in multi-unit prostheses for the evaluation of ideal implant positioning and spacing, as well as the desired contour, occlusal scheme and esthetics of the final restoration (Fig. 1).2 The use of Cone Beam CT (CBCT) imaging has dramatically improved diagnostic accuracy for the implant surgeon when compared to standard intraoral periapical and extraoral panoramic radiography. Surgical grafting and implant placement techniques, implant designs, and metal-ceramic prostheses are all evolving simultaneously and this sets our profession aside among others as an example of collective creativity and ingenuity. It is an exciting profession to be a part of. This case report describes a comprehensive fixed implant prosthetic rehabilitation of missing posterior dentition, as well as a correction of anterior natural dentition cross-bite.
Diagnostic wax-up showing proposed occlusal scheme as well as final tooth sizes, shapes and positions.
Surgical Considerations: Implant Connection Platform
Long-term maintenance of the peri-implant marginal bone level is an essential requirement for long-term success of an implant-supported fixed dental prosthesis, whether that is for a single-tooth site or for multiple edentulous sites. The preservation of this bone will help to prevent the development of deep peri-implant pockets and gingival recession, which could result in implant surface exposure and act as a reservoir for plaque accumulation.3
In the anterior maxilla, and within the Straumann© dental implant system, the Bone Level (BL) design, which uses a platform-switching concept, appears to preserve more bone at the facial aspect of the implant shoulder, and in some cases it even allows for the reconstruction of the facial bone wall coronal to the implant abutment junction, which helps to support the soft tissues mid-facially (Fig. 2). The BL design is therefore favoured for single-tooth replacement in the esthetic zone.4
Straumann© dental implant system–Bone Level (BL) and Tissue Level (TL) designs.
When considering single or multiple posterior edentulous sites, arguments can be made in favour of both the Straumann BL and Tissue Level (TL) implant designs. The TL design however has two features that may prove advantageous over the BL design in these sites, a trans-mucosal machined collar, and a rigid internal abutment connection (Fig. 2). These both have the potential to minimize stress exerted on the crestal bone around the implant. In a recent retrospective study of 1692 Straumann Tissue Level implants placed in 881 patients, of which 1013 implants had a prosthetic follow-up of at least five years, the average peri-implant bone loss was 0.09mm.3
In a retrospective study of the peri-implant bone resorption between Straumann TL and BL implants placed in alveolar ridges reconstructed by autogenous onlay bone grafts, involving 50 patients, 192 implants and a mean prosthetic follow-up of 33 months, peri-implant bone resorption was significantly lower for TL than BL implants, and particularly lower in the case of implants placed in iliac grafts, 0.36 versus 1.34 mm (p<0.0001).5
Surgical Considerations: Implant Placement with Simultaneous Lateral Guided Bone Regeneration (GBR)
Simultaneous GBR has three advantages over a staged GBR approach. First, it results in less morbidity as the bone augmentation and implant placement are performed in one versus two surgeries. Second, the overall treatment is less time-consuming. Third, the avoidance of a second surgery results in a decreased treatment cost for the patient. Simultaneous lateral GBR may be considered when the implant site satisfies two critical anatomical requirements: 1) a sufficient crest width at the future implant site to allow circumferential bone anchorage of the implant following bone healing, ideally the implant diameter plus 2 mm, and 2) a localized two-wall defect with the exposed implant surface within the “bony envelope”, as this will provide stability for the grafted site.4
Figure 3 displays a pre-operative CBCT image of an edentulous 43–46 region. The diagnostic wax-up and CBCT both revealed a mesial-distal prosthetic space of 26mm, satisfying the requirements for the safe rehabilitation with one canine, premolar, and molar implant-supported crown with mesial-distal dimensions of 7.2, 8.6, and 10.1 mm respectively, and sufficient minimum inter-implant distances. Particularly, at site 43 there was both sufficient crest width to stabilize the implant, as well as a localized two-walled defect that would provide stability for a simultaneous lateral GBR. Figure 4 displays the implants in position, prosthetic spaces demarcated on the ridge crest, as well as the exposed implant 43 surface within the “bony envelope”.
Pre-operative CBCT image with radiographic stent displaying prosthetic spaces, inter-implant distances, as well as sufficient crest width to stabilize implant 43 in a localized 2-wall defect.
Implant placement with simultaneous lateral guided bone regeneration (GBR)–exposed implant surface sits within a “bony envelope”.
A dual-layer technique may be considered when performing simultaneous lateral GBR. In the first layer, a small amount of autogenous bone chips may be harvested locally from the surrounding surgical site with a bone chisel (Fig. 5).4 Bone chips collected by bone scraper have been shown to contain a high number of live cells that can turn into osteoblasts and directly contribute to bone formation. The transplanted cells can also contribute to graft consolidation by the release of growth factors, especially bone morphogenetic protein-2 (BMP2) and vascular endothelial growth factor (VEGF). BMP2 is capable of stimulating rapid proliferation and differentiation of osteoblast progenitors, while VEGF is capable of stimulating angiogenesis.6
Dual-layer GBR technique–autogenous bone chips are applied to the implant surface.
The second layer may consist of either particulate xenograft, particulate allograft or a combination of both. Allograft, the first natural substitute used for autogenous graft, provides a major advantage of overcoming the limitations of harvesting autogenous bone which include availability issues as well as morbidity. Freeze-dried bone allografts (FDBA) undergo a process of sterilization and protein deactivation that includes freezing, defatting, and dehydration. Bone mineral is retained however and this provides the mechanical stability that makes FDBA suitable to provide space provision, blood clot stabilization, and an osteoconductive platform to allow bone regeneration to take place. Xenograft is the second natural substitute used for autogenous graft, and the best-documented xenograft used in implant dentistry is deproteinized bovine bone mineral (DBBM) (Fig. 6). DBBM may serve as a non-organic osteoconductive scaffold, and in daily practice may be considered close to non-resorbable once bone integration is achieved.4 The dimensional stability of DBBM may make it more suitable than FDBA as the second layer in a dual-layer simultaneous lateral GBR technique.
Dual-layer GBR technique–particulate xenograft is applied on top of the autogenous bone, and covered with a resorbable collagen membrane.
Prosthetic Considerations: Interocclusal Space and Restorative Material for Fixed Screw- and Cement-Retained Crowns
Having sufficient interocclusal space is critical for an implant prosthesis to be structurally stable, have proper physiologic contours and to be esthetic. When considering porcelain-fused-to-metal (PFM), zirconia, and lithium disilicate restorations, the minimal amount of vertical interocclusal space required is 4-5 mm for a screw-retained implant level crown, and 7-8 mm for a cement-retained crown (Fig. 7). Ideally, measurements of the interocclusal space should be taken from the implant platform to the opposing dentition.7 This may be possible in the pre-operative planning phase with the use of CBCT imaging and simulating the future implant position. In Figure 3, the interocclusal space for the future implant crown at site 43 was measured to be 13 mm. If a pre-operative CBCT image is not available, measurements can be taken from the soft tissue ridge to the opposing dentition on mounted casts.
The minimum vertical interocclusal space for screw- and cement-retained implant crowns are 4-5 and 7-8mm respectively.
For cement-retained crowns, the required interarch space for occlusal restorative materials, measured from the most coronal aspect of the abutment to the opposing dentition, may be as little as 1 mm for monolithic zirconia or lithium disilicate crowns, or as much as 2 mm for PFM crowns, considering 0.3 mm for opaque, 0.5 mm for metal, and 1 mm for porcelain.7 Figures 7-10 demonstrate three implants 43, 45 and 46 positioned as parallel as possible to each other and to adjacent teeth 42 and 47. Both the parallelism of the implants and the existing interocclusal space enabled restoration with either screw- or cement-retained crowns. In this case, the implants were restored with a hygienic splinted cement-retained bridge using prefabricated abutments. The implants are tissue level, which assists in reducing the risk of encountering excess cement by bringing the implant-abutment junction coronally.
Implants 43, 45, 46 with pre-fabricated abutments, with supra-gingival implant-abutment junctions and sufficient interarch space for restoration with PFM crowns.
Splinted porcelain-fused-to-metal (PFM) implant crowns with hygienic contours permitting adequate plaque control.
If there is excessive hard and soft tissue loss resulting in an interarch space of ≥15 mm, it may be difficult to restore the implants using conventional fixed PFM crown-and-bridgework as the laboratory may be less able to control for the metal distortion that occurs with multiple heating and cooling cycles in the fabrication of larger prostheses. In these cases, a titanium computer-aided design and computer-aided manufacturing (CAD-CAM)-fabricated framework and acrylic prosthetic teeth may have to be considered.7 Figure 11 demonstrates such a case. CoDiagnostiX© software was used to merge an intraoral scan and CBCT image to create a virtual diagnostic wax-up of a future implant supported bridge 16-x-14-x-12. This software was used to simulate both an ideal setup of prosthetic tooth heights, shapes, and contours, as well as a setup that would meet the implants positioned in the existing residual ridge. This diagnostic tool can help to facilitate materials section for a prosthesis that will be structurally stable, thus potentially avoiding a remake later on.
Virtual diagnostic wax-up of a future implant supported bridge 16-x-14-x-12 anticipating bothan ideal and actual tooth setup which can help to facilitate materials selection.
Prosthetic Considerations: Correction of Class III Malocclusion
Class III malocclusions may be divided into two categories. In category A, also known as a pseudo-Class III malocclusion, premature occlusal contacts may lead to functional forward shifting or positioning of the mandible at centric occlusion, resulting in a normal mandible and under-developed maxilla (Fig. 12). In category B, also known as a skeletal (true) Class III malocclusion, an inherent growth abnormality results in a large mandible. In patients with pseudo-Class III malocclusion, the mandible may be guided into a normal centric relationship, resulting in either a normal overjet or an edge-to-edge position of the incisors.8
Pseudo-Class III Malocclusion.
Patients with Class III malocclusion may be at increased risk for dental caries and periodontal disease due to their anterior end-to-end or anterior and/or posterior cross-bite. A comprehensive multidisciplinary approach to treatment planning involving prosthodontists, orthodontists, periodontists, and oral and maxillofacial surgeons is often indicated to ensure accurate diagnoses and appropriate treatment. This approach to the treatment of an adult patient with both skeletal Class III malocclusion and missing posterior teeth has been documented in the literature, and included both orthodontic treatment and orthognathic surgery to correct the skeletal discrepancy, before implant therapy.9
Prosthetic Considerations: Screw versus Cement-Retained Implant Prostheses
Screw-retained implant prostheses allow the advantage of being removed for cleaning and possible repairs. They also tend to show less marginal misfit at the crown-implant interface.10 Screw-retained prostheses, however, are also subject to higher rates of prosthetic complications than cement-retained prostheses, including screw loosening or fracturing. Cement-retained prostheses have been found to exert less stress on supporting bone tissue. They also have been found to encounter less marginal bone loss over follow-up periods of up to 15 years, fewer prosthetic complications, and higher implant survival rates than screw-retained prostheses. Special precautions must be taken however with cement-retained prostheses in avoiding the use of excess cement, both with prefabricated and custom abutments.11
The case presented is a healthy 45 year-old male (Fig. 12). Posterior teeth were lost 10 years prior due to decay. The patient’s primary concern was the replacement of his missing teeth. His secondary concern was the correction of his anterior cross-bite. He had an otherwise healthy periodontium.
The patient was diagnosed with an anterior cross-bite due to suspected Pseudo Class III Malocclusion, and a left-side shift of his dental mid-line (Fig. 13). The bilateral posterior vertical dimension of occlusion was deemed adequate to support fixed implant-supported prostheses. Teeth 18 / (47 – 48) and 24 / (35-34) were the last remaining posterior occlusal units.
Clinical presentation–cross-bite across teeth (11–24 / 35–41), and left-side shift of the dental midline.
Initial Radiographic Assessment
The panoramic image revealed normal osseous tissue structures, and bilateral pneumatisation of the maxillary sinuses (Fig. 14).
Radiographic presentation–Panoramic image reveals bilateral pneumatisation of the maxillary sinuses.
Proposed correction of the anterior cross-bite, as well as anterior and posterior tooth sizes, shapes, and positions were discussed with the patient using the tooth wax-up (Fig. 15). The wax-up revealed tooth prosthetic mesial-distal dimensions of 7 mm for teeth 37, 36, 8mm for teeth 15, 25, 43, 45, 9mm for tooth 27, and 10 mm for teeth 16, 26, 46. The wax-up also revealed insufficient prosthetic dimensions to replace missing teeth 14, 44.
Prosthetic tooth wax-up showing proposed correction of anterior cross-bite, final tooth sizes, shapes and positions.
Cone Beam CT Scan
The scan was taken using radiographic templates to help guide simulated implant diameters and connection platforms, inter-implant distances, and to optimize restorative emergence profiles (Fig. 16). Residual ridge heights were adequate to support simultaneous sinus augmentation and implant placement in the maxilla. Residual ridge widths were adequate to support simultaneous implant placement and lateral guided bone regeneration in the mandible. Both the wax-up and the CT scan assisted in the decision to plan two narrow implants supporting premolars at sites 37, 36 with M-D dimensions of 7 mm each.
Pre-operative CBCT image with maxillary and mandibular radiographic templates, and simulated implants.
Treatment Plan Discussion
The risks and benefits of a staged surgical and prosthetic rehabilitation of missing posterior teeth, and correction of anterior cross-bite to achieve ideal overbite and overjet, were discussed with the patient. Informed consent was obtained.
Treatment was delivered in four procedures, spaced two months apart. Procedures were performed under intravenous moderate conscious sedation, for optimal patient comfort and recovery. The radiographic stents in all sites were converted into surgical stents by removing the radiopaque markers. Prosthetic implant crown spaces were also demarcated on the ridge crests at all sites to help facilitate implant positioning. In the maxilla, simultaneous direct sinus augmentation (open lateral wall approach) and implant placement were performed. Figures 17-18 depict implant surgery at sites 25, 56 and 27. Internal and external resorbable collagen membranes (Mem-Lok©-BioHorizons) were used to house the grafted bone. A 50/50 ratio of particulate xenograft (Bio-Oss© -Geistlich) and allograft (MinerOss©-BioHorizons) was used. Site closure was achieved using a combination of chromic gut absorbable sutures and monofilament poly-tetra-fluoroethylene non-absorbable sutures (Cytoplast™-Osteogenics).
Direct sinus augmentation (open lateral wall approach) and simultaneous implant placement at sites 25, 26, 27.
Placement of bone graft, external resorbable collagen membrane, and site closure.
In the mandible, simultaneous implant placement and lateral guided bone regeneration were performed. Figures 19-20 depict implant surgery at sites 43, 45 and 46. The suspected extraction socket defect at site 43 was fully exposed. The residual ridge was flattened slightly to increase ridge width, using a bone chisel. This bone was collected and later applied as the first layer in a dual-layer technique lateral to the 43 implant. Particulate xenograft was used as the second layer over the 43 implant, and as the first layer over the 45 and 46 implants. A resorbable collagen membrane was draped over the 43 and 45 implants, and retaining tacks (J-TAC©-Citagenix) were used to stabilize both the graft and membrane. Straumann© SP SLA implants were used in the maxilla and mandible. Implant sites and corresponding dimensions were: 16, 26, 46 (4.8 x 12 mm WN), 15 & 25 (4.1 x 14 mm RN), 27 (4.8 x 12 mm RN), 37 (3.3 x 10 mm NNC), 36 (3.3 x1 2 mm NNC), 43 (3.3 x 14 mm NNC), 45 (4.1 x 12 mm RN). The retaining tacks placed in quadrant 4 were asymptomatic and left mucosalized. Figure 21 depicts intraoral radiographs of all implants at the time of placement.
Exposure of suspected previous extraction defect, and implant placement at sites 43, 45, 46.
Simultaneous lateral guided bone regeneration, placement of resorbable collagen membrane with retaining tacks, and site closure.
Intraoral radiographs of all implants at placement.
All implants were allowed a six-month healing period before integration was verified (Fig. 22). Teeth 13 – 24 were prepared to receive full-coverage crowns. A vaccuform acetate template of the wax-up was used to fabricate the splinted fixed provisional restoration 13 – 24, with correction of the cross-bite, and the development of an ideal overbite. Implant level impressions were then taken using a closed-tray technique. Splinted porcelain-metal bridgework was fabricated for both the teeth and implants. A combination of both prefabricated and custom abutments were used to support the implant bridgework. Verification jigs were used to help with seating of the abutments. Crown and bridgework was fabricated with hygienic contours to permit plaque control. Vertical dimension of occlusion was left unaltered. Treatment was completed in maximum intercuspation position (Fig. 25-29). PA radiographs were take to verify full seating of all prostheses before cementation (Fig. 30). Care was taken to use the minimum cement necessary to retain the bridgework. The patient was very satisfied with his overall experience, and the final functional and esthetic outcome (Fig. 31). The periodontist was Dr. Valentin Dabuleanu. The restorative dentist was Dr. Tudor Dabuleanu. The registered dental technician was Gus Tserotas at Poly-Dent Dental Ceramic Laboratory.
Beginning of prosthetic treatment phase.
Preparation of teeth 13–24 to receive full-coverage crowns. Facial-lingual discrepancy due to the cross-bite is evident. Cross-bite is then corrected.
Implant level upper and lower impressions were taken with the closed-tray technique.
Maxillary and mandibular laboratory casts showing crown preparations, prefabricated and custom abutments in position, as well as verification jigs.
Porcelain-metal bridgework on laboratory casts. Crowns are splinted on both teeth and implants(16-15), (13-12-11), (21-22-23), (24), (25-26-27), (37-36), (43-45-46).
Restorative abutments in position. Screw channel accesses are sealed.
Intraoral view of cemented tooth and implant crowns.
Occlusal and buccal intraoral views of the completed case.
Radiographs of cement-retained implant bridgework (16-15), (25-26-27), (37-36),(43-45-46).
Extraoral frontal and right lateral smile.
This case report discussed a staged approach for the delivery of implant therapy to replace missing posterior dentition, and prosthetic correction of anterior dentition cross-bite. A wax-up, radiographic templates, and Cone Beam CT scan were used as guides for a crown-down surgery approach, and to help determine the ideal prosthetic locations for implant placement. Through good communication and with a teamwork approach, the restorative dentist, the dental technician, and the periodontist can work together to achieve a good result. Selecting the right procedure and discussing all aspects of it with the patient will ensure a healthy balance between likely outcomes and patient expectations.
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About The Author
Dr. Valentin Dabuleanu maintains a private practice in Toronto limited to periodontics and implant surgery in a combined periodontal, endodontic, and orthodontic practice. Valentin is a Fellow of the Royal College of Dentists of Canada in Periodontology. He can be reached at firstname.lastname@example.org.
Dr. Tudor Dabuleanu is a 1968 graduate of the University of Bucharest and he completed his Canadian National Board Examinations in London, Ontario in 1977. He may be reached at email@example.com.
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