Dr. Adithi Prabhu1, Dr. Naisargi Shah2, Dr. Anshul Khanna3, Dr. Rahul Malu1, Dr. Nikita Gharat1, Dr.Anuradha Govardhane1
2Professor and Head of Department,
Department of Prosthodontics, Crown & Bridge.,
T.P.C.T’s Terna Dental College, Navi Mumbai
The goal of implant dentistry is to accurately restore the patient’s dentition. These goals are best achieved by a prosthetically driven approach which includes a three-dimensional (3D) diagnostic and treatment planning in the placement of dental implants. The focus of modern implant therapy has shifted from a surgical driven approach to a prosthetic one with a goal to optimize and maintain the form function and aesthetics of the patient. With a prosthetically driven approach and a combination of 3D imaging technologies, these have revolutionized dental implant diagnosis and treatment. This review article describes the various parameters that need to be assessed from a prosthetic point of view while planning for an implant prosthesis.
Keywords: implants, treatment planning, guided surgery, surgical guides, navigation systems
Citations: Prabhu A, Shah N, Khanna A, Malu R, Gharat N, Govardhane. A critique of prosthetically driven implantology. J Prosthodont Dent Mater 2021;2(2):3-10.
The goal of prosthetic dentistry is to replace a patient’s missing teeth to normal contour, comfort, function, aesthetics, and health regardless of the previous atrophy, disease or injury of the stomatognathic system. Implants in the last two decades have evolved and created a paradigm shift in restorative dentistry from function to aesthetics, with aesthetics enhancing the patient’s need and desires. Literature has also observed that it is the final prosthesis, not the implants, that accomplishes these goals. Therefore, planning for an implant placement should be done in such a way, so as to achieve the requirements of the prosthesis.
TREATMENT PLANNING: While planning for an implant supported prosthesis, the location of the edentulous space should be considered as well, as different protocols are needed to achieve the desired prosthetically driven outcome.
PROSTHETIC DRIVEN IMPLANTOLOGY IN THE ESTHETIC ZONE1,2:
When it comes to the aesthetic zone, it can be defined objectively as any dentoalveolar segment that is visible at full smile, while subjectively it is defined as any dentoalveolar area of aesthetic importance to the patient. A variety of factors need to be evaluated before placing an implant in the aesthetic zone to achieve the desired location of the prosthesis.
1) SMILE LINE :
Tjan and coworkers (1984) defined three general categories of normalcy for smile type: high, average and low smiles. Based on this smile line, the type of prosthesis can be determined. The displayed gingival tissues and interdental papillae are often key determinants of treatment risk, as they are most difficult to replace when deficient. Therefore, while evaluating the smile line, if soft-tissue defects exist that cannot be addressed surgically, prosthetic planning for gingival tissue replacement should be initiated before placing the dental implant(s). An FP2 or FP3 prosthesis should be planned with a special emphasis on pink esthetics (pink power concept 2,10,11).
2) WIDTH OF THE EDENTULOUS SPAN :
If interdental and inter-root space is adequate (>/= 7 mm for regular connection/diameter implants and = 6mm for Narrow connection/diameter implants), sufficient support will be available for the periimplant tissues and the prosthetic volume will be adequate for the abutment and restorative materials to create natural appearing contours. As the interdental and inter-root space decreases, the implant and restorative component options become limited and the prosthetic volume for ideal restoration emergence and contours suffer increasing the aesthetic risk. When the edentulous space increases to include multiple missing teeth, the aesthetic risk also increases due to the unpredictable nature of the inter implant soft- and hard tissue support and the increased difficulty to maintain symmetric mucosal contours. The placement of adjacent implants must therefore not only be restoration-driven, but also biologically driven in an effort to promote the best sulcular anatomy through preservation of the soft and hard tissues after implant placement.
3) GINGIVAL PHENOTYPE :
THICK GINGIVAL PHENOTYPE: The gingival tissue in these patients are often characterized by a predominance of thick, broad-banded keratinized tissue that is typically resistant to recession after surgical procedures. The thickness of the gingival tissue effectively masks the color of the implant(s) and any subgingival metallic components, reducing the risk of mucogingival discoloration. A tissue level implant can be planned for such a gingival biotype as they are more resistant to recession.
THIN GINGIVAL PHENOTYPE: Thin and friable nature is conducive to formation and maintenance of natural and predictable interproximal papillae. A bone level implant may be indicated for such type of phenotype. Restorative and surgical planning for these patients also requires implants to be placed further palatally allowing for maximum hard-tissue and soft-tissue coverage of implant surface.
4) SOFT TISSUE ANATOMY
If soft-tissue defects are present prior to implant placement, in particular a lack of sufficient keratinized mucosa, an increased aesthetic risk should be recognized. Connective-tissue grafts (CTG); vascularized pedicle grafts (VPG) or free gingival grafts (FGG) should be planned accordingly. These grafting procedures are dependent upon good underlying bone support and should not be considered a replacement of bone augmentation.
5) ANATOMY OF ALVEOLAR CREST
Deficiencies in horizontal and vertical bone dimensions will increase the esthetic risk. Horizontal ridge augmentation (regenerative procedures) and vertical bone augmentation with autogenous grafts combined with barrier membranes and xenograft bone fillers should be planned accordingly. Recently autogenous tooth grafts have also been introduced for such regenerative procedures. They can also be used in conjunction with other bone graft materials.7,8,9
When there is a vertical excess of the alveolar crest as in cases of congenitally missing teeth, bonescalloping procedures can be done wherein a surgical template that highlights the proposed mucosal margin of the restoration will dictate the degree of scalloping needed to position the implant in the correct submucosal position, allowing for an ideal emergence profile of the restoration. The Surgical template is made by a diagnostic mock-up, imitating the emergence profile and contour of the missing teeth and then duplicating it with an alginate impression.
6) IMPLANT DIAMETERS IN ANTERIOR MAXILLA1:
Appropriate implant diameter depends on the width of the alveolar crest and the size of the tooth to be restored. Narrow diameter implants (NDI) are usually indicated in single-tooth sites with a narrow gap, such as lateral incisors, where bone level NDI are most often used. They are also indicated at implant sites with a borderline crest width of 5 to 6 mm, where NDI allow a simultaneous GBR procedure to avoid staged ridge augmentation.
Keeping all the above factors in mind while considering to plan a prosthesis in the aesthetic zone, a diagnostic wax up is made to accurately evaluate all aspects of aesthetics and function, including tooth positions and angulations, tooth widths and lengths, occlusal interactions, and cervical/interproximal relationships between the teeth and soft tissue. For example, in cases of existing soft-tissue defects, the clinician can evaluate whether pink ceramics would make for an adequate substitute or whether more invasive surgical modes of hard- or soft tissue augmentation are required.
PROSTHETICALLY DRIVEN 3D IMPLANT POSITIONING IN THE ESTHETIC ZONE2:
After clinically evaluating the aesthetic risks and deciding on what type of implant should be placed, a prosthetically driven 3D positioning of the implant is an important aspect. According to the International Team of Implantology1, when planning for an ideal three-dimensional implant position, a distinction is made between “comfort” and “danger” zones in each dimension. The selection of the implant type and the placement of the dental implant should be based on the restorations planned in these zones. If implant shoulders are positioned within the danger zones, complications such as peri-implant bone resorption followed by soft-tissue recession may occur, resulting in aesthetic complications. Comfort and danger zones are defined in the mesiodistal, orofacial, and coronoapical dimensions.Apart from these dimensions, a correct implant angulation axis should be located roughly 1 mm palatally of the future incisal edge, allowing trans occlusal screw retention of the implant restoration in the cingulum area.
IMPLANT PROTECTED OCCLUSION:
It must be kept in mind that implants should be in a mutually protected occlusion wherein during excursion the posterior teeth are protected by the anterior guidance, whereas during centric occlusion the anterior teeth have only light contact and are protected by the posterior teeth.12 It must be kept in mind that the anterior guidance of the implant prosthesis with anterior implants should be as shallow as practicable. The steeper the anterior guidance, the greater are the anticipated forces on anterior implants. In case of a single tooth implant replacing a canine, no occlusal contact is recommended on the implant crown during excursion to the opposite side. The rationale of mutually protected occlusion is that the forces are distributed to segments of the jaws with an overall decrease in force magnitudes. It must also be kept in mind that if anterior implants must disocclude the posterior teeth, two or more implants splinted together should help dissipate lateral forces whenever possible.
EVALUATING PARTIALLY AND COMPLETELY EDENTULOUS MAXILLARY AND MANDIBULAR ARCHES:
For evaluating partially and completely edentulous spaces in the maxillary and mandibular arches, Misch has mentioned five elements that need to be assessed:
- Existing occlusal vertical dimension (OVD)
- Maxillary occlusal plane,
- Mandibular occlusal plane
- Crown height space (CHS)
1. EXISTING OCCLUSAL VERTICAL DIMENSION (OVD) :
To determine the crown height space (CHS), the overall issue of OVD must be addressed. It might affect the potential number, size, position, and angulation requirements of the implants. In addition, any change in the OVD will also modify the horizontal dimension relationship of the maxilla to the mandible; therefore, a change in OVD will modify the anterior guidance, range of function, and aesthetics.
2. MAXILLARY AND MANDIBULAR OCCLUSAL PLANE :
The occlusal plane of existing teeth is critical in evaluating partially edentulous patients in relationship to the final implant prosthesis. Occlusal modification, endodontic therapy, or crowns are indicated to remedy tipping or extrusions of adjacent or opposing natural teeth. A pre-treatment diagnostic wax-up is strongly suggested to evaluate the needed changes before implant placement. Occlusal plane analyser is then used to evaluate and correct an improper occlusal plane.
3. CROWN HEIGHT SPACE (CHS)
CHS for implant dentistry is measured from the crest of the bone to the plane of occlusion in the posterior region and the incisal edge of the arch in question in the anterior region. Ideal CHS for fixed implant supported restoration: 8-12mm (considering biological width, abutment height for cement retention or prosthesis screw fixation, occlusal material strength, aesthetics, and hygiene considerations). Removable prosthesis: >12mm (denture teeth and acrylic resin base strength, attachments, bars, and oral hygiene considerations)
Planning for a cantilever implant supported prosthesis should be done with other factors in mind such as the amount of force a patient places on the cantilever. When cantilevers are used in the final restoration, the occlusion on the cantilevered pontics should be reduced, with no contact on the pontic during mandibular excursions. Placing cantilever abutments in the anterior region has a better prognosis as compared to a posterior cantilever since the amount of force acting on the abutment is comparatively higher.
TREATMENT PLANNING IN COMPLETELY EDENTULOUS ARCHES:
Four phases of treatment planning have been emphasized by a review article by Tunkiwala A.14 which involves:
I. PLANNING AND DIAGNOSTIC EVALUATION - involves decisions made on the type of prosthesis based on which, the number and location of implants is finalized.
II. SURGICAL PHASE- decisions regarding need for soft/hard tissue augmentation and free handed or guided surgery are made depending on the evaluation in the planning phase.
III. PROVISIONAL PHASE- immediate or delayed loading of the implants based on implant stability and adjunctive surgical procedures performed during the surgical phase.
IV. FINAL RESTORATION PHASE- achieving ideal aesthetics, phonetics, function for the patient.
In order to simplify this decision-making process and allow predictability in final treatment outcomes, the “ABCD” classification system proposes the treatment planning in completely edentulous maxillary and/or mandibular arches which includes vital parameters of age, bone volume, cosmetic display and degree of resorption to create an algorithm that satisfies the treatment needs of every patient. This classification of the patient, based on these four parameters, helps choose the final prosthesis design at the planning stage.
Additionally, the decision whether to opt for a fixed or a removable prosthesis also should be kept in mind. The factors involved include the amount of tooth display in an existing denture and the available inter ridge distance. An increased inter ridge distance or crown height space necessitates the need for a removable implant supported prosthesis to avoid vertical cantilever and, it also increases the bulk of the prosthesis. Apart from these prosthetic factors, patient factors related to the oral hygiene maintenance should also be considered during the treatment planning itself.
GUIDED IMPLANT SURGERY:
The goal of a guided implant surgery is to primarily achieve the predicted ideal implant position surgically without damaging the surrounding anatomical structures and to have a prosthetically driven outcome. Guided implant surgery systems use a combination of hardware and software to facilitate the planning of implant positions. The resulting positions are then converted into surgical guides or loaded into positioning software using a variety of methods. Jung and co-workers (2009) have categorized these methods as either static or dynamic systems.
Surgical Implant Placement with Surgical Template:
A surgical template (guide) is defined as a prosthesis used to assist in the surgical placement of implants. In the literature, three different surgical template designs are based on surgical restriction: nonlimiting, partial-limiting, complete-limiting design.
No actual directional guide built into this template other than possibly the buccal or lingual contours of ideal positioning of teeth Partial-Limiting Design
- Allows for angulation of one drill size, usually the pilot drill.
- Remainder of the osteotomy sites are completed freehand.
In this design, the position, angulation, depth of the osteotomy is dictated by guided tubes or sleeves, thus restricting any variation by the implant surgeon. It prevents any osteotomy error in the buccolingual and mesiodistal planes.
Surgical Guidance Templates and Navigation Systems Optical systems: With an optical system, also referred to as an infrared system, infrared sensors along with light reflectors are fixed to the patient’s head and a handheld probe to track the position of the instruments within the surgical field.
Electromagnetic systems: Electromagnetic systems use an electromagnetic field and reference points on a device that is attached to the patient’s head and a wired surgical instrument.
Every treatment should start with the endpoint in mind as the final restoration will drive the planning and treatment process similar to that of a blueprint with a house. The dimensions of the final restoration will highlight the necessary interdental and interocclusal space requirements as well as the contours of the soft and hard tissues. When adequate restorative space and tissue support is determined, the restoration will drive the 3D positioning of the implant placement.
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