A Standardized Protocol for Successful Mandibular Implant Overdentures
Implant, Attachment, Overdenture, Mandibular.
In a recent meeting at McGill University (Canada) 1, a consensus was reached concerning the mandibular overdenture retained by two symphyseal implants. It must now be considered the minimal prosthetic treatment that most edentulous patients should expect from their dentist. However, the implant procedure must follow a well-controlled protocol since its success is neither the goal in and of itself nor is it a solution to the prosthetic handicap. Placing the implants is only the means to foster a higher success rate for the denture 2.
Beyond the psychological problem related to its removability, the mandibular implant overdenture (MIO) is considered by many patients to be satisfactory on the aesthetic and functional levels, together with the comfort and ease of maintenance that it provides 3-4. If this treatment modality meets a certain number of criteria, it will be able to respond to the demand of a large number of totally edentulous patients and, this, in a simple, rapid and reliable way, what is more for a reasonable cost.
The MIO, as will be described in this paper 4-5-6-7-8, aims at increasing the stability and more importantly the retention of the denture via the use of one ball attachment on each of the two implants, or a round bar joining the two implants together, both designs containing a stress-breaking mechanism.
A standardized sequence of treatment is needed, beginning with the decision making to the execution per se of the prosthesis, the only way to enable the practitioner to anticipate and thus avoid the many possible pitfalls during the different steps of treatment.
The MIO procedure’s aim is to reconcile the patient’s desire with the treatment options available, those being related to the practitioner’s skills on one hand and the patient’s local anatomic and general health conditions on the other hand. Many steps are involved 6-8-9:
It seeks to identify the patient’s complaints, whether objective and/or subjective, and assists in determining the reality of his problems so as to address them precisely.
The clinical examination must take into consideration a certain number of criteria:
1. The maximum range of opening, particularly when opposed to natural dentition.
2. The value of the supporting tissues, to compare with the radiographic exam;
3. The muscular coordination, especially the muscles of the tongue.
4. The neuromuscular coordination, needed for a precise and reproducible inter-maxillary relation.
The contra-indications of implant surgery are those of every oral surgery in general. The MIO being mostly prescribed for old patients, the chances of encountering a counter-indicative pathology are elevated. In case of a doubtful assessment of the general heath, it is always advised to discuss the decision to operate or not and the needed precautions with the treating physician.
Before making any decision, the patient should be informed of the:
1. Different possible implant modalities.
2. Different prosthetic modalities.
3. Different possible surgical protocols.
4. Total cost of the treatment.
5. Consequences of any foreseeable failure.
6. Main steps of the treatment plan.
1st criteria: The conventional complete denture (CCD) 10-11-12
The practitioner begins the treatment with the fabrication of a CCD. A precise mounting of teeth and waxing of the record base is essential in order to provide him with complete information, an inadequate mounting being useless for subsequent stages. The success of the MIO and therefore of the feasibility of the implant therapy cannot be guaranteed unless it is based on a well-designed CD. It is therefore imperative to validate the CD at the waxing-up before processing the prosthesis.
The fabrication of the CD falls therefore within the scope of the implant treatment. It must meet the criteria of quality described by SATO et al10 which allow to:
- Validate the occluso-prosthetic concept chosen for the rehabilitation and to treat the maxillary occlusal plan.
- Visualize the final aesthetic result.
- Determine the prosthetic volume available for the different surgical and prosthetic components.
- Guarantee continuity and ease of transition between the surgical and prosthetic treatments.
- Provide for temporization that is safe for the implant since a stable prosthesis induces less micro-movements.
The use of a conventional prosthesis as a template for all future stages of treatment allows a better management of the transition between the prosthetic and surgical therapies and thus guarantees an optimal placement of the implant.
2nd criteria: The radiologic stent
The objective of the radiologic stent is to determine the optimal implant positioning on the oblique reconstructions of the scanner. The study is based on the CD that has already been validated aesthetically as well as functionally, and the implant sites are determined with precision and fall within the prosthetic envelope defined by the denture.
The actual trend is rather to use entirely radio-opaque stents. They are obtained by the addition of barium sulfate (20% maximum) to the resin of the denture duplicate8. The implant sites are materialized in negative by drilling through the radio-opaque material. The technique greatly simplifies the pre-operative stage since the different information concerning the prosthetic volume and the soft tissue thicknesses are easily and directly read on the radiography without any further alterations of the stent.
It is important to note that whatever the selected treatment option is, the preparation of the radiological stent remains the same.
The stent must be worn before proceeding with the scanning. The patient must have been trained to find his occlusion. Eventually, a resin bite block can be used to insure a proper occlusal stabilization.
Radiologic stent with barium sulfate
3rd criteria: Radiologic Analysis
The radiographic reconstructions offer the following readings:
· Residual ridge volume and density,
· Matching between remaining bone volume and the desired prosthetic orientation,
· Relation of the denture to the anatomical structures,
· Possible presence of neighboring bone lesions.
Using the classification of Lekholm and Zarb13, the residual bone is analyzed quantitatively as well as qualitatively at the level of the selected implant sites. This initial study is essential since the collected information will allow indicating which surgical technique applies, namely choosing between a one- or two-stage technique.
Based on the implant possibilities suggested by the radiographic stent, the CT scan examination also allows the determination of the most appropriate implant positioning, based on both the anatomic as well as the prosthetic stand point, the latest requiring that the implant angulation fits that of the connecting components within the denture.
CT Scan Interpretation
Axial view of reference: the prospective implant emerging points are clearly visualized.
Implant orientation and choice of attachment system8-14-15-16-17
Three implant situations can be envisaged:
– In the lateral incisor position.
If a bar is chosen, the inter-implant distance should allow at least one rider to be to be used.
– In the canine position.
It is the most often employed option. It is also the most distalized position that still offers a choice between a ball and a bar attachment system.
– In the first premolar position.
It allows only ball attachment systems and necessitates a larger prosthetic height due to the presence of the denture tooth over the attachment.
The final choice is made prior to the surgery and according to the:
– CT Scan analysis, i.e. the anatomy of the symphyseal region.
– Available prosthetic space, the required height being variable among attachment systems, with a 7mm distance considered average.
Regarding the selection between different types of attachments, it is often determined in relation to the:
– Practitioner preferences,
– Implants’ position on the arch: a straight bar can only be used for implants separated by a moderate distance and on square-shaped or large ovoid ridges so as to avoid any cantilevering.
– implants’ long axis: if the implants are divergent by more than 10°, the chances of using a ball attachment system are reduced or at least confined to some brands that specifically allow for such divergence.
– Bulkiness: while it is true that a bar system might need a lower vertical room directly over the implants than the ball attachment system, its bulkiness still affects the anterior part of the ridge.
– Splinting the implants or not: if the implants are connected together though a bar, stress distribution is improved. Still, distal extensions are contraindicated since they might create a risky cantilevering effect that endangers the longevity of the implants. Retention, on the other hand, will not be improved.
At the end of this study, the practitioner must be able to determine the:
· Implant sites and angulations,
· Implant dimensions,
· Attachment system (bar or ball attachments),
· Method of fabrication for the MOI.
Three solutions are possible for fabricating the MOI:
- Transforming the existing CD into an implant overdenture by a chair-side direct connection with self-cure acrylic resin. The technique is delicate, particularly with bars, and the used resins have lower performance and quality than the heat-cure densified denture resins. However, the technique is less costly to the patient.
- Readapting the existing CD by a complete rebasing procedure, the impression being made with the denture. However the patient must accept to remain without his appliance for the few days that are needed for the lab-side transformation.
- Fabrication of a new CD by using the duplicate of the existing CD as a functional special tray 8-11-12. This option answers the previous disadvantages and also provides the patient with a breakdown service denture; however it is the most expensive of all.
The surgical phase begins right after the final decision is made.
4th criteria: the surgical stent
One important difficulty in implant treatment for the mandibular edentulous patients resides in the accuracy of the data transfer between the prosthetic planning phase to the surgical phase. It is related to the loss of landmarks following the loss of the teeth. The use of a surgical stent can overcome this problem.
The tissue surface of the stent is cut back at the area concerned by the flap reflection in order not to hinder its seating. Moreover the teeth at the implant sites are cut down so not to hold back the cutting action of the surgical drills down to the full needed depth of drilling.
In order to optimize the precision in implant positioning, metallic drilling channels (tubes of internal diameter of 2,1mm) can be included in the surgical stent. This takes into consideration the modifications of the implant orientations decided in the course of the radiographic study.
Surgical Stent with Drilling Channels
5th criteria: The Impression
The success of a MOI depends on the accuracy of recording with the same functional impression the characteristics of two different structures: the denture supporting tissues and the implant positions. These multiple objectives are best attained via a single functional impression that allows one working model on which the different parts of the prosthesis are constructed. Moreover, the impression technique must be able to appreciate the gradient of compressibility existing between the mucosal tissue and the implants. Whether the fabrication of a MOI is based on an existing CD or on a duplicate to be used as a special tray in occlusion, it must account for this tissue duality.
However, the impression technique is not exactly the same with ball attachments and bar systems.
Impression in a ball-attachment case
The ball abutments are connected to the implants and tightened up to 20 Ncm. The existing CD (in case of rebasing) or the denture duplicate (in case of a new denture fabrication) is perforated at the attachment sites. These perforations are closed with silicone material to allow for an impression of the whole ridge in conditions similar to a CCD.
For that purpose, a bite registration (wax wafer) is made using either an impression compound material (Kerr™) or self-curing acrylic resin (Duralayâ). Then, a circumferential peripheral seal can eventually be obtained using the high viscosity polyether material (Permadyneâ orange). It is followed by an adhesive coating specific to the wash material (Permlastic lightâ from Kerr™ or Impregumâ from 3M-ESPE™). It is applied to the mucosal side of the tray, whether the CD or the duplicate, except where facing the implant abutments.
As for the impression per se, the mucosal supporting tissues are first recorded through a wash of material made in occlusion, followed by functional muscle trimming of the borders with the mouth opened wide and tongue protracted. When full setting is attained, the tray is retrieved, the excesses of material are cut away and the silicone placed at implant sites is removed.
The impression tray is then replaced on the ridge in order to record the exact implant positions in a closed mouth technique. A low or medium viscosity polyether material is injected in the perforations and the patient is instructed to bite in maximum intercuspation until the full setting. The impression material of choice is the medium (Impregumâ) or low viscosity (Permadyneâ blue) polyether due to its rigidity after setting, a property required for a correct and precise connection of the abutment analogs. The impression together with the analogs is treated in a conventional way and the obtained cast is mounted on the articulator using the bite record already made.
Impression of rebasing for ball attachments
Impression for the ball attachments using an adapted special tray and in occlusion
Impression in a bar-attachment case
The use of impression copings, either for the implant or for the abutment, is needed. These must be connected and tightened up to 20 Ncm.
Then the special tray (duplicate of the existing CD) is hollowed at the copings site and any interference is removed.
The tray is loaded with a medium viscosity polyether (Impregumâ) and the impression is made under occlusal pressure first and then with finger pressure. After complete setting of the material, the transfer screws are loosened, the tray retrieved and the impression checked and validated. Lab analogs are then connected and a working cast is obtained and mounted on the articulator.
After setting the denture teeth, two indices are made in heavy silicone, one buccal and one lingual. They allow the bar to be designed according to the polished surface profile and to the setting of teeth.
No matter how the impression is made, the lab technician must use a spacer during the positioning of the female counterpart, whether for independent axial attachments or the rider of a bar.
Impression for a bar
6th criteria: The maintenance
Like any implant prosthesis, the MOI needs follow-up on a regular basis.
The eventual repairs 8-19-20-21, mainly in the first year following implant placement, are:
- Activation or replacement of the female matrix attached to the denture.
- Replacement of the ball abutment for its wear, a situation that is less frequent than the previous one.
- Activation or replacement of the clips or riders in case of bars.
However, any loss or fracture of prosthetic components or rapid atrophy of the edentulous ridge necessitating frequent rebasing or repair are, rather, the result of an inappropriate prosthetic design.
The scientific data currently available show that there is enough evidence that MOIs on two implants can be considered as the treatment of reference for the edentulous mandible.
This paper describes a systematic clinical approach in 4 points and according to 6 criteria leading to a long term success of the MIO:
- Fabrication of a conventional complete denture meeting quality criteria,
- Fabrication of a radiologic stent, based on the finished denture, with the goal of exploring the implant scopes related to each patient,
- Transformation into a surgical stent for a precise placement of the implants,
- Ultimate modification of this stent into a special tray that will pick-up the implant positions.
This protocol guaranties predictability and durability of the implant prosthetic therapy.
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by Dr. Marwan DAAS, Dr. André ASSAF, Dr. Karim DADA