|Year : 2014 | Volume
| Issue : 1 | Page : 86-90
Guidelines for treatment planning of mandibular implant overdenture
Siddharth Bansal, Meena A Aras, Vidya Chitre
Department of Prosthodontics, Goa Dental College and Hospital, Panaji, Bambolim, Goa, India
|Date of Web Publication||19-Apr-2014|
Department of Prosthodontics, Goa Dental College and Hospital, Panaji, Bambolim, Goa - 403 202
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Implant overdenture (OD) is the common treatment modality for the rehabilitation of complete mandibular edentulism with dental implants. The purpose of this review was to collect the data regarding various factors contributing to the selection of implant OD design and to provide comprehensive guidelines for the clinicians in planning the OD design.
Keywords: Attachments, implant overdenture, implant retained prosthesis, implant supported prosthesis
|How to cite this article:|
Bansal S, Aras MA, Chitre V. Guidelines for treatment planning of mandibular implant overdenture. J Dent Implant 2014;4:86-90
| Introduction|| |
Edentulous patients with severely resorbed maxillary and mandibular arches commonly experience problems with retention, stability and support and the related compromise in chewing ability with conventional complete dentures. As the successful use of dental implants in the treatment of mandibular edentulism is well-documented in the literature  for both fixed and removable prosthetic rehabilitations,  these problems can be easily solved by using implant retained prosthesis (IRP)/implant supported prosthesis (ISP). IRP achieves support from both implants and tissues whereas ISP achieves support only from implants.
The purpose of the review was to collect the data regarding factors contributing to the selection of implant overdenture (OD) design and to provide comprehensive guidelines for the clinicians in planning the OD design. Factors which contribute to the determination of the OD design are grouped under the following subheadings: indications, biomechanical principles, prosthetic space analysis and type of attachment (interconnected vs. solitary implants), optimum number of implants required, prosthesis design, implant location, prosthesis maintenance, patient satisfaction and success rate. 
| Indications|| |
IRP with 2 implants is contraindicated in younger patients or those who are edentulous for <10 years due to anterior posterior rotation of prosthesis, which causes increased bone resorption in posterior edentulous region [Table 1]. ,,
| Biomechanical Principles|| |
According to Misch, as ISP is stabilized on multiple bars between implants, the attachment clips located on each bar are frequently not parallel to one another or perpendicular to the posterior ridges. Therefore, the clips can bind in function, limiting prosthesis movement. This can produce a reduced range of motion between the prosthesis and bar attachment, increased prosthesis support from the implants and increased applied torsional forces to the implants.  In clinical situations involving poor posterior ridge form, reducing posterior mucosal support in this manner may be advantageous as it prevents rotational movements of the prosthesis. Similar to a fixed prosthesis, it creates a stable occlusal plane and prosthesis position reducing possible jaw resorption in posterior mandibular and anterior maxillary regions. ,
IRP with ball or bar and clip attachments design allows a significant amount of rotation and vertical movement due to soft-tissue resiliency and leads to residual ridge bone loss. Therefore for the functional success of an IRP, an optimal extension and fit of the denture is important.
| Prosthetic Space Analysis|| |
For successful implant OD treatment planning prosthetic space analysis should be taken under consideration for selection of the prosthetic components of the implant attachment system. At least 13-14 mm interocclusal space is required for bar supported OD considering teeth size, denture base thickness, bar thickness for the rigidity, the space from the mucosa to the bar for hygiene and the soft-tissue thickness.  Minimum space requirement , for ball attachment is 10-12 mm and for locators is 8.5 mm. Inadequate space for prosthetic components can result in an overcontoured prosthesis, excessive occlusal vertical dimension, fractured teeth adjacent to the attachments, attachments separating from the denture, fracture of the prosthesis and overall patient dissatisfaction.
| Type of Attachments (Interconnected vs Solitary Implants)|| |
Next is to decide which type of attachment is to be used, whether to splint implants with a rigid bar fixation or an independent implant attachment system is to be used. Guidelines to assist with this treatment decision are also limited and controversial in the literature. The attachment selection is affected by the implant number, distribution and alignment, bone quality, arch shape, retention, and denture design. 
The attachments used for implant ODs are mainly divided into the bar type and the solitary type and into the resilient type and the rigid type, depending on the movement allowance. Popular OD attachments used are:
- Ball attachments with rubber o-rings and/or metal housings
- Bar attachments with clips
- Bar with locators cast or tapped into the framework.
Abutment parallelism is very critical for the solitary implants as abutment non parallelism leads to faster wear of the matrix. Therefore with increase in number of implants, splinting should be done as abutment parallelism becomes more difficult.
In a V shaped anterior mandibular ridge, if bar is placed at canine locations, it encroaches on the tongue space and if placed anteriorly, length of the bar becomes inadequate. Therefore in such cases, ball attachments or 3-4 implants with a connecting bar supported OD is indicated. 
Use of a bar may complicate the procedure, increase the cost of the prosthesis, is more technique sensitive  and generally require more space than individual attachments. One perceived advantage of the bar is that it can accommodate divergent implants.  However individual attachments can also be used for divergent implants. 
The available data supports the use of independent implants for a mandibular OD. ,, Stress transmitted to implants by ball attachment or bar attachment is controversial in the literature.
A study by Kenney and Richards  evaluated the photo elastic stress patterns produced by implant-retained ODs. They found that independent O-ring attachments transferred less stress to implants than the bar-clip attachments when their model was subjected to posterior vertical load.
Celik and Uludag  in their study have reported that more stress was observed in the solitary type than in the bar splinting type when the photoelastic stress distribution was assessed in ODs with three mandibular implants according to the retention mechanism.
Wismeijer et al.  studied 110 patients who had received mandibular implant OD treatment. Subjects received either 2 implants with ball attachments, 2 implants with an interconnecting bar, or 4 interconnected implants. Subjects completed questionnaires designed to elicit opinion regarding individual treatment outcome. Sixteen months after treatment, almost all subjects were generally satisfied. No significant difference was found between the 3 treatment strategies.
Meijer et al.  using three-dimensional finite-element analysis, studied the stress distribution in the anterior mandibular bone around implants under conditions in which either 2 or 4 implants were used. They concluded that there was no reduction of the principal stresses in bone when the occlusal load was distributed over an increasing number of implants.
| Optimum Number of Implants|| |
Next question comes about what is the optimum number of implants required for removable IRP/ISP. The answer to this question is controversial because adequate data to address this concern is lacking. Some practitioners believe that using more implants for OD treatment results in a better treatment outcome, but supporting evidence is limited.
Zarb and Schmitt et al.  support the concept that fewer implants can be equally effective for the OD prosthesis. However, the placement of additional implants for the proposed prosthodontic treatment probably provides a means for contingency planning against the loss of implants, if tissue integration fails.
Sadowsky suggested multiple implants for mandibular OD when sensitive jaw anatomy, increased occlusal forces, or high retention needs are present or when implant length <8 mm or implant width <3.5 mm are employed. 
One of the most recent studies is by Thomason et al. The study aimed to present the current evidence and rationale to support the McGill (2002) and York (2009) consensus statements. , The conclusion was that there is overwhelming evidence to support the proposal that a two-implant OD should become the first choice of treatment for the edentulous mandible. No information was, however, given regarding the various options in attachment systems, i.e. bars, ball attachment, locator and/or possible adjunctive benefits with the use of additional implants. 
Four to six implants for Implant-supported ODs splinted with a bar, are usually prescribed to achieve a sufficient amount of support, stability and retention. In this type of prosthesis, more support is derived from the implants than the alveolar ridge mucosa eliminating the need for extension of denture base.
According to Wismeijer et al.,  Timmerman et al.  , Visser et al.  , Meijer et al.  , increase in number of implants did not significantly improve the patient satisfaction. 
| Location of Implant Placement|| |
According to Taylor,  for a 2-implant-retained mandibular OD, placement of implants in the lateral incisor area rather than the canine position offers a mechanical advantage, providing better stability for the OD.
The implants act as a fulcrum with 2 potential lever arms: (1) From the fulcrum to the posterior extension of the denture and (2) from the fulcrum anteriorly to the incisal edge. Forces on either lever arm will produce rotation. However, the primary and secondary bearing areas of the OD will resist occlusal forces placed on the posterior lever arm, but forces on the anterior lever arm, such as incisive movements, may cause more noticeable rotation. By moving the implants from the canine to the lateral incisor position, the effective anterior lever arm is reduced, thus minimizing the tipping forces on the OD. Various implant overdenture prostheses designs are described based on number of implants and type of attachments [Table 2]. 
| Prosthesis Design|| |
Maintenance and complications
One of the main problems with implant ODs is the potential complication associated with the attachment mechanisms. Controversy persisted as to whether the ball or bar design requires more maintenance. Some studies suggest that a bar attachment requires less maintenance , whereas others suggest the opposite. , However in recent literature, studies have shown that bar supported ODs requires less prosthetic maintenance than ball attachments. ,
In one of the study, Walton et al.  found a high complication rate with a ball attachment matrix which could be due to misaligned implants. Most common prosthetic maintenance and complications occurred with magnetic attachments are due to wear and corrosion.  Various complications are loss of retention, clip or attachment fracture, opposing prosthesis fracture, acrylic resin base fracture, prosthesis or abutment screw loosening and implant fracture. 
Success rates (as measured by the continual osseointegration of implants) of 1-10 years which supported the ODs in the mandible, ranged from 91.7% to 100% and the mean implant survival rate was over 98%, both of this supports the presumption that this treatment has a good prognosis in a long-term perspective. , No difference has been found among the success rates of the different attachment mechanisms.  However, direct comparisons of these reports are difficult owing to the variety of attachment mechanisms and different implant manufacturing systems employed.
| Summary|| |
Following objectives should be taken into considerations for planning of ISP/IRP:
- Determine the optimum location and number of implants in the context of the morphological aspects of the residual ridge.
- Design a favorable distribution for occlusal stresses on the implants and the prosthesis bearing tissues.
- Avoid discrepancies among the design of the dentures, the implant's location and the attachment system.
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[Table 1], [Table 2]