Introduction

Edentulism is considered a major health problem as it affects the overall well-being of an individual.^1-3 Traditionally, complete dentures were the only treatment option available for edentulous patients.⁴ The lack of prosthesis retention and stability coupled with poor masticatory efficiency led to dissatisfaction among denture patients, forcing them to seek alternative therapy.^5,6 The advent of implants and implant prostheses have positively impacted the quality of life of many edentulous patients.^6-10

An implant-supported and/or retained prosthesis can be fixed or removable. However, most dentists and patients believe that the most suitable restoration (that will eliminate all of the patient’s existing problems) is a “fixed” implant prosthesis.^11-13 The choice of the definitive prosthesis should **not** be based on the preference of the dentist or the patient, rather, it should be determined by comprehensive diagnosis and treatment planning.¹³ The bone quantity and quality, the number, location and distribution of implants, the available restorative space, smile line, lip length, opposing arch, oral hygiene compliance, financial implications, and the time required for fabrication and maintenance of the prosthesis should be taken into consideration while choosing the type of implant prosthesis.^11,12,14 Removable implant prosthesis can be indicated in most situations which may be inconducive to the fabrication of fixed implant prosthesis.^13,15,16

Removable implant restorations (Implant overdentures) may be either implant-retained and mucosa-supported or implant-retained and implant-supported. When the prosthesis is retained by implants and supported by mucosal tissues, it is termed as an implant-retained and mucosa-supported prosthesis. When the prosthesis is retained and supported by implants, it is an implant-retained and implant-supported prosthesis. Implant overdentures have several advantages compared to conventional complete dentures and removable partial dentures, including decreased bone resorption (in locations where implants are placed),¹⁷ reduced prosthesis movement, improved mastication, nutrition,^18,19 appearance²⁰ (teeth placement can be dictated by esthetics), ²¹ and quality of life²² and maintenance of the occlusal vertical dimension (OVD). When compared to fixed implant prostheses, implant overdentures have the following advantages: (These can be also be interpreted as disadvantages of fixed implant prostheses.)

1. Fewer implants needed: Fabrication of implant-supported overdentures generally requires a lesser number of implants compared to fixed implant prostheses.

2. Improvement of esthetics: Patients with significant resorption of the labial aspect of the anterior maxilla require lip support to optimize the esthetics. The labial flange of the overdenture can be contoured to provide the desired lip support.^20,23 Managing this situation with a fixed implant prosthesis would necessitate bone augmentation procedures or the development of non-cleansable contours in the fixed restoration.

3. Improvement in speech: The overdenture surfaces can be appropriately contoured and the prosthetic teeth can be optimally positioned to permit an improvement in speech. Also, the problem of air leakage between the prosthesis and the ridges is minimized with an overdenture.^20,24,25

4. Easy to clean: The oral cavity, attachments and the prosthesis are easy to clean because the prosthesis can be easily removed from the mouth.^20,26

5. Cost-effective: Removable implant restorations are less expensive to fabricate and repair than fixed implant restorations.

However, there are a few disadvantages associated with implant overdentures (which can also be interpreted as advantages of fixed implant prosthesis) they include the following:

1. All removable restorations must be kept out of the mouth for 6 to 8 hours in a 24-hour period which may be objectionable to some of the patients.²⁷

2. Removable implant prostheses generally require more restorative space than fixed implant restorations.²⁸ Note: Restorative space is the 3-dimensional oral space available (specific to the arch under consideration) to receive the proposed prosthodontics restoration.²⁹ Vertical restorative space may be assessed using measuring tools, such as a Boley gauge and existing complete dentures, wax rims (Figure 1A), or wax trial dentures. The tips of the measuring caliper should simultaneously contact the intaglio surface of the denture or record base and the deepest aspect of the overlying occlusal surface or the wax rim.²⁹ Vertical space may also be directly calculated using mounted casts with adjusted wax rims (Figure 1B) or wax trial dentures.²⁹ Also, computed tomography (CT) scans with computer-based planning can also be used to determine the available restorative space. The minimum vertical space requirement of various implant prostheses are shown in Table 1:^28-30

   Minimum Vertical Space Requirement of Various Implant Prostheses28-30
   Fixed screw-retained (implant level) restoration (porcelain fused to metal/Zirconia) replacing a single/few teeth	4-5 millimeters
   Fixed screw-retained (abutment level) prosthesis (porcelain fused to metal/Zirconia) replacing a single/few teeth	7.5mm
   Fixed cement-retained prosthesis (porcelain fused to metal/Zirconia) replacing a single/few teeth	7-8mm
   Fixed full arch implant prosthesis (porcelain fused to metal/Zirconia)	12mm
   Fixed screw-retained complete denture (acrylic and metal)	15mm*
   Overdenture supported by locator attachments	8.5mm
   Overdenture supported by bar attachment system	13-14mm
   *All the different types of fixed implant prostheses require lesser vertical restorative space compared to the removable implant prostheses except the fixed screw-retained complete denture.

3. There is a need for constant replacement of the retentive elements of the stud attachments when used in situations where the implants are not placed parallel to each other.²⁷

4. Since fewer implants are placed, the osseous structures will continue to resorb in locations where there are no implants.²⁷

All these factors should be taken into consideration while choosing the type of implant prosthesis.

Figure 1A. Evaluation of restorative space using the wax rim and Boley gauge.

Figure 1A. Evaluation of restorative space using the wax rim and Boley gauge.

Figure 1B. Evaluation of restorative space using wax rims on mounted casts.

Figure 1B. Evaluation of restorative space using wax rims on mounted casts.