The impact of this assessment is critical and must include an evaluation of the available restorative space. For the minimally resorbed patient, additional space may need to be obtained prosthetically (ie, increasing the vertical dimension with a new denture) and/or surgically (ie, by reducing the residual ridge) to create the necessary space for attachments or a bar. Although the exact vertical space needed will vary with the selected attachments and the height of the abutments, a minimal guideline for this space is approximately 7 mm.

ATTACHMENT OPTIONS

There is considerable variation in the design of anchorage systems used for overdentures within and between countries. In vitro studies have shown a large range of retention capacity and differences in load transfers among different retentive elements (eg, ball, magnet, or bar attachments). However, the amount of force that would be detrimental to the implant and surrounding hard tissues is not well defined. Nonetheless, findings reported in several randomized controlled trials indicate no clinically significant differences among retentive systems with regard to survival rate, peri-implant tissue parameters, and marginal bone.^1-3 Selecting appropriate attachments is further complicated by factors such as simplicity, cost, retentive capacity, parallelism of the implants, and aftercare.

Attachment devices specifically designed to retain and support overdentures are recommended for utilization with the two-implant overdenture. Given the comparable performance of competing attachment designs and products, personal preference, training, experience, and laboratory support may be most relevant to specific abutment selection, which typically takes place within two basic categories.

• Stud-Type Attachments: Ball and socket-type attachments in which one component is attached to an implant and the other element is retained in the prosthesis. These devices are generally used as individual overdenture abutments although it is technically possible that they may be incorporated within a bar assembly as well.
• Bar-Type Attachments: Attachments that span the edentulous ridge and connect or splint implants to each other. The overdenture fits over and connects to the bar with retentive sleeves, riders or clips.

Table 1. Characteristics of Ideal Overdenture Abutment for the Unsplinted Overdenture

• Easy to use: The attachment should be easy to use and should readily facilitate conversion of an existing denture to a two-implant overdenture.
• Resilient (nonrigid in function): The attachment should provide adequate retention and stability with some movement to compensate for the soft-tissue support. Movement may range from a Class 3 attachment (simple hinge-type action around a given point to a Class 6 attachment (a universal, omni-planar attachment that allows movement in any plane).
• Lowest vertical height possible: Because overdentures are space sensitive, the overdenture abutment with the lowest profile is most desirable. It will provide an optimal dimension for denture base thickness while avoiding excess overbulking lingually.
• Fabricated with retentive metal housing: For ideal retention within acrylic as well as ease of maintenance, ie, the metal housing most ideally maintains the long-term position of retentive element and facilitates its replacement.
• Maximum angulation correction without the need for custom components: Mal-aligned implants utilizing premachined components will result in excess wear of attachments, which has significant maintenance manifestations.
• Plastic retentive components: Clinical preference for plastic retentive components over metal alloy types is emerging because of the relative ease of adjusting retention without adjusting the metal retentive parts and because plastic is more easily replaced than the metal overdenture abutment when wear occurs.
• Minimal long-term maintenance requirements and simplicity of replacement: All components possess a specific amount of retention and wear characteristics unique to the component

FINAL ABUTMENT CONNECTION FOR EARLY LOADING

At approximately 8 weeks postsurgery, full maturation of tissue and bone around the implant is to be expected. If one-stage implant treatment has been undertaken, the patient will present with the healing abutments in place. The healing abutments will be removed and the final overdenture abutments placed. It is critical to choose the final overdenture abutment with the height as minimal as possible. Since premachined (ie, not custom) overdenture abutments are the components of choice, their height is uniform--this reemphasizes the need for a flat residual ridge. As previously mentioned, the final overdenture abutments may already be placed as an alternative approach.

NOTE: Restorative problems are likely to ensue when a case is planned for an unsplinted overdenture but, because of poor implant position or angulation, is subsequently changed to a bar overdenture design. Potential problems include the possibility of an insufficient amount of restorative space, a difficult conversion process, and increased cost to the patient. Some clinicians allow for as much as 40 degrees of divergence between new implants with innovative designs.

RESTORATIVE DESIGN OPTIONS

The two-implant-supported overdenture can be restored using either 1) an unsplinted overdenture or 2) a bar overdenture. The efficacy of the unsplinted overdenture for restoration of two mandibular implants is validated by a substantial body of scientific research, including studies demonstrating that equal success rates are achieved in splinted and unsplinted cases.⁴

The unsplinted design is the method of choice for the mandibular two-implant overdenture except when specifically contraindicated (ie, malpositioned implants that need realignment with a bar superstructure). It is especially well-suited to narrow or V-shaped jaw anatomy, where reduced bulk will not interfere with tongue space.

Advantages of an Unsplinted Implant Overdenture  

• More affordable to the patient;
•  Simple and Easier for dentist;
• Less technique sensitive for the clinician;
• Less technically demanding for the laboratory;
• Easily converted from adapted existing denture;
• Considerably less costly vs a bar prosthesis; and
• In most cases, requires less restorative space versus a bar overdenture.

Disadvantagesof the Unsplinted Overdenture

• None when contraindications are not present.

Conversion of an Existing Denture for the Unsplinted Restoration

The conversion of the existing denture is a practical goal that makes this treatment more accessible and affordable to a large segment of the edentulous patient population.

Converting an existing denture to a two-implant overdenture supports this goal and is the method of choice when feasible.

Indications

The conversion of an existing denture to a unsplinted implant-supported overdenture is planned for the following (Figure 2):

• An existing, previously fabricated conventional denture that fulfills the basic requirements of classic denture fabrication:
• Aesthetics and phonetics meet the patient’s needs and expectations;
• Acceptable vertical dimension;
• Accurate denture base adaptation and flange extension;
• Acceptable tooth position properly arranged within the neutral zone and with minimal wear; and
• Correct occlusal scheme;
• Sufficient restorative space (minimum of 7mm).

ADVANTAGES of adapting an existing denture:

• Significantly less cost to the patient
• Less chairtime
• Shorter treatment duration to completion
• Increased treatment acceptance

DISADVANTAGES of converting existing denture:

Aesthetics may be “acceptable” but not “ideal.”

 The success of two-implant overdenture treatment is dependent upon the classical principles of denture fabrication, and the placement of implants should not be a substitute for these tenets. Adequate denture base extension and adaptation to the remaining structures are basic requirements. If these guidelines cannot be met, then a new prosthesis must be fabricated.

Technique

Depending on the nature of flap design and the extent of the osteoplasty, the first step in the conversion process is to evaluate the fit of the denture base. The degree and location of reline needed will serve as a guideline to choose the most appropriate method. For a small or moderate reline, a direct technique is the method of choice. A small sectional or full reline may be accomplished as either a separate step, connecting attachments later, or in combination with a attachment connection process. Depending on one’s experience, it is easier to reline first and then connect retentive elements to the existing denture as a secondary procedure. Attachments may be connected one at a time or together as one procedure. For a full or extensive reline, the indirect technique is the method of choice.

Direct Technique

The retentive elements are connected directly to the existing denture in the patient’s mouth in a direct (ie, chairside) procedure (Figures 3 through 27).

Advantages of the Direct Technique

• Simple and easy;
• Can be completed in one office visit;
• Allows patient to “keep” teeth at all times;
• Requires no impression procedures;
• Requires no master cast or analogs;
• Generates no laboratory fees; and
• Minimizes polymerization errors by avoiding indirect processing.

Disadvantages of the Direct Technique

The direct technique involves more initial chairtime and is more labor-intensive. It can also result in inadvertent “locking on” to attachments or removal of denture before acrylic is actually set (premature set of acrylic).

Indirect Technique

The retentive elements are connected indirectly to the existing denture in the dental laboratory using a “closed-mouth” reline impression (Figures 28 through 41).

Advantages of the Indirect Technique

• Requires less initial chairtime;
• Eliminates the possibility of inadvertently “locking on” to attachments or premature set of acrylic; and
• Easier when extensive reline is indicated.

Disadvantages of Indirect Technique

• Requires two office visits;
• Requires patient to function without teeth while denture is sent to laboratory;
• Requires impression technique (often with impression copings and always with analogs);
• Requires accurate master cast;
• Requires appropriate laboratory support for the technique of indirect processing;
• Introduces the possibility of lab errors that may result in an increased number of adjustments; and
• More costly due to the need for analogs and laboratory expense.

Fabrication of a New Two-Implant-Supported Overdenture

Indications

• Existing dentures that do not fulfill the basic requirements of classic denture fabrication and, instead, exhibit one or more of the following characteristics:
• Unsatisfactory aesthetics and phonetics;
• Unacceptable vertical dimension;
• Poor denture base adaptation (ie, poor fit), improper flange extension (ie, too short);
• Incorrect tooth position, with excessive wear; and
• Unacceptable occlusal scheme;
• Restorative space is insufficient to accommodate conversion of an existing denture and an osteoplasty would reduce the ridge too much.

Technique

For the fabrication of a new two-implant unsplinted overdenture, the following technique can be utilized with success and predictability (Figures 42 through 54).

Advantages of restoration with a new overdenture:

• Provides opportunity to idealize tooth position, color, and shape and vertical dimension of occlusion
• Can be reinforced with a metal casting to minimize the potential of denture breakage

Disadvantages of restoration with a new overdenture:

• Greater cost
• More time for fabrication than conversion
• Necessity of finalizing tooth position for the new denture and for the fabrication of a surgical guide prior to implant placement.

A new two-implant overdenture may be fabricated with two approaches: 1) Process new conventional denture and then convert it or 2) Final impressions of implants and denture base.

OCCLUSION FOR THE TWO-IMPLANT OVERDENTURE

Although there has been consensus that bilateral balanced occlusion can provide better stability for overdentures,⁵ there are no clinical studies that demonstrate the advantages of bilaterally balanced occlusion compared to other occlusal schemes. Currently, there is no evidence-based implant specific concept of occlusion. Further studies are needed to clarify the relationship between implant occlusion and longevity.⁶

COST OF THE TWO-IMPLANT OVERDENTURE

The mandibular two-implant overdenture represents the most cost-effective implant rehabilitation to restore the fully edentulous patient. Although studies indicate that this treatment is at least two times more costly than conventional dentures,⁷ it is considerably less expensive than an implant-supported fixed prosthesis or multi-implant bar prostheses. Understanding the predictability of the unsplinted design and the conversion process will further aid in the accessibility and affordability of this important treatment modality.

Utilizing a one-stage surgical procedure and early loading protocol, the entire treatment time–from implant surgery to completion of final prosthesis (activation of overdenture abutments)—should be as short as 6 to 8 weeks.

Bar Overdenture

Indications

The indications for the bar overdenture include malpositioned implants, whereby their position will exceed the parallelism requirements of overdenture abutments (Figures 55 and 56). If unsplinted abutments are used in these cases, excessive wear of the retentive components and significantly more aftercare will result. A bar in such cases will minimize bulk and create ideal parallelism for necessary attachments.

Technique

For the fabrication of a bar overdenture, a repeatable technique can be utilized for the two-implant overdenture (Figures 57 through 72).

Advantages of a Bar Overdenture

• A bar connector may provide additional horizontal stability and advanced alveolar bone resorption (ie, Class IV).

Disadvantages of a bar overdenture

• More costly to the patient and the clinician due to the involved laboratory fees;
• More difficult for the dentist;
• More technically challenging to convert the existing denture;
• More technique-sensitive (requires master cast, proper bar position, precise fit);
• More technically difficult for laboratory technicians; and
• In most cases, bar overdentures require more restorative space than the unsplinted overdenture (minimum of 10 mm and 7 mm, respectively).

Contraindications

• Narrow or V-shaped jaw anatomy: A bar overdenture will leave insufficient tongue space.
• Lack of sufficient restorative space (see above). 

References:

1. Naert I, Gizani S, Vuylsteke M, van Steenberghe D. A randomized clinical trial of the influence of splinted and unsplined oral implants in the mandibular overdenture therapy: A 3-year report. Clin Oral Investigat 1997;1(2):81-88.
2. Gotfredsen K, Holm B. Implant-supported mandibular overdentures retained with ball or bar attachments: A randomized prospective 5-year study. Int J Prosthodont 2000;13(2):125-130.
3. Naert I, Hooghe M, Quirynen M, van Steenberghe D. The reliability of implant-hinging overdentures for the fully edentulous mandible. An up to 9-year longitudinal study. Clin Oral Investigat 1997;1(3):119-124.
4. Naert I, Alsaadi G, van Steenberghe D, Quirynen M. A 10-year randomized clinical trial on the influence of splinted and unsplinted oral implants retaining mandibular overdentures: Peri-implant outcome. Int J Oral Maxillofac Implants 2004;19(5):695-702.
5. Engelman MJ. Occlusion. In: Clinical Decision Making and Treatment Planning in Osseointegration. Carol Stream, IL: Quintessence Publishing, 1996:169-176.
6. Kim Y, Oh TJ, Misch CE, Wang HL. Occlusal consideration in implant therapy: Clinical guidelines with biomechanical rationale. Clin Oral Implants Res 2005;16(1):26-35.
7. Takanashi Y, Penrod JR, Lund JP, Feine JS. A cost comparison of mandibular two-implant overdenture and conventional denture treatment. Int J Prosthodont 2004;17(2):181-186.