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Implant Supported Overdentures

Osseointegrated implants, coupled with various precision attachment systems, have been utilised to enhance the retention and stability of complete overdentures, significantly improving patient satisfaction and quality of life (Kutkut et al., 2018; Thomason et al., 2012).

The McGill Consensus Statement, based on expert opinion, concluded that "The available evidence suggests that the restoration of the edentulous mandible with a conventional denture is no longer the most appropriate first-choice prosthodontic treatment, and there is overwhelming evidence that a two-implant overdenture should become the first choice of treatment for the edentulous mandible" (Feine et al., 2002).

In 2009, the York Consensus Statement by the British Society for the Study of Prosthetic Dentistry (BSSPD) further supported this view, stating that "A substantial body of evidence demonstrates that patients' satisfaction and quality of life with implant-supported mandibular overdentures are significantly greater than with conventional dentures. While the two-implant overdenture is not the gold standard of implant treatment, it is the minimum standard that should suffice for most people, considering performance, patient satisfaction, cost, and clinical time" (Thomason et al., 2009).

For mandibular overdentures, the recommended approach is to use two implants placed in the canine regions (Roccuzzo et al., 2012). In contrast, maxillary implant-supported overdentures generally require 4 to 6 implants. However, higher implant failure rates have been observed in the maxilla compared to the mandible, particularly with machined and short implants (less than 10 mm in length) and when fewer than four implants with non-splinted anchorage are used (Sadowsky and Zitzmann, 2016; Raghoebar et al., 2014).

Although both splinted and solitary implant attachment systems are viable, solitary attachments tend to require more maintenance. Additionally, soft tissue inflammation is more common under bar attachments, and long-term maintenance care is essential for all attachment designs (Sadowsky and Zitzmann, 2016).

 

Implant-Supported Precision Attachments for Removable Overdentures

  1. Ball Attachment:
    • Advantages:
      • Improved retention and stability: Ball attachments are well-regarded for enhancing the retention and stability of overdentures, contributing to improved patient satisfaction and function (Mericske-Stern, 1998).
      • Straightforward procedure: The simplicity of the ball attachment design facilitates easier placement and reduces complications during the fitting process (Naert et al., 1993).
    • Disadvantages:
      • Need for adequate space (15 mm): Adequate vertical space is necessary for the attachment, which can pose challenges in certain clinical scenarios (Walton et al., 2000).
      • Need for parallelism: Ball attachments require precise alignment to function optimally; a lack of parallelism can lead to increased wear and reduced retention (Sadowsky, 2007).
      • Loss of retention over time: Components of ball attachments can wear out over time, leading to a decrease in retention, necessitating regular maintenance or replacement (Feine et al., 1998).
  2. Locator Attachment:
    • Advantages:
      • Improved retention and stability: Locator attachments are known for providing strong retention, especially in situations involving divergent implants (Cehreli et al., 2010).
      • Straightforward procedure: The design of locator attachments simplifies placement, making them a popular choice in clinical practice (Bidra et al., 2014).
      • Minimal space requirement: Locator attachments require less vertical space compared to ball attachments, making them ideal for cases with limited inter-arch space (Goiato et al., 2013).
      • Can correct angulated implants: Locator attachments can accommodate implants that are not perfectly parallel, adding flexibility to their application (D'haese et al., 2012).
    • Disadvantages:
      • Loss of retention over time: Similar to ball attachments, locators may lose retention due to wear, necessitating ongoing maintenance (Wismeijer et al., 2011).
      • Locator maintenance required: Regular maintenance is necessary to ensure the longevity and effectiveness of locator attachments, including the replacement of nylon inserts (Sadowsky, 2007).
  3. Magnets:
    • Advantages:
      • Improved retention and stability: Magnetic attachments offer consistent retention, especially beneficial for patients who have difficulty with other attachment types (Goiato et al., 2009).
      • Self-adjusting: Magnets can adjust naturally within the prosthesis, making them easier to use for patients with limited dexterity (Ribeiro et al., 2010).
      • Easy to wear: Patients often find magnet-retained overdentures easier to wear and remove, leading to higher satisfaction levels (Sadowsky, 2007).
    • Disadvantages:
      • Need for adequate space: Magnets require sufficient space within the prosthesis to function properly (Goiato et al., 2009).
      • Magnet complications: Magnetic attachments can be prone to corrosion and demagnetization over time, affecting their performance (Freitas et al., 2012).
      • Need for maintenance: Regular checks and maintenance are necessary to ensure magnets continue to function effectively (Ribeiro et al., 2010).
  4. Bar and Clip:
    • Advantages:
      • Improved retention and stability: Bar and clip systems provide excellent retention and stability, particularly effective for overdentures in the mandible (Wismeijer et al., 1999).
      • Splinting effect on implants: This system offers a splinting effect, distributing occlusal forces more evenly across the implants, potentially reducing the risk of implant overload (Naert et al., 1991).
    • Disadvantages:
      • Need for adequate space: The bar and clip system requires adequate vertical and horizontal space, which may not be available in all cases (Sadowsky, 2007).
      • Difficulty cleaning under the bar: Maintaining hygiene can be challenging as food and plaque can accumulate under the bar, increasing the risk of peri-implant diseases (Mombelli et al., 1995).
      • Potential for peri-implant inflammatory disease: The accumulation of debris and plaque under the bar can lead to peri-implantitis if not properly managed (Wismeijer et al., 1999).
  5. Milled Bar Combined with Other Types of Attachments:
    • Advantages:
      • Highly improved retention and stability: Milled bars, particularly when combined with other attachments, offer superior retention and stability, making them suitable for patients with challenging anatomical conditions (Sadowsky, 2007).
      • Splinting effect on implants: Like the bar and clip system, milled bars provide a splinting effect, reducing the risk of implant overload and subsequent failure (Bidra et al., 2014).
      • CAD-CAM technology: The use of CAD-CAM technology in fabricating milled bars ensures a precise fit, improving both the function and longevity of the prosthesis (Bidra et al., 2014).
    • Disadvantages:
      • Need for adequate space: As with other bar systems, adequate space is essential for the proper functioning of milled bars (Goiato et al., 2013).
      • Difficulty cleaning under the bar: The intricate design of milled bars can make oral hygiene maintenance difficult, increasing the risk of peri-implant complications (Mombelli et al., 1995).
      • Potential for peri-implant inflammatory disease: Poor oral hygiene and plaque accumulation around the bar can lead to peri-implant inflammatory diseases (Wismeijer et al., 1999).
      • High cost: The advanced technology and precision required for milled bars make them a more expensive option, which may not be affordable for all patients (Bidra et al., 2014).

When planning for full-arch implant-supported fixed restorations, an initial diagnostic tooth setup is crucial. If the patient’s existing denture is satisfactory, it can be duplicated with radiopaque markers and converted into a radiographic and/or surgical stent to aid in implant placement.

For full-arch implant-supported prostheses, placing 4 to 6 implants is a well-established treatment option, with high survival rates reported for both the implants and the prostheses (Heydecke et al., 2012).

 

Mandible Considerations:

In a non-resorbed mandible, implants can be positioned above the inferior alveolar nerve, with the distal implants angled slightly mesially to support the restoration.

In a resorbed mandible, implants are generally placed in the anterior region, mesial to the mental nerve. Here, distal implants should ideally be angled distally to allow for the distal extension of the prosthesis, following the "All-on-Four" concept (Malo et al., 2005). Alternatively, short implants may be used in resorbed mandibular ridges, or vertical ridge augmentation can be considered.

 

Maxilla Considerations:

In a resorbed maxilla, distal implants can be angled distally, parallel to the mesial wall of the maxillary sinus, also following the "All-on-Four" concept. Alternatively, short implants, long implants with sinus augmentation (Thoma et al., 2015), or zygomatic implants (Aparicio et al., 2014) may be utilised.

Systematic reviews indicate that the All-on-Four treatment concept is a reliable approach for managing atrophic jaws, particularly for patients who prefer to avoid ridge augmentation procedures, with reported survival rates of 99.8% over 24 months (Soto-Penaloza et al., 2017).

 

Framework and Material Considerations:

The framework for full-arch prostheses can be fabricated from various materials, including gold, base metal alloys, titanium, zirconia, and polyetheretherketone (PEEK). Several techniques are available for processing the metal framework, including:

  • One-piece casting
  • Sectioning and reconnection
  • Spark erosion using an electric discharge machine
  • Framework bonding to prefabricated abutment cylinders
  • Computer-aided design/computer-assisted manufacturing (CAD/CAM), which ensures an excellent fit for frameworks (Abduo et al., 2011).

The prosthetic facings can be made of acrylic teeth or porcelain. Pink acrylic or porcelain is particularly useful in cases where hard or soft tissue deficiencies cannot be fully corrected with ridge or soft tissue augmentation.

For optimal prosthetic results, a minimum of 15 mm of vertical space from the ridge crest to the planned incisal edge is recommended. If there is insufficient vertical space for a fixed full-arch prosthesis, the edentulous arch can be restored with a series of short-span, fixed implant-supported bridges, eliminating the need for the precision fit required by a full-arch prosthesis.

 

Cantilever Extensions:

Cantilever extensions were initially advocated in the Toronto study (Zarb and Schmitt, 1990). However, complications may arise if the cantilever extension exceeds 15 mm. Distal cantilevers should be avoided, particularly in patients who are bruxists.

 

Occlusal Schemes for Full-Arch Implant-Supported Fixed Prostheses:

  • Bilaterally balanced occlusion: Recommended if the opposing arch is a complete denture.
  • Group function occlusion or canine-guided occlusion: With shallow anterior guidance, this is recommended if the opposing arch has natural dentition.
  • Group function occlusion: Suggested if both arches are implant-supported fixed prostheses.
  • No working or balancing occlusal contacts: On cantilever extensions.
  • Lighter occlusion: Recommended on cantilever segments (Kim et al., 2005; Sheridan et al., 2016).