Mandidular Figure 1

FIGURE 1: A schematic representation of U-shaped mandibular edentulous ridges. Two implants are splinted with a single bar (A) around which a rotational movement of the overdenture may occur. In (B), four implants are placed in the anterior region of the mandible and connected with a bar of three segments. In this case, the bar can be cantilevered on both sides distal to the terminal abutments. In (C), four implants are placed on the same line. The spaces between the implants are limited; therefore, a clip with an adequate length that provides the required retention is not achievable (modified from Mericske-Stern et al.). 12

Factors to be considered when selecting an attachment type
Selection of an attachment system that is suitable for a specific clinical situation is sometimes difficult. A good knowledge of the different systems and their mechanical properties, and the way in which they distribute load, is important. For instance, when short implants are used resilient attachments should be applied to ensure a degree of relief on the supporting implants. This allows denture movements to occur and enables the edentulous ridge to absorb the masticatory forces. Consequently, a significant amount of masticatory force is dissipated by the edentulous ridge. Some factors that should be identified and considered in order to obtain the best treatment option with the use of RISOs include the following:

1. Quantity and quality of available residual ridge
When the alveolar residual ridge is severely resorbed, a bar attachment and a telescopic attachment provide better horizontal stability, and most occlusal loads are dissipated through the supporting implants as previously stated. However, the potential risk for mechanical failure of the implant or its components is a concern if an adequate number, size and length of the implants are not appreciated.
On the other hand, when bone resorption is minimal, individual attachments such as a Locator, ball or magnet can be used. In this case the denture is mainly tissue supported and the attachments may just be used to retain the denture.

2. Shape of the dental arch

Mandibular Figure 2

FIGURE 2: A schematic representation of v-shaped (narrow) mandibular edentulous ridges. A bar attachment on two implants (splinted) is not always indicated as it may encroach on the tongue space and interfere with function (A). Three implants connected with two bar segments is indicated when good retention and stability are not possible with two non-splinted implants (B). Four splinted implants with three segments can also be used with an RISO and a fixed prosthesis (modified from Mericske-Stern et al.).12

When the residual alveolar ridge is narrow and has a v-shape, the use of two splinted implants is not recommended because the bar may encroach upon tongue space and interfere with function and speech (Figure 2). If the bar is placed more labially, it may interfere with the lower lip and also affect the denture stability, and may have a negative impact on the aesthetic outcome. Therefore, individual attachments are ideal for such clinical problems when an adequate space between the implants can be granted. Three splinted implants may also provide a good alternative (Figure 2B). With three splinted implants, biomechanical risk may be increased, particularly when the implants are short and narrow. In such a situation, the denture should be totally tissue supported whenever possible.
A U-shaped residual ridge with adequate bone permits placement of four implants that are connected with three bar segments (Figure 1). However, the inter-implant distances should be wide enough to accommodate the bar and the clips in order to avoid distortion and unsatisfactory retention. The distance between the most anterior implants and the most posterior implant (antero-posterior spread) dictates the cantilever extension if required.

3. Angle between implants
There is general agreement that when individual (non-splinted) attachments, such as a ball system, are used, implants should be installed parallel to each other to gain the best retention and to reduce the wear rate of the matrices. If this is not possible, other options such as the use of angled abutments or bars may provide an alternative solution. Furthermore, Locators and magnet attachments may also provide a solution when the implants are not parallel.

4. Amount of required retention
Bar attachments usually provide more retention than individual attachments. Thus, in patients who require maximum retention, bar attachments may fulfil this requirement and represent the most ideal option. A bar that has multiple segments can be combined with individual attachments in order to maximise the degree of retention, support and stability. Also, an FISO may be an alternative.
When a single bar is used with two implants, its length should be between 20 and 22mm to obtain good retention and stability. In this case, one or two clips can be used to gain the optimal retention. When the bar is too short, stability and retention are not achievable. If the bar is too long, it may bend when it is loaded and consequently is distorted and may be broken.

5. Restorative space
Restorative space is a three-dimensional space that is available to accommodate various parts of the overdenture and its attachment system. This space is surrounded bucco-lingually by cheek, lips and tongue, and vertically by the edentulous ridge and the occlusal plane of the prospective overdenture. Therefore, it should be appreciated in vertical and horizontal dimensions. Hence, room for the attachment, the superstructure, the acrylic and the teeth is required. If this room is not available, the outcome is negatively affected and, for example, mechanical failure of the denture is a possibility.
The aesthetic requirement of the final restoration (overdenture) is significantly influenced by the available restorative space, as well as by the used attachment system. As an example, when vertical space is limited, the use of a bar attachment may violate the inter-occlusal (free-way) space to accommodate the restoration, which results in an inferior aesthetic outcome of the overdenture, as well as possibly leading to other complications that arise as a result of this error. In such a case the attachments with a low profile are the best option; however, other factors should also be considered.
A minimum of 12mm of vertical restorative space from the crest of the ridge to the incisal edge is usually required with the bar system. This distance consists of 4mm for the bar, at least 1mm for the space between the inferior surface of the bar and the ridge, and 7mm for the teeth, the acrylic and the clip. A space between the bar and the tissue is required to facilitate oral hygiene, and reduce the possibility of plaque and calculus deposition. When the Locator attachment is used, a minimum of 8.5mm is required, while the ball attachment requires 10-12mm.
The horizontal restorative space in the bucco-lingual direction should also be considered, and the attachment should be placed on the crest of the ridge to achieve the best biomechanical advantages of the attachment. As well as this, the horizontal space in the mesio-distal direction should also be considered. Thus, a good distance between the adjacent implants, which provides a good biomechanical advantage and facilitates oral hygiene, should be considered.

6. Treatment costs
Treatment and repair costs should be considered when the treatment plan is made, and the patient should be made aware of these costs. For instance, a bar or telescopic attachment is more expensive when compared with other attachment systems. Furthermore, when bar repair or replacement is required, they cannot be carried out clinically and, therefore, laboratory work will be needed. This is a lengthy and expensive process. Also, the denture will be held for some time and the patient may have to leave without the denture. Also, when an implant tissue-supported overdenture is considered, bone resorption of the tissue-borne regions will continue. Therefore, relining and occlusal adjustments are needed on a regular basis.
The cost of treatment and its inter-relationship with other factors should be investigated and considered. However, the treatment cost may compromise and interfere with providing the best treatment option.

7. Other factors
Factors such as the patient’s expectations of the prosthetic, personal choice, and knowledge and skills of the dentist and laboratory technicians, as well as opposing (maxillary) arch, may play a role in the selection of a specific attachment system and type of overdenture. All of these factors are interrelated, and their individual and combined effects should be considered and appreciated when the treatment plan is made.

The anterior region of the mandible is an ideal site for implants
There is strong evidence indicating that implant success is highly reliant on the volume and quality of the peri-implant bone. Bone quality in the anterior area of the mandible is much better than that in the posterior region,15 and implant failure rate in this site is lower than in any other site in the mandible, as well as in the maxilla.23 Implant placement in the inter-foraminal area is less critical than in other areas of the mandible, and a success rate of ≥95% was reported for implants placed in this region.10 Furthermore, after tooth extraction, bone resorption in this area is usually less than that in the posterior region, which gives an opportunity for installing the implants without the need for other surgical methods such as ridge augmentation and the use of graft materials. Therefore, the mandibular anterior area represents an ideal site for placement of dental implants in the edentulous mandible with a high success rate.

Two-implant RISOs as a first choice standard of care for the edentulous mandible
The decision on the number of implants that suits a particular clinical situation is to some extent subjective and depends on the clinician’s knowledge and experience, as well as on many other factors. However, two consensus statements on the number of implants to be used with RISOs were published in 2002 and 2009, respectively. Both consensus statements recommended that RISOs with two implants should be considered as the first-choice standard of care for an edentulous mandible.24,25 A more recent survey was carried out by an expert panel of 16 representatives of academic prosthodontists to investigate if there was agreement on the gold standard for an edentulous mandible between the two-implant-supported mandibular overdentures and a conventional mandibular complete denture.26 The survey results concurred with the two consensus statements of 2002 and 2009.
Even though the RISO with two implants is a reliable clinical decision and is considered as the minimum standard that should be appropriate for most patients, it does not suit all patients and the abovementioned factors should be taken into account when planning for mandibular RISOs.24,25
As a general guide, two implants of 4mm diameter and a minimum of 8mm length are sufficient in order to obtain satisfactory retention and stability. The two implants are preferably located in the lateral incisor areas and not in the canine region.27-29 This location is regarded as the best site for installation of the implants for several reasons. For instance, if a removable RISO is supported by two implants that are placed in the canine regions, and the RISO then needs to be altered into a fixed one where five implants are considered essential, placement of three additional implants between the two existing implants may not be possible as the space would be insufficient. Additionally, it has been suggested that the anterior mandibular area should be divided into five equal spaces, in which prospective implants may be positioned.15 By this strategy, more implants may be installed if the overdenture needs to be changed from a two-implant-supported overdenture to a four-implant-supported overdenture. Furthermore, if four implants were installed with an equal distance between each, placement of an additional implant when it is required is not possible, as there will be insufficient space to accommodate the fifth implant.

Splinted versus non-splinted implants

FIGURE 3: A schematic representation of the ball attachment system showing the vertical restorative space required when a ball attachment system is used. Room for the attachment components, denture acrylic base and the denture teeth is needed.

FIGURE 3: A schematic representation of the ball attachment system showing the vertical restorative space required when a ball attachment system is used. Room for the attachment components, denture acrylic base and the denture teeth is needed.

In general, when two or more implants are used to retain an RISO, two basic techniques are described and widely used: connecting the implants to each other (splinted); or, using them individually (non-splinted) (Figures 1, 2 and 3). For example, two implants may be joined together with a straight bar that has an ovoid/circle cross-section, which permits denture movement vertically; thus, the implants as well as the mucosa will be involved in the dissipation of the occlusal force.5,15 Two implants can also be used individually in a non-splinted fashion with a ball, Locator, magnetic or telescopic attachment (see later). However, patient satisfaction regarding retention and stability of the RISO declined significantly with the use of the two non-splinted ball attachments, while patient satisfaction with single-bar and triple-bar (two and four implants) attachments did not change with time.30
When two implants are used in the construction of the RISO and to reduce mechanical failures of the prosthesis and/or implants, the overdenture should have a single axis of rotation during function. This rotation will allow the alveolar ridge to take part in dissipation of the occlusal loadings. This design requires that the denture base be extended in a similar fashion, as in the case of a conventional complete denture. This allows a maximum area of tissue to support the denture. However, when a bar with multiple axes is used, the denture is exposed to torque that consequently leads to damage of the attachment and the denture, in addition to overloading of the implants.11,31

Single-implant-retained RISOs
In an attempt to simplify the technique while still maintaining denture stability, retention, and patient comfort and satisfaction, a single implant mandibular complete overdenture can be used.32 In this case only one implant was installed in the midline of the mandible with a ball, a Locator or a magnet attachment. However, because of a limited number of well-controlled clinical studies, it is difficult to draw a sound conclusion regarding the viability of one-implant-supported overdentures.33

Use of mini-implants in RISOs
Mini dental implants (MDIs) are one-piece implants with a diameter of less than 3mm (range from 1.8mm to 3mm). They may be used as an alternative option when a conventional implant is not possible, as when the alveolar ridge is severely resorbed or in systemically compromised patients. The advantages of using mini-implants include relatively low cost, simplicity of placement, minimally invasive surgery and significantly shorter healing periods than those for conventional implants.34 However, mini-implants have a reduced diameter and surface area; therefore, they are subjected to greater occlusal loading, which may lead to mechanical failure, such as deformation and fracture. Nevertheless, when four mini-implants were used to support and retain the RISO for three years, high survival rates were reported irrespective of whether mini-implants were immediately or early loaded (91.7% and 96.7%, respectively).35 Nevertheless, well-controlled randomised clinical studies are required.

Complications and attachment failure
Mandibular Table 3RISO failure is most commonly of a mechanical nature.18 Types of failure include: fracture of the acrylic base, teeth and retentive clip; reduction of retention as a result of wear of the retentive elements or loosening of matrices and screws; fracture or wear of the clip and matrix; fracture of solder joints; and, dislodgement of the attachments. Complications as a result of plaque accumulation, such as mucosal enlargement and peri-implantitis, are also reported.34,36 Relining of the denture is also required regularly and may need to be carried out every four years37 to compensate for the changes in the alveolar ridge that may occur. A summary of several laboratory studies on attachment systems is presented in Table 3.

 

 

 

 

 

Conclusion
There is a general consensus that RISOs provide significantly greater patient satisfaction than conventional complete dentures. The use of two implants to support a mandibular complete denture is a reliable solution to resolve problems that are associated with conventional complete dentures. A comprehensive and careful treatment plan, which begins with the construction of a complete denture even before placement of the implant, is a wise approach to obtain the greatest benefit from the attachment systems.
As maintenance and aftercare are more frequently required in RISO wearers than in patients with conventional complete dentures, and elderly patients are more susceptible to oral diseases, the possibility of detecting oral disease in its early stages would be an additional important advantage in the use of RISOs.

Acknowledgement
The authors would like to thank Dr Osama Omar for Figures 4 and 5.

References

  1. Vecchia, M.P., Regis, R.R., Cunha, T.R., de Andrade, I.M., da Matta, J.C., de Souza, R.F. A randomized trial on simplified and conventional methods for complete denture fabrication: cost analysis. J Prosthodont 2014; 23 (3): 182-191.
  2. Carlsson, G.E., Omar, R. The future of complete dentures in oral rehabilitation. A critical review. J Oral Rehabil 2010; 37 (2): 143-156.
  3. Sohrabi, K., Mushantat, A., Esfandiari, S., Feine, J. How successful are small-diameter implants? A literature review. Clin Oral Implants Res 2012; 23 (5): 515-525.
  4. Massad, J.J., Ahuja, S., Cagna, D. Implant overdentures: selections for attachment systems. Dent Today 2013; 32 (2): 128, 130-132.
  5. Burns, D.R. Mandibular implant overdenture treatment: consensus and controversy. J Prosthodont 2000; 9 (1): 37-46.
  6. Hillerup, S. Preprosthetic mandibular vestibuloplasty with buccal mucosal graft. A 2-year follow-up study. Int J Oral Surg 1982; 11 (2): 81-88.
  7. Hochstedler, J.L., Finger, I.M. Preprosthetic surgery. Gen Dent 1998; 46 (6): 626-630.
  8. Matras, H. A review of surgical procedures designed to increase the functional height of the resorbed alveolar ridge. Int Dent J 1983; 33 (4): 332-338.
  9. Doundoulakis, J.H., Eckert, S.E., Lindquist, C.C., Jeffcoat, M.K. The implant-supported overdenture as an alternative to the complete mandibular denture. J Am Dent Assoc 2003; 134 (11): 1455-1458.
  10. Chee, W., Jivrai, S. Treatment planning of the edentulous mandible. Br Dent J 2006; 201: 337-347.
  11. Hoffmann, O., Beaumont, C., Tatakis, D.N., Zafiropoulos, G.G. Telescopic crowns as attachments for implant supported restorations: A case series. J Oral Implantol 2006; 32 (6): 291-299.
  12. Mericske-Stern, R. Prosthodontic management of maxillary and mandibular overdentures. In: Feine, J.S., Carlsson, G.E. Implant Overdentures: The standard of care for edentulous patients (1st ed.). Quintessence Publishing, 2003: 83-98.
  13. Simon, H., Yanase, T.R. Terminology for implant prostheses. Int J Oral Maxillofac implants 2003; 18 (4): 539-543.
  14. Tomasi, C., Idmyr, B.O., Wennström, J.L. Patient satisfaction with mini-implant stabilised full dentures. A 1-year prospective study. J Oral Rehabil 2013; 40 (7): 526-534.
  15. Misch, C.E. An organised approach to implant-support overdenture. In: Misch, C.E. Contemporary Implant Dentistry (3rd ed.). St Louis, Mo: CV Mosby; 2008: 293-313.
  16. Naert, I., Alsaadi, G., Quirynen, M. Prosthetic aspects and patient satisfaction with two-implant-retained mandibular overdentures: a 10-year randomized clinical study. Int J Prosthodont 2004; 17 (4): 401-410.
  17. Lee, J.Y., Kim, H.Y., Shin, S.W., Bryant, S.R. Number of implants for mandibular implant overdentures: a systematic review. J Adv Prosthodont 2012; 4 (4): 204-209.
  18. Alsabeeha, N.H., Payne, A.G., Swain, M.V. Attachment systems for mandibular two-implant overdentures: a review of in vitro investigations on retention and wear features. Int J Prosthodont 2009 (5); 22: 429-440.
  19. Kleis, W.K., Kämmerer, P.W., Hartmann, S., Al-Nawas, B., Wagner, W. A comparison of three different attachment systems for mandibular two-implant overdentures: one-year report. Clin Implant Dent Relat Res 2010; 12 (3): 209-218.
  20. Riley, M.A., Williams, A.J., Speight, J.D., Walmsley, A.D., Harris, I.R. Investigations into the failure of dental magnets. Int J Prosthodont 1999; 12 (3): 249-254.
  21. Allen, P.F., Ulhuq, A., Kearney, J. Strategic use of a new dental magnet system to retain partial and complete overdentures. Eur J Prosthodont Restor Dent. 2005; 13 (2): 81-86.
  22. Heckmann, S.M., Schrott, A., Graef, F., Wichmann, M.G., Weber, H.P. Mandibular two-implant telescopic overdentures. Clin Oral Implants Res 2004; 15 (5): 560-569.
  23. Gokcen-Rohlig, B., Yaltirik, M., Ozer, S., Tuncer, E.D., Evlioglua, G. Survival and success of ITI implants and prostheses: retrospective study of cases with 5-year follow-up. Eur J Dent 2009; 3 (1): 42-49.
  24. Feine, J.S., Carlsson, G.E., Awad, M.A., Chehade, A., Duncan, W.J., et al. The McGill consensus statement on overdentures. Mandibular two-implant overdentures as first choice standard of care for edentulous patients. Montreal, Quebec, May 24-25, 2002. Int J Oral Maxillofac Implants 2002; 17 (4): 601-602.
  25. British Society for the Study of Prosthetic Dentistry. The York consensus statement on implant-supported overdentures. Eur J Prosthodont Restor Dent 2009; 17 (4): 164-165.
  26. Das, K.P., Jahangiri, L., Katz, R.V. The first-choice standard of care for an edentulous mandible: a Delphi method survey of academic prosthodontists in the United States. J Am Dent Assoc 2012; 143 (8): 881-889.
  27. Scherer, M.D., McGlumphy, E.A., Seghi, R.R., Campagni, W.V. Comparison of retention and stability of two implant-retained overdentures based on implant location. J Prosthet Dent 2014; 112: 515-521.
  28. Hong, H.R., Pae, A., Kim, Y., Paek, J., Kim, H.S., Kwon, K.R. Effect of implant position, angulation, and attachment height on peri-implant bone stress associated with mandibular two-implant overdentures: a finite element analysis. Int J Oral Maxillofac Implants 2012; 27 (5): e69-76.
  29. Al-Ansari, A. No difference between splinted and unsplinted implants to support overdentures. Evid Based Dent 2012; 13 (2): 54-55.
  30. Timmerman, R., Stoker, G.T., Wismeijer, D., Oosterveld, P., Vermeeren, J.I., van Waas, M.A. An eight-year follow-up to a randomized clinical trial of participant satisfaction with three types of mandibular implant-retained overdentures. J Dent Res 2004; 83 (8): 630-633.
  31. Bryant, S.R., Walton, J.N., MacEntee, M.I. A 5-year randomized trial to compare 1 or 2 implants for implant overdentures. J Dent Res 2015; 94: 36-43.
  32. Cordioli, G., Majzoub, Z., Castagna, S. Mandibular overdentures anchored to single implants: a five-year prospective study. J Prosthet Dent 1997; 78 (2): 159-165.
  33. Alsabeeha, N., Payne, A.G., De Silva, R.K., Swain, M.V. Mandibular single-implant overdentures: a review with surgical and prosthodontic perspectives of a novel approach. Clin Oral Implants Res 2009; 20 (4): 356-365.
  34. Steffen, R.P., White, V., Markowitz, N.R. The use of ball-clip attachments with an implant-supported primary-secondary bar overdenture. J Oral Implantol 2004; 30: 234-239.
  35. Maryod, W.H., Ali, S.M., Shawky, A.F. Immediate versus early loading of min-implants supporting mandibular over dentures: A preliminary 3-year clinical outcome report. Int J Prosthodont 2014; 27: 553-560.
  36. Kuoppala, R., Näpänkangas, R., Raustia, A. Outcome of implant-supported overdenture treatment – a survey of 58 patients. Gerodontology 2012; 29 (2): e577-584.
  37. Attard, N.J., Zarb, G.A. Long-term treatment outcomes in edentulous patients with implant overdentures: the Toronto study. Int J Prosthodont 2004; 17 (4): 425-433.