Prosthetic Options for Implant-Supported Overdentures
Myriad choices make it possible to restore health and function
US Dentists are fortunate to practice in a country that boasts the finest medical and dental care the world has to offer. In an age of modern technology with information streaming from the Internet, it may be hard to believe that the more things change, the more they remain the same. The treatment paradigm has not changed from what it was a century ago; a dentist’s charge is not just to replace missing teeth, but to preserve the remaining dentition.1 Many times, preserving what is left may not be possible, and therefore, after extraction, the patient is either a candidate for a complete denture or implant placement.
This article describes several prosthetic options for implant overdentures that can be successfully treatment planned for longevity, esthetics, and patient comfort. As teeth are lost, it becomes difficult for dentists to provide the patient with acceptable levels of comfort and cleaning ability. Because the primary function of teeth is to masticate, their loss presents a challenge in restoring form and function. In these cases, implants can be invaluable.2-4
Treatment Planning Considerations
An implant overdenture is a removable complete denture or overdenture partial that is either totally supported by implants or partly supported by implants and soft tissue. The rationale for support depends on the number of implants placed; the quantity and quality of remaining bone; the arch used for placement; the opposing dentition; parafunctional habits; the diameter, length and width of the implants; the medical history of the patient; and the cost of the treatment plan. The advantages of implant overdenture treatment are well documented, and implants serve to:
• maintain alveolar height.
• maintain vertical dimension.
• increase chewing efficiency.
• allow for reduced denture flanges and open palates.
• provide greater patient psychological security and confidence.
Implants can also be placed in very favorable symmetrical positions so that at a later period, other implants can be placed that would allow for a fixed implant bridge. Implant overdentures can be fabricated directly over the implant healing caps as they were at one time fabricated directly over root copings. However, this only allows for the implants to bear some or the entire occlusal load, but it does not provide for retention.
Attachments
Dental attachments were used for years as retentive elements for root overdentures and are now being used almost exclusively for implant overdentures. By redirecting the occlusal forces, they serve as rigid or resilient connectors. As such, the prosthesis will transfer occlusal forces to the implants. A resilient or nonrigid connector will distribute the forces to the soft tissues. Knowing how attachments function is important for selecting an attachment that not only will provide the retention needed, but also will not allow the implant to be overloaded and cause implant failure.5-7
Table 1 demonstrates how attachments are classified by the way they allow the prosthesis to function. Class 1 attachments are rigid and do not allow the transfer of occlusal loads to soft tissue. Class 2 and Class 3 allow movement and loading of the soft tissue in only one direction. Class 4 through Class 6 allow for a vertical component of soft tissue support and compression, which allows the prosthesis to redirect occlusal forces to the soft tissue. The degree to which this is possible depends on the dental attachment itself. The vertical resiliency component allows the prosthesis to compress the soft tissue in function from 2 mm to 0.6 mm before the resulting occlusal forces load the implant. How a prosthesis loads or unloads an implant or implants is critical for the treatment planning process and implant longevity.8
Implant Supported vs. Implant Retained
One of the first treatment planning decisions is whether the prosthesis will be implant supported or implant retained. The main difference is that a sufficient number of implants must be placed for the prosthesis to be totally implant supported. Usually, six implants positioned in the lower arch and eight implants placed in the upper arch will suffice. In most cases, the maxillary arch needs at least two more implants placed than the mandibular arch because the quality and quantity of maxillary bone is not as good as mandibular bone. In contrast, an implant-retained case will rely on support from the implants as well as the soft tissue. Two to five implants placed in the mandible will necessitate an implant-retained, soft tissue-supported prosthesis. Both implant and soft tissue will be supporting areas and will share the occlusal load. The prosthesis will be stable, retentive, rigid, or resilient, depending on the attachments used.2-4
Types of Implant-Supported Overdentures
Many variations in treatment planning implant overdentures are available. One approach is to use freestanding implant attachment abutments, and it is not unusual for dentists to utilize anywhere from two to six in any given arch. Their ability to allow the prosthesis to be rigid or resilient enables an improved treatment planning flexibility with greater decision-making regarding how much load the implants would endure. Also, loadbearing areas of the palate and tuberosities can be utilized to minimize the amount of denture base in other areas.7,8 Bars can also be made resilient in the vertical direction by using a Dolder bar (Preat Corporation, www.preat.com) or an Ackermann clip on a Hader Bar® (Sterngold) with 0.6-mm spacers, which can mitigate occlusal loads on implants. Intra- and extra-bar attachments can also be utilized, which can be rigid or resilient.9
The following guidelines can be used to treatment plan implant overdentures. As the number of implants decrease, the length of a possible cantilever also decreases and the amount of tissue support increases (Table 2).6
Type 1
This overdenture uses a combination of implant and soft tissue support and is usually more effective in the mandible than in the maxilla. Two implants are generally used in the cuspid positions. The prosthesis is implant retained. The freestanding attachment abutments can be rigid and allow for both implant and soft tissue support concurrently or allow for soft tissue support up to 0.6 mm, which protects the implants from being overloaded. The attachment abutments can be ball attachments (eg, Preci Clix, Preat Corporation, www.preat.com), extracoronal resilient attachments, (eg, ERA®, Sterngold), o-ring attachments (eg, ORA, Sterngold), or LOCATOR® attachments (Zest Anchors, www.zestanchors.com), depending on how much soft tissue support is necessary.10,11
Type 2
This overdenture uses a straight round bar connecting two or more implants or a curved bar supporting two or more implants with two distal rotational attachments. The prosthesis is implant retained and supported by soft tissue and implants or, depending on the selection of implant attachment abutments, soft tissue supported. The distinguishing feature is that the overdenture is supported by a round or oval bar that allows for rotation. Four implants may be used in this scenario.
Type 3
This overdenture is similar to a Type 2 treatment planning option. However, if four implants are placed in and around the cuspid area, the anterior rotational clip and distal ball or other rotational attachments will work together to generate less torquing forces around the implants. With four implants, the midline clip functions as an indirect retainer, preventing the posterior base portion of the overdenture from rotating away from the posterior edentulous ridge areas during function. Another advantage of the midline clip is that it provides for a tripod effect, or third reference point to determine a plane. By balancing the mucosal-supported area with the bar/implant-supported area, rotation of the overdenture can be minimized. The midline clip should provide only a positive seat when the overdenture is engaged. It functions in a Type 3 case as an indirect retainer, which is basically a vertical stop for a removable partial denture framework, so the free-end saddles do not move from the tissue.
Type 4
Type 1, Type 2, and Type 3 overdentures receive a portion of their support from the soft tissue areas. The distinguishing feature of Type 4 overdenture is that the overdenture is completely supported by bars and implants. The overdenture base may actually contact the mucosal tissue, but any support is inadvertent. Because this overdenture is completely implant supported, it requires the same number of implants for support as a fixed prosthesis. Type 1, Type 2, and Type 3 options are usually treatment planned for the lower arch, while Type 4 is frequently planned for the upper arch. A minimum of five implants is planned for the mandibular arch, whereas six are planned for the maxilla. This all depends on many factors, and many practitioners will opt for six implants in the mandible and eight in the maxilla, just to ensure that the implants are not overloaded.2,13,14
Fixed, Detachable Implant-Supported Prosthesis
Although not a true overdenture by definition because only the dentist can remove it, this “fixed denture” restoration is also known as a hybrid (Figure 1). This prosthetic option allows for fabrication of a rigid fixed restoration when facial esthetics, lack of premaxillary bone, or other defects would obviate the need for an overdenture. It is usually fabricated from a metal framework with denture teeth and acrylic added over the frame. The bar and frame are then screwed into the implants. The holes where the screws attach are filled with cotton and sealed with a composite resin material to be retrieved at a later time when necessary. It is treatment planned as a Type 4 overdenture in terms of implant requirements.4
Narrow-Body Implant Overdentures
Many narrow-body implant systems on the market are used for implant overdentures. Four implants are usually placed on the mandibular arch between the mental foramen, and an overdenture is fabricated over them (Figure 2). Most allow the overdenture to rotate only (eg, MDI Mini Dental Implants, 3M ESPE, www.3mespe.com). Some allow the prosthesis to rotate as well as allow for vertical movement and soft tissue compression (eg, Shatkin FIRST, www.shatkinfirst.com). The Atlas® narrow body implant system (Dentatus, www.dentatus.com) provides for a total soft tissue-supported appliance by using a tough silicone material called Tuf-Link™, which functions as both a reline and a retentive element.15,16
Conclusion
This article attempts to simplify the various prosthetic implant treatment planning options by explaining how the number of implants placed dictates the amount of implant and/or soft tissue support necessary for success. Other factors must be considered, such as implant arch location; quantity and quality of bone; size of the implants; and medical, physiologic, and psychological conditions. These factors may mitigate or complicate the final design of the prosthesis and may require the placement of more implants or the use of attachments that are more rigid or resilient. The plethora and diversity of today’s treatment options makes this an exciting time to be a dentist.
About the Author
George E. Bambara, MS, DMD, FACD, FICD
Faculty
Rutgers School of Dental Medicine
Adjunct Assistant Professor
College of Staten Island
Lecturer
Attachment Dentistry Seminars
References
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