Over the last 20 years, implants have become part of the standard of dental care and have offered new treatment planning alternatives. We now have the ability to replace missing roots with metal screw-like structures imbedded into the jawbone. At a glance, it would seem that restoring an implant would be the same as restoring a natural tooth. Careful evaluation reveals that there are differences that require special considerations. Research, continuing education, innovative treatment planning, and new product developments have all contributed to the success and acceptance of dental implants. The incorporation of precision and semi-precision attachments has offered greater treatment planning flexibility and patient satisfaction.
Implants differ from natural teeth in many ways. The implant surface osseointegrates with bone to form a solid, immovable unit. There is no fibrous ligamentous attachment with an accompanying neurovascular bundle as there is with a natural tooth. Implants, therefore, lack proprioception, which allows us to feel pain, pressure, and a sense of positioning as to where and how our teeth fit together. This protective feedback mechanism is important and is the one essential difference that must be considered. Natural teeth under occlusal trauma can become hyperemic, resulting in pain, tenderness, and sensitivity. Relief is accomplished in many cases by a simple occlusal adjustment. Implants under occlusal trauma do not become hyperemic or temperature sensitive, since they are rigidly fixated. Excessive forces can cause an implant to fail over a period of time. The patient may never have known a problem existed; therefore periodic recalls are mandated. This reasoning dictates careful treatment planning and occlusal evaluation.
One of the first decisions to be made is whether a prosthesis will be implant supported or implant retained. The main difference is that there must be a sufficient number of implants placed for the prosthesis to be totally implant supported. Usually 6 implants placed in the lower arch and 8 implants placed in the upper arch will suffice in a completely edentulous case. In most cases, the maxillary arch needs at least 2 more implants placed than the lower arch since the quality and quantity of maxillary bone is not as good. In contrast, an implant-retained case will rely on support from the implants as well as the soft tissue. In the mandible, 2 to 5 implants placed 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.
Attachments are simply rigid or resilient connectors that redirect the forces of occlusion. Rigid connectors will transfer forces to the implants; a more resilient connector will distribute the forces to the soft tissues. It is important to know how attachments function to be able to treatment plan fixed and removable prostheses successfully.
FUNCTIONAL CLASSIFICATIONS OF ATTACHMENTS
- Class 1A—Solid, rigid, nonresilient
- Class 1B—Solid, rigid, lockable
- Class 2—Vertical resilient
- Class 3—Hinge resilient
- Class 4—Vertical and hinge resilient
- Class 5—Rotational and vertical resilient
- Class 6—Universal, omni-planer
This article attempts to simplify implant treatment planning by having the number of implants placed dictate 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 and physiological conditions. These factors may mitigate or complicate the final design of the prosthesis. Such factors may require the placement of more implants or the use of attachments that are more rigid or resilient.
The anterior-posterior spread is a measurement of the distance from the most anterior implant in the arch to the most posterior implant. A line is drawn from the center of the most anterior implant and measured to a line joining the distal aspects of the two most distal implants (Figure 1). The A-P spread indicates the reasonable amount of cantilever allowed off the most distal implants bilaterally. Although the amount of cantilever is usually 2.5 times the A-P spread, the actual length of cantilever depends on factors such as stress, parafunctional habits, crown heights, implant width, number of implants, and the opposing teeth or denture.
The following is a list of guidelines that can be used to treatment plan implants for fixed or removable prosthetics. Generally, as the number of implants decease, the length of the cantilever also decreases and the amount of tissue support increases.
Edentulous/Fixed: 5 to 6 implants located between the mental foramen; implant supported, cementable, and screw retained; allows for a 15-mm cantilever.
Edentulous/Removable Overdenture: 5 to 6 implants located between the mental foramen; clip bar/implant supported; allows for a 15-mm cantilever.
Edentulous/Removable Overdenture: 5 implants located between the mental foramen; clip bar and tissue supported with distal bar attachment ERAs (Sterngold); allows for 10- to 20-mm cantilever.
Edentulous/Removable Overdenture: 4 implants located between the mental foramen; clip bar and tissue supported with O-rings (Attachments International) and/or distal ERAs; allows for 5 to 10 mm of cantilever. Four separate attachments can also be used without a bar.
Edentulous/Removable Overdenture: 3 implants located between the mental foramen; clip bar and tissue-supported O-ring attachments and/or distal ERAs. Three separate attachments can also be used without a bar.
Edentulous/Removable Overdenture: 2 implants in anterior; clip bar and tissue supported. Two separate attachments can be used without a bar.
Edentulous/Fixed: 8 implants dispersed among the molar areas; implant supported, cementable, or screw retained; cantilevers optional.
Edentulous/Removable Overdenture: 8 implants dispersed among the molar areas; clip bar and implant supported without a palate; cantilevers optional.
Edentulous/Removable Overdenture: 6 implants dispersed among the premolar areas; clip bar and implant supported without a palate; cantilever 5 to 10 mm.
Edentulous/Removable Overdenture: 4 implants dispersed among the first premolar areas; clip bar and tissue supported, O-ring and ERAs or ball attachments, palate optional; no cantilevers.
Treatment planning is not an easy task, and no single plan fits everyone. Over the years we have developed techniques that have ensured success in dentistry. It is important to understand that implant design and concepts will vary. Plan ahead for success. In most cases, fewer attachments are better.
Figure 1. Anterior-posterior spread.
Figure 2. Canted mandibular arch.
Figure 3. Alginate for custom tray.
Figure 4. Visualizing parallelism.
Figure 5. Impression copings in place.
Figure 6. Hader Bar with ERAs.
A 56-year-old woman with an unremarkable medical history was referred to the author for the restoration of 4 Nobel Biocare Replace Select implants. The implants were placed in the maxillary arch by an oral surgeon approximately 8 months previously. The patient’s maxillary arch had been edentulous for many years, and her mandibular arch revealed a full complement of teeth. The pa-tient’s lower occlusal plane had been canted approximately 3 mm noticeably to the left as a result of a traumatic accident 20 years ago (Figure 2). After a lengthy discussion of the treatment planning alternatives, including leveling the lower occlusal plane, the patient opted to leave the lower arch alone and accept the aesthetics as directed by the lower occlusal anatomy. The patient requested that her palate remain as open as possible, and she did not want full coverage of the palatal tissues. Palatal anatomy was broad based with a normal arch form and normal tuberosities. The implants were placed in the 5, 7, 10, and 12 positions with an anterior-posterior spread of approximately 10 mm. After screwing the transfer copings onto the implants, an alginate impression was taken to de-termine the angulations of the placed implants, which would serve to determine the type of attachments appropriate for use (Figure 3). The impression was then sent to Tetra Dynamics Laboratory in Babylon, NY, for the fabrication of a closed custom tray and case completion. Careful assessment of the implant placement revealed that the emergence of each of the implants was splayed buccally and angulated severely to each other (Figure 4). This placement causes path of in-sertion problems that can lead to wear and a lack of retention, should individual overdenture attachments be selected (Figure 5). It also creates a rigid prosthesis, as even the most resilient of attachments would be restricted in their functional movement, placing more load on the implants. The matter became more complicated since a primary stress-bearing area, the palate, was not planned for utilization. After careful evaluation, a bar was to be utilized as the attachment of choice.
Four implants in the maxilla are not considered sufficient to plan a prosthesis that is totally implant supported, especially against a full complement of mandibular teeth. The rationalized plan outlined to the laboratory was to create a totally tissue-borne prosthesis by using a castable Hader Bar (Sterngold) unit and 2 ERA attachments cantilevered off the distal ends of the bar. The bar provides a common path of insertion for the prosthesis and allows lateral forces to be transferred down the long axis of the implants. Bars provide splinting, which shares the occlusal load among the implants (Figure 6).
Figure 7. Ackerman clip with 0.6-mm spacer.
Figure 8. Ackerman clip with spacer on bar.
Figure 9. ERAs with black processing male.
Figure 10. ERAs with 0.4-mm resilient male.
Figure 11. No palate, metal-mesh framework.
Figure 12. Completed case.
The plan was for the anterior palate and the tuberosities to be the primary stress-bearing areas, and the bar and distal attachments would provide retention and resiliency only. To accomplish this, 3 Ackerman clips with 0.6-mm spacers were used (Attachments International, Sterngold) as clip bar connectors (Figures 7 and 8). This allows the prosthesis to have vertical and rotational resiliency and ensures soft-tissue support. Two ERA attachments were placed on the distal ends of the bar. The ERAs provide a vertical resiliency of up to 0.4 mm as well as hinge rotation (Figures 9 and 10). Custom tray transfer impressions were taken, and a soft-tissue master model was fabricated along with the overdenture fabricated in wax. The bar was then cast and verified for fit. The prosthesis was sent back to the lab for final fabrication. The bar and the ERAs provide a means of retention and resiliency, while the prosthesis is totally supported by the anterior palate and tuberosities. A horseshoe palate was utilized with a steel metal framework (Figure 11). The Ackerman clips and spacers and the ERA attachments provide vertical and rotational resiliency.
Upon insertion and subsequent postoperative visits, the patient expressed satisfaction and gratitude for having her expectations exceeded, and was extremely happy with her new look (Figure 12).
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Dr. Bambara is a graduate and faculty member of the University of Medicine and Dentistry of New Jersey. He is past president of the Richmond County Dental Society and holds Fellowships in the American College of Dentists, the International College of Dentists, and the International Academy of Dento-Facial Esthetics. He is an adjunct assistant professor at the College of Staten Island and is on the attending staff at Methodist Hospital in Brooklyn and Seaview Hospital in Staten Island. He lectures nationally and internationally on attachment prosthetics and has been selected as one of the Top Clinicians in Continuing Education for 2006 and 2007 by Dentistry Today magazine. He is a general practitioner with a private practice in Staten Island. He can be reached at (718) 356-9700.