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AFFORDABLE IMPLANT PROSTHETICS 

AFFORDABLE IMPLANT PROSTHETICS

 

 
 
Tags:  affordable dentures  cosmetic dentistry crowns  crown and bridge dentistry  crown dentistry 
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Published:  December 19, 2011
 
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Slide 1: A D J A  ✷ CON T ✷ IO N N U IN G ED U C ARTICLE 3 A T I AFFORDABLE IMPLANT PROSTHETICS USING A SCREWLESS IMPLANT SYSTEM NORMAN J. SHEPHERD, D.M.D. A B S T R A C T It is ironic that use of one of dentistry’s most important develop- Many dentists have been reluctant to place dental implants because they have found that most implants are costly and time-consuming to place and have longterm maintenance problems. Most of these problems are caused by using screws to connect the abutment to the implant, the crown to the abutment or both. The use of a screwless implant system and conventional prosthetics, the author contends, can make implant dentistry affordable, versatile and easy to incorporate into all general dental practices. ments of the last 15 years, predictable implant dentistry, is actually being discouraged by many dentists.1,2 Patients who want implants often have to search for dentists to provide this care. Predictable osseointegrated implants have been used in the United States since 1982, when the Nobelpharma implant (Brånemark system, Nobel Biocare, Gothenburg, Sweden) was introduced into North America. Unfortunately, many of the techniques associated with that implant made it difficult and costly to treat partially edentulous patients. The excessive chair time, component costs and laboratory expense have limited the use of implants to only a few dentists and relatively affluent patients. Although it has been reported that as many as 40, 50 or even 60 percent of general dentists are restoring implants, most of them have used implants in very few cases.3 It has been estimated that only 4,000 of the 110,000 general dentists in the United States restore four or more implant cases a year.4 In an attempt to promote affordable implant dentistry, and thereby encourage more restorative dentists to treat more of their average patients with implants, I reviewed the literature on the techniques being used, which perhaps were based more on historical use than dental common sense. With the use of a screwless implant system and elimination or modification of some of the old techniques, implant prosthetics can be just as affordable as conventional crown-and-bridge dentistry. OPERATIVE SETTING The first thing to examine is the reported need for sterility at the level found in hospital operating rooms, or ORs. Professor PerIngmar Brånemark, who introduced osseointegration techniques into North America, had a background as an orthopedist, and as such was used to operating under very sterile conditions in hospital ORs.5 It made sense to his team of surgeons and engineers to design an implant fixture consistent with their orthopedic experience, and to design a procedure for placing it that required an OR environment. However, this requirement created problems when dentists in 1732 JADA, Vol. 129, December 1998 Copyright ©1998-2001 American Dental Association. All rights reserved.
Slide 2: CLINICAL PRACTICE which allowed easy access to the individual fixtures for examination during the experimentation years. It also allowed easy access to the stacked components for maintenance purposes. Unfortunately, screw-retained prosthetics created a multiplicity of problems, such as screw loosening and breakage.7-13 Screws are time-consuming to place, costly and unpredictable. Connecting a crown to an implant has to be one of the most basic acts of implant dentistry; yet, after 15 years, we still see journal articles about how to keep screws tight. This fact alone raises questions about the entire process. If screws are used, the dentist needs not only a variety of screwdrivers, but also expensive torque drivers. An increasing number of clinicians have begun to realize that use of cement-retained prosthetics is a better technique. One large commercial laboratory in the United States sent letters to all of its dentist customers explaining that it would no longer fabricate implants that used screw-retained prosthetics (James R. Glidewell, written communication, Oct. 5, 1993). There are many advantages to cement-retained crowns, such as the ability to maintain normal anatomy, improved esthetics and compensation for casting inaccuracies. On the other hand, there are many factors that cause implant screws to loosen: dpoor occlusion; dparafunctional habits; dcantilevers; dpoor component tolerances; dinaccurate castings; dinadequate tightening; dtoo much tightening; dsingle tooth torque. 1733 Figure 1. Cross-section of the Bicon implant (Bicon Implant System) demonstrating locking taper. Compare with conventional implant components. Figure 2. Bicon abutment ready for final impression. Note the soft-tissue maturation and the preformed gingival sulcus. the United States attempted to duplicate their suggestions. Most health insurance programs would not cover hospitalization for dental implants, and converting a typical dental operatory into an OR is extremely time-consuming and expensive. In 1993, a group from New York University published an ar- ticle demonstrating that implants could be placed successfully in a dental office or a clinic.6 This eliminates the need for the more costly hospital OR setting. PROBLEMS WITH SCREWRETAINED PROSTHETICS The Brånemark group used screw-retained prosthetics, JADA, Vol. 129, December 1998 Copyright ©1998-2001 American Dental Association. All rights reserved.
Slide 3: CLINICAL PRACTICE The literature indicates that the main advantage of screw-retained prosthetics is retrievability, but bridgework retained by temporary cement is much more easily retrievable than a prosthesis retained by multiple screws. Since neither caries nor sensitive dentin is a problem if cement washes out, there is no real reason not to use temporary cement. Use of cementable prosthetics for treating natural teeth is the standard in dental schools, and dentists are familiar and comfortable with this approach. Screw-retained prosthetics require totally different techniques and instrumentation that must be learned if a dentist wishes to practice implant dentistry. PROBLEMS WITH EXTERNAL HEX IMPLANTS Figure 3. Crown cemented on Bicon abutment with either temporary or permanent cement. Another concern is the external hex-top implant, a configuration that has been copied by more than a dozen implant companies and has become the industry standard for prosthetic attachments. Unfortunately, this design has two inherent problems: metal dilation and embedment relaxation. Metal dilation. The hex top was designed as a rotational torque transfer mechanism to turn the implant into the bone, and it successfully performs that task.14 Unfortunately, the classical external hex is a poor way to connect other components. It is not a true antirotational device, and this leads to micromotion, bacterial leakage and bone cratering.15,16 The 0.7-millimeter height of the hex gives very little protection against lateral forces, and the thinness of the hex walls makes them subject to metal dilation, which in turn creates looseness at the top threads. 1734 This looseness is a predictor of broken screws. The distance across the flats of the external hex is 2.68 millimeters, and the diameter of the internal thread that passes through the top is 2.02 mm. The distance across the flats should be greater than 3.03 mm to prevent the hex wall from dilating. Unfortunately, this dilation, which is caused by the wedging action of the screw as it is tightened, remains throughout the life of the implant.17 Embedment relaxation. A second problem is embedment relaxation. When two machined components are screwed together with a torque wrench, it will tighten to that particular torque. After a few minutes, the clinician will find that he or she can turn the wrench again slightly to reach that same torque. What is happening is that the slight machining irregularities of the surfaces actually flatten and allow the two components to be more closely approximated. This flattening or creep continues under occlusal forces; therefore, the preload that originally had been used when tightening was first accomplished loses its effect and contributes to broken screw problems.18 COST-EFFECTIVE TECHNIQUES FOR IMPLANT DENTISTRY Eliminating these expensive problems is one part of creating costeffective implant dentistry. The other part is to use less expensive techniques for fabricating prosthetics. The first prerequisite, then, is to use a true nonrotational system. Most of these have some sort of a bevel at the abutment-implant interface to provide good metal-to-metal contact. Friction between the two surfaces prevents the rotation. Some systems use screws to hold the components together. One system, the Bicon Implant (Bicon Implant System), uses a locking taper that requires no screw at the abutment-implant interface (Figure 1). This is a true nonrotational connection that has been used in or- JADA, Vol. 129, December 1998 Copyright ©1998-2001 American Dental Association. All rights reserved.
Slide 4: CLINICAL PRACTICE TABLE COMPARISON OF LABORATORY COSTS. BICON IMPLANT SYSTEM Implant Type Bicon Bar* O-rings: implants O-rings: implants $190 $80 ($40 if done chairside) $160 ($80 if done chairside) Cost OTHER IMPLANT SYSTEMS Implant Type Cast Overdenture Bar Two implants Four implants $450-$1,000 $900-$1,400 Cost Single Crown Porcelain fused to metal $80-$130 (lab fees same as those for natural teeth) Single Crown on Hex Top Cera One-type abutment (Nobel Biocare Inc.) UCLA $500-$700 $300-$400 * Bicon Implant System. thopedic hip replacements for many years. It also is used to hold the chuck to the shaft of the dental lathe. This locking taper allows no micromotion and also provides a bacterial seal.16 When a true nonrotational abutment such as this is used, the final abutment can be placed during second-stage surgery, and it then is treated like a natural tooth. There is no need for healing caps, impression posts or transfer copings. This eliminates a great deal of inventory and extra office visits. If the abutment needs modification, it can be done either out of the mouth before placement or in the mouth after placement. A number 1557 carbide bur (S.S. White Burs Inc.) cuts the titanium very effectively with minimum heat.19 A temporary crown can be placed at the secondstage visit if needed (areas that are not esthetically important require no temporary crown). The final impression is taken after soft-tissue maturation has occurred, and the crown is then cemented in place (Figures 2 and 3). Temporary cement is used for large bridges; permanent cement generally is used for single-tooth implants. The laboratory bills will be the same as those for natural tooth pros- The ability to use conventional prosthetic techniques to restore the implant abutment is critical for truly cost-effective implant dentistry. thetics, and the chair time is actually less than that for the fabrication of a crown for a natural tooth. The ability to use conventional prosthetic techniques to restore the implant abutment is critical for truly cost-effective implant dentistry. The restorative dentist does not need to purchase an implant prosthetic kit. All of the necessary restorative procedures are familiar to the dentist, the office staff and the dental laboratory. Ironically, the only other screwless implant systems that can be used in this manner are those that were used in the 1960s and 1970s. These systems used one-piece implants, such as blades, screws and subperiosteal implants, in which the abutment was part of the implant and therefore treated similarly to a natural tooth. Unfortunately, osseointegration was not as predictable as it was with the two-stage implant systems or the more recent onestage unloaded systems. Modern implant manufacturers are aware of the problem and have tried to find an answer. The ITI implant (Strauman) and the Astratech implant systems (Astratech) use a beveled abutment-implant interface to address the antirotational problem. The Calcitek (Sulzer Calcitek) spline has a mechanical interlock, and Screw-Vent (Paragon) has an internal hex with a slight taper lock. All of these implant systems, however, still require a screw to mechanically clamp the parts together. In the case of an edentulous patient, the nonrotational nature of these abutments eliminates the need for a cast bar, and this 1735 JADA, Vol. 129, December 1998 Copyright ©1998-2001 American Dental Association. All rights reserved.
Slide 5: CLINICAL PRACTICE in turn allows individual balls or O-ring abutments to be used. If a patient can afford the surgery necessary to place two implants, then the prosthetic cost of an Oring–retained overdenture is minimal. These are simpler techniques with significantly lower laboratory cost (Table). Much of the fabrication can be done at chairside if the patient already has a well-made denture.20 The locking taper shaft of the Bicon Implant allows for the use of 15degree O-ring abutments, which is a considerable help in treating maxillary cases. If a patient already has denture support of two balls or O-rings, the dentist can later add additional implants and convert the overdenture to fixed bridgework, even if the patient has to wear a laboratorycured provisional bridge for a year or two before he or she can afford the final metal-porcelain bridge. In other words, definitive treatment does not have to be done at one time. A patient can start with a relatively inexpensive treatment and then, as he or she can afford it, graduate to a more ideal fixed prosthesis. A true overdenture should be soft-tissue–borne and retained only by the mechanical device attached to the implant. For this reason, it is not necessary to link the implants. Such linkage certainly is not needed to prevent rotation of the individual abutments. The flexibility of these abutments allows a regular menu of treatment options that permit implant dentistry to become affordable to the majority of dental patients, instead of just the wealthy. The single-tooth implant restoration—which in many systems is extremely difficult, if not impossible, to place in posterior areas because of the 1736 A B Figure 4. A. Four individual premolar and molar crowns—three on Bicon abutments and the anterior crown on a natural tooth. B. Radiographic appearance. rotational torque—becomes a very simple post-and-core prosthetic restoration with screwless implants. Single-molar implants can be restored and several single crowns can even be placed side by side (Figure 4). This obviates the problem of inaccurate casting for multiple crowns. It more closely duplicates normal anatomy and allows the patient the luxu- ry of normal hygiene techniques. Unlike a natural-tooth crown, the margin of an implant crown needs to be placed subgingivally only in esthetically important areas. The casting can be at or above the gingival level in the mesial, distal or palatal/lingual areas (Figure 5). The single-tooth implant restoration allows the cost of a single implant and crown to be JADA, Vol. 129, December 1998 Copyright ©1998-2001 American Dental Association. All rights reserved.
Slide 6: CLINICAL PRACTICE competitive with that of a threeunit bridge. If a dental practice has a prosthetic unit charge of $600, a three-unit bridge would cost $1,800. The single implant can be placed for $900 to $1,100, and the crown would cost $600, the same as that for a crown on a natural tooth; therefore, the implant and crown would cost $1,500 to $1,700. This allows implant dentistry to be affordable to most dental patients. Because of the flexibility of angled abutments, full-arch fixed bridges also can be created with abutments prepared for cemented prostheses. These angled abutments can be rotated a full 360 degrees, and it is therefore relatively easy to mix straight and angled abutments to achieve parallelism. A very important requirement of cost-effective implant dentistry is that the implant is placed in the proper position. To do this, an accurate, userfriendly stent or stents should be used. I recommend that a palatal or lingual stent be used to position the implant, and a vacu-press clear stent be used to ascertain the position of the abutment. In most stents that are discussed in the literature, either there is an occlusal positional hole or the buccal cusps have been retained. This type of stent looks very good on a laboratory bench, but once in the mouth it often obstructs the surgeon’s vision and prevents him or her from seeing the tip of the guide bur and the bone at the same time. When a surgeon is uncomfortable with the amount of visibility, he or she often will not use the stent that has been made. The stents I recommend are made from waxed-up articulated study models, because they mechani- Figure 5. Schematic showing that the crown casting is subgingival only in the esthetic facial area. It may be at or above the gingival margin in the mesial, distal or palatal/lingual areas. cally direct the surgeon’s bur, and the implant will be in the precise location that the restorative dentist requires. Such a stent is shown in The stents I recommended are made from waxed-up articulated study models, because they mechanically direct the surgeon’s bur, and the implant will be in the precise location that the restorative dentist requires. Figure 6, along with the outcome of the treatment. SUMMARY ever, to perform implant prosthetics in a cost-effective manner, which in turn greatly increases the availability of the treatment modality to the general dental population. By combining a screwless implant system with simple and familiar prosthetic techniques, all restorative dentists can provide care for their patients who may benefit from implant dentistry. The four main prerequisites for cost-effective implant dentistry are as follows: dplace the implants in the proper position; duse nonrotational abutments; dprovide for the patient’s financial and functional needs; duse conventional dental prosthetics. s Dr. Shepherd is in private practice with Northern Essex Oral Surgery Associates Inc., 390 Water St., Haverhill, Mass. 01830. Address reprint requests to Dr. Shepherd. Dr. Shepherd holds an equity interest in Bicon Implant System. 1. Christensen G. Bicon Dental Implants. Clin Res Assoc Newsletter 1996;20(12):2. 2. Park N. Implants now simpler. Nobel Biocare Update 1997;8(3):2. For the most part, implant dentistry today is more expensive and time-consuming than conventional crown-and-bridge prosthetics. It is possible, how- JADA, Vol. 129, December 1998 Copyright ©1998-2001 American Dental Association. All rights reserved. 1737
Slide 7: CLINICAL PRACTICE sterile versus clean conditions. J Periodontol 1993;64:954-6. 7. Zarb GA, Schmitt A. The longitudinal clinical effectiveness of osseointegrated dental implants: The Toronto Study. Part III: problems and complications encountered. J Prosthet Dent 1990;64:185-94. 8. Jemt T. Failures and complications in 391 consecutively inserted fixed prostheses supported by Brånemark implants in edentulous jaws: a study of treatment from the time of prosthesis placement to the first annual checkup. Int J Oral Maxillofac Implants 1991;6:270-6. 9. Tolman DE, Laney WR. Tissue-integrated prosthesis complications. Int J Oral Maxillofac Implants 1992;7:477-84. 10. Naert I, Quirynen M, Theuniers G, van Steenberghe D. Prosthetic aspects of osseointegrated fixtures supporting over-dentures: a 4-year report. J Prosthet Dent 1991;65:67180. 11. Jemt T, Linden B, Lekholm U. Failures and complications in 127 consecutively placed fixed partial prostheses supported by Brånemark implants: from prosthetic treatment to first annual checkup. Int J Oral Maxillofac Implants 1992;7:40-4. 12. Naert I, Quirynen M, van Steenberghe D. A six-year prosthodontic study of 509 consecutively inserted implants for the treatment of partial edentulism. J Prosthet Dent 1992;67:236-45. 13. Kallus T, Bessing C. Loose gold screws frequently occur in full-arch fixed prostheses supported by osseointegrated implants after 5 years. Int J Oral Maxillofac Implants 1994;9:169-78. 14. Beaty K. The role of screws in implant systems. Int J Oral Maxillofac Implants 1994; 9(special supplement):52-4. 15. Balfour A, O’Brien GR. Comparative study of anti-rotational single tooth abutments. J Prosthet Dent 1995;73(1):36-43. 16. Muftu A, Mulcahy HL, Chapman R. Comparison of Streptococcus sanguis penetration through various implant connection mechanisms (abstract 585). J Dent Res 1997;76(special issue):87. 17. Blake A. What every engineer should know about threaded fasteners. New York: Marcel Dekker; 1986:32-5. 18. Dixon DL, Breeding LC, Sadler JP, McKay ML. Comparison of screw loosening, rotation, and deflection among three implant designs. J Prosthet Dent 1995;74:270-8. 19. Gros M, Laufer BZ, Oriniamar Z. An investigation on heat transfer to the implantbone interface due to abutment preparation with high-speed cutting instruments. Int J Oral Maxillofac Implants 1995;10:207-12. 20. Shepherd N. A general dentist’s guide to proper dental implant placement from an oral surgeon’s perspective. Compend Contin Educ Dent 1996;17(2):118-30. A B Figure 6. A. Panoramic radiograph of the mouth in the finished case. B. Clinical view of the mouth in the finished case. 3. Watson MT. Implant dentistry: a tenyear retrospective. Dental Products Report 1996;30(12):26-31. 4. Dental implants: emerging technology trends and oral and maxillofacial and periodontal surgery. Irvine, Calif.: Medical Data International Inc.; 1993:1-27. 5. Johansson F. Brånemark System: surgical operatory set-up procedures. Westmont, Ill.: Nobelpharma USA Inc.; 1994. 6. Scharf DR, Tarnow DP. Success rates of osseointegration for implants placed under 1738 JADA, Vol. 129, December 1998 Copyright ©1998-2001 American Dental Association. All rights reserved.

   
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