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A SCREW LOCK FOR SINGLE-TOOTH IMPLANT SUPERSTRUCTURES
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ARTICLE 2
A SCREW LOCK FOR SINGLE-TOOTH IMPLANT SUPERSTRUCTURES ZVI ARTZI, D.M.D.; ARIE DREIANGEL, D.M.D.
The predictabili-
secure the prosA B S T R A C T ty of long-lasting thetic superstrucsuccess of osseointure on the imBackground. The most common complication in a sintegrated implants plant in a stable gle-tooth implant restoration is abutment screw loosening. was originally position. Instability of the prosthetic superstructure is expressed by diffibased on their use culty in chewing and functioning, as well as soft-tissue soreness MATERIALS in edentulous paand/or swelling that could lead to screw fracture. Manufacturers AND METHODS tients who had reof oral implants have attempted to refine the connecting parts of movable or fixed, The most common the prosthesis to achieve a more predictable tightening method detachable applitype of oral imfor the screws. ances. For partialplant is a 3.75-milMethods. To maintain the abutment screw tightly in its ly edentulous palimeter root-form correct position, the authors developed a technique in which an tients, screwscrew-type implant elongated hexagonal titanium bar is inserted into the hexed fixed retained fixed with an external screw head. The screw is locked, and the bar is then fixed with a prostheses became light-cured composite resin material that serves to seal the retain- hex head, which a popular form of ing screw access hole. The occlusal hexagonal bar thus serves as a should stabilize prosthetic reconand verify the acsecure screw lock that can be easily removed if needed. struction. Prosthocurate position of Results. The authors have used the hexagonal bar for aldontic complicamost three years on 120 single-tooth screw-retained prostheses in its prosthetic comtions arise, 100 patients (65 in the first and second premolar region, 40 in the ponent. In this arhowever, when sin- incisor region and 15 in the posterior molar region). All of these ticle, we describe a gle-tooth implants prostheses functioned successfully, including those with wider oc- technique to enare placed. The sure prosthetic suclusal planes and increasing occlusal forces. No screw loosening mechanical type of perstructure stabilor fractures were noted in any of the fixtures. screw-retained sinClinical Implications. This technique secures and ity based on gle-tooth restorainterlocking the instabilizes the single-tooth prosthesis, reduces chair time on foltion under functernal hexed fixalow-up procedures and reduces unnecessary frustration in pation, which has to tion screw head tients and dental team members. withstand rotationwith the hexagonal bar. al forces, often exThe armamentarium consists of a titanium periences loosening of the screw abutment.1-5 hexagonal bar, a custom-fit rubber band, a modiAttempts have been made to solve this probfied female 1.27-mm hexagonal drive key and the lem,2,6,7 but it is still a persistent obstacle in implant manufacturer’s fixation screw (Figure 1). achieving success. The bar should fit precisely and firmly into the About three years ago, we developed a hexagoinner (female) fixation screw. Before the screw is nal bar that interlocks with the fixation screw to JADA, Vol. 130, May 1999 Copyright ©1998-2001 American Dental Association. All rights reserved.
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CLINICAL PRACTICE prostheses were initially placed at the required occlusal equilibration and with the fixation screw tightened; however, the hexagonal bar was not inserted for one month to allow any undesirable vertical and rotational forces to be detected and eliminated. RESULTS
Figure 1. Components for the screw lock technique are shown: a 1.27millimeter titanium hexagonal bar (hb), an isolating rubber band (rb), the superstructure fixation screw (fs) and the hexagonal bar carrier (hbc).
fastened, the dentist verifies that the prosthetic superstructure is passively stabilized by the external hex connection; he or she then inserts the fixation screw and secures it with the drive key. Before the prosthesis is placed, the unit is tested with the hexagonal bar and fixation screw inserted into the working model (Figure 2). The dentist positions the single-tooth implant prosthesis passively on the osseointegrated implant and tightly secures it with the screw according to the manufacturer’s recommended torque force. An orthoradial periapical radiograph should verify its position. The hexagonal bar is then inserted securely into the fixation screw. The custom-fit rubber band, serving as an isolating sleeve layer, is placed next to the bar and the screw head connection. This prevents the composite layer on top from penetrating into the hexed head screw. The dentist then applies the light-polymerizing composite resin filler. Once set, it will 678
serve as a fixative material for the bar, as well as an access hole sealer. Figure 3 is a schematic drawing of the fixation screw and hexagonal bar in the interlocking position, em-
Once set, the composite resin will serve as a fixative material for the bar, as well as an access hole sealer. braced by the isolated and hardened resin layer. In the final steps, the dentist cuts the bar down to the occlusal crown level, smooths it and polishes it (Figure 4). The hexagonal bar cannot be rotated or moved and the screw is locked securely in place. During follow-up visits, the composite resin material can be removed with a high-speed turbine, and the screw and prosthetic superstructure retrieved. We should point out that the
We placed a total of 120 singletooth implant screw-retained prostheses in 100 patients (60 women, 40 men), ranging in age from 19 to 61 years. Sixty-five of these prostheses were placed in the first and second premolar region, 40 in the incisor region and 15 in the posterior molar region. The distribution of implants in the mandible and maxilla was equivalent. During monthly follow-up visits, we examined the patients and found all of the prostheses to be securely in place. They all functioned successfully, especially those in the molar region, with wider occlusal planes and greater occlusal forces. We have used this technique for almost three years with no complications, including screw loosening and fractures. DISCUSSION
Several multicenter prospective studies have examined prosthetic appliances supported by osseointegrated implants and their associated complications and outcomes.2,3,8-11 Complications such as fractured acrylic resin materials, soft-tissue problems, oral hygiene difficulties, temporomandibular symptoms, loosening of superstructures, decreased occlusal vertical dimension and increased salivation have been observed.11 Carlson and Carlsson11 stated that a maintenance pro-
JADA, Vol. 130, May 1999 Copyright ©1998-2001 American Dental Association. All rights reserved.
CLINICAL PRACTICE
Figure 2. The hexagonal titanium bar is tested on the prosthetic superstructure on the working model.
gram, consisting of total occlusal equilibration and prosthetic stability, is essential. Jemt8 suggested that at least five to seven appointments are needed during the first year after the implant has been placed to verify stability. Neither the study by Carlson and Carlsson11 nor the one by Jemt8 mentioned using a singletooth restoration. In the past several years, the use of screw-retained prostheses over osseointegrated implants to replace single teeth has grown in popularity.1-5,12-15 Difficult complications, unlike those encountered in the traditional Bränemark reconstruc-
Figure 3. This schematic drawing illustrates all components in their final position. The custom-fit rubber band (rb) serves as an inner isolating sleeve layer to prevent undesired penetration of the composite (co) resin onto the hexed head screw. The composite resin secures the hexagonal bar in position.
tion,8-11 have been the focus of attention by researchers. In a multicenter study, Laney and colleagues3 placed single-implant prostheses in 82 patients and screened them for three years. They recorded several types of complications, such as abutment screw fracture, softtissue penetration, mucosal inflammation and screw loosening. Prosthetic superstructure loosening occurred in 10 patients. After one year, this number had significantly reduced. Use of gold screws. By replacing titanium abutment screws with gold ones, Laney and colleagues thought the
problem of screw loosening had been eliminated.3 Using a nonlinear finite contact analysis, Sakaguchi and Borgersen16 determined that this conclusion is incorrect. In a recent study,17 15 percent of titanium and gold screws became loose during initial loading and at the first follow-up visit. Similar findings were found for full-arch fixed prostheses.18 The process of fastening the screw inside the implant body results in a secure butt-joint connection between the prosthesis and the implant. This connection produces a load, which
JADA, Vol. 130, May 1999 Copyright ©1998-2001 American Dental Association. All rights reserved.
679
CLINICAL PRACTICE
Figure 4. The light-cured composite layer seals the access hole and locks the hexagonal bar. After the composite resin is placed and hardens, the bar is cut and smoothed to the proper occlusal level.
is known as preload.19 A properly preloaded abutment screw should be strong enough to resist torques from occlusal loadings for implant-supported prostheses. However, according to several reports, screws have loosened in 15 to 57 percent of single-tooth implant restorations.1,2,4,5,12,15,17 In extreme instances, abutment screws have even fractured.3,12,15 Abutment screw loosening occurs because of improper occlusion and/or excessive loading. When it occurs, patients complain of soreness at the interface between the soft tissue and the fixture, swelling and/or fistula formation, difficulty in chewing and prosthetic instability because of poor contact between the implant neck and its prosthetic component (as a result of a weak fit in the interface zone of the prosthesis and the implant).20 Causes of loose superstructures. A loose superstructure may result from an inadequate framework design. Essentially, the machining accuracy of the implant and its 680
prosthetic superstructure, especially between the patrix (the prosthetic recess that fits precisely around the external hex) hexagonal extension and the matrix (the external hex of the implant neck) hexagonal abut-
Abutment screw loosening occurs because of improper occlusion and/or excessive loading. ment, should be observed by the research team.21,22 Dentists may also encounter rough surfaces on parts of the prosthesis, settling effects of the implant– prosthetic superstructure connection, mechanical fatigue resistance of the metal portion of the fixation screw, persistent torque and micromovement forces.6,7,23 Binon21,22 examined the effect of the relationship between the abutment and implant on the stability of the superstructure.
He found a direct correlation between the hexagonal misfit of the external hex of the implant neck and its female abutment recess, as well as screw joint loosening; the greater the misfit, the greater the probability of screw loosening. Conversely, a small hexagonal misfit would result in increased surface contact area between the external implant hex and its female abutment recess; consequently, a greater number of rotational cycles would be required for the screw to loosen. A rotational misfit of less than 2 degrees provides the most stable and predictable screw joint. Prosthetic modifications. Manufacturers of oral implants have attempted to refine the prosthetic connecting parts to achieve a more predictable method of tightening the screws. One system (Paragon Implant Surgical System, Paragon) has a Friction Lock design that claims to achieve superstructure stability,24 but it did not demonstrate superiority over other implants in an objective comparative study.7 Furthermore, a cross-sectional scanning electron microscopic view clearly demonstrated an interface discrepancy that reached 75 micrometers (mean discrepancy, 45 µm).22 Arvidson and colleagues25 conducted a clinical study of an abutment design that was introduced in the early 1990s. The conical seal design of this system (Astra Tech, Molndal) demonstrated increased resistance to bending movements at the implant-abutment interface, compared with results for the regular external hex connection.26 Consequently, screw loosening should be alleviated with a one-cone screw
JADA, Vol. 130, May 1999 Copyright ©1998-2001 American Dental Association. All rights reserved.
CLINICAL PRACTICE connection as an abutment that fits into the implant rather than on top of the external hex implant body. Another new dental implant system (Spline, Calcitek) produces a stable loading platform and high-fidelity fatigue strength.27 Longterm studies of both systems are needed to substantiate their promising mechanical properties. However, both exhibited minimal rotational movement. The question is whether minimal rotational movement (that is, almost complete rigidity) could jeopardize osseointegration via excessive forces and trauma on the surrounding living hard tissues. Other modifications, such as flat-headed titanium screws14 and gold screws2,6 reduce, but do not eliminate, the problem of screw loosening. Some studies15,18 on gold abutment screws actually have demonstrated many incidents of screw loosening. Jorneus and colleagues6 suggested that the screw be elongated to achieve better stability, but objective mechanical limitations resulting from the limited number of screw threads dictate that the solution be found elsewhere. A modified single-tooth implant restoration is the CeraOne system (Nobel Biocare).28 In this system, the post is shaped and then cast before the screw is fastened to the implant body. However, a mechanical testing study29 demonstrated that the abutment screw in that system was the weakest link in the implant post. Most studies6,13,14 emphasize the need for total equilibrium of occlusion and a more predictable tightening method for screws to withstand the occlusal forces.
Figure 5. A single-molar implant prosthesis and screw lock with hexagonal bar that has functioned for two years.
Hexagonal bar. The technique we have described above solves the problem of screw loosening. It can be used in areas of dominant occlusal forces, such as the first molar region (Figure 5). However, this technique is effective only after undesirable vertical and rotational forces have been detected and eliminated. To accomplish this, the metal bar is inserted into the fixation screw one month after the screw-retained prosthesis has been placed; this period serves as an initial diagnostic function-
Dr Artzi is a lecturer and director of the
Dr. Dreiangel is an in-
Graduate Program in
structor and coordi-
Periodontology,
nator of the
Department of
Undergraduate
Periodontology, The
Clinics in
Maurice and Gabriela
Periodontology,
Goldschleger School
Department of
of Dental Medicine,
Periodontology, The
Ramet Aviv, Tel Aviv
Maurice and Gabriela
University, Tel Aviv
Goldschleger School
169978, Israel.
of Dental Medicine,
Address reprint re-
Tel Aviv University,
quests to Dr. Artzi.
Tel Aviv, Israel.
al phase. Because the fixation screw did not loosen, the forces were transferred to the matrix/patrix connection or even to the implant/bone interface (that is, the zone of osseointegration). Nevertheless, no disintegration and/or other deterioration was observed on these single-tooth interlocking screw units. Ease of maintenance. Since the intercuspal peaks of the occlusal premolar/molar plane and/or lingual aspect of the anterior crowns range from 5 to 9 mm, the occlusal access hole should not exceed 4 mm in diameter. Thus, the 2-mm hexagonal bar can be easily inserted, and the hardened lightcured composite resin material can be removed with a small round high-speed bur. On routine maintenance visits, when the composite material is removed, the bar is securely positioned at the hexed screw head, and the dentist can gently remove it with forceps. Since most reports of screw loosening2,3-5,15,17,18 occurred during the first year after place-
JADA, Vol. 130, May 1999 Copyright ©1998-2001 American Dental Association. All rights reserved.
681
CLINICAL PRACTICE ment, the efficacy of the hexagonal bar technique, which has been used successfully for almost three years, has been shown. The securing system locks the antirotational superstructure in place and complications, such as soft-tissue soreness, fistula formation and screw fracture, are avoided. We should emphasize that use of the hexagonal bar does not reduce the need for frequent maintenance visits or ensure proper functional occlusion. However, this technique secures and stabilizes the single-tooth prosthesis, reduces chair time for follow-up procedures and eliminates unnecessary frustration in patients and dental team members. CONCLUSION
We have presented a simple method of securing the singletooth implant prosthesis by locking a hexagonal bar into the hex head of the fixation screw and sealing the access hole with a light-cured composite resin. The technique has been used successfully for almost three years on 120 implants in 100 patients. During the follow-up period, we found no instances of loosened screws. This technique enables a single-implant prosthetic superstructure to function successfully over the long term and, at the same time, to be easily retrieved. ■ The authors thank Rita Lazar for her editorial assistance and Rellu Samuel for providing photographs.
682
1. Jemt T, Lekholm U, Grondahl K. A 3year follow-up study of early single implant restorations ad modum Bränemark. Int J Periodontics Restorative Dent 1990;10:341-9. 2. Jemt T, Laney WR, Harris D, et al. Osseointegrated implants for single tooth replacement: a 1-year report from a multicenter prospective study. Int J Oral Maxillofac Implants 1991;6(1):29-36. 3. Laney WR, Jemt T, Harris D, et al. Osseointegrated implants for single-tooth replacement: progress report from a multicenter prospective study after 3 years. Int J Oral Maxillofac Implants 1994;9(1):49-54. 4. Ekfeldt A, Carlsson GE, Borjesson G. Clinical evaluation of single-tooth restorations supported by osseointegrated implants: a retrospective study. Int J Oral Maxillofac Implants 1994;9(2):179-83. 5. Haas R, Mensdorff-Pouilly N, Mailath G, Watzek G. Bränemark single tooth implants: a preliminary report of 76 implants. J Prosthet Dent 1995;73(3):274-9. 6. Jorneus L, Jemt T, Carlsson L. Loads and designs of screw joints for single crowns supported by osseointegrated implants. Int J Oral Maxillofac Implants 1992;7(3):353-9. 7. Dixon DL, Breeding LC, Sadler JP, McKay ML. Comparison of screw loosening, rotation, and deflection among three implant designs. J Prosthet Dent 1995;74(3):270-8. 8. Jemt T. Failures and complications in 391 consecutively inserted fixed prostheses supported by Bränemark 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(3):270-6. 9. 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(2):185-94. 10. van Steenberghe D, Lekholm U, Bolender C, et al. Applicability of osseointegrated oral implants in the rehabilitation of partial edentulism: a prospective multicenter study on 558 fixtures. Int J Oral Maxillofac Implants 1990;5(3):272-81. 11. Carlson B, Carlsson GE. Prosthodontic complications in osseointegrated dental implant treatment. Int J Oral Maxillofac Implants 1994;9(1):90-4. 12. Henry PJ, Laney WR, Jemt T, et al. Osseointegrated implants for single-tooth replacement: a prospective 5-year multicenter study. Int J Oral Maxillofac Implants 1996;11(4):450-5. 13. Avivi-Arber L, Zarb GA. Clinical effectiveness of implant-supported single-tooth replacement: the Toronto study. Int J Oral Maxillofac Implants 1996;11(3):311-21. 14. Lekholm U, Jemt T. Principles for single tooth replacement. In: Albrektsson T, Zarb GA, eds. The Bränemark osseointegrated im-
plant. Chicago: Quintessence; 1989:117-26. 15. Becker W, Becker BE. Replacement of maxillary and mandibular molars with single endosseous implant restorations: a retrospective study. J Prosthet Dent 1995;74(1):51-5. 16. Sakaguchi RL, Borgersen SE. Nonlinear finite element contact analysis of dental implant components. Int J Oral Maxillofac Implants 1993;8(6):655-61. 17. Wie H. Registration of localization, occlusion and occluding materials for failing screw joints in the Bränemark implant system. Clin Oral Implants Res 1995;6(1):47-53. 18. 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(2):169-78. 19. Carr AB, Grunski JB, Hurley E. Effects of fabrication, finishing, and polishing procedures on preload in prostheses using conventional gold and plastic cylinders. Int J Oral Maxillofac Implants 1996;11(5):589-98. 20. Weinberg LA. The biomechanics of force distribution in implant-supported prostheses. Int J Oral Maxillofac Implants 1993;8(1):1931. 21. Binon PP. The effect of implant/abutment hexagonal misfit on screw joint stability. Int J Prosthodont 1996;9(2):149-60. 22. Binon PP. Evaluation of machining accuracy and consistency of selected implants, standard abutments, and laboratory analogs. Int J Prosthodont 1995;8(2):162-78. 23. Burguete RL, Johns RB, King T, Patterson EA. Tightening characteristics for screw joints in osseointegrated dental implants. J Prosthet Dent 1994;71:592-9. 24. Balfour A, O’Brien GR. Comparative study of antirotational single tooth abutments. J Prosthet Dent 1995;73(1):36-43. 25. Arvidson K, Bystedt H, Frykholm A, von Konow L, Lothigius E. A 3-year clinical study of Astra dental implants in the treatment of edentulous mandibles. Int J Oral Maxillofac Implants 1992;7(3):321-9. 26. Norton MR. An in vitro evaluation of the strength of an internal conical interface compared to a butt joint interface in implant design. Clin Oral Implants Res 1997;8(4):290-8. 27. Binon PP. The spline implant: design, engineering and evaluation. Int J Prosthodont 1996;9:419-33. 28. Andersson B, Odman P, Carlsson L, Bränemark PI. A new Bränemark single tooth abutment: handling and early clinical experiences. Int J Oral Maxillofac Implants 1992;7(1):105-11. 29. Andersson B, Odman P, Boss A, Jorneus L. Mechanical testing of superstructures on the CeraOne abutment in the Bränemark system. Int J Oral Maxillofac Implants 1994;9:665-72.
JADA, Vol. 130, May 1999 Copyright ©1998-2001 American Dental Association. All rights reserved.
D
J
A
✷
IO N
A
T
T
CON I
N U
IN G
ED
U
ARTICLE 2
A SCREW LOCK FOR SINGLE-TOOTH IMPLANT SUPERSTRUCTURES ZVI ARTZI, D.M.D.; ARIE DREIANGEL, D.M.D.
The predictabili-
secure the prosA B S T R A C T ty of long-lasting thetic superstrucsuccess of osseointure on the imBackground. The most common complication in a sintegrated implants plant in a stable gle-tooth implant restoration is abutment screw loosening. was originally position. Instability of the prosthetic superstructure is expressed by diffibased on their use culty in chewing and functioning, as well as soft-tissue soreness MATERIALS in edentulous paand/or swelling that could lead to screw fracture. Manufacturers AND METHODS tients who had reof oral implants have attempted to refine the connecting parts of movable or fixed, The most common the prosthesis to achieve a more predictable tightening method detachable applitype of oral imfor the screws. ances. For partialplant is a 3.75-milMethods. To maintain the abutment screw tightly in its ly edentulous palimeter root-form correct position, the authors developed a technique in which an tients, screwscrew-type implant elongated hexagonal titanium bar is inserted into the hexed fixed retained fixed with an external screw head. The screw is locked, and the bar is then fixed with a prostheses became light-cured composite resin material that serves to seal the retain- hex head, which a popular form of ing screw access hole. The occlusal hexagonal bar thus serves as a should stabilize prosthetic reconand verify the acsecure screw lock that can be easily removed if needed. struction. Prosthocurate position of Results. The authors have used the hexagonal bar for aldontic complicamost three years on 120 single-tooth screw-retained prostheses in its prosthetic comtions arise, 100 patients (65 in the first and second premolar region, 40 in the ponent. In this arhowever, when sin- incisor region and 15 in the posterior molar region). All of these ticle, we describe a gle-tooth implants prostheses functioned successfully, including those with wider oc- technique to enare placed. The sure prosthetic suclusal planes and increasing occlusal forces. No screw loosening mechanical type of perstructure stabilor fractures were noted in any of the fixtures. screw-retained sinClinical Implications. This technique secures and ity based on gle-tooth restorainterlocking the instabilizes the single-tooth prosthesis, reduces chair time on foltion under functernal hexed fixalow-up procedures and reduces unnecessary frustration in pation, which has to tion screw head tients and dental team members. withstand rotationwith the hexagonal bar. al forces, often exThe armamentarium consists of a titanium periences loosening of the screw abutment.1-5 hexagonal bar, a custom-fit rubber band, a modiAttempts have been made to solve this probfied female 1.27-mm hexagonal drive key and the lem,2,6,7 but it is still a persistent obstacle in implant manufacturer’s fixation screw (Figure 1). achieving success. The bar should fit precisely and firmly into the About three years ago, we developed a hexagoinner (female) fixation screw. Before the screw is nal bar that interlocks with the fixation screw to JADA, Vol. 130, May 1999 Copyright ©1998-2001 American Dental Association. All rights reserved.
✷
677
C
CLINICAL PRACTICE prostheses were initially placed at the required occlusal equilibration and with the fixation screw tightened; however, the hexagonal bar was not inserted for one month to allow any undesirable vertical and rotational forces to be detected and eliminated. RESULTS
Figure 1. Components for the screw lock technique are shown: a 1.27millimeter titanium hexagonal bar (hb), an isolating rubber band (rb), the superstructure fixation screw (fs) and the hexagonal bar carrier (hbc).
fastened, the dentist verifies that the prosthetic superstructure is passively stabilized by the external hex connection; he or she then inserts the fixation screw and secures it with the drive key. Before the prosthesis is placed, the unit is tested with the hexagonal bar and fixation screw inserted into the working model (Figure 2). The dentist positions the single-tooth implant prosthesis passively on the osseointegrated implant and tightly secures it with the screw according to the manufacturer’s recommended torque force. An orthoradial periapical radiograph should verify its position. The hexagonal bar is then inserted securely into the fixation screw. The custom-fit rubber band, serving as an isolating sleeve layer, is placed next to the bar and the screw head connection. This prevents the composite layer on top from penetrating into the hexed head screw. The dentist then applies the light-polymerizing composite resin filler. Once set, it will 678
serve as a fixative material for the bar, as well as an access hole sealer. Figure 3 is a schematic drawing of the fixation screw and hexagonal bar in the interlocking position, em-
Once set, the composite resin will serve as a fixative material for the bar, as well as an access hole sealer. braced by the isolated and hardened resin layer. In the final steps, the dentist cuts the bar down to the occlusal crown level, smooths it and polishes it (Figure 4). The hexagonal bar cannot be rotated or moved and the screw is locked securely in place. During follow-up visits, the composite resin material can be removed with a high-speed turbine, and the screw and prosthetic superstructure retrieved. We should point out that the
We placed a total of 120 singletooth implant screw-retained prostheses in 100 patients (60 women, 40 men), ranging in age from 19 to 61 years. Sixty-five of these prostheses were placed in the first and second premolar region, 40 in the incisor region and 15 in the posterior molar region. The distribution of implants in the mandible and maxilla was equivalent. During monthly follow-up visits, we examined the patients and found all of the prostheses to be securely in place. They all functioned successfully, especially those in the molar region, with wider occlusal planes and greater occlusal forces. We have used this technique for almost three years with no complications, including screw loosening and fractures. DISCUSSION
Several multicenter prospective studies have examined prosthetic appliances supported by osseointegrated implants and their associated complications and outcomes.2,3,8-11 Complications such as fractured acrylic resin materials, soft-tissue problems, oral hygiene difficulties, temporomandibular symptoms, loosening of superstructures, decreased occlusal vertical dimension and increased salivation have been observed.11 Carlson and Carlsson11 stated that a maintenance pro-
JADA, Vol. 130, May 1999 Copyright ©1998-2001 American Dental Association. All rights reserved.
CLINICAL PRACTICE
Figure 2. The hexagonal titanium bar is tested on the prosthetic superstructure on the working model.
gram, consisting of total occlusal equilibration and prosthetic stability, is essential. Jemt8 suggested that at least five to seven appointments are needed during the first year after the implant has been placed to verify stability. Neither the study by Carlson and Carlsson11 nor the one by Jemt8 mentioned using a singletooth restoration. In the past several years, the use of screw-retained prostheses over osseointegrated implants to replace single teeth has grown in popularity.1-5,12-15 Difficult complications, unlike those encountered in the traditional Bränemark reconstruc-
Figure 3. This schematic drawing illustrates all components in their final position. The custom-fit rubber band (rb) serves as an inner isolating sleeve layer to prevent undesired penetration of the composite (co) resin onto the hexed head screw. The composite resin secures the hexagonal bar in position.
tion,8-11 have been the focus of attention by researchers. In a multicenter study, Laney and colleagues3 placed single-implant prostheses in 82 patients and screened them for three years. They recorded several types of complications, such as abutment screw fracture, softtissue penetration, mucosal inflammation and screw loosening. Prosthetic superstructure loosening occurred in 10 patients. After one year, this number had significantly reduced. Use of gold screws. By replacing titanium abutment screws with gold ones, Laney and colleagues thought the
problem of screw loosening had been eliminated.3 Using a nonlinear finite contact analysis, Sakaguchi and Borgersen16 determined that this conclusion is incorrect. In a recent study,17 15 percent of titanium and gold screws became loose during initial loading and at the first follow-up visit. Similar findings were found for full-arch fixed prostheses.18 The process of fastening the screw inside the implant body results in a secure butt-joint connection between the prosthesis and the implant. This connection produces a load, which
JADA, Vol. 130, May 1999 Copyright ©1998-2001 American Dental Association. All rights reserved.
679
CLINICAL PRACTICE
Figure 4. The light-cured composite layer seals the access hole and locks the hexagonal bar. After the composite resin is placed and hardens, the bar is cut and smoothed to the proper occlusal level.
is known as preload.19 A properly preloaded abutment screw should be strong enough to resist torques from occlusal loadings for implant-supported prostheses. However, according to several reports, screws have loosened in 15 to 57 percent of single-tooth implant restorations.1,2,4,5,12,15,17 In extreme instances, abutment screws have even fractured.3,12,15 Abutment screw loosening occurs because of improper occlusion and/or excessive loading. When it occurs, patients complain of soreness at the interface between the soft tissue and the fixture, swelling and/or fistula formation, difficulty in chewing and prosthetic instability because of poor contact between the implant neck and its prosthetic component (as a result of a weak fit in the interface zone of the prosthesis and the implant).20 Causes of loose superstructures. A loose superstructure may result from an inadequate framework design. Essentially, the machining accuracy of the implant and its 680
prosthetic superstructure, especially between the patrix (the prosthetic recess that fits precisely around the external hex) hexagonal extension and the matrix (the external hex of the implant neck) hexagonal abut-
Abutment screw loosening occurs because of improper occlusion and/or excessive loading. ment, should be observed by the research team.21,22 Dentists may also encounter rough surfaces on parts of the prosthesis, settling effects of the implant– prosthetic superstructure connection, mechanical fatigue resistance of the metal portion of the fixation screw, persistent torque and micromovement forces.6,7,23 Binon21,22 examined the effect of the relationship between the abutment and implant on the stability of the superstructure.
He found a direct correlation between the hexagonal misfit of the external hex of the implant neck and its female abutment recess, as well as screw joint loosening; the greater the misfit, the greater the probability of screw loosening. Conversely, a small hexagonal misfit would result in increased surface contact area between the external implant hex and its female abutment recess; consequently, a greater number of rotational cycles would be required for the screw to loosen. A rotational misfit of less than 2 degrees provides the most stable and predictable screw joint. Prosthetic modifications. Manufacturers of oral implants have attempted to refine the prosthetic connecting parts to achieve a more predictable method of tightening the screws. One system (Paragon Implant Surgical System, Paragon) has a Friction Lock design that claims to achieve superstructure stability,24 but it did not demonstrate superiority over other implants in an objective comparative study.7 Furthermore, a cross-sectional scanning electron microscopic view clearly demonstrated an interface discrepancy that reached 75 micrometers (mean discrepancy, 45 µm).22 Arvidson and colleagues25 conducted a clinical study of an abutment design that was introduced in the early 1990s. The conical seal design of this system (Astra Tech, Molndal) demonstrated increased resistance to bending movements at the implant-abutment interface, compared with results for the regular external hex connection.26 Consequently, screw loosening should be alleviated with a one-cone screw
JADA, Vol. 130, May 1999 Copyright ©1998-2001 American Dental Association. All rights reserved.
CLINICAL PRACTICE connection as an abutment that fits into the implant rather than on top of the external hex implant body. Another new dental implant system (Spline, Calcitek) produces a stable loading platform and high-fidelity fatigue strength.27 Longterm studies of both systems are needed to substantiate their promising mechanical properties. However, both exhibited minimal rotational movement. The question is whether minimal rotational movement (that is, almost complete rigidity) could jeopardize osseointegration via excessive forces and trauma on the surrounding living hard tissues. Other modifications, such as flat-headed titanium screws14 and gold screws2,6 reduce, but do not eliminate, the problem of screw loosening. Some studies15,18 on gold abutment screws actually have demonstrated many incidents of screw loosening. Jorneus and colleagues6 suggested that the screw be elongated to achieve better stability, but objective mechanical limitations resulting from the limited number of screw threads dictate that the solution be found elsewhere. A modified single-tooth implant restoration is the CeraOne system (Nobel Biocare).28 In this system, the post is shaped and then cast before the screw is fastened to the implant body. However, a mechanical testing study29 demonstrated that the abutment screw in that system was the weakest link in the implant post. Most studies6,13,14 emphasize the need for total equilibrium of occlusion and a more predictable tightening method for screws to withstand the occlusal forces.
Figure 5. A single-molar implant prosthesis and screw lock with hexagonal bar that has functioned for two years.
Hexagonal bar. The technique we have described above solves the problem of screw loosening. It can be used in areas of dominant occlusal forces, such as the first molar region (Figure 5). However, this technique is effective only after undesirable vertical and rotational forces have been detected and eliminated. To accomplish this, the metal bar is inserted into the fixation screw one month after the screw-retained prosthesis has been placed; this period serves as an initial diagnostic function-
Dr Artzi is a lecturer and director of the
Dr. Dreiangel is an in-
Graduate Program in
structor and coordi-
Periodontology,
nator of the
Department of
Undergraduate
Periodontology, The
Clinics in
Maurice and Gabriela
Periodontology,
Goldschleger School
Department of
of Dental Medicine,
Periodontology, The
Ramet Aviv, Tel Aviv
Maurice and Gabriela
University, Tel Aviv
Goldschleger School
169978, Israel.
of Dental Medicine,
Address reprint re-
Tel Aviv University,
quests to Dr. Artzi.
Tel Aviv, Israel.
al phase. Because the fixation screw did not loosen, the forces were transferred to the matrix/patrix connection or even to the implant/bone interface (that is, the zone of osseointegration). Nevertheless, no disintegration and/or other deterioration was observed on these single-tooth interlocking screw units. Ease of maintenance. Since the intercuspal peaks of the occlusal premolar/molar plane and/or lingual aspect of the anterior crowns range from 5 to 9 mm, the occlusal access hole should not exceed 4 mm in diameter. Thus, the 2-mm hexagonal bar can be easily inserted, and the hardened lightcured composite resin material can be removed with a small round high-speed bur. On routine maintenance visits, when the composite material is removed, the bar is securely positioned at the hexed screw head, and the dentist can gently remove it with forceps. Since most reports of screw loosening2,3-5,15,17,18 occurred during the first year after place-
JADA, Vol. 130, May 1999 Copyright ©1998-2001 American Dental Association. All rights reserved.
681
CLINICAL PRACTICE ment, the efficacy of the hexagonal bar technique, which has been used successfully for almost three years, has been shown. The securing system locks the antirotational superstructure in place and complications, such as soft-tissue soreness, fistula formation and screw fracture, are avoided. We should emphasize that use of the hexagonal bar does not reduce the need for frequent maintenance visits or ensure proper functional occlusion. However, this technique secures and stabilizes the single-tooth prosthesis, reduces chair time for follow-up procedures and eliminates unnecessary frustration in patients and dental team members. CONCLUSION
We have presented a simple method of securing the singletooth implant prosthesis by locking a hexagonal bar into the hex head of the fixation screw and sealing the access hole with a light-cured composite resin. The technique has been used successfully for almost three years on 120 implants in 100 patients. During the follow-up period, we found no instances of loosened screws. This technique enables a single-implant prosthetic superstructure to function successfully over the long term and, at the same time, to be easily retrieved. ■ The authors thank Rita Lazar for her editorial assistance and Rellu Samuel for providing photographs.
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1. Jemt T, Lekholm U, Grondahl K. A 3year follow-up study of early single implant restorations ad modum Bränemark. Int J Periodontics Restorative Dent 1990;10:341-9. 2. Jemt T, Laney WR, Harris D, et al. Osseointegrated implants for single tooth replacement: a 1-year report from a multicenter prospective study. Int J Oral Maxillofac Implants 1991;6(1):29-36. 3. Laney WR, Jemt T, Harris D, et al. Osseointegrated implants for single-tooth replacement: progress report from a multicenter prospective study after 3 years. Int J Oral Maxillofac Implants 1994;9(1):49-54. 4. Ekfeldt A, Carlsson GE, Borjesson G. Clinical evaluation of single-tooth restorations supported by osseointegrated implants: a retrospective study. Int J Oral Maxillofac Implants 1994;9(2):179-83. 5. Haas R, Mensdorff-Pouilly N, Mailath G, Watzek G. Bränemark single tooth implants: a preliminary report of 76 implants. J Prosthet Dent 1995;73(3):274-9. 6. Jorneus L, Jemt T, Carlsson L. Loads and designs of screw joints for single crowns supported by osseointegrated implants. Int J Oral Maxillofac Implants 1992;7(3):353-9. 7. Dixon DL, Breeding LC, Sadler JP, McKay ML. Comparison of screw loosening, rotation, and deflection among three implant designs. J Prosthet Dent 1995;74(3):270-8. 8. Jemt T. Failures and complications in 391 consecutively inserted fixed prostheses supported by Bränemark 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(3):270-6. 9. 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(2):185-94. 10. van Steenberghe D, Lekholm U, Bolender C, et al. Applicability of osseointegrated oral implants in the rehabilitation of partial edentulism: a prospective multicenter study on 558 fixtures. Int J Oral Maxillofac Implants 1990;5(3):272-81. 11. Carlson B, Carlsson GE. Prosthodontic complications in osseointegrated dental implant treatment. Int J Oral Maxillofac Implants 1994;9(1):90-4. 12. Henry PJ, Laney WR, Jemt T, et al. Osseointegrated implants for single-tooth replacement: a prospective 5-year multicenter study. Int J Oral Maxillofac Implants 1996;11(4):450-5. 13. Avivi-Arber L, Zarb GA. Clinical effectiveness of implant-supported single-tooth replacement: the Toronto study. Int J Oral Maxillofac Implants 1996;11(3):311-21. 14. Lekholm U, Jemt T. Principles for single tooth replacement. In: Albrektsson T, Zarb GA, eds. The Bränemark osseointegrated im-
plant. Chicago: Quintessence; 1989:117-26. 15. Becker W, Becker BE. Replacement of maxillary and mandibular molars with single endosseous implant restorations: a retrospective study. J Prosthet Dent 1995;74(1):51-5. 16. Sakaguchi RL, Borgersen SE. Nonlinear finite element contact analysis of dental implant components. Int J Oral Maxillofac Implants 1993;8(6):655-61. 17. Wie H. Registration of localization, occlusion and occluding materials for failing screw joints in the Bränemark implant system. Clin Oral Implants Res 1995;6(1):47-53. 18. 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(2):169-78. 19. Carr AB, Grunski JB, Hurley E. Effects of fabrication, finishing, and polishing procedures on preload in prostheses using conventional gold and plastic cylinders. Int J Oral Maxillofac Implants 1996;11(5):589-98. 20. Weinberg LA. The biomechanics of force distribution in implant-supported prostheses. Int J Oral Maxillofac Implants 1993;8(1):1931. 21. Binon PP. The effect of implant/abutment hexagonal misfit on screw joint stability. Int J Prosthodont 1996;9(2):149-60. 22. Binon PP. Evaluation of machining accuracy and consistency of selected implants, standard abutments, and laboratory analogs. Int J Prosthodont 1995;8(2):162-78. 23. Burguete RL, Johns RB, King T, Patterson EA. Tightening characteristics for screw joints in osseointegrated dental implants. J Prosthet Dent 1994;71:592-9. 24. Balfour A, O’Brien GR. Comparative study of antirotational single tooth abutments. J Prosthet Dent 1995;73(1):36-43. 25. Arvidson K, Bystedt H, Frykholm A, von Konow L, Lothigius E. A 3-year clinical study of Astra dental implants in the treatment of edentulous mandibles. Int J Oral Maxillofac Implants 1992;7(3):321-9. 26. Norton MR. An in vitro evaluation of the strength of an internal conical interface compared to a butt joint interface in implant design. Clin Oral Implants Res 1997;8(4):290-8. 27. Binon PP. The spline implant: design, engineering and evaluation. Int J Prosthodont 1996;9:419-33. 28. Andersson B, Odman P, Carlsson L, Bränemark PI. A new Bränemark single tooth abutment: handling and early clinical experiences. Int J Oral Maxillofac Implants 1992;7(1):105-11. 29. Andersson B, Odman P, Boss A, Jorneus L. Mechanical testing of superstructures on the CeraOne abutment in the Bränemark system. Int J Oral Maxillofac Implants 1994;9:665-72.
JADA, Vol. 130, May 1999 Copyright ©1998-2001 American Dental Association. All rights reserved.