Medical knowledge | Dentistry » Robert-Rutger - A 3 year prospective study of a single tooth implant prosthodontic complications

A 3-Year Prospective Study of a Single-Tooth Implant Prosthodontic Complications Robert H. Johnson, DDS, MSDa G. Rutger Persson, DDS, Dr Odonta Purpose: The objective of this 3-year prospective study was to evaluate a new titanium dental implant system and to identify any mechanical concerns or deficiencies in its design. Materials and Methods: One periodontist placed 78 single-tooth implants in 59 subjects. One restorative dentist using one dental laboratory fabricated all of the crowns Each patient then was seen eight times by the authors for clinical, radiographic, and laboratory testing. Results: One of the 78 implants failed to integrate and was removed There were no structural failures per se, although six of the cemented crowns and six of the restorative posts and cores (abutments) became loose. On a few occasions, the emergence profile was esthetically or functionally unsatisfactory. The manufacturer introduced changes in design and created an adjustable torque wrench, a

torqueadjusting beam scale, and die replicas of the abutment posts in response to the identified concerns. Conclusion: The clinical trial helped in the development of an implant system that is user friendly, cost effective, and able to withstand parafunctional forces in the absence of antirotational features. Int J Prosthodont 2001;14:183–189 F actors that allegedly influence the survival rates of dental implants include the quantity and quality of bone in the selected site,1–4 the lack of keratinized tissue around the implant,5–10 the patient’s periodontal status,11–13 the type of microflora in the sulci/pockets of the remaining natural teeth and implants,14–19 the patient’s plaque control, 20 professional maintenance,21–28 smoking,29–32 parafunctional habits,33-35 and the surface characteristics of dental implants.36–39 The features of the implant system itself can affect its ultimate success. Is it relatively easy to place and restore? How complicated and

expensive are laboratory procedures? Can it withstand potentially destructive occlusal forces? Does its design facilitate periodontal monitoring and maintenance by dental professionals and patients? The objectives of this 3-year prospective clinical trial were twofold. The first objective was to conduct a series of clinical, radiographic, and laboratory tests on single-tooth commercially pure titanium (Ti) implants and to compare the results with those on natural teeth and coated Ti alloy implants in the same patients. This material has been reported elsewhere40 The second objective was to assess the design and mechanical properties of the implant system (Genetics Implant Systems), to notify the manufacturer of any problems, and to report any changes in design made by the company to overcome the identified shortcomings. The findings of this part of the investigation constitute the current article a Professor of Periodontics, School of Dentistry, University of Washington, Seattle.

Reprint requests: Dr Robert H. Johnson, Department of Periodontics, PO Box 357444, University of Washington, Seattle, Washington 98195-7444. Fax: + (206) 616-7478 e-mail: rhjperio@u.washingtonedu Volume 14, Number 2, 2001 183 The International Journal of Prosthodontics Prosthodontic Complications with a Single-Tooth Implant Johnson/Persson implant’s collar and after 2 mm was tapered at 6 degrees. The core was modified in height and circumference at chairside to become the “prepared tooth” There were no antirotational features and no occlusal retention screws. If more than one implant was placed in a patient, the second and third were of identical design, but were double-coated Ti alloy (Ti-6Al-4V) implants. The first coat was a 35-µm thickness of Ti plasma; the second layer was a 25-µm thickness of porous hydroxyapatite (HA). In the current study, 3.75- and 475-mm-diameter implants were selected depending on the thickness of the alveolar ridge. The clinical director of

the implant company (a periodontist) placed all of the implants under local anesthesia between September 1992 and August 1993. The surgical site usually was bounded by teeth mesially and distally (sometimes just mesially). A stent was not used for alignment purposes. A crestal incision was made, generally within keratinized tissue, and full-thickness buccal and lingual/palatal flaps were reflected. A # 4 long-shank surgical bur was used to create a starting point in the cortical plate. The initial hole was drilled with a 16-mm internally and externally irrigated spade drill to the predetermined length, and a guide pin was placed to check the alignment. Needed adjustments in alignment were made as the diameter of the site was increased through sequential use of larger spade drills until 3.2 or 42 mm (ie, the minor diameter of 3.75- and 475-mm implants, respectively) was reached A counter bore was employed to create a space that was 1 mm deep and either 3.75 or 475 mm wide to accept the

collar of the implant. This design preserves valuable crestal supporting bone because there is no need for extensive countersinking in depth and width to accommodate a collar, an external hex, and a cover screw. The site was then threaded to the major diameter using a thread former. In the presence of dense bone, the site was threaded the full length of the implant; in soft bone, just the coronal half was threaded to permit selftapping of the implant for the remainder of the length. The top of the implant was left flush with the crest of the bone. A cover screw was placed, and the flaps were sutured to achieve primary closure. An immediate postoperative periapical radiograph was taken Six cases required ridge augmentation. Depending on the clinical findings, the following materials and methods were used: expanded polytetrafluoroethylene (e-PTFE) membrane (3i/WL Gore) alone, 200- to 300µm corticocancellous demineralized freeze-dried bone allograft (American Red Cross) alone, e-PTFE and

bone allograft, and crestal expansion and e-PTFE. Membranes that became exposed were removed approximately 6 weeks postoperative; otherwise, they were removed at the time of the second-stage surgery. Fig 1 Coronal surface of implant designed with a 30-degree internal bevel to interface with 30-degree external bevel of restorative post and core. Materials and Methods The Human Subjects Review Committee of the University of Washington approved the study. To qualify, individuals had to be in good general and periodontal health and have relatively intact dentitions. The feasibility of placing an implant was determined by clinical examination with the aid of periapical radiographs and an orthopantographic image. There had to be adequate mesiodistal space between the bordering teeth and at least 9 mm of vertical bone height between the alveolar crest and underlying structures such as the maxillary sinus or mandibular canal. A thin buccolingual dimension of the alveolar ridge could result

in exclusion, although the surgeon was prepared to use ridge augmentation techniques during implant placement. A maximum of three freestanding implants could be placed in any given patient At least 4 months had to have elapsed between extraction of the tooth and placement of the implant. The system as tested was a two-part implant: the actual implant and a one-piece restorative abutment, the latter more accurately described as a “post and core.” The implant was threaded and manufactured from noncoated, commercially pure grade 2 Ti. It had a collar of 1 mm in height with the same diameter as the major diameter of the external threads of the implant. The top, or coronal, surface of the implant was not flat, but designed with an internal 30-degree bevel (Fig 1). The second component, the restorative post and core, was made of a Ti alloy (Ti-6Al-4V) and had a machined 30-degree external bevel to interface with the internally beveled top of the implant. The threaded apical portion, the

“post,” was 2.5 mm in diameter and extended 5.5 mm into the implant The coronal portion, the “core,” started flush with the outside of the The International Journal of Prosthodontics 184 Volume 14, Number 2, 2001 Johnson/Persson Prosthodontic Complications with a Single-Tooth Implant Patients were seen 1 week postsurgical for suture removal and again at 2, 6, 14, and 22 weeks. Periapical radiographs were taken at the last three of these visits. The time between implant placement and the second-stage surgery averaged 6.5 months in the mandible and 8.2 months for maxillary sites Full-thickness flaps were reflected from an incision made just palatal/lingual to the middle of the alveolar crest. Any overlying bone and the cover screw were removed. The internal threads of the implant were flushed with saline, dried with air, and the restorative post and core was placed and tightened with a hand wrench until it felt snug. A minimal scallop was created in the coronal margin of

the facial flap to accommodate the post, and the flaps were secured with silk sutures. The emergent core was modified at chairside to fit the occlusal scheme. A temporary crown was immediately fabricated for those implants located in the anterior part of the mouth. The sutures were removed in 1 week. One restorative dentist made all impressions for the final crowns approximately 8 weeks later. Crowns were fabricated using die spacer and were cemented with Temp-Bond (Kerr) modified with 20% petroleum jelly by volume. The occlusion of each new crown was carefully checked and adjusted, if necessary, as follows. Light centric stops were generated, and any working and balancing contacts were eliminated in the posterior sextants. In the anterior region, light centric stops and excursive contacts were generated The subjects then reported to the authors at the University of Washington School of Dentistry to start the actual clinical investigation. The manufacturer made no demands and placed no

restrictions on the design of the trial. The patients were seen at baseline and at 3, 6, 12, 18, 24, 30, and 36 months for examination, plaque control instructions as needed, and to undergo a series of clinical and laboratory tests. Annual intraoral radiographs were taken. Analyses of these data are reported elsewhere.40 implant that was coated with both Ti plasma and porous HA. Two coated Ti alloy implants were placed in two individuals. Forty-eight of the implants had a diameter of 4.75 mm; the other 30 were 375 mm in diameter Relative to length, 33 (42%) implants were 10 mm, 31 (40%) were 13 mm, and 14 (18%) were 16 mm. One implant of the 78 had to be removed within the first year. In preparing the surgical site, the mesial aspect of the drill hole encroached on a prominent nasopalatine canal Integration of that side of the implant never occurred; the defect could be probed to the apex. A second patient moved and was lost to followup immediately after the crown was cemented Thus,

the 3-year survival rate was 98.7% (76 of 77 implants that were followed). Any problems encountered in the remaining 76 implants in 57 subjects over the next 3 years were reported to the company’s representative and form the basis for the current article. There were no structural failures per se. None of the implants or restorative posts were distorted or fractured during the 3-year clinical trial. Six of the 76 crowns (8%) cemented with Temp-Bond and petroleum jelly became loose and had to be recemented. On six occasions (8%), the restorative post loosened In the original design of the post and core being tested, there was no outward flare from the top of the implant. The core portion started flush with the outside of the implant (diameter 3.75 or 475 mm) and after 2 mm tapered 6 degrees as it passed through the soft tissue into the oral cavity. Thus, the cervical diameter of the implant–restorative post-and-core interface sometimes was significantly smaller than the desired

dimension of the cervical portion of the crown; this could compromise the creation of an esthetically pleasing emergence profile (Fig 2a). Moreover, the marginal soft tissue adhered tightly around the tapered core, making it difficult to retract the tissue to prepare a subgingival crown margin and obtain a good subgingival impression (Fig 2b). On a few occasions, this led to the fabrication of crowns with bulky ledges or overhangs. Results Discussion One hundred ninety-two individuals were screened; among them, 59 subjects were accepted for implant placement. The most common reasons for rejection were an inadequate amount of bone, multiple missing teeth, and complicating medical conditions. Seventyeight single-tooth implants were placed (45 mandibular posterior, 20 maxillary anterior, 12 maxillary posterior, and one mandibular anterior) Six surgical sites exhibited a bony dehiscence or narrow alveolar housing that required regenerative procedures or splitting and expansion of the

ridge. Each subject received a noncoated, commercially pure grade 2 Ti implant. Fifteen of the participants also received one Ti alloy Volume 14, Number 2, 2001 Loosening of crown-attachment screws has been a commonly reported problem, with some incidences approaching 40% to 50%.33,41–44 The more precise the fit and the larger the surface area between components, the less chance there is for micromovement, which can lead to loosening.45 It also seems reasonable that the fewer components in a system, the less chance there is for micromovement. There are only two parts in the implant system under scrutiny, the implant and the restorative post and core. Moreover, the two components are machined to contact via a large 30-degree interface. They are not subject to casting 185 The International Journal of Prosthodontics Prosthodontic Complications with a Single-Tooth Implant Johnson/Persson Fig 2a Three-year intraoral radiograph demonstrates extreme flare of crown contour required

to extend from the post and core to adjacent contact points. Fig 2b (right) Clinical view of case shown in Fig 2a. Difficulty in obtaining a subgingival impression results in exaggerated flare because the crown margin begins at the level of the soft tissue. Ncm.48 To overcome the loosening of the restorative posts, the company designed an infinitely adjustable torque wrench and a torque-adjusting beam scale. The latter has a range of 10 to 50 Ncm. The torque wrench has a standard drive head that can be used with a variety of implant-placement tools and can be calibrated to deliver the recommended force using the beam scale. Thereafter, none of the restorative posts tightened with the wrench became loose in spite of the fact that the system has no antirotational features. Temporary cement has been advised for the placement of crowns or superstructure restorations on copings and implants.49 Caries is not a concern, and temporary cements permit removal of the crowns Eight percent (six of

76) of the crowns loosened in the current study. This was an inconvenience and a possible hazard had a crown been swallowed. The restorative dentist elected to recement the crowns with Dycal (Dentsply/Caulk), which solved the problem. Loosening and fracture of screws and implants, especially in the molar region, have been attributed to excessive forces from parafunctional habits.33,34 The importance of controlling occlusal forces on singletooth implants has been stressed in the literature.35 This precaution was heeded in the current trial, and no breakage or distortion was observed. Another reason the system was free from fractured components may lie in the nature of the interface between the implant and the restorative post placed. It is speculated that when a lateral force is placed on the crown of a flattopped implant, the bulk of the stress resisting dislodgment of the crown from the implant is placed on the screw. Most crown-retention screws have an outside, or major, diameter of

17 to 20 mm and a thread length of 2.5 to 30 mm Significant lateral stress can stretch and fracture screws of these small dimensions. The post and core under investigation was fabricated from a Ti alloy that is up to three or four times stronger and heating cycles in the fabrication of the crown that could adversely affect their original fit. Even so, 8% of the restorative posts loosened. This problem was overcome in two ways. The first was the use of a lubricant Friction is created when two dry metal surfaces (such as a nut and bolt) are turned and tightened against each other at a given force. This is termed galling. If a lubricant is applied to the two surfaces, friction is decreased and the two components can be made tighter using the same amount of force.46 In the current study, the loose restorative post was removed, and thread seal (Omni-Lite, Attachments and Implants International) was applied to its threads and the interfacing bevel between the post and the top of the

implant. The post and core was retightened into the implant and remained in place over the course of the study. Thread seal may provide an added bonus In 24 hours, it sets to a consistency similar to silicone. This may help to prevent bacterial percolation into the internal connection between the implant and restorative post. This, in turn, may reduce the possibility of inflammation in the adjacent soft tissue. Additional study is needed to confirm this hypothesis. The second modification designed to prevent loosening of the restorative post and core focused on establishing an adequate, standardized turning force to ensure that the post had been sufficiently tightened in the first place. In the current trial, the clinician used a hand wrench to tighten the post until it seemed snug. In any implant system, insufficient tightening causes loosening, while overtightening leads to breakage of posts or screws.47 All metal components have fatigue or tensile fracture strength values that are

specific for their diameter and composition. The recommended force used to tighten threaded components is approximately 60% of their tensile fracture strength values. In the case of the 25-mm-diameter Ti alloy post and core used in the current study, this equals 40 The International Journal of Prosthodontics 186 Volume 14, Number 2, 2001 Johnson/Persson Prosthodontic Complications with a Single-Tooth Implant Fig 3 Computer-generated stress distribution images after the application of horizontal force of 140 N. To resist tipping the post and core from the top of the implant, greater stress is placed on the shoulder and screw of flat-topped (90-degree) implant (left) than on 30-degree beveled implant (right). than commercially pure Ti.48,50 The post extends 55 mm into the implant and has a major diameter of 2.5 mm. Moreover, the top of the implant is not flat; it is designed with a large 30-degree internal bevel, against which the collar of the post is machined to fit (Fig 1).

Lateral forces are absorbed mainly by a combination of large surface area and bevel, thus reducing the stress on the threaded portion of the post. To confirm this claim, the manufacturer generated two sets of computer models. In the first model, the implant and post interfaced at a 30-degree bevel; in the second, the top of the implant was flat and the two components abutted at 90 degrees. Finite element analysis, which is a method to analyze the distribution of stress within a model when a force is applied, was used to test this hypothesis. 51 Independent investigators from the Department of Mechanical Engineering, University of Victoria, Canada applied a horizontal force of 140 N and compared the stress distribution in the beveled design with that of the flat-top design.52 At least twice as much force was borne by the region where the post was inserted into the flat-top implant than in the 30degree beveled implant (Fig 3). In this stress test, both posts were 2.5-mm-diameter Ti alloy

Had the post been smaller or fabricated from a material that is not as strong as the alloy, breakage may have occurred. To improve the emergence profile, the restorative post and core was redesigned. It still is one piece, but it now consists of three sections: an apical screw (the post), a central collar, and a tapered coronal core. The post portion was not changed. It is 55 mm in length, the apical 2.0 mm of which is a nonthreaded cylindric lead to ease insertion and prevent cross threading. The coronal end, or core, maintained the original 6-degree taper, but instead of being circular was changed to a hexagonal shape for more accurate seating of the ultimate crown (Fig 4). Its 6-mm height provides adequate retention and in most instances sufficient clearance of Volume 14, Number 2, 2001 Fig 4 Comparison of original core prepared at chairside (right) and new, preprepared tapered hexagonal core (left). the opposing teeth. Significant overeruption of the opposing teeth does require

modification of the plane of occlusion. The changes made to the collar addressed the problems identified with the emergence profile. The collar now comes either straight or flared and either 1 or 2 mm in vertical height. This permits the clinician to select the appropriate post and core to facilitate the fabrication of a crown with a pleasing emergence profile (Fig 5). Devoid of undercuts or overcuts, the system is designed to facilitate periodontal monitoring and maintenance. The need for accurate subgingival impressions to capture the cervical margins of the restorative post was eliminated by the production of stainless steel die replicas of each size and style of post and core. An impression that does not have to extend subgingivally is taken, and the die is placed into it, thus producing a working model with well-defined margins at the apical edge of the collar (Fig 6). The laboratory technician now has an adequate distance with which to work in 187 The International Journal of

Prosthodontics Prosthodontic Complications with a Single-Tooth Implant Johnson/Persson Fig 5a Intraoral radiograph of four implants displays a variety of new posts and cores that can be used to generate proper crown profiles. Fig 5b Intraoral radiograph of the restored implants seen in Fig 5a. The smooth contours from implant to crown simplify periodontal maintenance The manufacturer redesigned its second-stage restorative posts and cores to create more acceptable emergence profiles. Stainless steel die replicas were fabricated for each size and shape of post and core, which eliminated the need for the restorative dentist to capture subgingival margins in the impression. This simplified the task of fabricating a functionally sound and esthetically pleasing crown both in the dental office and the laboratory. The harmonious emergence profile facilitates maintenance of the restored implant by dental professionals and patients. Acknowledgments Fig 6 Two of three die replicas placed

and stabilized in the supragingival impression. The resultant cast will exhibit accurate margins for the fabrication of the crown. The actual investigation was supported by the Elam M. and Georgina Hack Memorial Research Fund, University of Washington, Department of Periodontics, and the Regional Clinical Dental Research Center, NIH/NIDR grant No. P30 DEO9743 The authors thank Dr Stan Sapkos of Genetic Implant Systems for placing the implants, and Dr James W. Cherberg for fabricating the crowns the creation of an attractive crown contour and emergence profile. These modifications have simplified the life of the restorative dentist and the dental laboratory. A crown can be fabricated for the implant as it would be for a prepared tooth. This, in turn, reduces costs An informal survey of regional dental laboratories revealed that the fee for a crown in the implant system being tested was approximately 60% of that for an implant system employing a screw-retained crown. References 1. 2.

3. 4. Conclusion In the current prospective study, 78 implants were placed in all regions of the maxilla and mandible, in a variety of types of bone, and in the presence of facial dehiscences. The 3-year survival rate was 987% Six (8%) of the restorative posts and cores loosened in the implant. The company therefore designed an adjustable torque wrench and a torque-adjusting beam scale to ensure that an adequate calibrated force was used to tighten the post and core. The application of thread seal to the interface between the post and core and the implant also helped to prevent loosening. The International Journal of Prosthodontics 5. 6. 7. 8. 188 Jaffin RA, Berman CL. The excessive loss of Brånemark fixtures in type IV bone: A 5-year analysis. J Periodontol 1991;62:2–4 Haas R, Mensdorff-Pouilly N, Mailath G, Watzek G. Survival of 1,920 IMZ implants followed for up to 100 months. Int J Oral Maxillofac Implants 1996;11:581–588. Listrom RD, Smith D, Symington JM. A clinical

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of four implant designs. Postgrad Dent 1995;2:4–14 51. Zienkiewicz OC The Finite Element Method in Engineering Science, ed 4. New York: McGraw-Hill, 1989 52. Berezon BB, Tabarrok B Determinants of critical stresses in a dental prosthesis Presented at the Pacific Coast Society of Prosthodontists meeting, 21–24 June 1995, San Francisco. 189 The International Journal of Prosthodontics