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CLINICAL SCIENCE AND TECHNIQUES

Resistance to Crown Displacement on a Hexagonal Implant Abutment

Kwan, Norman DDS*; Yang, Silvia BSc; Guillaume, Didier DMD; Aboyoussef, Hoda DMD, MS§; Ganz, Scott D. DMD||; Weiner, Saul DDS

Author Information
doi: 10.1097/01.ID.0000127519.68325.D7
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Abstract

During the early 1980s, successful surgical and restorative protocols were established for predictable root-form titanium implants mainly through innovative work by Brånemark and colleagues. 1 Osseointegration was thought to occur only as a 2-stage surgical approach of initially placing implants into the bone, and then, protected under the gingival tissue without function, allowed to heal and osseointegrate for a predetermined period of time. The abutment to retain the prosthesis was then connected to the implant with a screw that was subsequently inserted after osseointegration had occurred. 2

The abutment-to-implant connection became a cause for concern as a result of limitations of the 0.7-mm high standard external hexagonal interface. Originally designed to help deliver and rotate the fixture into the osteotomy site, the hex was used to orient the abutment intraorally, transfer its location to a working cast, and prevent antirotation when required in a single-tooth application. However, it was not uncommon for fixation screws to loosen, stretch, or even break under normal functional masticatory cyclic loading. To alleviate these problems, improvements in manufacturing tolerance, redesign of fixation screws, and the development of alternate methods of fixation such as the Morse taper or an internal hex have been adopted. 3

In a more recent development, a 1-piece implant system has been introduced in which the root-form and the abutment are milled from 1 piece of solid titanium. The transmucosal abutment design requires a 1-stage surgical procedure. In addition, no abutment screw is needed because the root-form and abutment are a “unibody.” The abutment shape is a hexagon 3 mm high and 4.1 mm at its widest point with rounded corners. It features a central screw receiving-hole to allow for the option of providing either a screw-retained or a cementable restoration.

In the case of implants, screw retention initially was the commonly used method, especially for retrievability, to retain the crowns on the fixture. 4 However, more recently, clinicians are adapting a more traditional cementable abutment design that incorporates relative parallelism with adequate abutment height. 5 Resistance and retention-form for implant restorations have the same requirements as for natural teeth. Among the available cementable abutments are a series of prefabricated flat-walled abutment forms, including the CeraOne abutment (Nobel BioCare, Yorba Linda, CA), the STA abutment (3i, West Palm Beach, FL), the Hex-Lock abutment (Centerpulse, Carlsbad, CA), the Hexed abutment (BioLok International, Inc., Deerfield Park, FL), and the Octalink abutment (ITI America, Waltham, MA) that are widely used. Although their surface areas are relatively small, the parallel walls and angled-hexed design seem to allow retention and resistance of a crown of greater dimension than could be expected from comparable natural tooth preparations. 6

This novel implant system uses a prefabricated flatwall abutment design similar to those noted previously. Covey et al. 7 compared the retentive strength of 3 different diameter Cera-One abutments: the standard, wider, and narrower diameter abutments. Matching milled cylinders, cast to create crown forms, were cemented with zinc phosphate or zinc oxide and eugenol provisional cement. Retention strengths of the restorations were similar for all 3 abutment diameters.

Clinical experience with this system suggests that this abutment is also “highly retentive” and is useful in a variety of clinical situations. In the case of multiple splinted units, the treatment protocol permits trimming of up to 3 adjacent surfaces (sides) of an individual nonremovable solid abutment to establish parallelism between adjacent abutments. To maintain retention, it is required that there be at least 6 surfaces (sides) untouched between any 2 abutments. Although past clinical experience might question the retentiveness of this design, retrospective analysis has demonstrated the long-term effectiveness of this approach. 8

It is unclear, however, if the retentive characteristics of the hexagon abutment also are observed during application of shear forces. To further extend the clinical observations, this series of experiments examines, in vitro, the resistance of the hexed abutment of the 1-piece implant system to crown displacement. Resistance-form is a clinically realistic factor because most of the load applied to the crown during oral function is not vertical, but has a lateral, shear component. 9 The object of these experiments was to observe the displacement of the cemented crowns of obliquely loaded 1-piece implant-abutment specimens. In this experiment, modified abutments with 3 walls trimmed were used. Although full in vitro simulation is difficult to accomplish, oblique loading, similar to mastication, can readily be achieved by modifying the load angle. The hypothesis of this study is that this abutment, with 3 sides modified, provides sufficient resistance-form to prevent crown displacement with an oblique load.

Methods and Materials

Specimen Preparation

A gypsum block (Modern Materials, South Bend, IN) was trimmed to the shape of a cube with a uniform width and length, 22 mm × 22 mm, with a height that varied from 11 mm to 14 mm. Impressions were taken of the block using alginate impression material (Dentsply Caulk, Dentsply International Inc., Milford, DE). Autopolymerizing resin (Jet Denture repair Acrylic; Lang Manufacturing Co., Inc., Wheeling, IL) was mixed according to the manufacturer’s instructions. The mixture was poured into the alginate molds and allowed to polymerize. The blocks were then trimmed to remove all flash, mounted in an 8-inch drill press, and 12 mm deep holes were drilled perpendicular to the top of the block using a HS #29 drill bit at 3100 RPM. The resultant hole was slightly larger than 4 mm in diameter. A standardized 3:1 mixture of resin was mixed and used to individually cement the solid 1-piece titanium implants, 4 mm × 10 mm (Fig. 1) (Biomedical Implant Technology, St. Catharines, Ontario, Canada) into the resin blocks. The resin was polymerized under 20 pounds of pressure for 20 minutes.

Fig. 1.
Fig. 1.:
A resin block single a 3.75 × 10-mm implant cemented at a 30° angle to the vertical.

Crown Fabrication

Crown waxups were fabricated for each of the 30 implant samples. Plastic transfer copings supplied by the manufacturer were used to form a core for the application of blue inlay wax (Jelenko Dental Health Products, Armonk, NY) to create cylindrical crowns 6.5 mm in diameter with heights of 10 mm, 12 mm, and 14 mm for 10 samples each. The center hole was maintained in all crowns. The occlusal surface of each crown was made parallel with the base of the resin block with cusp projections on the mesial and distal sides of the crowns to receive cyclic loading (Fig. 2). Point A designates the cusp where off-axis loading was applied. Point B represented the cusp where loading resulted in an axial load on the crown. The height of the crown was measured from the margin to the highest point on the crown. The wax patterns were then invested in CeraFina casting investment (WhipMix Corp., Lexington, KY) without a ring liner. The crowns were burned out at 1250°F in an Accu-Therm II, XL-L oven (Jelenko, Inc., Armonk, NY) and cast with 4 to 5 pennyweights of Midigold 50, a type III Alloy (Ivoclar Vivadent, Inc., Amherst, NY). After casting, the crowns were fitted to the implant abutments with FitChecker (GC America, Ilsip, IL) for adjustment of potential interferences and to develop a consistent frictional fit.

Fig. 2.
Fig. 2.:
A crown cemented onto a resin block sample with zinc phosphate cement. The 2 stops, points A and B, are the sites at which the loads were applied.

Using a clinical methodology, the seating of the crowns was assessed with a new fine-tine explorer. Gaps were deemed acceptable if the tine of the explorer failed to enter under the crown margin.

Stability was assessed by applying finger pressure vertically to the crown while seated on the abutment and deemed acceptable if the crown did not have any rotational movement on the abutment. Crowns were cemented onto the abutments with a standardized mix of zinc–phosphate cement (Fleck’s Zinc Phosphate Cement; Mizzy, Inc., Cherry Hill, NJ; 0.8 g of powder with 0.3 mL of liquid on a 16°C glass slab) and held between a vise grip for 20 minutes until the cement was set. Testing was done 24 hours after storage in 100% humidity.

Testing Procedure

An Instron testing machine, model 8511.20 (Canton, MA), was used to vertically load the crowns with a 200-N force at a rate of 10 Hz (Fig. 3). Calibration of the machine and load cell was done before the testing series. The machine was set to terminate the test at 1 million cycles or when there was a 10% change in abutment position. A change in abutment position would result from a break in the cement seal, a bend in the crown, or a bend in the 1-piece implant abutment device.

Fig. 3.
Fig. 3.:
A sample loaded occlusally is with a force of 200 N at 10 Hz in an Instron testing machine.

Evaluation Protocol

The marginal gap was measured in microns on images captured on Ultra-Speed DF-58, Super-Polysoft Size 2 periapical film (Kodak Canada, Inc., Toronto, Ontario, Canada). One was taken from the mesial and a second from the facial by placing each implant specimen on a flat surface and supporting the film behind it (Fig. 4). The x-ray beam was directed perpendicular (horizontally) to the block at a fixed distance. Films were exposed and developed with a standard protocol. After processing, films were scanned and saved as JPEG image files and imported into AutoCAD 2000 software (AutoDesk, Inc., San Rafael, CA). To establish a baseline for measurements, an initial reference line was drawn overlying the image of the abutment shoulder. A second line was drawn along the crown margin. The measurement for the marginal gap was recorded with a perpendicular line drawn to connect the 2 horizontal lines (Fig. 5). Differences in marginal gap size between groups were compared with a 3-way analysis of variance with factors of load application, crown height, and site of loading. Pairwise comparisons were done with the Neuman-Keuls test.

Fig. 4.
Fig. 4.:
The radiography protocol used in these experiments. The film is placed behind the sample. The x-ray beam is perpendicular to the plane of the crown.
Fig. 5.
Fig. 5.:
An image scanned from a radiograph in which measurements were made of the marginal gap using the AutoCAD software.

Results

The 10-mm Crown Experimental Group

These experimental groups did not show implant fracture or crown displacement at either point A or point B.

The 12-mm Crown Experimental Group

This experimental group experienced fractures of the 1-piece implant body for loading at point A for 4 of the 5 specimens. The mean number of cycles until implant fracture for the point A subgroup was 320,717 cycles. The average marginal gap (measured in microns) for the point A subgroup before testing was 18.56 μm ± 41.50 μm and after testing it was measured to be 193.56 μm ± 138.62 μm.

The point B subgroup did not show any implant fractures. The group B specimens endured the complete 1 million cycle test. The average marginal gap for 12-mm point B before testing was 46.80 μm ± 54.93 μm and after testing it was measured to be 81.41 μm ± 49.80 μm.

The 14-mm Crown Experimental Group

This experimental group experienced fractures of the 1-piece implant body for loading at point A for 4 of the 5 specimens. The mean number of cycles for the point A subgroup was 137,275 cycles. The average marginal gap for the point A subgroup before testing was 42.58 μm ± 58.36 μm and after testing it was measured to be 400.18 μm ± 644.31 μm. Of all samples tested, only 1 crown or specimen in this subgroup 14 mm at point A was dislodged (ie, cement seal broken).

The point B loaded group did not show any implant fractures. The group B specimens endured the complete 1 million cycles. The average marginal gap for the point B subgroup before testing was 33.99 μm ± 46.56 μm and after testing it was measured to be 41.54 μm ± 57.26 μm.

The analysis of variance suggested that the factors of loading (P <0.0001) and the site of loading (P <0.0027) were significant in causing crown displacement. Crown height was not a significant factor (P <0.453).

TABLE

Table 1
Table 1:
Analysis of Variance Summary Table

Discussion

The findings of this study suggest that a parallel-walled, 6-sided abutment could provide adequate retention and resistance-form for stabilization of crowns, supported by implants, against occlusal forces, with either a screw-retained or cementable retention design. The height of the abutment provides flexibility for clinical use in a variety of circumstances in which the interarch space could be small. Use of an abutment shape, significantly different than that seen with natural teeth, with external parallel line angles and a hex design appears to compensate for a small surface area. 7 The parallel walls of the preparation could also significantly contribute to resistance displacement. 10 In addition, the 1-mm wide shoulder at the base of the abutment could provide an enhanced resistance-form.

A factor that is important in obtaining resistance to crown displacement is the nature of the luting cement. The interface between the crown and abutment is subject to a combination of compressive and shear forces. 11 Cements with high compressive strength have been proven to be more retentive and resistant to dislodging forces. 12 This would appear to be true for implant-supported restorations as well. Recurrent caries is not a concern for dental implant-supported restorations. Nevertheless, dissolution of the cement could result in either crown displacement or marginal gingival inflammation. Thus, use of permanent cements could be a significant factor in crown retention for implant-supported restorations. The disadvantage to this approach, of course, is that the crowns are not retrievable. Screw-retained restorations are retrievable and do not require as elaborate a resistance and retention-form. For this study, however, a cementable type of restoration was used to test the stability of the abutment design. Clinically, use of a cementable design generally results in a more esthetic outcome. Sealing of the access hole of the screw-retained restoration is problematic esthetically, because it is difficult to obtain an exact color match.

Use of the 1-piece implant design eliminates 1 of the 2 microgaps present in most internally and externally hexed implant systems. 13 In most systems, microgaps exist at both the fixture–abutment interface and the abutment–crown interface. The 1-piece implant system does not have a fixture–abutment interface. It has been suggested that these interfaces could allow bacterial infiltration into the implant system and serve as a reservoir for release of byproducts that provoke an inflammatory response near the bone crest 14 that could, in part, be responsible for peri-implant crestal bone resorption. Thus, use of a 1-piece system could minimize this problem and allow maintenance of the level of the osseous crest. However, further work is required to validate this concept.

The loading done in this in vitro study was applied off-axis. The rationale for this experimental design was to test the resistance-form of the preparation. The findings of this study suggest that at physiological loads, that are nevertheless higher than those observed during normal mastication (200 N), 15 crown displacement is unlikely. The implants were embedded in poly-methyl methacrylate acrylic (PMMA) resin, which is rigid although its modulus of elasticity is similar to that of cancellous bone. 16 However, PMMA lacks anisotropy as well as a true osseointegrated interface. Thus, the bending of the implants observed in this study would not be usual in a clinical situation. Loosening of the implant would be a more likely outcome. Clinically, off-axis loading, like done in this study, is not advisable because it is perceived as deleterious to the bone–implant interface. 17 Furthermore, crowns 12 mm and 14 mm high generate moment arms that are larger than that usually observed clinically.

Nevertheless, the factors of selection of implant length and width, the type of occlusal pattern, and evidence of bruxing behavior are all important factors that should be carefully considered in implant-treatment planning.

Summary

An in vitro study was done to test the resistance-form of a small (3 mm high × 4.7 mm diameter) hexed abutment that is standardized and has a potentially wide application clinically. Loading was applied off-axis to groups of crowns of varying heights. The results of the study demonstrated that with physiological loads, this abutment design resists crown displacement. Although this was an in vitro study, these findings support other clinical design reports of the efficacy of this system.

Disclosure

The authors, except for Dr. Kwan, claim to have no financial interest in any company or any of the products mentioned in this article. Dr. Kwan is part owner of Biomedical Implant Technology Inc., manufacturer of the 1-piece implant-abutment device in this study.

References

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Abstract Translations [German, Spanish, Portugese, Japanese]

AUTOR(EN): Norman Kwan, DDS*, Silvia Yang, BSc**, Didier Guillaume DMD***, Hoda Aboyoussef DMD, MS#, Scott D. Ganz##, Saul Weiner###. *Direktor, Kanadisches Institut für Zahnimplantation, St. Catherines, Ontario, Kanada. ** Mitglied des Forschungsteams, Kanadisches Institut für Zahnimplantation, St. Catherines, Ontario, Kanada. *** Ehemals Mitglied des Post-graduiertenkollegs, Abteilung für wiederher-stellende Zahnheilkunde, zahnmedizinische Fakultät New Jersey, Newark, NJ 07103. # A.O. Professor, Abteilung für wiederherstellende Zahnheilkunde, zahnmedizinische Fakultät New Jersey, Newark, NJ 07103. ##Klinischer Assistenzprofessor, Abteilung für wiederherstellende Zahnheilkunde, zahn-medizinische Fakultät New Jersey, Newark, NJ 07103, und privat praktizierender Arzt, Fort Lee, NJ. ### Professor, Abteilung für wiederherstellende Zahnheilkunde, zahn-medizinische Fakultät New Jersey, Newark, NJ 07103. Schriftverkehr: Saul Weiner DDS, Abteilung für wiederherstellende Zahnheilkunde (Dept. of Restorative Dentistry), zahn-medizinische Fakultät New Jersey (New Jersey Dental School), 110 Bergen Street, Newark, NJ 07103. Telefon: 973 - 972–4246, Fax: 973 - 972–0370. eMail:[email protected]

Hexagonale Implantatstützzähne und deren Widerstandsfähigkeit gegenüber Kronenluxation

ZUSAMMENFASSUNG:Zielsetzung: Der hexagonale Stützzahn war bezüglich seiner Widerstandsfähigkeit und Beständigkeit gegenüber Kronenluxation unter Verwendung unterschiedlicher Kronenhöhen (10, 12 und 14 mm) und variierender Belastungsachsen zu bewerten. Materialien und Methoden: Implantate wurden in 30° zur Vertikalachse in Harzblöcke eingesetzt. Es erfolgte ein Kronenaufbau in 10, 12 und 14 mm Höhe. Für jede der drei unterschiedlichen Kronenhöhen wurden Gruppen von jeweils fünf Kronen angelegt. Diese wurden sowohl über die Längs- als auch über die Querachse mit 200N belastet. zur Messung der Randspalten vor und nach Belastung wurden standardisierte Periapikalröntgenaufnahmen herangezogen. Der statistische Vergleich der Spalten wurde anhand einer 3-Wege ANOVA durchgeführt. Die Vergleichsfaktoren waren hierbei Belastung, Kronenhöhe sowie Belastungspunkt. Die Kronen wurden maximal in 106 Zyklen oder bis zum Versagen belastet. Ergebnisse: Die 10 mm hohen Kronen zeigten keinerlei Luxationstendenzen. Für die Gruppen der Kronen mit 12 mm bzw. 14 mm Höhe zeigte sich die Luxation ausschließlich bei der Belastung in der Querachse. Die durchschnittliche Randluxation bei Kronenversagen betrug 193,56 μ (SD ± 138,62 μ) in 320.717 Zyklen bzw. 400,18 μ (SD ± 644,31) in 134.278 Zyklen. Schlussfolgerungen: Ein einteiliges Implantat in standardisierter Form verfügt über ausreichende Widerstandsfähigkeit und Beständigkeit für Kronen unterschiedlicher Ausmessungen. Voraussetzung ist allerdings, dass die Belastung über die Längsachse der Krone erfolgt.

SCHLÜSSELWÖRTER: Widerstandsfähigkeit, Krone, Zahnzement, Implantat

AUTOR(ES): Norman Kwan, DDS, Silvia Yang, BSc**, Didier Guillaume DMD***, Hoda Aboyoussef DMD, MS#, Scott D Ganz DMD##, Saul Weiner DDS###. *Director, Instituto Canadiense de Implantes Dentales, St. Catharines, Ontario, Canadá. **Asistente de Investigación, Instituto Canadiense de Implantes Dentales, St. Catharines, Ontario, Canadá. ***Antiguamente, estudiante de posgrado, Departamento de Odontología de Restauración, Escuela de Odontología de Nueva Jersey, Newark, NJ 07103. #Profesor Asociado, Departamento de Odontología de Restauración, Escuela de Odontología de Nueva Jersey, Newark, NJ 07103. ##Profesor Clínico Asistente, Departamento de Odontología de Restauración, Escuela de Odontología de Nueva Jersey, Newark, NJ 07103 y Práctica Privada, Fort Lee, NJ. ###Profesor, Departamento de Odontología de Restauración, Escuela de Odontología de Nueva Jersey, Newark, NJ 07103. Correspondencia a: Saul Weiner DDS, Dept. of Restorative Dentistry, New Jersey Dental School, 110 Bergen Street, Newark, NJ 07103. Teléfono: 973-972-4246, Fax: 973-972-0370. Correo electrónico:[email protected]

Resistencia al desplazamiento de la corona en el pilar hexagonal de un implante

ABSTRACTO:Propósito: Evaluar la resistencia y retención de un pilar hexagonal al desplazamiento de una corona con distintas alturas de la corona (10, 12 y 14 mm) y ejes de carga. Materiales Y Métodos: Los implantes fueron colocados en bloques de resina a 30° del plano vertical. Se prepararon coronas con alturas de 10, 12 y 14 mm. Se cargaron grupos de cinco (5) coronas de cada una de las tres (3) alturas de la corona (200N) bajo el eje largo y del eje inclinado. Los espacios marginales se midieron usando radiografías periapicales estandarizadas antes y después de la carga. Se compararon estadísticamente los espacios usando un ANOVA de 3 vías con factores de carga, altura de la corona y punto de carga. Las coronas se cargaron en ciclos de 106 o punto de falla. Resultados: Las coronas de 10mm no mostraron ningún desplazamiento. Los grupos de 12 y 14 mm solamente demostraron un desplazamiento de las coronas cargadas en el eje inclinado. El desplazamiento marginal promedio fue 193,56 μ (desviación estándar ± 138,62μ) en el punto de falla, (320.717 ciclos) y 400,18 μ (desviación estándar ± 644,31) en el punto de falla (134.278 ciclos) respectivamente. Conclusiones: Un implante de una pieza con un diseño estandarizado puede proporcionar suficiente resistencia y retención para coronas de distintas dimensiones si las cargas se centran sobre el eje largo de la corona.

PALABRAS CLAVES: Resistencia, corona, cemento, implante

AUTOR(ES): Norman Kwan, Doutor em Ciência Dentária, Silvia Yang, Bacharela em Ciências,**, Didier Guillaume, Doutor em Medicina Dentária***, Hoda Aboyoussef, Doutor em Medicina Dentária, Mestre em Ciências#, Scott D. Ganz, Doutor em Medicina Dentária##, Saul Weiner, Doutor em Ciência Dentária###. *Diretor, Instituto Canadense de Implante Dentário, St. Catharines, Ontario, Canadá. **Assistente de Pesquisa, Instituto Canadense de Implante Dentário, St. Catharines, Ontario, Canadá. ***Antigamente, Estudante de Pós-Graduação, Depto. De Odontologia Restauradora, New Jersey Dental School, Newark, NJ 07103. #Professor Associado, Depto. de Odontologia Restauradora, New Jersey Dental School, Newark, NJ 07103. ##Professor Clínico Assistente, Depto. de Odontologia Restauradora, New Jersey Dental School, Newark, NJ 07103 e Clínica Privada, Fort Lee, NJ. ###Professor, Depto. de Odontologia Restauradora, New Jersey Dental School, Newark, NJ 07103. Pedidos de reimpressão e correspondência para: Saul Weiner DDS, Dept. of Restorative Dentistry, New Jersey Dental School, 110 Bergen Street, Newark, NJ 07103. Telefone: 973-972-4246, Fax: 973-972-0370. E-mail:[email protected]

Resistência ao Deslocamento da Coroa num Suporte de Implante Hexagonal

RESUMO:Propósito: Avaliar a resistência e retenção de uma suporte hexagonal para deslocamento de coroa com alturas de coroa variadas (10, 12 e 14mm) e eixo de carga. Materiais & Métodos: Os implantes estavam encaixados em blocos de resina a 30° para a vertical. As coroas foram feitas em alturas de 10, 12 e 14mm. Grupos de cinco (5) coroas para cada uma das três (3) alturas de coroa foram carregadas (200N) tanto sob o eixo longo quanto o eixo secundário. Os espaços marginais foram medidos usando-se radiografias padronizadas periapicais antes e depois da carga. Os espaçoes foram comparados estatisticamente usando-se uma ANOVA tríplice com fatores de carga, altura da coroa e ponto de carga. As coroas foram carregadas em ciclos de 10 6 ou ponto de falha. Resultados: As coroas de 10mm não mostraram nenhum deslocamento. Os grupos de 12mm e 14mm apenas mostraram deslocamento das coroas carregadas do eixo secundário. O deslocamento marginal médio foi de 193.56u (SD +- 138.62u) no ponto de falha (320,717 ciclos) e 400.18u (SD +- 644.3l) no ponto de falha (134,278 ciclos) respectivamente. Conclusões: Um implante único com design padronizado pode proporcionar resistência e retenção suficientes para coroas de dimensão variada se as cargas estiverem centralizadas no eixo longo da coroa.

PALAVRAS-CHAVE: Resistência, coroa, cimento, implante

FIGURE

FIGURE
FIGURE
Keywords:

resistance; crown; cement; implant

© 2004 Lippincott Williams & Wilkins, Inc.