Resistance to Crown Displacement on a Hexagonal Implant Abutment

Resistance to Crown Displacement on a Hexagonal Implant Abutment
June 2004
Kwan, Norman DDS*; Yang, Silvia BScÇƒÜ; Guillaume, Didier DMDÇƒ?; Aboyoussef, Hoda DMD, MS¨?; Ganz, Scott D. DMD||; Weiner, Saul DDS
Implant Dentistry: Volume 13(2) June 2004 pp 112-119
Lippincott Williams & Wilkins
*Director, Canadian Dental Implant Institute, St. Catharines, Ontario, Canada.
ÇƒÜResearch Assistant, Canadian Dental Implant Institute, St. Catharines, Ontario, Canada.
Çƒ?Formerly a postgraduate student, Department of Restorative Dentistry, New Jersey Dental School, Newark, New Jersey.
¨?Associate Professor, Department of Restorative Dentistry, New Jersey Dental School, Newark, New Jersey.
||Assistant Clinical Professor, Department of Restorative Dentistry, New Jersey Dental School, Newark, New Jersey and Private Practice, Fort Lee, New Jersey.
¨?Professor, Department of Restorative Dentistry, New Jersey Dental School, Newark, New Jersey.
Reprint requests and correspondence to: Saul Weiner, DDS, Department of Restorative Dentistry, New Jersey Dental School, 110 Bergen Street, Newark, NJ 07103, Phone: (973) 972-4246, Fax: (973) 972-0370, E-mail: weiner@umdnj.edu
Abstract
The purpose of this study was to evaluate the resistance and retention of a hexagonal abutment to crown displacement with varying crown heights (10, 12, and 14 mm) and axis of load. Implants were embedded in resin blocks at a 30¨? angle to the vertical. Crowns were made at heights of 10, 12, and 14 mm. Groups of 5 crowns for each of the 3 crown heights were loaded (200 N) both under the long axis and off axis. The marginal gaps were measured using standardized periapical radiographs before and after loading. The gaps were statistically compared using a 3-way analysis of variance with factors of load, crown height, and point of load. Crowns were loaded to 106 cycles or point of fail ure. The 10-mm crowns did not show any displacement. The 12-mm and 14-mm groups only showed displacement of the off axis-loaded crowns. The average marginal displacement was 193.56 ¨µ (standard deviation [SD] ¨± 138.62 ¨µ) at point of failure (320,717 cycles) and 400.18 ¨µ (SD ¨± 644.31) at point of failure (134,278 cycles), respectively. A 1-piece implant with a standardized abutment design can provide sufficient resistance and retention form for crowns of varying dimension if loads are centered over the long axis of the crown.
During the early 1980s, successful surgical and restorative protocols were established for predictable root-form titanium implants mainly through innovative work by Br?ïnemark 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.
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