Brazilian Journal of Oral Sciences Piracicaba Dental School - UNICAMP EISSN: 1677-3225 Vol. 4, Num. 15, 2005, pp. 919-922

Brazilian Journal of Oral Sciences, Vol. 4, No. 15, Oct./Dec. 2005, pp. 919-922

Effect of different torque levels on the implant/abutment interface using castable “UCLA” abutments

Gustavo Augusto Seabra Barbosa¹ Paulo Cézar Simamoto Junior² Alfredo Júlio Fernandes Neto³ Maria da Glória Chiarello de Mattos⁴ Flávio Domingues das Neves⁵

¹DDS, MS, PhD Student, School of Dentistry of Ribeirão Preto, University of Sao Paulo. Substitute Professor at Department of Fixed Prosthodontics, Occlusion and Dental Materials, Federal University of Uberlandia. ²DDS, MS, PhD Student, Piracicaba Dental School, State University of Campinas ³DDS, MS, PhD, Titular Professor at Department of Fixed Prosthodontics, Occlusion and Dental Materials, Federal University of Uberlandia ⁴DDS, MS, PhD, Professor at Department of Dental Materials and Prosthesis at School of Dentistry of Ribeirão Preto, University of Sao Paulo ⁵DDS, MS, PhD, Assistant Professor at Department of Fixed Prosthodontics, Occlusion and Dental Materials, Federal University of Uberlandia

Correspondence to: Gustavo Augusto Seabra Barbosa Av: Brasil , 4800, Bl. C; Apt 102 – Umuarama CEP: 38405-312 – Uberlândia/MG Phone: (34) 3232-7356 E-mail: seabrabarbosa@uol.com.br / gustavoseabra@forp.usp.br

Received for publication: May 24, 2005
Accepted: November 21, 2005

Code Number: os05040

Abstract

The abutment/implant interface has been reported as a significant factor on tensile transfer, adverse biological responses or prosthetic restoration complications. The abutment/implant junction success is directly related to pre-load attained during torque application. The purpose of this work was to assess the vertical fit/misfit degree on abutment/implant interface, when castable “UCLA” type abutments are used on fixed prosthesis process on implants, after different torque levels applications. Four three-unit fixed prosthesis were manufactured by the same laboratory. Torque was applied on the same framework using initially a 10 Ncm (T1) and later, a 20 Ncm torque level (T2) with a screwdriver hand Torquimeter. Trough scanning electronic microscope (SEM), it was obtained two photographs corresponding to mesial and distal regions of each prosthesis unit, summing 24 assessed regions to each applied torque value. After submitting the results to statistical analyses Paired T-Test (p<0.05), a significant difference was seen between the average vertical fit/misfit degree: T1 = 23.53mm ± 20.20; T2 = 9.01m ± 11.69 (p=0.000). The misfit degree decrease when applied manufacturers recommended torques, could decrease mechanical (loosening or fracture of the screw) complications.

Key Words: dental implants, prosthetic adjustment, torque

Introduction

The abutment/implant interface has been reported as a significant factor on stress transfer, adverse biological response or prosthetic restoration complications¹. Michalakis et al.² reported two likely complications types inherent to a misfit of the prosthetic framework: a) biological – increased load transfer to bone, bone loss, and microflora development in the micro-gap between implant and abutment^3-4; and b) prosthetics – loosening or fracture of the screw and implant loss⁵.

Due to periodontal ligament absence, implants are incapable to readily adapt to misfit restorations induced stresses⁶. It should than, have a relative precision on fit between prosthesis and abutments, as also between prosthesis and implants⁷, to the implant-supported restorations long term success⁸. However, there’s no perfect fit, and thus, the clinicians should accept certain misfit level, which doesn’t affect the bone/implant interface health⁹. According to early information about osseointegration, it is considered that an acceptable fit, values less than 10µm¹⁰.

The screw tightening aim to maintain the union between components. However, during function, micro-gaps could be formed on the abutment/implant interface due to an untight abutment fixture, favoring bacterial invasion, as well as mechanical problems¹¹.

On every implant system the fixture joint system efficiency depends on several factors: the component design; connection geometry between implant and abutment; mechanical adjustment between fixture and its set surface on abutment; the mechanical and physical component properties and torque application¹².

The tension needed to maintain two parts of the same junction tightly united, either under static or dynamic conditions, is referred as preload¹³. Surface imperfections lead to friction rising and to decreasing of such pre-load. Increasing the same is due to screw take off and consequent tightening^13-14. During abutment screw tightening, a compressive strength is generated to keep implant and abutment surfaces in touch. This joint success is directly related to attained pre-load during torque and for this preload maintenance with time. The fixture untightening, as also the pre-load reduction to critical levels, can compromise the joint stability, making a clinical failure more powerful¹⁵. Regarding to the screw tightening, some authors report that increasing the torque applied on screws, the compression between the facing surfaces is also increased, producing a greater stability on the joint¹⁶.

In one study comparing the micro-leakage level on abutment/ implant interface of five implant systems, varying the torque between 10 Ncm and 20 Ncm, the authors observed significant differences on micro-leakage between applied torques. There was a significant decrease when the torque was near to recommended values¹⁷.

Due to a great misfit presented by UCLA abutment, this study aims to evaluate the vertical fit/misfit degree on abutment/ implant interface, when using castable “UCLA” type abutments, with different torque levels.

Four three unit fixed prosthesis were manufactured from a metallic matrix with three compatible Brånemark implants (3,75x10mm) (Conexão Sistemas de Prótese, Sao Paulo, SP, Brazil). Those were made of castable UCLA type abutments (Conexão Sistemas de Prótese, Sao Paulo, SP, Brazil), with nickel-chrome alloy (Verabond II, Albadent Inc., USA). The frameworks were united with conventional solder method. They were placed on the matrix, first with 10 Ncm torque (screwdriver hand Torquimeter, Conexão Sistemas de Prótese, Sao Paulo, SP, Brazil) (Treatment 1 – T1) so the first analyze could be done, using the scanning electronic microscopy (SEM, LEO 435 VP, GERMANY), at a x500 magnification. Later, the same frameworks were placed, with a 20 Ncm torque (Treatment 2 – T2), to the last analyze.

For each framework “dental unit”, two images were obtained, one mesial and other distal, making a total of 24 images. The images were printed using a laser printer (Laser Jet 1500L, Hewlett Packard, USA). From the printed photos, a ruler was built with the micrometers scale existing on the lower portion of the images, by a rule of three.

Tracing a line paralleled to the implant platform and other parallel to lower surface of the abutment the vertical fit/misfit value can be obtained, if it’s present. Looking for a better view of the results, the measurement site received a number (Table 1). Factors as visual accuracy, view angle, light and professional experience, were described by Kan et al.¹⁸, as the factors that could interfere on the accurate determination of misfit. To minimize those interference factors, three examiners were trained to the measurements. Statistical processing was done using the SPSS data program (SPSS/ PC for Windows Inc., Chicago, IL). Parametric statistical test were used, with the Paired T-test for paired samples. A p-value less than 0.05 was considered statistically significant.

The three examiners mean values for vertical misfit after two torque levels apply (T1 and T2) are expressed in µm, on Table 2. As showed on table 2, all misfit values were larger when 10 Ncm load was applied, decreasing, in some cases, more than 35µm after applied torques of 20 Ncm load (as show regions 13 and 19, table 2). After 20 Ncm torque application sixteen analyzed regions presented values less than 10µm and figures 1 and 2 exemplify the best fit obtained with this torque level. The statistical analysis (Paired T-test) presented significant difference in the vertical fit among the two levels of applied torques (p=0.000), being lower when used 20 Ncm.