USE OF BASE METAL CASTING ALLOYS FOR IMPLANT FRAMEWORK : MARGINAL ACCURACY ANALYSIS

1DDS, MS, PhD student, Department of Prosthodontics, Bauru Dental School, University of São Paulo, Bauru SP, Brazil. 2DDS, MS, PhD Associate Professor, Department of Prosthodontics, Bauru Dental School, University of São Paulo, Bauru SP, Brazil. 3Resident, Hospital of Rehabilitation of Craniofacial Anomalies, University of São Paulo, Bauru SP Brazil. 4DDS, MS, Student Federal University of Santa Catarina UFSC SC Brazil.


INTRODUCTION
The development of osseointegration allowed a longterm predictable solution for totally and partially edentulous patients.In spite of the high prosthetic success rates 7,17 , long-term studies have demonstrated a high prevalence of prosthodontic complications 3,7,8,20 and the most common complication is screw loosening.Jemt, Linden, Lekholm 7 showed a 49% rate of screw failure for maxilla and 20.8% for the mandible, accounting for 13% of prosthesis mobility.
The literature has emphasized the need of a passive fit restoration for long-term success of osseointegration 15,18,20 and many techniques have been proposed to achieve a passive fit restoration 19 .In fact, each step of prosthesis fabrication influences the final fit 14 .To improve or even assure a better fit, the use of pre-made components has been highly recommended 1 .However, pre-made cylinders use high gold and silver-palladium alloys for framework casting, and their melting ranges are not compatible to alternative alloys 12 .The high cost of this type of framework has led to the development of plastic components allowing the use of alternative alloys, like base metal alloys.
The aim of this paper is to compare the marginal accuracy obtained by plastic components cast with two chromium type alloys and the marginal fit of pre-made metal cylinders.

MATERIAL AND METHODS
Plastic cylinders and metal cylinders for standard abutments were used in this study according to the following groups: Group 1 -Five plastic cylinders (Conexão Sistema de Próteses Ltda, São Paulo, Brazil) for standard abutment were cast with Cobalt-Chromium (Co-Cr) alloy (Rexilium® NBF, Jeneric Pentron® Incorporated, MT, USA).
Group 3 -Five metal silver-palladium pre-made cylinders ( Conexão Sistema de Próteses Ltda, São Paulo, Brazil) for standard abutment were analyzed as received, and used as a positive control group.
The cast specimens of groups 1 and 2 were produced using the plastic cylinders that were invested in phosphatebonded investment (Talladium Micro-fine 1700, Talladium do Brasil Inc. -Comércio de Materiais de Prótese Odontológica, Curitiba, Paraná, Brasil) according to the manufacturer's instructions.Each component was cast individually.Alloys composition and melting range are listed in Table 1.Divesting was carefully performed and no further finishing and polishing procedure was performed.
The abutment/cylinder interface for each specimen studied was analyzed at 8 different locations around the interface, according to the resin model design, using an optical microscope Mitutoyo TM 505 (Mitutoyo Corporation, Tokyo, Japan), with 150x magnification equipped with Digimatic Micrometer Heads 164-162 (Mitutoyo Corporation, Tokyo, Japan).
Three analyses were carried out for all groups: (A) vertical and (B) horizontal gap and (C) horizontal depth gap (Figure 1) Figure 2 illustrates the dimension of the expected horizontal surface of contact at the abutment/cylinder interface of the conventional abutment and prosthetic cylinders used in this study.
Mean values for each analysis were obtained for each specimen and for each group and then were submitted to one-way analysis of variance (p<0.05) and to the Student-Newman-Keuls test.

RESULTS
Mean values and standard deviations for vertical and horizontal gap, as well as the horizontal depth of gap, are shown in Table 2.
Significant differences were found for all analyses for p<0.05.(Table 3 to 5).Student-Newman-Keuls test revealed that the control group presented significantly better fit than both tested groups for all analyses.Significant differences (p<0.05) were also found when group 1 was compared to group 2 for the horizontal gap analysis (Table 2).

DISCUSSION
The use of alternative alloys in conventional prosthodontics is widely accepted due to the good properties of these alloys, but mainly for their low cost when compared to gold alloys.More than 80% of dentists in the United States use metal base alloys containing nickel, chromium and beryllium 16 .
For implant prosthesis, framework manufacture consists of casting-on to prefabricated gold alloy cylinders.In this manner, casting alloys must be chosen on the basis of the thermal properties relative to the cylinders and the temperature of cast molten alloy should not closely approach the solidus temperature of the cylinder.Usually, metal premade cylinders are made of gold or silver-palladium alloy with melting range around 1280 o C to 1350 o C (melting range for standard gold cylinders, Nobelbiocare) and therefore chosen casting alloys should not have a liquidus temperature much above 1000 o C. 21 Since Nickel-Chromium and Cobalt-Chromium alloys have a melting range around 1200 o C and 1315 o C (Table 1), they should not be used with pre-made gold cylinders.Plastic cylinders are the only available option if this type of alloy is to be used.When plastic cylinder components are used, the conventional lost wax technique is used and the contact surface of the cylinder as well as the internal cylinder surface is influenced by a variety of processing and handling conditions of the fabrication process, as well as the casting accuracy of the alloy 11 .
In fact, for all analyses carried out in this study, marginal discrepancies for cast components obtained from plastic cylinders were statistically higher than for pre-made cylinders.Figure 3 shows the aspect of a cylinder and Figures 4 and 5 show the aspect of a plastic cylinder after casting procedure.When comparing Figures 4 and 5 to Figure 3, the superiority of pre-made cylinders is evident.Figures 6 and 7 show the abutment/cylinder interface when a cast cylinder and a pre-made cylinder is used.
A clinical trial carried out by Jemt, Book 6 could not found a statistically significant correlation between marginal bone level and prosthesis misfit.But, when considering the mechanical aspect of the implant prosthesis, poor fitting prosthesis with 6µm to 10µm vertical misfit may lead to screw loosening 5,8 .In this study, vertical gap of cast cylinders of 23.18µm (Ni-Cr) and 25.06 (Co-Cr) were found, 5 to 6 times greater than for pre-made cylinders (4.13µm).
In a previous study, Kano, et al. 9 compared the vertical misfit obtained after casting procedures when plastic cylinders and gold cylinders were used.Even though higher values were obtained for all groups when compared to present study, both studies clearly demonstrate that casting procedures do influence the final fit of prosthetic components when plastic cylinders are used, no matter what type of alloy is used.The initial fit of plastic cylinders can be responsible for the results obtained since even pre-made metal cylinders present a marginal misfit.Marginal discrepancies of the as-received metal cylinders can vary from 0.5µm to 5.04µm, depending on the implant system used and as high as 46.9µm when components from different manufacturers are combined 10 .
The inability to produce a surface free of irregularities and the impossibility to finish and polish the final surface with reliability may be the reasons for discrepancies observed.
When analyzing horizontal misfit frameworks, Patterson, Johns 13 have stated that when implant parts are not correctly aligned, internal connecting screws are more susceptible to fracture or loosening.According to White 21 , this can happen because abutment replicas have a lateral fitting error.In this study, the mean horizontal misfit of pre-made cylinders was 14.5µm before submitted to any laboratory procedure.This means that no matter how careful is the framework fabrication process, the final prosthesis will be delivered with some degree of horizontal misfit.And when plastic cylinders are cast, significant higher horizontal misfit is to be expected, as seen in this study where horizontal misfit of 33.2µm for Ni-Cr alloy and 51.8µm for Co-Cr was observed.
When implant prosthesis is screwed in place, the torque applied to the screw creates a preload which keeps the components together.The higher the preload, the higher will be the ability of the screw to withstand the external loads without loosening.Changes in the surface of the cylinder can influence preload because 50% of the torque is influenced by friction of this area and part of the preload can be lost when the screw is used to bring components together 4 .
During microscopic analysis of specimens, vertical and horizontal marginal discrepancies of cast components were evident, but a careful look at the abutment/cylinder interface of cast components demonstrated that contact surface between both components was very compromised by the depth of the gap. Figure 2 shows that the area of contact at the abutment/prosthetic component interface is a 0.20mm (200µm) flat surface for the standard abutment and a 0.25mm (250µm) surface for the prosthetic cylinders.When plastic components were cast in Co-Cr alloy, a mean depth gap of 134.5µm was created, for example, only one third of the abutment contact surface was really in contact with cast cylinder (Figure 8A and 8B).This lack of contact between  Although the present literature lacks in providing conclusive results on the role of marginal fit of implant prosthesis in the screw loosening phenomena, and the great variables encountered in the use of plastic components for the fabrication of implant superstructure, the use of plastic components should be viewed with caution when precision and predictability is desired.Longitudinal clinical studies

CONCLUSIONS
Under the conditions of this study, the following conclusions can be made: 1.The mean marginal discrepancies for plastic cylinders cast with Co-Cr and Ni-Cr alloy were statistically different when compared to the pre-made metal cylinder, for vertical, horizontal and depth gap analysis, except for the horizontal misfit of the plastic cylinder cast with Ni-Cr and pre-made cylinder.
2. The mean marginal discrepancies of plastic cylinders cast with Co-Cr and Ni-Cr were not statistically different when compared to each other.

FIGURE 2 -
FIGURE 2-Contact surface of prosthetic components: a) abutment ; b) metal cylinder; c) plastic cylinder 2a 2b 2c both components may lead to tilting and rocking of the final restoration, which may cause the screw loosening and fracture4,2 .

FIGURE 7 -
FIGURE 7-Marginal fit of pre-made metal cylinder at the abutment/cylinder interface

FIGURE 6 -
FIGURE 6-Abutment/cast cylinder interface: vertical and horizontal gap and horizontal depth gap

FIGURE 8 -
FIGURE 8-Horizontal Depth Gap at the abutment/cylinder interface of a cast cylinder: a) without screwing, note how small is the contact surface; b) components are screwed together, note that no contact exists at the interface

TABLE 2 -
Mean values (µm) for each group.Bars represent values that do not differ significantly (Student-Newman-Keuls -p<0.05)

TABLE 3 -
Results of 1-way analysis of variance for Vertical Misfit

TABLE 4 -
Results of 1-way analysis of variance for Horizontal Misfit

TABLE 5 -
Results of 1-way analysis of variance for Gap Depth