The influence of screw type, alloy and cylinder position on the marginal fit of implant frameworks before and after laser welding.

UNLABELLED: Misfit at the abutment-prosthetic cylinder interface can cause loss of preload, leading to loosening or fracture of gold and titanium screws.
OBJECTIVES: To evaluate the influence of screw type, alloy, and cylinder position on marginal fit of implant frameworks before and after laser welding.
METHODS: After Estheticone-like abutments were screwed to the implants, thirty plastic prosthetic cylinders were mounted and waxed-up to fifteen cylindrical bars. Each specimen had three interconnected prosthetic components. Five specimens were one-piece cast in titanium and five in cobalt-chromium alloy. On each specimen, tests were conducted with hexagonal titanium and slotted gold screws separately, performing a total of thirty tested screws. Measurements at the interfaces were performed using an optical microscope with 5mm accuracy. After sectioning, specimens were laser welded and new measurements were obtained. Data were submitted to a four-way ANOVA and Tukey's multiple comparisons test (alpha=0.05).
RESULTS: Slotted and hexagonal screws did not present significant differences regarding to the fit of cylinders cast in titanium, either in one-piece casting framework or after laser welding. When slotted and hexagonal screws were tested on the cobalt-chromium specimens, statistically significant differences were found for the one-piece casting condition, with the slotted screws presenting better fit (24.13 microm) than the hexagonal screws (27.93 microm). Besides, no statistically significant differences were found after laser welding.
CONCLUSIONS: 1) The use of different metal alloys do exert influence on the marginal fit, 2) The slotted and hexagonal screws play the exclusive role of fixing the prosthesis, and did not improve the fit of cylinders, and 3) cylinder position did not affect marginal fit values.

INTRODUCTION

The osseointegration between titanium and bone has been investigated and confirmed by anatomical and histological studies1. However, the clinical success of implant-supported restorations depends on several biomechanical factors. The precision between machined parts is one of them7,8. High precision components are necessary to provide stability, resistance and excellent esthetical results.

Nowadays, titanium is very important as a restorative material due to its mechanical resistance, biocompatibility, and inertness. In addition, it is the only metal that can be soldered intra-orally9,11. Furthermore, it has adequate thickness to provide support for the dental porcelain19.

Nevertheless, Co-Cr alloys have been advocated in dentistry for more than 60 years to cast removable partial dentures. These alloys have low density values, good resistance to tarnish and erosion, low cost and high elastic modulus2,3. However, they demonstrate poor castability, difficulties in finish and polishing, and great shrinkage after casting procedures1.

A study showed that one-piece casting FPDs have predictable degree of distortion (100μm)27. In this sense, any attempts to reduce non-axial forces to the surrounding bone could be advantageous. One paradigm in implant prosthodontics is to obtain passive fit between abutment and prosthetic cylinders incorporated into the framework18,20,23,24,26when gold alloys are used. To solve this problem, laser welding procedures have been proposed since 1987 as an alternative method for implant-supported prostheses8. Thus, pre-selected parts can be joined together with less residual stresses5,6,12,22,29,30.

Even thus, screw loosening has been observed either in single or extensive implant-supported restorations. It is not certain to what extent the screw material or its configuration could exert some influence at the abutment-cylinder interface. Also, the interaction between laser welding and screw material on marginal fit results is unknown.

In this way, the purpose of this study was to evaluate the influence of screw type, alloy, and cylinder position on the marginal fit before and after laser welding procedure.

PROPOSITION

The present study evaluated, before and after laser welding:

Marginal fit at abutment-prosthetic cylinder interface, cast in titanium and Co-Cr alloys;

Marginal fit at abutment-prosthetic cylinder interface, with different screw types (slotted or hexagonal);

Marginal fit at abutment-prosthetic cylinder interface, according to the cylinder position.

MATERIAL AND METHODS

The master model used in this study was constructed as follows: three implants with external hexagon design, regular platform (10mm-long, 3.75mm-diameter; Conexão Sistemas de Prótese – São Paulo– SP – Brazil) were embedded with self-cured acrylic resin (Artigos Odontológicos Clássico LTDA, São Paulo) in a rectangular metallic base (40x10x15mm) (Figure 1). Estheticone-like abutments (22CNB-A 3mm, Conexão Sistemas de Prótese – São Paulo – SP – Brazil) were screwed on each implant with a 20Ncm torque (Nobelpharma – Torque Controller™ Gotemburg, Sweden).

FIGURE 1

Waxed-up bar incorporating prosthetic cylinders

To obtain prosthetic frameworks, plastic cylinders were screwed to the abutments. Then, cylindrical bars were attached to the plastic components (105CNB Conexão Sistemas de Prótese – São Paulo – SP – Brazil). To verify the influence of alloy type on marginal fit, wax patterns were divided into two groups: G1- Titanium alloy (5 specimens) (Rematitan – Dentaurum Pforzheim – Germany) and G2- Cobalt-Chromium alloy (5 specimens) (Rexillium N.B.F. – Jeneric/Pentron Incorporated). Both alloys were invested and cast according to manufacturer's instructions. Titanium was cast in a specific atmosphere to avoid bulk contamination. Samples were divested and internally cleaned with 45% hydrofluoric acid. To protect external margins from scratching after cleaning, abutment analogs were screwed and frameworks blasted with glass beads particles.

The prosthetic frameworks were placed in the master model with a 10Ncm torque. The cylinders were named C1, C2 and C3. To verify the influence of screw type on marginal fit, slotted (S) and hexagonal (H) screws were tested (Figure 2). Thus, four subgroups were formed: G1-S, G1-H, G2-S and G2-H. Analysis of marginal fit in each subgroup was performed with a digital microscope (Mitutoyo, Tokyo, Japan) (Figure 3A,B). Measurements were made at abutment-cylinder interface. Each point was measured 3 times, which resulted in 12 measurements. Final marginal fit corresponded to the overall mean of these values.

FIGURE 2

Titanium and gold screw general configuration

FIGURE 3A

Specimen positioned in the microscope for analysis

FIGURE 3B

Cross-sectional view of points for measurement

To verify the influence of laser welding on marginal fit, frameworks were cross-sectioned with a carborundum disc under water cooling (ref.9500.900.220.GEBR – Brasseler Gmbh & Co.KG, Lemgo, Germany) at low speed rotation. The separated parts had their marginal fit as already described and were screwed again. A laser welding machine (DL 2002s – Laser Dentaurum Pforzheim, Germany) was used to obtain the solder joints. The laser weld was scheduled to 300V/10msec at a focus zero frequency. After laser welding, new marginal fit measurements were performed using the same criteria in both subgroups.

Statistical analysis

The Kolmogorov-Smirnov test was used to verify homogeneity in both subgroups. Data were submitted to a four-way ANOVA analysis (alloy, cylinders, laser welding, and screws) to identify significant differences among groups (α=0.05). In the event that differences were found, a Tukey's multiple comparisons test was performed to determine which groups were similar or different (α=0.05).

RESULTS AND DISCUSSION

One of the greatest difficulties in implant-supported prosthodontics is to obtain precise fit when the metal framework is screwed onto the abutments. This primary goal has been the subject of several studies4,15,16,21,25,28. According to Carlsson and Carlsson1, an implant prosthesis with passive fit means that it can be screwed without tensile stress transmission to the implants or associated components. However, there are several designs and materials for prosthetic screws. Manufacturers state that screws are interchangeable, regardless of their material or mechanical properties. In addition, prosthetic screws that generate higher torque values are less prone to fracture or deformation; however, this assumption was not confirmed under clinical conditions. The main objective of this work was to answer the question: what really would be the influence of different screws, alloys and cylinder position on the marginal fit before and after laser welding?

One limitation of this study could be the generated torque applied to the screws; however, a torque controller device was used since manual torque is prone to fatigue operator. Machining defects in the thread configuration also influence final results and irregular screws were discarded before the experiment begins.

In the present study, non-statistically significant differences were observed at abutment-prosthetic cylinder interface when the screw type was the source of variation. For the marginal fit values, statistically significant differences were found between cast material (titanium and Co-Cr alloys), and between cast techniques (one-piece castings and laser welding procedures) (Table 1).

TABLE 1

Marginal fit values before and after laser welding (μm)

Cylinder	Subgroups	Before	After
C1	G1-S	25.08+6.30 a	15.83+4.57 a
C2		25.99±8.01	19.49±4.30
C3		24.83±3.73 b	15.75±3.60 b
C1	G1-H	24.99+4.27	19.08+6.86
C2		22.80±2.70	18.99±4.86
C3		25.33±6.90	16.33±2.86
C1	G2-S	22.91+3.97	23.91+4.52
C2		25.33±3.02	21.41±4.60
C3		24.16±2.58	22.83±3.10
C1	G2-H	29.83+7.66	24.83+6.07
C2		26.74±3.54	21.07±5.39
C3		27.24±4.96 c	19.33±3.12 c

C= cylinder, S= slotted screw, H= hexagonal screw, G1 = titanium alloy. G2 = cobalt-chromium alloy Small letters represent statiscally significant differences in the same line (α<0.05)

Marginal fit values for slotted screws were not different with or without laser welding (Table 1), but significant differences were found for hexagonal screws in the same conditions. Slotted and hexagonal screws did not present significant differences regarding to the fit of cylinders cast in titanium, either in one-piece casting framework or after laser welding. When slotted and hexagonal screws were tested on the cobalt-chromium specimens, statistically significant differences were found for the one-piece casting framework condition, with the slotted screws presenting better fit (24.13μm) than the hexagonal screws (27.93 μm). Besides, no statistically significant difference was found after laser welding. This observation is in accordance with previous studies that recommend sectioning and soldering to achieve better fit17,22,26. Furthermore, slotted gold screws have an elastic modulus two times lower than the titanium screws. This condition generates larger deformations at the first screw threads to achieve higher pre-load values.

Significant differences were observed for both screws when one-piece castings were obtained. Thus, these differences are due to casting techniques and not to the screws. For example, titanium has to be cast in special atmospheres (e.g., noble gases) given its high oxide formation due to contamination. Similarly, Co-Cr alloy provided higher marginal fit values compared to titanium alloy. This can be explained by differences in castability of both alloys that are not related to the screw configuration. Even thus, the Co-Cr alloys have an important role in dentistry due to their low cost14,15. An acceptable fit has been observed when Co-Cr is cast for implant frameworks13,17,31.

JEMT, et al.16suggested that acceptable marginal fit levels were less than 150 mm. Moreover, even with the little range of marginal fit values (16 to 29mm) (Table 1) obtained, gaps with 3mm or less are harmful since a number of microorganisms have this diameter. Potential plaque accumulation in these areas produces one type of electrolyte, and the saliva provides another electrolyte at the occlusal surface. Therefore, electrochemical corrosion (concentration cell or crevice corrosion) occurs, with preferential attack of the metal surface occurring underneath the layer of food debris29. In the same way, stress and pitting corrosion occur in chromium alloys due to chloride ions. The corrosion products can spread into the periimplant sulcus leading to inflammation, bone loss, and lack of osseointegration. Regardless of values obtained, adequate oral hygiene procedures are mandatory to guarantee long-term stability of prosthesis.

The use of different metal alloys do exert influence on the marginal fit;

The slotted and hexagonal screws play the exclusive role of fixing the prosthesis, and did not improve the fit of the cylinders;

Cylinder position did not affect marginal fit values.