Determination of strain around distal implants as abutments for partial and fixed implant-supported prostheses with posterior cantilever.

This study aimed to compare strain around implants used as abutments for removable partial dentures with wrought wires and fixed partial dentures with ball attachments and fixed dentures with posterior cantilever. An edentulous mandibular model was constructed using epoxy resin with four parallel implants in the area between the two mental foramina. Four strain gauges were attached to the buccal, lingual, mesial, and distal aspects of each implant. One fixed prosthesis with cantilever and two removable partial dentures were considered as prosthetic treatments. A vertical 500-N force was applied with 10-N intervals. The maximum strain in the fixed prosthesis was higher than that of the partial removable denture; in the removable denture with a wrought wire arm, it was higher than that in the denture with a ball attachment (P < 0.001). The lowest rate of strain was recorded on the mesial aspect (P < 0.05). However, the highest rate of strain was recorded on the lingual and distal aspects of the removable denture with a wrought wire in the buccal aspect and the removable prosthesis with the ball attachment on the buccal and lingual aspects (P < 0.05). Finally, despite minor differences in the maximum strain rate in each implant position, the differences were not statistically significant (P > 0.05). Partial removable denture with a ball attachment decreased strain more than that by the removable portal denture with a wrought wire arm. The worst type of prosthesis in terms of the overall strain rate was the fixed prosthesis with cantilever. Copyright:

INTRODUCTION

Different treatment options with implants are available for edentulous patients. The use of implants in mandibular reconstruction has been associated with excellent results.[1] Although implant-supported prostheses are associated with numerous advantages, it is not possible to render such treatment to all patients without osseous grafts. Therefore, removable implant-supported prostheses are considered a favorable option.[2]

Previous studies have reported a high success rate with the use of four implants in the area between the mental foramina in edentulous patients.[3]

Excessive occlusal loads exerted due to inattention to the design of implant-supported prostheses are one of the most important factors for bone loss around implants and implant-supported prostheses.[4]

Bone biology studies have shown that when implants undergo excessive occlusal loads, significant deformation occurs at 2000–3000 microstrain levels at the peri-implant bone. When pathologic and excessive loads occur at microstrain levels of > 4000, the stress and strain exceed the bone tolerance threshold, finally resulting in microfractures at implant-bone interface.[5]

Some studies have suggested the use of implants as an abutment for removable partial dentures, with successful outcomes.[6] However, strain rate and distribution in the bone around the implant used as an abutment for the removable partial denture treatment are still unknown.

Different treatment plans are available for placing partial dentures with implant abutments in the anterior edentulous mandible: (1) use of cantilever fixed short-arch prostheses and (2) implant-supported overdentures.[7]

According to previous studies, cantilevers in implant-supported prostheses directly affect stress distribution around implants and stresses increase with an increase in the cantilever length.[8]

In the present study, the treatment plan consisted of the reconstruction of the anterior segment with implants with a fixed prosthesis and the use of partial dentures with different attachments for posterior areas to evaluate the distribution of stress in posterior implants for plans 2 and 3.

MATERIALS AND METHODS

Preparation of the study model

A patient with mandibular edentulism and moderate resorption of the mandibular ridge, who was a candidate for an implant-supported overdenture, was selected for this study.[9] An impression of the mandible was taken and poured with the Type III stone (Mold Stone, Pars Dandan, Iran). Special trays with acrylic spacers were fabricated for both jaws (Acropars 200, Eshtehard Industrial, Iran).

The resulting cast was used to take an impression with a monophasic material (Panasil Monophasic Medium, Kettenbach, GmbH, Germany) and preserved for later use. A carbide bur (Tizkavan, Iran) was used to remove a layer, measuring approximately 2 mm, which is almost equal to the thickness of the mucosa in the mandible, from the surface of the stone case.[10] An impression was then made from the prepared cast using a monophasic material (Panasil Monophasic Medium, Kettenbach, GmbH, Germany) and poured with autopolymerized resin (POLYUROCK; Metalor technologies, Stuttgart, Switzerland). The resin's flexural coefficient was 3000 MPa, which is similar to the trabecular bone.[11] Subsequently, using the first monophasic impression, Gingifast (Zhermack, A-silicone for gingival mask, Italy) was added to simulate the mucosa on the cast prepared using the Polyurock resin, and the final resin model was prepared.[5]

Four fixtures (Dentis, South Korea), measuring 12 mm in length and 3.7 mm in diameter, were placed at positions A, B, C, and D between the two mental foramina parallel to each other and perpendicular to the ridge crest, similar to the OD-4 plan of mandibular overdenture.[9] Stain gauges measuring 2 mm × 3 mm were used in the present study. Four strain gauges were selected for each implant and placed on the buccal, lingual, mesial, and distal aspects in a central position perpendicular to the surface. Cyanoacrylate glue (Grandil Co., Iran) was used to attach the strain gauges [Figure 1].

Figure 1

Connecting the gauge strains to the implants

A try-in base was fabricated using VLC plates (Mega-Light Tray, Mega Dental, GmbH, Germany), measuring 2 mm in thickness to mount on the mandibular model.[12]

Wax occlusal rims (Cavex set up regular, Modeling wax, Netherlands) were placed on the try-in bases. The models were adjusted using bases and rims and mounted on an articulator (Hanau, Arcon, Germany) in a moderate Class I jaw relationship.[5] The analogs were connected, and the impressions were poured with Type III dental stone (Mold Stone, Pars Dandan, Iran). An alginate impression was taken from the maxillary model using a stock tray, and mounting was carried out on the articulator with the help of the existing base and rim. Straight abutments were then connected to the analogs. In the next stage, a 12-unit fixed prosthesis was fabricated on the cast and abutments [Figure 2].[13]

Figure 2

The 12-unit fixed prosthesis

A resin index was prepared from this 12-unit bridge with the use of additional silicone impression material to ensure the similarity of contours and transfer the contours. Two 8-unit fixed prosthesis plans were designed and fabricated on the casts after preparing the silicone index so that the removable dentures could be fabricated on these two types of prosthesis with distal extension. These two types of prosthesis were fabricated by extension from the premolar area on one side to the premolar area; on the other side, their main difference was in their retention mechanism:

Prosthesis no. 1: Rests were placed on the mesial aspect of the first premolar teeth and the cingulum of the canine teeth, with guiding planes on the distal aspect of first premolar teeth. Two wrought wires were used as retention arms. Due to the round cross-section of wrought wires, this form of retention is more flexible than circular clasps [Figures 3 and 4] [7]

Figure 3

The removable denture with a ball attachment

Figure 4

The removable denture with a wrought wire arm

Prosthesis no. 2: Two ball attachments were used distal to the first premolar teeth to fabricate this prosthesis. Ball abutments with a length of 4 mm manufactured by the South Korean company DIO were used in this investigation.

After fabricating two 8-unit fixed prostheses, one removable prosthesis with distal extension was fabricated on each of these designs. The first removable denture was fabricated and prepared by considering rests on the fixed prosthesis and placing two wrought wires. The second removable denture was fabricated without wrought wire retentive arms, using an extracoronal ball attachment system. The ball attachment was placed distal to the first premolar teeth on both sides. The prepared dentures were mounted on the chief model.[5] The strain gauge ends were connected to the signal conditioning amplifiers, which were monitored by MATLAB software.

Connection to the apparatus simulating jaw movements

A mechanical device that simulated the masticatory movements of the jaws was used to apply force. The device was able to apply 0–300-N force at 10-N intervals perpendicular to the occlusal surface of the teeth in an articulator, forcing the articulator to move laterally and protrusively with a range of 0–3 mm (GNATUS, NON-ARCON, BRAZIL).[5]

The device was adjusted to apply loads at 0–30-N range with 10-N intervals.[14] The ends of strain gauges were connected to the signal conditioning amplifiers, which were monitored by the MATLAB software.[5]

The strains were separately measured at different points in peri-implant bone and around the final prosthesis, using the following formula:

Stress = Strain × Elasticity (3000 MPa).

RESULTS

(One-waye ANOVA) as shown in Table 1 indicates that strain in the implants and different areas of the fixed prosthesis was significantly higher than the two other prostheses. The maximum strain in the removable partial denture with wrought wire was significantly higher than the removable partial prosthesis with a ball attachment (P < 0.001).

Table 1

The maximum strain values in the three prostheses

Prosthesis type	Maximum mean strain±SD	P
Fixed	86.3±12.3	<0.001*
Partial removable with wrought wire	64.2±5.3	<0.001*
Partial removable with ball attachment	54.1±7.4	<0.001*

*Statistical significance. SD: Standard deviation

Besides, Figures 5 and 6 compare the maximum strain rates in the three prosthesis types in terms of the surface and implant, respectively.

Figure 5

Comparison of the maximum strain of different prostheses in terms of surface

Figure 6

Comparison of the maximum strain of different prostheses in terms of the implant

Table 2 presents the maximum strains measured on the four surfaces and in four implants. Figure 7 compares the maximum strain values on the four surfaces between the four implants, with no significant difference between different implants (P > 0.05).

Table 2

The maximum strain values on different surfaces and implants in the fixed prosthesis

	Maximum mean strain
Buccal	87.2±6.4
Lingual	89±6.7
Mesial	72.7±5.1
Distal	96.2±16
Implant A	88.7±11.5
Implant B	86±7.5
Implant D	86.7±8.6
Implant E	93.7±17

Figure 7

Comparison of the maximum strain of different implants between different surfaces in the fixed prosthesis

However, the maximum strain on the mesial aspect was significantly lower than the lingual and distal aspects (P = 0.004 and P = 0.01, respectively).

Table 3 and Figure 8 presents the maximum strain values measured on four surfaces and four implants in the removable partial denture with wrought wire. Besides, Table 4 compares the maximum strain values between the four surfaces in four implants. The results showed no significant differences between the four different implants (P > 0.05). The maximum strains on the buccal aspect were significantly higher than the mesial aspects (P = 0.001 and P = 0.007, respectively) and significantly lower on the mesial aspect than the distal aspect (P = 0.04).

Table 3

The mean maximum stain values on different surfaces and in different implants in removable partial dentures with wrought wire

	Maximum mean strain
Buccal	87.2±6.4
Lingual	89±6.7
Mesial	72.7±5.1
Distal	96.2±16
Implant A	66.5±7.6
Implant B	63.7±4.6
Implant D	63.7±2.6
Implant E	64.6±3.4

Figure 8

Comparison of maximum strain values of different implants between different surfaces in removable partial denture with wrought wire

Table 4

The mean maximum strain values on different surfaces and implants in the removable partial denture with the ball attachment

	Maximum mean strain
Buccal	58.7±3.1
Lingual	60.7±4.5
Mesial	44±2.8
Distal	62.5±3.5
Implant A	57±10.7
Implant B	52.7±8.1
Implant D	52.2±5.7
Implant E	54.2±6.7

Table 4 presents the maximum strain values measured on four surfaces and four implants in the removable partial denture with the ball attachment. Also, Figure 9 compares the maximum strain values on four surfaces and in four implants. The results showed no significant differences between the different implants (P > 0.05). However, the maximum strain on the distal surface was significantly higher than the lingual surface (P = 0.02) and significantly lower on the mesial surface than the three other surfaces (P = 0.02, P = 0.01, and P = 0.04 for the buccal, lingual, and distal surfaces, respectively).

Figure 9

Comparison of the mean maximum strain values between the different surfaces in the removable partial denture with the ball attachment

DISCUSSION

The present study aimed to determine and compare peri-implant strains in partial removable denture design with a wrought wire arm with a partial removable denture with ball attachment retention and cantilever prosthesis.

Based on the results, the overall strain was the lightest in the fixed prosthesis with cantilever and on the distal aspects of the two distal implants. In a study by Tashkandi et al., the maximum strain was recorded in the cortical bone around the most distal implant, consistent with the present study.[15]

Rodrigue et al. reported that the amount of stress transferred to the cortical bone might decrease with an increase in the number of implants and a decrease in the distance from the distal abutment. Considering the results of the studies above, it might be concluded that in fixed implant-supported prostheses with posterior cantilever, there is a higher strain in distal implants, which might be because when a force is applied to cantilever prostheses, the distal implant is subjected to higher tensile and shear forces.[16]

Clayton and Jaslow reported higher stresses and force on the abutment in wrought wire clasps than cast clasp bars.[17]

The flexibility of a wrought wire clasp marks it possible to place it in larger undercuts, which is one of the advantages of these clasps. However, based on the present and previous studies, this clasp can exert excessive forces on the abutment teeth, and the overall strain in this retainer design is significant, which might be attributed to factors such as incorrect bending and manipulation of this retentive arm.

The present study showed a significantly lower overall strain in the removable denture with a ball attachment than the two other designs; however, no significant differences were observed between the different implant surfaces.

Takahashi et al. evaluated the effects of three different types of attachment on maxillary implant-supported overdentures, reporting higher strain levels in the ball attachment design than the magnet and locator attachments.[18]

The present study showed a lower overall strain in the prosthesis with a ball attachment than removable prosthesis with a wrought wire retainer and fixed prosthesis with posterior cantilever, which might be attributed to more favorable stress-breaking property and better distribution of stresses in this extracoronal retainer.

In all three prosthetic designs, the lowest strain was recorded on the mesial aspect. However, the highest strain in the fixed prosthesis was recorded on the lingual and distal aspects, in the removable partial denture with a wrought wire on the buccal aspect, and in the removable partial denture with a ball attachment on the buccal and lingual aspects. Finally, despite minor differences in the maximum strain in each implant, the difference was not significant on any prosthetic type.

In the present study, operators with similar skills and similar techniques were used. Besides, the conditions and factors selected for the study were similar for each area order study. The results showed different strain rates in terms of the prosthesis type, implant position, and the area evaluated.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.