Retention of Implant-Supported Overdenture With Bar/Clip and Stud Attachment Designs

There are some problems in the treatment of fully edentulous patients such as residual ridge resorbtion, excess salivary flow, muscle tone reduction, and other factors that affect the retention quality of complete dentures. These patients need greater retention for chewing and psychological reasons.^1,2 

Implant prostheses are a great improvement over conventional dentures. Implant-supported overdentures increase patient satisfaction and quality of life. It has been suggested that an overdenture with 2 implants is the first choice of treatment in the edentulous mandible.³  Retention is one of the most important factors for determining patient satisfaction with removable prostheses.^4–6  A clinically acceptable degree of retention depends on the amount of dislodging forces, the behavior of the prosthesis during function, and the patient's ability to place and remove the prosthesis.

Implants and attachments provide retention and support for the removable denture. This can improve patient satisfaction, psychologic profile, and emotional status.⁷  There are many different attachment systems for retaining overdentures. These can be classified as clips and bars, balls, magnets, and telescopic copings. The practitioner and/or dental technician selects the attachment system based on his or her experience and training. Previous studies have shown that bar and magnet attachments have the highest and the lowest retention force, respectively.^5,8–14  A decrease in the retention forces after 5 years of follow-up has been reported for magnet (70%), ball attachments (33%), and bar attachments (44%).^8,10 

Different values of retention forces have been reported for various attachment systems.^7,14  The purpose of this study was to evaluate the retention of an implant-supported mandibular overdenture with 8 different attachment designs. In this study, we focus on some of the current attachment systems and compare splinted and unsplinted implants.

An edentulous mandibular model (140-032 Straumann AG, Basel, Switzerland) with two 4.1- × 10-mm ITI implants (043.032, Straumann AG) in the symphyseal region was used. There was no undercut in this model. Five specimens of 8 attachment designs with corresponding abutments and superstructures were investigated: ITI Dolder bar with 1 metal clip (048.414, regular Dolder bar matrix, Straumann AG), ITI Dolder bar with cantilever and 3 metal clips, Hader bar with 1 plastic clip (Preci-Horix, Alphadent NV, Waregem, Belgium), Hader bar with cantilever and 3 plastic clips (Figure 1), Sphero block abutment with Rhein plastic caps (green, white, and pink; Rhein 83Srl, Bologna, Italy), and retentive anchor with ITI elliptical matrix (048.456, Straumann AG; Figure 2). Figure 3 shows the studied abutments and superstructures. For making the bars, castable rotating abutments (108 BFT, Rhein Slr 83) were screwed into the implants. A regular plastic bar (048.460, Straumann AG) for metal clips and a plastic bar pattern (Preci-Horix, Alphadent NV) for plastic clips were fitted between the 2 abutments and 1-cm length at the distal of the abutments as the cantilever. Pattern resin (GC America, Alsip, IL) was used to attach bars to the abutments. The abutments and plastic bars were invested and cast in a noble alloy (Degobond 4, DeguDent, Hanau, Germany). The castings were divested, finished, and inserted on the model. To create a proper space for the attachment housings, a 3-mm thickness of base plate wax (Modeling wax, Dentsply, Surrey, UK) was adapted on the bar, and an irreversible hydrocolloid impression was made of the model. After making a master cast, an overdenture base was made with clear heat-polymerizing acrylic resin (Meliodent, Heraeus Kulzer, Hanau, Germany). Two nuts were inserted into the base in the anterior region. These nuts with corresponding screws allowed the housings that contained different attachment systems to be secured and fastened into the overdenture (Figure 4). Retentive anchors, Sphero block abutments, and/or bars systems were screwed into the implants, and their counterpart attachments were positioned on them with the spacers. The overdenture housings were fabricated by filling the space between the lubricated overdenture base and the attachments with light-polymerizing acrylic resin (Megatray, Megadenta, Dental Product, Radeberg, Germany). These housings contained 2 nuts and were secured into the overdenture base nuts by screws (Figure 5). The advantage of using a single overdenture base was that this base remained constant for all tests and enabled us to use the same model and overdenture for all attachment systems. Three withdrawal hooks were attached to the overdenture base (1 in the anterior and 2 in the first molar areas) with autopolymerizing acrylic resin (Meliodent, Heraeus Kulzer; Figure 4).

The model and overdenture with each of the studied attachments were secured in a universal testing machine (Instron Corp, Canton, Mass). The hooks were connected by three 10-cm metal chains to the movable head of the universal testing machine. The chains were adjusted to reduce slack to a minimum. The universal testing machine was set at a constant crosshead speed, 50.8 mm/min, which has been reported to approximate the speed of the movement of the denture away from the ridge during mastication.¹⁵  Dislodging forces were applied in a vertical direction to the path of withdrawal of the overdenture. In the next experiment, the anterior chain was disconnected, which resulted in a posteroanterior dislodging force simulating function. The retromolar pads were used as a reference for the entire seating of the overdenture base on the model. After 8 pullings, the plastic clips and caps were renewed and the metal clips and elliptical matrix were activated.

The vertical and posteroanterior peak loads were analyzed. Mean retention values (N) were subjected to 1-way analysis of variance and Duncan tests to determine differences (α = .05).

The mean retention value of studied attachments in both vertical and posteroanterior directions are presented in Tables 1 and 2. There was a significant difference between retention values of the attachments (Table 3).

The highest average value of retention was recorded for the ITI Dolder bar with cantilever and 3 metal clips in both vertical and posteroanterior directions, respectively (P < .05; Table 1). The Rhein pink caps had the lowest retention in the posteroanterior direction (P < .05; Table 2). Retention decreased over the course of consecutive pulls for all attachments in both vertical and posteroanterior directions (Figures 6 and 7).

One of the treatment options for the edentulous mandible is an implant-supported and/or retained removable prosthesis with 2 splinted or unsplinted implants and corresponding attachment systems.^3,16  Adequate retention can improve patient satisfaction, and it has been shown that the clinician empirically selects the attachment systems based on his or her presumed retention.^5,6  In this study, the retention value of a mandibular overdenture with 8 attachment designs in the vertical and posteroanterior directions was investigated.

A wide range of retention forces has been reported for different attachment systems under static and simulated function.^7,14,17–26  This difference may be the result of different conditions of the studies. Some studies reported a large variation in retention of the same attachment systems.^17,27–30 

In this study, the highest average retention value was recorded for the ITI Dolder bar with 3 metal clips in both vertical and posteroanterior directions. The retention of 1 metal clip was significantly less than 3 metal clips in vertical direction (P < .05). Increasing the number of plastic clips in the vertical and posteroanterior directions did not significantly increase retention (P = .12 and P = .07). There was no significant difference between the retention value of 1 metal clip and plastic clips (P = .11 and P = .07). The retention of 3 metal clips is more than 3 plastic clips in the both directions. This may be related to the difference in material and design of these attachments. Physical properties of attachment alloys (modulus of elasticity in particular) can be the reason, and it can also demonstrate the wear behavior of these attachments.³¹  Polymeric (plastic, nylon, and rubber) components of attachment systems are more susceptible to wear than metallic ones are.^32,33 

Breeding et al²⁵  suggested a higher retention for 2 plastic clips than for 1 plastic clip, while Williams et al⁷  reported that the number of clips did not affect the retention value of the maxillary overdenture. They stated that this difference in their results with the work of Breeding et al²⁵  may be related to the functional position of the retentive clips on the bar or to placement of the attachment housing on the maxillary model.

Petropoulos et al¹⁴  and Naert et al^8–10  also showed that the bar and clip design are the most retentive overdentures. The metal clips are adjustable by the clinician, so retention of metal clips might be variable in different studies. In this study, the distance between the parallel sleeves was measured to be 2 mm, and they were adjusted after every 8 consecutive pulls.

In this study, there was no significant difference among retention values of 1 and 3 plastic clips, 1 metal clip, and Rhein green caps (P = .11). Rhein green caps had significantly higher retention values than pink caps did in posteroanterior directions (P < .05). There was no significant difference between white and pink caps in both directions. According to the manufacturer's claim, Rhein green caps have a higher retention than white and pink ones do.

It has been suggested that ball systems are more favorable in the mandible because of the favorable ratio between the supporting bone structures and the forces acting during function and parafunction.³⁴  The use of a bar for connecting implants can simplify attachment placement in a more favorable position. On the other hand, splinting of the implants is indicated for increasing the stability of implants with less than ideal prognosis and for providing more vertical support under functional load.⁷  Some of the disadvantages of a bar and its corresponding attachments are the higher cost and more complicated clinical and laboratory procedures compared with stud attachments.⁷ 

Several factors can influence the retentive force of attachment systems and their wear features under simulated function. Some of these factors include implant and attachment angulation,^20,28  interimplant distance,^19,29  the direction of applied dislodging forces,^14,26  material,^17,19  design,^17,20  dimension,^18,26  and mode of retention of attachment systems.^19,22,29 

The retentive value of the studied attachments was decreased over the course of consecutive pulls in both vertical and posteroanterior directions. Some of the previous studies also reported decreasing initial retention in different attachment systems.^7,24,25  Evtimovska et al²⁴  studied Locator attachments and Hader bar clips and found a significant difference in the percentage reduction in peak load to dislodgement between the Hader clips, white and green Locator attachments after the first and the final pulls. They concluded that retentive values of the Locator attachments are reduced significantly after multiple pulls.

This study investigated only the retention value of overdenture attachments. Other factors such as amount of available space, maintenance requirements, stress distribution to the mucosa and to the implants, and cost should also be considered in selection of attachment systems. The cantilever design with 2 implants for supporting the mandibular overdenture should be considered only when there is good bone quality and stress distribution condition.³⁵  This in vitro study evaluated the retention of overdenture attachments only in a static situation and revealed the initial retention of the tested attachments. Further investigations to evaluate the retention value of the attachments in an oral environment and simulated function are needed. On the other hand, a wide range of attachments can be used in clinical practice, including bar/clip (with variations), O-rings, ERA, Locators, and so forth. This study focused on only bar/clip and some of the stud attachments. Investigation of the other attachments should be considered in future studies.