Removable Attachment Systems for Fixed, Full-Arch Restorations

Fortunately, many patients with fully edentulous arches are candidates for implant-supported or -retained denture prostheses, which eliminate many of the problems experienced with traditional dentures.³ When full-arch prostheses are supported and/or retained by dental implants, they are more secure and stable, offer greater support, and are more comfortable for patients.⁴

Technological advancements have contributed to more predictable treatment planning of implant-supported or -retained, full-arch prostheses to restore edentulous arches. These include 3D implant treatment planning software applications, precision surgical guides, and enhanced imaging technologies, such as cone-beam computed tomography (CBCT), that capture the necessary intraoral and anatomical data to help direct implant and abutment placement.⁵

Although CBCT scans are very sophisticated and provide tremendous amounts of information, they do present limitations that may prevent a definitive prosthesis from being loaded at the surgical appointment. For example, CBCT scans have been shown to underestimate jaw sizes (by an average of about 0.5 mm) and bone height, and they may not allow the accurate visualization of areas with thin bone.⁶ Additionally, when CBCT scans are used to plan implant positions, the deviation between the surgical plan and the actual position of the implant could range from 0.3 to 2.3 mm at the crest and from 0.3 to 2.4 mm at the apex.⁷ This may help to explain the frequency of clinicians' attempts to place a full-arch, screw-retained prosthesis according to a CBCT-based treatment plan, only to experience deviations in the implant/abutment position that result in a prosthesis that does not passively fit.

A number of approaches have been taken to overcome and/or compensate for this deviation in positioning. Among them is planning the full-arch restoration and then having the laboratory technician finish only one implant site on the prosthesis, leaving the additional sites to be completed chairside immediately following implant placement. More recently, a hybrid solution has been adopted that involves placing a cement-retainable prosthetic component on an abutment in combination with a screw-retained metal cylinder. In situations where angle correction is indicated, an angled abutment may be used in conjunction with the metal cylinder.

However, there are several caveats associated with implementing this abutment solution. First, consideration must be given to the potential for retained cement, which has been associated with peri-implant diseases and subsequent implant failure.^8,9 Second, although this hybrid option enables realization of the ideal angulation for connecting the full-arch prosthesis to the supporting implant, the location of the screw access hole also requires consideration and planning.¹⁰ In particular, the screw-retained component requires careful planning to ensure that the exit point of the screw access hole is not on the facial aspect. Angulation issues should be resolved by positioning the implant fixture so that the screw access hole can be redirected to either the occlusal or lingual aspect of the prosthesis.

Implant abutment jigs, which are fabricated and provided by the laboratory, can be helpful when placing and correcting this type of angled abutment. Once all of the abutments are placed, the screw-retained cylinders can be added and the acrylic can be placed into the overdenture at the same time. After pick-up and finishing, the prosthesis is screwed into place, and the access holes are filled with Teflon tape and light-cured restorative material. Although few complications have been associated with this technique, for some patients, whether due to occlusion or diet, maintenance issues have been observed involving the restorative material used to seal the screw access hole in which the composite has either worn or dislodged, requiring replacement.¹¹

Although the aforementioned techniques are not complicated or technically difficult, they can be extremely time-consuming for the dentist, laboratory technician, and patient. An alternative to cement- or screw-retained implant attachments is a fixed locator attachment system for rigid connection of full-arch restorations to endosseous dental implants (LOCATOR F-Tx, Zest Dental Solutions). Capable of being removed and reattached by the dentist as needed, such a system supports full-arch prostheses in the mandible or maxilla without the need for prosthetic screws, screw access channels, or cement.

Considering the potential discrepancies and deviations in the implant/abutment position that can result in a prosthesis not fitting passively,^6,7 the fact that the fixed locator attachment system includesspherical abutment geometry that allows the denture attachment housing to pivot in any direction is significant. In fact, this pivoting ability enables alignment of the attachment housings in a way that creates a more parallel draw for the prosthesis, which contributes to a stress-free and passive fit. This also enables angulation correction of up to 20° from a common reference point between implants without the need for angled abutments.

The spherical abutment is coated with titanium carbo-nitride (TiCN), and as a result, it features a hard, smooth, and wear-resistant exterior. Compared with abutments coated in titanium nitride (TiN), it is 32% harder, demonstrating 26% greater wear resistance and a 64% reduction in roughness. These characteristics may explain why a locator system showed superior clinical results in a study examining complications with attachments for implant-supported overdentures (eg, retention loss, attachment dislodgement).¹²

Aggressive grooves and flats on the denture attachment housing-which is passively picked up in the prosthesis during a chairside procedure-limit vertical and rotational movement, contributing to stability and essentially locking the prosthesis into place. After placement, the prosthesis's degree of retention is such that only a dentist can remove it. Research has demonstrated that locator attachments could dramatically improve the stability of dentures, and that they have a stronger effect on preventing horizontal denture movement.¹³

This attachment housing is internally threaded to accept the polyether ether ketone (PEEK) retention balls and processing ball, which snap into the abutment. When combined, these components eliminate the need for abutment cylinder preparation, screw access channels, filling composite, and intensive chairside procedures to ensure retention of the denture prosthesis. When compared with ball attachments, locator attachments have shown better retentive properties,¹⁴ and in terms of prosthodontic maintenance, cost, and ease of denture preparation, locator  systems demonstrate superior clinical results.¹⁵

During recall appointments and/or yearly maintenance appointments, the prosthesis can be quickly and easily removed using the F-Tx Seating/Removal tool. Only the retentive balls require replacement. This is much simpler than the complex process associated with removing screw-retained, full-arch prostheses, which requires drilling out composite material, unscrewing all components, cleaning the access channels and components, and then refilling them.

Indicated for full-arch prostheses, the removable attachment system for fixed, full-arch restorations is not appropriate for single tooth restorations, unilateral bridges, or cases that require a resilient connection. It is also not appropriate for use with implants that require a vertical divergence greater than 20°. The system can be used to stabilize newly fabricated full-arch restorations (eg, all-ceramic, prosthetic acrylic) or a patient's existing full denture if the position of the teeth is acceptable.

The basic prosthodontic principles that are followed for most cases are also applicable to the removable attachment system for fixed, full-arch restorations. After a clinical examination, the determination was made to extract a patient's maxillary teeth and restore the arch with an implant-retained prosthesis (Figure 1). An analysis of the patient's anatomy was performed with CBCT scans. Using implant treatment planning software, the CBCT data was then converted to enable visualization, planning, and precise preoperative selection of correct implant positions in relation to the available bone. Adjustments can also be made to the site prior to ordering the surgical guide.

The implant planning software was then used to design the full-arch, implant-retained prosthesis in the context of where the implants would be placed, ensuring that there would be adequate space for the planned attachments. This enables the preoperative correction of any angle issues so that the implants can be placed ideally through the surgical guide. Upon receipt, the surgical guide can be forwarded to the laboratory for use in fabricating a bite registration to help stabilize the guide during implant placement as well as for designing the full-arch prosthesis and fabricating an immediate load prosthesis.

Following implant placement (Figure 2), the spherical abutments are screwed into place (Figure 3 and Figure 4), which enables the denture attachment housings to pivot in any direction for better alignment of the full-arch prosthesis. The denture attachment housings-including the processing balls-are then placed and pivoted into the correct position and angulation (Figure 5 through Figure 7), which creates an easily drawn path of insertion and removal for the prosthesis.

When an immediate denture is made, the inside is hollowed out to create recessed holes wide enough to accept the denture attachment housings. The prosthesis is tried in to ensure midline alignment (Figure 8), and then it is removed. Chairside acrylic material (eg, dual/light-cured syringeable material) is placed to enable pick-up of the denture attachment housings, after which the prosthesis is returned to the mouth, and the patient is instructed to close his or her mouth into proper occlusion. After the material is light-cured and the prosthesis is removed and allowed to sit, it is finished with laboratory burs to cut back any excess acrylic and polished.

As the technologies and techniques for providing implant-supported and -retained prostheses have advanced, dentists can now provide edentulous patients with sophisticated options that are simultaneously simpler to place, more precise, and easier to maintain. Using a removable attachment system for fixed, full-arch prostheses eliminates the need for cement, screw access channels, screws, and composite filling materials. Furthermore, it helps to resolve the issues that can result from deviations in implant/abutment location after placement.

The author received an honorarium from Zest Dental Solutions for the preparation of this article.

Robert Del Castillo, DMD
Private Practice
Miami Lakes, Florida