Precision attachments in partial removable prosthodontics: an update for the practitioner part 1

Definition, advantages and disadvantages

Definitions for precision attachments include the following:

Precision attachments offer potential advantages in restorative dentistry, particularly in removable prosthodontics. In spite of this, their use in general dental practice tends to be limited.5 In general, the uses of precision attachments in dentistry are for bridgework: intracoronal and extracoronal attachments can be used to overcome the problem of non–parallel abutments; partial dentures, including free-end saddle dentures (unilateral and bilateral); overdentures and implants.

The advantages of precision attachments in removable partial denture design include:

There are several disadvantages associated with precision attachments. These include:

These disadvantages need to be clearly considered before treatment planning precision attachments. They also emphasize the importance of case selection.

The literature contains several classification systems for attachments. This seems to be due to a potential for category overlap, along with the fact that many of these attachments can be classified in different ways, for example the position of placement, material, accuracy of fit, appearance and flexibility. Below is a classification which is deemed the simplest for the purposes of this article and is adapted from several sources.4^,5^,7

Classification of precision attachments

Precision attachments can be classified according to:

The sub-categorical classification based on rigidity merits further discussion. Rigid attachments are those that theoretically allow no movement of their component parts during function.6 Clinically, however, micromovement does occur during function, with the amount of movement being a function of the degree of wear of the components.6 The resilient attachment allows a precalculated amount of movement when the attachment is fully seated. The movement is usually of a hinge type and serves to distribute potentially harmful forces towards the saddle and away from abutment teeth.5^,6 These are especially indicated in tooth-and mucosa-borne dentures associated with large edentulous spans.

When classified by design, this is related to the benefits they provide. Precision attachment is a term used generally for those units that provide benefits in terms of support, retention, reciprocation (bracing) and stability. The term semi-precision attachment is reserved for those intracoronal attachments with tapered sides. These attachments will supply a similar degree of support, but cannot be relied upon for retention and reciprocation.7 This is due to the fact that the fabrication methods for semi-precision attachments yield a less precise tolerance.6 Resilient attachments provide a more defined and predetermined amount of movement in their component parts, permitting functional movement of the denture base towards the ridge during function, theoretically reducing stress on the dependent abutment. It is in this way that the semi-precision attachment acts as a ‘stress director’.6

This article will deal with the former classification, by location, and specifically in relation to removable prosthodontics.

Intracoronal attachments

These consist of two parts, one part joined to the removable prosthesis (flange or patrix) and one part embedded in the restoration (slot or matrix). The two units are linked intra-orally.7 Intracoronal attachments ideally should be completely incorporated within the contour of the crown (Figure 1). This is advantageous because the occlusal forces are then applied close to the long axis of the tooth. If the tooth is underprepared, then an overcontoured crown is produced placing part of the intracoronal attachment outside the natural crown contour, thereby losing this perceived advantage. The size of the tooth, the size of the pulp and the chosen attachment will all influence this. This advantage has to be offset against the degree of preparation required so as not to cause loss of too much tooth structure or vitality. These attachments usually require a box preparation in line with the manufacturer's instructions. It is often useful to have a plastic model or the actual attachment nearby, so that a try-in may be performed and the clinician can be assured of adequate reduction to house the attachment. It will also act as a guide prior to preparation to ensure that its choice is appropriate relative to the tooth and pulp size; minimizing the risk of irreversible pulp damage.5 All intracoronal attachments are rigid. This can make insertion and removal more difficult for patients.

Extracoronal attachments

This term is best defined as those units having part or all of their mechanism outside the contour of a tooth (Figure 2). Commonly, these attachments have their main application with distal extension prostheses, but may be used in bounded saddles.7 The advantage of this type of attachment is that tooth reduction is less than an intra-coronal attachment, minimizing tooth reduction. One disadvantage is that maintaining hygiene below the attachment can be more difficult (Figure 3). This depends on the height of the tooth, the dimension and shape of the attachment used and the contour of the soft tissue adjacent to the edentulous span. It is important to provide the patient with specific oral hygiene instruction to overcome this problem. This should include the use of superfloss or interdental brushes, so enabling adequate plaque removal. In free-end saddle situations, for example, it may be advantageous to use a cantilever pontic unit with the attachment on the pontic to allow for soft tissue clearance (Figure 4). For patients with limited dexterity, insertion and removal is often easier with extracoronal attachments compared with intracoronal attachments. Since teeth retaining extracoronal precision attachments can be subject to considerable torque (especially if opposing teeth are natural), it is suggested that distal facing attachments in free-end saddle situations be retained by two or more crowns (double abutted) to resist these adverse forces.5 The disadvantages of this approach is the additional tooth preparation, difficulties in interproximal cleaning and therefore risk of caries or periodontal disease. Individual circumstances need to be considered, such as root size and shape, as well the size of the crown and the opposing occlusion before committing to a double abutment construction. Extracoronal attachments can be either rigid or resilient.5

Radicular attachments

By definition, radicular precision attachments are connected to a root preparation (Figure 5). These have been shown to improve the retention of removable partial dentures significantly.8 Their use can minimize the need for retention and stability components in aesthetic areas.9 The majority are soldered or cast to a coping, although there are pre-formed ones available. The use of pre-formed ones could be limited as there is no scope to change their angulations. It is critical that an assessment of the vertical height of both the attachment and coping is made so that there is sufficient space for the denture tooth. Case selection is important not only with respect to attachment choice, but also in relationship to root length, root morphology and quality of endodontic treatment. An appropriate length post is required to resist decementation on removal of the prosthesis. This has become less of a problem more recently with the availability of adhesive cements.

Retained roots provide many advantages, even when not restored with precision attachments.10 Their maintenance continues to provide proprioceptive feedback, support psychological benefits of tooth retention and tactile sensitivity discrimination to the patient. Crum and Rooney, in 1978, used lateral skull radiographs to assess quantitatively vertical alveolar bone loss in lateral skull radiographs to compare edentulous and overdenture patients with retained canine teeth. A mean loss of 5.2 mm mandibular alveolar height was reported for the edentulous group, whereas the comparable mean for the overdenture group was 0.6 mm.11 The problem of combination syndrome can be avoided if roots are maintained.12 The maintenance of roots has also been advocated in patients who have had radiotherapy to reduce the risk of osteoradionecrosis following tooth extractions.13

The use of radicular precision attachments in retained roots offer additional advantages.8 Those that are soldered onto gold copings cover the root face and provide protection of the root face from caries. If ferrule production is possible, they can resist stresses which may propagate vertical root fracture.8 Some systems enable the level of retention to be modified. This can allow retention to be increased or decreased depending on the patient's needs.

There are, however, disadvantages when using teeth for overdentures, and these include problems of plaque retention, secondary caries, endodontic problems and the need for continuing care and maintenance. Once again, case selection is critically important as the dexterity of the patient should influence the choice of radicular attachment.14^,15^,16^,17 Like all the aforementioned classified precision attachments, their disadvantages include cost and technical demands. In addition to these, the bulk of the attachments need to be considered, not only in height, but also mesiodistally. Not considering these can leave insufficient space to house the component that is encased in the denture. This can cause a potential weakness of the prosthesis, which may be more susceptible to fracture. It is always sensible, therefore, to do a tooth try-in to assess space in all dimensions before choosing an attachment. Table 2 highlights the heights of a number of commonly used attachments.

Öwall found that the long-term results for intra-radicular stud retained removable partial dentures were favourable.18 At 10 years, the success rate was reported to be 70% and at 15 years 60%.18

Various types of radicular attachment exist, these include:

Auxiliary attachments 5

This category contains a large collection of miscellaneous items. For this reason, only those most commonly used will be detailed here:

Screw and tube attachments consist of a screw with a tapered head and threaded tube. A collar for the screw may be incorporated into the design. These attachments are for connecting prostheses and allowing contingency planning of long span bridgework. Removal of these screws would allow removal of a bridge suprastructure and examination or treatment of supporting abutments. Their brief discussion warrant mention as they facilitate removal rather than retention of prostheses. The other auxiliary attachments fall outside the scope of this article.

Conclusion

The success of partial dentures is primarily dependent upon sound prosthetic principles; the selection and design of precision attachments should be based around these, not vice versa. With so many different systems on the market, it is better to be fully aware of a small number and to be clinically competent at using them, rather than have knowledge of many systems. It is also equally important to work with a technician who is knowledgeable in their use. Knowing when to use the appropriate one is fundamental to their long term success. In the second article, these clinical considerations will be illustrated with appropriate cases.

All precision attachments require a varying amount of vertical height (Table 2). It is critical in the planning that there will be enough space for the vertical height of the attachment, along with 1–2 mm clearance from the occlusal level, so the attachment is completely housed within the denture and therefore not subject to undue stress, which might lead to early failure. Adequate tooth preparation can be ensured by having a tooth try-in stage prior to committing to a particular attachment.