Treatment options for the free end saddle

Simple rigid design

Accurate fully extended working impressions ensure optimal load distribution. Mandibular free end saddles should be extended posteriorly on to the anterior third of the retromolar pad, and laterally they should be limited by the buccinator muscle (buccally), and the palatoglossal arch, superior constrictor, and mylohyoid muscle (lingually)21 (Figure 2). A reduced area of occlusal table has been shown to reduce loads generated during mastication and this can be achieved by narrowing, shortening and even omitting artificial posterior teeth.5

Other aspects of design may include the use of wrought clasps which do not rigidly hold the tooth, mesial placement of rest seats to allow more even load distribution, and indirect retention. This term refers to those elements of an RPD that counteract the rotational movement seen in an occlusal direction about a fulcrum line along the clasp axis. The portion of the RPD mesial to this fulcrum will sink in towards the tissue, which is resisted by the indirect retainer components, typically made up of one or more rest seats (occlusal rests, cingulum rests) and the supporting minor connectors. They are usually placed as mesially as possible from the distal extension saddle, in order to optimize the leverage against dislodgment5,22 (Figure 3a–c).

RPI system

The RPI system is designed to avoid harmful torque on the distal abutment tooth. It comprises an occlusal rest (R), distal guide plate (P) and a gingivally approaching I bar (I) clasp (Figure 3). These components allow vertical rotation of the distal extension saddle into the underlying mucosa during loading. As the saddle is pressed into the mucosa, the denture rotates about a point close to the mesial rest located on the distal abutment tooth. The distal guide plate and I bar disengage from the tooth and avoid potentially harmful torquing forces to the tooth and supporting structures.5

Balance of forces

A modification of the RPI system is available commercially as Equipoise (www.equipoisedental.com). It consists of a mesial rest seat and guide surface with lingual cusp arm extending into a distal undercut from the rest.5 It has the opposite effect from the RPI system as the abutment tooth is held in a rigid manner.

Precision attachments

Studies have demonstrated that RPDs retained with precision attachments can positively alter patient attitudes towards wearing RPDs, being preferred in most cases.23 They are usually made up of two components, one located on the abutment tooth and the other housed in the RPD. When the corresponding parts are coupled, they produce positive retention and stability. There are five main groups of precision attachments (intracoronal, extracoronal, studs, bars and auxiliary) and readers are directed to the recent review of precision attachments by Preiskel for details regarding indications for use and descriptions of these main groups24 (Figure 4a, b).

Overdentures and telescopic crown retained dentures

Where part of the roots or crown of a fractured molar remain, simple preparation may allow these teeth to serve as overdenture abutments, often without the need for elective root canal therapy.25

Overdentures are useful for the maintenance of alveolar bone and proprioception, and are known to enhance denture retention.5 An oral hygiene programme to include brushing and/or fluoride application would aid in the prevention of caries.5

Telescopic crowns have also been shown to aid in the retention of partial dentures and are widely used on the continent.26 They are made up of a primary coping on the abutment tooth and a secondary coping (crown) located in the RPD, which is an accurate negative duplication of the primary part. The primary and secondary parts interlock, thus aiding retention. Although studies indicate a high rate of patient acceptance, clinical data on the long-term survival of these prostheses is currently rather limited.26

As seen in other fields of healthcare, treatment options available are always under review and constantly evolving. The altered cast impression technique is one such example. Whereas it was once believed that taking an impression of the alveolar mucosa under controlled pressure would improve load distribution and thus denture stability and support, a recent randomized clinical trial has suggested that this technique offers no significant advantages over non-specialized careful impression techniques.27 The periodontal tissue destruction associated with stress breaker designed dentures provides another example of a technique no longer favoured, furthermore highlighting the need for continued evidence-based work.18,19

Cantilevered bridgework

Cantilevered bridgework provides a method of extending an SDA by a maximum of one unit bilaterally. Conventional bridgework would be indicated when faced with a heavily restored abutment tooth. This option can often improve the appearance of the abutment but at the cost of heavy tooth destruction, increasing the risk of pulpal complications.28 Treatment costs are likely to be higher than compared to an RPD, but this option has reasonable longevity with studies indicating a mean survival rate of between 15–20 years.29

Whilst resin-retained bridges are unable to improve the aesthetics of an abutment, they have the chief advantage of being minimally invasive, typically relying on bonding to enamel on the abutment tooth.30 With appropriate case selection and design, resin-retained bridgework has become more predictable, with data indicating a survival rate of 74% after 4 years, the most frequent reported complaint being that of debonding.30,31

The advent of fibre-reinforced composites in recent years has permitted the use of fibre-reinforced bridgework. This technique employs fibres to reinforce laboratory composite material, thereby enhancing the physical properties. It offers the advantage of improved aesthetics with the use of tooth-coloured retainers as opposed to cast metal wings, but there is currently limited clinical data as to the longevity of fibre-reinforced composites.32

Implant-retained prostheses (crown or bridge)

The development of predictable osseo-integration techniques has led to more sophisticated fixed options when considering the distal extension saddle. Implant-retained prostheses provide greater scope for SDA extension than that offered by cantilevered bridgework. Studies indicate good long-term survival rates33 and high levels of patient satisfaction, making this an excellent alternative in those patients unable to tolerate RPDs, despite their greater expense.33,34

Treatment is lengthy and involves careful planning with regard to implant positioning, placement, design of the fixed prosthesis and maintenance. The finished prosthesis should restore function and aesthetics whilst limiting the occlusal loads distributed to the supporting implants to within physiological tolerances. Great care must be exercised with this option as even the smallest technical and clinical errors can be extremely difficult and costly to correct33 (Figure 5 a, b).