Pin-Retained Amalgam Restorations

A pin-retained restoration is defined as any restoration requiring the placement of one or more pins in dentin to provide adequate resistance and retention forms. Pins are used whenever adequate resistance and retention forms cannot be established with slots, locks, or undercuts only.⁵ The pin-retained amalgam is an important adjunct in the restoration of teeth with extensive caries or fractures.⁶ Amalgam restorations including pins have significantly greater retention compared with restorations using boxes only or restorations relying solely on bonding systems.⁷ However, caution is indicated when using pins. Preparing pinholes and placing pins may create craze lines or fractures and internal stresses in dentin.^8–10 Such craze lines and internal stress may have little or no clinical significance, but they can be important when minimal dentin is present. Pin retention increases the risk of penetrating into the pulp or perforating the external tooth surface. The use of pins decreases the tensile and horizontal strength of pin-retained amalgam restorations.^11,12

Slot-Retained Amalgam Restorations

For a complex restoration, a slot is a horizontal retention groove in dentin (Fig. 16-5). Slot retention can be used in conjunction with pin retention or as an alternative to it.¹³

Figure 16-6 illustrates the use of coves (placed with a No. bur) to provide additional retention form in a preparation that uses pins. Coves also may be used in preparations using slots (see Fig. 16-5). Proximal locks, as described in Chapter 14, also are placed in the proximal box and in other locations where sufficient vertical tooth preparation permits (Figs. 16-7 and 16-8).

Some operators use slot retention and pin retention interchangeably. Others more frequently use slot retention in preparations with vertical walls that allow retention locks to oppose one another. Pin retention is used more frequently in preparations with few or no vertical walls. Slots are particularly indicated in short clinical crowns and in cusps that have been reduced 2 to 3 mm for amalgam.¹³ Compared with pin placement, more tooth structure is removed in slot preparation. Slots are less likely to create microfractures in dentin, however, and to perforate the tooth or penetrate into the pulp. Medium-sized self-threading pins may elicit an inflammatory response if placed within 0.5 mm of the pulp, whereas slot placement does not.¹⁴ The retention potential of pins and slots is similar.^15–18

Amalgam Foundations

A foundation is an initial restoration of a severely involved tooth. The tooth is restored so that the restorative material (amalgam, composite, or other) serves in place of tooth structure to provide the retention and resistance forms during the development of the final indirect restoration. A foundation is indicated for a tooth that lacks the resistance and retention forms needed for an indirect restoration. The retention of the foundation material should not be compromised by tooth reduction during the final preparation for the indirect restoration. The foundation also should provide the resistance form against forces that otherwise might fracture the remaining tooth structure.

In contrast to a conventional amalgam restoration, an amalgam foundation might not depend primarily on the remaining coronal tooth structure for support. Instead, it may rely mainly on secondary preparation retention features (pins, slots, coves, and proximal retention locks). A temporary or caries-control restoration may serve as a foundation, but only if the retention and resistance forms of the restoration are appropriate.

A temporary or caries-control restoration is used to restore a tooth when definitive treatment is uncertain or when several teeth require immediate attention for control of caries. It also can be used when a tooth’s prognosis is questionable. A temporary or control restoration might depend only on the remaining coronal tooth structure for support, however, using few auxiliary retention features. When preparing a tooth for either a foundation or a temporary restoration, remaining unsupported enamel may be left except at the gingival aspect to aid in forming a matrix for amalgam condensation. In each case, the remaining unsupported enamel is removed when the indirect restoration is placed. Occasionally, when providing a temporary or control restoration, sufficient retention and resistance forms are included in the preparation to meet the requirements of a foundation.

As a rule, foundations are placed in anticipation of a full-coverage indirect restoration. Not all teeth with foundations, however, need to be immediately restored with full-coverage crowns. For example, amalgam can be used as a definitive partial-coverage restoration if only minimal coronal damage has occurred in endodontically treated teeth.¹⁹ The greatest influence on fracture resistance is the amount of remaining tooth structure.²⁰

The restorative materials used for foundations include amalgam, composite, and occasionally resin-modified glass ionomers (RMGIs). Of the direct filling materials, amalgam may be preferred by some clinicians because it is easy to use and is strong. Threaded pins and slots can be used for retention in vital teeth. Prefabricated posts and cast post and cores also may be used to provide additional retention for the foundation material in endodontically treated teeth receiving foundations. The use of prefabricated posts and cast post and cores is limited to endodontically treated teeth and is used generally on anterior teeth or single-canal premolars with little or no remaining coronal tooth structure. On endodontically treated molars, the pulp chamber or canals typically provide retention for the foundation, and it is not necessary to use any form of intra-radicular retention.

Initial Tooth Preparation

The general concept of the initial tooth preparation is presented in Chapter 14, and it applies to the pin-retained complex amalgam restorations described here. When caries is extensive, reduction of one or more of the cusps for capping may be indicated. For cusps prone to fracture, capping of cusps reduces the risk of cusp fracture and extends the life of the restoration.^21,22 Complex amalgam restorations with one or more capped cusps have documented longevity of 72% after 15 years and show no differences in the survival rate of cusp-covered and non–cusp-covered amalgam restorations, whether or not pins were used.^4,23

When the facial or lingual extension exceeds two-thirds the distance from a primary groove toward the cusp tip (or when the faciolingual extension of the occlusal preparation exceeds two-thirds the distance between the facial and lingual cusp tips), reduction of the cusp for amalgam usually is required for the development of adequate resistance form (Fig. 16-9, A). Reduction should be accomplished during the initial tooth preparation because it improves access and visibility for subsequent steps. If the cusp to be capped is located at the correct occlusal height before preparation, depth cuts should be made on the remaining occlusal surface of each cusp to be capped, using the side of a carbide fissure bur or a suitable diamond instrument (see Fig. 16-9, B). The depth cuts should be a minimum of 2 mm for functional cusps and 1.5 mm for nonfunctional cusps.²⁴ To correct an occlusal relationship, if the unreduced cusp height is located at less than the correct occlusal height, the depth cuts may be less. Likewise, if the unreduced cusp height is located at more than the correct occlusal height, the depth cuts may be deeper. The goal is to ensure that the final restoration has restored cusps with a minimal thickness of 2 mm of amalgam for functional cusps and 1.5 mm of amalgam for nonfunctional cusps (see Fig. 16-9, C), while developing an appropriate occlusal relationship.

Using the depth cuts as a guide, the reduction is completed to provide for a uniform reduction of tooth structure (see Fig. 16-9, D). The occlusal contour of the reduced cusp should be similar to the normal contour of the unreduced cusp. Any sharp internal corners of the tooth preparation formed at the junction of prepared surfaces should be rounded to reduce stress concentration in the amalgam and improve its resistance to fracture from occlusal forces (see Fig. 16-9, E). When reducing only one of two facial or lingual cusps, the cusp reduction should be extended just past the facial or lingual groove, creating a vertical wall against the adjacent unreduced cusp. Figure 16-9, F and G, illustrates a final restoration. The procedure for capping the distolingual cusp of a maxillary first molar is illustrated in Figure 14-68. Extending the facial or lingual wall of a proximal box to include the entire cusp is indicated only when necessary to include carious or unsupported tooth structure or existing restorative material. The typical extension of the proximal box for restoring an entire cusp is illustrated in Figures 14-69 and 14-70, B.

When possible, opposing vertical walls should be formed to converge occlusally, to enhance the primary retention form. Also, a facial or lingual groove can be extended arbitrarily to increase the retention form. The pulpal and gingival walls should be relatively flat and perpendicular to the long axis of the tooth.

Final Tooth Preparation

After the initial tooth preparation of a severely involved tooth, removal of any remaining infected carious dentin or remaining old restorative material is usually necessary and is accomplished as described previously. An RMGI base can be applied, if needed; if used, a liner or base should not extend closer than 1 mm to a slot or a pin.

Pins placed into prepared pinholes (also referred to as pin channels) provide auxiliary resistance and retention forms. Coves and retention locks should be prepared when possible (Figs. 16-10, and 16-11). Coves are prepared in a horizontal plane, and locks are prepared in a vertical plane. These locks and coves should be prepared before preparing the pinholes and inserting the pins. Cusp reduction significantly diminishes the retention form by decreasing the height of the vertical walls. When additional retention is indicated, pins may be inserted in carefully positioned pinholes, thus increasing retention. Slots may be prepared along the gingival floor, axial to the dentinoenamel junction (DEJ) instead of, or in addition to, pinholes (see Fig. 16-10, B). Slot preparation is discussed later in this chapter.

Types of Pins

The most frequently used pin type is the self-threading pin. Friction-locked and cemented pins, although still available, are rarely used (Fig. 16-12). The pin-retained amalgam restoration using self-threading pins originally was described by Going in 1966.²⁵ The diameter of the prepared pinhole is 0.0015 to 0.004 inch smaller than the diameter of the pin (Table 16-1). The threads engage dentin as the pin is inserted, thus retaining it. The elasticity (resiliency) of dentin permits insertion of a threaded pin into a hole of smaller diameter.²⁶ Although the threads of self-threading pins do not engage dentin for their entire width, self-threading pins are the most retentive of the three types of pins (Fig. 16-13), being three to six times more retentive than cemented pins.^27–29

Table 16-1

The Thread Mate System (TMS) Pins

Name	Illustration (not to scale)	Color Code	Pin Diameter (inches/mm)*	Drill Diameter (inches/mm)*	Total Pin Length (mm)	Pin Length Extending from Dentin (mm)
Regular (standard)		Gold	0.031/0.78	0.027/0.68	7.1	5.1
Regular (self-shearing)		Gold	0.031/0.78	0.027/0.68	8.2	3.2
Regular (two-in-one)		Gold	0.031/0.78	0.027/0.68	9.5	2.8
Minim (standard)		Silver	0.024/0.61	0.021/0.53	6.7	4.7
Minim (two-in-one)		Silver	0.024/0.61	0.021/0.53	9.5	2.8
Minikin (self-shearing)		Red	0.019/0.48	0.017/0.43	7.1	1.5
Minuta (self-shearing)		Pink	0.015/0.38	0.0135/0.34	6.2	1

^*1 mm = 0.03937 inch.

Vertical and horizontal stresses can be generated in dentin when a self-threading pin is inserted. Craze lines in dentin may be related to the size of the pin. The insertion of 0.031-inch self-threading pins produces more dentinal craze lines than does the insertion of 0.021-inch self-threading pins.³⁰ Some evidence suggests, however, that self-threading pins may not cause dentinal crazing.²⁶ Pulpal stress is maximal when the self-threading pin is inserted perpendicular to the pulp.³¹ The depth of the pinhole varies from 1.3 to 2 mm, depending on the diameter of the pin used.³² A general guideline for pinhole depth is 2 mm.

Several styles of self-threading pins are available. The Thread Mate System (TMS) (Coltène/Whaledent Inc., Mahwah, NJ) is the most widely used self-threading pin because of its (1) versatility, (2) wide range of pin sizes, (3) color-coding system, and (4) greater retentiveness.33,34 TMS pins are available in gold-plated stainless steel or in titanium. Other titanium alloy pins (Max system, Coltène/Whaledent Inc.) are available.

Factors Affecting Retention of the Pin in Dentin and Amalgam

Pin Placement Factors and Techniques