Johns RB. Pinned retention for amalgam. Dent Update. 1973; 1:(1)9-14
Ibbetson R. Auxiliary retention and the role of the core in fixed prosthodontics. Dent Update. 2002; 29:(6)284-290
Markley MR. Pin-retained and pin-reinforced amalgam. J Am Dent Assoc. 1966; 73:(6)1295-1300
Baum L., 2nd edn. Philadelphia and London: WB Saunders; 1985
Brunton PA, Sharif MO, Creanor S, Burke FJ, Wilson NH. Contemporary dental practice in the UK in 2008: indirect restorations and fixed prosthodontics. Br Dent J. 2012; 212:(3)115-119
Restorative Dentistry: an Integrated Approach.Oxford and Boston: Butterworth-Heinemann; 1998
Butchart DG. The retention in dentine of wider pitch self-shearing dentine pins. Br Dent J. 1987; 162:(1)21-23
Webb EL, Straka WF, Phillips CL. Tooth crazing associated with threaded pins: a three-dimensional model. J Prosthet Dent. 1989; 61:(5)624-628
Dilts WE, Welk DA, Laswell HR, George L. Crazing of tooth structure associated with placement of pins for amalgam restorations. J Am Dent Assoc. 1970; 81:(2)387-391
Dhuru VB, McLachlan K, Kasloff Z. A photoelastic study of stress concentrations produced by retention pins in amalgam restorations. J Dent Res. 1979; 58:(3)1060-1064
Anil N, Keyf F. Determining the temperature rise in the pulp and at the root surface during pin hole drilling and placing of dentine pins. Eur J Prosthodont. 1998; 6:(3)111-114
Biagioni PA, Hussey D, Mitchell CA, Russell DM, Lamey PJ. Thermographic assessment of dentine pin placement. J Dent. 1996; 24:(6)443-447
Tjan AH, Dunn JR, Grant BE. Fracture resistance of composite and amalgam cores retained by pins coated with new adhesive resins. J Prosthet Dent. 1992; 67:(6)752-760
Nayyar A, Walton RE, Leonard LA. An amalgam coronal-radicular dowel and core technique for endodontically treated posterior teeth. J Prosthet Dent. 1980; 43:(5)511-515
Bonsor SJ. Bonded amalgams and their use in clinical practice. Dent Update. 2011; 38:(4)222-230
Staninec M, Setcos JC. Bonded amalgam restorations: current research and clinical procedure. Dent Update. 2003; 30:(8)430-436
Setcos JC, Staninec M, Wilson NH. Bonding of amalgam restorations: existing knowledge and future prospects. Oper Dent. 2000; 25:(2)121-129
Bonsor SJ, Chadwick RG. Longevity of conventional and bonded (sealed) amalgam restorations in a private general dental practice. Br Dent J. 2009; 206:(2)
Summitt JB, Burgess JO, Berry TG, Robbins JW, Osborne JW, Haveman CW. The performance of bonded vs. pin-retained complex amalgam restorations: a five-year clinical evaluation. J Am Dent Assoc. 2001; 132:(7)923-931
Fischer GM, Stewart GP, Panelli J. Amalgam retention using pins, boxes, and Amalgambond. Am J Dent. 1993; 6:(4)173-175
Fedorowicz Z, Nasser M, Wilson N. Adhesively bonded versus non-bonded amalgam restorations for dental caries. Cochrane Database Syst Rev. 2009; (4)
Burke FJT, Shortall ACC. Successful restoration of load-bearing cavities in posterior teeth with direct-replacement resin-based composite. Dent Update. 2001; 28:(8)388-398
Burke FJT, Wilson NH, Cheung SW, Mjör IA. Influence of patient factors on age of restorations at failure and reasons for their placement and replacement. J Dent. 2001; 29:(5)317-324
Reinhardt JW, Capilouto ML. Composite resin esthetic dentistry survey in New England. J Am Dent Assoc. 1990; 120:(5)541-544
Lynch CD, Shortall ACC, Stewardson D, Tomson PL, Burke FJT. Teaching posterior composite resin restorations in the United Kingdom and Ireland: consensus views of teachers. Br Dent J. 2007; 203:(4)183-187
Dentine pins have been used in operative dentistry for many years to retain a non-adhesive restorative material (such as dental amalgam) in a cavity which has little or no inherent mechanical retention. However, dentine pin placement can be a hazardous procedure and there are a number of problems associated with their use. Alternative techniques have been described which may be utilized and are effective at retaining the restorative material without the shortcomings of dentine pins. There is a strong argument therefore that dentine pins have no place in contemporary dental practice and have consequently become obsolete.
Clinical Relevance: Auxiliary retention may be required to retain direct intra-coronal dental restorations in unretentive cavities.
Article
The need for retention
As traditional dental materials, in particular dental amalgam, have no inherent ability to bond to tooth tissue, they have to be retained in an unretentive cavity by mechanical means, this being a topic covered in the first issue of Dental Update.1 This retention is required in two planes: axially and laterally. Retention may be defined as ‘the features of the cavity preventing withdrawal of the restoration in the long axis of the preparation whilst resistance form describes those features preventing dislodgement of the restoration under all other forms of loading’.2
Dentine pins were invented at the end of the 1960s3 as a means of mechanically retaining dental amalgam in unretentive cavities, such as where a cusp or proximal surface was missing4 (Figure 1). Dentine pins became widely used in operative dentistry and have also been used to retain and support a subsequently placed cast restoration indirectly by retaining the restorative core material in extensively damaged teeth.
Figure 1. A large unretentive cavity which would require some form of auxiliary retention to retain the direct intra-coronal restorative material.
Types of dentine pins
Many types of dentine pins became available. They have been manufactured from stainless steel, titanium alloy, pure titanium or gold anodized stainless steel, with the former two being the most popular.5 As such they are corrosion resistant and are available in different sizes (Figure 2).
Figure 2. A narrow and a medium diameter self-shearing self-threading dentine pin (Stabilok, Fairfax Dental Ltd, London, UK).
Self-threading pins are placed by the dentist cutting a pin hole, usually 2mm deep, using a twist drill in a low speed handpiece rotating clockwise. The diameter of this hole is slightly smaller than that of the pin which is to be used. This system relies on the elasticity of the dentine to retain the pin. This is also the case for the friction fit system, which is similar to the self-tapping variety except that the pin is pushed home using a holder.6 The last type is the cemented pin and, as the term would suggest, the pin is placed into a slightly oversized hole being retained with a low viscosity cement such as zinc polycarboxylate.6 The cemented, friction-retained and original self-threading pins have all been superseded by the self-shearing, self-threading pins (Figure 3). Once the pin hole has been prepared, the dentine pin (with its plastic carrier) is then inserted into the slow speed handpiece. The rotating pin and carrier is placed into the hole and, when resistance is met at the base of the pin hole, the dentine pin shears from its carrier, so leaving the pin in situ. These are also called self-tapping7 and are considered to be the most convenient type of dentine pin.6
Figure 3. The drill used to prepare the hole in the dentine with its corresponding self-shearing self-threading dentine pin (Stabilok, Fairfax Dental Ltd, London, UK).
Dentine pins may be serrated, which increases the retention of the pin in the dentine and also enhances the retention of the restorative material to the pin. The threads of a self-tapping pin will function in the same way (Figure 4).
Figure 4. A high magnification view of the thread of a self-shearing self-threading dentine pin (Stabilok, Fairfax Dental Ltd, London, UK).
Choice of number and size of pin
There is no firm recommendation regarding the number of pins which should be used in any given case. A rough rule of thumb is that one pin is indicated for each missing cusp and one pin for each missing proximal surface4 up to a maximum of four.8 This is because the more pins that are placed, the weaker the restoration and tooth become, and so it is recommended that the minimum number of pins are used. Furthermore, the presence of multiple pins makes condensing the amalgam around them difficult.
Larger diameter pins have been recommended where possible.8,9 However, one study demonstrated that more extensive cracks occurred in dentine with the larger size pins and that crack communication with the pulp chamber occurred more frequently with the larger pins.10
Problems associated with dentine pins
Procedural accidents
Dentine pin placement is a hazardous procedure. The pulp could be penetrated during pin hole preparation and, whilst not ideal, this could be managed by preventing microbial infection from reaching the dental pulp (ideally by firstly isolating the tooth with rubber dam) and placing a material to create a barrier. Another pin hole could then be (carefully) prepared in another part of the tooth.
Pins may also be placed into the periodontal membrane (Figure 5), particularly into furcations. This iatrogenic damage is much more difficult to salvage compared to pulpal perforation. Some operators recommend sealing the hole up, smoothing the pin off on the root surface or extending the cavity margin to include the defect.7
Figure 5. A bitewing radiograph showing the perforation of the periodontal membrane by a dentine pin in the upper right first molar tooth. Note the vertical periodontal bone loss mesially which has occurred as a result. Note also that the tooth (for some reason?!) has had six dentine pins placed in it.
Tooth fracture
The preparation of a pin hole or placement of the pin itself may cause the tooth to fracture. This may start as dentinal crazing or the initiation and subsequent propagation of a crack under low levels of loading during function, leading to fracture of the tooth.11 As enamel is very brittle, pins should only be placed into dentine with at least 1 mm between the pin and the amelo-dentinal junction.7 It is also possible that a local fracture of the dentine may occur at the site of the pin. In this case, the pin will not be retained.6
Fracture of the tooth substance is more likely to occur in an endodontically treated tooth. This is because such teeth are compromised owing to the large decrease in the bulk of dentine remaining due to access cavity and canal preparation. This renders the tooth greatly weaker and more liable to fracture during pin hole preparation and pin insertion. For this reason, Ibbetson stated that dentine pin placement is an absolute contra-indication in root-filled teeth.2
It is frequently necessary to bend pins so that they do not interfere in the occlusion of the opposing tooth. Unfortunately, the bending of pins can cause the dentine to craze with the potential for cracks to develop and propagate.
Fracture of drills or pins
Drills can fracture during pin hole preparation. This occurs most commonly if the drill is stalled whilst cutting.7 Pins may fracture on placement and they may or may not be removed. If they are able to be removed then the pin hole is likely to be damaged and so unable to retain another pin by friction fit. Both cases will necessitate another pin hole having to be prepared.
Pin hole incorrectly prepared
This can occur if the bearings of the handpiece are not running true, causing the bur to rotate eccentrically. This results in an overprepared hole which will not retain the pin. This can also result from poor technique on the part of the operator. The hole should be cut with slow speed using a rapid once only in and out movement.
Failure of the pin to seat fully
Incomplete seating of the dentine pin is usually caused by the handpiece running too quickly or by variations in the elasticity of dentine. An incompletely seated pin may lead to it loosening as it is not so well retained in the dentine. Fracture of the dentine or restorative material during function may also occur. The pin will protrude more out of the cavity so possibly interfering with the occlusion. The pin may therefore have to be bent or shortened by cutting with neither option being ideal.
Insufficient interocclusal clearance
A dentine pin extends 2 mm in the restorative material and a sufficient amount of material is required to cover the pin to provide adequate strength.12 This may be impossible to achieve if insufficient interocclusal clearance is present.
Heat generation
Studies have shown that heat is produced during dentine pin hole preparation and subsequent pin placement.13,14 This may have a detrimental effect on the dental pulp which may lead to irreversible pulpitis and loss of vitality of the tooth.
Stress concentrations
Dentine
Owing to the nature of the retention of the dentine pin with the self-threading and friction grip pin systems relying on the elasticity of the dentine, stress concentrations can occur around the pin. This has been shown by a photoelastic stress analysis study.15 This was not seen in pins which had been cemented.
Restorative material
Similarly, stress concentrations can occur around dentine pins within the restorative material which may lead to fracture of the material. Stress within amalgam restorations can be reduced if a chemical bond exists,12 so some workers have advocated the use of an adhesive cement such as Panavia Ex (Kuraray, Osaka, Japan) or 4-META (Cover-Up, Parkell, Edgewood NY, USA).16
Concern has also been raised that little or no bonding occurs between the restorative material and the dentine pin. This may also lead to stress concentrations at this point. This is particularly true if a dissimilar, non-metallic material is used, as the difference in the coefficients of thermal expansion of each material is greater. For this reason, only amalgam should be used in combination with dentine pins. Furthermore, in the case of resin composite, pins cause discoloration.7
Alternative methods to create retention
Rather than using dentine pins, there are other methods and techniques which may be employed in contemporary dental practice to create retention.
Mechanical devices
Other mechanical devices may be prepared in the cavity as an alternative to the placement of dentine pins. Slots, grooves and pits have all been described. These are well-defined, sharp internal forms which are placed on the cavity floor or in opposing walls (Figure 6). Amalgam is condensed into these features, thereby becoming an amalgam cleat,4 and by that means the restorative material is retained.6 They must not be placed where they may be removed with subsequent tooth preparation, such as the preparation for a crown. Whilst they provide less stress in both the dentine and restorative material, they weaken the remaining walls and tooth tissue as they must be of adequate depth and width so that sufficient restorative material can be held in them for strength. These retentive features have been described in traditional operative textbooks4 and their use has seen a revival in recent years.8
Figure 6. A groove prepared in dentine into which amalgam will be condensed so providing some mechanical retention for the restoration.
Modification of cavity shape
Often, and particularly in root-filled posterior teeth, the dentist can utilize the geometry of the pulp chamber by paralleling opposing walls into which the restorative material is placed (Figure 7). These days, this material would be further enhanced by utilizing a bonding technique (see later). For many years, a Nayyar core was advocated in which 3–4 mm of gutta-percha was removed from each root canal and dental amalgam condensed into the space (Figure 8).17 However, whether significantly increased retentive benefit is gained from this remains to be seen.
Figure 7. An endodontically treated lower left second molar tooth ready for restoration. The walls of the pulp chamber have been paralleled to each other to provide mechanical retention and resistance form for the subsequent amalgam restoration. A resin-modified glass ionomer lining material has been placed on the floor of the pulp chamber and over the gutta-percha, obturating the root canals to prevent potential microleakage.Figure 8. A diagram of a mandibular first molar demonstrating a Nayyar core. Note the gutta-percha has been removed from the coronal 3–4mm of the root canals to permit the amalgam to be condensed.
Bonding
Of course, in contemporary practice, a bonding technique would now be used to afford or to enhance retention of the restorative material. Both dental amalgam and resin-based composite may be placed, in combination with an intermediate resin-based bonding agent.
Bonded amalgam restorations
There has been a move in recent years for operative dentists to use the benefits of adhesive technology when placing dental amalgam restorations. There are many potential benefits of this approach 18,19,20 and these include:
Decreased microleakage between the cavity wall and the restorative material. This in turn may decrease post-operative sensitivity, pulpal inflammation and the incidence of recurrent caries;
Decreased flexure of the remaining tooth tissue during function and, in the case of teeth exhibiting cracked cusp, this may alleviate or eliminate symptoms;
The ability to support weakened tooth tissue which otherwise would have to be removed, so rendering cavities more conservative and potentially increasing the fracture resistance of the tooth;
Extra retention for the restoration.
This means that traditional retention and resistance form of cavities is decreased or even eliminated and so precious tooth tissue is conserved. Other mechanical methods of auxiliary retention, including dentine pins, are not required.
The use of a bonding technique means that large amalgam restorations replacing cusps may be successfully placed and retained without the use of dentine pins21,22,23 (Figure 9). However, a Cochrane Review was carried out in 2009 by Fedorowicz, Nasser and Wilson24 which concluded that ‘there was no evidence to either claim or refute a difference in survival between bonded and non-bonded amalgam restorations’ and that there was ‘no significant difference in the in-service performance of moderately sized adhesively bonded amalgam restorations, in terms of their survival rate and marginal integrity, in comparison to non-bonded amalgam restorations over a 2-year period’. Both this group of workers and Bonsor and Chadwick21 warned clinicians to be mindful of the additional costs that may be incurred of providing adhesively bonded amalgam restorations when the additional benefit was not proven.
Figure 9. A post-operative view of a bonded amalgam in an unretentive cavity in an upper first molar tooth, 14 years after placement. No auxiliary mechanical retention was provided for this restoration.
Resin-based composite restorations
It is true to say that all of the indications and potential benefits for bonding amalgam can be extended to resin composite used in cavities in posterior teeth.25 Indeed, with the use of resin composite increasing26 due to improvements in the materials, the techniques employed to place them and patient demand,25,27,28 some clinicians would postulate that resin composite has superseded the use of amalgam.29 That said, it may be prudent to avoid using resin composite in large cavities where the occlusion is unfavourable due to the increased wear resistance of the material and the technical difficulties of placing the restoration directly. In these cases, the clinician may wish to consider prescribing an indirect resin composite restoration in an attempt to overcome these shortcomings. In either case, as has been discussed earlier, dentine pins are contra-indicated in combination with resin composite.
Indirect resin composite restorations
Heavily restored teeth, especially those which are missing one or more cusps, have been traditionally restored with an amalgam core retained with dentine pins which has subsequently been crowned. The improvement in materials and techniques has meant that alternative, more conservative techniques may be utilized. One such restoration is the provision of resin composite which has been fabricated in the dental laboratory and then bonded into and/or onto the tooth surface. The advantages of this technique are that it is easier to fabricate the restoration on a model and not in the patient's mouth, with the problems of access and moisture control not being issues. The material may be more completely cured, with the effects of polymerization shrinkage being more compensated for, which may result in improved performance of the restoration.
Conclusion
Although dentine pins are still widely used in general dental practice, their use is in decline.5 Dentine pins are used more frequently by older practitioners than younger colleagues.5 This may be explained by the fact that many dental schools in the UK now no longer teach the placement of dentine pins. This, coupled with younger dentists being more used to bonding techniques and perhaps practising more evidence-based dentistry, accounts for the decline in dentine pin usage. In this author's own clinical practice, he has not placed a dentine pin in over 12 years! In this regard, sales of dentine pins have been reducing. In personal correspondence with this author, two dental wholesale companies reported that, in the 10-year period between 2002 and 2011, inclusive, sales of dentine pins have decreased by between 25% and 33%.
Thus, when the risks and shortcomings of dentine pins are balanced against the alternative contemporary successful techniques, both with respect to function, biology and longevity, then there is a strong argument that dentine pins have no place in contemporary dental practice and have therefore become obsolete.
