Bleaching Complications: ‘Greening’ in a Post-crowned Tooth

A 30-year-old medically fit and well male patient attended the Restorative Department at King's College Hospital, London, following a referral from his general dental practitioner. The patient reported a history of trauma to the upper right central and lateral incisors and the upper left central incisor more than 5 years previously. According to the patient, his dental practitioner had repaired the broken upper right central and lateral incisors with a ‘post and some composite’, but this resulted in gradual discolouration and then development of intermittent symptoms from those teeth. The main issue for the patient was the grey colour and mis-matched shape of the upper right central incisor (Figure 1).

Clinical examination

The grey discolouration of the upper right central and lateral incisors along with some poorly contoured composite restorations were noted.

Chronological hypoplasia was noted from UR3 to UL3 and from LR3 to LL3 (Figure 2). On direct questioning the patient confirmed that he had suffered from several childhood illnesses, which would be consistent with that dental appearance.

Radiographic examination

Peri-apical radiographs of the upper central incisors and upper right lateral incisor showed that the composite restorations had been retained with prefabricated metal screw-posts in the upper right central and lateral incisors, but that root canal treatments had not been carried out before doing this (Figure 3).

There were peri-apical radiolucencies associated with the upper right central and lateral incisors, consistent with chronic peri-apical periodontitis. The position of the prefabricated metal screw post in the upper right lateral incisor showed significant deviation from the root canal anatomy and had resulted in a near lateral perforation.

Pragmatic treatment after prolonged discussions about the poor prognosis and the realistic options to satisfy ‘Montgomery’ consent issues

The composite restorations were removed and the metal screw-posts removed under rubber dam. They were identified as prefabricated Dentatus (Spånga, Sweden) screw-posts.

Root canal therapy was completed in both central and lateral incisors. There was no perforation noted on the upper right lateral incisor and so copious amounts of hypochlorite was used during the chemo-mechanical preparation. The iatrogenic root damage was repaired, and following obturation both central and lateral incisors were restored pragmatically with RelyX Fibre Posts (3M, Bracknell, UK) and bonded composite restorations.

However, while the endodontic and restorative aspects of the treatment were completed and the symptoms disappeared, the cervical third of the clinical crown structure of the upper right central and lateral incisors remained discoloured.

In dental trauma, the source of the discolouration is located on the pulpal side of the dentine, rather than close to the labial dentine–enamel interface. The significance of that is that this worrying discolouration for patients is managed more effectively with inside-outside bleaching as described by Poyser et al,¹ rather than by conventional nightguard vital bleaching using 10% carbamide peroxide as described by Haywood et al.²

In this case, however, external bleaching was the only practical option as the tooth had already been restored pragmatically, following a repair of the iatrogenic damage, with the fibre post-retained direct resin composite. This had been carried out immediately on completion of the root canal obturation, mainly to stop coronal micro-leakage and consequent re-contamination of the root canals. The original failure to seal the root canals effectively from bacterial invasion was probably the main cause of the obvious peri-apical radiolucencies visible on the pre-operative radiographs.

The ‘greening’ complications following bleaching

The patient was prescribed a course of external bleaching using a mouthguard with 10% carbamide peroxide, following which an interesting phenomenon occurred. Rather than experiencing a lightening in colour, as hoped for with such bleaching, the upper right central incisor progressively turned more green instead. This unexpected colour change noted with the bleaching with 10% carbamide peroxide (releasing just 3.5% hydrogen peroxide) suggested that the discolouration had probably not been caused by blood products in the cervical dentinal tubules, as previously assumed, but rather by the oxidation of the metal corrosion products leaking from the previous metal screw-posts.

Pragmatic additive composite treatment

After extensive discussions with the patient, it was agreed to carry out pragmatic additive composite bonding on all the upper anterior teeth. While treatment might not have been absolutely necessary, it was surmised that additive direct bonding would result in better uniformity of colour and surface smoothness.

The superficial, pitted, stained hypoplastic enamel on these teeth was roughened very gently and check-etched to confirm enamel frostiness after washing and drying. A three-bottle bonding system (etch, prime and bond) was employed, along with shade A2 Gradia (GC, Tokyo, Japan) composite being applied directly in bulk, and sculpted freehand with no matrix.

A similar process was carried out on the central incisors and upper right lateral incisor, with minor roughening of the existing composite restoration for increased retention. Gradia Opaque A3 composite was used cervically to attempt to conceal the green discolouration there.

Care was taken to ensure the composite restorations were not over-contoured cervically by using a Kerr (Uxbridge, UK) Jet TC Bur Needle Trim and Finishing FG 7901 bur inserted into the gingival crevices at a 45-degree angle to the long axis of the teeth.

Mild cervical discolouration was visible at the 4-week post-operative review appointment. However, the gingival tissues appeared reasonably healthy and the patient was very pleased with the improvement of the appearance of his teeth.

Discussion

Dentatus Classic Surtex Posts were initially made of gold-plated brass, but then became available in stainless steel in 1989. These stainless steel posts are the most commonly used at present because they are readily available in various sizes, are cheap, and have reasonable mechanical and retention properties.³ Both stainless steel and gold-plated brass posts are made of non-precious metal alloys, being composed of iron, chromium and nickel, and copper and zinc, respectively.⁴ In the presence of an electrolytic fluid (such as retained moisture in the root canal system, or fluid micro-leakage from the oral cavity, or from the periodontium), metal alloys can undergo galvanic corrosion, resulting in the migration of corrosion products into adjacent dentinal tubules thereby causing discolouration of teeth.⁵

An in vitro study of the microstructure of screw-posts demonstrated that stainless steel posts can discolour dental hard tissues producing a brownish-black colour with the distribution of chromium and iron ions along the post-canal walls.⁶ In the case of gold-plated brass posts, areas of the post unprotected by gold can result in the exposure of the copper–zinc alloy to electrolytes. The subsequent corrosion of brass results in the deposition of copper and zinc ions in hard and soft tissues.⁷ In this case, copper oxide was the probable cause of the green-blue discolouration.⁸

Metal ions such as chromium, iron and copper, have the potential to cause discolourations owing to their transitional nature and ability to form colourful ions and complexes. The gradual green discolouration observed here was probably due to copper oxide being produced by the oxidizing effects of the 3.5% hydrogen peroxide, which was released from the 10% carbamide peroxide employed here. However, there may have been some oxidation of any chromium, iron or nickel ions within the alloy of the screw post, which had leached out in to the tooth before its removal during endodontic treatment.

Practical clinical aspects and conclusions

• Great care needs to be taken with bleaching teeth known to have been restored with metallic restorations such as metal posts, or with old palatal amalgams often previously used to seal palatal pits. Any such amalgams should be replaced carefully before bleaching.
• Oxidizing any copper (or other transition metals) within teeth can result in unattractive greening of teeth.
• Careful and appropriate management of traumatized anterior teeth, while time consuming and probably inconvenient at that time, can repay that investment by dramatically reducing later discolouration or other problems.
• Serious trauma to teeth usually causes pulpal haemorrhage with the blood, which contains iron, spreading outwards from the pulp in to the dentinal tubules. This results in impregnation of dentinal tubules with haematinic products subsequently breaking down deep within the tooth. It is very important to remove any iron-containing products physically early on, by prolonged use of ultrasonic vibration using very fine tips and plenty of irrigation within the chamber generally, but especially in the critical aesthetic areas of the cervical region where the enamel is only 0.7 mm thick. One should then switch to a fine hooked ultrasonic tip for cleaning the pulp cornuae meticulously. Failure to do both first, for at least 5 minutes or more, before then carrying on with the endodontic treatment is a frequent cause of the unsightly discolouration about which many patients complain so bitterly.¹⁰
• Leaving any iron-containing compounds within the dentinal tubules on the pulpal side, or in the pulp cornuae, is responsible for the disappointing and frustrating regression in colour sometimes found following bleaching. In other words, these perennial problems are caused by incomplete removal of blood breakdown products physically by ultrasonics early on.¹⁰
• Bleaching using 10% carbamide peroxide will change the valency of any residual iron molecules, so producing a lighter colour, but the unstable iron compound regresses gradually to a more stable version that causes the tooth to look dark again. The only effective solution is to be persistent early on in removing all the blood products tediously with prolonged use of different fine ultrasonic tips intracoronally, and only then continuing on to do the conventional root canal treatment.
• The bonus in that approach of first using ultrasonics diligently is that it will also remove roughly 80% of the bacterial contamination known to be in the coronal chamber.¹¹ Removing that volume of oral bacteria first minimizes the chances of transporting pathogenic bacteria to the apical region, thereby reducing the chances of a flare-up.¹²
• A further benefit is that using an ultrasonic tip first removes all the blood predictably and gently, without altering the root canal anatomy, and thereby allows easy visualization of the existing root canal position. Gentle ultrasonic vibration avoids deviating from the root canal anatomy and causing any structural damage by a bur, which had occurred in this case by creating a near-perforation problem and that then necessitated the use of mineral trioxide aggregate (MTA), which, rather perversely, is known to cause discolouration itself.¹³