Clinical spotlight: Dental material allergies

This article was first published in the ADA News Bulletin, February 2023.

Fletcher and colleagues have recently published a review paper on allergies and soft tissue reactions to dental materials. The key points cited by the authors are:

- ‘most reactions to dental materials occur due to amalgam restorations’;

- ‘reactions to denture materials will cause erythema of the denture-bearing area only’; and

- ‘contact sensitivity (patch) testing has a limited role to play in identifying suitable restorative materials in an individual patient.’

Reactions to dental materials, although known for many years, rely mainly on eye and personal experience for identification and elimination. Reactions may be inflammatory or lichenoid, and in rare cases may predispose to oral cancer. When patch testing, there is a difference between cutaneous and mucosal sensitivity, which it is
important to understand. In the context of dental materials, ‘allergy’ and ‘hypersensitivity’ are largely
interchangeable; allergy is preferred and most often equates to a Type IV hypersensitivity.

Adverse Reaction Units have been established in Norway, Sweden and the UK, and reporting to these units is voluntary. The frequency of adverse reactions ranges from 1:700 – 1:10,000, mostly, in the UK, a lichenoid reaction to metals and especially amalgam. Adverse reactions are poorly reported, and the World Health Organization has called for more robust global reporting.

Skin-based, ‘delayed hypersensitivity testing’ has been used to evaluate dental materials. Oral mucosa and skin, although embryologically related, differ in sensitivity; skin may show a reaction when oral mucosa does not. In addition, the results are subjective, which means that the results are best interpreted by a dermatologist with extensive experience in testing dental materials, rather than one with little dental experience. The authors provide two summary references concerning contact sensitivity testing in dentistry.

In delayed hypersensitivity testing, the material is placed in aluminium cups on the skin of the back for 2–4 days. The skin change may range from mild erythema to blistering, but cellular examination is not routinely possible; irritant reactions and mild true allergy may look the same clinically. Another issue is that a reaction to metals may take two weeks, leading to false negatives if standard 2–4 day readings are used.

Metal salts are mostly used for contact sensitivity testing, but different salts have different immunogenicity,
which causes major differences between test results. Many laboratories have a ‘standard dental series’ of metal salts which have been shown to demonstrate clinical correlation. However, other laboratories have their own ‘standard series’, which may be different from national and international series. Clinicians therefore need to ensure that the materials they use are included in the standard set. It may be possible to apply a dental material such as acrylic, metal or amalgam directly to the skin, however metals are not a substitute for metalsalt preparations. If there are visible mucosal reactions to materials in the mouth, it may be cheaper and more reliable to change the material (e.g., replace amalgam with resin-based composite), rather than undertake costly and possibly unreliable contact sensitivity testing.

Metals are the most common dental materials to produce symptomatic mucosal lesions. The mechanism is not well understood, since some metals, e.g., amalgam, produce lichenoid reactions, while others, e.g., bonding alloys, produce Type IV hypersensitivity, which are histologically distinct. Demonstrated skin hypersensitivity may not predict an oral lichenoid reaction, and thus removal of non-mucosa-contacting materials is usually of no benefit. For reasons which are unclear, it may be some years before a mucosal reaction to metals becomes evident; a reaction to corrosion products or to surface bacteria have been proposed. Nevertheless, if there is a lichenoid reaction to a metal, the metal should be replaced because of the possibility of malignant change in the lesion.

Amalgam contact lichenoid reactions are the most frequent reactions seen in the oral cavity. They usually appear several years after amalgam placement, and are unpredictable in who or where they will appear. Even in the same mouth, some amalgams may cause a reaction; others may not. Lichen planus is a T-cell reaction, suggesting an immune reaction to metal(s) in the amalgam, to corrosion products or to bacterial colonisation. About 80% of patients show a positive reaction to contact testing, and > 90% of lesions improve after replacement of the amalgam, even if the patch test is negative. Thus, there is no clinical benefit in patch testing in addition to removing amalgams incontact with the mucosa. The authors highlight theissues of replacing amalgam restorations, includingadditional loss of tooth structure, however thepatient may opt for amalgam removal given the 1% likelihood of malignant transformation. It isvnot unreasonable to arrange a biopsy before amalgam replacement.

Bonding alloys, such as palladium and platinum,appear to be more widely used given the increasing gold price, and reactions to palladium are increasing. The authors cite a good review of palladium allergy in dentistry. Testing for palladium allergy can be complex because of nickel cross-reactivity. The benefit of skin testing is unclear, and the most practical treatment is replacement of the restoration. Cobalt chromium allergies in the mouth are rare, and contact testing is rarely useful. Titanium has been reported to be the cause of allergy, however titanium reactions are usually granulomatous in the soft tissues with low-level pain in the bone.

The authors state that referral for allergy testing to acrylic is the second most common dental referral after amalgam. Dental acrylic is well known as a cause of contact stomatitis, a Type IV hypersensitivity reaction rather than a lichenoid change. A true reaction to acrylic affects the entire denture bearing area, and resolves if the denture is left out for a few days; candidiasis should be eliminated by a biopsy as a swab is unreliable. The authors cite a systematic review which identified Type IV reactions to resin cements, orthodontic adhesives and methyl methacrylate resin. Although there is a range of reactionsto ‘plastic’ materials, lichenoid reactions are
comparatively rare. If a there is a suspected allergy to a denture base, it should be determined if this is a replacement or new denture. If a denture has been worn previously without a problem, then residual monomer in the new denture might be the cause. ‘Cold-cure’ acrylic contains more monomer than heat-cure acrylic, and is thus more commonly associated with a tissue reaction.

Essentially, resin-based composites (‘composites’) are not considered to elicit sensitivity reactions. However, there is a wide range of materials on the market with varying compositions, and no materials can be considered absolutely safe. Reactions to resin-based materials are usually Type IV sensitivity reactions, rather than lichenoid, and are anyway extremely rare. Patch testing is rarely useful. Regarding glass-ionomer (glass polyalkenoate) cement materials, there is limited information on biocompatibility. Resin-modified glass-ionomer
cements are somewhat less biocompatible due to the resin component (commonly 2-hydroxyethyl methacrylate) with the possibility of pupal irritation and contact dermatitis.

Sodium hypochlorite used in endodontics can cause surface injury if spilt onto soft tissues, but allergy is extremely rare. Extrusion into the apical tissues can cause localised necrosis and urgent surgical management is required. Endodontic gutta percha is highly biocompatible, however eugenol-containing endodontic sealers can cause inflammation in the soft tissues so apical extrusion should be avoided. Paraformaldehyde-containing sealants are highly damaging to surrounding tissues and should not be used, whereas resin-based sealants may be slightly toxic while unset. There is scant information on adverse reactions to sealant materials.

The authors conclude by listing practice learning points, and in bullet form prescribe the strategy for management of asymptomatic contact lichenoid lesions.