Orthodontic pain

Pain pathways

The sensation of pain is initiated in one of two ways, either by the direct stimulation of nociceptive nerve fibres by a mechanical, chemical or thermal stimulus; or indirect stimulation following the release of any one of a number of inflammatory mediators, including histamine, bradykinin, substance P, prostaglandins and serotonin. These mediators are released as a result of tissue injury. Once stimulated, nociceptive fibres undergo depolarization and, in the case of orthodontics, the sensation of pain is transmitted from the pulp and/or periodontium to the central nervous system via nociceptive afferent fibres. These are mainly lightly myelinated primary afferent Aδ fibres, which respond to mechanical or thermal stimulae and are associated with acute pain. The slower primary afferent unmyelinated C fibres respond to mechanical, thermal or chemical stimulant and are associated with a slower, persistant pain. These primary afferent nociceptive fibres are those of the trigeminal nerve, whose cell bodies lie within the Gasserian or trigeminal ganglion (Figure 1).

Once initiated, the pain stimulus is transmitted along the neuron axon by propagation of an action potential. At rest a neuron axon is negatively charged intracellularly (-70 mV) relative to the area surrounding the neuron. Action potential propagation is caused by the activation of voltage-gated sodium channels in response to depolarization of the neighbouring axon segment. This leads to a rapid influx of positively charged sodium ions into the cell along a concentration gradient. This reverses the polarity in that segment of the axon. The polarity is then reinstated by the sodium channels entering an inactive state, halting further ion flow, and the opening of voltage-gated potassium channels that allow positively charged potassium ions to flow out of the axon down their concentration gradient. Each segment depolarizing triggers further voltage-gated sodium channels to activate downstream and the action potential cascades along the axon. In myelinated Aδ fibres, the axon is surrounded by a series of Schwann cells that electrically insulate the axon. There are spaces between the Schwann cells called nodes of Ranvier. The action potential jumps from node to node down the axon length significantly speeding up conduction (Figure 2).¹⁴

Factors affecting the perception of pain

The degree of pain experienced by individuals in response to an identical noxious stimulus can vary greatly from person to person.¹⁵ The perception of pain may be influenced by a number of different factors. Anxiety has been shown to be a factor affecting dental pain, lowering the reported threshold⁹ and increasing the likelihood of the avoidance of dental treatment.¹⁵ It is therefore important that the level of anxiety associated with orthodontic treatment is minimized in order to optimize the patient care experience.

It has also been suggested that both age and gender might have an effect on perceived orthodontic pain, but the results of studies investigating these factors are somewhat equivocal. While some studies have demonstrated age has no effect,^8,9 others have found the level of pain reported by adolescents to be greater than in other age groups.^2,16 Similarly, the evidence for the effect of gender on pain perception is mixed. At the time of appliance or separator placement, two studies have reported pain experience to be greater in girls than in boys,^4,16 whilst others have concluded that gender has no effect on pain perception following the placement of initial archwires.^5,8,9

When does orthodontic pain occur?

It is generally considered that orthodontic pain starts at around 2 hours after appliance placement, with some studies reporting that this is the case for between 91% and 97% of orthodontic patients.^4,16 It then usually peaks at 24 hours⁵ before gradually subsiding over the next 5 to 7 days.^7,8 Interestingly, it would seem that up to 25% of orthodontic patients report experiencing pain for longer than 7 days.¹⁶

Location of orthodontic pain

A number of investigations have been carried out to determine which sites within the dental arch experience the most pain during orthodontic treatment. In almost all cases, the teeth which experience the most significant pain are the incisors, in particular the lower incisors, and this is the case during both initial alignment and at debond.^{5,10,11,16,17}

Does bracket or wire type have an effect on the pain experienced?

In recent years a number of studies have considered the possible effect of both bracket and wire type on reported pain. In particular, comparing self-ligating brackets with conventional brackets, which utilize elastomeric ligation (Figure 3). Perhaps due to the complex and also subjective nature of pain, the effect of bracket type is not conclusive. Some studies report less pain when using self-ligating brackets,⁹ others report greater pain with self-ligating brackets, but only during insertion or removal of the archwire,⁷ and others report no difference at all between the two bracket types.⁸

There have also been a number of investigations into the pain experienced with lingual versus labial fixed appliances. It would seem that the pain related to the teeth and the periodontium is most probably the same, but what is different is the site of any soft tissue-related discomfort, which unsurprisingly is usually greater in the tongue in the case of lingual brackets, and greater in the lips and cheeks in the case of labial brackets.¹⁸ A recent systematic review and meta-analysis concluded that, overall, there is more oral discomfort experienced with lingual appliances.¹⁹

With respect to archwire choice and pain, a recent Cochrane review has found no evidence that the use of any particular type of wire for initial alignment has any effect on perceived pain.²⁰

Pain at debond

Debonding requires the removal of the brackets and bands at the completion of treatment, along with residual adhesive from the enamel surfaces of the teeth. The most commonly used method of bracket removal is to use debonding pliers (Figure 4).² In the case of metal brackets, this works by the pliers distorting the bracket base, leading to crack propagation and either cohesive failure within the bonding material, or adhesive failure at the interface between the bonding material and the bracket base or enamel surface. Care needs to be taken to make this process as pain free and efficient as possible. The risk of damage to the enamel is greater in the case of the removal of ceramic brackets because both the bracket material and enamel are relatively brittle. The pliers must therefore initiate crack propagation within the bonding material, or at its interface with the bracket base or enamel surface. With both metal and ceramic brackets, pain may be experienced if forces are transmitted to the teeth, particularly rotational or torqueing forces. Alternative methods of debonding, such as the use of thermal and laser instruments, have been investigated, but to date there is little evidence to indicate whether their use is associated with any more or less pain at debond.

Although there are a considerable number of studies centred on the pain associated with orthodontic procedures, such as separator placement and archwire placement, there is limited research on the pain associated with debonding. A study in Iowa, USA, assessed patient discomfort levels at debond and found that a threshold force existed above which discomfort and pain was felt.¹⁰ The level of this threshold was not uniform throughout the mouth and was significantly influenced by tooth mobility and the direction of any force applied. Intrusive forces were the best tolerated by patients and with the rather surprisingly high mean threshold being 934 g. This study also reported that pain varied according to the teeth being debonded. The mandibular incisors were found to display the greatest pain at debond, with a mean discomfort threshold of below 830 g. However, it should be noted that this study only involved 15 patients and pain was assessed in 16 teeth per patient (first premolar to first premolar). No data were provided for pain experienced in the molar regions.

A larger randomized controlled trial¹¹ assessed pain at debond in 90 subjects, half of which were biting on a soft acrylic wafer during debond. It showed similar results to the Iowa study, in that the lower anterior teeth were deemed to be the most painful by patients. This study also reported that subjects who bit on an acrylic wafer during the debond process reported reduced pain levels in the posterior dentition. This may have been due to the intrusive forces applied to the teeth during biting. From the raw data it would seem that 93% of the patients experienced some degree of pain during debond. Although the sample size was reasonably large, the power calculation was based on research around clinically significant pain reduction within an accident and emergency setting²¹ for acute pain, which may not translate well to pain in orthodontic patients.

Management of orthodontic pain

Summary

In conclusion, pain may be experienced during all of the stages of orthodontic treatment with fixed appliances, with up to 95% of patients being impacted. Pain is very subjective in nature and the experience of pain can differ considerably from one individual to the next, for the same procedure. Pre-emptive analgesics, such as ibuprofen or paracetamol, are worthwhile, as is the use of post-operative sugar-free chewing gum.

The identification and management of orthodontic pain is important in order to improve the acceptance of orthodontic treatment, patient compliance and overall satisfaction with the final outcome.