In this blog we will describe one of the most common side effects of orthodontic treatment; root resorption, Part 1 covers the ‘why’ and ‘how’ root resorption occurs, and the relationship between mechanics and biology based on recent publications. Part 2 will answer how to manage root resorption based on current guidelines
What causes root resorption in orthodontics?
The biological mechanism responsible for orthodontic tooth movement is also the culprit for orthodontic root resorption. When orthodontic forces exceed blood capillary pressure, occlusion of the blood vessels results in necrosis, inflammation, and degradation of the outer layers of the tooth, known as hyalinized zones. The loss of the outer pre-cementum activates osteoclastic cells, if there is an imbalance of repair of the cemental layer and its destruction, then root resorption occurs. The outcome of localised inflammation is one of three possibilities;
1. Surface resorption: Outer layer of cementum resorbed, then fully repaired
2. Deep resorption: Cementum and outer dentin is resorbed and is usually repaired with cementum only, which may or may not result in loss of root length
3. Circumferential apical root resorption: Tridimensional loss of cementum and dentin, with recognizable loss of root length (Tridimensional – I know great word, from the enlightened James Andrews)
How common is it?
Root resorption during orthodontic treatment is expected, occurring in almost every treated orthodontic case, with histological assessment of orthodontically treated teeth showing 90%+ occurrence Weltman 2010.
Mild – moderate root resorption: The most common effect of orthodontic root resorption (48-66%) is fortunately its most common and mildest form, with loss of root length up to 2.5 mm Sondeijker 2019. This loss of root length is considered inconsequential to biomechanics and long-term survival of the dentition Killiany 1999.
Severe root resorption:
An uncommon occurrence with 1-5% of orthodontic patients effected, with loss of more than 4mm of root length or a third of the root length Tieu 2014. The longevity of teeth with severe root resorption (less than 10mm) was shown to be compromised in a 5 year follow up, with increased mobility and loss of teeth Jönsson 2007.
How to assess root resorption?
The established index of root resorption was produced by Malmgren in 1982 , and uses periapical radiographs and the below grading.
· Grade 0 – no resorption
· Grade 1 mild resorption, root with normal length and only displaying an irregular contour
· Grade 2 – moderate resorption, small area of root loss with apex exhibiting almost straight contour
· Grade 3 – accentuated resorption, loss of almost one-third of root length
· Grade 4 – extreme resorption, loss of more than one third root length.
American Journal of Orthodontics and Dentofacial Orthopedics 2017 151, 397-406DOI: (10.1016/j.ajodo.2016.06.048)
Although CBCT assessment has been found to be of greater accuracy, infrequent changes to treatment plans occur after CBCT analysis when compared to periapical radiographs Alqerban 2014.
There are two principal factors which result in either repair or loss of root length when localized inflammation occurs, they are:
1. Host response
2. Mechanical factors
The focus of the remainder of the blog will be to explore the effects of orthodontic mechanical factors on root resorption, the main findings are taken from Currell’s 2019 systematic review on the topic
The greater the orthodontic force, the greater the magnitude of local inflammation, resulting in greater root resorption occurring, in the studies a light force of 25g was compared to 225g Paetyangkul 2009, Wu 2011,
Continuous Vs intermittent forces
Intermittent force (such as elastomeric chain or loop mechanics) lead to less root resorption when compared to continuous forces (such as closing coils) Sawicka 2014, Aras 2012. This is an interesting finding which demonstrates periods of rest between forces allows repair to occur.
This is a pertinent point as some appliance manufacturers have claimed no or little root resorption to occur, however studies investigating self ligating appliances have shown similar occurrences of root resorption to conventional fixed appliances, with a mean difference of 0.1mm Leite 2012, aligners have also been investigated where the occurrence of severe root resorption was similar to that of fixed appliances at 3.7% Gay 2017
TADs and root resorption
In interesting study by Barros 2017 showed retraction of anterior teeth with direct traction from TADs Vs conventional retraction showed significantly more cases of moderate to severe root resorption in the TADs group of 47% Vs 18%, a potential explanation being greater retraction of anterior teeth in the TADs group.
Image from Barros 2017 American Journal of Orthodontics and Dentofacial Orthopedics 151.2 (2017): 397-406. Copyright American Journal of Orthodontics and Dentofacial Orthopedics
Type of tooth movement
Specific type of tooth movement are united by the weak nature of the evidence available, below are the details of the effect, however it is to be taken with caution due to the strength of evidence being weak.
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