David Healey BDS, MDS, MOrthRCSEd
David works as a Senior Lecturer in Orthodontics in the Dept of Oral Sciences, University of Otago.
A central tenet of the Hippocratic oath is encapsulated by the phrase, 'First, do no harm.' As clinicians we must preserve this goal in all of our treatments, but even in the best of hands, and with every positive intention, this is not always possible. Root resorption is one of the few deleterious consequences of orthodontic treatment and perhaps the most commonly suffered, if not actually seen. The clinician who claims that this never happens to his patients just isn't taking good enough post-treatment records.
Even though there is a paucity of studies examining long-term survivability of root-resorbed teeth (Lee et al, 2003) it would appear unusual for the amount of resorption from orthodontic treatment to produce a reduction in the life of a tooth. Premature tooth loss has not been reported in the literature (Killiany, 1999).
Here are some questions about this topic that a discerning clinician may wish the answers to, and as such, help clarify some of the misconceptions that abound:
What is root resorption?
Physiological root resorption is an entirely normal process which happens with the timely loss of deciduous teeth. Root resorption has been compared to the scar that is the consequence of surgery (Watson, 2004). It may be defined as the loss of apical root tissue, however this in some ways overly simplifies a complex multifactorial phenomenon. During tooth movement (orthodontic or otherwise), forces are transmitted through the tooth to the periodontal ligament.
Areas of compression of the ligament result in the generation and influx of osteoclasts resulting in bone resorption. Areas of tension result in osteoblast differentiation and the deposition of bone. An imbalance between resorption and deposition combined with the loss of some of the protective characteristics of cementum may contribute to the cementoclasts / osteoclasts resorbing areas of the root. (Ngan et al, 2004).
It is postulated that resorption occurs during periods of active tooth movement when the balance is tipped in its favour. The opposite deposition or cemental repair occurs during periods of quiescence. If these two mechanisms are disrupted or an imbalance towards resorption is maintained then a permanent loss of root tissue occurs. (Brezniak and Wasserstein, 1993). Root shortening has also been reported in individuals who have never undergone orthodontic treatment (Harris et al, 1993).
|Fig 1: Pre treatment||Fig 2: Post treatment same patient: generalized root resorption|
|Fig 3: Idiopathically occurring root resorption (untreated individual)||Fig 4: Idiopathically occurring root resorption (Same untreated individual)|
Are there different types of root resorption?
Yes. Root resorption is known to occur after trauma and in response to inflammation (Tronstad, 1988). Ankylosis may well be related to reparative processes in replacement resorption (Andreasen,1985). Orthodontists are principally interested in External Apical Root Resorption (EARR), which is a form of surface resorption. EARR refers to the loss of apical root material only, other forms of resorption can be observed ie. lateral (cervical) resorption on the buccal surface of molar roots during rapid maxillary expansion treatment (Heithersay 1999).
When and where does it occur?
Micro CT studies have identified microscopic areas of EARR of 0.1mm in depth in 90% of non-orthodontically treated permanent teeth. Small resorption lacunae particularly on the lateral aspects of roots are thought to be harmless and a normal finding reflecting the on-going balance between resorption and deposition. The incidence of EARR shows a wide range of reporting, with histological studies showing a higher incidence than clinical studies. Females are more susceptible than males (Linge and Linge 1983). The most readily affected sites are (in order) maxillary laterals, maxillary centrals, mandibular incisors, the distal root of mandibular first molars, mandibular second premolars and maxillary second premolars (Brezniak and Wasserstein, 1993). The risks of resorption do seem to increase after the age of 11 years (Linge and Linge, 1983).
How much of the root is affected?
Unfavorable and detrimental EARR is thought to occur in 3-5% of orthodontic patients with mean losses of between 1 and 2mm (Kennedy et al, 1983). However, because of its unpredictable nature and difficulty in detection if small, the incidence may be much higher. In the majority of patients root loss is limited to 2mm or less (Sameshima and Sinclair, 2001)
Is it related to any other condition?
Researchers have examined the connections with other conditions such as asthma (McNab et al, 1999), and its relationship to endocrine disturbances such as hyperthyroidism (Levander and Malmgren, 1988) but these associations have only proven to be weak.
Is there a familial basis?
Very probably. Recent papers have pointed more strongly to a genetic basis for root resorption. The importance of genetic factors in bone metabolism have been confirmed, and recently twin studies have indicated a strong genetic component to EARR (Ngan et al, 2003). A genetic susceptibility to EARR has been supported (Al-Qawasmi et al, 2003), with 70% genetic contribution being proposed by one researcher (Harris et al, 1997).
How is it detected?
Most commonly resorption is detected towards the end of treatment on a root paralleling or end of treatment orthopantomogram. Long cone parallel technique periapicals have the highest chance of reflecting the severity of the case. Carefully performed, they allow some quantification of the problem. They are not traditionally taken as part of an orthodontic pretreatment work-up. Animal and histological studies show the problem has been traditionally under-reported, whilst clinical studies have a tendency to minimise it.
What are the risk factors for root resorption?
Root form has been targeted as a method of pre-treatment prediction of those roots most likely to undergo greater amounts of EARR. Unfortunately there is no widespread agreement on this (Kjaer, 1995). Levanger and Malmgren (1988) identified blunted roots and pipette shaped roots as predictors of EARR.
A past history of root resorption in the individual is a good indicator of future activity. Already shortened roots have been identified as risk factors (Linge and Linge, 1983) A familial history (siblings) may be an indicator. Trauma prior to orthodontic treatment has been positively correlated by a number of authors (Andreasen, 1985).
Longer treatment times seem to be associated with greater amounts of resorption (Baumrind 1996; Snellgrove, 1995). Conflicting evidence exists as to whether in increase in force levels during orthodontic treatment will increase the severity of resorption (Owman-Moll, 1995; Owman-Moll et al, 1996). On balance, heavier forces do seem to lead to greater amounts of resorption (Chan et al, 2004), but there is a great amount of individual variation (Acar et al,1999). Different techniques have variably been reported as being able to reduce or cause root resorption but little consistency exists in these studies.
What should we do if we find it?
If root resorption is found, the literature supports an inactive phase of 4-6 months (Sameshima and Sinclair, 2001). The orthodontic appliances can be left passively in place, or removed. The clinician should be mindful if they are removed to the possibility of relapse and therefore the issue of uncontrolled tooth movement.
How can we minimalise or prevent it?
Detect it early (4-6 month in treatment screening OPG), be aware of the risk factors prior to treatment and inform the patient as soon as it is detected. Keep treatment duration to a minimum (largely a patient centered requirement), and utilise light forces if possible.
Whose problem is it?
Everyones! Severe resorption (over 1/4 of the root length) (Kaley and Phillips, 1991) may have adverse effects on the longevity of the tooth. The orthodontist would naturally be disappointed to discover extensive resorption. The patient may question the longevity of the remaining tooth, and the General Dental Practitioner would be concerned about the crown root ratio for future prosthetic treatments and possibly the impact that further loss of periodontal attachment might have for the patient.
The finding of EARR is a common sequalae to orthodontic treatment. As such it needs to be part of the informed consent process. Fortunately, it is rare that root resorption is ever extensive enough to cause any detrimental effect for the patient. In the vast majority of cases root resorption will continue to be a finding that only the researcher or occasionally the orthodontist will notice on post treatment radiographs.
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