Cortical Plates: Cliff-Edge or Forgiving Friend?

Occasionally, we read a paper that makes us question our practice and, indeed, our strongly held beliefs. I was a trainee during the frenzied promotion of self-ligating systems, which were effectively marketed as ‘bone-growing’ appliances. The implications were potentially seismic, obviating the need for extractions or more aggressive forms of expansion. A wealth of independent research later debunked this promise and concluded cortical plates were unforgiving and could not be breached. I have continued to hold this concept close and incessantly drone on to my students (and anyone else who will listen) about the need to consider safe boundaries underpinned by aesthetics, prospective stability and periodontal health. 

But am I correct? Can we be more optimistic and adventurous? This study, was a collaborative effort from a team in China and Wisconsin, addressed these questions? The American Journal of Orthodontics and Dentofacial Orthopedics published the paper.

What did they ask?

They asked this question

“What were the labial and lingual cortical bone remodelling characteristics of mandibular central incisors after retraction”

What did they do? 

They conducted a retrospective study involving 33 adults who had undergone fixed appliance-based treatment involving the loss of lower first premolars.

Participants: 

Participants had retraction of the mandibular incisor edges of 4mm or more. They did not have pre-treatment periodontal or gingival issues. 

All had CBCT scans (0.3mm voxel size) taken before and after orthodontic treatment. 

They superimposed the scans by scans by voxel-based registration on the mandible from the external symphysis to the first molar. Slices were taken at 3mm (crestal), 6mm (mid-root), and 9mm (apical) below the cementoenamel junction of the mandibular right central incisor. 

They categorised the planes as crestal (S1), midroot (S2), and apical (S3) levels of the mandibular central incisor. They used the pre-treatment (T0) image to outline the labial and lingual cortical border and incisor edge.

Outcomes: 

The team identified four remodelling patterns lingual to the mandibular incisors as follows: 

• Type I: No lingual bone remodelling without root penetration of the original lingual cortical bone border.
• Type II: Lingual bone remodelling without bone defects when the root penetrates the original lingual
• cortical bone border.
• Type III: Lingual bone remodelling with bone defects when the root penetrates the original lingual
• cortical bone border.
• Type IV: No lingual bone remodelling when the root penetrates the original lingual cortical bone border.

What did they find?

• At the cervical level, the mandibular incisor roots moved just over 3mm lingually. Movement occurred both due to bodily movement and tipping. 
• There was no type I bone remodelling cervically on the lingual aspect, indicating that all roots moved beyond the original lingual alveolar bone boundary.
• Most patients presented type III patterns involving fenestration and dehiscence in conjunction with bone formation.
• Approx. 12% had type II bone remodelling with no denudation of the alveolus despite retraction.
• Apically, more type I bone remodelling was observed (48%), while 43% had type III patterns.
• Only 3 type IV patterns indicative of a complete lack of bony apposition lingually were noted overall at any of the three levels.
• They did not find a correlation between lingual bone remodelling and the magnitude of lingual tooth movement. Remodelling capacity was not linked to the vertical skeletal dimensions. Generally, the extent of labial cortical bone resorption exceeded lingual bone apposition.

What did I think?  

This study was interesting. As the authors acknowledge, it lacks significantly from a methodological perspective because it is retrospective and involves a small sample. Therefore, we cannot speculate whether certain mechanics or movement rates are more or less likely to lead to adaptation or fenestration. The assessment is also purely radiographic, with no clinical evaluation of periodontal parameters. Nevertheless, it does provide some food for thought. 

The authors infer that, during lower incisor retraction, labial alveolar bone is resorbed, while lingual alveolar bone may either be resorbed or undergo more favourable remodelling. 

Does this give us a license to retract with abandon? I don’t think so. However, it does imply that our capacity to camouflage in Class III malocclusion may be greater than we thought. Is this, for example, a contributor to the increased use of buccal shelf TADs for Class III camouflage? Mandibular occlusal plane rotation is believed to partly explain its potency. This study suggests that we may be understating the impact of retraction alone.

The use of repeated CBCT is illuminating but does present ethical challenges. The sample was a convenience sample in whom these were taken. We would not apply for or be approved for repeat CBCT in the U.K. or Ireland. The authors acknowledge some limitations, including the inability to reveal particularly thin cortical plates. Equally, they refer to the possibility of long-term self-repair. These caveats suggest that the risk of fenestration may be overstated in the present study. It is essential to balance this inference with the reality that soft tissue changes often occur in the medium term.

What can we conclude?

Based on this retrospective study involving the superimposition of serial CBCTs, the lingual alveolar plates do appear to have the capacity to remodel. However, we don’t yet understand the factors that affect the extent of remodelling or, indeed, that predispose to lingual dehiscence and fenestration.

Are there implications for other forms of tooth movement? We know that similar findings have been observed concerning the retraction of maxillary incisors. 

I can only speculate whether this data is relevant to buccal transverse expansion in the maxillary and mandibular arches and the advancement of the mandibular incisors. Conventional wisdom would suggest a lack of forgiveness in these regions. But maybe I shouldn’t pre-judge?