ALVEOLAR BONE RESPONSE TO MAXILLARY RETRACTION

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Response of alveolar bone to maxillary retraction has been the subject of interest for many decades. Both the quantity and quality of the alveolar bone surrounding the retracted teeth are important to examine as it reflects on the post treatment stability and safety.

During orthodontic tooth movement, initially only the periodontal ligament was believed to be involved. Later studies proved the remodeling of the cortical bone as well. Furthermore, studies conducted in the early 1960s showed that bone remodeling was proportionate to the tooth movement, but recent CBCT studies have proven otherwise.

Premise of this discussion

All earlier studies had used tooth axis as the reference line which had an inherent limitation of being an internal and variable reference landmark. There was a need for an external, stable reference line which has been used in this CBCT study and hence the discussion.

Alveolar bone response to maxillary incisor retraction using stable skeletal structures as a reference

Teerapat Eksriwong; Udom Thongudompor

Angle Orthodontist Vol 91, No 1 , 2021

What this study intended to do?

  • To evaluate the alveolar bone change in relation to dental position change after maxillary incisor retraction using stable skeletal structures as a reference.
  • The parameters that correlated with alveolar bone change were also examined.

Null hypothesis of this study

There was no significant change in the alveolar bone position following maxillary incisor retraction and that the B/T ratio was not equal to 1:1.

Type of study

Prospective single blinded study

The methodology…

Inclusion criteria

A total of 17 female patients in the age group 18 to 30 years old who required retraction of maxillary incisors of >/-2 mm with no significant medical history, drug intake history and no history trauma to the anterior teeth affecting the periodontal ligament.

Interventional description

Brackets: Preadjusted bidimensional edgewise (Roth prescription; 0.018 slot on incisors, 0.022 slot on canines, and posterior teeth and buccal tubes on second molars)

Archwires: Sequential 0.012’’, 0.014’’, 0.016’’, and 0.016’’x 0.016’’ nickel–titanium and maxillary canine distalization initiated with 0.016’’ x 0.016’’ stainless-steel archwires

Sequence:

  • Pre -retraction CBCT images (CT0) were taken after the completion of canine distalization
  • Incisor retraction was performed using 0.016’’ x 0.022’’ titanium–molybdenum alloy retraction T-loop (8-mm loop length and height) continuous archwires with an accentuated curve.
  • Each loop was activated by 1.5 mm per month until the incisors were completely retracted.
  • 0.016’’ x 0.022’’ passive stainless-steel archwires for 3 months to facilitate completion of the alveolar bone remodeling process
  • Post – retraction CBCT images (CT1) were taken at the end of the 3-month period

How they did it using CBCT

Reference lines:

  • Horizontal reference line (H Line): ANS to PNS
  • Vertical reference line (V Line): Perpendicular to the H-line halfway between ANS and PNS

For the measurements of labial (L) and palatal (P) root (R) and bone (B) positions on the CT0 images, each tooth was measured at the crestal (S1), mid-root (S2), and apical (S3) levels apical to the labial cemento-enamel junction every 2 mm along the V-line.

Vertical changes assessment

Dental

  • Distance between the maxillary incisor edge and the H-line;
  • Difference at CT0 and CT0 was defined as vertical dental change

Horizontal changes assessment

Dental & skeletal

  • Perpendicular distance between the maxillary incisor edge and V-line
  • Difference at CT0 and CT1 was defined as retraction distance.
  • The rate of tooth movement was calculated as the retraction distance divided by the retraction time.
  • All parameters were calculated and averaged for the upper incisors in each individual
  • The root and bone positions were defined as the distances between each point (LR1-3, PR1-3, LB1-3, PB1-3) and the V-line measured parallel to the H-line
  • To assess horizontal root and bone changes, H-line, V-line, and horizontal lines parallel to the H-line at the level of S1, S2, and S3 from the sagittal slice of the CT0 image were transferred to and superimposed on the H-line and V-line of the sagittal slice of the CT1 image.
  • Changes were defined as the horizontal distances between LR1-3, PR1-3, LB1-3, and PB1-3 points of the two images on the S1, S2, and S3 lines.

What was their CBCT interpretation?

  • Using stable skeletal structures as a reference, the present study revealed that alveolar bone on the labial side remodeled concomitantly with root movement during orthodontic retraction.
  • The B/T ratio of nearly 1:1 observed for all root levels was in agreement with previous CBCT studies that used the tooth axis as a reference except one study that found a 1:2 B/T ratio on the labial side after maxillary incisor retraction. The mismatch may be due to the different inclinations of the maxillary incisors
  • Nonsignificant change in the palatal alveolar bone position during incisor retraction suggests the palatal bone to be a posterior limit and therefore biomechanical considerations during retraction should take the distance between the palatal bone and the incisor root into consideration
  • The correlation between alveolar bone change at the labio-apical level (LB3) and inclination change was not statistically significant. This may be explained by the fact that the incisor movement was not translatory and therefore the negligibile root movement could not have caused a notable remodelling
  • The results of this study highlight the importance of assessing the distance between the root surface and palatal cortical plate in cases undergoing maxillary incisor retraction.

What they deciphered

Labial alveolar bone: Remodeled in accordance with tooth movement in a 1:1 B/T ratio

Palatal alveolar bone: Remained unchanged

Maxillary incisor inclination change was the only factor associated with alveolar bone change

Our thoughts and suggestions

  • A bigger sample size was warranted
  • Despite using external stable landmarks the results of this study did not reveal information that was not already known.
  • Since the authors used a 3 dimensional assessment, they could have used additional reference lines which could have resulted in a more comprehensive evaluation
  • Findings in this study could not be generalized to all patients, because the vertical height was not significant. So, in cases where the maxillary incisors are intruded or extruded, this finding cannot be applied.
  • The results are indicating that the maxillary incisor inclination affects the alveolar bone position, thus in cases where the maxillary incisors were severely protruded or retroclined, the findings of this study are questionable.
  • The only one factor the authors attributed to the bone change were axial inclination, which is insufficient to know the true changes in both labial and palatal alveolar bone change. More parameters could have contributed more in the assessment of the actual response of alveolar bone to maxillary incisor retraction.
  • The authors assessed only female patients in this study. So, gender as a possible confounding factor was not addressed.

One would assume the use of a single prescription was for standardization. However, it stirs the thought of whether other prescriptions could have yielded different results?