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Updated: Aug 5, 2026
A Postoperative Evaluation Guideline for Computer-Assisted Reconstruction of the Mandible
Published on: January 28, 2020
Comparing the Frankfort Horizontal and Adjusted Natural Head Position as Upper Facial Horizontal Reference Planes
Simon Luu1, Jeffrey Posnick2, Myra Rana3
1Resident-in-Training, Department of Oral and Maxillofacial Surgery, University of Alabama at Birmingham School of Dentistry, Birmingham, AL.
Background:
In orthognathic surgery planning, the upper facial horizontal reference plane (UFHRP) designated during computer aided surgical simulation (CASS) is known to influence the final position of the maxillomandibular complex. However, the extent of the potential differences that occur due to using different UFHRPs through CASS has yet to be evaluated.
Purpose:
The study purpose was to compare the final positional (mm) differences in bimaxillary orthognathic surgery when planned through CASS with the UFHRP oriented to adjusted natural head position (aNHP) versus Frankfort horizontal (FH).
Study Design, Setting, Sample:
A retrospective cohort study was implemented with patients who were planned for orthognathic surgery at the University of Alabama-Birmingham between 2020 and 2022. Patients with incomplete records, unrecoverable plan files, patients for whom all necessary landmarks on photographs and CBCT could not be identified, and patients planned for single-jaw mandible-only surgery were excluded.
Predictor Variable:
The UFHRP used for CASS; defined as aNHP or FH.
Main Outcome Variable:
Three-dimensional positional difference between the plans with aNHP versus FH as the UFHRP.
Covariates:
Covariates included age, sex, race, primary diagnosis, angle between aNHP and FH (∠aNHP-FH), and vertical and horizontal planned movements.
Analyses:
Descriptive statistics and multivariable regression models were computed to compare the final surgical positions. P value ≤ .05 was considered statistically significant.
Results:
In the final sample of 49 subjects, the mean age was 26.1 ± 12.5 years and 25 (51%) were female. The mean and mean absolute value of ∠aNHP-FH were 0.6° ± 5.1° and 3° ± 4.1°, respectively. The mean absolute value of vertical (ΔZ) and anteroposterior (ΔY) discrepancies between aNHP and FH plans were 0.26 ± 0.32 mm and 0.11 ± 0.21 mm, respectively. There was a statistically significant positive association between ∠aNHP-FH for both ΔZ and ΔY (P < .001).
Conclusions:
In a consecutive series of subjects planned for orthognathic surgery, the positional differences of the maxillomandibular complex through CASS using aNHP versus FH had mean vertical and anteroposterior discrepancies < 2 mm for ∠aNHP-FH between [-10°, 25°] and maxillary movements between [-4,12] mm.
