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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.
Computer-aided surgical simulation (CASS) for orthognathic surgery shows minimal positional differences in maxillomandibular complex planning when using adjusted natural head position (aNHP) versus Frankfort horizontal (FH) reference planes. These findings suggest that either UFHRP is acceptable for CASS, impacting surgical outcomes minimally.
Area of Science:
- Oral and Maxillofacial Surgery
- Orthodontics
- Computer-Aided Surgery
Background:
- Computer-aided surgical simulation (CASS) in orthognathic surgery relies on the upper facial horizontal reference plane (UFHRP) for planning.
- The choice of UFHRP, such as adjusted natural head position (aNHP) or Frankfort horizontal (FH), can influence the final maxillomandibular complex position.
- Previous studies have not fully quantified the positional discrepancies resulting from different UFHRPs in CASS.
Purpose of the Study:
- To compare the three-dimensional positional differences of the maxillomandibular complex in bimaxillary orthognathic surgery planned using CASS with UFHRPs oriented to aNHP versus FH.
- To evaluate the clinical significance of discrepancies arising from the selection of aNHP or FH as the UFHRP in CASS.
Main Methods:
- A retrospective cohort study included 49 patients undergoing bimaxillary orthognathic surgery planned with CASS.
- The upper facial horizontal reference plane (UFHRP) was defined as either adjusted natural head position (aNHP) or Frankfort horizontal (FH).
- Three-dimensional positional differences (ΔZ, ΔY) between plans using aNHP and FH were analyzed using descriptive statistics and multivariable regression, controlling for covariates like age, sex, race, diagnosis, ∠aNHP-FH, and planned movements.
Main Results:
- The mean absolute angle between aNHP and FH (∠aNHP-FH) was 3° ± 4.1°.
- Mean absolute discrepancies in vertical (ΔZ) and anteroposterior (ΔY) positions were 0.26 ± 0.32 mm and 0.11 ± 0.21 mm, respectively.
- A statistically significant positive association was found between ∠aNHP-FH and both ΔZ and ΔY discrepancies (P < .001).
Conclusions:
- Positional differences in maxillomandibular complex planning using CASS with aNHP versus FH resulted in mean vertical and anteroposterior discrepancies less than 2 mm.
- These small discrepancies were observed within a range of ∠aNHP-FH between -10° and 25° and maxillary movements between -4 mm and 12 mm.
- The choice between aNHP and FH as the UFHRP in CASS for bimaxillary orthognathic surgery appears to have minimal impact on the final planned position of the maxillomandibular complex.
