2.1

Study design and sample

This was a retrospective cohort study. The study sample consisted of 16 patients (13 female, 3 male) who underwent Le Fort I osteotomy combined with horseshoe osteotomy for posterosuperior repositioning of the maxilla between 2020 and 2022 at the Kyushu Dental University Hospital, Japan. Bilateral sagittal split ramus osteotomy (BSSRO) was performed simultaneously in 14 patients.

The inclusion criteria for this study were complete medical records, identification of maxillary excess, and history of Le Fort I osteotomy combined with horseshoe osteotomy. The exclusion criteria were: 1) patients with otorhinolaryngologic conditions, such as allergic rhinitis within the past 4 weeks, 2) patients with incomplete medical records, and 3) patients younger than 16 years of age.

The research protocol was approved by the Research Ethics Committee of Kyushu Dental University (20–42). All procedures were performed in accordance with the ethical standards of the committee responsible for human experimentation and the Declaration of Helsinki (2013). All the participants provided written informed consent.

2.2

Surgical procedure

All surgeries were performed by 2 experienced surgeons (Dr MH and Dr IY) at Kyushu Dental University Hospital, with consistent surgical procedures. A horseshoe osteotomy combined with a Le Fort I osteotomy was performed to maintain the posterior nasal cavity, including the interior nasal concha. This surgery followed conventional methods for anaesthesia, incision, bone cuts, mobilization, and fixation [ ]. Two patients used double splint positioning technique, and 14 patients used maxillary positioning guide [ ].

Fixation plates were secured on both sides of the pyriform aperture and zygomatic buttress. The anterior nasal spine (ANS) was not contoured during surgery. After osteotomy, an alar base cinch suture was performed in all 16 patients, and the nasal alar width postoperatively remained unchanged from the preoperative measurement.

2.3

Variables

The primary predictor variable was the change of anterior nasal cavity volume (mm ³ ). Other predictor variables included the maxillary posterior movement (mm), and maxillary superior movement (mm). The primary outcome variable was nasal airway resistance (Pa/(cm ³ /s)).

Covariates included age (years), sex (male/female), smoking status (yes/no), body mass index (BMI) before surgery (body weight/height ² ), and mouth breathing habits (yes/no). Treatment-related variables included the use of absorbable plates (yes/no) and changes in the nasal alar width after surgery (mm).

2.4

Data collection methods

Rhinomanometry [ ] was conducted to measure nasal airway resistance, and Pro Plan software (Materialize, Leuven, Belgium) was used to calculate the anterior nasal cavity volume from the computed tomography (CT) data obtained one month before and three months after surgery.

A minimum of 3–5 breaths were recorded at a fixed transnasal pressure of 100 Pa during quiet breathing. The total nasal airway resistance and airflow were measured, and the values were expressed as the ratio of transnasal pressure to flow. The investigators recorded the ratio and anterior nasal cavity volume one month before surgery and three months after surgery and then calculated the changes.

The anterior nasal cavity was defined as the space bounded by the bilateral nasal walls and one plane, the coronal plane perpendicular to the Frankfort horizontal (FH) plane passing through the nasion (N) ( Fig. 1 ). The volume of this space was measured, excluding the nasal mucosa, septum, and conchae.

Computed tomographic image of anterior nasal cavity volume. The space bounded by the bilateral nasal walls and one plane: the coronal plane perpendicular to the FH plane passing through the Nasion (N).

Additionally, the investigators traced lateral cephalometric radiographs taken one month before and three months after surgery, with the patients oriented to the FH plane and their teeth in centric occlusion, to evaluate the operative movements of the maxilla. They then superimposed the preoperative and postoperative cephalometric tracings on the FH plane. Operative changes in the UMT (upper first molar cusp tip) were measured. The FH plane served as the X-axis and the coronal plane perpendicular to the FH plane passing through the nasion (N) served as the Y-axis. The perpendicular distance from the UMT to the X- and Y-axes was recorded to calculate the changes in the maxillary posterior and superior movements. The movements of the UMT were represented as linear measurements in millimetres on both axes ( Fig. 2 ).

Method of analyzing lateral cephalometric radiographs for postoperative changes at the UMT with the construction of the X- and Y-axes ² .

X-axis = FH plane, Y-axis = The coronal plane perpendicular to the FH plane passing through the Nasion (N).

To maintain impartiality, all cephalometric evaluations were performed by one of the authors, who was unaware of the identity of the cephalometric radiographs being examined.

Smoking status, BMI, mouth breathing habits, absorbable plate use, and maxillary positioning technique were obtained from the medical records.

2.5

Data analysis

All statistical analyses were performed using the 3DD-IN software (OMS Publishing Inc., Saitama, Japan) in Microsoft Excel (Microsoft, Redmond, WA, USA). Regression analysis was conducted to examine the relationship between changes in the anterior nasal cavity volume and nasal airway resistance, as well as the relationship between the posterior, superior movement of maxilla and nasal airway resistance. Continuous variables were reported as mean ± standard deviation (95 % confidence interval). The level of significance was set at P < 0.05. Categorical data are presented as frequencies and percentages (%).