Reproducibility of Manual Transfer of the Clinical Natural Head Position: Influence on the Soft Tissue and Hard Tissue Position of 3-Dimensional Virtual Surgical Planning

Abstract

The purpose of this study was to assess the reproducibility of manually transferring the clinical natural head position (NHP) to the 3-dimensional (3D) virtual surgical planning and its subsequent influence on the soft tissue and maxillary hard tissue position. A retrospective cohort study was set up. The study population consisted of subjects who underwent bimaxillary osteotomies between 2016 and 2020 at the Department of Oral and Maxillofacial Surgery in Radboud University Medical Centre (Nijmegen, the Netherlands). Cone beam computed tomography scans, dentition data, and clinical photographs were acquired 4weeks before surgery. Two attempts (NHP1 and NHP2) were performed by a single examiner to manually transfer the NHP. 3D transformation matrices were used to quantify the transferred NHP in 3 degrees of freedom (pitch, roll, and yaw). Landmarks and surface-based matching were used to quantify the influence on the soft tissue and hard tissue positions in 6 degrees of freedom. The primary outcome variable was the reproducibility of manually aligning the NHP. The secondary and tertiary outcome variables were the effect of the reproducibility of the manually aligned NHP on the soft tissue and hard tissue displacements in the 3D virtual surgical planning. The study population consisted of 109 subjects: 37 males (33.9%) and 72 females (66.1%) with a mean age of 29.1±10.3years (range, 17.0 to 59.0). The manual transfer of pitch alignment (2.24±1.64⁰; 95% confidence interval [CI], 1.93 to 2.55) was significantly less reproducible than the roll (0.56±0.44⁰; 95% CI, 0.48 to 0.64; P<.001) and yaw (0.67±0.92⁰; 95% CI, 0.50 to 0.85; P<.001). Subsequently, this alignment error influenced the position of the maxilla (incisal point) and soft tissue menton by 0.85±0.86mm and 1.01±1.00mm vertically and 0.78±1.10mm and 0.80±1.18mm sagittally. The present study demonstrated that the manual transfer of the NHP from the clinical situation to the virtual environment influenced the soft tissue and hard tissue position and that a more reproducible method of transferring the clinical NHP is recommended.