Technical Modifications for the Application of the Total Difference Method for Frontal Sinus Comparison

SIMPLE SUMMARY: The individuality observed in the frontal sinus cavity (located in the skull) can be used to corroborate an identification of an unknown skeleton, with a success rate that is comparable to that obtained by using dentition or DNA. For admissibility in court, however, forensic methods should be continually tested to assess their repeatability and accuracy. This study tests one technical method for obtaining the frontal sinus shape: the Total Difference Method, which quantifies the sinus shape from 59 linear measurements. We develop a more streamlined application by using a measurement aid (a tracing overlay) and a semi-automated macro to collect the multiple required measurements. The radiographs of 10 adults were assessed by two observers. The data were collected following the original method in addition to the use of a measurement aid with and without the semi-automated macro. The results indicate that this technique for assessing the frontal sinus is reliable and repeatable between and within observers, both for the original method and when using the newly developed applications. Further, the semi-automated macro improved the accuracy and efficiency over the original method. Importantly, the measurement aid will allow researchers to conduct the Total Difference Method on a larger scale, creating comparative databases that can be utilized for future forensic research and practice. ABSTRACT: Despite being used in personal identification since the 1920s, frontal sinus-based methods are rarely validated. This study is a validation test of the Total Difference Method (TDM). The posterior-anterior radiographs of 10 adults were assessed by two observers using three modes: the Freehand Mode largely followed the original protocols; the Overlay Mode utilized a tracing overlay; and the Semi-Auto Mode used the overlay and macro, walking the user through multiple steps. The modes were evaluated for the time taken to complete each image and the accuracy and repeatability of the line lengths, midline assessment, and angle placement. The repeated measures analysis of variance results for the intra-observer error revealed differences in bias in the angle placement and line length mean error between the rounds and modes. The differences between the rounds were approximately consistent for each mode, suggesting observer error. Significant differences in the inaccuracy of the angle placement and the line lengths between observers by mode were evident; in post hoc testing, the Freehand Mode and Overlay Mode had the greatest error in both variables (p.adj < 0.0001). The Semi-Auto Mode retained no significant error for angle inaccuracy and had the fewest errors for line length inaccuracy (p.adj < 0.01). When using the Semi-Auto Mode, the time was 46.1% faster than that of the Freehand Mode and 34% faster than that of the Overlay Mode (F(2,18) = 52.71, p < 0.0001); time slightly improved with method familiarity. The results suggest that the technique required for the TDM can reliably be repeated, and the semi-automated macro improves accuracy and efficiency, but only after the users are familiar with the method and macro procedures. All resources needed to repeat this study are freely available on GitHub.