Different types of bone samples require different methods of preparation and analysis to obtain an accurate measurement. The current trend of using virtual bone samples includes the MSCT scan of bones, for the development of population standards to establish a biological profile of unknown skeletal remains (Franklin et al. 2016). Computed tomography (CT) bone samples can be measured using different techniques and software, ranging from free, open-source software to state-of-the-art, expensive software. This study compared the agreement of two different techniques for subpubic angle measurement by Checkpoint (Method 1), a subscription-based software, and MeshLab + OnScreenProtractor (Method 2), which were free software. Both methods were relatively easy to use, in the measurement of subpubic angle on reconstructed 3D pelvic models.
Technically, the 3D pelvic models resemble more of a real pelvic bone compared to the 2D pelvic images (e.g., photographs, radiographs, projection images). During measurement of the subpubic angle, the observer can manipulate and move a 3D bone model better than in 2D, to determine the subpubic angle. Models in 3D are flexible in 360°, while in 2D are relatively fixed in position, imposing some restrictions in their measurements. However, measurements using a 3D model with a software that measures in 2D can lead to some errors associated with measurement in 2D (Muñoz-Muñoz and Perpiñán 2010; Kula et al. 2017).
Some lack of agreement is inevitable when using different methods of measurement, to measure the same variable, on the same sample (Bland and Altman 1986). However, the amount of disagreement between the methods matters in deciding whether the new method can replace the “standard” method or the two methods (new and “standard”) can be used interchangeably (Bland and Altman 1986). It is vital that the disagreement amount does not affect the interpretation of the results (Bland and Altman 1999). Ideally, both methods in this study should be compared to the traditional method of subpubic angle measurement, using a protractor. However, this is impossible to perform as the samples were obtained from living individuals. Thus, measurement protocols nearest to the “standard” method (protractor) may be regarded as the protocol that will provide measurement closest to that measured by a protractor, which was the protocols in Method 1 using the Checkpoint (measured in 3D).
Measurement of the subpubic angle by Method 1 resembled more of a measurement on a real bone. The 3D pelvic model can be rotated 360° to determine the exact anatomical landmarks for the subpubic angle measurement defined in the study protocol (Table 1, Fig. 1). Landmarks can be moved (after the angle was initially measured using the angle measurement tool of the software) if required. The software automatically re-determined the “new” angle, until the observer was satisfied with the location of the landmarks.
In Method 2, the 3D pelvic bone model was viewed in 360° and properly oriented to visualize the subpubic angle before the measurement using the OnScreenProtractor. The OnScreenProtractor measured the subpubic angle on the model, on the computer screen, in 2 dimensions (2D). The pelvic model was rendered static during the measurement. If the pelvic model was moved, the OnScreenProtractor had to be repositioned to measure the subpubic angle.
According to Bland and Altman (Bland and Altman 1999, 2003), repeatability is also a vital characteristic in the method comparison study. In this study, both methods had achieved high repeatability between repeated measurements (using the ICC). Both methods were able to measure the subpubic angle on a 3D pelvic models reconstructed from MSCT scans. However, measurement on the MeshLab required well-defined positioning of the 3D models, before the measurement by the OnScreenProtractor. The OnScreenProtractor measured the subpubic angle in 2D, albeit the pelvic bone model was in 3D. Different positioning of the 3D model of pelvis, not according to the study protocol, will give different measurement values.
In Method 1 and Method 2 comparison, the confidence intervals for the lower limit of agreement was − 0.2° to − 2.6°, and for the upper limit of agreement was 10.2° to 12.6°. Narrow confidence intervals reflected a large sample size and small variation of the differences. Meanwhile, wide confidence intervals indicated a small sample size and large variation of the differences (Bland and Altman 2003). This study demonstrated relatively small intervals for the mean difference (4.28° to 5.66°), and the limits of agreement (lower, − 0.2° to − 2.6° and upper, 10.2° to 12.6°), between the two methods. Nevertheless, the limits of agreement were quite large (− 1.4° and 11.4°). The decision for how small the limits of agreement, and the acceptability of the degree of agreement between the two methods of measurement remains a clinical judgment, and not a statistical one (Bland and Altman 1986). Experts in physical anthropology should decide whether the limits of agreement are small enough for the two methods to be used interchangeably.
The study compared two methods to measure the subpubic angle on a virtual pelvic bone reconstructed from CT scan images, to determine the difference/agreement between these two methods. It is of practical value, which may be used in forensic studies that use similar sample (i.e., virtual pelvic bones) to measure the subpubic angle (as one of the parameters) to establish population standards. We recommended to use Method 1 because it resembles more of subpubic angle measurement on a real bone, as this method measures in 3D, while Method 2 measures in 2D. Nonetheless, the measurements using the two methods were still within the 95% limits of agreement, which means that measurement using Method 2 may also be used. The difference in subpubic angle between the two methods were about 5°. The next step from this study, we wished to use Method 1 in a study for sex estimation in the Malaysian population, with the subpubic angle as one of the parameters (using virtual bones).