Linear and angular measurements of computer-generated models: Are they accurate, valid, and reliable?

Abstract

Traditional advanced imaging modalities such as CT and MRI are limitedin their ability to perform accurate linear distance and angularmeasurements regardless of anatomical orientation. The constructionof 3D models has been used to perform anthropometric analyses aswell as in the reconstruction of rapid prototypes. We hypothesizedthat such measurements would be precise to within 2 mm or 2 degreesof measurements performed with a coordinate measurement machine (CMM).We also hypothesized that there would be a high degree of interobserverreliability with these measurements.Multiple aluminum screws wereimplanted in various positions in three foam pelvises which weresubsequently scanned by CT and rendered as 3D models using a commerciallyavailable software package (Mimics). Linear and angular measurementswere performed using a CMM machine, the software package, and a dialcaliper or goniometer. The deviation of the measurements from theCMM data was compared using ANOVA. The interobserver reliabilityof both the manual and computer-generated measurements was calculated.Themean difference between the CMM distances and those measured manuallyand with the software was 2.12 +/- 1.20 mm and 1.57 +/- 1.05 mm,respectively. The mean difference between the CMM angular measurementsand the angular measurements performed manually and with the softwarewas 4.07 +/- 4.70 degrees and 1.62 +/- 1.32 degrees, respectively.In all cases, the manual measurements were significantly less accurate(p < 0.0001) and there was a high degree of interobserver reliability.Computer-generatedmeasurements taken from three-dimensionally reconstructed modelsare more accurate than manual measurements and are within 2 mm and2 degrees of measurements performed with a CMM. These measurementshave high interobserver reliability.