Measurement Accuracy of 3-Dimensional Mapping Technologies versus Standard Goniometry for Angle Assessment

BACKGROUND A specific aspect of the hypospadias phenotype that may contribute to long-term outcomes is the presence of ventral penile curvature and the adequacy of its surgical correction. The current gold standard to assess this angle is intraoperative goniometry of an erect penis. 3-dimensional (3D) mapping technologies may overcome the limitations of these traditional methods through their combination of digital image and geometric replication to produce consistent 3D digital forms of a physical structure. The aim of this study is to evaluate the measurement accuracy and reliability of handheld 3D mapping technologies versus standard goniometry for angle assessment in a laboratory setting. METHODS Blocks with specified angles (10-45°) were printed using a Zortrax M200 3D printer (±0.2% accuracy). Following the completion of standardized training, blinded participants measured each block angle using a baseline digit goniometer. Additionally, complete digital models of the blocks were created using 3D mapping technologies. Structured light scanning was completed using an Artec Space Spider and Artec Studio 13. Traditional photogrammetry was completed using a Canon Eos Rebel T5i DSLR camera and Agisoft Metashape Pro. Photogrammetry with a 3D camera was completed using the VECTRA H1 and VECTRA Analysis Module. All 3D models were imported into the software Autodesk Inventor in which automated angle measurements through the central plane were obtained. Statistical analysis was performed to determine the accuracy, precision and reliability of each modality using SAS 9.4 software. The reliability of goniometry and each mapping technology was evaluated using two-way random effect models with absolute agreement. RESULTS Six 3D printed blocks were evaluated. 5 digital models per block were created using each of the 3 mapping technologies. Inter-rater reliability of goniometry was moderate (ICC 0.76, 95% CI 0.46, 0.92), whereas all mapping technologies demonstrated excellent test-retest reliability: structured light scanning (ICC 0.99; 95% CI 0.999, 0.999); traditional photogrammetry (0.99; 0.99, 0.99); 3D camera (0.99; 0.99, 0.99). Mean angle measurements and standard error for each angle and modality are provided in the table. CONCLUSIONS This study demonstrated excellent accuracy, precision and reliability of off-the-shelf, handheld 3D mapping technologies and moderate reliability for goniometry when applied to measurements of angulation in a laboratory setting. The described methods developed in the laboratory for optimization of angle analysis from 3D models are an important step toward reliable, reproducible phenotypic analysis of congenital genitourinary conditions in future intraoperative and database development applications.