Development of a novel method to perform morphological measurements on 3D models of the calcaneus

Abstract

Currently, morphological measurements on conventional radiographs are used to assess calcaneal fractures. The two most commonly used angles for determining the degree of depression and displacement of such a fracture are Böhler’s angle (BA) and the Critical angle of Gissane (CAG). However, conventional radiographs only provide limited amount of information and are increasingly being omitted, while computerized tomography (CT) scans are increasingly being used as they contain much more information regarding fracture configuration and displacement. It is therefore important that morphological measurements can be performed using CT-scans. The purpose of this thesis were to design a novel method to perform semi-automated morphological measurements based on 3D models of the calcaneus. The results of the novel method were compared with conventional 2D measurements of the same calcaneus.

119 lateral foot radiographs and CT-scan of the same foot were obtained. The data consisted out of three populations: 40 non-fractured, 39 fractured and 40 reconstructed calcanei. Based on the CT-scans, 3D models were reconstructed through interactive segmentation processes. Measurements on the radiographs and 3D models were performed in a specially designed Graphical User Interface. Three observers measured BA and the CAG, after which they repeated the measurements after a minimum of one week. The inter- and intra-observers reliability were calculated separately for all three populations, using the intra-class correlation coefficient (ICC). Additionally, the frequency of consensus given an allowed discrepancy was calculated.

The ICC for inter- and intra-observer reliability of the 3D measurements showed an improvement for BA and the CAG for all populations compared with the ICC of the 2D measurements. The ICC for inter- and intra-observer reliability of the 3D measured BA ranged from 0.97 to 1.00, indicating an almost perfect agreement between the observers. For the CAG, the inter- and intra-observer ICC ranged from 0.89 to 0.92 and from 0.87 to 0.96, respectively. To achieve 80% consensus for the BA among the observers, an allowable discrepancy of 20 degrees was needed for the 2D measurements, while only 5 degrees was needed for the 3D measurements. To achieve 80% of consensus for the 2D CAG measurements, an allowable discrepancy of at least 20 degrees was needed. For the fractured and reconstructed population, even an allowed discrepancy of 30 degrees was needed. For the 3D CAG measurements, 80% of consensus was achieved with an allowable discrepancy of 5 degrees, 20 degrees and 10 degrees for the non-fractured, fractured and reconstructed calcanei populations, respectively.

3D morphological measurements based on 3D models of the calcaneus showed better reliability and repeatability compared to the 2D measurements. However, the novel method of performing the measurements is not yet safe for use in clinical practice, as the measurements first should be validated. Nevertheless, the method is well suited to be employed as a reference for the development of further (semi-)automated methods to perform morphological measurements.