Impact of the rotational position of the hindfoot on measurements assessing the integrity of the distal tibio-fibular syndesmosis

Abstract

BackgroundApproximately 20% of patients with ankle fractures demonstrate evidence of syndesmotic injuries. As intra-operative measurements have high specifity but low sensitivity for identifying injuries to the distal tibio-fibular syndesmosis, numerous measurements have been developed to assess pre-operative syndesmosis integrity. Several factors affecting measurements on conventional radiographs and computed tomography (CT) images have been identified. The influence of the rotational position of the hindfoot during imaging, however, remains unclear. MethodsTwenty (20) healthy volunteers (mean age 49, standard deviation [SD] 7.5, range 40–66 years) underwent a weightbearing cone beam CT scan. From this dataset, digitally reconstructed radiographs (DRRs) and axial CT images of the hindfoot were reconstructed. For each image, an antero-posterior view (defined as a plane perpendicular to the longitudinal axis of the second metatarsal) was reconstructed. Then, internal and external rotation of the hindfoot was simulated in 10° increments (maximum rotation of 30°). The tibio-fibular clear space (TFCS), tibio-fibular overlap (TFO), and medial clear space (MCS) were measured on each reconstructed DRR and axial CT image. ResultsInternal rotation of the hindfoot substantially impacted inter-observer agreement for TFCS measurements on DRRs. Intra- und inter-observer agreement of measurements (MCS, TFCS, TFO) on axial CT images was minimally affected by hindfoot rotation. Hindfoot rotation highly impacted on absolute values of each measurement. As little as 10° of internal or external rotation significantly (P<0.05) impacted MCS and TFO measurements (DRRs and axial CT images). External rotation increased, while internal rotation decreased, MCS and TFO measurements. TFCS measurements performed on DRRs did not significantly differ for 10° (P=0.0931) and 20° (P=0.486) of external rotation or for 10° of internal (P=0.33) rotation. DiscussionThe rotational position of the hindfoot during imaging has a major impact on MCS, TFCS, and TFO measurements when using DRRs and axial CT images. To avoid misinterpretation of measurements, the position of the hindfoot during imaging must be standardized. ConclusionsDRRs and axial CT images require reliable reconstructions to allow accurate assessment of the TFCS, TFO, and MCS.