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Three-dimensional (3D) reconstruction of lower limbs is essential for surgical planning and clinical outcome evaluation. 3D reconstruction from biplanar calibrated radiographs may be an alternative to irradiation issues of CT-scan. A previous study proposed a two-step reconstruction method based on parametric models and statistical inferences leading to a fast Initial Solution (IS) followed by manual adjustments. This study aims to improve the IS using a new 3D database, a novel parametric model of the tibia and a different regression approach. The IS was evaluated in terms of shape accuracy on 9 lower limbs and reproducibility of clinical measurements on 22 lower limbs. Reconstruction time was also evaluated. Comparison to the previous method showed an improvement of the IS in terms of shape accuracy (1.3 vs. 1.6 and 2 mm respectively for both femur and tibia) and reproducibility of clinical measurements (i.e. 3.1° vs. 8.3° for neck-shaft-angle; 4.2° and 5° vs. 5° and 6° for tibial and femoral torsion respectively). The proposed approach constitutes a considerable step towards an automatic 3D reconstruction of lower limb.

International audience; In the context of patient-specific 3D bone reconstruction, enhancing the surface with cortical thickness (COT) opens a large field of applications for research and medicine. This functionality calls for database analysis for better knowledge of COT. Our study provides a new approach to reconstruct 3D internal and external cortical surfaces from computer tomography (CT) scans and analyses COT distribution and variability on a set of asymptomatic femurs. The reconstruction method relies on a short (∼5 min) initialisation phase based on 3D reconstruction from biplanar CT-based virtual X-rays and an automatic optimisation phase based on intensity-based cortical structure detection in the CT volume, the COT being the distance between internal and external cortical surfaces. Surfaces and COT show root mean square reconstruction errors below 1 and 1.3 mm. Descriptions of the COT distributions by anatomical regions are provided and principal component analysis has been applied. The first mode, 16–50% of the variance, corresponds to the variation of the mean COT around its averaged shape; the second mode, 9–28%, corresponds to a fine variation of its shape. A femur COT model can, therefore, be described as the averaged COT distribution in which the first parameter adjusts its mean value and a second parameter adjusts its shape.

Three-dimensional (3D) reconstruction of the skeleton from biplanar X-rays relies on scarce information digitalised by an operator on both frontal and lateral radiographs. In clinical routine, difficulties occur for non-skilled operators to discriminate the medial from the lateral femur condyle on the lateral view. Our study proposes an algorithm able to detect automatically a possible inversion of the two condyles by the operator at an early stage of the reconstruction process. It relies on the computation of two 3D femur surfaces, one directly from the operator digitalisation and the other from the same digitalisation with medial and lateral condyles automatically swapped. Pairs of virtual biplanar X-rays are computed for both reconstructions and the closest pair to the original X-rays is selected on the basis of similarity measures, pointing the correct 3D surface. The algorithm shows a success rate higher than 85% for both asymptomatic and pathological femurs whatever the initial condyle digitalisation of the operator, bringing automatically non-skilled operators acting in clinical routine to the level of skilled operators. This study validates moreover the proof-of-concept of automatic shape adjustments of a 3D surface on the basis of similarity measures in the process of 3D reconstruction from biplanar X-rays.

Resume Introduction Des variations physiologiques des torsions des membres inferieurs surviennent au cours de la croissance chez l’enfant. Les donnees de la litterature disponibles ne sont que des valeurs cliniques semi-quantitatives ou des mesures 2D. Des methodes de mesure quantitatives 3D sont necessaires. La stereoradiographie basse-dose avec reconstruction 3D apparait comme une bonne alternative. Son utilisation se generalise en pediatrie grâce a son irradiation minimale. Les etudes anterieures ont demontre sa fiabilite et sa reproductibilite sur les reconstructions de membres inferieurs chez l’adulte et l’enfant, mais les parametres torsionnels segmentaires n’ont jamais encore ete etudies. Cette etude reporte les valeurs normales de torsion des membres inferieurs et leur evolution au cours de la croissance sur une population d’enfants et de jeunes adultes sains en utilisant le systeme biplanaire basse-dose EOS et methode de reconstruction 3D. Hypothese La reconstruction 3D EOS est une technique precise et reproductible des mesures des torsions chez l’enfant. Materiel et methodes La torsion femorale (FT) et la torsion tibiale (TT) ont ete etudiees sur une population de 114 volontaires (228 membres inferieurs), divises en 5 tranches d’âge de 6 a 30 ans. Les acquisitions EOS ont ete realisees selon le protocole debout pieds-decales. Resultats La FT decroit avec l’âge, passant de 21,6° a 18°. La TT augmente de 26,8° a 34,7°. Il y avait une difference statistiquement significative entre les deux groupes extremes mais pas entre les groupes d’âge adjacent. L’ICC intra-observateur etait de 0,96 pour la FT et de 0,95 pour la TT concernant le premier operateur, et de 0,79 et 0,83 respectivement pour le second operateur. L’ICC inter-observateur etait de 0,84 pour le FT et 0,82 pour la TT. Discussion Le profil torsionnel obtenu est concordant avec les mesures cliniques et 2D connues dans la litterature. La reconstruction 3D avec le systeme biplanaire basse-dose EOS est une technique fiable et sure pour les mesures des membres inferieurs, particulierement pour l’etude des parametres torsionnels. Niveau de preuve IV.

Introduction Lower limb torsion varies substantially among healthy children during growth. Values reported in the literature to date have been obtained using semi-quantitative clinical or 2D measurement methods. Quantitative 3D measurement would help determine the physiological range of lower limb torsion. Low-dose stereoradiography with 3D reconstruction provides a good alternative. Its use increases in pediatrics because of radiation minimization. Previous studies have shown accurate and reproducible results of lower limbs reconstruction in adults and children but the torsional parameters haven’t been measured yet. The present study reports the values of lower limb segmental torsion and its course during growth in a cohort of healthy children and young adults using the EOS low-dose biplanar X-ray. Hypothesis EOS 3D reconstruction is an accurate and reproducible method to measure the torsional parameters in children. Materials and methods Femoral torsion (FT) and tibial torsion (TT) were studied on 114 volunteers (228 lower limbs) from 6 to 30 years of age divided by age into 5 groups. The EOS™ acquisitions were obtained in subjects standing with their feet offset. Results Mean FT decreased during growth, passing from 21.6° to 18°, whereas mean TT increased from 26.8° to 34.7°. There was a statistically significant difference between the 2 extreme age groups, but no difference was found between any other age groups. The ICC for intra-observer reproducibility was 0.96 and 0.95 for FT and TT for the first operator, and 0.79 and 0.83 for the second operator respectively. The ICC for inter-observer reproducibility was 0.84 and 0.82 respectively. Discussion The course of lower limb segmental torsion observed was consistent with literature reports based upon clinical and 2D measurements. 3D reconstruction of EOS low-dose biplanar imaging appears to be a safe and reliable tool for lower limbs measurements, especially for investigating lower limb segmental torsion in children and adults. Level of evidence Level IV.