Reconstructions en 3D du pied de l’enfant à partir de radiographies biplanes

 Foot conditions are a common motive of paediatric orthopaedic consultation, going from congenital abnormalities to physiological deformities related to growth. These deformities may have an impact on gait and posture. (Mickle et al. 2011), but are difficult to quantify because of lack of quantitative tools allowing a global evaluation of the bone lower limb morphology. Foot and lower limb morphology are traditionally evaluated using semi-quantitative clinical measurement methods and radiological measurements in the weight-bearing position. Clinical measurements can potentially be subjected to biases related to personal interpretation, and are therefore insufficient to either completely describe abnormalities or to plan treatment or surgery. Moreover, to assess the efficiency of a treatment and the changes induced in the foot, objective quantitative measurements of the angles used to describe the geometrical structure of the foot are required. Conventional radiological measurements are more accurate than clinical ones (1-4), but are prone to errors when frontal plane deviations are combined with transverse or sagittal plane deformities (referred to as projection biases)(5). As an example, it has been shown in cavus feet that the talus1 st metatarsal angle of Meary measured on a lateral view can be strongly biased by either the adduction of the calcaneopedal unit or the rotation of the tibio-talo-fibular unit (6). In addition to the key role of the radiographic evaluation of the paediatric foot, an additional challenge is related to the variation in osseous geometry resulting from skeletal immaturity. Other imaging techniques, such as the CT scan or the MRI, are available and used in clinical routine. Despite some attempts to simulate weight-bearing position (weight-bearing CT scan (WBCT)), this technology is not yet widespread in clinical routine and, owing to the limited field of imaging, the analysis of the complete lower limb cannot be performed (7). Indeed, CT and MRI acquisitions are routinely performed in the supine position, while the foot has to be evaluated in the standing weight-bearing position. This is of critical importance for the correct assessment of the mutual impact of the foot on the lower limb bone and joints morphologies. Recently, a 3D model-based reconstruction technique using 2D calibrated lowdose biplanar X-ray images has been proposed and validated for assessing clinical parameters of the spine (8), thoracic cage (9), upper limb (10) and lower limb (11- 13), and has been shown to be a promising alternative to the standard aforementioned procedures (14,15). Concerning the adult foot, preliminary work on the 3D-reconstruction of the foot and ankle in the weight-bearing position from low-dose bi-planar X-ray radiographs with clinical measurement assessment for clinical routine also appears encouraging (16). This could potentially help to address phenomenon, which cannot be clinically quantified, such as the calcaneal pitch angle or the twist angle, which are of major interest for physio-pathological analysis of feet deformities and particularly cavus and flat feet (17).

Material and methods 

Population and acquisition protocol After Institutional Review Board approval (CPP 0 2013-A01568-37, n ° 76 09 2013), ten typically developing children (6 girls, 4 boys aged 9 to 13 years old) were enrolled in the study. All participants and their parents were previously informed of the protocol and had signed a written informed consent, allowing both collection and use of the data reported in this work. For the X-Ray acquisition, subjects were positioned in specific position adapted from Rohan et al. (16): the left foot was in complete weight-bearing position, while the right one rested on the tip of the foot as illustrated on Fig. 4.1.Two calibrated orthogonal Biplanar X-rays (EOS Imaging, Paris, France) were then simultaneously acquired in 10 seconds (exposure parameters: antero posterior (AP) view radiograph: 60 kV, 200 mA, 400 mGy/cm2 ; profile: 70 kV, 200 mA, 500 mGy/cm2 ). (2.2) Simplified parametric model A Simplified Parametric Model (SPM) of the paediatric foot was defined by representing the bones by 3D geometric primitives (3D points, cylinders, and sphere). In our case, a collection of 8 stereo corresponding landmarks was defined on both the AP and profile radiographs of each foot (Fig. 4.2).Then, the foot reconstruction method was performed using a dedicated software developed in the Institut de Biomecanique Humaine Georges Charpak Arts et Métiers ParisTech), with an adaptation of the method proposed in Chaibi et al. (11) and Rohan et al. (16). From information that was numerized, an initial parametric model of the foot was built, and then retroprojected on the AP and profile radiographs. Manual adjustments were then performed to improve the superposition of the projected elements on bone contours.