7 9 11 However, conclusions were based on 2D imaging a thorough morphometric evaluation in healthy individuals utilizing 3D surface reconstructions is still lacking. In addition, recent studies investigated alterations at the level of the subtalar joint in flat foot deformity. 6 12 13 15 The underlying mechanism still remains unclear due to 2D imaging analysis limitations. 6 12 13 From this prior analysis, the authors concluded that the movement of the subtalar joint has the potential to compensate for ankle deformities. 6 12– 15 Thus, measurements performed on weightbearing conventional radiographs and single CT images have been used to assess the orientation of the posterior facet in the coronal plane at different stages of ankle osteoarthritis. The impact of the osseous configuration of the posterior facet of the subtalar joint on the evolution of ankle joint osteoarthritis has been recently recognized as a clinical concern. However, in practice it can be very difficult to extract clinically-relevant shape metrics from 3D surface reconstructions. 5– 12 Improved understanding of the spectrum of hindfoot deformities and joint morphometrics could assist in the treatment of patients with conditions such as posttraumatic osteoarthritis (OA) or instability of the hindfoot. 3 4 Weightbearing cone beam CT technology provides an added benefit to assess pathology such as joint space narrowing, bony malalignment or impingement that may be undiagnosed without the presence of load. 1 2 The advent of volumetric imaging, including computed tomography (CT) and magnet resonance imaging (MRI) has made it possible to generate three-dimensional (3D) reconstructions of the hindfoot however these imaging modalities are commonly performed in a supine non-weightbearing position. Historically, the morphometric understanding of the subtalar joint has been derived primarily from two-dimensional (2D) measurements of conventional radiographs. The described SSM approach also shows promise for clinical evaluation of the subtalar joint from 3D surface reconstructions of WBCT images. Results provide a detailed 3D analysis of the subtalar joint under a weightbearing condition in a healthy population which can be used for comparisons to pathological patient populations. Despite considerable shape variation across the population, the posterior and anteromedial articular facets of the subtalar joint were consistently congruent. The four most relevant findings were: I) talar and calcaneal anatomical differences were found regarding the presence of a) the talar posterior process, b) calcaneal pitch, and c) curvature of the calcaneal posterior facet II) the talar posterior facet articular surface area was significantly greater than the calcaneal posterior facet articular surface area III) the posterior facet varied in joint space distance, whereas the anteromedial facet was even and IV) the posterior and anteromedial facet of the subtalar joint were consistently congruent. Herein, we employed statistical shape modeling to evaluate three-dimensional (3D) shape variation, coverage, space and congruency of the subtalar joint using WBCT data of 27 asymptomatic healthy individuals. The shape and orientation of the healthy/normal subtalar joint, in particular, is not well-understood, which makes it very difficult to diagnose subtalar pathoanatomy. Clinical measurements of WBCT images are generally limited to two-dimensions, which reduces the ability to quantify complex morphology of individual osseous structures as well as the alignment between two or more bones. Weightbearing computed tomography (WBCT) enables visualization of the foot and ankle as patients stand under load.
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