Biomechanical Insights Afforded by Shape Modeling in the Foot and Ankle

Abstract

Advancements in volumetric imaging makes it possible to generate high-resolution three-dimensional reconstructions of bones in throughout the foot and ankle. The use of weightbearing computed tomography allows for the analysis of joint relationships in a consistent natural position that can be used for statistical shape modeling. Using statistical shape modeling, a population-based statistical model is created that can be used to compare mean bone shape morphology and identify anatomical modes of variation. A review is presented to highlight the current work using statistical shape modeling in the foot and ankle with a future view of the impact on clinical care.

Keywords: Anatomy; Foot and ankle; Imaging; Morphology; Morphometrics; Statistical shape modeling; Three-dimensional.

Figure 1:

Hindfoot visualization of a healthy individual with three distinct methods of assessing morphology: A) Hindfoot radiograph for the Saltzman view, b) WBCT coronal slice, and C) 3D bone reconstruction for the tibia, fibula, talus, calcaneus, cuboid, and navicular.

Figure 2:

The same talus is visualized in three different viewing perspectives (medial, lateral, and anterior/superior). The first row demonstrated a native imaging resolution of 0.6 mm x 0.6 mm in the XY plane and 2 mm in the Z axis. The 3D segmentation shows this pixelation and stair stepping in the Z-direction. Whereas the second-row segmentations began with a native imaging resolution of 0.6 mm in all three planes.

Figure 3:

Examples of the talus and calcaneus 3D reconstructions with high-quality anatomical details on the left. As the degree of smoothing increases, the level of detail decreases moving to the right. Excessive smoothing can create erroneous features that did not exist prior in some areas and remove meaningful features in other areas.

Figure 4:

First and second modes of variation of the multi-domain model consisting of the talus, calcaneus, navicular and cuboid. The shape variation from these modes is demonstrated by point-to-mesh surface distances (CloudCompare). Surfaces expanding outward of the mean shape surface are highlighted in red and surfaces reducing into the surface of the mean shape are highlighted in blue. Key observed variations are indicated by an arrow.

Figure 5:

Linear discriminant model displaying shape scores for bone alignment and morphology between two groups with ankle osteoarthritis (OA): non-compensated (orange) and compensated (blue).