21 A non-invasive scheme for parametric characterisation of the natural ankle geometry

Abstract

Background The mechanics of the natural ankle joint is dictated by its geometry and morphology. Enhanced parametric characterisation of its articulating surfaces in 3D would significantly help the development of in-silico simulations of the joint as well as facilitate better methodologies for pre-clinical testing of novel interventions for chronic ankle instability and osteochondral defects. Objective To identify key morphological parameters of the trochlear surface in 3D, related to the talo-tibial articulation. Design Experimental. Setting Engineering laboratory. Participants Two cadaveric feet (age >76) sectioned from the distal tibia. Assessment of risk factors The ankles were scanned using a high resolution CT and the image data were processed to produce 3D-surface renderings of the talus. The surface points lying on the lateral and the medial margins as well as the mid-sagittal line of the trochlear surface were recorded using 3D coordinates, as separate sets. A computational algorithm was developed to obtain the best-fit sphere for the each of the point sets using a least-square error method. The alignment of the spheres, their centres, and the sphere radii were evaluated as parameters for characterising the articulating surface. Main outcome measurements Radii, centres of the best-fit spheres to talus trochlear surface. Results Best-fit sphere dimensions were determined as follows: lateral margin (x = 58.0 mm, y = 59.3 mm, z = 51.7 mm, radius = 21.5 mm); mid-sagittal (x = 68.6 mm, y = 57.3 mm, z = 49.1 mm, radius = 22.8 mm); medial margin (x = 78.1 mm, y = 55.2 mm, z = 49.4 mm, radius = 22.8 mm). The centres of the spheres were on a straight line on the transverse plane with the medial end of this line positioned more towards anterior. The sphere representing the lateral margin was smaller compared to the other two and was located slightly at a higher level axially. Conclusions From the results it is seen that surface (sphere) fitting allows derivation of characteristic parameters for the talotibial articulation. Further, these fitted surfaces are compatible with pronation/supination, and inversion/eversion motion, and hence provide a method for evaluation of these movements.