Fig. 7

a and b illustrate initial and subsequent tibial sesamoid positions on the ridge and groove. a represents the initial tibial sesamoid contact point in its groove (B’), whereas B would be its position if the metatarsal were spherical. In b, it can be seen that there is a change in the CR (red normal line) due to the groove having a different tangent line (B’) from the ridge (B); it has shifted proximal because the elliptical tibial groove makes the tangent lines of A and B’ more obtuse than the angle between the tangent lines at A and B, and as a result, it has shifted proximal in b