Ankle Joint
There are 12 patient-specific model parameters (7 translational and 5 rotational)
that must be customized to determine a pair of patient-specific functional ankle joint axes
(Figure 3-7, Table 3-4). Comparable to Bogert et al. (1994), the talocrural and subtalar
joints connect the tibia, talus, and foot segments. Within the tibia segment, 3
translational model parameters (p6, P7, and ps) and 2 rotational model parameters (pl and
p2) position the nominal talocrural joint axis. The talus origin corresponds to the
talocrural joint center; therefore, it is not necessary to prescribe model parameters
defining the talocrural joint axis in the talus segment. The talus coordinate system is
created where the y-axis extends along the line perpendicular to both the talocrural joint
axis and the subtalar joint axis. The heel and toe markers, in combination with the tibia
y-axis, define the foot coordinate system. There are 3 translational model parameters
(plo, p11, and p12) and 2 rotational model parameters (p4 and ps) (Inman, 1976) that place
the nominal subtalar joint axis in the foot coordinate system.
Given the anatomical ankle joint DOFs are found within the articular capsule, a
cube with side lengths equal to the distance between ankle markers and its center located
at the midpoint of the nominal talocrural joint axis provides the geometric constraints for
the optimization of each translational model parameter. The rotational model parameters
of the talocrural joint axis are restricted within a circular cone defined by the 3600
revolution of the nominal talocrural joint axis varied by + 300. The rotational model
parameters of the subtalar joint axis are confined within a circular cone defined by the
3600 revolution of the nominal subtalar joint axis altered by + 300 (Figure 3-5, Table A-3,
Table B-3).