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).