Chimpanzee Foot Model

A 3D FE model of the chimpanzee foot was developed based on the CT scan data of a frozen cadaveric lower leg (female, age 44 years at the time of death; weight not available; foot length, 25.2 cm) (Figure 1). Cross-sectional images were reconstructed at 0.25 mm intervals, with a pixel size of 0.25 mm. The chimpanzee model was developed in the same manner as described for the human foot model. However, the following differences existed between the human and chimpanzee foot models: 1) The chimpanzee foot did not exhibit an anterior talofibular ligament and a transverse metatarsal ligament between the first and second metatarsals (Raven, 1936; Lovejoy et al., 2009); 2) the long plantar ligament of the chimpanzee foot originated from the plantar surface of the cuboid, whereas that of the human foot originated from the anteroplantar surface of the calcaneal tuberosity (Gomberg, 1985); and 3) the chimpanzee foot had an additional sesamoid for each second, third, and fourth digit, as well as two additional sesamoids for the fifth digit; therefore, all PA elements were modeled with the sesamoids. The PA of the chimpanzee foot was previously suggested to be rudimentary (Lovejoy et al., 2009), but a recent study estimated the stiffness of the PA of chimpanzees to be about one-half of that of humans (Wareing, 2016). Therefore, the spring constants of the PA were assumed to be one-tenth (10%) and one-half (50%) of those of the human values in the chimpanzee foot. The length of the ligaments and the PA elements when the foot was in the CT-scanned posture was assumed to be the natural length of the ligaments and PA elements, as the chimpanzee cadaver foot was scannable via CT in a natural plantigrade posture. Again, some adjacent phalanges were extracted as one entity. Therefore, there were a total of 31 bones constituting the chimpanzee foot model: tibia, fibula, talus, calcaneus, navicular, cuboid, three cuneiforms, five metatarsals, proximal and distal hallucal phalanges, one, three, two and two phalangeal bones of the second, third, fourth, and fifth rays (proximal, middle and distal phalanges were fused in the second ray and the middle and distal phalanges were fused in the fourth and fifth rays), and seven sesamoids.

The female chimpanzee foot is much smaller than the human foot. To facilitate biomechanical comparisons, the chimpanzee foot was enlarged by a factor of 1.323, i.e., the cube root of the total bone volume of the human foot divided by that of the chimpanzee foot. The material parameters were assumed to be identical for both the human and chimpanzee models.