Quantifying lateral femoral condyle ellipticalness in chimpanzees, gorillas, and humans

Articular surfaces of limb bones provide information for understanding animal locomotion because their size and shape are a reflection of habitual postures and movements. Here we present a novel method for quantifying the ellipticalness (i.e., departure from perfectly circular) of the lateral femoral condyle (LFC), applying this technique to hominid femora. Three-dimensional surface models were created for 49 Homo sapiens, 34 Pan troglodytes and 25 Gorilla gorilla femora. Software was developed that fit separate cylinders to each of the femoral condyles. These cylinders were constrained to have a single axis, but could have different radii. The cylinder fit to the LFC was allowed to assume an elliptical cross-section, while the cylinder fit to the medial condyle was constrained to remain circular. The shape of the elliptical cylinder (ratio of the major and minor axes of the ellipse) was recorded, and the orientation of the elliptical cylinder quantified as angles between the major axis of the ellipse and the anatomical and mechanical axes of the femur. Species were compared using analysis of variance and post hoc multiple comparisons tests. Confirming qualitative descriptions, human LFCs are more elliptical than those of chimpanzees and gorillas. Human femora exhibit a narrow range for the angle between the major axis of the elliptical cylinder and femoral axes. Conversely, the chimpanzee sample is bimodal for these angles, exhibiting two ellipse orientations, while Gorilla shows no preferred angle. Our results suggest that like modern human femora, chimpanzee femoral condyles have preferentially used regions.