Meeting Abstract
The patella (kneecap) is a bone found within the tendon of the knee extensor muscle group. An often-cited and presumably important function of the patella is to increase the moment arm of the extensor muscle(s), and so reduce the force required from these muscles. This mechanical benefit may partly explain why evolution of the patella in birds seems to have coincided with greater knee flexion and a more crouched limb posture. The nature and magnitude of the patella’s mechanical benefit(s) in birds is unclear, however. Ostriches diverge from the avian anatomical norm, with two patellar bones per knee rather than one, in a proximal-distal arrangement. The proximal patella is small and appears homologous to the single kneecap of most other birds, whereas the distal patella is elongate, closely attached to the tibia, and reminiscent of the extended tibial crest (a retroarticular process) seen in some birds. Does this specialized morphology correlate with alterations in the mechanical function(s) of the knee joint sesamoids and their associated muscles? Here we modify a previously published ostrich musculoskeletal model, adding patellae and kinematics obtained from biplanar fluoroscopy (XROMM) of an adult cadaver, to explore the mechanics (particularly moment arm ratios) of these sesamoids. We find that the distal patella is functionally more similar to a sesamoid than a retroarticular process, rotating to remain pressed against the femur as the knee bends. Both patellae appear to provide a low mechanical advantage throughout knee extension, though this gearing effect increases as the knee joint extends.
