1. The patella may be absent, due to congenital or hereditary causes.
2. Congenital absence of the patella is associated with other isolated grotesque anomalies and with marked impairment in function of the knee joint.
3. The hereditary absence of the patella is usually associated with but one remote anomaly, absence of the thumbnails, and there is no evident impairment in function of the knee joint.
4. The patella is not a lever.
5. The femoral condyle is not an efficient inclined plane.
6. The patella owes its efficiency to increasing the distance between the axis of rotation of the tibia and the line of direction of the force that pulls on the tibial tubercle by means of the patellar ligament, thereby decreasing the magnitude of the force factor in the product (magnitude of force times the perpendicular distance between the center of rotation and the line of direction in which the force is exerting itself) that gives the moment of force, sufficient to rotate the tibia.