The human knee dates back 320 million years in the evolutionary scale to Eryops, the common ancestor of reptiles, birds, and mammals (Figure 2.1). The Eryops knee was bicondylar, with a femorofibular articulation, cruciate ligaments, and asymmetric collateral ligaments. The patella was not yet present. As evolution continued, the fibula migrated distally, away from the joint line; and the bicondylar femur rotated internally and developed a medial offset, bringing the joint progression closer to the midline. The osseous patella developed separately in birds, some reptiles, and in mammals about 70 million years ago. This was a late development compared with the cruciates or the condylar surfaces. The anterior femoral articular surface extended proximally beneath the patella to form the sulcus and completed the development of the patellofemoral joint (Figure 2.2).1,2

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Embryology and Anatomy of the Patella - Springer