(Chapter 32) Footnote 16:

The developing cartilaginous patella frequently ossifies from more than one center between the third and sixth year. If these centers of ossification fail to fuse into a single bone, a bipartite or tripartite patella could develop that may fracture into fragments under a direct blow or sudden contraction of m. quadriceps femoris; the proximal fragment is pulled superiorly by the quadriceps tendon, leaving the distal fragment with the ligamentum patellae (K.L. Moore, Clinically Oriented Anatomy 3rd edn. Baltimore: Williams & Williams, 1992, p. 393).