Sideview of the knee bones, showing a transverse fracture of the patella.
Showing a patellar fracture (middle image) compared to other ruptures of the extensor mechanism (ie the chain of structures that together straighten the knee).
How kneecap fractures occur
The kneecap may fracture from a direct impact, such as a fall on the knee. Or it may fracture from excessive extension force, such as in a weightlifter.
Consequences of a fractured patella
Because the patella may fracture in different ways there are different consequences. A small fragment may be pulled off, causing pain but not a huge amount of disability, while a complete transverse rupture may render incompetent the whole of the extensor mechanism of the knee, and the person may be unable to fully extend the knee or bear weight through it. The worse kind of fracture is a 'comminuted fracture, where the kneecap is shattered, and the parts may be forced apart, making any healing virtually impossible unless the surgeon is able to pull the pieces back into position and hold them there with wires for a sufficient time. The transverse fracture will also need wires to hold the parts together, as the extensor mechanism of the quads muscle will tend to pull the fragments apart.
Stiffness is a common consequence of patellar fracture.
Imaging of patellar fractures. Jarraya M, Diaz LE, Arndt WF, Roemer FW and Guermazi A. Insights Imaging. 2017 Feb; 8(1): 49–57.
Current concepts review: Fractures of the patella. Gwinner C, Märdian S, Schwabe P, Schaser K-D, Krapohl BD and Jung TM. GMS Interdiscip Plast Reconstr Surg DGPW. 2016; 5: Doc01.