When a person straightens, or 'extends', their knee, they use the muscle-bone-tendon complex that we call 'the extensor mechanism'. This includes -
Contraction of the quads muscles thus puts tension on this entire complex to pull the leg straight, and the patella acts to increase the leverage of the whole. During extension, the patella and the two tendons are subjected to high tensile forces, and the patella is also subjected to high compressive forces where it makes contact with the femur bone. Under extreme stress, or in conditions where the structures are otherwise weakened, the extensor mechanism can fail -
In all three of these ruptures, the mechanism of injury can be 'direct' or 'indirect'. A direct injury is likely to be a blow to the front of the knee or a fall onto the knee. An indirect injruy may occur when a person lands from a height and uses the quads to control the landing.
Although a patellar fracture may occur at any age, a quads tendon rupture usually occurs in patients over 40 years of age and a patellar tendon rupture in sporting patients under 40 years of age.
In all three a failure to restore continuity early is likely to lead to weak quads and an extensor lag, ie the person is likely to be unable to get the knee to fully extend.
In terms of the main differences, the big factor with a patellar fracture is that the joint surface is often disrupted, and the joint cartilage damaged. With the other two types of rupture, the challenge is to secure the soft tissue back onto the bone without there being any re-rupture.
In quads tendon rupture the bulk of the muscle may recoil with a large gap above the patella. This may be missed because these injuries are often associated with tremendous localised swelling. In patellar tendon rupture, the patella is pulled up into an abnormally high position from which it must be retrieved and the anatomy restored as soon as possible.