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What is a Bipartite Patella? - Types & Treatment

Instructor: Artem Cheprasov
This lesson describes a relatively uncommon condition known as bipartite patella. You'll learn what it is, why it occurs, what types there are, and two major ways by which it can be treated.

Bipartite Patella

How many patellae, or kneecaps, do you have? Two, right? Well, there are some people who have more than two due to bipartite patella, a kneecap that seems to be in two parts. In other words, it's composed of two separate bones as opposed to just the one.

A normal patella of the knee.
patella

Let's find out why this occurs, what kinds there are, and how one might be treated.

Causes

So why does this happen? Well, first a little developmental anatomy is in order.

See, when you were a 9-week-old embryo inside your mother's womb, you got yourself a pair of nice cartilaginous patellae. But only at around age 3 - 6 years of age did the cartilage begin its transformation into solid bone, called primary ossification.

Around early adolescence, secondary ossification centers might appear in the patella. Generally speaking, primary ossification centers help the patella grow more longitudinally while secondary ossification centers help the patella grow more spherically. All of these ossification centers should fuse together to form one solid and bony kneecap by early adulthood.

However, sometimes these ossification centers don't fuse together, and you're left with a bipartite patella. Sometimes, the patella is actually split into more than two pieces. Either happens upwards of 6% of the time. These pieces are commonly connected to one another thanks to cartilage or fibrous tissue.

There are, however, other possible causes for a bipartite patella. For instance, trauma that fractures the patella could end up in nonunion of the two halves, resulting in the bipartite patella. That trauma could come from, say, a peeved mob boss who decides to 'bust' a kneecap to prove a point.

Types

The different types of bipartite patellae are often classified according to the Saupe classification system, which goes by the position of the accessory ossification center

  • Type I, where the accessory fragment is located in the inferior (bottom-most) pole of the patella. This accounts for about 5% of all cases.
  • Type II, where the accessory fragment is located at the lateral margin of the patella. In other words, it's on the same side as the outer thigh. This accounts for roughly 20% of all cases.
  • Type III, where the accessory fragment is at the superolateral pole. This is towards the top and outer edge of the patella. This amounts to about 75% of all cases.

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