A bipartite patella is where the kneecap is made up of two separate bones, instead of one, that have failed to fuse together during childhood. It is a rare condition, affecting only 1-2% of the population.
In most cases, it doesn’t cause any problems and may go completely unnoticed – it is often only discovered when an x-ray is done for an unrelated knee problem.
However, in some instances it can cause pain at the front of the knee, particularly when squatting down, and limit function.
Here we will look at what causes a bipartite patella, how they are classified, the symptoms that can develop and how to treat them.
The patella, or kneecap, is a triangular shaped bone. When you are born, there is essentially no bone in the kneecap – it is made up of cartilage and blood vessels. During early childhood, as you grow, the kneecap cartilage gradually expands.
At around age 4, the cartilage starts to ossify, forming small portions of bone. These bone segments gradually fuse together and by the age of 12 a single kneecap has usually formed.
Occasionally, the bones fail to fuse completely leaving two distinct pieces of bone. There tends to be one large piece of bone and one smaller fragment.
This is what is known as bipartite patella and no one knows why this happens. The two portions of bone remain unfused, connected by fibrocartilaginous tissue known as synchondrosis.
Bipartite patella is more common in males with a male:female ratio of 9:1.
The first classification system for bipartite patella was introduced in 1921 by Saupe. He used the location of the fragment to classify the condition into 3 types:
Type 1: Found at the inferior (bottom) portion of the patella. Incidence approximately 5%
Type 2: Found on the lateral margin (outer side) of the patella. Incidence approximately 20%
Type 3: Found on superolateral (upper, outer) portion of the patella. Incidence approximately 75%
So as you can see, by far the most common place to have the synchondrosis is at the top of the kneecap, on the outer side.
In just under half of all cases, both kneecaps are affected.
In approximately 98% of cases of bipartite patella, people are completely asymptomatic. This means there are no obvious signs that the kneecap hasn’t fully fused and it doesn’t cause any problems. In most cases, the condition is only discovered when the knee is x-rayed or scanned following an unrelated injury.
In 2% of cases, it does go on to cause problems. This usually happens when the tissue between the bones (synchondrosis) is damaged, causing irritation and inflammation either from a direct blow to the front of the knee, a fall or repetitive overloading. Activities that typically cause problems are those where there is repetitive knee bending, jumping or squatting such as with skiing, cycling, hill walking or basketball.
The most common symptoms associated with bipartite patella are:
1) Pain: usually at the top of the patella on the outer side of the knee. It tends to be worse with squatting activities
2) Tenderness: when pressure is placed over the fragment
3) Swelling: this tends to be fairly localised
4) Instability: the knee may feel like it is going to give way
5) Bony Ridge: you may be able to feel a slight ridge on the patella
6) Large Patella: The kneecap may well be larger than usual
The unfused area of the kneecap shows up clearly on an x-ray or MRI scan of the knee. The triangular bone will have a gap in it where the fibrocartilaginous tissue has failed to ossify.
It may initially be mistaken for a patella fracture but the tell-tale signs of bipartite patella are:
1) Rounded Edges: rather than the characteristic jagged edges seen in patella fractures
2) Location: A gap on the superolateral aspect of the kneecap is characteristic of bipartite patella rather than a fracture
3) Bilateral: If both knees show similar findings on x-ray it is most likely bipartite patella rather than a fracture
Most symptomatic cases can be treated conservatively i.e. non-operatively. Treatment for bipartite patella usually consists of:
It is important to rest from all aggravating activities. If an activity causes pain, don’t do it. The joint needs time to settle down
Regularly applying ice to the knee can help to reduce inflammation in the synchondrosis. In the ice treatment section we look at the most effective ways to apply ice safely and effectively
It is really important to strengthen and stretch the knee muscles to allow the kneecap to move properly without the synchondrosis getting irritated and inflamed. Visit the knee exercises section to find a whole range of exercises that can help.
It can help to wear a knee brace to unload the kneecap. In some cases the knee may be immobilised in a brace that holds the knee in 30 degrees of flexion. Visit the knee braces section to find the right brace for you.
Your doctor may give you an injection of corticosteroid mixed with local anaesthetic to reduce pain and inflammation. You can find out all about how they work and the possible side effects in the knee injections section.
Your doctor may prescribe painkillers and anti-inflammatory medication can help to reduce any pain and inflammation.
Regular session of physical therapy can also be helpful. The will work on a rehab programme with you and may also use ultrasound to help reduce inflammation and improve healing.
Symptoms usually settle after two or three months with these treatment methods. However if they persist for more than six months then your doctor may advise surgery.
There are three surgical options with bipartite patella:
1) Open Excision: this is when the smaller fragment of bone is removed. This is the most common surgical treatment technique for bipartite patella
2) Open Reduction Internal Fixation (ORIF): A screw is inserted to hold the bone fragments together. This tends to be done for larger fragments
3) Lateral Release: If there is tightness in the lateral retinaculum or vastus lateralis that is pulling the patella out of alignment, then the tight structures are cut to allow the kneecap to sit centrally. You can find out more about what surgery involves and the recovery process in the lateral release section.
People usually make a full recovery from surgery within a few months and are able to get back to their usual activities. It is important to do both strengthening and stretching exercises for the first few months to ensure you regain full strength and mobility in the knee.
If there are no symptoms associated with bipartite patella, then no treatment is required.
Most people with bipartite patella will go through life completely unaware they have the condition.
There are a number of other conditions that are more likely to cause pain at the front of the knee – visit the front knee pain section to find out what they are.
If you need some help working out what might be wrong with your knee, visit the knee pain diagnosis section.