What is a popliteal cist?  
Why is it produced?  
How is the diagnosis established?  
Which is the treatment? 

Some children show a prominence at the back of their knee, on the named popliteal fosse. It is usually produced by a cist with liquid or semi liquid contents.

We don't know why children have this kind of cist. As opposed to adults, it doesn't have communication with the joint and it doesn't have any relationship with intrarticular lesions of the knee.

The popliteal cist is formed by the distension of a anatomic sac full of gelatine-like stuff which comes from the posterior tendons of the knee, in special, semimembranosus and intern gastrocnemius tendon (fig. 1).

Fig. 1: Location of the popliteal cist: in the posterior part of the knee.

How is the diagnosis established?

Parents usually discover this kind of cist by chance, for example, after a contusion. The cist doesn't hurt and it doesn't affect the range of knee movement.

Exploration has to be done with the patient lying on the stomach and the knee has to be completely extended.

Which is the treatment?

Physical exploration leads to the clinical diagnosis. A study by ultrasonography confirms the diagnosis and allows to size the volume of the cist. This is very important to know because the cist may grow.

These cists have a tendency to disappear spontaneously. This is the reason why we don't recommend any treatment except observation.

If the cist grows a lot or it produces pain, the surgeon has to think about a surgical treatment, but this is only carried out in few cases. Parents and children should to know that after a surgical approach the cist can reappear.