mainly polymers are mixed with HAp , to impart the required plasticity, mouldability, control the rate of degradation in vivo, etc.. and mixing polymers with HAp will increase the proccessability of HAp using various methods such as extrusion , etc..Either polymers can be used as a binder for HAp powder or HAp powder can be used as reinforcements for the polymer.
I agree with Arun Kumar Rajendran. Many polymers are combined with hydroxyapatite and other calcium orthophosphates to create composites. To get further details, I suggest you to study reviews on the subject (there are a lot of them).