Unipolar vs. Bipolar vs. Total Hip

I'm an MS4 trying to differentiate between total hip vs. hemi with unipolar or bipolar

I understand that in a hemiarthroplasty you are resurfacing only 1 part of the joint, the femur

I don't get the unipolar vs. bipolar. I searched online and read that with bipolar hemiarthroplasty you have motion within the implant, while in unipolar it is the femoral implant in contact with the acetabulum. So is the difference that you're putting in a cup with bipolar implants? And if that's true, isn't this a total hip?

In unipolar hemi, there is still an ARTICULATION between the prosthetic head and the native acetabulum. So there is motion between the prosthesis and the patient's own acetabulum.

In bipolar hemis, the prosthesis has a "built-in" articulation...i.e. there is a prosthetic head inside an inner bearing insert....the key principle of the bipolar design is to reduce motion, stress and wear of the patient's own native acetabulum...so the motion occurs between the prosthetic head and the prosthetic "cup".

Difference between a bipolar and THA is that with a bipolar, you don't do anything to the pt's acetabulum. The inner bearing insert and the prothestic head are all "one piece" and fit onto the femoral stem. But with THA, you ream out the acetabulum and the cup is press fit.

hope this helps. To really understand the difference, you have to hold each implant in your hand in the OR before implanting it. As a med student it's difficult to have the opportunity to do that...but if you ask your senior resident I'm sure they will give you the chance.

thanks!

another thing to keep in mind is that the value of a bipolar over a unipolar is questionable b/c how much motion occurs at the "second" interface is up for debate (several articles can be found on this) and they are quite a bit more expensive. The only time I have done a bipolar was b/c for some bizzare reason they did not have the size of unipolar head that we needed, and we do a LOT of hip fractures.

Some believe that there is a decreased risk of dislocation with a bipolar hip, however if they do dislocate, they are a bear to get back in whereas some believe unipolar is easier to get back in because that outer shell isn't flopping around like the bipolar has, if it's (unipolar) going to go back in.

I still haven't reduced a biipolar yet. It will be interesting the first time I have to. Hopefully it is less likely and I won't have to do it. But it seems like it would be hard with that second articulation flopping around. Has anyone reduced one of these?