choosing cup size

[podgornymd]

Does anyone know if the doctor decides to put in a larger size ball because it was between 2 sizes does that necessitate a larger cup size? For example if they go with a size 50 ball on the femur instead of 48 does that mean the cup needs to be larger?

[Granton]

Hi podgornymd,

I guess so. In the case of the BHR, and surely all other types, certain ball sizes only fit with certain cup sizes. I cannot see why it should be a problem - your natural acetabulum must match the size of your natural femoral head.

David

[Jason0411]

My understanding they all come as matchyed pairs.

[chuckm]

Many surgeons will template everything before surgery and know the sizes to be implanted before the operation. It is the size of the femoral component that the bone structure can support that will determine the cup size chosen. So the surgeon carefully re-measures the head and neck during surgery to determine which femoral components will fit and often there is more than one size that can be implanted. A good surgeon will aim for putting in the largest component due to better statistics for larger components. Once the femoral component size is selected, the cup is put in first. The BHR will have two different cups that fit one ball size. Both are the same on the inside but one cup is thicker that could be used if the standard thinner one wasn't seating right and the surgeon had to ream the acetabulum further.

chuckm

[Dannywayoflife]

Good reply there from chuckm!

[toby]

 Chuck,
'A good surgeon will aim for putting in the largest component due to better statistics for larger components.' I seem to remember this topic being discussed before and my understanding is as follows:
 Correct that larger components show lower revision rates but surely you need the best/closest match to the natural anatomy.If the surgeon goes for a larger component this will require increased and unnecessary reaming/natural bone loss of the acetabulum- which is obviously less desirable (particularly in the event of a revision being required).
Toby

[chuckm]

Cool point Toby but no hip resurfacing surgeons are trying to fit smaller components on their patients. Like I said, they measure for the ones that fit.
Chuckm

[Dannywayoflife]

I think most surgeons will look at the body type and activitys etc and also decide weather the bigger head or thicker cup will benefit the patient most. Generally I think they tend to go with the bigger head as it keeps the head neck ratio closer to the natural anatomy 

[chuckm]

Also don't forget, when you ream more for the larger cup, conversely you don't need to take as much from the femoral head to fit the larger piece. And that is the point, if a certain ball is measured to fit, why would you grind more to fit a smaller one? And there is a chance if for some reason, the surgeon went back to implant the ball after the cup and found the large ball was a bit to big or there were cysts that were unknown, he could pull the cup and install the thicker one and then go for the smaller head. Not much room for error if you started by grinding right down to the smallest ball first.
Chuckm

[toby]

Chuck and Danny, my point was that I thought they don't go for the smallest or largest but the very closest fit  to the natural anatomy.
Toby

[Dannywayoflife]

Yes I understand what you mean but within those permitters I believe they can decide on a bigger head and a thinner cup or a thicker cup and a smaller head. If you see what I mean? I would think that generally they would go for the bigger head as it would lessen the loss of head neck ratio

[chuckm]

Toby, the surgeon measures the bones to see which size femoral component will fit. There's not much deciding going on at that point because if your bones fit a 54 mm head, for example, that's what you'll get. It's the space inside the component that the bone must fit into. Generally speaking it appears the component that fits usually has a ball that is similar in size to the one it is replacing.
Chuckm

[toby]

Chuck,
Yeah the point you are making is precisely the same as mine-isn't it?
Toby

[chuckm]

Maybe. With the BHR there's eleven femoral sizes. Usually one size fits. If there happens to be two that fit, you are probably going to be fitted with the larger. And a bigger femoral component requires a larger cup size.

[chuckm]

Hi Toby, sorry to keep posting about this but I do find the subject interesting. I think we are somewhat saying the same thing but not quite. I don't think resurfacing surgeons are trying to fit and choose components to fit the natural anatomy as much as using it as a consideration for reaching their main goal which is to restore the function of the hip joint. Especially with OA patients, the natural anatomy at the time of resurfacing is one that is often very dysfunctional. A surgeon could guess at the exact anatomy well before OA ever set in but duplicating that might not necessarily restore function of the hip considering how much the bones have changed over time. With resurfacing, the neck of the femur is preserved as well as much of the head. While the head is reshaped during the surgery, the neck remains mostly the way it is, so trying to put the same size head that was there might not lead to better function if there will be impingement. So as Dannywayoflife said, a larger head might lead to better ROM in some cases by restoring the head/neck offset. But I am sure there are cases where bone structure is quite functional and the resurfacing was needed for other reasons. That might be a situation where trying to replace the parts with the same size stuff will work best and I would agree with you there. 

Chuckm

[toby]

Chuck,
Sure is a very interesting topic as is so much about Hip Resurfacing. Thanks for all your thoughtful and insightful responses in this quest to achieve a precise understanding. It's wonderful that we can all discuss and raise questions to become better informed. Because my posts were fairly brief I think there may have been some misunderstandings about what I was suggesting. I think generally we are pretty much in agreement.
Looking forward to the next discussion.
Toby