The Hip Prosthesis
I Use for Total Hip
Replacements

0:00 · Start

Hi,, I'm Dr Chien-Wen Liew. In today's episode, we're going to discuss a little bit about hip prosthesis. I'm a hip and knee replacement surgeon from Adelaide, South Australia. Today, I'd like to go into something that a few of my patients often ask me. And certainly, it is something of great interest to us as orthopaedic surgeons and also to our patient cohort. Today, we're going to be going over some of the options of what hip replacements are made of,

as well as some of the different funny aspects about hip replacements, such as the weight of a hip replacement, don't worry, when we do a hip replacement, you really don't add a lot of weight on. However, we are going to go into some exact figures today.

0:49 · Direct Anterior Approach Hip Replacements

A hip replacement that I perform is performed via the direct anterior approach. I use this approach which comes in at the front of the hip. Which means that we do not cut or detach any muscles, tendons or ligaments to access the hip joint. When we look at the hip prosthesis that we place in, these are the standard hip prosthesis that have been slightly modified to allow perfect access into the hip via this approach.

Along with a lot of our specialised instruments, we actually have a very refined process now, which is both elegant and reproducible. I'd like to go into a little bit about what I use first. I like to use an uncemented implant for most of my patients with this particular prosthesis.

1:32 · The prosthesis I normally use

You'll see it has a composition made of metal and hydroxyapatite. The metal that this is composed of is niobium vanadium titanium alloy. The coating that you can see here is composed of hydroxyapatite. Hydroxyapatite is a type of artificial bone where bone will actually grow into the hydroxyapatite over the first six weeks or so after the operation has been completed. The actual stem and the liner of the hip are both composed of almost exactly the same material, and they're both composed of this hydroxyapatite.

2:23 · Ceramic on Ceramic articulation

On the outer surface, where the bone grows in, the articulating components of the hip replacement are made of ceramic on ceramic, this pink, and yes, it is pink when we place these in place as well is a ceramic on ceramic hip articulation. This means that there are no metal or plastic parts touching each other during the surgery, the ceramic is placed onto the thermal component during the surgery and impacted in place, the liner is placed within the socket and is also impacted in place.

These sizes and positions have often been pre-designated. From your three-dimensional scan before the surgery, as many of my patients will know, a three-dimensional scan is done so that I can perform the operation on my computer before it comes time for the actual procedure itself. This allows me a great deal of comfort in knowing exactly how the prosthesis will be placed and any small nuances with a patient's own anatomy that we may need to take into consideration for the surgery. Once the hip prosthesis is placed in there, you can see that there is metal, ceramic, ceramic and metal.

3:30 · Will my hip go off at the airport?

A commonly asked question at this stage is, Will my hip beep at the airport? The answer is certainly yes. It is composed of enough metal that most metal detectors will be able to pick up a hip replacement inside your body. But don't worry, this is something that very, very adept to handle. But don't worry. With over one hundred thousand hip and knee replacement performed in Australia every single year, airports are very trained to handle these situations without any fuss to yourself. Another type of hip replacement that may be placed in is a cemented implant.

4:04 · When I use cemented implants

As you can see from this implant, there is no coating, it is polished, it is tapered, and it is extremely smooth. This is designed to be cemented into a femoral shaft. The situations where we may use a cemented stem include those patients with severe osteoporosis or those with significant anatomical differences within the proximal femur. That may make the positioning of a cementless stem a little bit more difficult.

It is not common for me to place a cemented stem in place unless during a surgery. We find that the bone quality is insufficient to handle the biological fixation of a cementless implant. Along with that, we have different options for cups. cups can also be cemented in place. The type of articulation of a hip replacement can also be different. What I've shown you so far is a conventional one, which is generally a 32 or 36 millimeter ball placed on the end of the femoral component. That is coupled with a perfectly matching liner made of ceramic or plastic.

5:19 · What is a dual mobility implant

There is another type of implant called a dual mobility implant, if we remove this now and place on this specialised type of implant. You can see that there is a ball within a ball that circulates within the actual capture mechanism of this larger plastic ball. The dual mobility implant is something that is used in high dislocation risk scenarios or revisions. We find that the dual mobility implant has a higher level of stability than a standard implant, with some potential downsides in the future and some potential mechanical differences.

That make us preference a standard hip replacement for flexibility and movement more so than the dual mobility. However, for those patients with neurological disorders, any balance issues, or any connective tissues, such as Ehlers-Danlos syndrome. Or anything that will make you more likely to dislocate than a dual mobility implant can be a good choice. The dual mobility implant is called such because it has one level of mobility at the small bowl, which locks into the larger ball, and then that ball actually sits within another shell the shell, or the acetabulum, is perfectly smooth on the inside.

We don't have one of those to show you today, but it looks similar to this when it's inside the body. So, the dual mobility implant will move with the larger ball within the socket, and then the smaller ball within the larger bowl. As many of my patients know, we use three-dimensional planning before the surgery.

6:55 · Patient Specific Technology - Jigs

Today, I've also got to show you some of the models that we use during the operation, this is the ephemeral and the acetabular model. And that gives us the confidence and the tools during surgery to ensure that we place the hip replacement and prepare the bone extremely accurately. We have reference guides and jigs that attach to the hip to allow us to perform the surgery with a very high degree of accuracy.

Ensuring that we are very accurate with our leg length, offset, size and position, this allows us to essentially pre-navigate the hip replacement, knowing exactly what we're going to achieve before the surgery takes place. As a surgeon, that gives me a huge amount of confidence in knowing all of the little issues or potential differences that a patient may have. Now, it wants us a little bit more of something interesting, more than useful.

7:47 · How much does a hip replacement weigh?

I brought along my scale today to weigh a few of the components to start with, we have the femoral stem now this is a size five. These implants actually go from size double zero all the way up to a size nine, and within that range, this is approximately the average size, maybe a touch bigger. Our normal is probably a size three or four, and this particular implant weighs 147 grams.

So the smaller sizes will probably go all the way down to 110 grams or so, and the larger sizes may be up to about 180 grams. The socket itself I have a indicative socket just here. This is a size 56 and this again, is slightly larger than what I would normally place in most patients. However, as you can imagine, there is a big individual variation. The size 56 is designated due to the diameter 56 millimeters for this socket, and this socket weighs 84 grams.

So combined, we have approximately 230 grams worth of components. Without the ceramic, if we now add the ceramic liner and the ceramic ball. The ceramic ball weighs 85 grams. The ceramic liner weighs 72 grams. So as you can see, approximately adding to your total weight, maybe around 400 grams. With the actual prosthesis itself, a cemented implant, or the cemented stem, in comparison, also a size five, weighs in at 264 grams, a much heavier implant.

And in addition to that, we would add cement to the construct, which adds a little bit further weight. Having said that, the weight of the implant is certainly not very important. The weight of the implant plays no bearing as to any of the post-operative period or any of the recovery. It is just really for your interest, as I get a lot of people asking me, how much does my prosthesis weigh?

I hope you found this little video a little bit useful but also interesting. Please join us for the next video. Please like and subscribe this YouTube channel and I look forward to seeing you for the next one. Thank you.

Transcript edited lightly for readability.