Hip Implant Corner

The gold standard implant for hip replacement surgery today is a ceramic ball, titanium socket, and polyethylene liner.

Product Review: Stryker Accolade II Total Hip Construct

There are horses for courses. In 1973 Secretariat won the Kentucky Derby, running Churchill Down’s in 1:59 2/5. It was a race for the ages. You may know that ecretariat set a track record that, to this day, has never been broken. He went on to set course records at the Belmont Stakes and Preakness which still stand as well. Secretariat is nearly universally considered the greatest racehorses of all time. But have you ever heard of Sham? Sham came in second at the Derby in ‘73; not quite as fast as Secretariat, but Sham’s time was also a record when he ran it. What almost no one remembers is that two weeks before the Derby Sham actually beat Secretariat in the Woods Memorial. Secretariat may be the GOAT; but in 1973 Sham was the racehorse for the Woods Memorial course.

Kind of the same thing is true for hip replacement implants. In 2003 Dr. Joel Matta started teaching orthopedic surgeons courses on anterior hip replacement. Those surgeons intuitively saw the anterior approach, which cuts no muscles, as a sure fire means of doing a stronger hip replacement. They were right: patients showed lower rates of dislocation; and less drastic surgery produced a lower rate of infection. Boom! But it wasn’t all flash bulbs and roses in the winner’s circle. It turned out those first anterior hip replacements also had a higher rate of leg bone fracture after the surgery. And there was another problem: the implants were sinking in more than usual, called subsidence. To make matters worse, some of the implants just didn’t fit as well as the surgeons expected form their experience. There are horses for courses. The implants that ran like champs for the traditional surgery were not right for the new anterior approach.

The engineers at the big four manufacturers (Zimmer, Stryker, Depuy Synthes, and Smith and Nephew) kicked into action. They calculated. They measured. They designed. And pretty soon they had a new horse for the anterior approach course. Stryker’s entry into this race was the Accolade stem (2000) which was updated with the addition of the Accolade II in 2012. By now nearly 1,000,000 Accolade II femoral stems have been implanted around the world. Sure enough, they’ve done it again. The fracture rate fell more than four times according to a study from surgeons at New England Baptist; subsidence all but disappeared, and the fit was snugguly snug. How did Stryker do it?

The Accolade II stem has three parts: a top, middle, and bottom. Each has smart features that make it a champion.

The top of the stem, which is wonkily called the trunnion, is designed with a taper to create a cold weld to the femoral head. Humma na humma na. Welding. A cold weld is nearly as strong as a traditional weld, but without the inconvenience of a super-hot flame which causes fires in operating rooms which are full of pure, combustible oxygen. The metals have to be the same type. Sprinkle in exactly the right taper, and pressure alone is enough to trick two metal surfaces into bonding as if they’re one. Sure, cold welds are, well, really cool; but the best bond is still no bond.

Since the goal is to replace the ball of the leg bone, why not just produce one combined part that is a ball and stem?

Because not everyone is the same height. Stryker makes 2 stems. The length of the neck is proportional to the overall length of the stem, but there are two different angles, and 17 sizes of sockets. That means there are zillions of possible combinations of these three components for every hip that needs to be replaced. Crazy complexity? Not entirely. It’s helpful if your surgeon can personalize the implant construct to your size, leg length, bone quality, and hip structure. And it matters. The size of the construct will determine not only whether your legs are the same length after the surgery, but whether or not your hip still hurts.

The proof of the pudding is in the eating. The good news is, all this wonkery is totally working. In the real world the hip replacement components almost never fail or fall apart. The Aussies had the foresight to start keeping track of everyone who had a total joint replacement way back in 1999. So, by 2019 they had the scoop on 643,567 hip replacements, some going back the full 20 years. They analyze this data carefully every year to see how changes in implants and surgical techniques pan out. In the Australian registry database, the Stryker components have a 99.2% survival rate after three and a half years. The Trident shell, which is used with the Accolade stem, actually has the lowest failure rate of any implant in the Australian as well as the British registries going back 20 years. The best is, well, darn good. Geez. No wonder even his mother-in-law actually likes him.

Like the crowd after a Post Malone concert, the middle of the Accolade II stem looks rough. That’s because its sprayed with a titanium phosphate compound. One of the most common ways for hip replacement to fail long term is to loosen. This rough area allows bone to grow onto the implant, securing it for the long term. Ahem. 99.2%.

The shape of the middle part of the stem is where they got really smart. Stryker dusted off their collection of 16,500 high-resolution CT scans of the leg bone and modeled the perfect shape for the perfect fit. What they found was that the optimal shape changed as the length of the stem changed. That allowed them to create shafts that fit so well they don’t sink in after placement. Well, they do sink in, because everything does; but subsidence for the Accolade II was shown to be less than 0.1 mm after 2 years. That’s the thickness of something really thin. Like a lick on a lollipop. Sweet.

The bottom of the stem forms a tapered wedge. The tapered wedge makes it easier to insert on the angle of the anterior approach. Bada boom, bada bing. With this implant your surgeon can make a smaller incision and take no muscle. Minimally invasive and muscle sparing. Lower dislocation rate. Femur fracture avoided. Tight fit. Nearly perfect. Mischief managed.
Behind every great stem there’s an even greater femoral head. In 2018 the wizards at Stryker came out with the Trident II.


You recognize the shell, which fits into the socket, is created with additive manufactured (3D printing) titanium to allow bone to grow into it. That means no icky cement, which is another source of failure for hip implants in the long term. The holes are for a screw. And their precise location was determined using the CT imaging data. It takes six months for your bone to grow in and secure the cup; until then the perfect screw prevents the socket from coming loose. Nice.

The coolest thing about the Trident 2 is how they make it. Like a New York Housewife, its impossibly thin. Stryker tamed the natural enemies of strength and thinness by using additive manufacturing; what most of us would call 3D printing. Thin wins. We know from registry data that the risk of dislocation goes way down if your surgeon can replace the ball in your socket with a size 36 head instead of a 32. So, you want to be a 36, not a 32. The thinner the shell which lines your socket, the larger the head that can be placed to accommodate it. Et voila. By 3D printing the shell, surgeons can fit a 36 head with its lower dislocation rate into a lot of us who would previously have been 32s.

It would be a really great concert if Iron Maiden would open for Metallica; but we learned the hard way that —when it comes to joint implants— metal on metal sucks. Metal on metal implants produce debris that gums up your implant so badly it may need to be replaced. The debris can even get into your blood an make you sick. So, like all modern implants, Stryker’s metal cup is lined by a super strong plastic liner to prevent debris from forming.

But this is Stryker, so you know the liner is going to be special. Stryker manufacturers their polyethylene plastic liner using a special process in which it is heated just below its melting point to prevent the plastic from weakening. Oh, my. So clever. The integrity of the plastic liner is crucial to keeping down the amount of debris that is formed as the ball rubs on the socket over the years. The wizards at Gryffindor, excuse me, Stryker, estimate this construct makes less than 2 red blood cells worth of debris a year. Adult humans have 20 – 30 trillion red blood cells. So, two seems like, well, not very many.

Of course, the Stryker system comes with an available ceramic ball. Studies of the afterlife of hip replacement constructs suggest ceramic on plastic produces the least wear debris. Like Mormons on the Simpsons, a ceramic ball with a polyethylene plastic liner appears to be the right answer.

So, how do you get one of these killer constructs in your hip replacement? You’d think you could just ask your surgeon for one. “Hey, doc, I dig that Stryker implant. Can we use that one?” No. That would be like going to your local Ford dealer to buy an F150 and saying, “I love the truck. Can I get it with a Chevy engine?” No. You can’t. Surgeons stick to one implant because every time they use a new one, there’s a learning curve. You don’t want to have pain every day for the rest of your life because you were on somebody’s learning curve. To get the implant you want you have to pick a surgeon who uses that implant. Luckily, you can do that. Stryker’s website for patients https://patients.stryker.com/ has a cool jump table where you can find a surgeon who uses the Accolade II with the Trident II and ceramic ball near you.

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