“This is an unnecessary evil. It’s not required for function and may be associated with implant failure, ” says David Blaha. Fares Haddad notes, “There are still indications for modularity, but we need to get the design right and simplify the technique. And it has to be generalizable.
This week’s Orthopaedic Crossfire® debate is “Femoral Neck Modularity: An Unnecessary Evil.” For the proposition is J. David Blaha, M.D. from the University of Michigan; against the proposition is Fares S. Haddad, M.B., F.R.C.S. from University College Hospital in London, United Kingdom. Moderating is Thomas S. Thornhill, M.D. from Harvard Medical School in Boston.
Dr. Blaha: “At previous meetings I’ve been quoted as saying, ‘The modular neck is the keystone to the functional restoration of the hip…period.’ I never really said that. (Picture of President Obama appears onscreen.) What I said was, ‘The modular neck is the keystone to the functional restoration of the hip—if it doesn’t turn out to have a problem with fracture and corrosion…period.’”
“All taper junctions corrode. The question becomes, ‘Is the advantage provided by the modular neck worth the corrosion?’ The advantages of the modular neck are that you can uncouple the placement of the stem and final position of the head. Also, you can fine tune the ‘sweet spot’ where all of the muscles have the most normal lever arms. They also allow implantation of a total hip stem through a smaller, less invasive approach, but that’s not something that I’ve found to be an advantage.”
“The disadvantage of the modular neck is corrosion. Titanium alloy, cobalt chromium and all biocompatible metals corrode when exposed to the oxygen in room air. This chemical reaction ‘passivates’ the material and renders it resistant to further corrosion. If the passivation layer remains intact nothing else happens.”
“The modular neck, however, is in a mechanically disadvantageous position, with the loads at the head potentially moving the male portion of the taper and knocking off the passivation layer. If this continues, corrosion releasing titanium, hydrogen gas and chloride will result in hydrogen gas collecting below the implant; and the entire effects of this mechanically accelerated crevice corrosion is that it can ruin the entire fixation of the modular neck. You can change the material, the shape, and the roughness, etc. to change the modular neck. Ions from metal-metal articulations can make it worse. Still, corrosion is inevitable.”
“We’ve had several cases where we had gas in the joint without fracture; when we aspirated the gas it turned out to be hydrogen gas. To remove a fractured modular neck you must drill through it. When you do that you find some combustion of the hydrogen gas. And if you use a slap hammer to remove a well fixed neck the combustion can be impressive in the OR.”
“Stryker has recalled their modular neck because of corrosion. We put in 390; 275 were in primary cases. In order to get a unified series we looked at just those with the PROFEMUR-Z and with the large diameter metal-metal head. This is the same metal-metal that we used for the surface replacement; 21.5% of our cases required revision. With the exception of limb length discrepancy, everything else such as modular neck fracture and gas in the joint can be an effect of corrosion.”
“If we compare our large femoral head stem replacements to our surface replacements, the latter only had a 7% failure rate. Amstutz had a low failure rate (94.4% survival at four years), as did McMinn (99%, 97%, 96% survival at 5, 10, 13 years, respectively). The addition of the modular neck seems to increase the failure rate.
“Mike Berend and Adolph Lombardi did 810 stem replacements with a fixed neck and a 38mm metal-metal articulation; they had a failure rate of 7.8%. And because of the corrosion they abandoned this.”
“We want to know, ‘Could the corrosion products from the modular neck be increasing the problems associated with large diameter metal-metal?’ Is the modular neck necessary? The majority of my necks were varus; a few were varus retroverted, which I could have done by rotating the stem slightly in the canal.”
Mr. Haddad: “First of all, I know very little about flip-flopping. Also, we never allow explosions in our operating theater. In order to give patients functional hips, we have to tailor implants to their bone stock and anatomy. Here, modularity has advantages in terms of addressing version, length, and offset. If we fail to balance the hip, we get impingement, loss of soft tissue tension, and instability. We also get increased wear, and we get those patients who limp or are unhappy with their prosthesis.
“The early solution that we used for these problems was customized stems…but having one bullet when you go to battle seems suboptimal. Modularity gives us more intraoperative options, allows us to hit the sweet spot by uncoupling the head from the stem, and allows us to correct deformity. Also, we have better stability, less impingement, more inventory, and if you have to do a revision it is easier.
“My particular argument is that in revision surgery this is something you really want because modern revisions have aimed much higher by allowing us constructs that adjust for leg length and stability. The problem here has occurred due to some uncontrolled introductions of suboptimal implants in the primary setting. We’ve seen failures in the Australian registry, but bear in mind that a lot of these failures are due to loosening and instability…not necessarily due to corrosion. Nevertheless, they are failures. But if you drill down and look at the implants, you see that the ones recalled account for a big proportion of the failures. There are some long term studies with modular necks that show good results for instability and survivorship.”
“We need to look at how we innovate, but also be careful not to get rid of things just because we don’t understand them. We have to take a step back and understand the taper. The taper relies on frictional interlock, and that is dependent on a number of factors. These include taper size, taper length/engagement, the material, surface finish, neck length and offset.”
“In our quest for thinner femoral necks, greater offsets, and bigger femoral heads, we have created an environment that can generate fretting corrosion at modular junctions and leads to premature implant failure. If we take a step back and see that tapers are complex systems, we can begin to see how we can use them now and in the future. We have the data…the science is there, and we know what the important factors are. The way you clean the area before impacting the taper, how you position it, the impaction force, and the environment are under your control.”
“There is a risk-benefit equation here. We’ve been looking at this in the lab, specifically examining the electrochemical nature of this. The biggest key is the bending moment. The greater the bending moment the higher the risk of corrosion. The more the torque, the higher the risk of corrosion. That’s where metal-metal large heads possibly come in. The higher the offset, the bigger the head size, the greater the torque, the greater the current, the greater the corrosion. We can establish patterns for that…depending on the scale you get and on the materials that you use.”
“If you use ceramic or a different surface you will see that the scale is much less. There is potential for alternative surfaces to allow us to continue to use these tapers. Jacob Cartner and colleagues did a retrieval study comparing cobalt chrome and oxidized zirconium. Their conclusion was that offset was important, but also that head material was important. In 2013 we’re beginning to understand why corrosion happens at the taper. It’s partly due to large femoral heads, high offsets…partly surface area and surface finish…and to a great extent it’s surgical technique.”
“I think we have to be careful in how we innovate, but we should also take care not to overreact. We’ve created a problem; we have to understand it and not lose its advantages when we move forward with the next generation of hip implants.”
Moderator Thornhill: “Why can’t we model these things better so that this doesn’t occur?”
Dr. Blaha: “I don’t know. The data from the Rizzoli Institute convinced me that the production of wear debris was going to be minimal, as was the corrosion. The difference here is that when I assembled the modular necks with a slight varus and slight retroverted neck, when I impacted that firmly and according to instructions I probably didn’t get the neck exactly down the pocket. And that left room for corrosion. So we didn’t model the way we put it in the OR; instead we modeled perfect implantation of the modular neck.”
Mr. Haddad: “We have a great operation, but we’re in danger of ruining it by not evaluating and innovating carefully.”
Moderator Thornhill: “Should we have something that controls impaction force?”
Mr. Haddad: “You must make sure you’re completely clean at the taper junction and that you’re hitting it hard in the right direction. If you’re going to do sequential taps the last one has to be the hardest.”
Dr. Blaha: “It’s important that the impaction be directly along the length of the taper.”
Dr. Thornhill: “David, can you—by serum levels of cobalt and chromium and the differential between the two—have some determination of whether this is coming from the articulation or from one of the modular junctions?”
Dr. Blaha: “I don’t know any way to do that. What I’m using is cobalt and chromium to look at articulation…and the cobalt chromium ball on the titanium neck. But I’m looking at titanium levels as a surrogate for corrosion at the pocket junction.”
Dr. Thornhill: “Does it matter whether it’s a titanium stem with a cobalt chrome neck or a cobalt chrome stem with a cobalt chrome neck?”
Dr. Blaha: “Yes, but I don’t know how it matters.”
Mr. Haddad: “There is a difference between what you see with a mechanical effect load edge bearing at a surface like metal-metal where you get both cobalt and chrome elevated. When you’re looking at this sort of phenomenon at a taper, the chromium precipitates so you get a differentially high cobalt and a low chromium. And you should not be reassured by the low chromium. We don’t have any data to be able to look at that differential and say, ‘This is good. This is bad.’’
Moderator Thornhill: “David, at the head-neck junction are we better off using ceramic?
Dr. Blaha: “There’s no question that at that junction ceramic has a lower amount of corrosion. There’s no way to test that with a modular neck. If we found some way to use ceramic there it might be better…but we have to test that in the lab and in limited clinical situations…and see if it fails in the long term.”
Mr. Haddad: “That’s what the electrochemical tests will tell you. If you put a ceramic head—same torque, same bending force—you will get less of a current.”
Moderator Thornhill: “Thank you gentlemen.”
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