This week’s Orthopaedic Crossfire® debate is “Femoral Neck Modularity: A Bridge Too Far.” For the proposition is Michael J. Dunbar, M.D., F.R.C.S.(C), Ph.D. from Dalhousie University in Halifax, Nova Scotia; against the proposition is Stephen B. Murphy, M.D. from Tufts University in Boston, Massachusetts. Moderating is Thomas S. Thornhill, M.D. from Harvard Medical School.
Dr. Dunbar: “Stephen is going to tell you that you need modularity because it reduces impingement, reduces dislocation, and you can better balance the hip. Impingement can be an issue with ceramics and this is largely why modular necks were developed out of Europe—to prevent impingement on the rim of the acetabular component and subsequent fracture. My answer to that is: just don’t use ceramics. Others say we need impingement to balance mal-positioning related to the acetabular component. The obvious answer to that is: get the component positioning correct.”
“The other issue that arises is dislocation or instability. This can be improved, but if you use the right approach dislocation isn’t the problem it used to be. In fact, it’s paradoxical. Looking at reasons for revision on the Australian registry we see that dislocation is the second highest reason.”
“What about fine tuning the hip? Clearly there’s some ability to change your center of rotation, particularly when you’ve seated the femoral component and it’s fixed. But if you use a cemented component you have all the same options. You can change the leg length by where you place it within the cement mantle, you can change the offset by where you put it within the cement mantle, and you can do the same thing with the version.”
“Major disadvantages: There is additional mechanical interface, which increases the potential for new problems such as fretting and corrosion, as well as fractures. The midterm results with respect to these procedures are just not as good, and there is increased cost.”
“As for fretting, there is a paper out of Vancouver that won the John Charnley Award that looks at metal-metal resurfacing versus total hip with another metal-metal articulation. They found a much higher incidence of urine and serum cobalt chrome levels in the total hip, not the resurfacing. The issue was the mechanical interface of the new coupling. So by adding a new interface the metal ions were increased.”
“The 800 pound gorilla in the room, however, is fracture. There are numerous reports of fractures. One 2010 report is from Europe on a series of 5, 000 with a fracture rate of 2.4%. Who wants that?”
“We’ve produced a case report at our center using the ProFemur. We have a larger series of 452 and at about six years out we have over 20 fractures. They’ve occurred in the long necks at about two years out. The real problem is what to do with the person’s other femur.”
“When you take all of these things together, it’s not panning out. Modular necks have a higher revision rate…there’s not an advantage to survivorship. Coming down to more subtle points, there are the biomechanics. What is really happening when we change the version of our components, particularly through the modular neck? I think it introduces a new biomechanical stress strain relationship that may be disadvantageous.”
“A paper from Oxford led by Gill looked at radiostereometric data and what happens when you retrovert a femur. Think of it this way: the real issue isn’t standing. It’s when you’re getting out of a chair or climbing stairs…we’re talking about the forces that drive this component into rotation. So if you put a component into a regular anteverted position, you’re offset is small in terms of your deforming forces. But just by tuning that femur back through the neck you’re actually increasing the biomechanical lever arm, and you’re putting substantially more rotational forces on the femoral component.”
“There are increased costs, which are coming at a time when markets are tanking. We can’t afford these premium products without improved outcomes. Would the money have been better spent on surgical technique/training? Yes!”
“Another moral hazard is metal ion concerns at the taper. Do I live with the risk of toxicity or do I undergo a complicated revision? This is happening with data that we don’t know a lot about; few people know what is going on with MARS-MRIs and ion levels. So in conclusion the use of modular necks is not justified in routine primary hip arthroplasty.”
Dr. Murphy: “I think we can agree that control at the neck shaft junction is the primary biomechanical control that we have, both in joint preserving surgery and in total hip arthroplasty.”
“There are certainly many advantages to modularity, already outlined by Michael. Variation in native femoral anatomy is quite wide: 60 degrees or more. I think that fixed necks have some difficulty in controlling and correcting these variables properly. You can use a cemented component, and cement more or less in anteversion; a canal filling component has great difficulty doing that. If you fail to correct the anteversion properly that leads to impingement, dislocation, polyethylene fracture, and potentially, revision.”
“If you have significant rotational problems you can burrow away some bone and put a fixed neck in a different direction…or use a component that has free control of anteversion. But these all lack the potential revision opportunities of being able to remove and replace a modular neck.”
“Our personal experience of titanium neck on titanium stem is with 792 patients and a minimum of 3 – 10 years follow up. There were two neck fractures in this group (0.25%), however there were four revisions that were much easier because of the presence of a modular neck. We saw no adverse local tissue reaction, and had a 0.25% dislocation rate. There is a 19-fold difference in the fracture rate we had compared to the fracture rate that Michael just quoted—using the exact same neck. If you look at those stems they had variable wall thickness, and a very thin wall thickness in one area. The stems we used had a more uniform wall thickness, which may affect the fretting. And assembly force is the greatest single force affecting fretting, corrosion, and fracture resistance.”
“Looking at our cobalt chrome neck on titanium stem experience, we had 559 patients with no neck fractures, no dislocations, one revision that was made easier because of the presence of a modular neck, and one patient with a metal allergy and elevated cobalt chrome level (and fluid collection around his greater trochanter).”
“I would like to discuss the recalled Stryker Rejuvenate modular neck stems. That is totally different than the other stems: it’s a cobalt chrome neck on a beta titanium alloy, which is made of TMZF. There were a number of these stems used in our institution with pseudotumors, and I think that’s a corrosion/fretting issue which may be metallurgical in its origin.”
“Control of that junction is the right way to go, but it’s a question of designing things so that they are successful.”
Moderator Thornhill: “Michael, Steve says the reason you have trouble when it’s breaking is because you’re using the wrong implant and you’re too weak to assemble it.”
Dr. Dunbar: “That’s my point. We’re not the best surgeons in the world, but we’re not the worst…and these things pop up. Total hip is the golden goose of arthroplasty; all of a sudden we bring in a new variable and our golden goose starts laying lead.”
Moderator Thornhill: “Steve, if you need to change the modular neck at five years do you have concerns about the female side on the stem in terms of taking a new neck?”
Dr. Murphy: “This is not risk free. Let’s say you had an unstable patient with a femoral anteversion problem…and you had a choice of removing that entire component that was well fixed and potentially extensively porously coated, versus popping a neck off, changing the angle and putting it back on. Well, the risk tradeoff is strongly in favor of putting another neck on, compared to an extended trochanteric osteotomy.”
Moderator Thornhill: “When I run into trouble it’s either developmental dysplasia of the hip where I have too much anteversion, or a femoroacetabular impingement with femoral retroversion. I tend to use a modular implant. Mike, can I be less worried about that articulation of that modularity versus the neck?”
Dr. Dunbar: “I assume you’re referring to a prosthesis that’s been around a long time. I agree with Stephen that the future will be balancing the hip. But why did we release all these things in such large numbers and then find out after that depending on how you impact this taper may make a difference?”
Moderator Thornhill: “Steve, how much can you change in terms of anteversion, retroversion, and offset with your modular necks? And are the extremes of that putting more risk on that couple?”
Dr. Murphy: “You have a 30 degree range in anteversion, retroversion, and a 16 degree range in varus/valgus. Regarding femoral anteversion, I think the concept of combined anteversion is very important, but it’s greatly oversimplified and overused. If you take a femur with 60 degrees of anteversion and you decrease it to 20 degrees then the patient will dislocate posteriorly. If you take a femur with 0 degrees of anteversion and increase it to 25 degrees the risk would be anterior dislocation. I think the change in anteversion from what the patient had to what you set it to is a much more important variable than absolute number of anteversion. I think that’s one of the reasons why, in the Australian registry, they had a higher dislocation rate…because they had the ability to overcorrect.”
Moderator Thornhill: “Is it fair to say that the relationship between the final construct of the femur and the acetabular isn’t critical for stability?”
Dr. Murphy: “It is critical. I think that using rigid numbers, and not keeping track of what it was compared to what it is, is the problem.”
Moderator Thornhill: “Mike, you used the term ‘moral hazard.’ To me that means that you make a decision where the results of that don’t impact you. You were discussing fracture and ions. I’d think that wouldn’t be a moral hazard because it would impact the patient and surgeon.”
Dr. Dunbar: “This is the point. We are falling into traps.”
Moderator Thornhill: “Thank you, gentlemen.”
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