“End the tyranny of the tibia!” exclaims Mark Pagnano. “The data show that for 80% of knee replacements mechanical axis alignment works very well, and it’s been a durable, reliable way to do knee replacement.” Counters Tom Thornhill, “We’re going to look back years from now and see that the stuff we’re putting in now is completely obsolete. I’m interested to see if the satisfaction improves if we improve the kinematics.”
This week’s Orthopaedic Crossfire® debate is “Kinematic Alignment of the Knee.” For the proposition is Mark Pagnano, M.D. of Mayo Clinic in Rochester, Minnesota; against the proposition is Thomas Thornhill, M.D. from Harvard Medical School in Boston. Moderating is William Maloney III, M.D. from Stanford University in California.
Dr. Pagnano: “Is it time to end the tyranny of the tibia? Historically in total knee replacement [TKR/A] we have focused on durability, which has been appropriate because most surgery has been for marked deformity in older patients. But today we tend to deal with smaller deformities, thus as we move forward more of the attention is going to be focused on function.”
“I propose that the 3D position of the femoral component will be proven to be the prime driver of total knee function…and that we surgeons systematically get this wrong in most TKA. By ‘systematically’ I mean that our surgical technique and our instrumentation do this.”
“For three decades TKA has been dictated by two rules surrounding the tibia: ‘Cut me at 90 degrees and minimize the thickness of bone you cut.’ There are unintended consequences that impair function. First, why zero degrees varus/valgus? In the early 80s some total knee designs had greater failures when the tibia was in more than three degrees of varus. And why a minimal thickness cut? Because 1980s biomechanics work suggested increased risk of loosening with greater levels of resection.”
“What’s wrong with those rules? First, the total knee designs that highlighted these problems are not currently used on a routine basis. And many designs with coronal plane deformity have done reasonably well in mild degrees of varus. Second, early total knees had few sizes; today there are multiple sizes.”
“And the unintended consequences? If we cut at zero it changes the joint surface in most, but not all, knees; it tends to over-resect the lateral side in both extension and flexion. That then dictates specific changes to the femoral side. A minimal tibial cut causes relatively tight flexion and extension gaps. Then the surgeon cuts more distal femur, cuts more posterior femur, and these biases are built into the instruments that we use.”
“In a typical varus knee we do all these things to compensate for a minimal thickness tibia cut at 90 degrees: over-resect the femur medially in extension and flexion, change the joint line position in extension and flexion, underestimate the anteroposterior AP size of the femur, change the posterior offset (both medially and laterally), and externally rotate the femur to artificially fill a loose lateral side. Is it any surprise that some TKAs don’t function well?”
“There’s evidence that many modern knees seem to tolerate small degrees of tibial varus, and the depth of resection has no effect on durability. We can explore a new paradigm in which the femur assumes the preeminent position. And within some—for instance, no tibia more than 3 degrees of varus—make the tibia accommodate the femur.”
“Our new femoral component designs have multiple sizes and we can carefully match the tibial inserts to the femur and maximize the kinematics of current knee designs. So what would this look like? Cut the distal femur to match the anatomy. Make the thickness of the cut match the thickness of the implant, recognizing that cartilage loss tends to make you over-resect. You should match the AP size to the native femur, recognizing that posterior wear tends to make us undersize by one half or one size in modern designs. Remember that modern designs have three millimeter sizing increments. And in femoral rotation with the typical varus knee any additional rotation is abnormal. Your posterior sizing and rotation guide already positions the guide in some degree of external rotation. When you account for cartilage loss, the proper rotation relative to native anatomy is close to zero.”
“Then you balance the knee, but that’s done on the tibial side. If the native is zero, cut at zero; if it’s 2 degrees of varus, cut at 2 degrees of varus. If it’s more than 3 degrees, then just go to 3 degrees. You should resect enough tibia to make an extension gap. Then re-cut as needed.”
“So in 2013 modern knee replacement is durable enough to let us explore ways to get better function. One approach is to consider the femur as the prime driver of function.”
Dr. Thornhill: “I agree with virtually everything Mark said, but you’ve got to limit it to 3 degrees of varus. So is kinematic alignment in TKR a new concept? Not really.”
“Kinematic alignment aligns the best fitting component along the transverse axis of the femur (more valgus) and then aligns the tibial component perpendicular to the femoral axis, risking varus. You remove the osteophytes to restore ligament length and balance, and then the knee rotates around the femoral condylar axis…and you don’t have to do as much soft tissue balancing.”
“Some of the drivers of this are custom instruments, custom prostheses, and some of the newer technology knees. Many of the companies have attempted to recreate normal kinematics to improve patient satisfaction, assuming that those two things are linked.”
“When I say that it’s not completely new, I mean that in terms of the coronal alignment if you go back to the PCA Universal Instruments you see that it put the femur in 7, 9, or 11 degrees of valgus and the tibia in 3 degree of varus. One of the reasons for failure was that if you aim for 3 degrees you may get 5 degrees or even more.”
“Regarding coronal alignment and early failure due to tibia vara, Stephen Howell’s 2013 study involved 214 knees and a mean follow up of only 38 months…but there were no differences in the kinematic versus the mechanically aligned knees. But if you examine the scattergram, over 75% of the tibial components were in varus and the function was good regardless of the alignment category. In a 2010 study from Mayo Clinic they defined the mechanically aligned knee as being in the central portion; in their study, almost 25% were malaligned by their criteria. These are some of the best knee surgeons in the world and I think there are rotational errors. This axis is also influenced by the hip, the hindfoot, and the forefoot…and by dynamic forces including the abduction moment.”
“Total knees are not real knees! And varus is poorly tolerated. In a 2012 JBJS study from Germany the authors found that a deviation of 1 degree in an instrumented knee with strain gauges increased the medial load by 5%. In a 2004 study from Merrill Ritter and his group there were over 3, 000 knees. They found that varus tibial alignment of more than 3 degrees was associated with a higher incidence of failure. The femoral J curve…whether you should subscribe to the big ball/little ball or a single radius of curvature with less contact area or a gradual reduction in radius, I think remains to be seen.”
“Do we really want to reproduce ‘normal’ kinematics with our current materials, especially because the soft tissues are bad? As for the constitutional varus issue, we can refer to Johan Belleman’s study with 250 asymptomatic healthy volunteers. Of these, 32% of males and 17.2% of females were in constitutional varus. When you’re doing a total knee on someone like this I don’t think you want to put them back into the classic mechanical alignment because you’re going to overstress and have to balance more.”
“So I end with George Santayana: ‘Those who cannot remember the past are doomed to repeat it.’”
Moderator Maloney: “Mark, a rebuttal?”
Dr. Pagnano: “The data show for 80% of knee replacements that mechanical axis alignment works very well, and it’s been a durable, reliable way to do knee replacement. If we’ll primarily be dealing with patients with marked bone deformities or bone loss that remains the right way to do knee replacement. But for a subset of patients where function is the prime driver it’s not going to be changes in implant design, but changes in surgical technique that get us to better function for patients who are not quite satisfied with their knee.”
Moderator Maloney: “Tom?”
Dr. Thornhill: “We’re going to look back years from now and see that the stuff we’re putting in now is completely obsolete. I’m interested to see if the satisfaction improves if we improve the kinematics. And if we improve the kinematics and have the component drive it to normal kinematics and you get a conflict with the soft tissues then you’re going to make it harder to put in…and maybe have a higher failure rate.”
Moderator Maloney: “How do we know if we are improving the kinematics?”
Dr. Thornhill: “There are some good simulators. I can’t really tell you that those simulators are in fact reproducing, but I think it’s going to be radiostereometrics (RSA) and doing studies where you’re confirming in vitro the kind of things you see on simulators.”
Moderator Maloney: “I agree that improving kinematics is important, but I think we’re a long way from being able to accurately measure that during activities of daily living. Mark, I’m more confused than ever on alignment. Is it the same for every patient?”
Dr. Pagnano: “One of the fundamental problems is trying to come up with one rule for all patients, when realistically there’s probably an ideal alignment for every individual patient…and there is some penalty to pay as soon as you deviate from that.”
Moderator Maloney: “Tom, what’s your goal for femoral component positioning? Is it the same for every patient?”
Dr. Thornhill: “I’d never put a valgus knee in more than 5 degrees of femoral valgus by my cut. A varus knee I’d put that in 5, 6, or 7 degrees.”
Moderator Maloney: “How about the rotational alignment?”
Dr. Whiteside: “I’ve changed. I’ve used predominantly the transtrochlear axis.”
Moderator Maloney: “Using the transepicondylar axis is hard to do with small incision surgery, so are you making a more standard incision to do that?”
Dr. Thornhill: “I make an incision so that I can see everything I need to do.”
Moderator Maloney: “Mark?”
Dr. Pagnano: “We’ve seen that the size of the incision makes little difference when you consider all the other things we do in contemporary knee replacement with regard to pain management, rapid rehabilitation, and patient expectations. Surgical technique as far as the size of incision or approach really makes no difference.”
Moderator Maloney: “So for the valgus knee do you not want to go more than 5 degrees?”
Dr. Pagnano: “I don’t accept an arbitrary number, so the caveat would be that if a patient has a post-traumatic deformity that’s a totally different situation. But by and large I’m looking at the native anatomy and as long as that doesn’t result in some bizarre alignment like 15 degrees of varus or valgus I’m letting that be my starting point.”
Moderator Maloney: “You talked about tibial varus and looking at the preop X-ray. How do you estimate that when a patient’s got fairly bad wear on the medial side?”
Dr. Pagnano: “That can be difficult. Many of those patients with marked wear actually have posteromedial wear so you can sometimes define a point anteriorly to get that slope. Other times you can look at the opposite knee and get some idea.”
Moderator Maloney: “Thank you both.”
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