Yes, says Mark Pagnano, there are really very few issues with regards to malalignment in total knee arthroplasty (TKA) because, frankly, it is irrelevant to long term outcomes. Counters Dr. Michael Berend: “We do know that there are many factors regarding long term survivability. I say that alignment is the most important.”
This week’s Orthopaedic Crossfire® debate is “The Consequences of Malalignment in TKA: Few If Any.” For the proposition was Mark W. Pagnano, M.D. from Mayo Clinic in Minnesota. Against the proposition was Michael E. Berend, M.D. of the Center for Hip & Knee Surgery in Indiana; moderating was Steven J. MacDonald, M.D., F.R.C.S.(C) of the University of Western Ontario.
Dr. Pagnano: “The title is preposterous. The prefix ‘mal’ comes from Latin and it means ‘abnormal or defective.’ By definition alone, no one wants a malaligned knee. But can we routinely tell what malalignment really looks like?”
“Mr. Smith: 64 at the time of his total knee [arthroplasty] (TKA); his tibia was left in a bit of varus. His overall limb alignment was in varus; but his one year knee score was 95. Mrs. Jones: 64 at the time of her TKA; a 90 degree tibial cut, a perfect neutral mechanical axis, and a one year knee score of 95. Mr. Smith died 22 years into his total knee—with a perfect knee score. Mrs. Jones was revised last week at five years for loosening of her tibia.”
“In 2009, if we’re intellectually honest, we’ll admit that our knowledge about ideal limb alignment isn’t better off than it was in the 1970s. And if you’re intellectually curious, that is not acceptable. If we continue to accept broad, average targets, we can expect broad, average results.”
“What is the ideal limb alignment after TKA? Is there really one simple target for all patients? Is that a broad target like a mechanical axis within three degrees? Or is it a narrow target that is specific for each patient…that has a penalty if you deviate even one degree?”
“Some of the scientific support for always aiming at a neutral mechanical axis is weak. Some of the reports routinely quoted involve a rudimentary knee design with an all-polyethylene tibial component…and that’s fairly routine for many of these studies.”
“We looked at the effect of the mechanical axis alignment on the 15 year survival of modern cemented total knee designs. We looked at 399 knees…our results showed that survival was not better at 15 years with a neutral mechanical axis when we looked at revision for any reason, for aseptic loosening, or wear. We were unable to confirm the premise that underlies the use of the computer in TKA; we couldn’t show improved durability with a neutral mechanical axis…suggesting that factors other than alignment are more important.”
“The ideal alignment after total knees is likely specific for any given patient…and it’s based on individual differences in gait dynamics…involving a complex interplay between limb alignment, component rotation, sizing, and ligament balancing. Also, implant specific things, particularly with ‘flat-on-flat’ designs.”
“What we can say in 2009 is that the obsessive pursuit of broad target values like a neutral mechanical axis +/- 3 degrees is unlikely to pay off with better survival. Our understanding of ideal limb alignment after TKA remains rudimentary. While alignment is one factor in determining durability, factors other than axial alignment appear to be more important to survival.”
“You still need to aim for something, and the traditional target of a neutral mechanical axis is a reasonable guide until we know more. Ultimately, patient-specific targets that account for individual variations in gait are required to improve function and durability.”
Dr. Berend: “I believe that alignment is the central variable in the failure of TKA. A number of other factors work in conjunction with alignment to predict failure in TKA, like surgical technique, patient factors, and how well we do as surgeons…all critical factors for long term survivability.”
“From our data…When you combine varus malalignment of the tibial component in patients with a BMI [body mass index] greater than 33, you can see a precipitous drop in survivorship at mid- and long-term intervals.”
“We looked at the most common mechanisms of failure in our cohort and varus malalignment leading to tibial collapse was the single most common failure mechanism we observed. Alignment based failure represented only 0.6% of all total knee replacements implanted. So while alignment is the most common predictor of failure, it continues to be quite rare in our series and in Dr. Pagnano’s series.”
“In our data the failure rate in varus malalignment below 2.5 degrees is 1.8%. The failure rate of knees aligned between a single standard deviation of the mean is 4.5 degrees in our series. So between 2.4 and 7.2 degrees of tibial femoral valgus, the failure rate is exactly the same at roughly 0.5%. The interesting finding here is that if we shot for excessive valgus, greater than 7.2 degrees, we also had a higher failure rate at 1.5%. So there’s no question in a cohort of 6, 000 total knee replacements that varus malalignment has increased failure rates somewhere between 2 and 4%.”
“If you look at the failure mechanism, failure itself, if it’s infection or instability, may or may not be related to alignment. In our series varus malalignment less than 2.5 degrees had significantly increased failure through a certain mechanism of medial tibial collapse. The converse was true for valgus malalignment, with instability being more common; if you look in aggregate, varus knees failed almost 7x more commonly with medial collapse. And valgus knees failed 3.7x more frequently through instability. So malalignment predicts a certain mode of failure. If you look at this in survivorship, those between 2.4 and 7.2 had improved survivorship compared to the valgus and varus outliers.”
“In an example of one of our knees…where it is one degree of varus, six degrees on the femur, seven degrees varus malalignment on the tibia… and looking at the long term response of varus malalignment in the medial tibia… we saw an increased uptake—we believe—related to the abnormal loading conditions in the medial tibia. We looked at neutral aligned knees compared to varus malaligned knees and when we quantified the strain in the proximal tibia with a photoelastic coating mechanism, we saw a large hot spot in the posterior medial aspect of the tibia, which we believe is related to increased bone turnover, increased strain, which leads to implant loosening. Neutral alignment in the laboratory seemed to have the protective effect of no significant overload in the medial tibial bone.”
“When we performed a finite element analysis we found that both metal-backed and all-polyethylene implants provided relatively balanced predicted strains in the medial and lateral tibia. Then with varus condylar loading we did see significantly increased strain in the medial tibia. So, we concluded, alignment overloads the medial tibia and predicts a certain type of failure mechanism. As you examine the literature it is critical to look at the reason for failure more than alignment itself. So alignment remains the single most important variable in long term survivorship.”
Moderator MacDonald: “We need a target when we go in the OR. What is your target and when do you look at a film and say, ‘I missed my target.’”
Dr. Pagnano: “Use the same target values that you’ve looked at for the past 35 years because we don’t have better evidence on what to select for any individual patient. We’re really arguing over the difference between 0.5% failure and 1.5% failure, according to Mike’s data. What I’m trying to figure out is an explanation for those 0.5% as well because it’s only when we understand why the 0.5% fail that we’ll also be able to address the 1.5%.”
Moderator MacDonald: “Mike, your target?”
Dr. Berend: “4.8 degrees. But other factors are also important. Is a modest tibial resection important? We used to cut below any defect—and we’ve now found with 16mm polys and higher, a 4% failure rate…so we think a modest tibial resection, larger tibial surface area is important. We think metal backing of the implant plays a significant role; we think the size of the femoral component compared to the tibial component also plays a role, as well as the patient’s BMI in correlation with the size of their tibia. So I think the target’s bigger—2 to 7—however in certain patients it may be related to their gait, to their preoperative deformity, and it’s most likely related to the bone that the tibia rests on.”
Dr. Pagnano: “I’m thinking over time that the target is very narrow and is specific for any individual patient, and as soon as you deviate even one degree from that ideal you will incur a penalty. And that’s why some of your knees that are aligned between 2 and 7 do fail because of aseptic loosening. It’s because 2 to 7 doesn’t describe the perfect alignment for that subset of patients. I think some of those 0.5% failures have something different about their gait mechanics that changes the pattern of alignment so that 2-7 degrees of valgus is no longer the appropriate target.”
Moderator MacDonald: “We focus on the alignment visible on a radiograph…how do you judge interoperatively your femoral component rotation?”
Dr. Berend: “We look at the flexion gap and make it a rectangle in the knees that have an intact cruciate…so for nonfixed deformity we’ll do a balanced gap technique. We’ll also draw a Whiteside’s line, draw the upper condylar axis, and most importantly we’ll go with a rectangular flexion gap. So rotation in the femur is just as important for all kinds of problems rather than coronal alignment of the tibia or overall alignment of the leg.”
Dr. Pagnano: “I use the trans-epicondylar axis…I want to be within 1-2 degrees of that for simplicity during surgery. Oftentimes I will start in a varus knee with 3 degrees of external rotation relative to the posterior condyle; in a valgus knee I start with 5 degrees, but then compare it to the trans-epicondylar axis and to the flexion gap.”
Moderator MacDonald: “Obese patients…how do you set your tibial jig?”
Dr. Berend: “I draw a ‘t’ on the preoperative X-ray to note where the perpendicular exits the tibial metaphysis; then we place the center of the cutting block on the proximal tibia at that same mark…trying to reestablish the rotation of the tibia on our proximal cut. The most important thing we’ve changed in the last 10 years is to always check the tibial cut with a long alignment rod.”
Dr. Pagnano: “I do preoperatively plan the resections so I get a quick check during the surgery. In the MIS realm these smaller cutting guides have been helpful. Also, I make sure there are no drapes on the anterior part of the tibia. Lastly, if I have questions after checking with the long alignment rod, I will get an intraoperative x-ray in really heavy patients.”
Moderator MacDonald: “Thank you.”
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