“Tensioners optimize stability and alignment…and stability controls alignment, ” argues Kelly Vince. “Look, we share the same goals, ” says Jan Victor. But there are several hidden “icebergs” which can sink the operation.
This week’s Orthopaedic Crossfire® debate is “Ligament Tensioners Optimize Stability and Alignment.” For the proposition is Kelly G. Vince, M.D., F.R.C.S.(C) of Whangarei Hospital in New Zealand; against the proposition is Jan Victor, M.D., Ph.D. from the University of Ghent in Belgium. Moderating is Michael J. Dunbar, M.D., F.R.C.S.(C), Ph.D. from Dalhousie University in Halifax.
Dr. Vince: “I think what trumps Level I evidence is physics, logic and common sense. So if we look at lever arms we see that the bigger the lever arm the larger the load that it balances on the other side; conversely, the smaller the lever arm the smaller the load it balances. If we don’t correct deformity then the patient is left with bad biomechanics and bad loads in strategic points on the joint.”
“There was a 2010 paper in JBJS that looked at mechanical axis alignment (15 year survival). The paper doesn’t say that alignment is not important; it says that maybe there is a permissible range of alignments within which knees function well. But there are certainly boundaries beyond which we would call a knee ‘mal-aligned’ and expect bad things to happen.”
“Whether you have varus/neutral/valgus alignment, the knee that is stable will function better right off the bat. But even if you don’t have the optimal alignment, you at least only have one alignment. That is why stability is important…it controls alignment. By contrast, if you have instability it means that you have many alignments; you usually have the worst one for any given loading pattern.”
“We need alignment; the knee must be stable, and that process is one of tensioning. And if you’re going to break those rules you have to ask yourself if you are recreating anatomy for your patients or are you recreating pathology. I use the famous on-board computer as my tensioner, but any device that helps with that has got to be good.”
Dr. Victor: “I share common goals with Dr. Vince. Both of us want to achieve the desired alignment in a reliable and consistent way. We both want to obtain functional stability throughout ROM [range of motion], with gap equality (flexion-extension) and gap symmetry (medial-lateral). Most of all, we would like to restore the flexion axis of the knee because that is what will return the patient to normal kinematics.”
“We share the same goals, so the debate is basically over how to achieve these goals. If I were to visit Kelly and make my way around Antarctica, I could take the Eastern route or the Western passage—both will bring me to my goal. But one route might be more dangerous than the other.”
“With tensioners they cut the tibia. Then they crank up the femur until the ligaments are tight. They position the block parallel to the tibial cut then make the femoral cuts and put in the prosthesis.”
“Where are the icebergs? The first one is that the ligaments do not have a linear load elongation curve. Secondly, it’s incorrect to think that the knee is symmetric. The medial side does not equal the lateral side. We know that the central fibers of the medial collateral ligament (MCL) are isometric; in contrast, the lateral side of the knee is non-isometric, is lax in flexion, and is stabilized by muscles.”
“Thirdly, tensioners check balance at 0 degrees and 90 degrees of flexion, but there is no information on mid- or deep flexion. Fourth, there is a dependent relative position in the sagittal plane of the tibia and the femur. If you crank up the femur you basically verticalize the PCL by pushing the femur backwards and pulling the tibia forwards. That is what you do with a tensioner.”
“It might not be much of a problem if it weren’t that your prosthesis will define the sagittal relationship between the tibia and femur…and that might not be the same as what you did with your tensioner. Fifth, there are uncontrolled variables such as a dislocated patella, or chronic ligament injury or insufficiency. So with these five icebergs I think it’s fair to state that there is room for improvement.”
“We should go for adapted measured resection. We know the medial side of the knee is the stable isometric side. Therefore, we want to restore the medial joint line level and the medial posterior condylar offset. To do the former, you insert an intramedullary rod, you hit the medial condyle distally, perform the cut, and you restore the original joint line level.”
“But what about the varus knee where there is cartilage and bone wear? Without realizing it you have already raised the joint line by 4mm. More importantly, you might do the same in flexion. As you put on this block and you have central pivoting when you apply external rotation you will take out more bone on the medial side.”
“If you look at the medial condyle you see that flexion-extension occurs around the single point of pivot. If you insert a prosthesis with the same geometry and the same size then the isometry of the MCL will be maintained. However, if you perform the cuts with a proximalization of your femoral component and an increased cut on the posterior medial side, then you raise the femoral component and place it more anteriorly.”
“So the center of rotation has been shifted proximally and anteriorly. In kinematic terms the knee will now flex and extend around this new center of rotation. As you get to 90 degrees there is normal tension; as you reach 120 degrees of flexion you have a tight MCL and that may be painful for the patient.”
“It’s better to pivot around the posterior side of the medial condyle in order to preserve your posterior medial offset. You do that by putting on a spacer if there is cartilage or bone wear posterior medially, and then apply the desired amount of rotation that you want to add to that reference line. As for reference lines, we looked at the surgical transepicondylar axis and found this to be very close to the optimal flexion axis. However, we found that the posterior condylar axis had the smallest standard deviation.”
“My conclusive algorithm: I perform measured resection, compensating for cartilage or bone loss on the medial side. I try to use the posterior condyles as a reference; I add three degrees of external rotation, and except for severely deformed knees where I perform a CT scan, measure the condylar twist and then correct the measured value. Finally, I confirm if these settings align with gap balance.”
Moderator Dunbar: “So Jan, you’re advocating that we should avoid the tensioners because it gives us a false sense of security since we’re looking at flexion-extension and the metrics are perhaps old. Is that a fair assessment?”
Dr. Victor: “My point is: use as much information as you can. For me, the geometry is the basis. The ligaments will adapt to that. If I see a big mismatch I will cheat a bit on the geometry and try to go a little bit towards the ligaments.”
Moderator Dunbar: “If we could provide a tool that gave you on-board information that was more reliable and reproducible…and more importantly we could give it to our colleagues who are starting their practices so they could start with your knowledge.”
Dr. Vince: “I think most of what I know goes on a napkin. These are very sophisticated and important concepts that we need to think about more thoroughly. To translate this into correcting the entire range of deformities that we’re going to face on any given morning is a tough job.”
Moderator Dunbar: “Excellent point. We have a lot of happy patients despite the fact that we’re all kind of doing it this blunt way.”
Dr. Victor: “It’s very simple. You restore the medial joint line level, and you can do that in extension and in flexion. Then you adapt your rotation in order to have a rotational axis that is close to the optimal flexion axis. That is simple and reproducible.”
Moderator Dunbar: “So why do we get so many good patients just doing it conventionally?”
Dr. Victor: “There is a next step, which is to keep that original coronal orientation of the tibia. What we do now is adapt for the fact that we make a perpendicular cut on the tibia. That means externally rotating the femoral component. If you cut parallel to the original orientation of the joint line you would not need that rotational correction. That’s a new concept.”
Dr. Vince: “If you’re going to change the rotational axis by making the rotation of the femoral component a function of something else—instead of where we would like it to be anatomically—then we know empirically that this can play havoc with patellar tracking. And if you’re saying that the axis is important because it represents the function of the ligaments, the ligamentous environment is part of the pathology in many cases. Get the mechanics right and then adjust the soft tissues with surgical techniques.”
Moderator Dunbar: “In the future will we be looking at biomarkers to say that ‘this collagen type is different than this patient and should tension it differently?’”
Dr. Vince: “That’s a long way down the track.”
Dr. Victor: “The amount of laxity you accept at the end of the operation will be a bit different from patient to patient.”
Moderator Dunbar: “Thank you both.”
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