Dr. Springer: I do have to admit this is a really difficult, intimidating situation to have to come up and debate one of the most influential mentors in your career. It always leaves me with a little bit of a lump in my throat to try to figure out how to approach that.
We had many long conversations when I was a fellow about the wisdom of measured resection and he was such an influential mentor that as soon as I left the Brigham, I immediately started doing gap balancing as my preferred technique for total knee arthroplasty.
But, I think he’s right. I think our goals are really aligned in what we’re trying to achieve with total knee arthroplasty, but I think the technique remains controversial and I think it’s interesting for debate to be a pure gap balancing side and a pure measured resection side.
But I think we all kind of blend somewhere in the middle.
I think that’s what is appealing to me about gap balancing, it’s really a step-wise sequential approach. I tend to put my underwear on before I put my pants on.
Where we’ve really differed though is how we look at, evaluate and set femoral component rotation.
In gap balancing, we’re going to do this based on ligament tension to create a symmetric flexion gap.
In measured resection the rotation is really based on your ability to accurately locate those bony landmarks.
I contend this makes the difference between an A student and a C student.
So, again it’s a step-wise approach. We cut the distal femur. We cut the proximal tibia and we balance the knee in extension. Once we’ve achieved that balance in extension, our goal is creating that rectangle through a series of releases in extension.
Then we want to match that in flexion. You can use a tensioner. You can use lamina spreaders. You can use spacer blocks. The concept is still the same. You put symmetric tension on your ligaments in flexion. That you rotate the femoral component to create that symmetric flexion gap and then you match your extension gap.
One of the biggest issues you have to understand with this technique, no matter which technique you do, is that those releases that you do in extension very clearly have an effect on the medial side of the knee in flexion. That’s particularly true depending on what and when you release.
You can release in extension if we’re tight on the medial side. Our releases are mainly going to focus on bone, osteophyte and the posterior medial corner of the knee. If you aggressively release down into that superficial MCL [medial cruciate ligament] you’re going to create medial-sided laxity on the knee. This is not an uncommon scenario that oftentimes you will find when you over-release the superficial MCL.
I think there are issues with using fixed bony landmarks for femoral rotation. There’s significant difficulty in accurately and reproducingly finding them every single time. This leads to substantial variations in rotation, sizing and gap symmetry, and oftentimes can lead to arguments in the operating room.
Let’s just look at those landmarks from studies.
Transepicondylar axis…remember these are all ranges and averages based off of studies. Within 3 degrees only 75% of the time. The AP axis, up to a 32-degree range of error in femoral component rotation.
Using solely the posterior condylar axis, particularly in the valgus knee. Inaccurate 30-45% of the time. And what you gain from all of these curves is that using bony landmarks generally gets it right about 70% of the time in most knees. And that’s why really the bible of measured resection is the bell-shaped curve.
In West Virginia or at Harvard, that is still a C in my book.
If we look at just a couple of studies that are out there…this is one from our group (Fehring et al, CORR 2000)…100 consecutive posterior stabilized total knees, two techniques compared, measured resection and gap balancing. Rotational errors greater than 3 degrees were present almost half the time when bone landmarks were utilized.
And then Doug Dennis’ work (CORR 209) looking at femoral condylar lift-off—and I do agree with what Dr. Thornhill said about the differences between PS and CR for the most part—that a PS gap balanced total knee results in less condylar lift-off when you’re using a gap balancing technique.
I think our goals are the same regardless of the technique that you use. But the surgeon’s ability to accurately locate important bony landmarks still remains limited and can lead to rotational errors and imbalance in flexion. And that gap balancing, at least in my hands, can reliably and reproducibly produce a more balanced knee.
And there is also another movie that was just released. The most recent “Star Wars” and it addresses the issue of gap balancing. I think you’ll see that the force is strong with gap balancing.
