A very powerful counter argument about this neutral target we’re aiming for and fitting to a widely variable patient population.
This is data from our center where we did a randomized trial looking at the shape/match system with navigation. We have one group in the navigation going for neutral mechanical axis and the shape/match was wherever we put them. What was interesting to us was that even though we were trying for neutral, we ended up in a bit of varus in both populations. And the two distributions looked more or less the same for hip/knee/ankle angle.
When we looked at the tibial component angle, that’s where the action is, we saw the difference between kinematic alignment and neutral mechanical alignment. It’s not necessarily the residual alignment. It’s where you put the components within the soft tissue envelope.
You don’t have to make something straight to make it last. We look at coronal alignment as if that’s the way our patients are going to stand straight. But our patients have the audacity to climb stairs, get in and out of bathtubs and cars. This is not a 2-dimensional or 3-dimensional; it’s a 4-dimensional operation.
I can’t help but think that this might have something to do with the fact that we have this glass ceiling of around 18% dissatisfaction with total knee arthroplasty—which is not improving with time.
The number one reason for patients are dissatisfied is unmet expectations.
Think about it. Neutral mechanical alignment. This is how we’re taught. I would submit this is wrong target for patients.
Consider a patient, now standing, mid-stance, or walking, their newly implanted low friction, slippery surface is loading all their proprioceptive fibers in valgus, when their whole life they’ve been loading in varus. The patient says, “This sucks. I don’t know what’s going on, but it feels wrong.” Unmet expectations.
Only 5% of the population is naturally in neutral tibia. Yet, we’re told to cut them all in neutral because that’s the safest thing we can do. We don’t want to be outside that ±3 degrees. Then we transpose that into flexion, set up a whole series of machinations where we have to chase our tail by externally rotating the femoral component. We make non-anatomic cuts, then we put an implant in place.
We take time to level these cuts at 0 and 90 degrees and we balance the knee both in flexion and extension and we think we’re doing a good job, but we completely forget about what’s happening mid-flexion. It’s not a coincidence, in my opinion, that instability is a leading cause of early revision in total knee arthroplasty.
There is evidence coming out. In an RCT on 60 patients; 30 and 30 in each group (Matsumoto et al., Bone Joint J 2017). No increased complications in the kinematic alignment (KA) group. Also, better flexion in the KA group and a better Knee Society functional score indicating better function. Perhaps the component is in the right position.
A meta-analysis on the same topic (Courtney et al., J Arthroplasty 2017), looking at nine studies; 877 patients; 38 months follow-up. No increased complications with the KA group. Using the Knee Society functional score, KA was favored over mechanical alignment.
In conclusion…neutral mechanical alignment is the historic paradigm. But it ignores individual alignment, morphology, and biomechanics variation. Under the current surgical model, zero degrees is often considered the safest position. But that’s changing as the new tools are becoming available.
We’re not at a dead end. We can do better.
