Virtual Knee Balancing with Navigation
Introduction
Surgical navigation and robotics have gained popularity in knee replacement surgery over the past few decades, but why?
Two decades ago, navigation surgeons claimed navigated knee replacements were superior to manually instrumented knee replacements because of improved accuracy, but skeptics showed that improved accuracy did not improved clinical outcomes. Back then, surgical navigation followed the same knee replacement techniques as manual instrumented knee replacements, namely mechanical alignment. This discussion is going to explain how newer navigation systems have improved and allow knee balancing techniques that were previously impossible.
Manually instrumented knee replacements rely on serial decision-making. A surgeon makes a decision, executes that decision, and then makes the next decision based of effects of the first decision. Once the first decision is made and executed, there is no going back. Surgical navigated knee replacements allow parallel decision-making. A surgeon can simultaneously analyze multiple variables, adjust these variables, analyze the results, make final decision, and then execute the plan. A surgeon does not have to execute any decision until all of the decisions are made and the results are known.
Serial vs. Parallel Processing in the real world
The most obvious real-world example of the benefits of parallel processing over serial processing is the GPU vs. CPU processor. A CPU executes one task at a time in a sequential order. A GPU executes different tasks simultaneously which allows GPUs to do computer graphics and machine learning (AI). Intel, the maker of CPUs, has a market cap of $100 billion. Nvidia, the maker of GPUs, has a market cap of $2.7 trillion.
A good physician utilizes parallel processing to make a clinical diagnosis. The physician will take a history and physical, review appropriate studies and lab work and then analyze all the available information to make a diagnosis. A bad physician may only look at one variable, make a diagnosis, and then start treatment.
A good entrepreneur will consider their product, the market demand, competitors, and customer feedback before they start their company. A bad entrepreneur may build a product without thinking about all these down stream decisions until after the product is finished.
A surgeon using manual instrumentation typically starts with making a distal femoral cut and/or a proximal tibial cut. They balance the extension gap with ligament releases and then rotate the femoral component to balance the flexion gap. Unfortunately, the surgeon made serial decisions and must live with the outcomes of the previous decisions.
As an analogy, consider driving home during rush hour traffic. You may have multiple ways to get home. You can cut through a neighbor with slower speed limits or use the highway. When you get to the highway on ramp, you look to see how backed up the highway is. You instantly decide to get on the highway or take the neighborhood route. Maybe the visual of the on ramp is the best guide to get you home the fastest, but maybe your Waze app can analyze all the different routes and find a faster way home.
A parallel knee balancing strategy means a surgeon can analyze all these variable in a virtual environment before they must execute their surgical plan. They are not beholden to previous decisions and forced to correct for the balancing issues that arise through serial decision-making.
Where to start
With both manual instruments and navigation, surgeons must have a priority stack. That is to say some decisions are more important than others, and surgeons prioritize the most important decisions. Surgeons set a reference point on the knee as their starting point and work through their surgical algorithm according to their priority stack. This reference point is typically the most distal articular surface on the medial femoral condyle and/or the lowest articular surface on the proximal tibial. The distal femoral and proximal tibial cuts determine the resultant knee coronal alignment, which was historically high in most surgeon’s priority stack, but many surgeons now feel less constrained by the coronal alignment. With manual instruments, the posterior femoral cut references the distal femoral cut, so the distal femoral cut must happen first. The posterior femoral cut (i.e. femoral component rotation) is easier to adjust and with less adverse effects than the distal femur and proximal tibial cuts. Therefore, the posterior femoral cut has historically been the lowest on surgeon’s priority stack and resulted in the largest variation from the patient’s normal anatomy.
Many surgeons want to restore the normal knee anatomy and kinematics. The surgeon can either reference the least worn articular surface or reference an arthritic articular surface and estimate the amount of worn cartilage. Many surgeons take 10 mm off the lateral tibial surface and/or 2 mm off the medial tibial surface for a varus knee. The greatest amount of worn cartilage occurs in the cartilage that sees the most weight bearing, namely the distal femoral surface and proximal tibial surface. In most knees, the posterior femoral surface sees the least weight bearing and has the least worn cartilage.
Navigation Unlocks
This section discusses options that are possible with newer navigation systems that were previously impossible with just manual instrumentation.
Flexion of the femoral component
Surgical navigation allows a surgeon to adjust the flexion of the femoral component to make fine adjustments in the flexion gap. Historically, surgeons with manual instruments could not change the flexion of the femoral component because they had already cut the distal femoral surface. Surgeons could only upsize or downsize their femoral component to change their flexion gap which increases or decreases the flexion gap by ~3 mm. Surgeons with manual instruments could also choose to allow the anterior flange of the femoral component to be prominent compared to the anterior femoral cortex (i.e. posterior referencing). With surgical navigation, the surgeon can visualize the flexion gap and the location of their anterior flange before they cut the distal femur, so they can still adjust the distal and posterior femoral cuts.
Posterior femoral condyle first technique
Surgical navigation allows a surgeon to use the patient’s posterior femoral surface as their reference point. Since the posterior femoral condyle typically is the least worn surface, the posterior femoral surface should be the best reference point. I often start my virtual balancing of a knee replacement with setting the posterior femoral bone resection at 9 mm (implant thickness) for both the medial and lateral posterior femoral condyles. I will adjust the flexion of the femoral component to provide an ideal anterior femoral cut (i.e. no notching, no air ball). I will then adjust my tibial cut (raise/lower & varus/valgus) to balance the flexion gap. I then adjust the distal femoral cut (raise/lower & varus/valgus) to balance the extension gap. I then analyze the thickness of the six bone resections (medial & lateral distal femur, medial and lateral posterior femur, medial and lateral tibia) and compare those thicknesses to the implant thickness. My goal is to have a balanced knee, the least cumulative difference between the resected bone and implant, and a coronal alignment +/- 3 degrees.
Surgical Navigation of Hip Replacements
Navigated hip replacement surgery offers least parallel processing and is why hip navigation is not as popular as knee navigation. The acetabular component positioning is less dependent on the femoral component positioning because One example of parallel surgical decision-making in hip replacements is combined version. If a surgeon knows the patient’s femoral component version, then the surgeon can adjust the patient’s acetabular cup anteversion to create a combined version of 45 degrees. Another example of parallel decision making would be leg length and offset. A surgeon could broach the femur and predict where their femoral component might sit and then adjust their acetabular reaming relative to the change in femoral leg length. When I see a hip with increased total offset (dysplasia), I will plan on using a high offset femoral component and deliberately under ream the acetabulum to preserve total offset.
