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.
Fig. 1 - Serial Knee Balancing
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.
Fig. 2 - Parallel Knee Balancing
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.
Surgical workflow for knee replacement
Many surgeons attempt to restore 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. Surgeons may set their distal femoral resection at ~9mm (i.e. the thickness of the femoral component.)
There are only 6 possible ways to do any three sequential steps. Surgeons must cut the distal femoral, proximal tibial and posterior femoral bone, but in what order? Surgeons have a priority stack and prioritize the most important decisions at the top of their list. For 98% of surgeons, they prioritize the extension gap and cut the distal femur and proximal tibia first as shown in columns I and II (Fig. 3). The distal femoral and proximal tibial cuts determine the leg’s coronal alignment, which has historically been at the top of surgeons’ priority stack. Many KA surgeons are now comfortable with some variation in coronal alignment if it improves their ligament tension. With manual instruments, the posterior femoral cut must be performed after the distal femoral cut which is another reason way columns I and II are most popular. The flexion gap is typically low on surgeons’ priority stack because surgeons feel they can correct the flexion gap with rotating the femoral component.
Fig. 3 - 6 ways to cut a knee replacement
Things only possible with navigation
Referencing the posterior femoral condyle
Columns I, II and III are possible with both manual and navigation instruments. Columns IV, V and VI are only possible with navigation instruments.
Surgical navigation allows surgeons to prioritize restoring the patient’s posterior femoral offset. Since the posterior femoral condyle typically is the least worn surface, the posterior femur should be the best reference point. I often begin my virtual balancing of a knee replacement by setting the posterior femoral bone resection at 9 mm (implant thickness) for both the medial and lateral posterior femoral condyles. In other words, I virtually cut the posterior femur at 0 degrees to the posterior condylar axis. I will adjust the femoral component size and flexion to provide an ideal anterior femoral cut (i.e. no notching, no air ball). I will then virtually adjust my tibial cut (raise/lower & varus/valgus) to balance the flexion gap. I then virtually 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 balance the knee ligaments, to have all six bone resection match the implant thickness as much as possible, to accept a coronal alignment +/- 3 degrees, and to accept 0 to 3 degrees of varus on the tibial cut.
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 “airball” and allow the anterior flange of the femoral component to not contact the anterior femoral cortex. 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.
Axial Rotation
Navigation allows a surgeon to measure axial rotation in extension and flexion which is not possible with manual instruments. During surgery, surgeons can rotate the tibial component to recreate the patient’s pre-op knee axial rotation.
Surgical Navigation of Hip Replacements
Navigated hip replacement surgery offers less parallel processing and is why hip navigation is not as popular as knee navigation. Navigated hip surgery does not allow surgeons to do things that are only possible with navigation. The acetabular component positioning is independent of femoral component positioning. In other words, surgeons put their acetabular component in the bone without much thought to where the femoral component may be located (i.e. serial processing). One possible minor example of parallel surgical decision-making in hip replacements is combined version. If a surgeon knows the femoral component version, then the surgeon may adjust the 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.
