Introduction
When considering all of the steps involved with a total knee arthroplasty (TKA), placing the components in proper alignment and with appropriate balance may be the most important. An unstable knee can lead to persistent pain, effusions, and difficulty on unstable ground or stairs, all of which will compromise the patient’s outcome ( Box 8.1 ). TKA instability is one of the leading causes for primary and revision TKA failure. In addition, even if patients do well initially, an unbalanced knee can lead to increased wear of the components and a need for early revision. In fact, it has been reported that around 20% of patients undergoing revision TKA do so for instability. One study found that soft-tissue imbalance and instability caused over 25% of early failures and almost 20% of late failures for TKAs. Furthermore, TKAs that are revised for instability appear to have worse outcomes than TKAs revised for other mechanisms of failure, , and an unstable knee can lead to catastrophic injuries ( Fig. 8.1 ). Therefore, obtaining the proper balance and stability for a TKA at the time of surgery is of upmost importance.
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Recurrent effusions
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Difficulty ascending/descending stairs
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Pain about the iliotibial band and hamstring insertions
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Quadriceps weakness
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Repeated feelings of the knee “giving out”
The stability of a TKA is influenced by surgical technique, host factors, bone cuts, the selection and placement of implants and soft-tissue tension. , Because of all of these variables, it is crucial that surgeons develop an algorithm for checking and obtaining proper balance during TKA procedures, some of which have been published by previous authors. , However, judging the balance and stability of a TKA intraoperatively is largely a subjective activity that cannot simply be accomplished by moving from one step to another. This chapter will review some of the main concepts and techniques regarding this crucial aspect of surgery so that surgeons will be able to attain an optimal outcome no matter the circumstances.
Preoperative Considerations—What Is Instability?
In general, instability is typically categorized as occurring during flexion, mid-flexion, or extension, with global instability referring to instability during all points in range of motion ( Fig. 8.2 ). However, because of the subjective nature of this diagnosis, discussions and definitions surrounding instability are difficult to compare. While the goal of any TKA is to allow the knee to maintain the appropriate position during periods of function, what may be considered stable for one surgeon or patient may feel unstable to another. In fact, fluoroscopic studies have found that there is a wide variation in joint contact points for TKA patients during activities of daily living and that these differences likely persist to some degree no matter what TKA implant is chosen. , Therefore, when it comes to determining stability of a TKA, a broader definition of stability can be considered. In short, a TKA is considered unstable when changes in motion about the knee create large and unsettling changes in contact points within the joint.
Intraoperative Total Knee Arthroplasty Balancing
Mechanical Alignment Versus Kinematic Alignment
The mechanical axis of the lower extremity is defined as a line connecting the center of the femoral head with the center of the ankle joint. Most native proximal tibias are in 3 degrees of varus compared with the mechanical axis of the limb, whereas most native distal femurs are in 9 degrees of valgus. Therefore, cutting the distal femur in 6 degrees of valgus and cutting the tibia perpendicular to the mechanical alignment allows the TKA to be placed in neutral mechanical alignment. The thought behind achieving mechanical alignment with a TKA is that creating a TKA that has a bearing surface perpendicular to the mechanical axis will equally distribute the loads across the joint and minimize the accelerated wear of the TKA components. While it is important to remember that every patient may have unique anatomic features, there is a large amount of literature suggesting that placing TKAs in neutral mechanical alignment is likely to improve the long-term outcomes and decrease early failure of TKA implants regardless of the patient’s preoperative limb alignment. , Although there is some debate regarding how much deviation from the neutral mechanical alignment can be accepted before an increased rate of failure for TKAs is realized, it is generally accepted that placing components with more than 3 to 5 degrees of varus or valgus relative to the mechanical axis can increase their rate of failure. Still, determining how important pre- and postoperative coronal plane balancing and alignment is for the outcome of a TKA remains a point of contention among surgeons. Notably, there are plenty of data suggesting that having an ideal mechanical axis alone does not lead to a satisfactory outcome for many TKA patients. Therefore, some surgeons have started to question whether a mechanically aligned knee should be the goal of surgery instead of creating a knee that is more in line with a patient’s native alignment. These thoughts have led to a resurgence of investigation regarding kinematicall or anatomically aligned TKAs.
Proponents of kinematically aligned TKAs argue that creating a mechanically aligned knee for a patient may alter the soft-tissue tension and natural biomechanics of patients, leading to an unsatisfactory outcome or a TKA that does not feel “normal.” Some studies have found that around 20% to 30% of patients have mechanical axes about their knee that are more than 3 degrees of varus and that correcting these knees to a neutral mechanical axis could distalize the lateral joint line and alter the natural biomechanics of their knee. While the initial attempts of kinematic alignment demonstrated high failure rates, this may have had more to do with the TKA implant designs rather than a failure of kinematic alignment technique. Recent data regarding the short-term outcomes of this technique are more promising than that in previous literature. However, further long-term data are needed to evaluate this technique. ,
Gap Balancing Versus Measured Resection
Although there have been many articles evaluating the potential benefits of gap-balancing and measure-resection techniques for TKA, in reality most surgeons employ a combination of both techniques. Furthermore, there are no data clearly showing that one technique is superior to the other in terms of patient outcomes. ,
In the measured-resection technique, surgeons aim to remove the exact amount of bone that is being replaced by the implants and determine femoral rotation based on anatomic references (the femoral posterior condylar axis, transepicondylar axis, and anteroposterior axis) before addressing the soft-tissue balance. The posterior condylar axis (PCA) is the most commonly used landmark for determining femoral rotation with this technique and is typically believed to be internally rotated about 6 degrees relative to the transepicondylar axis (TEA). The TEA is a line bisecting the high point of the lateral epicondyle with the sulcus of the medial epicondyle, defining the flexion-extension axis of the knee. It defines the axis on which knee flexion and extension occurs and is thought to be the most reliable landmark for determining femoral rotation, if not the most difficult to find intraoperatively. , Lastly, the anteroposterior axis, commonly referred to as the Whiteside line, is marked by finding the deepest part of the trochlea. This axis should be perpendicular to the TEA. Using this line as a reference has been found to be quite reliable, especially in valgus knees. In practice, many surgeons will use multiple femoral anatomic references when using this technique, as obtaining accurate femoral rotation with one or more of these anatomic landmarks is a challenge. This is especially true in valgus knees in which the lateral posterior femoral condyle may be hypoplastic and where the TEA may be externally rotated from the posterior condylar axis much more than the typical 3 degrees seen in most patients.
The femoral cutting blocks used during measured-resection techniques can be either anterior or posterior referencing. Anterior-referencing guides are designed so that component size changes will not take any additional bone anteriorly, reducing the risk of notching. However, this also may allow for additional bone to be removed from the posterior condyles, which can lead to flexion instability by increasing the flexion gap. Conversely, posterior referencing guides are designed so that any additional bone removed is from the anterior surface of the femur, allowing the flexion gap to be tightened if desired.
In the traditional gap-balancing technique, the surgeon will make the distal femur and tibial cuts first and then use that “gap” or soft-tissue tension to determine the posterior femoral cut and distal femoral rotation. The goal of this technique is to ensure that the distal and posterior femoral cuts parallel the tibial cut at 0 and 90 degrees of flexion, respectively, so that there is equal ligament tension in both flexion and extension. When using this technique, the surgeon has to be sure to balance the medial and lateral gaps before performing the final bone cuts since both gaps will be dependent on their position. This is especially true when performing the posterior femoral cuts, as leaving posterior femoral osteophytes in place prior to tensioning the flexion gap and performing the posterior femoral cuts can lead to an unbalanced flexion gap once the cuts are made.
One of the benefits of the gap-balancing technique is that it is not dependent on any of the femoral landmarks that may be difficult to identify during surgery using the measured-resection technique. Furthermore, there are some data suggesting that surgeons are more readily able to produce equal flexion and extension gaps and decrease the number of TKAs in which the femoral rotation is more than 5 degrees from neutral when using this technique compared with the measured-resection technique. , The disadvantages to this technique include its dependence on a precise tibial cut in addition to the danger of performing an over-resection of the posterior femoral condyles. Errors at either of these steps can lead to either a misaligned TKA or difficulties in obtaining a properly balanced joint or adequate patella tracking. There are also data showing that the gap-balancing technique may lead to elevations of the joint line, mid-flexion instability, and the possibility of creating an excessive flexion gap when the posterior cruciate ligament (PCL) is sacrificed. In fact, there are some data suggesting that over one-third of patients undergoing a TKA using the gap-balancing technique may have symmetric gaps at both full extension and 90 degrees of flexion but have substantial mid-flexion instability. Lastly, while some surgeons will use spacer blocks to help determine their flexion and extension gaps, recent literature suggests that using these blocks may not help the surgeon balance these gaps as accurately as desired.
Patellar Tracking
Patella maltracking ( Fig. 8.3 ) is a common problem after a TKA. While there are many variables that can contribute to this problem, the most common reasons for patella maltracking during a TKA are internal rotation of the femoral or tibial component, medialization of the femoral component, placing a resurfaced patella button on the lateral side of the patella, or performing an oblique patella resurfacing. All of these actions can lead to an increased Q angle in which the forces of the extensor mechanism pull the patella lateral to the trochlea, causing either lateral subluxation of the patella or a lateral patella tilt. Checking the tracking of the patella during a TKA can be performed after the trial components have been placed under direct visualization or by using specific intraoperative tests designed for this purpose.
Sagittal Plane Balance
The flexion gap of the knee is largely controlled by the PCL, posterior femoral condyle offset, and the posterior capsule in addition to the medial and lateral ligamentous structures. There are many schools of thought regarding how to best manage the PCL during TKA. Some surgeons remove the PCL and place a posterior substituting or anterior stabilized polyethylene TKA. Others routinely leave the PCL intact in and place a less confirming polyethylene TKA. While removing the PCL during a TKA may allow patients to obtain improved flexion, surgeons need to be aware that resecting the PCL will open up the flexion gap by 3 to 4 mm , and care needs to be taken to restore posterior femoral offset and not perform an overly aggressive posterior femoral resection to ensure that this gap is not larger than the extension gap. If the surgeon chooses to leave the PCL intact, consideration should be given to distalizing the distal femur cut so that equally tight flexion and extension gaps are created and, if necessary, loosening of both gaps can be accomplished by removing more proximal tibia. The PCL can also be recessed from its tibial or femoral attachments to impart more space in the flexion gap. However, it is also possible that the PCL may attenuate or tear over time, which could lead to a balanced knee becoming unstable. Surgeons should also be aware that the PCL may contract in the control plane, which may make coronal plan balancing in flexion more difficult to obtain if it is left in place and a cruciate-retaining TKA is desired. Regardless, there is no concrete data suggesting that either technique with regard to the PCL portends better outcomes over another.
The tibial slope also needs to be considered when addressing the flexion gap of a TKA. Any decrease in the tibial slope will diminish the flexion gap, while any increase in the tibial slope will increase it. It is common that PCL-substituting implants are designed with an anticipated tibial slope of 0 degrees to help tighten the flexion gap created when the PCL is resected. Conversely, many surgeons who use cruciate-retaining designs will plan to provide 3 degrees of tibial slope to help increase the flexion gap.
Distal femoral resection will also affect sagittal-plane balancing. This resection will have a direct impact on the extension gap of the knee. Removing too little distal femur will make the knee tight in extension and may predispose the patient to a flexion contraction. If the extension gap is too tight and the patient cannot obtain full extension, stripping of the posterior capsule off of the femoral condyle should be considered. This is often done while the knee is in flexion after all bony cuts have been made and consists of placing a Cobb elevator at the capsule/bone interface to elevate the capsule off of the bone. If the extension gap is still tight after this maneuver, the surgeon can consider removing more distal femur. While this will raise the joint line some, studies have shown that raising the joint line 2 mm may not have deleterious effects on the outcome or balance of a TKA. However, surgeons should be aware that raising the joint line may impart some laxity to the collateral ligaments during mid-flexion, which can lead to mid-flexion instability. Conversely, removing too much may lead to recurvatum or instability with terminal extension. If too much distal femur is resected, then one technique that can be used to build back up the distal femur is the placement of cancellous under the implant in order to distalize the joint line ( Fig. 8.4 ). However, this will only add another 2 to 3 mm to the joint line; thus, an overly aggressive distal femoral resection should be avoided.
Related posts:
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Preoperative Planning for Total Knee Arthroplasty
Marlex Mesh Reconstruction of Extensor Mechanism Disruption in Total Knee Arthroplasty
Painful Total Knee Arthroplasty Workup
Troubleshooting the Direct Anterior Approach
Antibiotic Spacer Techniques in Hip and Knee
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