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Achieving Balance and Stability During a Total Knee Arthroplasty
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.
BOX 8.1
Symptoms Associated With Total Knee Arthroplasty Instability
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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. ,
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